To Bernice, my beloved wife, for being so patient, supportive, and
understanding in the past eight years when I started out to fulfil my
dream as an author. We did not know how it would turn out, and
though there were tough and uncertain times, you stayed by my side
and cheered me on, knowing that my works will make a difference,
however small, in the lives of my readers, and leave a lasting legacy
when both of us fade into history.
I would not have come this far without you. Thank you for believing
in me. With heartfelt gratitude, I dedicate this book to you, just as
I did for the first.

Diagnostic
/ˌdʌɪəɡˈnɒstɪk/
noun
plural: diagnostics
1. a distinctive symptom or characteristic.
2. the practice or techniques of diagnosis.

An apprentice is one who practices until
he gets it right.
A craftsman is one who practices until
he can't get it wrong.

Copyright © 2023 by Ng Keng Tiong. All rights reserved.
Cover design by the author.

Products and services mentioned in this book are trademarks or registered trademarks of their
respective companies. All trademarks and registered trademarks are the property of their respective
holders.
No part of this book may be reproduced in any form, or stored in a database or retrieval system, or
transmitted or distributed in any form, by any means, electronic, mechanical, photocopying, recording,
or otherwise, without the prior written permission of the author.

LIMIT OF LIABILITY AND DISCLAIMER OF WARRANTY
The information, examples, illustrations, documentation, and other references in this book are
provided "as is", without warranty of any kind, expressed or implied, including without limitation any
warranty concerning the accuracy, adequacy, or completeness of the material or the results obtained
from using the material. Neither the publisher nor the author shall be responsible for any claims
attributable to errors, omissions, or other inaccuracies in the material in this book. In no event shall
the publisher or author be liable for direct, indirect, special, incidental, or consequential damages in
connection with, or arising out of, the construction, performance, or other use of the materials
contained herein.

Print copy ISBN-13 : 979-8-37-838890-5

PREFACE
PCB diagnostics is as much an art as it is a skill to be mastered, because
it demands creative innovation and complete immersion from the one
who practices this peculiar craft.
Printed circuit boards can be found in almost everything that runs on electricity in our modern
world today. From simple to complex, single-sided to multi-layered, commercial to military,
stand-alone to whole system, these electronic circuit boards perform myriads of functions they
are designed for, 24/7 daily or occasionally when required. In time, however, these PCBs will
fail——whether it's due to design deficiency, limited operational lifespan or reasons attributed
to human negligence and errors.
Every day, countless engineers and technicians engage in the practice of diagnosing PCB
failures, either in-house during the manufacturing process, on-site as part of customer support
or in a workshop that provides repair services. Diagnostic methods will vary depending on the
availability of resources such as equipment setup, test specifications and procedures, and
more importantly, PCB documentation. But what if we lack these resources to carry out fault
diagnosis? That's where innovation and invaluable experience distinguishes the pro from the
amateur.
PCB diagnostics is a skill that has to be developed and refined over time. Nothing can replace
the knowledge and know-how of troubleshooting different types of PCBs. It requires not only
hands-on experience but in-depth exposure to a wide enough spectrum of faults to enhance
understanding in the dynamics involved in this challenging yet rewarding endeavor. This book
is a culmination of the author's 30 years of experience in the field of PCB repair, both in the
military setting as well as the commercial sector.
Just as every culture has its own unique culinary recipes, each genre of PCB requires specific
approach when it comes to identifying failure or determining the cause of its malfunction.
Thankfully, there are common tools of the trade to rely on, as well as an arsenal of powerful
test equipment that can deliver faster, more reliable and consistent results. It is my hope that
this book will expand your horizon and open up new possibilities of what PCB diagnostics
entails.
Ng Keng Tiong
February 28, 2023

PCB Diagnostics

vii

TABLE OF CONTENTS
GETTING STARTED
1

Introduction

13

What is PCB Diagnostics? My Personal Journey. A Holistic Approach.
The TCM Analogy. Models and Methodologies. Setting Up a Workbench.
PCB-Related Equipment. Anti-Static Mat & Wrist Strap.
Summary.

2

Pre-Requisites

37

APPRENTICE LEVEL: Electronic Components. Circuit Topologies.
Reading Schematic Diagrams. Using Benchtop Equipment. Soldering
and Rework.
PROFICIENT LEVEL: Embedded Systems. Firmware Hacking.
Programming Languages. Test Jigs and Interfaces. Prototyping and Testing.
Summary.

LEARNING THE ROPES
3

Basic Diagnostic Skills

71

Preliminary Diagnosis. Visual Observation. Sensory Evaluations.
Past History. Basic Measurements. Passive Checks. Power Up Checks.
Common PCB Failures. Physical Damage. Open Circuit. Short Circuit.
Missing, Misaligned or Misoriented Components. Component Failure.
High-Risk Components. Intermittent Faults. Thermal. Mechanical. Erratic.
Summary.

4

Building Test Jigs
Test Jigs and Fixtures. Types of Test Jigs. Design Considerations.
Example 1: Programmable Attenuator Test Set (PATS)
Example 2: An Arduino LCD Testbench
Example 3: A Budget Test Rig for Low-Volume Production
Summary.

viii

97

5

Signature Analysis

121

What is Analog Signature Analysis? The Concept Behind V-I Test.
The Four Basic Signatures. Resistive Signatures. Capacitive Signatures.
Inductive Signatures. Semiconductive Signatures. Diodes. Zener Diodes.
Transistors. Integrated Circuits. Digital ICs. Analog ICs. V-I Testers.
Summary.

6

Clip-n-Test

143

In-Circuit Benchtop Testers. ABI System 8 Diagnostic Tools.
Advanced Matrix Scanner (AMS). Advanced Test Module (ATM).
Analogue IC Testers (AICT). Board Fault Locator (BFL).
Multiple Instrument Station 4 (AIS 4). Programmer Power Supply (PPS).
ABI System 8 Test Types. Case Study: Tenta CPCI SCOM-0800.
Summary.

7

Automated Testing

171

Automated Test Equipment (ATE). Hardware. Software. Test Interface.
Test Program Set (TPS). Test Fixture and Panel. Interface Cables.
Test Program. Test Program Set Document (TPSD). Flying Probe Testers.
Example 1: The RADCOM WSTS
Example 2: The Factron S730 In-Circuit Tester
Test Head. Test Software. Test Philosophy.
ICTR 1: Testing a 7400 Logic Chip
ICTR 2: Testing an IDT71256 Static RAM
Summary.

8

Thermal Imaging

207

Infrared Vision. IR Detection and Calibration. Types of IR Cameras.
Calibrating the IR Camera. Thermal Profiling a PCB.
Power Source and Cables.
Example 1: Water Damaged iPhone SE
Example 2: iPhone 7 Battery Drain Problem
Summary.

9

Other Techniques

225

Boundary Scan Test. JTAG Chip Architecture. Test Access Port Controller.
How BST Works. Boundary Scan Description Language.
Example: Testing an 80486DX2 CPU

PCB Diagnostics

ix

XJTAG. Example: The XJDemo Board.
Automated Optical Inspection (AOI). AOI Lenses. Inspection Methods.
Image-based System. Algorithm-based System. AOI Programming.
Inspection Process. Challenges in AOI. X-Ray Inspection.
Comparison of Inspection Systems.
Summary.

APPENDICES

x

A

Tables & References

B

Common Failures

C

Conformal Coatings

D

Counterfeit Parts

E

486DX2 Databook (Partial)

F

Glossary

He who works with his hands is a laborer. He who works with his heart
is a craftsman.
Francis of Assisi

Since its inception the PCB has undergone tremendous revolution, from the initial primitive
wire-wrapped boards to the sophisticated multi-layered incarnation found in today's electronic
systems. This has given rise not only to a new generation of hardware designers and powerful
EDA tools, but also the need for a new breed of PCB diagnosticians.

What is PCB Diagnostics?
I prefer the term 'diagnostics' over 'troubleshooting',
'fault-finding' or 'repair' because of the holistic nature in
which this word encompasses. Just as a traditional
Chinese medicine (TCM) physician views the human
body as a complete entity with inter-related biological
constituents and functionalities, and employs various
means and methods to ascertain the underlying root
problem of the patient and prescribe the right remedy
to bring about total recovery——so too a skilled and
experienced PCB diagnostician, whether an engineer or
technician, approaches and treats a defective PCB in an
analogous manner.
This requires a paradigm shift from the usual prognosis
based on symptomatic guesswork to identifying the real
cause via systematic observation and examination. Of
course, attaining this level of proficiency does not come
easy. A TCM practitioner not only needs to spend years
studying and familiarizing the human body, but also be
well-versed in pulse reading and the art of acupuncture
on top of knowing what to prescribe from the hundreds
of medicinal herbs. Similarly, a good PCB diagnostician
must not only be acquainted with the different types of
PCB and electronic components, but also be able to use
various tools and test equipment to aid in isolating and
rectifying both hard and intermittent failures.
Perhaps a brief memoir of my engineering career will help set the tone for this book…
PCB Diagnostics

13

Chapter 1

My Personal Journey
The year was 1978. I was in grade nine1 class 3A and the last privileged batch of students to
study electricity and electronics. One of the deepest impressions I had back then was doing
an electronic project, a superheterodyne AM receiver:

We were each given a kit set of components, a piece of
wooden board with a white laminate on one side, a bag
of nails and a length of single core wire.2 We can either
reproduce the schematic on the surface of the white
laminate surface or simply paste it over (I chose to do
the former). Using the schematic as reference, the nails
were then fixated on the nodes to facilitate soldering of
the components and wires according to their
orientations. This is a primitive form of PCB that we
were exposed to, something akin to breadboarding but
of a more permanent nature.
Most of us didn't get it right on the first try and we had to diagnose the problems, from miswiring to wrong component value or orientation, you name it we had it. Thankfully the majority
of the class managed to get the AM receiver working after some corrective actions and tuning
the various stages under the patient guidance of the teacher. That was my first attempt at
building a circuit and performing circuit diagnosis. It fired up my interest in electronics and
subsequently led to my enrollment in the Polytechnics to major in this subject.
1

Secondary three in the Singapore education system.

2

The wire-wrapping type, size 26 AWG.

14

GETTING STARTED

Introduction

I did two projects in the second year of my tertiary study: an adjustable dual DC outputs linear
power supply and a manual IC tester. Both were interesting hobbyist projects using singlesided PCB artworks and I had little difficulty fabricating it using photo-resist developer kit on
copper laminated boards.3 This was followed by a Z80 microprocessor-based project in my
final year. This time, I manually designed a double-layered artwork from the schematic using
Bishop tapes and component transfer templates. The holes are not plated-through so they
had to be precisely aligned such that the component legs could act as solder bridges on both
sides. It was quite challenging but fun, nonetheless.
Upon graduation, I signed up with the Republic of Singapore
Air Force (RSAF) and due to good academic performance, I was
earmarked for the E-2C program's radar team. But that was not
to be my destiny. During my attachment to the ground radar
squadron, I was assigned to work with a group of technicians
to maintain a Line Equipment Cabinet (LEC) which processed
data acquired from two height finders located on the hilltop
and relayed it to radar operators stationed within a secured
complex at the foot of the hill.
One common problem faced was lightning strike on the high
ground which often took out the cards in the LEC. After repair,
there was a need to check serviceability before plugging them
back into the cabinet rack. That's when we decided to build a
mock-up testbed. I was tasked to design the signal handling
portion of the semi-automatic tester. We used standard Eurosize stripboards for quick prototyping and within three months
came up with a working model.4 The commanding officer was
impressed.
Words got out and I was transferred to another squadron's repair bay that serviced the A4
Skyhawk using an automated test equipment (ATE) operated by a PDP-11 host computer. It
was my first encounter with a real-life sophisticated test equipment. Then someone from the
ATE team pulled out and naturally I became the replacement choice. So after completing my
on-the-job training (OJT), I was flown to the US with my teammates for a six month training
stint on two of the most advanced ATEs designed by Grumman Aerospace——the CAT-IIID and
the RADCOM. The year was 1987. It was one of the best times of my career life.
Positiv-20 was a well-known brand many electronic hobbyists used to produce their PCBs. It employed a positive
photoresist chemical in a spray can that can be applied evenly on a clean, grease-free copper surface and dried
using a hair dryer. A transparency with printed PCB artwork is then laid over it and the whole unit exposed to
ultraviolet light for 10 minutes. The transparency is then removed and the copper plated board immersed in
ferric chloride solution to etch out the artwork.
3

We took this route because designing and building the PCBs would be too costly (we would need to buy an EDA
tool like OrCAD SDT and PCB) and time-consuming (there would be a learning curve to use the tool). The fastest
and most practical approach was wire-wrapping since we weren't going to mass-produce them anyway.
4

PCB Diagnostics

15

Chapter 1

The ATE team was split into two groups, one handling the CAT-IIID and the other the RADCOM.
An US certified engineering test specialist (CETS) was assigned to each group to further train
us to operate all the test program sets (TPS) developed to test and repair the E-2C avionics. I
was fortunate to work on the RADCOM under the guidance of Mr. Ronald Dykeman, one of the
most experienced and meticulous engineers you could ever find. The RADCOM alone serviced
75% of the E-2C's electronic systems, including radar, communications, navigation, display
and passive detection system, which added up to over a hundred test programs!

The RADCOM Test Station
Due to the stringent requirement imposed on the repair quality of these airborne PCBs, we
were sent to the Air Engineering Training Institute (AETI) for the much-coveted PACE repair
and rework training course. 5 Our soldering skill underwent a dramatic transformation and
complete makeover we could no longer settle for anything less than perfection when it comes
to repairing PCBs.6
The three full years of working and training new batches of local technicians in the RSAF was
an enriching experience and laid a solid foundation for my future engineering endeavors.

Those who've been through the grueling process will know that it's a baptism of fire——a paradigm shift that
completely changes one's perception on soldering and desoldering. What used to be thought of as a low-level
menial work becomes a high-standard quality job that reflects the skill of a PACE certified engineer.
5

Incidentally, one of the E-2C teams who was trained for three months to operate the EMTC——Electronic Module
Test Cabinet, underwent the US Navy's version of PACE training which was comparable to the rigors of Navy SEAL
in technical terms.
6

16

GETTING STARTED

Introduction

A month before my contract with the RSAF run out, I was approached by a project engineer
from the Electronic subsidiary arm of the home-grown industry of Singapore Technologies (ST)
group, in part due to my invaluable experience with the RADCOM test station.7 I gladly took up
the offer since it was familiar ground and I got to do what I loved to do anyway. In my initial
two years with the company, I assisted two engineers in their TPS projects to completion and
successful delivery to the RSAF.
Meantime, I was introduced to a different genre
of ATEs——the Factron S700 Series automated
testers.8 When I first came on board, there was
just the S720 functional tester which belonged
to the Republic of Singapore Navy (RSN), and
was transferred to our work center for the same
purpose as the RADCOM. A few years later, we
acquired a refurbished S750 combinational 9
tester followed by an S730 in-circuit tester from
the RSAF two years later. Together, these three
ATEs churned out over two hundred and fifty test
programs to service the Singapore Armed Forces’
(SAF) various weapon systems. In the heyday,
our department employed over thirty engineers
who concurrently developed test programs on
these three testers.
The text based Factron testers served the center well for several years, but soon felt out of
favor with younger engineers who joined us to replace those who left for greener pastures. In
the end, the management decided to invest in two Teradyne ATEs that run on the modern
Microsoft Windows NT with a graphical test development interface——the Spectrum Series
8851 in-circuit and 8852 combinational testers. As we gradually phased out the aging Factron
testers, some of the test fixtures for the SAF's newer weapon systems needed to be migrated
over to the new test platforms for reuse with minimum modifications.10 We decided that the
best course of action was to build an adapter interface unit for mounting these Factron test
fixtures on the Spectrum testbeds and come up with some sort of conversion program to
translate existing test programs to run on these new test machines.

RSAF owned two of these stations but one was later transferred to ST Electronics for test program set (TPS)
development purposes.
7

Schlumberger was the company that produced these table-top style testers. Although it was not the top three
(Teradyne, HP and Genrad), I personally felt that it had one of the best test program development process that
is user-friendly and well thought out by the engineers who designed these ATEs.
8

9

A combinational tester has the capabilities of both in-circuit and functional testing combined into one.

Each of these test fixtures cost between $3,000–$5000 so it wouldn't be economical to build new ones for
use on the Spectrum testers.
10

PCB Diagnostics

17

Chapter 1

Retrofitting of test equipment was also a skill that I picked up as part of providing solutions to
obsolescence which the military faces. Believe it or not, the RADCOM test programs were
stored on magnetic cartridges mounted on HP7906 disc drive and run off an HP1000 host
computer. Over time the disc access heads of the drive built up ferrite dirt and caused disc
head crashes that scratched the magnetic surface of the cartridges, rendering them unusable
and had to be scrapped. The RSAF approached Grumman Aerospace for help but was given
an interim upgrade solution priced at 1.5 million dollars. Constrained by cost and desperate,
the project manager came to us for a possible alternative option. After some research, we
proposed to migrate the test programs to a magneto-optical (MO) drive that could emulate the
HP7906 and run flawlessly on the existing host computer. Best of all, it cost the RSAF less
than half of what Grumman asked for.

Another quirk about working with the military was the unusual requests that were put forth to
us. One memorable incident was resurrecting an old tester that serviced the Puma helicopter's
signal data converter (SDC) unit. In the process of moving this piece of equipment to its new
location within the air base, it suffered some structural damage due to poor handling. The
maintenance staff did not realize it and when he tried to power up the tester, there was a
catastrophic power outage that burned up the internal supply strip. We were called up to
assess the extent of the damage and if possible, bring the equipment back to life. Within two
weeks we had the power issue resolved but a first run of the self-test revealed that there were
other failures associated with that unfortunate episode. It was another two months before we
finally got the tester back on its feet. But the saga was far from over.
The technical staff who was trained to operate the tester was leaving in about a month's time
and the squadron project officer was hard pressed to find a replacement due to manpower
squeeze. As expected, my manager assigned me to understudy and pick up as much as I could
18

GETTING STARTED

Introduction

from that technical staff who was completing his service term. It wasn't exactly a smooth
handover but at the very least I was able to operate the old dinosaur11 by the time he left. Still,
it's not the end of the story. As it turned out, the RSAF decided to let our sister subsidiary (ST
Aerospace) take over the tester's operation and maintenance.12 I was tasked to come up with
a training course for its engineers and the military staff overseeing the transference exercise.
That's the irony of life.

Sometime in early 2003, we were awarded a project to develop 18 test program sets for ST
Aerospace who secured the contract to maintain RSAF's fleet of F-16 aircrafts. These test
programs were to be run on a WesTest 2000 DATS tester which the authority acquired for incountry servicing purpose and was housed in ST Aerospace's premise. It took our team of four
test engineers nearly three years to complete and deliver the goods. All in all, in my 25 years
with the company, I had worked on several ATE platforms and developed numerous test
program sets for a wide range of PCBs.
Now, you would think that test engineers are only responsible for developing test programs.
Not in our case——the business model of the work center requires every engineer to take up
PCB repair on top of writing and debugging test programs. At any one time, a senior test

This tester was built by an Israeli company more than ten years ago at the time we came on the scene to
salvage the mess. Most of the cards in the equipment bay were wire-wrapped, including the backplane harness.
The host industrial computer was a 486DX motherboard that operated on Windows 3.11 (more like a graphical
application layered on top of good old DOS 6.22). See above figure for an idea of its built.
11

It's not surprising that ST Aerospace always get the best deal from the RSAF because they service most of the
airframe and engine of our country's military planes, so the authority would rather choose to deal with just one
main contractor and leave it to us to divide the pie.
12

PCB Diagnostics

19

Chapter 1

engineer like me could have between 3-5 projects on hand while keeping track of 10-20 repair
jobs.13 Sounds crazy? You bet!
These are some of the major milestones in my engineering career, both in the military setting
and commercial sector. I have intentionally skipped many details to make the storyline less of
a drag and more of an inspiration to my readers. But rest assured, as we progress on I will fill
in the missing pieces as and if needed. It's always important to focus on the essentials and
not get sidetracked by trivialities that do not contribute to the overall learning process, or if I
feel that my readers may not be ready for such details. I like the advice of Dan Millman, a
personal development speaker and author, who said:
I learned that we could do anything, but we can't do everything… at least not
at the same time. So think of your priorities not in terms of what activities you
do but when you do them. Timing is everything.

A Holistic Approach
There is no one size fits all approach when it comes to PCB diagnostics; more often than not,
it depends on what kind of PCB and failure you're up against. While there may be visible and
obvious signs that point you to the source of a problem such as a burnt or blown component,
you shouldn't expect these as the norm. There are engineers who swear by a specific tool or
technique that they claim to be as reliable as clockwork, but even that is no guarantee to work
every time or fool-proof against any PCB. "Well, it's fine with me so long as I'm comfortable
with the method of my choice." Sure. But why limit yourself if there are better and more
effective ways to do a job? After all, if you intend to make repairing PCB a lifelong career or
passion, it makes sense to learn as much as there is to learn about this trade, to seek
continuous improvement and a whole new level of experience. This is what distinguishes a
professional from an amateur, or a craftsman from a mere worker.
What I'm advocating in this book is a holistic approach to PCB diagnostics that is based on
practical, real-life examples. This includes case studies from my own work experience using
various tools and techniques, as well as that of some field engineer friends of mine who are
generous enough to share their expertise. There is a Chinese saying, "It is not beyond one's
dignity to learn from the wisdom of others." In a company of three we can always find a teacher
who can endow new knowledge, or a master who can impart a useful skill. One of the most
important traits of a good engineer is a willingness to be open to opportunities of learning to
further enhance one's abilities. This is something that cannot be attained in a classroom, but
only by emulating other good engineers.

Whether it's a new job input, work-in-progress, awaiting spares, conformal removal or re-application, etc. the
repair work inflow never stops. It meant constant income for the center and management was certainly happy
but not for those of us who were under much duress. Except for the core group who could put up with such work
regime, most engineers who joined us usually quit after 2-3 years.
13

20

GETTING STARTED

Introduction

It brought to mind a painful experience I had several years ago, while I was developing test
programs for the F-16 project at ST Aerospace. One day during work, an engineer there asked
me to assist in moving a power supply module to a new location. That equipment must've
weighed almost 200kg and being foolhardy, we did not enlist the help of more people or use
a roller platform. Instead, by sheer force of the will we lifted it up with our bare hands and
carried it to the designated storage area.
I didn't feel anything amiss apart from a tinkling sensation on my lower back after the exertion
and went back to my work. That night, however, I began to experience a gnawing pain creeping
up and the next morning quickly went to a clinic near my home. The doctor said I probably
sprained my back and gave me some painkillers and a day's medical leave. Boy was he wrong!
The pain got so bad I could not even turn over when I sleep, and the painkiller did little to
alleviate my misery. I had to take another two days off work because I struggled to even walk
normally. A colleague of mine heard what happened and recommended a Chinese physician
who had treated his wife with a similar condition.
The next morning, I wasted no time and made my way from the west (where I live) to the east
(where the TCM clinic was). The queue was long and I had to wait about an hour for my turn.
When I finally stepped into the room, the Chinese physician observed my awkward body
movement and commented, "You have a serious back problem." He made me lie face down
on a massage bed and ran his fingers along my spine, lightly pressing and feeling every part
of the vertebrae. When he came to the lumbar section, he exclaimed, "Here's the problem!
Both L3-L4 are out of position." He proceeded to massage my legs to relax the muscles. Then
bending over, he bent my right leg and pressed it against his right chest, at the same time
gripping my right thigh with his right hand as he positioned his left fingers over the L3-L4 area.
He then asked me a non-related question and as I was answering, he gave my right thigh a
firm jerk upwards as he pressed hard on the lumbar area with his left fingers. I was startled
by the sudden action and heard a 'click'.14 The Chinese physician put down my right leg, gave
my lumbar area a short massage and told me to stand up and do squats and jumps. It was
incredible! The pain was gone and I could move freely and do what I couldn't a while ago!
What this episode illustrates is there are many ways to go about solving a problem but not all
methods are effective or equal. If I were to go to the hospital, the medical staff might subject
me to a series of tests and X-ray before referring me to a specialist or physiotherapist. It would
probably take weeks or longer to improve my condition, or I might even have to go under the
knife to correct the problem. I would end up with a hefty medical bill and be out of action which
would probably affect my work performance. This Chinese physician, by his knowledge and
expertise, however, solved my back problem in less than half an hour——without surgery or
medication. What's amazing is that it cost me only thirty bucks! This personal anecdote
underscores an important point: there is often a better way to solve a problem, so be open to
such possibilities.

The question was meant to be a distraction to reduce my anxiety. It certainly worked and I didn't feel any pain
except for a slight sensation as the L3-L4 segments were restored to their original positions.
14

PCB Diagnostics

21

Chapter 1

The TCM Analogy
A Chinese physician relies on three main components to form a diagnosis and find the underlying cause of an illness, namely:
▪
▪
▪

The balance of yin and yang15
The state of the twelve organs and their five elements16
The vital substances17

Similarly, we can apply this correlation to a PCB diagnostician who relies on the following traits
to determine the cause of a failure:
▪
▪
▪

The distribution of voltage and current
The functional states of devices and their characteristics
The signal parameters

Every PCB has its own operating voltage requirements. Upon power up, current is supplied to
the components and the board reaches a quiescent state when the node voltages stabilize at
the working temperature. This is the equilibrium or balanced state of the PCB. Any failure in
the PCB will result in a deviation from this normal condition, ranging from power outage (dead)
to erratic performances (intermittent or fixed faults).
Like any biological entity, a PCB is made up of different component parts that function as a
whole. It can be a single-sided board with just discrete passive and semiconductor devices, or
a multi-layered PCB comprising large-scale integrated circuits and sub-modules. Whether it’s
simple or complex, these parts are the vital organs of a PCB which are made up of different
composite materials (elements).
Signals are the vital signs of a PCB. A board that is working properly will have normal signals
flowing and interacting between its components. Signals can have AC and DC parameters
depending on the type of devices that produce and condition them, so if any of these devises
malfunctions the associated signal(s) will either be missing or messed up.
Hopefully the above analogy will give you a better conceptual introduction into the diagnosis
of defective PCBs. Such a holistic approach not only broadens your understanding of the PCB
repair process but makes it enriching and enjoyable at the same time.
Everything in the universe can be described as a combination of these two fundamental forces which are in a
constant state of flux. Yin and yang attribute to the duality, interaction, interdependence and transformation of
life.
15

The twelve organs are grouped into yin and yang as well, the former being the lung, spleen, heart, kidney, liver
and pericardium; the latter comprises the stomach, bladder, gall bladder, triple burner, and the large and small
intestines. The five elements are metal, wood, water, fire and earth.
16

These are the basic constituents of our body functions that determine the state of our health. Vital substances
include qi (energy), blood, jing (essence), body fluids, and shen (mind-spirit).
17

22

GETTING STARTED

Introduction

Models and Methodologies
TCM diagnosis is a way of establishing the body's functional status based on the vegetative
system of biology. The guiding criteria of practice in interpreting a patient's clinical signs is
based on four physiological models of body regulation:
▪
▪
▪
▪

Neuro-vegetative origin (fullness vs emptiness)
Humoro-vegetative origin (heat vs cold)
Neuro-immunological stages (exterior vs interior)
Yin-yang balance (structural vs primary regulatory deficiency)

In TCM, the concept of syndrome or pattern is crucial to establishing the diagnosis, and entails
the patient's overall physiological and pathological conditions. In the process of assessing the
symptoms and signs to determine the root cause, a Chinese physician may utilize one or more
of the following methods of diagnosis:
▪
▪
▪
▪

Tongue and facial examination
Pulse taking18
Voice or handwriting analysis
Questioning patient’s diets and habits

When it comes to electronics, there are four categories of circuit designs:19
▪
▪
▪
▪

Digital (ON vs OFF)
Analog (periodic vs sporadic)
Hybrid (discrete vs integrated)
Power (linear vs switched)

A PCB diagnostician can also employ various methodologies to determine the cause of failure
in a circuit board:
▪
▪
▪
▪
▪
▪

Basic diagnostic skills
Test jigs for manual (simple) or automated (complex) testing
JTAG for PCBs with boundary scan features
In-system and built-in self-test
Signature analysis
Thermal imaging and profiling

And just as a TCM practitioner has his own clinical setup with his tools of the trade, so too a
PCB diagnostician requires a workspace equipped with at least a basic set of some common
electronic gadgets to do a decent repair job.
Western medicine recognizes only one pulse. TCM recognizes three on each wrist, each of which can be taken
on the surface or by pressing deeply.
18

19

There’s actually a fifth——RF, but it is too specialized and will not be included in this book.

PCB Diagnostics

23

Chapter 1

Setting Up a Workbench

A basic PCB repair workbench setup
Setting up an electronic lab like a commercial repair house is certainly out of the question
unless you're prepared to fork out a fortune. But if you're thinking of doing PCB diagnostics on
your own, you'll need a good set of basic but essential equipment:20
▪
▪
▪
▪
▪
▪

Digital Multimeter (DMM)
SMD LCR Meter
Adjustable Power Supplies
Function Generator
Mixed Signal Oscilloscope
Multi-Protocol Adapter

▪
▪
▪
▪
▪

Universal Device Programmer
Digital Microscope
Soldering & Rework Station
Hand Tools
Anti-Static Mat & Wrist Strap

I reckon that most engineers will have at least 6–7 out of the above listed items, and the most
meticulous ones probably have all of them and more. There is no hard and fast rules to what
type of models to procure but try not to go for the cheap ones with minimal functionalities if
you can afford it. Good and reliable equipment will last longer and make your work a tat easier
and more enjoyable, instead of unnecessary frustrations that may result in wrong diagnosis
or wasted time and effort. Of course, you are expected to know how to use these tools and if
you could utilize their full potentials, the better you will be at solving PCB defects.
The suggested list is meant for freelance engineers. In this book, you will be introduced to other mid and highend equipment used by repair centers for automated and more comprehensive testing. These are certainly out
of reach of most self-employed engineers, but it is still advantageous to be aware and have some ideas how they
operate. You never know when you may have the opportunity to work with them!
20

24

GETTING STARTED

Introduction

Let's briefly go through these items.
Digital Multimeter (DMM)
The DMM is perhaps the most versatile instrument an
engineer or technician will ever get to learn and use at
work. Increasingly, digital multimeters are preferred over
analog ones for their accuracy, functions and ease of
operation, since they come with a numerical display that
provides quick reading of the electrical properties being
measured.
Choosing a DMM would seem like a no brainer but there
are subtle yet important differences that separate a good
model from an average one. Alongside quality and
reliability, resolution and accuracy are two main factors
that should not be overlooked.
A low-cost digital multimeter may fit the bill for hobbyist
and general tasks but consider a higher grade digital
multimeter for professional or commercial applications or
for high voltage work, since it is more cost effective in the
long term due to better specifications and durability,
besides the safety factor and ratings. You get what you
pay for so you should consider the overall performance
and requirement, not just the basic ranges and cost.

SMD LCR Meter
Surface-mounted components are becoming popular with board designers due to the small
sizes and space economy they afford. Some of these devices are so miniature there is hardly
space for part numbers or even abbreviated values to be printed on them. Using the probes
of a DMM to measure these devices can be challenging and error-prone. Enter the SMD LCR
meter.

UT116A SMD Multimeter from Uni-T
PCB Diagnostics

25

Chapter 1

This remarkable and portable one-hand operated instrument identifies both marked and
unmarked SMD-components with ease——passive or through-hole. A standard model can
measure capacitance, inductance and resistance with speed and precision. Advance model
can even check secondary parameters such as capacitor's ESR, quality factor (Q), dissipation
factor (D) and impedance (Z).
SMD LCR meters come in different makes and functions. Those providing only basic functions
may cost just between $20-$50 apiece, whereas more advanced models can run into $150$300 a unit.
Adjustable Power Supplies
PCBs usually operate from DC power sources, except for
some which may need AC references such as synchroresolver circuits. Common voltages include +5V, ±12V,
±15V, and +24V.
Logic boards normally operate from +5V for TTL devices
though digital ICs are increasingly employing sub-range
voltages like +3.3V and +1.8V to attain higher switching
speeds. These voltages are usually produced by LDO21
circuits that convert either from the +5V or +12V input
instead of having separate external sources.
Analog PCBs containing operational amplifiers and DAC
devices require the ±15V for bipolar rail voltages on top
of +5V for their mixed signal requirements. If there are
electromechanical relays present, then +24V may be a
necessity.
So if you have a hybrid PCB comprising analog and digital components, you will need at least
three different power sources to fire it up for a live run and test. This implies that you need to
have a set of power supplies to give a combination of three DC outputs. You may think that
getting a fixed triple output power supply module will be sufficient but it's not that simple. You
need to consider not just the current requirement of the board under repair, which may exceed
what a power module can deliver, but also voltage and current limit protection in the event of
a catastrophic power outage on the faulty board. The safest bet is still to get a pair of single
and dual DC output adjustable power supply units that provide visual indications as well as
higher power ratings, besides the flexibility and protection they offer.

These Low Dropout (LDO) regulators are used to derive lower output voltages from a primary DC supply. The
output voltage should be ideally stable with line and load variations, immune to changes in ambient temperature,
and remain contant over time.
21

26

GETTING STARTED

Introduction

Function Generator22

Owon XDG3252 Arbitrary Waveform Generator
A function generator produces basic waveforms like sine, square, sawtooth, pulse and noise
signals. Advanced models such as the Owon XDG3000 series dual channel multi-function
arbitrary waveform generator can generate complex modulation signals such as AM, FM, PM,
PWM, FSK, 3FSK, 4FSK, PSK, OSK, ASK, BPSK, linear–logarithmic sweep and burst, etc. from
its 150 built-in waveform library, and even allow you to write your own functions up to a million
points.23
This surprisingly affordable range of function generators from Owon comes with an impressive
8-inch color TFT LCD at an 800 x 600 pixel resolution, and depending on the model, is capable
of waveform frequencies up to 250MHz. The USB and LAN interfaces also makes for versatile
connectivity to program arbitrary waveforms and its small form factor ensures space-saving
for a clutter-free workbench.
So while you can buy second-hand branded models from online auction websites like eBay at
a reasonable price, why not invest in a new and feature-rich model from Owon or Rigol for the
same amount or possibly lesser?
This piece of equipment is necessary only if you intend to test analog circuits as part of the PCB diagnostic
process. Of course, you'll need an accompanying digital oscilloscope to view the waveforms, and that subject to
availability of schematic and specifications of the PCB in question.
22

These benchtop equipment used to be quite expensive and those made by Agilent, LeCroy, Rohde & Schwarz
still are. More recently, new players from China such as Rigol and Owon are making a strong presence in the test
instrument market with their wide range of affordable benchtop products that are feature-rich and functionally
comparable to the big names.
23

PCB Diagnostics

27

Chapter 1

Mixed Signal Oscilloscope24

Owon MSO7102 Mixed Signal Oscilloscope
This is not your average digital storage oscilloscope. The Owon MSO7000 and 8000 series
Mixed Signal Oscilloscope are really two instruments combined into one, comprising a highperformance dual channel digital oscilloscope and a 16-channel logic analyzer.
It sports the same 8-inch color TFT LCD screen but with a lower resolution of 640 x 480. The
function menus are intuitive and easy to operate for both the digital oscilloscope and logic
analyzer. The former supports two channels with 200MHz bandwidth for the high-end models,
while the latter can measure digital signals up to 66MHz with various trigger modes25 and
supports binary, decimal and hexadecimal data bus format displays.
One of the flip side of this MSO is you are limited to either using the digital scope or the logic
analyzer. You can't use both at the same time. This can be a problem if you want to trigger off
an analog signal on the digital scope channel while monitoring some digital signals with the
logic analyzer——you can't do it with this scope. So if you intend to measure a mixed of signals
you may want to consider the Rigol brand MSOs.26

Besides space saving by having one integrated instead of two separate equipment, the MSO affords the
flexibility of checking digital or analog circuits using the same platform, simply by switching to the specific
function menu and using a different set of probes.
24

25

Trigger modes include edge, bus, state, data alignment and data width trigger.

Of course, products offered by different companies have their pros and cons in terms of specifications and
functionalities, so it is important to know what your requirements are and do a comparison of the features before
parting with your hard-earned cash.
26

28

GETTING STARTED

Introduction

What if you do not have the budget or the workspace27 to house these benchtop equipment?
If space and cost savings are your main concern, you may want to consider going for the USBbased alternatives. I strongly suggest looking up BitScope which has a broad range of data
acquisition products, one of which is the Mini model BS10:

Bitscope Mini BS10

Pinout and Logic Pod

The BS10 is a complete USB oscilloscope, logic analyzer, arbitrary waveform generator and
spectrum analyzer all rolled into one. You can download the various supporting software
interfaces to operate the hardware, two of which are shown below:

Bitscope DSO

Bitscope Logic

Learning how to use these benchtop equipment, whether via their hardware display controls
or through software virtual instrument interfaces, is just as important. These can be acquired
from manuals, online tutorials, or formal training provided by the suppliers, if available.

As equipment becomes digitalized, their footprints have also become smaller compared to their analogue
predecessors. Thus, even a decent 4' x 2' worktable should be sufficient with a little space organization and
management.
27

PCB Diagnostics

29

Chapter 1

Universal Device Programmer
If you want to go beyond just replacing defective ICs to performing content extraction from
memory and logic devices (EPROMs, PLDs, MCUs, etc.), then a universal device programmer
is indispensable. While it is good practice to backup these devices' data before attempting
PCB diagnostics, it may not always be possible28 or even feasible29 to do so.
Coming from my own experience, the risk of damaging these devices, however slight, is still
present so I would read out the content for archiving as a safeguard, whenever possible. As
to what kind of device programmer you'll need, that will depend on what level you're into the
game, of course. For beginners, I would recommend the TL866 USB Universal Programmer as
shown below:

The popular TL866 model comes with extra free accessories.
At less than $100, it comes with a whole set of accessories that will probably be all you'll ever
need to get started. Best of all, the enclosed CD-ROM contains the software to operate the
hardware at no additional cost. Surprisingly, it does not lag behind the more expensive brands
in terms of the number of devices supported. And the good thing is new devices are constantly
added to its ever-expanding library that is freely available for download on the manufacturer's
website.30

PCB designers and manufacturers are likely to use programmable ICs with security bit protection to prevent
copying or extracting of the data content.
28

This is especially true for Altera or Xilinx FPGAs (Field-Programmable Gate Arrays) which requires their own
specialized programming hardware and software to read out the binary content in their proprietary formats. And
the hardware cost and software licenses are not cheap either.
29

30

30

http://www.autoelectric.cn/en/tl866_main.html

GETTING STARTED

Introduction

Multi-Protocol Adapter
The adoption of Internet-of-Things (IoT) technology has seen an increased in implementation
of wireless communication protocols with in-system programmability functions in many PCBs.
Embedded system design became an in-demand discipline within the PCB design community
it's considered an indispensable skill for electronics engineers.
Different interfaces and protocols were introduced to allow firmware programmers to access
devices on PCBs that support embedded applications. These include JTAG, SPI, I2C, CAN and
asynchronous serial ports such as the RS-232 and RS-422 standards. This has given rise to
multi-protocol adapter toolkits that are used by hardware designers and hackers, for obviously
different reasons and purposes.

Amoza

J-Link-V7

TIAO (Recommended)

These days the chances of encountering PCBs
containing embedded ICs (MCUs, FLASHs,
PLDs, Serial PROMs, etc.) are probably higher
than before, so you may want to include a
multi-protocol adapter in your tool collection
and pick up some firmware debugging skill, if
necessary.
Some of the open-source embedded software
development tools include OpenOCD, UrJTAG,
FlashRom, JTAG Pro, XC3SPROG, etc. Online
tutorials on how to use these software are also
readily available. You may want to take time to
explore before deciding which combination of
hardware and software to adopt.
(TIAO-supported protocols on silkscreen)

PCB Diagnostics

31

Chapter 1

PCB-Related Equipment
When it comes to PCB-related equipment, you need to consider the following aspects:
▪
▪
▪

Conformal coating and treatment
Soldering and rework
Lighting and inspection

Conformal Coating and Treatment
Before you can perform diagnosis on a PCB, you may need to remove its conformal coating if
it's present. Certain types of conformal coating can be removed with the appropriate solvent
while others may require abrasive means such as controlled sandblasting. Determining the
conformal coating type and correct method for removal is beyond the scope of this book, but
if you intend to DIY this process you may want to refer to Appendix C to have a better idea of
what is involved.
Soldering and Rework
A good set of soldering and rework station like the PACE MBT-250 model with its replaceable
temperature-controlled soldering tips, vacuum pump activated solder extractor, and SMD
desoldering pincers, is Indispensable. Unfortunately, for freelance PCB repair engineers the
price is both prohibitive and unjustifiable.31

This model costs about $2,200 on Amazon. You can probably get a good bargain for a second-hand set on
eBay, though.
31

32

GETTING STARTED

Introduction

Thankfully, there are alternatives that can do a decent job and they're surprisingly affordable
and readily available.

Soldering tool kit

Vacuum desoldering pump

The soldering tool kit above with all the accessories costs a mere $15 with spare change, and
surprisingly the 60W iron has a temperature knob that allows adjustment of between 200450°C. The 30W electric vacuum desoldering pump costs about $40 and is a decent tool for
not too intensive desoldering work.
If you work on SMT boards, surface-mounted
devices such as SOIC, CHIP, QFP, PLCC, BGA,
etc. may be easily removed with the help of
a hot air heat gun as shown on the right. The
main unit comes with temperature and air
volume control for a maximum 450W power
output. This unit and its accessories sell for
about $60.

Note:
Prices are taken from Amazon online store.
The same product and model may vary from
seller to seller.

PCB Diagnostics

Hot air heat gun and nozzles

33

Chapter 1

Lighting and Inspection
It's not just age and deteriorating vision that demand good lighting and visual aids; the size of
components on SMT boards is an important and deciding factor. It's a necessity, not a luxury,
to reduce fatigue and eyestrains on the engineer working on such PCBs, more so if you need
clarity on details and to avoid costly mistakes.
There are many models of glass magnifier with a
swivel arm base, but given a choice go for one with
a rectangular viewing glass, like the Yp-86i model.
This unit has a 6.7" x 4.25" rectangular window——
wide enough to work on a large PCB without having
to shift the board too frequently.
A 22W fluorescent ring bulb surrounds the diopter
magnifying lens to flood the inspection area with
plenty of light for close-up inspection work. Prices
vary between $50-60. With a little work, you can
even replace the fluorescent light to a non-flicker
LED source to further reduce strain and fatigue, as
well as power consumption.
Digital Microscope
For tech-savvy engineers, a digital microscope might
just be the piece of gadget you're looking for to add a
level of sophistication and excitement to your mundane
daily routine.
The G1200 from MUSTOOL comes with a 12 megapixel
7-inch HD 1024 x 600 LCD display that provides up to
1200 times magnification and supports 16 languages.
Amazingly, it costs less than $100.
Unlike some models with only two-gear magnification,
the G1200 employs continuous zoom and a wide range
of viewing options. It is angle-adjustable which permit
tilting of viewpoints to reduce glare from reflection due
to shiny PCB surfaces such as thick conformal coating
or EMI-shielded component packaging. Light intensity is
adjustable as well.

34

GETTING STARTED

Introduction

Anti-Static Mat and Wrist Strap
Dry weather and aircon workplace are potential environments where static can quickly buildup and discharge.32 This can damage delicate components found on the PCB you're working
on, so it's paramount that you put on an anti-static wrist strap and lay out an anti-static mat
on your work desk to mitigate the risk. There is a correct way to setup an anti-static workspace
and properly maintain it, as depicted in the figure below:

It goes without saying too, that anti-static bags are indispensable when it comes to storing
and shelving your PCBs when you're done for the day's work. Also, never leave a PCB on top
of any material such as paper or plastic that has the potential to zap your board.

Even in a country with high humidity like Singapore, if you work in an aircon environment and don't hydrate
yourself often, static will still build up overtime——more so if you like to wear cotton clothing.
32

PCB Diagnostics

35

Chapter 1

Summary
After all that has been said,
A tool is only as good as the craftsman using it.
To effectively use the above hardware and software tools, the PCB engineer must possess
relevant knowledge and the necessary skillset. And this is what we will be looking at in the
next chapter.

36

GETTING STARTED

Knowledge is not skill. Knowledge plus practice is.
Shinichi Suzuki
If you've bought this book because you're new and wanted to learn how to do PCB diagnostics,
some basic knowledge and skillsets are needed. You should have some former education in
electronics or at least picked up some fundamentals during your apprenticeship.
At the very least, you should:
▪
▪
▪
▪
▪
▪

be able to identify and differentiate the different types of electronic components
understand the basic functions and workings of these components
know how to read schematic diagrams
possess an acceptable level of competence in handling PCBs
be able to operate standard benchtop equipment
be proficient in soldering and rework tasks

If you want to be really competent, then you should:
▪
▪
▪
▪
▪
▪

have a good working knowledge of embedded systems
know how to use a variety of hacker's tools
possess good programming language skill (C, C++, ASM, etc.)
be able to build test jigs to interface and test PCBs
understand the concept of PCB testing33
be willing to invest in a well-equipped workshop

Now, do not panic just yet. All the above prerequisites are usually acquired and built overtime
depending on what stage you have progressed. The very least requirements are mandatory
and will suffice to see you through to the intermediate level; the competent list is optional and
only necessary if you intend to advance further.
In this chapter, I will elaborate on these essentials. It's by no means exhaustive or complete
given the scope of this book, but hopefully the information will provide some guidelines and
point you in the right direction to supplement what you lack.
Work experiences with various kind of test equipment will broaden your mind to many alternate possibilities,
including those you innovate or invent out of necessity (or desperation).
33

PCB Diagnostics

37

Chapter 2

APPRENTICE LEVEL
Electronic Components
The most basic skill associated with PCB diagnostics that will enable you to make sense of
any circuit board is associated with the following three verbs:
1. IDENTIFY
One of the most fundamental aspects of PCB repair is the ability to identify the diverse types
of electronic components found on a PCB. After all, if you can't make out what the parts that
make up a circuit board, how are you going to diagnose and fix it?
In the electronics universe there are literally tens of thousands of electronic components in
existence. 34 These devices can be broadly categorized as discrete and integrated in their
construct, and further grouped into active, passive and electromechanical by their electrical
characteristics. More commonly though, they are known by their designated names, as listed
below:
▪ Resistors
▪ Fuses, Suppressors
▪ Transistors
▪ Capacitors
▪ Relays, Switches
▪ Optoelectronics
▪ Inductors
▪ Crystals, Oscillators
▪ Integrated Circuits
▪ Transformers
▪ Diodes, Rectifiers
▪ Converters
It's not an exhaustive list but does include most of the common components that are generally
used in PCBs today. 35 While it may seem straightforward enough to identify a component
based on its physical form, appearance can be deceptive sometimes. This is especially true
with the proliferation of SMD devices, many of which are vaguely similar due to their small
sizes and form factors that do not permit detailed markings.
For instance, consider the circled component shown in the photo
on the right. What do you think it is? If you guess it's a surfacemount resistor, you're wrong. If you guess it's a transistor device,
you're wrong again. 36 It's a thin film temperature variable chip
attenuator. The two keys to identifying components correctly are
exposure and experience. Horn your skill whenever you handle a
PCB; look out for unusual or unfamiliar devices and pay close
attention when reading component catalogues.
In the realm of resistors alone there are already more variety than all your fingers could number. Look at the
small samples overleaf and you'll have a rough idea what you're up against.
34

Heavy duty industry components are not listed since these are not likely to be found on PCBs due to their sizes,
electrical and thermal capacities.
35

Don't feel bad if you didn't get it right. Even seasoned electronics engineers may not be able to identify every
component they encounter.
36

38

GETTING STARTED

Pre-Requisites

PCB Diagnostics

39

Chapter 2

Obsolete components with odd part numbers can be challenging to identify and differentiate.
Some of these may be custom-made, have their original markings erased or covered with
opaque compound to hide their identities or manufacturer logos.
2. DIFFERENTIATE
A rose by any other name would smell just as sweet. Unfortunately, the same cannot be said
when it comes to differentiating electronic components. For example, consider the simple
resistor with its variants:
Resistors

Linear

Fixed

Non-Linear

Variable

Trimmers

Carbon Composite

Fusible

Pots

Thermistors

Rheostats

LDRs

Varistors

LDR – Light Dependent
Resistor

Wire-Wound

Thick Film

Thin Film

Cermet Film

Metal Oxide

Carbon Film

Metal Film

These variants come in all sorts of shapes and sizes, depending on their wattage ratings. Most
are discrete though the metal film fixed type may take on integrated forms such as SIP or DIL
packages, and are either same-value discrete collections, common arrays, or multiple network
configurations.
Conventional through-hole components are much easier to differentiate than surface mount
SMD types. Even so, sometimes an axial lead inductor can be mistaken for a resistor because
of their resemblance in physical appearance. Similarly, a three terminal SMD diode package
looks like any SOT-23 transistor or MOSFET, and the marking on the package isn't going to be
obvious until you look it up in an SMD code book.37

37

40

These SMD references are available online and usually free for download.

GETTING STARTED

Pre-Requisites

Talking about SMD components, chip resistors and capacitors share the same form factors
based on the IPC-7351 specifications:
L

W
T

C

It may not be too difficult to differentiate SMD resistors from capacitors, since the former
usually have values marked in coded form and the body color is mostly green or sometimes
light blue. SMD capacitors are seldom marked and generally dull yellow or brown in color with
slightly thicker bodies.

Chip Resistors

Chip Capacitors

Here’s an interesting question: Why is it that SMD capacitors are not marked?
SMD capacitors actually predate surface-mount techniques by several decades where they
were used in hybrid microcircuit assembly. Such hybrids have very few if any markings on the
components. SMD resistors are produced on large sheets then segmented into individual
parts. The marking is probably applied in the panelized version so the cost is insignificant.
SMD capacitors, however, are produced as individual units. That figures.

PCB Diagnostics

41

Chapter 2

3. DECIPHER
Except for ICs and components with large surface areas which permit printing of part numbers
most surface mount devices use some form of cryptic coding system to indicate their identities
or values. Some basic components like resistors, resistor networks, and capacitors may also
contain long strings of alphanumeric characters that require their manufacturers' references
to make sense of their package and content. Take for example the two types of through-hole
resistor coding:

Color Code

Alphanumeric

The pair on the left uses color coding, with four and five bands of color for the top and bottom
resistors, respectively.38 The pair on the right shows the same value in alphanumeric notation
wrapped around the resistor's body. Appendix A provides a list of references for deciphering
both color and alphanumeric codes found on resistors and capacitors.
Surface-mounted devices, passive and active, are
becoming common place these days so it's good
to keep a copy of SMD codebook at hand for easy
reference and be familiar with their outlines and
packaging.
Exercise:
Can you identify and decipher the components on
a PCB shown on the left?
According to their layout:
Chip Capacitor, Tantalum
1uF 35V
Diode, case SOD-80
Switching
Chip Resistor, size 0805
4.64K

Chip Resistor, size 0603
10 Ohms
Low-Capacitance
Diode Array (5V)

NPN Transistor, SOT-323
BFR92AW
Chip Capacitor, unknown

Chip Capacitor, unknown
There is also a six-band coloring which provides an additional temperature coefficient factor, but this is a rare
occurrence.
38

42

GETTING STARTED

Pre-Requisites

How many did you manage to get right?
▪

The tantalum chip capacitor (top left) should be easy to identify and decipher with its
values and polarity marking. The small 'k' indicates the manufacturer's code (Kemet).

▪

The diode is a glass type SOD-80 casing39 with a yellow stripe denoting it as a switching
diode; other than that, there is no information on its part number. Fortunately, the color
is all we need since some manufacturers provide references to identify their products.
In this case, it's one of three possibilities: BZV55, BZV80 or BZV81 series zener. As to
its breakdown voltage, you can determine by removing and subjecting it to standard
zener voltage check.

▪

Chip resistors are also easily identified and deciphered by their sizes and values. The
smaller of the two is a bit cryptic with the marking '01X' but doing an online search will
usually give the answer you're looking for. The other with a '4640' marking is decoded
as 4640 ohms40 or 4.64K in standard notation.

▪

The 8-pin small outline IC (SOIC) in the middle can be tricky since there's two lines of
alphanumeric codes but that should pose no problem if you do a search for both online.
It turned out that the lower line is the part number and the top line is the date code
'0102' which means week 01 of year 2002.41

▪

The SOT-323 case 3-terminal device can either be a diode or a transistor; crossreferencing 'P2p' yielded the result 'BFR92AW' which is an NPN wideband transistor.
The small 'p' can stand for 'Philips' but not always.42

▪

Chip capacitors without marking are a common sight so you should not be too uptight
or upset over this fact. If they're connected across power and ground near components,
most likely they're decoupling capacitors with a value of 100nF, which usually spot a
light to dull-brown color. If they're connected across signal and ground, or in op-amp
circuits, you'll have to take them out and measure with an SMD LCR meter mentioned
in Chapter 1 to determine their values.

Appendix A contains many quick references which should be helpful for most of your PCB-RE
needs in identifying and deciphering through-hole and SMD components. You can add on to
your own list as you progress in your undertakings of this discipline.

39

SOD-80 case code is a mini metal electrode leadless face (MELF) DO-2013AA size package.

40

Do not mistaken it as 464 ohms which is denoted '464R' instead of '4640'.

Sometimes the part number can be an abbreviated description, in this case LCDA stands for low capacitance
diode array. Referencing the datasheet from Semtech indicates it's an array of four TVS or transient voltage
suppressors used to protect high-speed data lines. The 'SC' before the date code probably stands for 'Semtech
Corporation'.
41

This part is manufactured by NXP Semiconductor, formerly known as Philips Semiconductor, but subsequently
renamed to NXP after it was sold to a consortium of private equity investors in 2006. (Source: Wikipedia)
42

PCB Diagnostics

43

Chapter 2

Circuit Topologies
Next to identifying and deciphering components, familiarity with electronic circuit topologies
that are commonly found in PCBs may prove crucial for successful PCB diagnostics. The term
'topology' used here is not referring to network theory in the strictest sense that is taught in
the classroom; rather, it refers to the 'configuration' or 'model' of a circuit, as in an op-amp
configuration or a filter model.
In this respect, any circuit——digital or analog——is made up of a collection of discrete and/or
integrated components that conforms to a certain pattern of connectivity to perform a known
function. For example, consider the various passive filter topologies made up of LC discrete
components:

Depending on the positions and values of the inductors and capacitors, these can form lowpass, high-pass or bandpass filters of either balanced or unbalanced configurations, as shown
below:

44

GETTING STARTED

Pre-Requisites

Then, of course, there are the active filter topologies made up of op-amps and RC discrete
components:

This is just a small sample of what circuit topologies is about. There are many types in analog,
digital and power circuits across the spectrum of electronics and PCB designs, too numerous
to include in this book. However, there is no reason to despair or become discourage since
these resources are available online in many electronics educational sites and discussion
forums. Articles and videos are also helpful sources of information or tutorials to increase your
knowledge and understanding of how circuits are designed to work. A list of websites and
books is provided in the following sub-sections to give you a head start in finding and learning
these electronics-related information.

Books
There are many great books by popular or renown authors that are available in print form and
they can be readily purchased through Amazon, the biggest online bookstore, at reasonably
affordable prices. My recommendations are the following series of books:

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Chapter 2

The first three volumes are by Charles Platt, a contributing editor and regular columnist for
Make: Magazine. You will benefit greatly from the author's insights based on his extensive
hands-on experience in working with electronics components——what a certain device does,
how it works, its characteristics and configurations, and how it's use in real-world circuits.
The next four volumes are the McGraw-Hill Circuit Encyclopedia & Troubleshooting Guide by
John D. Lenk, which have now become a classic collection with over 2,500 circuits in total. 43
In addition to the wealth of circuits, the author analyzes them for defects, performance issues
and real-world practical applications, which makes it a good reference for design engineers,
repair technicians, and even occasional hobbyists, despite its apparent age.

References & Tutorials
There are many websites that teaches electronic subjects, some of which are well-organized
and categorized, while others are mish-mashed and a pain to navigate. The following sites are
worth a look:44
▪
▪
▪
▪
▪
▪
▪
▪
▪

4QD-TEC Circuit Archives
All About Circuits
Discover Circuits
Electronics Tutorials
Electronics Hub
Electronics Notes
FC's Electronic Circuits
Instructables
Sam Electronic Circuits

Good circuit design engineers often study circuits designed by other engineers as well as those provided by
component manufacturers. The more you understand the basic principles behind different circuit designs and
their purposes, the better your chances of figuring out how components on a PCB are related and connected.
43

44

46

Simply Google by name and the websites should show up in the first few links.

GETTING STARTED

Pre-Requisites

YouTube is also the place to look for tutorial videos for those who learn more effectively
through audio and visual means.

Forums and Blogs
Of late, blogging has become a trend even among engineers who have taken to expressing
their opinions and sharing their knowledge via personalized blog sites or Facebook pages:
▪
▪
▪

EDAboard
EEVblog
Hackaday

▪
▪
▪

Quora
Seeed Studio
SparkFun

Magazines
Remember the good old days when we used to frequent magazine stand to browse and read
our favorite magazines on all thing electronics? While some still carry on the traditional print
distribution, many have since hopped onto the e-magazine wagon:
▪
▪
▪
▪
▪
▪
▪
▪

Electronics for You (EFY)
Elektor
Everyday Practical Electronics
EE Times
Electronics World
EDN
Nuts and Volts
Popular Electronics

PCB designers and assembly houses also have their selected picks:
▪
▪
▪
▪
▪
▪
▪

Circuits Assembly
Circuit Cellar
Design World
Military and Aerospace
New Electronics
PCB Magazine
Test and Measurement World

There are many other websites that are too numerous to list them all here. As you find and
veto through each one, bookmark and categorize those that are relevant and useful to you. In
time, you will build a collection of links that will give you the needed information at your
fingertips for doing PCB diagnostics, or for that matter, your engineering interests.

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47

Chapter 2

Reading Schematic Diagrams
Any respectable electronic engineer should have no problem reading and understanding the
schematic diagram of a simple to moderate circuit. The challenge though is making sense of
those that are either inconsistent in style or difficult to read in terms of circuit flow.

Where style matters, there are two aspects you need to know:
REPRESENTATION — ANSI, IEC or DIN standard
If you've read enough electronics books and magazines, work on PCB design projects, or repair
in-house or third-party PCB products, you'd most likely have come across schematic diagrams
with different circuit representations. Consider the eight basic logic gates and their symbolic
representations below:

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GETTING STARTED

Pre-Requisites

For many of us, the first column representation is familiar. This type of logic symbols is known
as distinctive shapes and are commonly found in traditional or simple schematic diagrams. It
has its origin from the US military under the MIL-STD-806 specifications drafted to standardize
all electronics drawing documents. It is a 'Made in America' thing which is why this style of
drawing is also known as the ANSI/MIL standard.45
The IEC46 style of representation, as can be seen from the middle column, is rectangular in
shape with logic notations to describe a device's functionality. US engineers who've come
across this style found it rather 'unfriendly' compared to the ANSI style, but it is widely adopted
by many European countries such as the UK and Germany, the powerhouses of electronics
innovation and design. The reason is because the IEC standard provides a consistent method
of describing complex logic functions for digital circuits than is impossible with the ANSI-style
notation. Engineers who are comfortable with mathematical symbol notations will appreciate
the elegance of this style, but those who don't will probably irk it.
The last column is known as the DIN or Deutsches Institut für Normung, which translated is
German Institute for Standardization. While it is not as common as the ANSI or IEC, some
countries in Europe still use them.47
Space does not permit me to give equal treatment to the ANSI and IEC standards, so I'll have
to limit myself to just using the ANSI style for all my examples. For those interested in the IEC
standard, there is a booklet by Texas Instruments which gives an overiew of the IEEE Standard
91-1984 with the same title. Just google to find and download it.
ORGANIZATION — Hierarchical or Flat
Drawing a schematic diagram requires some planning. One of the considerations is the layout
of schematic drawings that span multiple sheets or pages. Schematic diagrams that fit nicely
into one single A4 size paper are usually simple circuits with small PCB footprints. Often, a
PCB of medium complexity will contain an average of 50-75 SSI48 type ICs, in which case you
may be looking at three A3 size sheets or more, depending on how the placement of the
component symbols and wire runs are optimize and organize.

45

ANSI stands for American National Standards Institute, and MIL stands for military.

IEC stands for International Electrotechnical Commission, and is a non-profit, non-governmental international
standards organization that prepares and publishes international standards for all electrical, electronic and
related technologies—collectively known as 'electrotechnology'. (Source: Wikipedia)
46

They are such rare gems nowadays—but I had the honor of encountering some of these at work in my previous
company!
47

This is just an estimation since a PCB rarely consists of only SSI (small-scale integration) ICs but a good mix of
other peripheral (MSI, LSI) and processor (VLSI) ICs as well. You may roughly equate four SSI to one MSI, four
MSI to one LSI, and four LSI to one VLSI, etc. but that again is subjective to the type and functionality which are
simply too complicated to breakdown accurately.
48

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Chapter 2

A hierarchical layout is a top-down approach that allows many levels or layers of nesting,
progressing from simple to complex. Unless you are doing serious PCB design involving large
number of components, it is unlikely that you'll need to go beyond two levels of abstraction. A
flat layout, on the other hand, is a straightforward, single layered representation in which the
various sheets are inter-linked relationally and laterally (side-to-side).

Hierarchical (Layered)

Flat (Side-by-side)

The figure above shows the visual concept of these two models. It must be understood that
for the flat model, the level zero implies that there is only a single layer of representation,
while the arrows are indicative of the cross-border circuit interconnectivities between sheets.
For schematic diagrams spanning more than ten pages, a hierarchical layout is better because
the top-down organization makes it easier to search for circuit clusters based on block
diagram functionalities. I have worked with OEM schematic diagrams that spanned over 30
sheets while developing test programs for their PCBs, and I can honestly say that without a
hierarchical organization, it would be a nightmare just trying to find my way through the
schematic labyrinth!
And as far as readability is concerned, the following factors are important:
▪
▪
▪
▪
▪

Page size and orientation (landscape or portrait)
Reference designators and information (value, part number, etc.)
Circuit layout and signal flow (wires, buses, ports, etc.)
Component symbology and representation
Text and labelling (signal name notation)

The aim of a schematic diagram is to convey to the reader a better understanding of how the
circuit it is depicting works. This implies that clustering of related components and how they
are interconnected should make it easy for the reader to comprehend. The best way to learn
to read schematic diagrams is to study the works of engineers with good practices and from
circuit artworks found in reputable electronic magazines.
50

GETTING STARTED

Pre-Requisites

For a start, we must be able to identify the symbols that represent the electronic devices on a
schematic. Some examples are shown below:

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Chapter 2

A good schematic diagram doesn't just contain nameless symbols; each symbol will come with
at least an identifier or reference designator (R1, C2, U3, etc.) and possibly a value (1K, 2.2u,
etc.) or a corresponding part number in the case of an IC (74F00, LM358, etc.). Component
identifiers are a combination of prefixes and numbers, and these are usually standardized as
shown in the table below:
Components
Battery
Capacitor
Capacitor (decoupling)
Connector (Jack)
Connector (Plug)
Crystal Oscillator
Delay Line
Diode
Display, LED
Filter
Fuse
Hybrid Device
Inductor

Prefixes
B, BT
C
CU
J
P
Y
DL
D, CR
DS
FL
F
HY
L

Components
Integrated Circuit
Jumper, Link
Potentiometer
Power Supply
Relay
Resistor
Resistor Network
Switch
Test Point
Transformer
Transistor, MOSFET
Zener Diode

Prefixes
U, IC
JP
RV
PS
K, RL
R
RN
S
TP
T
Q
Z, VR

For discrete components like resistors, capacitors and inductors, their values are commonly
affixed with terms such as kilo (103), mega (106), milli (10-3), micro (10-6), nano (10-9), etc. You
should be familiar with how they are represented on the drawing, for example:
10K5 = 10,500
20R5 = 20.5
30mH = 0.03H
100uF = 0.0001F
82nF = 0.000000082F

3M8 = 3,800,000 (unit in ohms)
(units in henry and farads)

To make sense of what a circuit does, you need to know how the components are connected
and take advantage of any extra information present on the drawing, such as net or signal
names, connector labels and better yet——waveforms, if available. The logical flow of a circuit
will usually be from left to right, much like the way a book is written.49 Space constraints as
well as complexity of design may compel the person drafting the schematic diagram to adopt
unconventional methods of representation, either to simplify the drawing or to avoid clutter
due to the large number of interconnecting wires.50

There are exceptions such as closed loop feedbacks found in many analog circuits, bi-directional protocols in
digital communications and mixed-signal interfaces in signal processing applications.
49

Most schematic editors allow signal tagging instead of joining the components pin-to-pin which can quickly
obscure the functional flow of a circuit and makes it hard to trace specific signals. This feature simplifies the job
of the PCB designer but is a nightmare for the engineer studying the schematic diagram.
50

52

GETTING STARTED

Pre-Requisites

Consider the sample circuit below:

It's obviously analog in nature and is made up of four operational amplifiers (U1, OP11), two
voltage comparators (U2, LM139A), and a couple of discrete resistors and capacitors. It is not
difficult to figure out the circuit function if you know what kind of opamp configurations are at
play here——in this case, we have an integrator (U1A), an integrator with a T-filter (U1B), and
two inverting amplifiers (U1C and U1D). Base on the waveforms supplied, we can determine
that it is an oscillator circuit that generates four phase-shifted triangular signals with similar
amplitude and frequency.51
While it may be true that digital circuits are much easier to interpret than analog ones, PCBs
these days come with many miniaturized yet complex hybrid components that run a myriad of
functions, you really need to study their product documentation to know what these monolith
chips do. In fact, it is no longer possible to represent these large pin-count components with
just a single entity on the schematic diagram. More often than not, these devices are broken
up into various parts according to functionalities and their connectivity to relevant clusters of
adjacent components.
Next, we will look at using some benchtop equipment.
Don't be disheartened if you didn't figure it out. Familiarity comes with study and revision while experience
comes with practice. The more you brush up on your fundamentals the stronger your foundation in electronics
and the more confident you'll become.
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Chapter 2

Using Benchtop Equipment
Test equipment are indispensable for doing PCB diagnostics. Any electronics engineer would
have hands on a few common types during practical lab sessions in tertiary study, such as the
multimeter, DC power supply, oscilloscope, signal generator, etc., and be familiar with their
functions and operations.52 In this section, we will briefly touch on three basic but essential
benchtop equipment.

Digital Multimeter (DMM)
The DMM is perhaps the most common piece of personal equipment to have around for its
versatility in measuring a variety of electrical entities, from resistance and capacitance to live
voltages and currents. Some models like the DT830D even allows you to check transistor and
diode functions as well as measure square wave frequency!

Transistor test function of a DMM
In this age of the internet, learning to use any test equipment has never been easier and more accessible.
There are plenty of how-to videos and tutorials on YouTube. You can also join an interest group forum and ask
questions or seek advice from experts there, who will be more than happy to offer their help or point you to the
right resources.
52

54

GETTING STARTED

Pre-Requisites

Handheld DMMs are mainly used for quick and easy measurements where precision is not
critical. If accuracy and repeatability is paramount, then a proper benchtop model like the
Keithley 2000 series multimeter is a must.

6½–Digit USB Multimeter (Keithley 2100)
Benchtop multimeters are more complex in function and usage, but they offer the highest
performance and reliability at a higher cost. Besides providing faster rate of measurement 53
and better accuracy, these equipment come with higher resolution and sensitivity in terms of
value readings as well.
Resolution refers to the smallest portion
of the signal that can be measured and
displayed on any selected range. For a
7.5-digit DMM shown on the right, the
resolution for 100mV range is 10nV, and
for 1V range it is 100nV.
Sensitivity refers to the smallest change
in the input signal that can be detected.
On the 100mV range, this DMM is able to
display a sensitivity down to tens of nV.
Another advantage of benchtop multimeters are their ability to employ four-wire (Kelvin) type
measurements to minimize the impact of lead resistance, using the additional Sense Hi and
Sense Lo terminals. This is especially crucial for low resistance and small current readings in
the sub-milliohm or microvolt region where wire resistance of the probe leads may introduce
unwanted residual values to the actual readings.

Measurement rate is determined by the number of power line cycles (NPLC) and represents the duration of
signal sampling. A faster NPLC will sacrifice sensitivity due to noisier readings; a slower NPLC provides better
noise performance at the expense of speed.
53

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Chapter 2

Power Supply Unit (PSU)
Next to the DMM, the power supply unit is the most indispensable piece of equipment since
all PCBs invariably need some kind of DC power source to operate. But not all power supplies
are made equal. Often they come in different flavors too——from single to multiple outputs,
fixed or adjustable (i.e. programmable), linear-regulated or switched-mode, direct or remotely
sensed, well-protected or simply no-frills.
As mentioned in the previous chapter, digital PCBs will usually take in a primary source (+5V
or +12V) and convert it to some smaller voltages (+3.3V or +1.8V, etc.) using LDO circuits for
newer generation of ICs with higher switching speeds. Analog or mixed signal PCBs, however,
will require ±12V or ±15V on top of the +5V. What this means is that power supply unit with just
a single output will not make the cut; you'll need to have two or more combination of PSUs to
allow you the flexibility to satisfy your PCB's power requirements.

TTi-CPX400 Series Power Supplies
For those who only require fixed DC outputs and want to cut cost, the easiest way is to convert
an ATX power supply unit into a multiple DC outputs voltage source, as shown below:

ATX Power Supply Breakout Unit and Output Ratings

56

GETTING STARTED

Pre-Requisites

The behavior of an ATX power supply can be unpredictable when it is short-circuited. A welldesigned power supply will usually shut down; however, this is not guaranteed. Less expensive
power supplies may attempt to continue supplying voltage or fail catastrophically. In short:
use ATX power supplies with care! If you need a bench power supply for experimenting, a real
laboratory power supply is really the best option. It not only allows you to monitor current, but
also set a current limit beyond which the supply will either limit the voltage or just shut down.
It's a worthwhile investment and the safest solution.54
Power supply units can be configured for different applications, whether it's single output type
PSUs or one with multiple outputs, such as the examples below illustrate:
Double output voltage is used when one
PSU alone cannot provide the desired
operating voltage required by the PCB.
This is achieved by connecting two power
supplies in series to give a sum total
voltage output.
Double output current is used when one
PSU alone cannot provide the desired
operating current required by the PCB.
This is achieved by connecting two power
supplies in parallel to give a sum total
current output.
Bipolarity output voltages is used for PCB
that operated on two voltages of different
polarities. This is achieved by connecting
the positive terminal of one output to the
negative terminal of another output as a
common reference point.
Multi-channel power supplies should have isolated outputs to enable them to be combined in
either series or parallel for voltage or current configurations. Furthermore, even for PCBs with
analog and digital grounds, the board must be grounded at only one point to eliminate ground
currents that can create error voltages in conducting pathways.55

If you're adamant on using an ATX power supply, you can get a breakout module with fuses for each of the ATX
supply outputs as an extra protection. These are readily available online for less than $5.
54

55

A small amount of noise current can create a large error voltage in a high impedance circuit.

PCB Diagnostics

57

Chapter 2

Digital Storage Oscilloscope (DSO)
The basic function of an oscilloscope is to plot the amplitude of a signal under measurement
as it changes over time. These days, digital storage oscilloscopes are capable of fast Fourier
transform (FFT) and spectral analysis with their built-in advanced mathematical operations,
but we will just focus on the time analysis aspect of this benchtop equipment.56
In past era of the CRT-based analog oscilloscope, you need to manually set three functions to
effectively use it——the horizontal time base, the vertical scale, and the trigger setting. Modern
DSOs come with a magic 'Autoset' button (see arrow below) that allows quick measurement
of a signal without the need to touch any other button on the front panel, well almost. Not all
signals are well-behaved i.e. periodic and predictable. What if you need to measure a small
noisy signal, a random pulse, or a slow changing signal? Getting acquainted with the functions
of a DSO becomes crucial to successfully capturing that illusive signal.

There are five basic steps to setting up an oscilloscope for measurement:
1.
2.
3.
4.
5.

Connect the DSO input(s) to the circuit node of interest
Set vertical scale (signal amplitude)
Set horizontal scale (time base)
Set trigger conditions to begin acquisition
Read measured values

In the course of my previous employment, I conducted several interviews with potential candidates. One of
the questions I asked them was——what's the difference between an oscilloscope and a spectrum analyzer?
Unfortunately, not a single candidate could give me the answer. For those who study signal analysis and know
how to correlate it to differential and algebraic equations, it's a piece of cake. The former operates in the time
domain while the latter in the frequency domain. It's basically looking at the same thing from two different
perspectives, that's all.
56

58

GETTING STARTED

Pre-Requisites

Most DSOs come with two input channels and an auxiliary (trigger) input. Normally, one of the
channels can function as a trigger input. However, there are times when you may need a
separate trigger source and that’s where the extra trigger input comes in handy. It is also
important to check for probe compensation by calibrating it against a reference signal and
ground, located at the bottom right corner near to the BNC connectors as two exposed ring
terminals for the probes to conveniently hook onto.57
Adjustments to the vertical, horizontal and trigger function buttons will be reflected on the
display in real-time, as shown below:

Additional options are available as multi-level menus which are accessed via pushbuttons
located on the right (and sometimes bottom) area of the display. These functions allow for
vertical and horizontal cursor manipulations to obtain delta readings of time and amplitude,
various parameters of the signal under measurement (peak-t-peak, RMS, bandwidth, etc.), as
well as printing and saving of the displayed waveform to external devices.
Just as there are tools for diagnosis, there are also tools for treating the cause.
The reference signal is normally a 5V 1 kHz square wave while the ground ensures that the input channels are
taking the proper ground reference.
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Chapter 2

Soldering and Rework
Those who started out doing PCB repair often began with a basic soldering iron and possibly
a manual suction pump, alongside accessories such as a roll of solder, an iron holder with a
cleaning sponge, and a bottle of flux. But soon, the realization that a single pointed or beveled
tip soldering iron with just a hand-operated pump would prove to be insufficient against a wide
variety of PCBs.
To be proficient in PCB repair, you not only need a proper soldering and rework station with a
comprehensive set of soldering tips, you must possess a good understanding of the intricate
process and be properly trained to execute it.

Types of soldering tips

Good soldering technique for a perfect solder joint
Some repair centers have dedicated technicians who carry out such tasks after the engineers
diagnosed the failures and transfer the PCBs to them with the indicated faulty components to
be removed and replaced.

60

GETTING STARTED

Pre-Requisites

This is similar to Western medicine practices in which a general diagnosis often results in the
patient being referred to a specialist, where further assessments on a specific condition are
performed before a treatment can be prescribed. If in the process an underlying cause is
discovered that is beyond that specialist’s training, the patient will then need to consult a
different specialist to address the newfound problem. This can be a costly and long-drawn
medical process that adds stress to the patient in question. A TCM physician is trained to view
the human body as a whole eco-system where every organ and its functions are intimately
connected. The diagnosis does not follow a generic to specific path, but one that is holistic in
treating the pathological condition of the patient.
Coming back to PCB diagnostics, there are instances where a failure can only be confirmed
by first isolating certain component pins or leads, or in the case of surface-mounted devices,
progressively removing one or more parts. This would prove inefficient and may disrupt the
diagnostic process if the PCB has to change hands a couple of times just to get the soldering
work done to facilitate the next step. If the person involved in the repair work can handle the
soldering process, this concern then becomes non-existent. So if you lack this skill, consider
enrolling for a formal soldering course conducted by an IPC-certified training provider. One
such company is BEST Inc. which offers a mobile training center that drives to your facility to
provide IPC certified soldering workshop, across all states.

But if you're short on cash or cannot afford the time to attend a formal training, there are
online videos that you can watch and learn on your own. But without hands-on practices and
a real-life experienced trainer to guide you along, be prepared for some hard knocks when you
get down to working on a PCB.
The Apprentice level should be sufficient to see you through the normal PCB repair jobs. To
go beyond that, you’ll need to attain another level of excellence…

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Chapter 2

PROFICIENT58 LEVEL
According to a friend who was trained as a TCM physician, it takes about eight years of study
and practical hands-on to be certified. This includes a four-year program that chalks up over
2,500 hours of theory and practice, covering foundational subjects such as the language and
terminology of TCM, anatomy of the human body, Chinese herbs identification and usage,
clinical assessment and diagnostic processes. Once the basics are mastered, the apprentice
then progresses to acquire advanced skills——acupuncture, therapeutic massage, gynecology
and pediatrics, treating traumatic injuries, etc.59
Similarly, a PCB diagnostician can only reach a level of proficiency by acquiring peripheral yet
related skills to engage in more challenging PCB repair works.

Embedded Systems
At first glance, embedded systems seem to imply some kind of processor-based design with
firmware hard-wired into its core. That’s a general description at best. Embedded systems are
small form-factor processor boards that perform specific tasks. They are classified according
to performance and functional requirements, and the type of processors used. There are four
main categories:
▪

Real-time
Mission-critical controls and acquisition (defense, aerospace, medical, etc.)

▪

Standalone
Independent systems (digital gadgets, household appliances, etc.)

▪

Networked
Wired or wireless network communication systems (ATM, POS, CCTVs, etc.)

▪

Mobile
Small portable devices (cellphones, laptops, calculators, etc.)

While embedded systems consume less power, are robust and require little maintenance,
they have limited processing resources and usually perform simple task managements due
to the barebone operating systems (OS) that power the hardware.60
I have intentionally chosen 'proficient' over 'veteran' because it better describes competency and dexterity,
since a veteran usually implies one who has had long experience in a particular field or skill only. To be proficient
you need to continuously adapt and acquire new skills to meet evolving challenges.
58

These days there is an increasing integration of TCM methods and western medicine to get the best of both
worlds. So it’s hardly surprising that TCM courses may also incorporate basic western diagnostics as a means to
understand how pharmacology interacts and affects the use of Chinese herbology.
59

60

62

Scaled-down versions of Linux or even hand-crafted assembly codes for speed and efficiency.

GETTING STARTED

Pre-Requisites

Embedded System

Firmware Hacking

Firmware Hacking
When we think about firmware hacking, we tend to link it to the dark side which involves illegal
violations of copyright laws, something which unethical hackers do to steal codes, circumvent
software protection, and even espionage.61 Far from it, there are many professionals who
engaged in such activities to help law enforcers gather valid evidence to convict criminals,62
salvage a critical piece of equipment to save a company’s business, or test the strength of a
commercial product against security risks and exploitation.63
Besides relevant knowledge of programming languages and the use of proper hacking tools
for different kind of firmware security designs, the rate of success is very much dependent on
both the intuition of the one performing the task, and how resourceful and innovative to work
around restrictions and possible roadblocks imposed by the firmware designers. Certainly, it
will take a lot of grit and experience, but the breakthrough will be the greatest reward.

Programming Languages
Both embedded systems and firmware hacking require different degree of programming skill.
In most cases, though, you need to know some form of low-level assembly language because
firmware devices are programmed using hexadecimal codes, compiled or handcrafted. What
you read out from these devices will either be in the form of strings (ASCII) or hex-dump (binary
numbers).
61

We need a little thrill and excitement to spice up our otherwise mundane lives, don’t we?

This is a specialized branch of hacking known as digital forensics. It is covered in one of the chapters in my
book, PCB-RE: Real-World Examples.
62

63

Another specialized branch of hacking known as penetration testing, or pentesting in short.

PCB Diagnostics

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Chapter 2

So which programming language should you pick up and what advantage does one have over
the others? To give you a rough idea of how many flavors there are out there, have a glance
at the periodic table below:

Each row represents roughly a decade, starting on
the second row with the 1950’s up to the 2000’s on
the final row. The first row is pre-1950 with the two
mechanical programming systems for which all the
others have evolved—the first from around 1837
created by Charles Babbage and Ada Lovelace.

The colours denote the programming paradigm that
the language in question originally supported or the
primary paradigm for which it is known.
Certain languages could have evolved to support
other paradigms over time which are not shown.
© 2012 Paul Bowler

Some of these programming languages are dated and no longer supported as far as firmware
and embedded systems are concerned, so you can safely discount them. Also, most if not all
of the above are high-level programming language with an English-like syntax, meaning if you
intend to learn low-level assembly programming, you’ll need to decide which processor(s) to
focus on. My inclination is to familiarize yourself with the classic 8051 architecture as a first
cut and progress from there. In fact, some high-level programming languages do support inline assembly coding and firmware designers often take advantage of this feature to optimize
their codes for critical functions and subroutines.
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GETTING STARTED

Pre-Requisites

Once you have a basic foundation on x86 assembly programming, you may want to go into C
and C++ since these two are the most popular choices among embedded system designers.64
However, we should not forget the integrated development environment (IDE) each language
comes with, which adds to the learning curve or provides the necessary features to speed up
your development efforts. Different people have differing preferences so it’s really up to you
to discover the programming language that suits you.65

Test Jigs and Interfaces
If you’re a DIY person, then building test jigs will not be too much of a challenge, though it’s
by no means a piece of cake in some instances. Good planning and a lot of thoughts go into
the design and fabrication of a functional PCB test platform.66 A test jig can be as simple as
an interface for a PCB to power up for manual probing and measurement, or as complicated
as an in-circuit test fixture that can be interfaced to a full-fledge tester for a comprehensive
PCB diagnosis.

Simple Test Jig

In-Circuit Test Fixture

C++ still represents the benchmark for speed though it is barely faster than the old stalwart Fortran, and only
1.5–3 times faster than up-and-coming rivals among the high-level languages (especially when you allow for
hybrid programming to speed up the slowest algorithms).
64

These days, you can easily download a variety of program development kits to test drive their functions, and
learn popular programming languages on YouTube to get a feel. If in doubt, join a related discussion forum to
find out more from real life programmers. Most of these communities have FAQs and folks who will gladly help
you find your footing.
65

Test jig, testbed and test platform are just different ways of calling a rose, though there can be subtleties
depending on the types of test they are designed for. More of this in a dedicated chapter later.
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Learning how to build a test jig, interface or fixture requires a few skillsets:
▪

Mechanical design and fabrication
In the past, test jigs are built with aluminum
consoles and accessories that are readily
available from hobbyist shops and online
component suppliers. These days if you have
access to a 3D printer, you can easily design
and create your own customed mechanical
parts,67 or engage a third-party to do it for you.
If you’re building it yourself then you may also
need to know how to use certain type of hand
tools (screwdriver, files, cutter, motorized drill,
etc.)

▪

Electrical wiring and soldering
Plugging a PCB to the test jig necessitates some kind of electrical interfacing to mate
the unit-under-test (UUT) to a test circuit that is controlled by an onboard processor or
a peripheral PC. These related parts are connected by direct wires or assortment of pin
headers and sockets. Knowing the right wire gauges to use for power and signals, as
well as proper soldering technique is important to ensure you have a reliable working
test jig.

▪

Test circuit design and prototyping
Some PCBs have common interfaces like
USB or serial ports that you can connect
to a PC or laptop and perform simple but
limited tests. When such options are not
available or if you want a more thorough
diagnosis, additional support circuitries
will be required to provide the necessary
test signal excitation and acquisition. The
common practice is to come up with a
test circuit and then build a prototype
board based on the schematic diagram. A
prototype board can be wire-wrapped or
wire-soldered since it is a one-off effort.
These days, though, you can send your Gerber design files to a third-party vendor and
have your PCBs fabricated and delivered to your doorstep in less than a week at a
modest price.

This is made possible with the advent of The Fourth Industrial Revolution (aka Industry 4.0), an initiative to
promote connected manufacturing and a digital convergence between industry, businesses and other processes.
67

66

GETTING STARTED

Pre-Requisites

Prototyping and Testing
Before the advent of circuit simulators, breadboarding a circuit is the common practice among
electronic hobbyists and enthusiasts.68 This is a simple and convenient way of prototyping a
design concept without having to solder components and wires. Circuits can be quickly wired
up, easily modified and then readily dismantled once they served their purposes.

Messy and untidy

Clean and organized

Not everyone subscribed to good breadboarding practices, though there are reasons to do so.
It’s much easier to spot any wiring mistake and provides better access for measuring signals
or diagnosing failures.
Once a circuit is proven, prototypes can be built using stripboards as shown below:
Cutting

Component side

Solder side

The first time I came across the term ‘breadboard’, I thought it was some kind of board used to make bread
that engineers improvised for building and proving their circuit designs. I wasn’t wrong though——the idea could
have spun off in the form of a café where engineers meet to discuss and experiment their prototypes over
sandwich (bread) and coffee.
68

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Chapter 2

The disadvantage of using stripboards is you need to make cuttings (see figure above) at
certain intervals to isolate component connections where necessary. An alternative is to use
Vero boards which have individual, isolated solder pads but this means you’ll need to provide
the wiring connections for every node in the circuit. An example is shown below:69

Component side

Solder side

Summary
This chapter gives you a good idea of the various knowledge and skillsets needed to set you
on the PCB diagnostics road. You are not required to master all of these upfront, or even to
acquire them completely to begin doing PCB repair. Equipped with just enough of the basics
and a sound foundation in electronics, you are good to go as you explore, discover and pick
up whatever additional skills along the way. Be adventurous, stay open-minded, and see new
possibilities as your personal abilities attain greater heights than you could ever imagine!

This prototype video pattern generator was built by me for a test program set development project to test an
F16 video card. The final PCB was incorporated into a test interface fixture
69

68

GETTING STARTED

Move out of your comfort zone. You can only grow if you're willing to feel
awkward and even uncomfortable when you try something new.
Brian Tracy

Preliminary Diagnosis
A trained TCM physician is usually able to diagnose the root cause of an illness by applying
the following methods:
▪

Observe (wang): examining the entire body, which includes the tongue, complexion,
body posture, movement and vitality.

▪

Smell and listen (wen-ting): noting the smell of body odors, excretions and secretions;
listening to the voice, tone, and sound of respiration or cough.

▪

Question (wen): inquiring the main concerns or complaints, the onset and duration of
the problem, and relevant medical history and symptoms.

▪

Pulse-taking (qie): evaluating the pulse by pressing certain parts of the body such as
the wrists, muscles, acupoints, limbs, chest, abdomen, etc.

Like learning how to play chess, these basic diagnostic skills are easy to pick up but require
time to master. Some root causes are obvious and can be readily pinpointed while others are
more subtle and need further probing and analysis. In the same way, diagnosing a PCB failure
takes practice and in the case of intermittent or illusive faults, patience.
PCB failures occur due to various reasons and circumstances. It could be attributed to poor
circuit designs, deficient cooling measures, harsh operating environment, component defects
during manufacturing process or due to limited operational lifespan, and even human errors
arising from negligence or improper handling. Whatever the cause, any PCB will eventually
breakdown in a matter of time, whether it’s within the warranty period or after many years in
the field.70
When diagnosing a faulty PCB, there are some preliminary steps you can take:
▪
▪
▪
▪

Visual observation
Sensory evaluations
Refer to past failure and repair history
Perform basic measurements

The DEC VAX series of computers, built in the 1980s, were among the most robust systems ever, with some
still in operation today!
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Visual Observation
Some PCBs come with catastrophic failures such as badly burnt or cracked components that
are easy to pick up. However, it takes a keen eye to detect subtle visible defects like bulged
capacitor casings or IC packaging, discoloration, and even hairline cracks. Knowing what to
look for by studying and understanding how components fail is the key.
Below are some obvious visible signs of component-related failures:

Burned resistor

Blown fuse

Leaked capacitor

Burned transistor

Blown IC

Leaked battery

Components that suffered severe burnt or blown out are
categorized as catastrophic failures, and the affected areas
may not be limited to just the components themselves. This is
especially true if combustible materials are involved which
produce smoke and scatter ashes like a nuclear fallout. The
result would be a combusted PCB encased in arid debris like
the figure on the left.
Components that contain dielectric, such as electrolytic capacitors and batteries, tend to leak
as they age and the acid content will corrode their way out. When the leak involves a group of
capacitors causing widespread damage to the PCB, it is known as a ‘capacitor plague’. This
phenomenon is common in older motherboards and video cards, mainly due to the presence
of aluminum electrolytic capacitors which tend to become leaky due to aging. Unless the mess
is discovered early and quickly cleaned up, over time the residual stain will be cooked onto
the PCB and corrode the surrounding components. Modern PC motherboards today use solid
SMD electrolytic capacitors that avoid such problems.
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Here are some subtle visible signs:

Bulged capacitor

Miniature puncture

Hairline crack

Young engineers with good eyesight may still not detect these defects, either because they
lack experience or do not have the patience. For seasoned engineers, they will likely need
help with the use of either a handheld magnifier or digital microscope.71
Here are some solder-related failures:

Solder bridge

Solder splash

Cold solder joints

Solder bridging is a common occurrence these days with fine pitch component leads like the
PQFP and PLCC packages. Even for a trained technician performing solder reflow on these IC
devices, there is still a chance of bridging if the movement is not smooth, if the soldering iron
tarries a little longer at a spot, or if there is insufficient flux applied.
Solder splashes can happen during the solder wave process, or due to the careless action of
the person removing excess solder over a PCB by flicking the soldering iron or solder sucker.
While most of these solder debris can be easily cleaned off the PCB surface, there may still
be traces lurking in hard to reach component crevices or its underside.
A cold solder joint, also known as a dry joint, is formed when the solder is not able to create
proper contact between a component’s lead and the pad of a PCB. Dry joints can occur if the
pads being heated have large contact areas that dissipate heat faster than the soldering iron
can melt the solder. So even though the solder may look like it has melted on the surface, the
leads might not have made proper contact with the PCB pads and this is usually indicated by
dull looking solder joints.
71

An example of a digital microscope can be found on page 32.

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Another common source of PCB failure is socketed components that have come loose from
mechanical stress, or poor contact due to oxidation, corrosion, dust and rust:

1
2
1

Socketed ICs on a PCB
The figure above shows two dual-in-line package (DIP) RAMs (denoted 1) and a plastic lead
chip carrier (PLCC) CPU (denoted 2) on sockets. It’s always good to remove and examine all
socketed components for signs of oxidized leads, as well as dust or dirt lodged in the crevices
of their sockets. Oxidized leads can be cleaned using a pencil eraser and then wiping them
with an alcohol solution. Dust or dirt lodged in sockets can be flushed out using aerosol spray
cleaner, then brushed away and dried using lint free wipes.
There are several ways to prevent socketed ICs from coming loose as a result of mechanical
vibrations:

74

▪

Fasten the IC to its socket using waxed lace strings
(see right figure).

▪

Apply epoxy on both ends where the narrow edges
of the IC meet the socket. You have the option of
using either hard or soft epoxy to do the job. Hard
epoxy can be softened with the tip of a soldering
iron and then easily severed with a hand cutter.

▪

Solder opposite diagonal pins of the IC to its socket
for a more permanent grip. To remove the IC either
wick out the solder if you want to keep it, or simply
snip off the two soldered pins if you intend to
replace it.
LEARNING THE ROPES

Basic Diagnostic Skills

Since the advent of lead-free solder, there has been a fierce debate on whether it has led to
a new issue impacting PCB production——tin whiskers. What are these, anyway? Tin whiskers
are electrically conductive, crystalline structures that sometimes grow from surfaces where
tin, especially electroplated tin, is used as a final finish.72 Numerous electronic system failures
have been attributed to short circuits caused by these whiskers that bridge closely-spaced
circuit elements operating at different electrical potentials.

Almost invisible to the naked eye, tin whiskers are 200x thinner
than a human hair and can grow up to 10mm long.
Whenever you see a product marked with the RoHS logo,73 you can be certain that the PCB it
carries employed lead-free solder. That’s when you’ll need to pay particular attention to the
possibility of tin whiskers being a source of failure.
To recap:
▪

Visual observation should be the first approach to adopt at diagnosing a failed PCB. It
is always good to spend 10–15 minutes checking for visible signs of damage arising
from broken, burned, blown or leaked components before attempting anything else.

▪

When it comes to SMT boards with miniaturized components, use a magnifier or digital
microscope to assist you in finding defects that are not visually apparent, as well as to
reduce eye strain and improve productive time.

▪

Pay careful attention to components that are most likely to exhibit fault symptoms,
such as fuses, electrolytic capacitors, bent, broken, or reclused connector pins, etc.
The possibility of damages resulting from poor handling of the PCB should never be
discounted, including scratches, punctures, and even electrostatic discharge related
symptoms.

Tin whiskers have been observed to grow to lengths of several millimeters (mm) and in rare instances to
lengths in excess of 10mm. Tin is one of several metals that is known to be capable of growing whiskers, which
is not a new phenomenon with published reports dating back to the 1940s and 1950s.
72

Restriction of Hazardous Substances (RoHS) is an EU initiative that restricts the use of hazardous substances
such as lead in electrical and electronic equipment to protect the environment and public health.
73

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Sensory Evaluations
Sensory evaluations imply the use of other human senses (smell, hear, touch, taste) beside
visual observation which was covered in the previous section. It may sound absurd but some
engineers do have keen sense of smell and hearing that enable them to detect the likelihood
of a PCB exhibiting telltale signs of an impending or outright failure. Not all burnt components
display evident burn marks, especially those made of heat-resistant materials such as ceramic
packages and hermetically sealed casings. Still, the burnt odor emitting from within them is a
good indication that their internal structures have already been compromised.74
Is it possible to differentiate the type of component faults based on the odor they emit? Well,
yes and no. Certain kind of PCB faults do have their characteristic distinct smell, such as the
following three types illustrate:
▪

Leaked electrolytic capacitors
Some say they smell like urine or poo, while others link it to rotten fish.75 One thing is
sure, these are usually non-solid or ‘wet’ aluminum dielectric type capacitors.76

▪

Burnt components
Depending on the composition, burnt components may emit smoke and brown out
their surrounding area, glow red hot and then break apart into an open circuit, or in
the case of plastic casing melt down without proper heat dissipation in place. These
usually produce either a smoky (cigarette-like), metallic or acrid smell that can be toxic
and irritating to the nose.

▪

Insulation breakdown
Transformer windings are typically covered in shellac and if that insulation starts to
break down it can emit quite a sharp and pungent scent.77 Older power supplies used
rubber or polymer insulated wires instead of the Teflon type, and these materials are
prone to melt down under prolonged excessive current flowing through them, emitting
a burning rubber smell even before that happens.

This is very different from ‘new electronics’ which have a distinctive pleasant smell when they are first power
up, like a new TV or Hi-Fi audio system. This is because most electronic products contain glue, flame retardants,
protective coatings, and plasticizers and these materials are full of volatile organic compounds (VOCs) which
evaporate at room temperature and produce the ‘new electronics’ smell.
74

75

One engineer goes so far as to describe the smell as a mix of sugary piss and acetone with a 70/30 ratio.

In my second year tertiary study school holiday, I went to work in a flatted factory where there were different
companies operating at different levels of the building. The company I worked with took over a vacant lot as part
of its expansion and a few of us were tasked to clean up the place. It was formerly occupied by a company that
produced capacitors so you can imagine the amount of dielectric stains we needed to scrap off from the floor.
And the smell in that former production facility was unforgettable too!
76

If you smell something like a cheap blender trying to grind through ice at low speed, that will probably be what
it is like.
77

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Sometimes, a PCB may just give out a faint odor in the inactive state, not quite noticeable
except to those with an acute sense of smell. However, upon power up the scent will quickly
become evident. Such manifestation can be disturbing if not terrifying to an inexperienced
PCB repair technician, but a seasoned engineer will seize the opportunity to locate the source
of the problem. However, care must be exercised not to inhale too much of the invisible fumes
produced as the chemical composition of electronic components can be toxic. Have a fume
extractor fan placed next to the PCB, if necessary.
Next to smell, sound can be another indication of electronic malfunction. One example is the
switch mode power supply (SMPS) which can be found in desktop PCs and modern LCD TVs.
As the name implies, SMPS operates on high frequency voltage switching principles by means
of semiconductor devices (transistors, MOSFETs), controlled by a pulse-width modulating
(PWM) chip which converts AC or DC primary power into a DC filtered secondary output.78 One
frequent symptom encountered when repairing a faulty SMPS is the presence of an audible
clicking sound, presumably an over-current or over-voltage protection circuit operating in
response to abnormal load conditions, or a result of the filter capacitors drying out and losing
their ability to store charges.79
Talking about PCs, all motherboards invariably performs a power-on self-test (POST) when they
are turned on and upon successful completion, will produce a beep before the operating
system kicks in. If there is a fault in one of the PC’s sub-units, a series of long and short beeps
will be heard and the PC will fail to boot up. Different motherboard manufacturers have their
own sets of beep codes which can be referenced from the documentation provided, some of
which are common and similar. A familiar example I had is the video card problem, which
produces one long and three short beeps if it cannot be detected or is faulty. Another is the
memory modules which has one long and two short beeps as its signature.
One of the more memorable example was the plasma display unit I worked on in my previous
company. Upon power up, it would initialize and produce two audible beeps to indicate that
the self-check is good. If there was no sound at all, the touchscreen would be unresponsive
and I’d know the problem lies in the infrared matrix sub-assembly,80 or if there was no display
then it would be the power section of the plasma module.
Of course, not every PCB will emit audible sound when it fail. But when it does, that can be an
added bonus to your repair experience. In the case of PCBs with shorted power and ground
planes which make it unsuitable to power up, there are special test equipment that produce
sound to help pinpoint those short locations. More about this later.
There are many SMPS topologies, each with their unique characteristics, advantages and modes of operation
that determine how input power is transferred to the output.
78

Replacing these electrolytic capacitors often takes care of the problem. Look for signs of leakage or dried fluid
marks at the bottom of these capacitors, bulging top casings or discoloration in the surrounding area.
79

This sub-assembly was my first reverse engineering project and it took me about three weeks to re-create the
schematic diagram. It is mentioned in my book, The Art of PCB Reverse Engineering.
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Chapter 3

When it comes to personal touch, few engineers apart from the more experienced ones will
use their fingers to feel around a live PCB for anomalies. It’s like they have calibrated infrared
sensors built into their hands to detect the most subtle temperature changes on those PCBs
that they have worked on and are familiar with. There is a more reliable and consistent method
than using your bare hands, though.81
Nonetheless, the sense of touch can be useful when detecting problems in power devices
which produce heat during normal operation. If a device is cold to the touch, it could indicate
a dead component with probable internal open circuit that prevented power from reaching its
core integrated circuit. One rule of thumb when feeling around for overheated components is
the 70–degree rule——if you can keep your fingers on a hot surface for more than three
seconds, the temperature is below 70 degrees.82

Burnt resistors

Bulged capacitors

ESD-damaged IC

To recap:

81

▪

Smell can be an effective and quick method to detect problems in a PCB, especially if
there are leaky capacitors, burnt resistors, or transformers with insulation breakdown,
etc. Care must be observed not to over-inhale as the fumes emitted from chemicals in
these devices could be toxic.

▪

Sound is also a useful indication in some instances where it should not be present. In
high voltage circuits involving flyback transformers, low humming noise is normal but
any high-pitch, arcing or ticking sound is not. Always adhere to safety protocols when
handling such hazardous equipment.

▪

Touch should be carried out discretely, not only for personal safety but also to protect
sensitive components from electrostatic discharge damages.83

Chapter 8 is dedicated to using infrared imaging technology to diagnose faulty PCBs.

Most power transistors, MOSFETs and voltage regulators that have heatsinks have no problem maintaining
below this temperature. You should exercise caution when first touching a hot surface to prevent accidental burn
by momentarily tapping with the tip of your finger.
82

It is important to ground yourself when working in a dry environment where static builds up easily. While
touching the metal casing of an equipment may do the trick, a surer way is to wear a wrist strap that has proper
grounding.
83

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Past History
Most clinics maintain a record of their patients’ consultation history containing details of their
illnesses and the kind of medication or treatment prescribed. It is no different for a PCB repair
center. My previous company provides in-country support and repair services mainly to our
national defense agency (MINDEF) involving weapon systems from the army, air force and the
navy. On the company level, we use the popular SAP enterprise management software for
business operations and customer services; 84 within our department, however, we maintain
a separate database system that is customized to our operational needs, including process
tracking and report generation for every repair job.
Whenever a PCB is sent in for repair, the first thing an engineer assigned the job is to check
its repair history based on the PCB’s name, part number, serial number, and the symptoms
reported by the customer. Several scenarios can happen:
▪

If there is a match on the PCB and its symptoms, meaning this PCB had come in for
repair before with the same failure, then logically the next step would be to check the
faulty component(s) that had been found and replaced. This would be the easiest and
most straightforward case to resolve.

▪

If there is a match on the PCB but with different symptoms, then a further search for
another PCB of a different serial number but with similar failure is then carried out. If
a symptomatic match is found, the faulty component(s) recorded will be high suspect
and should be investigated first for malfunction. This additional step may help reduce
the time spent in diagnosing failures of a similar nature.

▪

If there is a match on the PCB but with no symptomatic match on the failure reported,
then it will be considered a new fault symptom. In-depth analysis and troubleshooting
will thus be necessary.

From the above exercise, we see the importance of maintaining a record of past PCB repair
history. Not only will it expedite the repair process of PCBs with similar fault symptoms, it will
also help engineers working on them for the first time as well.85 And besides, with sufficient
data collected, a failure pattern and profile for each PCB can be established to estimate the
mean-time-between-failures (MTBF), and the stock levels required for component spares to
reduce waiting and turn-around time for future repairs.86
In all honesty, SAP is a robust management system suited for big enterprises like our company, except for one
thorny issue——it’s too rigid (it’s a German company, after all) which makes it not user-friendly. The interfaces
and configurations are built by the IT department and more often than not, it is the users who have to adapt to
the software and not the other way around. It’s no wonder there’s a saying among us that SAP stands for ‘slow
and painful’ to use!
84

Of course, it would be nice to tap on past records of similar failures to rectify faulty PCBs but over-reliant will
not help you grow in experience and expertise in your diagnostic skills.
85

These are the primary concerns of any military organization since that will impact the operational readiness of
the weapon systems which is critical to national defense and survival.
86

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Basic Measurements
The preceding sections discussed diagnostic methods that do not involve any test equipment
and should first be attempted. Should these preliminary checks yield no conclusive results,
then it’s time to perform some basic measurements using a digital multimeter (DMM)87 and
a few benchtop equipment.

Passive Checks
Before you even consider powering up a faulty PCB, it is preferrable to carry out some passive
measurements to ensure it is safe to do so. The first priority is to check the integrity of the
power planes with respect to ground for possible short circuit or abnormal low impedances.88
It is always good practice to keep a record of normal impedances taken from functional or
serviced PCBs, to be used for comparison against faulty ones for verification later on.
Also, check the following components if they are present:
▪

Fuses
Not all fuses have transparent glass casing; resistor fuses and surface mounted ones
are usually opaque so only by measuring their continuity can you ascertain that they
are still intact.

▪

Low-value resistors
Typically in the range of 0.1 to 3 ohms, these resistors are primarily used for currentsensing purposes in power-related applications.

▪

Transformers
Check for insulation breakdown and short circuit between windings.

▪

Line filters
These EMI attenuators at the AC mains input are susceptible to power outage damage
and should be checked for open and short circuit conditions.

▪

Surge suppressors
Transient voltage suppressors (TVS) can absorb large amount of in-rush current within
a short period of time to protect a circuit. They too can burn out if power surges exceed
their allowable ratings.

Digital multimeter is preferred over analog multimeter not only because of better accuracy and resolution, but
also it has auto-ranging, auto-polarity and auto-zeroing functions so a user only needs to select the parameter
to measure (Ohm, AC, DC) and the rest will be taken care of.
87

It is not uncommon for digital PCBs with large IC counts to exhibit impedances in the range of between 10-50
ohms, so any reading below 5-10 ohms would constitute an abnormal condition.
88

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Power Up Checks
Once you’ve ascertained that a PCB is safe to power up, you are ready to do some voltage
checks. Depending on the power requirements of the PCB, you may need two or more power
supply sources to turn it on. If you have the documentation or schematic diagram then it’s
relatively easy to figure out; if not, you may need to investigate a bit further by doing a partial
reverse engineering on the power and ground connections.89
Here are the steps:
▪

Create a profile of the ICs present
Depending on the type of PCB, you may only have digital ICs or a mixed of analog and
digital ICs. Download their respective datasheets, if available. If not, pinouts should do
just fine.

▪

Determine their power and ground pins
You do this by studying the pinouts of these ICs. These days, digital ICs are not limited
to just +5V for their VCC pins. Some digital boards may contain ICs that use different
logic power levels (+5V, +3.3V, +1.8V, etc.). Also, analog and hybrid ICs with VDD and VEE
pins may not necessarily default to +15V and –15V; their datasheet may suggest an
operational voltage ranges, which is why the next step is essential.

▪

Verify their common connectivity
If the PCB contains digital ICs that operate on different logic voltages, look out for the
presence of low dropout (LDO) power devices which provide conversion of the primary
input voltage into lower operating voltages. These devices, if present, are usually
cascaded meaning one LDO will convert the +5V to +3.3V, another will in turn convert
the +3.3V to +1.8V, etc.
For analog and hybrid ICs such as operational amplifiers and ADCs/DACs, you should
determine the operating voltages based on aggregation across these ICs.90 Also, verify
whether the digital and analog grounds are connected onboard or separated up to the
PCB connector pins.

▪

Establish the power and ground connector pins
After verifying that similar voltage and ground pins are connected to their respective IC
pins, the last step would be to trace the primary input voltage and ground pins to the
PCB connector pins. These are the pins you would connect the benchtop power supply
units to power up the PCB.

I wrote a series of books on the subject of PCB reverse engineering. For those starting out on this journey, I
recommend The Art of PCB Reverse Engineering. Those who want to dive straight into the deep, you may want
to consider PCB-RE: Real-World Examples and Manual PCB-RE: The Essentials.
89

90

The IC with the lowest operating voltage will determine the overall supply required for that voltage.

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When you have the PCB’s power requirement figured out, the next thing is to connect up the
various power supplies to the PCB’s designated power connector pins. Most benchtop power
supplies have output terminals that fit power cables with banana plug ends.91

Banana plugs to crocodile clips

Banana plugs to hook clips

The same kind of output terminals are also available on an ATX power supply breakout module
though you are limited to only fixed output voltages. Fuses are provisioned because there is
no current limiting or overvoltage protection as in benchtop power supplies.

ATX power supply breakout module

Benchtop power supply

The challenge is on the PCB side which has all kinds of connector pins or terminals. Depending
on how accessible these pins or terminals are, you may have to connect directly to the PCB
instead. A common practice is to connect to the input filtering capacitor’s lead terminals using
crocodile or hook clips. If conformal coating is present, you may need to scrap it away from
those leads you intend to clip on.92
Crocodile clip ends will do just fine so long as the grip is good to prevent accidental slip and disconnection,
which can be disastrous to the PCB.
91

92

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It is important to ensure proper contact of power points and to prevent damage to the PCB.

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Some methods of connecting power to a PCB:93

Hook clips

Crocodile clips

Screw terminal block

Once a PCB is deemed safe to power up, two types of basic measurement can be performed
namely, voltage and frequency. Voltages can be measured using a DMM on devices such as
voltage regulators and references, and low dropout (LDO) voltage converters. But if you want
to measure the frequency parameter of oscillators or waveform generating devices, then an
oscilloscope will be necessary.94

Power up checks is definitely a lot more work than passive checks. It requires you to do quite
a bit of preparation before the PCB can be turned on. However, some faults will only manifest
when the PCB goes live and components start heating up and interact with each other. It also
allows you to become more acquainted with the PCB you’re diagnosing.
The preferred method is to connect power via the primary input entry of a PCB since it will detect faults arising
from poor or intermittent contacts, open or broken tracks, or failed input protection devices. If you bypass them
and connect the power directly to component leads, these faults may not surface or become evident.
93

Of course, you can use an oscilloscope to measure voltages as well but if you’re only doing voltages, then a
DMM will suffice.
94

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Common PCB Failures
There are many reasons why a PCB stops functioning. Five of the most common causes of
failures are:




Physical damage
Open circuit
Short circuit




Missing, misaligned, or misoriented component
Component failure

Physical Damage
A PCB can suffer physical damage due to mishandling, improper storage, mechanical, thermal
or environmental stress. Mishandling can arise from carelessness resulting in physical impact
such as knocking or dropping the PCB. Improper storage is often due to negligence and poor
workplace practices, for example, wrapping the PCB in a non-antistatic bag or stacking PCBs
in a heap. Such human-related factors can be remedied through education and improving the
workplace processes and practices.
PCBs that are subjected to constant vibrations will develop mechanical-related failures over
time. PCBs operating under fluctuating temperatures with poor ventilation or insufficient
cooling measures are prone to thermal-related issues. Avionics systems are more susceptible
to such problems, which is why the PCBs must be installed in specially fabricated housings to
secure them in place while at the same time provide good heat dissipation.95 Environmental
stress can refer to high dust or humidity which traps heat and cause corrosion or rust, such
as naval systems that operate out in the high seas. It’s not unusual to find PCBs coated with
thick layers of conformal substance for this reason.

Water damaged PCB
This is achieved via forced cool air through the housing’s cooling fins which conduct heat away from metalplated PCBs. Of course, besides cooling the components, these metal plates also act as EMI shielding to reduce
crosstalk and interferences.
95

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Open Circuit
An open circuit disrupts current flow and affects circuit operation. Although this kind of fault
is less serious compared to a short circuit, nonetheless it can impact a PCB’s functionality
and puts it out of action or makes it behave erratically. Open circuit failures are categorized
as either component-related or PCB-related.
Some components are more prone to open circuit than others. One common example is the
fuse which usually blows when the current flow exceeds its rating.96 Then there are devices
that tend to burn out when large current flows through them.
These include passive discrete components like resistors,
capacitors97 and inductors, as well as discrete semiconductors
such as diodes, transistors and MOSFETs. Open circuit can
also occur within an integrated circuit (IC) due to ESD or high
current surge damaging the bond wire that connects the
external pin to the internal silicon wafer.98
Stuck contacts in switches and relays can also be considered
as open circuits. Faults of this nature are attributed to either
mechanical failure, hot switching from high voltage that causes
welding, or oxidation due to high humidity trapped within the
degraded casing.

Bond wire breakage

PCB-related open circuits are often caused by mechanical or
thermal stress, degradation due to quality issues, or corrosion
attributed to operating environment or dielectric spillage from
electrolytic capacitors. Board warping usually causes breakage
of internal via links, as do prolonged heating from high heat
soldering, and not preheating the PCB before performing repair
and rework.
Internal open circuit due to via link breakage is one of the most
challenging problem to rectify manually,99 especially if the PCB
is of poor quality or has been severely compromised.
Corroded PCB trace
Unlike the conventional fuse, a polymer resettable fuse (PTC) does not blow but produces a high resistance
with a low holding current under fault conditions and cycles back to a conductive state after the current is
removed. There is an operational limit though——after a number of trip-reset cycles, the resettable fuse will
degrade and become less reliable before failure occurs.
96

When subjected to excessive operating voltages or ripple currents, capacitors will exhibit internal heat rise
and the buildup pressure will accelerate electrolyte evaporation, resulting in open circuits.
97

98

The same can be said of transistor and MOSFET devices.

99

I say manually because automated in-circuit testing can quickly find such problems.

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Short Circuit
A short circuit is an abnormal current path caused by either a component malfunction or PCB
breakdown. Passive components are just as capable of short circuit as do semiconductor
devices under electrical stress. This can also happen to integrated circuits which are primarily
made up of transistors. Sometimes, bad circuit designs are the cause because IC pins are tied
directly to power or ground instead of pull-up or pull-down resistors.100 This invariably subject
the delicate microelectronics to potential power outages which degrade the ICs and may lead
to either an open or short failure.
PCB-related short circuit is more challenging to resolve, and the most difficult condition is a
plane to plane short. A multimeter will not be of much help since it can only measure but not
trace the short to its source. Fortunately, there are benchtop equipment that can do the job
quickly and effectively——short locators. And one of the best models is the Toneohm 950 from
Polar Instruments:

Stimulus leads

Probes and clips
Toneohm 950 multilayer short locator
This piece of equipment comes with three sets of probes and a set of clip leads neatly stored
in its top compartment. The four-color plane stimulus leads are used to clip the four corners
of the PCB under check. The three cable sets comprise of:
▪
▪
▪

a blue plane probe and clip set for locating plane short
a pair of red/black needle probes for current/voltage tracking, and
a magnetic current trace probe for contactless current tracing

Unless the IC datasheet specifically states that its pins can be directly connected to power and ground, it is
safer to use pull-up and pull-down resistors as a current limiting measure.
100

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The Toneohm 950 allows you to locate the position of a short between two planes (e.g. GND
and VCC). With the four-color stimulus leads attached to four corners of the PCB (blue – top
left, red — top right, green — bottom left, yellow — bottom right) where the GND plane is, the
blue color plane probe is then used to probe around various GND points on the PCB with its
clip connected to any VCC point.
DRIVE
SOURCE

PLANE

0

200m

20

200

20K

0.03

TRACK CURRENT

TRACE &
DRIVE
SOURCE

PLANE STIMULUS

TRACK RESISTANCE

200mA

2A TRACE

VOLUME
NEEDLE
PROBES

TRACK VOLTAGE
2mV

Polar TONEOHM 950

20mV

20V

PLANE SHORTS
ACTIVE
STANDBY

UNCALIBRATED
PLANE
SHORTS

PROBES REVERSED
DRIVE SOURCE

MULTILAYER
SHORTSLOCATOR

Front panel controls and indicators of the Toneohm 950
The four arrow indicators on the bottom right below the readout guide you where to place the
plane probe on the PCB. If the probe is far from the short, only one or two arrow LEDs will light
up with a low pitch beeping sound. This will hint you which direction to move your probe. After
three to four tries, you are likely to be within a few millimeters of the short and all the arrow
LEDs will light up with a high pitch beeping sound.101 The majority of plane to plane shorts are
caused by problems on the outer layers (e.g. a shorted chip capacitor) and you can use this
short locator to quickly identify the location and cause of the short.
There is a kind of PCB-related short circuit known
as creep corrosion. It occurs when sulfur dioxide
and moisture condense onto the PCB surface to
form weak sulfuric acid. Oxidized copper then
reacts with the acid to produce copper sulfide
which in turn precipitates into dendrites that
grow across the PCB surface which result in
electrical shorts. Creep corrosion tend to attack
exposed copper on the PCB such as non-filled
vias, press-fit connectors, guard traces, and test
points. Areas with increased air flow will see
higher chances of this happening as sulfur and
air moisture are brought into contact with the
PCB.102
You can connect a headphone to the audio jack near the bottom left of the equipment so only you will hear
the beeping sound and not disturb or irritate your colleagues working in the same room.
101

Conformal coating will resolve this problem but increase PCB production and repair cost, which is why most
commercial products are usually not coated unless the operating environment requires it.
102

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Missing, Misaligned or Misoriented Components
PCBs with missing, misaligned or misoriented components are quite common these days as
surface-mounted PCB's density increases.

Missing, misplaced, or misaligned components
Such problems can arise during the assembly stage or because of improper process during
in-house repair, when removing a component from a densely packed area without preventing
the surrounding components from coming loose as a result of heat application. Socketed ICs
are also susceptible to being dislodged if not fastened to their sockets, or else misoriented
when putting back after they are removed for testing or cleaning.103
Visual inspection can be tedious and tiring to the eye on densely populated PCBs even with
the help of a magnifier. Automated optical inspection (AOI) is used in many PCB assembly
houses to detect such defects, as do some repair centers which invested in similar software
system for the same purpose.
I had a lady colleague who is fond of chatting while working. On one occasion, she had to remove many
socketed ICs from a navy PCB to clean the sockets due to dirt buildup. When she sent the PCB back for testing
after the job, the customer reported that it failed on installation. Upon return and inspection, it was discovered
that a majority of the ICs were misoriented! As a result, she was reprimanded by the department manager for
not focusing on her work.
103

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Component Failure
With the myriads of components that go into making a PCB and the harsh environment they
often operate in, failure is only a matter of time and chance. The most common reasons for
electronic component failures can be attributed to:
▪

Contamination
PCBs are susceptible to ionic contamination104 during fabrication and assembly if not
properly protected. Plating and etching chemicals, as well as dust (from drilling) in the
fabrication process; flux residue from wave soldering and biocides introduced during
pick and place of components in the assembly process, are examples of contaminant
sources.

▪

Temperature and moisture
Condensation can occur on PCBs and within component packages due to temperature
fluctuations. Conformal coating is essential for protection against these environmental
conditions.

▪

Power surge
In places where power stability is lacking, PCBs are subjected to constant electrical
stresses which lead to structural fatigue and degradation. Protection circuits must be
incorporated to prolong operational life and reliability.

▪

Radiation
Excessive noise can distort signal integrity, as do radiation sources from EMI affecting
today’s highly delicate and sensitive components. Hardening components and proper
shielding designs are basic requirements for mission critical applications in medical
and aerospace systems.

▪

End-of-life
Electronic components have a finite operational lifetime. Components sourced during
their end-of-life (EOL) stages are likely outdated and may not meet newer performance
requirements, which makes them liable to premature failures.

▪

Counterfeit parts
Counterfeit components can include trying to pass off parts with lower specifications
as higher-spec by altering part numbers, re-packaging, or even mixing them with higher
quality parts. Older parts being sold as new and defective parts marked as functioning
and resold. These components did not go through rigorous testing and qualification
processes expected to be rated for extreme operating environments.

Ionic contamination can be defined as the existence of residues that become charged in solution on a circuit
board following manufacturing that may interfere with its functionality and reliability later on.
104

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High-Risk Components
Certain people seem more predisposed to illnesses than others due to their diets, weaker
biological constitutions and the places they live in. Similarly, some components are more likely
to fail than others because of their design or fabrication, and the kind of operational risks they
are exposed to. Three examples are listed for discussion below.

Power Devices
Failed components may not exhibit visible symptoms. If the components on a faulty PCB look
fine, you will need to conduct some measurements. Consider the switch mode power supply
module below:

By segregating the primary/secondary circuits, and identifying/labeling the components, you
will have a better idea where to begin looking for the problem based on the failure reported.
If there is no output i.e. the module is dead, the primary side is the area to work on first. The
first suspect would be the fuse as it is usually the first to blow in a power fault. However, if a
single output voltage is missing, the primary side can be skipped and you should focus on the
secondary side instead.
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If after replacing the fuse and it immediately blows upon power-up, some other components
may be shorted and draining a huge amount of current. Lower or zero output voltages often
indicate that the switching transistors, voltage regulator or diodes along the voltage rail has a
short condition. In such cases, the damaged component will heat up quickly. Lightly tap on
these components to feel if any is giving off excessive heat. Be careful not to directly touch
any exposed surface or leads when the power supply is turned on as it can be extremely hot
and live voltage may be present. Remove and replace the overheated component and check
if the voltage has returned to the expected value. If the reading is still different, there could
be more components that are damaged down the voltage line. Refer to the labeled diagram
above or the schematic to determine the next component to be removed.

Input/Output (I/O) Ports
I/O ports are also common points of failure. Damage on I/O ports
seldom shuts down the whole PCB but it usually results in anomalies,
for example, an alarm controller that always senses an open door even
if it’s closed or a motor that is continuously activated. If the I/O is
protected by fuses, zener diodes, or varistors, ensure that they are
intact. If so, the logic or controller IC is likely bad. The only way to verify
is to replace the faulty part.
Power switching devices (transistors, MOSFETs) or Darlington array ICs
are often used to drive high current loads such as motors and relays.
These are also potential high-risk components to watch out for in a
faulty driver board.

Communication Devices
PCBs containing communication ports like
the Ethernet and RS422/485 transmitter–
receiver devices have a high risk of failure,
because these signal lines are normally
extended outside of the system. This makes
them susceptible to lightning strikes or high
voltage statics along the communication
cables. Protective circuits can reduce but
not totally eliminate the risks. Check for burned or cracked communication ICs, or anti-surge
protection diodes, if present.
The above are selective examples of high-risk components found in power supplies, digital
and communication circuits. More can be added as your repair portfolio improves.

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Intermittent Faults
The most frustrating kind of fault a PCB diagnostician struggles with is an intermittent failure
——one which does not manifest in a consistent nature, and experience has shown there are
at least three kinds.

Thermal
PCBs that exhibit thermal intermittent failures usually surface only after operating for some
time or even a few days later, when the system either hangs up or behaves abnormally. Once
the system is allowed to cool off and power is re-applied, the problem disappears but comes
back again when the PCBs and their components reach a certain operating temperature. To
speed up the occurrence of such faults, a heat gun or hair dryer can be used but care must
be exercised if there are plastic parts present. To pinpoint the fault, however, the popular
freeze spray technique may be employed.105

Applying freeze spray to detect failed component
Hot areas of a circuit board, solder joints, and components are a good indication of elevated
resistance or short. By applying a layer of frost over the area of interest, if the frost around a
component melts faster than the rest, it is the likely fault location. Depending on the failure
there are two options:
▪

For a PCB that hangs or shuts down when overheat, spray specific areas in sequence
until it starts up again. This will narrow down to the failing component.

▪

To test for bad solder joint, spray isolated areas of the PCB while it is in operation. The
sudden thermal shock will break any cold solder joint loose and make the failure
permanent.

Freeze sprays can be used to find intermittently failing components as well as identify cold solder joints,
cracks in PCBs and oxidized junctions. Only use freeze sprays that are anti-static to prevent damage to static
sensitive components.
105

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Mechanical
The all too well-known TV serviceman thumping on a television set analogy106 allures to this
quirky problem common in old PCB designs in household appliances. Not surprising, dry solder
joints resulting from poor workmanship and cyclic heating is often the cause and culprit.107
Modern PCBs with densely packed miniaturized SMD components with fine pitch and high pin
count connectors may also exhibit mechanical-related intermittent problems.

Example of a high-density HDI PCB
A comparison of some SMD discrete components
gives you an idea of the scale we’re dealing with
in today’s PCBs, and the likelihood of mechanical
damage associated with their delicate traces.
This is why we should never apply unnecessary
force on SMT PCBs, because the components can
be easily fractured or broken loose from their
solder joints.

In the analog age of the 60s up to 80s, TV sets are prone to this and reception problems. Some smaller TVs
have mobile antennas placed on top of them and were often moved around to improve signal reception.
106

I remembered an old TV serviceman whom my mum and neighbors engaged to solve such mechanical issues.
Most of the time, he would open up the casing or back panel of the TV, plugged his soldering iron to a nearby
socket, and after locating a specific area based on the model, then did some reflow work or replace an electrolytic
capacitor. That seemed to do the trick and he charged $20-$30 for each job. Easy money indeed.
107

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Erratic
The elusive kind of intermittent fault that evades even the most experienced engineers is one
which does not respond to heat or cold, nor mechanical force applied. It may appear at some
point during normal operation, and not surface at all when you take the PCB out to diagnose
the problem.108 Nonetheless, there are three ways you can ‘coax’ the problem to manifest——
with some daring maneuvers, that is:
1. Operate the PCB at its maximum ratings for an extended period of time
This method may not be applicable for all PCBs, and those that do are usually tested
in the equipment which they are installed. This means going on-site to perform the
checks and the customer must agree to let the equipment run at full speed or ratings
allowed under supervision, until the fault manifests or becomes permanent.
2. Subject the PCB to extreme temperature treatments
Extreme situations often requires extreme measures. Baking a PCB inside an oven at
high heat, or wrapping it air-tight109 in a freezer, might just be the straw needed to
break the camel’s back. Precautions must be observed not to overheat or damage any
component that cannot withstand high temperatures, or in the case of cold turkey
treatment, avoid possible precipitation or ice formation on the PCB.
3. Apply operating voltages above or below the PCB’s allowable ranges
Approach this with extreme care. Know what you are doing and the risks involved.
Some components can sustain permanent damage if their operating voltages go above
5% of their maximum ratings. Always use precision power supplies with voltage sensing
and check the readings before applying to the PCB. There is no turning back once the
power is switch on.
One final question before we round up this topic:
Are the failures really intermittent or are the tools and equipment we are using unable to keep
up with the PCB or system we're trying to diagnose and repair? For example, how fast does a
multimeter sample data? I’d say about 1,000 readings per second for a standard model, and
4,000 for the more costly ones. Sometimes, we need to look at what we are trying to diagnose
and determine if we have the right tools and the correct repair procedures to fix the problem,
especially the ‘intermittent’ ones.

The same can be said of patients who complain of certain pain or discomfort but when seeing a doctor, the
symptom simply disappear and after examination, everything seems normal. The doctor would suspect that it is
a psychological thing, whereas the patient would think the doctor is incompetent.
108

To prevent moisture from forming into ice on the PCB and later melting and causing short circuit during testing
at room temperature.
109

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Summary
Basic diagnostic skills can cover quite a bit of ground for PCBs that are simple to moderately
complex, using basic equipment such as the DMM, power supply and oscilloscope, and even
a digital microscope for visual inspection. For more complicated boards, however, advanced
diagnostic skills and sophisticated equipment are required. The flowchart below outlines the
tools and techniques covered in this book:110
1

2

3

Preparation

Pre-Requisites

Basic Diagnosis

Non-Power

3

Shorts

Short Locator

5

Power

Signature

Manual

V-I Analyzer

4

Fixtures
Test Jigs

6

Partial

Clip-n-Learn

8

7

Automated

ATE, FPT, JTAG

Infrared

Thermal Imager

The first three chapters are mandatory to getting you started on the PCB diagnostics journey,
while the rest of the book touches on specialized tools and skillsets that you can pick up or
learn as you progress and grow in experience and expertise. Of course, depending on the
company you work for, you may or may not have the opportunity to handle some of these
expensive equipment. Still, it is beneficial to know the concepts behind these machines which
may prove useful should you be fortunate enough to work on one in time to come.

The numbers on the top left are chapter references; the descriptors below the boxes are the diagnostic tools
and methodologies we will be covering or had touched on.
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It is easy to obtain a thousand prescriptions
but hard to get one single remedy.
Chinese Proverb

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Chinese herbology is a branch of TCM that uses plant elements and extracts to treat illness
and achieve holistic balance for the body's functions. Chinese herbs have been in use for
centuries.111 While plant elements are by far the most commonly used ingredients, animal
and mineral products are also utilized.
Herbal concoctions are made from the roots, stems, bark, leaves, seeds or flowers of many
plants, both wild and cultivated. Herbal medicines are usually taken in the form of a recipe
known as a prescription. An herbalist carefully blends together a number of herbs to achieve
a specific effect for a particular patient.112 Chinese medicines are frequently ingested in the
form of dried herbs, decocted into soup, as powders, or tinctures. Some external preparations
are also used on the skin as ointments, creams or herbal plasters.113
Just as these herbal prescriptions are customized to each patient receiving treatment for his
or her conditions, so too PCBs are tested differently based on their make and requirements.
PCB repair is not just about diagnosing failures and replacing faulty components. After a PCB
is serviced, it must be tested to ensure its functionalities are indeed restored. This can either
be accomplished by sending the PCB for on-site testing in the system, or some kind of test
platform can be constructed to carry out serviceability checks. The former approach, though
easier and preferred, is dependent on the availability of the system to perform the test and
requires approval and cooperation from the customer.114 The latter necessitates additional
construction cost, storage space, equipment resources and further test processes on top of
the repair efforts expended.115
There are over 10,000 types of herbs used in China and over 100,000 medicinal recipes recorded in ancient
literature.
111

112

There can be any number of ingredients in an herbal prescription, although six to eight is the norm.

When I was ten, I developed some kind of silvery boil rashes on the right side of my nose and face. My late
grand uncle who was an herbalist, went to the forest behind his house and gathered some plants, mixed and
pounded them into a paste and applied it on the affected area for a week. The rashes soon turned into a harden
mass and fell off, leaving just a faint scar till this day.
113

Some customers are reluctant to plug a serviced PCB into their system just to verify if it is good, as not only
will it impact their operations (because they need to make time to do it outside of their normal workloads), but
there is a risk that if the PCB is not properly serviced, it may bring down the system and inconvenience them.
114

Investing in such endeavors may not be a bad thing, though. Firstly, it adds value to repair work by designing
and building test jigs to test the PCBs after rectification; second and more importantly, it shortens turn-around
time and improves company image as well as increases customer confidence in our competency.
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Chapter 4

Test Jigs and Fixtures
First, we want to define what a test jig is in relation to PCB testing:
A test jig is a customized platform that is purposefully designed and built
for diagnosing and verifying the serviceability of a printed circuit board
or an electronic module.116
Engineers who operate automated test equipment (ATE) or assembly line PCBA testing will
infer a test jig to mean test fixture, a term they are more accustomed to. As someone who had
worked on a variety of automated testers myself, I can identify with this mindset.117 Of course,
these two terms can be used loosely and interchangeably but in this chapter, I want to make
a distinction between these two so readers know what I’m referring to:
Test Jigs118
As per the definition above. The term platform can mean a mechanical structure in which
test circuits are housed, or a CPU-based board that provides both interfacing and testing.
Test jigs can either be standalone, self-hosted test platforms, or PC-based that requires
an external computer to execute the test processes.
Test Fixtures
A test fixture’s design is based on a layout template that conforms to the test interface of
an automated test equipment (ATE), and can be an in-circuit (ICT) or functional (FCT) test
fixture, or a combination of both in a single fixture. These are usually straight-wired from
the ATE’s test interface to the PCB’s contact points, either via a vacuum activated bed-ofnails or lever operated mating connector(s). Signal conditioning or supporting circuit may
also be incorporated into the fixture to satisfy a PCB’s test requirements.
In this respect, there are more leeway and flexibility when it comes to building test jigs than
test fixtures. The one main factor that determines how you want to build a test jig is very much
dependent on how you want to interface and test a PCB or module. Other than that, it’s more
or less an ‘open-architecture’ approach in terms of design and build, as opposed to the fixed
form-factor requirement of test fixtures.

An electronic module can make up of two or more PCBs and chassis mounted components. It can be a subassembly or a standalone system on its own.
116

In the course of my 30 years of engineering career, I had the privilege of working with different test platforms,
from the Emerson A4 Skyhawk and the E2C CAT-IIID/RADCOM ATEs in my air force days, to the Factron S700
series text-driven and the Teradyne Spectrum 8800 series Windows-based testers, plus several special-to-type
equipment (STTE). In short, I’ve been there and done that!
117

Test jigs are sometimes referred to as test rigs or testbeds. A more complex form that mimics the operation
of an entire system is called a hot mock-up.
118

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Building Test Jigs

Types of Test Jigs
Now that we have a common understanding of what constitute a test jig, we can classify the
types of test jigs as follows:
▪

Standalone
Self-hosted test platforms can either be barebone CPU boards without enclosures, or
custom-designed circuits or off-the-shelf modules installed inside an enclosure that is
tailor-made to test specific PCBs.
Test PCB

Holder

Pogo pins

Barebone test jig119

Test jig with enclosure

Barebone test jigs have the advantage of portability in field deployment but may lack
comprehensiveness when it comes to faulty reporting. A simple implementation uses
a buzzer to produce beep codes, much like what a PC’s power on self-test (POST) does.
Pogo pins are used to interface with the PCB under test and requires some kind of
holder to keep the test subject in place, but otherwise is good for quick release which
reduces wear and tear from multiple mating actions. This is a low cost solution where
the test PCB is not too complex and accessibility is not a problem.
Standalone test jigs with enclosure cater to PCBs with limited accessibility (via their
primary interface connectors) and usually come with greater number of I/O pins or test
signals than a barebone CPU board. They usually have pushbuttons and indicators built
into the chassis that allow operator control over the test processes by following some
operational instructions. Serviceability of the test jig electronics is essential but selftest may not necessarily be included. In such cases, it is up to the operator to ascertain
the tester’s condition before carrying out testing on the PCB.120

119

Pogo pins stack mount test jig (courtesy of Adafruit). The CPU board is an Arduino UNO.

120

Periodic maintenance of these test jigs are paramount to ensure they are functional when needed.

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▪

PC-based
PC-based test platforms utilize some sort of communication ports to exercise direct
control on a PCB, or via a controller board inside a test jig to operate its test resources
indirectly. The end result is the same in both cases——to acquire test data or status for
each test executed and display it on screen.

PC to PCB direct

PC to PCB via test jig

The most common communication medium these days is the USB port,121 and there is
certainly no lack of USB adapters to provide the communication bridge to the test jig
electronics. Even in the case of direct PCB interfacing where some other serial ports
(SPI, I2C, CAN, etc.) except USB is available, it is not hard to find adapters that translate
from USB to the desired serial communication standards.
A rather peculiar test jig that is gaining popularity
among PC enthusiasts is this PC test bench
shown on the left, made up of a bench table that
is designed to fit multiple motherboard type form
factors for the expressed purpose of assembling
and testing PC-related hardware.
While it is mainly used as an alternative to the
standard enclosed PC casing, the potential of
diagnosing all kinds of PCI plug-in accessory
cards cannot be dismissed, once the test bench
is properly setup.

Older test jigs may still use the RS232 COM port but modern PCs and laptops no longer offer such legacy
interfaces, which renders the test programs obsolete and unusable since the codes will not be able to find the
required COM port in these new PC hardware anymore.
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Design Considerations
Engineers who build test jigs are a special breed of people with a certain level of creativity and
innovation. They love to DIY and are proud of their workmanship. That is not to say they build
everything from scratch, though given today’s 3D printing technology, it is possible to design
and produce custom parts that are not readily available or too costly to engage a third-party
to fabricate it.122
Generally, those who opt for the barebone CPU-based approach will have to decide on the
type of CPU platform to use. Two popular off-the-shelf choices are the Arduino and Raspberry
Pi with their respective specifications:

Arduino UNO

Raspberry Pi B+

Factors

Arduino

Raspberry Pi

License Type

Open source

Proprietary

CPU Make

ATMega-based

ARM-based

CPU Architecture

8-bit Microcontroller

64-bit Microprocessor123

Clock Frequency

16MHz

Up to 1.5GHz

Memory (RAM)

Small (2kB)

Large (1GB or more)

Logic Level

5V

3V

Power Consumption
Hardware Structure

200mW
Simple

700mW
Complex

The Fourth Industrial Revolution, or Industry 4.0, introduces fundamental shifts in how global production and
supply network operates through ongoing automation of traditional manufacturing and industrial practices using
modern smart technologies. 3D printing can be very useful for producing spare parts locally, thereby reducing
production cost, supplier dependence and supply lead time.
122

123

The latest Pi 4 model sports a quad-core Cortex-A72 (ARM v8) 64-bit SoC running at 1.5GHz.

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Alongside specifications, the following considerations are also important if you want to build
a test jig around a particular CPU board:
▪

Flexibility in customization
The Arduino is a better candidate for customized design since both its hardware and
software are open source, which gives you the liberty to adapt your own Arduino board
and use codes that are free and readily available, unlike the Raspberry Pi which is
proprietary and requires you to work within its inherent design.

▪

Resources for test
The Raspberry Pi, on the other hand, carries a large number of general purpose inputoutput (GPIO) pins124 to satisfy a wide range of test requirements, as opposed to the
modest resources found in the Arduino.125

▪

Ease of programming
Arduino test program development is very much limited to the integrated development
environment (IDE) that it supports, but is otherwise easy to learn since it is based on
the C/C++ language. The Raspberry Pi which runs natively on the Linux OS126 offers a
wider selection of programming tools.

▪

Scale of implementation
If you’re looking for a one-off, single purpose test jig, the Arduino would fit the bill and
cost, given its huge community of users and wide range of applications. If you want a
general platform with reusability and reconfigurability, then the single board computer
(SBC) architecture Raspberry Pi would be your best choice.

In short, go for the Arduino if your project is of a repetitive nature that simply requires providing
output based on sensory inputs. If your project demands complex functionalities and internet
connectivity, Raspberry Pi is your definite solution.
One additional consideration of using barebone CPU boards for test jigs will be the interface
medium. There are primarily two choices:
▪

Pogo pins. These are spring-loaded metal sticks with spear points to make electrical
contact on solder side pads or component leads.

▪

Header connectors. Although both Arduino (female) and the Raspberry Pi (male) have
these by default, it is often desirable to use separate or extended header connectors
to prevent wear and tear on the original board from frequent mating.

The Raspberry Pi 3 model B has 40 GPIO pins and can support a large number of sensors, making it a popular
test platform candidate.
124

125

The Arduino UNO carries 14 digital I/O pins and 6 analog input pins, enough for simple test requirements.

Other OSes can also be installed and run on the Raspberry Pi with sufficient hardware configurations and
device drivers support.
126

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It is a little more involved when designing standalone test jigs with enclosure. The following
factors need to be taken into account:
▪

Type of Controller. Simple designs which involve minimum test steps and only manual
operation may not require any controller at all. Complex designs requiring control of
multiple resources (buttons, indicators, relays, signal switching and conditioning, etc.)
will certainly need a controller to manage and execute test steps in response to the
PCB under test.127

▪

Power Source. Electronic circuits need power to operate, be it the test resources within
the test jigs enclosure or the PCB under test. The power can come from an external
source such as a benchtop power supply, or internally using power converter modules.
The former has the advantage of adjustability with greater current throughput, but will
require additional power connectors and cables. The latter is simpler to implement but
is limited to fixed voltages as well as smaller current ratings.

▪

Test Resources. These include control and conditioning circuits that may be customdesigned, commercial off-the-shelf, or a combination. They provide extension to the
controller that operate buttons and indicators to allow interaction with the operator,
and the necessary signal conditioning and conversion for PCB test and measurement
purposes.

▪

Test Interfaces. If the PCB under test has only a
single connector that is commercially available,
the straightforward implementation is to use an
equivalent mating connector for the job.128 But
if the PCB uses an unconventional connector
that is obsolete or is not easily obtainable or
expensive, then a special test interface may be
the only viable option.129 However, to get better
test coverage, it is necessary to directly access
the PCB via its solder side by means of crown-end test receptacles.

▪

Self-Test and Calibration. Though optional, it is good to include some kind of self-test
as a confidence check on the operational status of the test jig before use. Calibration
is required if precision measurement is involved.

Depending on what failure is encountered during execution of the main test steps, the controller may direct
the appropriate diagnostic path to take, and provide visual cues via light indicators or an LCD display panel. In
this respect, a controller is usually a CPU board that runs a dedicated test program upon power-on initialization
and may include some form of built-in self-test.
127

For example, a size-C VME card usually spots a pair of 96-pin male DIN connectors, so the test jig will require
a pair of 96-pin female DIN connectors for mating.
128

I noticed that most PCBs employ male type connectors at their end while the backplane connectors they
interface to tend to be of the female type. This convention seems to be intentional by design. So a special test
interface can make use of this fact and use cup-end pogo pins as mating receptacles to make contact with the
male connector pins (see figure above).
129

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PC-based test platforms have the added flexibility and power of a workstation and graphical
user interface for better test navigation and execution.130 A full-blown implementation would
come close to a special-to-type equipment (STTE) such as the Puma helicopter’s SDC tester
mentioned in chapter one of this book. The test program runs in a graphical environment that
is programmed using a form of BASIC:

Anatomy of a real-world test program using ATEasy
In most cases, it is not practical nor economical to build a test platform of this scale unless
your company happens to land on a multi-million dollar project to develop one.131 The reason
for including this illustration is to give readers a feel of what can be expected in the real-world
of test engineering.132 From this point on, I will provide three test jig examples built by me and
two other electronic engineers.
Linux OS test platform tend to favor text-based test programs for quick and no-frills operation. The ones who
designed and wrote the test programs often ended up being the operators, and for good reasons too!
130

You’ll need a team of engineers with solid backgrounds in test instrumentation, mechanical and harness
design, as well as software development. No small feat and not for the faint-hearted!
131

This is really a whole new area of engineering discipline altogether, a topic I had initially planned to write that
will probably span a four-volume series. Unfortunately, I do not have the time and energy, and frankly the job of
an indie author is not sustainable, so regrettably that will remain an unfulfilled dream for me. Nevertheless, I will
give readers a glimpse of it when we come to the chapter on automated testing later.
132

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Building Test Jigs

Example 1: Programmable Attenuator Test Set (PATS)
This item, designated as a Sensitivity Time Control/Automatic Gain Control (STC/AGC) module
was sent to my work center for repair evaluation. It operates in the X–band frequency range,
requiring some high–end test equipment, a synthesized RF signal generator and a spectrum
analyzer that can handle 9–10GHz frequencies with low noise distortion.

Attenuator module with connector pinout
The defect was due to a broken pin at the J3 connector. Replacing this connector was not an
easy task as it involved high temperature solder-welding the gold-plated exterior to the
machined casing of the module. The feasible solution was to source for a replacement casing
instead without subjecting the internal sensitive electronic parts to excessive heat.
After rectification, the next thing was to verify its performance based on the manufacturer's
specifications, which includes step programming the attenuator and measuring the nominal
attenuation at the three-tier frequency points (9GHz, 9.5GHz and 10GHz). For this, some kind
of test setup was necessary, which included fabricating a digital logic interface for manual
programming control, power source inputs, as well as RF connectors for signal injection and
measurement.
In other words, a test jig was needed.

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The manufacturer133 description states that it is an 8-bit digitally controlled voltage variable
pin diode attenuator with a 60dB dynamic range. What this means is that the attenuation can
be programmed digitally to operate from 0–60dB using the eight individual pins designated
on connector J3, so any RF signal entering J1 input can pass through without attenuation or
in discrete steps of binary weight, the smallest being 0.25dB and the largest at 32dB.
I’d figured that since a test jig was to be built to interface external power sources of ±15Vdc
as the pin diode attenuator’s operating voltages and eight digital logic signals are required to
select the attenuation level, a self-contained test module would do just fine without having
the added complication of hooking up a PC and running a test program. In other words, I opted
for the manual switch selection method instead. Designing the 8-bit digital logic circuits was
quite simple using a single-throw double pole slider switch for each bit, one pole to provide
the logic signal and the other to indicate the level. The schematic diagram was drafted in less
than half an hour (see overleaf).
The pull-up resistors (R1-R16) and the 7805 regulator (U1) would be mounted on a Veroboard
while the switches and LEDs were to be chassis mounted on the slope face of the test jig
enclosure. Wires would join all these entities to provide the power and signal routing to the
test interface that mates with the UUT’s primary J3 connector. Once the details were mapped
out, the next step was to decide on the casing for the job. Hammond enclosures are among
the popular choices so it was a no brainer. The instrument console series seemed a good
candidate and size 2 a suitable fit:
Dimensions (mm)

Off white top
& blue case

A

B

C

D

E

F

102

140

76

28

94

56

165

140

76

28

94

56

165

183

102

28

145

56

254

140

76

28

94

56

254

183

102

28

145

56

254

259

102

28

145

132

356

183

102

28

145

56

356

259

102

28

145

132

The materials and components were sourced and procured. The Veroboard was first cut to
size, then the pull-up resistors and 7805 regulator (heatsink) soldered before being wired up
using enamel coated wire strands. Header pins were affixed to bring out the power and logic
signals for connecting to wired sockets later.
133

American Microwave Corporation, Inc.

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LEARNING THE ROPES

PATS Schematic Diagram

Building Test Jigs

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Chapter 4

Once the circuit board is settled, I turned my attention to designing the enclosure’s layout. It’s
not too difficult to decide on the placement of the chassis mounted parts. I used Microsoft
Visio and the layout illustration was drafted within an hour or so:

There was enough room to include some basic test setup instructions and instrument settings
on the right side of the slope panel. The interface connector was placed on the top instead of
the slope panel for better stability of the UUT, as were the three power terminals. The switches
and LEDs were positioned on the slope panel for a natural inclination. When designing a test
jig, the primary consideration is not only the aesthetic appeal but also the practical aspect of
operating it from the user’s perspective.

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Next, it’s a straightforward task of making the cut-outs of the various chassis mounted parts
and printing it on paper to line up with the top and slope panels. Then, it’s just a matter of
drilling and filing out the holes:

Test jig cut-outs
For proper labelling, I printed the full-color layout drawings on a thick film paper and overlayed
it to the top and slope panels using adhesives. The final step was to fit all the parts into their
cut-outs and then solder the relevant wires to complete the electrical connections based on
the schematic diagram.
The end product is what you see on the right figure.
Quite professionally executed if I might say so. Of
course, after fabricating the test jig, I still had the
unenviable task of writing a formal test procedure
and a test report sheet for recording the results.134
Hopefully this will give you a better idea of the tasks
involved in designing a test jig with enclosure.
Next up…

The test procedure includes connecting the dual output benchtop power supply sources as well as the RF
signal generator and spectrum analyzer settings. You should take stock of what test equipment is available
before you even think of building any test jig.
134

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Example 2: An Arduino LCD Testbench135

1 VSS
2 VDD
3 V0
4 RS
5 RW
6 E
7 D0
8 D1
9 D2
10 D3
11 D4
12 D5
13 D6
14 D7
15 A
16 K

LCD display modules are popular with Arduino based projects, not just because they look cool
to have around, but more importantly they allow users to better interact with the hardware’s
built-in functionalities. So it’s no surprise that many LCD modules have been designed and
produced to target the growing Arduino market. These displays come in various configurations
two of which are shown below:

16x2 LCD module

20x4 LCD module

Notice that they spot the same number of interface pins. This standardization is intentional
regardless of the configurations to allow interchangeability without hardware changes. Taking
advantage of this fact, an engineer by the name Floris Wouterlood came up with the idea of
an Arduino-based LCD testbench to test these modules using an Arduino Nano as his CPU
board of choice:

This project was undertaken and shared by a Dutch engineer, Floris Wouterlood, in his blog post dated April
6, 2020. (Website: https://thesolaruniverse.wordpress.com/2020/04/06/1162/)
135

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He drafted a wiring diagram as a conceptual design, which I’ve reproduced below with some
enhancements:

Arduino LCD Testbench
Wiring Diagram

And here is the component parts list:
▪
▪
▪
▪
▪
▪
▪
▪

1x
2x
1x
1x
2x
1x
4x
1x

80×120 mm double-sided Veroboard
15 pin female pin header (designated ‘Nano support pin headers’)
16 pin female pin header (designated ‘LCD pin header’)
14 pin female pin header (designated ‘test bench pin header’)
4 pin male pin header (designated ‘power and GND pin headers’)
potentiometer 10K; 2x resistor 220 ohms; 1x LED
nylon spacers with nylon bolts
Arduino Nano (version 3.0)

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Based on the wiring diagram, we see that the Arduino Nano to LCD display uses only a 4-bit
addressing scheme, resulting in the following LCD breakout board pin designations:
LCD Breakout Board Pin Number
LCD Pin
1

Signal
VSS

Function
Logic Ground

Pins Used
GND

2

VDD

Logic +5V Supply

+5V

3

V0

Display Contrast

10K Pot

4

RS

Register Select

5

RW

Read/Write

Nano D12
GND

6

E

Enable

Nano D11

7

D0

Data Bit 0

Unused

8

D1

Data Bit 1

Unused

9
10

D2
D3

Data Bit 2
Data Bit 3

Unused
Unused

11

D4

Data Bit 4

Nano D5

12

D5

Data Bit 5

Nano D4

13

D6

Data Bit 6

Nano D3

14

D7

Data Bit 7

Nano D2

15

A

Backlit LED 3.3V136

16

K

Backlit LED Ground

+5V via 220R
GND

Pin 13 of the Arduino Nano is used to lit an LED via a 200 ohm resistor. This is useful to provide
visual activity indication. Additionally, the main supply (+5V) is provided by the mini-B USB jack
and routed to a red color 4-pin header, while the GND is similarly routed to a black color 4-pin
header. This allows the testbench to power external devices (e.g. sensors) if the need arises.
Since the Arduino Nano uses only 6 out of its 14 digital pins, there are plenty of resources left
that can be put to good use. Thus, a 14-pin header is provisioned that draws out the Nano’s
seven analog inputs A0-A6 and five digital pins D6-D10. To facilitate serial communication, the
Nano’s TX and RX pins are also made accessible via this header.137

LCD displays are generally ‘invisible’ and require some kind of backlight to provide contrast and make the
content visible to the human eye.
136

For further future extension of the testbench’s capability, the Nano’s remaining unused digital pins D0-D1
can be wired to an auxiliary test header at a later stage.
137

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With all the necessary information and parts ready, the next step was to layout the items on
the Veroboard, starting with the basic components and followed by the rest:

Modified by the author for better presentation.

Testbench layout (barebone)

Testbench layout (complete)
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Of course, before the testbench can be functional, you’ll need to program the Arduino Nano
with some test routines. Since this chapter is all about building test jigs, I won’t be covering
the software aspect of this project.138
Here’s the testbench in action:

Arduino LCD Testbench in operation

Acknowledgement:
Special thanks to Floris Wouterlood, an Arduino enthusiast, for his kind permission to use part of his
blog materials. He is an advocate of solar power over fossil and nuclear energy and a promoter of what
he calls an interesting and environmentally positive hobby.

Interested readers can write to the testbench designer and request for the source code. If you visit his blog,
you can find many more projects that he had built and shared. It’s a great place to learn how to build useful
stuffs with different Arduino boards.
138

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Building Test Jigs

Example 3: A Budget Test Rig for Low-Volume Production
Building prototypes and tinkering with them to prove design concepts is one thing; developing
a product and ensuring each unit that is shipped out is in good working condition is another
thing altogether. Keeping operating costs down to maintain a healthy profit margin is always
a challenge when it comes to low-volume production. And that means overseeing the whole
process from design to manufacturing, testing and shipping yourself instead of engaging a
third-party to do the job. A hardware and embedded software engineer by the name Greg did
just that while working with Smart Armaments, Inc. to design a Sigma automatic electric pistol
(AEP) MOSFET for Airsoft pistol products.139

An Airsoft Pistol fitted with the Sigma AEP MOSFET module
Designing and fabricating the Sigma AEP MOSFET module was simple enough for Greg, but
ensuring that it meets the specifications and is field-function is not that straightforward. Also,
flashing and testing every new board manually was time consuming when you’re talking about
a few hundred pieces per production run. Some form of test rig has to be built to allow quick
flashing, testing and validation of the module under real load conditions.
The main goal of installing a MOSFET in Airsoft pistols is to prevent trigger contacts from wearing. High power
batteries can cause micro-damage to the trigger contacts and slow down trigger response or eventually a
MOSFET failure, even though the high current does not damage any internal parts. The Sigma AEP MOSFET
module is a computerized version with additional features like burst mode, battery supervisor, cycle detection,
and haptic feedback.
139

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The test rig was built using some basic tools and a 3D printer. A 3mm PMMA plate was used
as a base on which all the components were mounted. Four black block stands are 3D printed
and attached to the corners of the PMMA plate. The test rig comprises:
1.
2.
3.
4.
5.

Raspberry Pi Model B version 1.2
Signal distribution board
DC motor (load)
Bed of nails PCB holder
User buttons and LEDs (mounted onto a 3D printed block)

2

5

4

3

1

The Sigma AEP MOSFET Test Rig
The signal distribution board that interfaces the Raspberry Pi with the other components on
the test rig is the main PCB. It includes relays, a fuse, LDOs (3V3 and 5V), ESD protection,
flyback diodes, as well as digital signal isolation.140 Green connectors are used for connecting
to the power supply (battery or benchtop power supply) while blue connectors are designated
for the device under test. Four large AWG wires are needed for power connections——two per
power and two per load, because of the high current amperage they carry. These are soldered
onto the Sigma AEP MOSFET module during test.
Signals from the Raspberry Pi are isolated via digital isolators (ADUM1401 chips) before interfacing with the
device under test on the Sigma AEP MOSFET test rig by means of pogo pins (bed of nails).
140

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Building Test Jigs

Signal distribution board

Sigma AEP MOSFET Test Rig

PCB Diagnostics

117

Sigma AEP MOSFET Test Rig Schematic Diagram

Chapter 4

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Building Test Jigs

The schematic diagram illustrates how the various components of the test rig are electrically
connected. The whole process from flashing, testing and validation takes approximately 30
seconds to complete. The bed of nails turned out to have the most issues due to the minuscule
test points (1mm) on the device under test (DUT).141 At the end of the day though, the test rig
works just fine and automates the work acceptably.142
Below are the pinout and functional diagrams of the digital isolator IC:143

Pinout

Functional block diagram

Acknowledgement:
Special thanks to Greg from Smart Armaments for his kind permission to use the Sigma AEP
MOSFET test jig as example for educational purposes.

Component density and small PCB footprint of the DUT also made it impractical to use through-hole test
points.
141

That’s provided the test rig operator needs to ensure the PCB is positioned correctly on the bed of nails and
the high current wires are securely connected to the power terminals.
142

Pins 2 and 8 are internally connected so connecting both to GND1 is recommended. Similarly, pins 9 and 15
are internally connected and should be connected to GND2.
143

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Chapter 4

Summary
In this chapter, we have looked at building test jigs as an extension of testing and diagnosing
faults in PCBs. While ad hoc PCB repairs may not necessitate going through the trouble of
designing and making a test jig, the need arises when you intend to design and fabricate your
own product, or if there is volume repair to justify building one.
Test jigs or rigs of such nature are usually custom-made according to the test requirements
of the device under test. Again, a lot depends on the accessibility of the board is and how
comprehensive you intend to test it, based on available resources and budget constraints.
There is no hard and fast rules, only general guidelines and your own experience to bring your
ideas to fruition and satisfaction.

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Some TCM practitioners diagnose patients’ conditions by their handwriting——examining their
personal letters rather than business letters. The first thing to notice is the general character
of the writing. Is it pleasing to look at and clear to read? Next comes the details:
▪
▪
▪
▪

the inclination of the letters (is it forward, backward or straight?),
the regularity of the spacing between the letters,
the consistency of the height of the letters, and
whether the written lines have a tendency to slant upwards or downwards on the paper.

Here are three samples of handwriting. Do you know which is yin and which is yang?144

The forward slant is more yin and the backward slant is more yang, while handwritings with
no slant is more balanced. It is difficult for children to write with a forward slant——they are
too yang (pun intended); women, however, tend to write with a forward slant. Yin people
coordinate the rhythms of their writing with their breathing. Yang people coordinate it more
with their heartbeat. In general, speaking is in harmony with breathing, and writing with the
heartbeat; but there is a tendency for yin people to be influenced by the rhythm of their
breathing when they write.
The spoken word is more yang than the written word. A yang person is drawn to speaking while
a yin person, to writing. It is difficult for a yin person to speak publicly; he will prefer writing.
Very few people can do both and most people have a preference. When we begin to write,
consciousness (yin) comes first, followed by the will (yang) to write; these are complementary
front and back. Then intellect begins to work——checking for misspelling, seeking clarity, etc.
and also determination. Then comes emotion——the desire to write beautifully, to be artistic
and sentimental. Then follows sensory desire and modification. Finally, the desire to complete
the task, which is an expression of our more mechanical nature.
Lines that slant upward are yin; lines that slant downward are yang. Vertical strokes are yin; horizontal strokes
are yang. In English, yin strokes predominate; yang strokes are used mainly for connecting. In between are the
circular strokes, which can be either yin or yang, depending on their form.
144

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All of these aspects of our personality succeed one another at enormous speed. These five
stages occur in every word, sentence, paragraph and chapter. The influence of the organs in
the body bears on these aspects of our personality as follows:

Head
Brain
1
Consciousness
Will

Lungs
Breathing
2
Intellect
Determination

BODY
Heart
Circulation
3
Art
Sentiment
MIND

Intestines
Sexual function
4
Sensory desire
Adjustment

Nerves
Reflexes
5
Instinct
Completion

Even one word, one sentence shows this order. A handwritten communication that is regular
at the beginning but turns irregular towards the end, indicates that the writer's intestines and
autonomic nervous system are in poor condition, although the brain, heart and lungs may be
functioning well. With this tool we can see the overall condition of the people who write to us,
and whether they are practical, romantic, cold or warm——as well as many other things about
their character and state of health.
So what am I driving at here?
Traditionally, the functional test method is used to find PCB faults. This entails applying power
to the board and performing the test procedure specific to that particular PCB. This is ideal
provided such commissioned test procedures and PCB documentation are available. Without
these resources, an experienced engineer may attempt simple measurements at strategic
points of a PCB he’s familiar with such as the outputs of a voltage regulator or divider circuit,
the threshold references of a 555 timer or window comparator, the waveform integrity of an
oscillator or generator circuit, etc. Further tests may then be performed on components or
circuit clusters suspected of malfunctioning, either by removing the suspected part for offboard testing or injecting external signals at a circuit cluster’s input to verify its functionality.
There is a lot of uncertainties because the engineer is operating in ‘blind’ mode, and runs the
risk of introducing additional failure since the PCB is in ‘live’ mode.145 The outcome is more
of a hit-and-miss affair since there are possible uncovered areas on the PCB or components
that are only partially tested.146
Well, it turns out that the components of a PCB exhibit similar traits much like a human body
except thankfully, it’s a lot less complicated to diagnose. I’m referring to analog signature
analysis (ASA) or what is more commonly known as V-I Test.

145

This is what we call a game of ‘double-jeopardy’ in tandem with ‘Russian roulette’.

Voltage and waveform measurements are ‘static’ in nature, whereas a circuit may have a ‘dynamic’ failure
that requires several iterations to capture.
146

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Signature Analysis

What is Analog Signature Analysis?
ASA is a powerful PCB fault-diagnostic technique and increasingly the preferred tool to use
whenever schematic diagrams or documentation are lacking. In such situations, comparative
analysis is used to match the analog signatures of a known-good PCB with those of a faulty
one and any deviation in the signature can indicate a potential fault. A major advantage of
using V-I testing is that the PCB under test does not need to be powered up. This makes the
technique ideal for evaluating so-called ‘dead boards’.
But before we get carried away, we need to realize that there are pros and cons to power-off
testing of PCBs, which are tabulated below:
Advantages
▪

Test is possible even if the PCB cannot
be powered up for functional test.

▪
▪

▪

▪

Limitations
▪

No risk to the PCB where powering it up
might damage it further.

ASA does not test for PCB or component
function, so it is not useful with logical
or program problems.

▪

PCB-related information is not needed
since ASA diagnose at the component
level for every network.

Cannot be used as a GO/NO-GO or RFI
(Ready for Issue) type testing since ASA
is not a functional test method.

▪

Requires functional PCBs as reference
for test development.

▪

Interpreting test results is a subjective
matter but can greatly improve with use
and experience.

Test setup time is minimal compared to
test procedure development times for
functional test.
ASA is based on comparisons to known
good data, so the test method is easy to
learn and understand.

So what type of faults will power-off diagnosis detect? ASA relies on a change in the electrical
characteristics of a circuit node to detect problems on a PCB. All analog signatures are made
up of four basic components:
▪
▪
▪
▪

resistive147
capacitive
inductive
non-linear conductive148

Understanding these basic signature shapes and curves will greatly simplify the analysis of
more complex signatures.
147

It can be any value from an open to a short.

148

Also known as semi-conductive, from semiconductor devices such as a diode junction.

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Chapter 5

The Concept Behind V-I Test
The key to understanding how analog signatures are derived from different component types
is knowing the underlying operating principles of the V-I tester. The block diagram below shows
how this technique works:

Rs

I

Vs @ Fs

Test

RL

DUT

Common

ASA functional block diagram
The analog signature displayed can be thought of as a visual representation of Ohm's Law. A
sine wave generator is used as the test signal source and is connected to a resistor voltage
divider made up of Rs and RL. Rs is the ASA's internal source impedance; RL is the load
impedance of the component under test. Since Rs is constant, both the voltage across the
component under test and the current through it is a sole function of RL.
The test signal source has three parameters: internal resistance Rs, source voltage Vs, and
frequency Fs. The objective is to select the range that gives the best overall signature display.
The source voltage Vs of the test signal can be used to enhance or disregard semiconductor
switching and avalanche characteristics. The source frequency Fs of the test signal can be
used to enhance or disregard the reactive factor (capacitance or inductance) of a component
or circuit node. The source resistance Rs is used to match the impedance of the load under
test to provide the best descriptive signature possible.
The voltage across the device under test controls the amount of horizontal trace deflection on
the display. When the component under test is removed creating an open circuit (i.e. RL = ∞),
the voltage at the output terminals will be at its maximum thus the display is a straight
horizontal line of full width. The amount of vertical trace deflection on the display is affected
by the voltage drop across the internal source impedance Rs. Since Rs is in series with the
load RL, this voltage will be proportional to the current flowing through RL and is represented
by the vertical part of the signature waveform. When RL is shorted (i.e. 0 ohm) there is no
voltage drop across RL resulting in no horizontal component in the analog signature and is
represented by a vertical line trace on the display.
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Signature Analysis

The Four Basic Signatures
All analog signatures are a composite of one or more of the four basic component signatures
——resistance, capacitance, inductance and semi-conductance. Recognizing these four basic
unique signatures is key to successful V-I test diagnosis.

Resistance

Capacitance

Inductance

Semi-conductance

The signature of a resistor always exhibit a straight line inclined at an angle from 0 to 90
degrees. The signature of a capacitor is always in the form of a circle or elliptical shape, as do
an inductor except it may also have internal resistance. Finally, semi-conductance signature
is always made up of two or more linear line segments that generally form an approximate
right angle, and can show conduction in both forward and reverse-bias.
When components are connected together to form a circuit, the signature at each circuit node
is a composite of the basic component signatures in that circuit. For example, a circuit with
both resistance and capacitance will have a signature that combines the analog signatures of
a resistor and capacitor, which is a slanted ellipsoid.
Two examples of good (solid) versus bad (dotted) signature comparisons on superimposed
displays are shown below:

Capacitor signatures

Transistor signatures

The signatures on the left show a good capacitor versus one with internal leakage, and on the
right are the signatures of a damaged transistor compared to a working one.
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Chapter 5

Resistive Signatures
Resistors have straight line signatures because of the linear relationship between voltage and
current. The slope of the signature is dependent on two factors——the V-I tester’s resistance
range selection and the resistance value at the probe point. The following signature groups
illustrate how different resistance values behave under different selected resistance ranges:
Resistance range: 10-ohm

10

1K

10K

100K

For 10-ohm resistance range selection of the V-I tester, high test resistance values tend to
produce horizontal signatures.
Resistance range: 100-ohm

100

1K

10K

100K

For 100-ohm resistance range selection of the V-I tester, high test resistance values will
still produce horizontal signatures.
Resistance range: 1K-ohm

100

1K

10K

100K

For 1K-ohm resistance range selection of the V-I tester, low test resistance values tend to
produce vertical signatures.
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LEARNING THE ROPES

Signature Analysis

Resistance range: 10K-ohm

100

1K

10K

100K

For 10K-ohm resistance range selection of the V-I tester, high test resistance values are
beginning to show signatures.
Resistance range: 100K-ohm

100

1K

10K

100K

For 100K-ohm resistance range selection of the V-I tester, low test resistance values tend
to produce vertical signatures.
The analog signature of a resistor does not change when voltage or frequency of the test
signal source is varied. Resistors are non-reactive devices and hence remain unchanged
compared to the internal source resistance of the V-I tester.

Capacitive Signatures
Unlike resistive circuit, the relationship between
induced voltage, current and capacitance is not
linear. In a capacitive circuit, voltage and current
are out of phase with current leading voltage.
This ‘time lapse’ is a function of the capacitive
reactance where the voltage lags the current
and is what produces the elliptical shape of the
signature. The width of the ellipse is directly
related to the capacitance value and the range
parameters selected on the V-I tester.

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Chapter 5

The following signature groups illustrate how different capacitance values behave under
different selected resistance ranges, but at a fixed voltage and frequency parameters:149
Resistance range: 10-ohm

220uF

10uF

10nF

1nF

As the capacitance value decreases the signature becomes more horizontal, hence this
range is best suited for large capacitors.
Resistance range: 100-ohm

220uF

10uF

10nF

1nF

At this resistance range, elliptical signatures are displayed for the 220uF and 10uF, and a
hardly noticeable one for the 10nF.
Resistance range: 1K-ohm

220uF

10uF

10nF

1nF

The 10uF and 10nF capacitors display a narrow elliptical signature at this resistance range
while the signatures for the two end values appear either as a vertical or horizontal line.
The analog signature of a capacitor does not change when the source voltage is varied. This is because the
capacitive reactance of the device under test remains unchanged compared to the internal source resistance of
the V-I tester.
149

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LEARNING THE ROPES

Signature Analysis

Resistance range: 10K-ohm

220uF

10uF

10nF

1nF

At this resistance range, large capacitors exhibit vertical lines while the 10nF displays a
typical elliptical signature and the 1nF a hardly noticeable one.
Resistance range: 100K-ohm

220uF

10uF

10nF

1nF

Similar to the 10K-ohm range, large capacitors exhibit vertical lines but the 10nF and 1nF
displays narrow vertical and horizontal ellipsoids, respectively.
The analog signature of a capacitor changes when the source frequency is varied since the
capacitive reactance is a function of frequency. Lower frequencies work better for testing
larger capacitances, while higher frequencies work better for testing smaller capacitances.
Capacitive leakage or dielectric failure is a common type of failure especially in electrolytic
capacitors as they age. While a good capacitor tend to exhibit a symmetrical ellipsoid, a bad
leaky one will show an angled orientation instead.

10uF – good

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10uF – bad

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Chapter 5

Inductive Signatures
Similar to capacitors, the relationship between
induced voltage, current and inductance is nonlinear. In an inductive circuit, voltage and current
are out of phase with current lagging voltage. This
'time lapse' is a function of the inductive
reactance where the voltage leads the current.
The width of the ellipsoid is directly related to the
inductance value and the range parameters set
on the V-I tester. Most inductive signatures
exhibit a resistive tilt with some distortion caused
by inductive hysteresis.
The following signature groups illustrate how different inductance values behave under
different selected resistance ranges, but at a fixed voltage and frequency parameters:150
Resistance range: 10-ohm

680uH

68uH

As the value of the inductance under test decreases, the signature displayed becomes
more vertical. The width of the signature will also change.
Resistance range: 100-ohm

680uH

68uH

The elliptical signatures have changed in both angle and shape compared to those using
the 10-ohm resistance range setting.
The analog signature of an inductor changes little when the source voltage is varied. The inductive reactance
of the device under test remains unchanged compared to the internal source resistance of the V-I tester.
150

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LEARNING THE ROPES

Signature Analysis

Resistance range: 1K-ohm

680uH

68uH

The signatures are now very close to vertical. Resistance range settings above 1K-ohm are
not suitable for testing these two inductor values.
As source frequency increases, the signature will become more horizontal due to increasing
inductive reactance within the inductor under test. Higher frequencies work best for large
inductor values while lower frequencies work well for small inductor values.

Suggested Range Settings
Based on the preceding observations, the following V-I range settings are suggested for the
respective capacitive and inductive values:
V-I Range Setting

Min Capacitive Value

Max Capacitive Value

10 ohms @ 20Hz

100uF

15,000uF

100K @ 5KHz

100pF

10nF

V-I Range Setting

Inductive Value

10 ohms @ 20Hz

< 50uH

100K @ 2KHz

> 50uH

Because inductors come in various types and values and can exhibit wide varieties of
signature distortion, troubleshooting inductive components is best accomplished using the
dual channel comparison function of the V-I tester.

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Chapter 5

Semiconductive Signatures
Semiconductor devices form the bulk of PCB components, from the basic diode and transistor
to complex integrated circuits. We will explore the V-I signatures of some of these devices.

Diodes
The V-I signature of a diode reflects the basic characteristics of
a semiconductor junction. As long as the anode to cathode
voltage remains below the threshold, the diode will behave as
an open circuit. As the anode to cathode voltage increases
positively the diode will begin to conduct. Once current flow
begins, a small increase in anode voltage will cause a large
increase in current flow. This is the 'knee effect’ or ‘breakdown
point’ and is characteristic of a semiconductor junction.
How does a diode signature respond to variations in the V-I test settings in terms of source
voltage, resistance, and frequency?
Voltage:

5V

10V

15V

20V

As the voltage increases the horizontal volts per division scale increases, giving the illusion
that the signature is changing, though it’s not.
Resistance:

Frequency:

1K

100K

2KHz

20Hz

The V-I characteristics inherent to a diode are not sensitive to frequency changes while
the slight variation observed in the case of resistance is due to the change in available
current from the instrument source.
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Signature Analysis

Semiconductor failures are generally resistive in nature besides open and short. This is similar
to the signature displayed when a resistor is added in series to the diode.

Normal

Internal Resistance

Leaky

A leaky diode, on the other hand, causes a slant in the usual horizontal portion of the curve,
indicating the presence of current flow through the device when it is supposed to be in a nonconducting state. Composite signatures exhibit characteristics of several different types of
components that are interconnected together, and are more indicative of the signatures
experienced in the real world of in-circuit troubleshooting. Below are two examples of parallel
diode combinations with a resistor and a capacitor:

Diode and Resistor

Diode and Capacitor

By manipulating the voltage and resistance range settings, signatures of parallel components
can be examined individually or in combination.

200mV @ 10KΩ

3V @ 10KΩ

200mV @ 100Ω

3V @ 100Ω

Examples 1 and 3 are 'passive' testing where the test voltage is set below the 0.6V breakdown
threshold of most silicon semiconductors, which essentially takes the diode out of the
signature equation. Similarly, by changing the resistance setting, the individual capacitor and
resistor signatures can be isolated.
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Chapter 5

Zener Diodes
Standard diodes conduct only when forward biased and act as an open when reversed biased.
A zener diode is designed to conduct current in both forward and reversed biased. When
forward biased, a zener diode acts like a standard diode and conducts current when the
forward voltage reaches 0.6V. When reversed biased, they act as an open until the reverse
voltage reaches the rated zener voltage at which point conduction begins.
For example, a zener diode with 5V rating will conduct reverse current when the reverse bias
voltage reaches 5V. Even if the voltage increases to higher than 5V, the measured voltage drop
across the device will remain at 5V. This clamping feature of zener diodes is useful for voltage
regulation. Since zener diodes conduct in both directions, their analog signature will display
two 'knees' or breakdown points.

Single Zener

Two Zeners in series

This type of signature is commonly referred to as a 'zener pattern'. Zener signatures are the
most common type of signature encountered in integrated circuits (ICs). Combining two zener
diodes in series essentially combines their voltage ratings. In view of this, the test voltage
should be chosen such that it is higher than the Zener voltage. A faulty Zener diode may exhibit
leakage in the reverse region of the curve instead of the expected well-defined ‘knee’.

Normal (solid) vs Leaky (dotted) knee curve
Zener diodes which exhibit significant reverse leakage will have a diagonal slant in the reverse
region, similar to that of a resistor.
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Signature Analysis

Transistors
In order to better understand the nature of transistor signatures we can model these devices
in terms of equivalent diode circuits shown below. This diagram shows the collector to base
junction appears as a simple diode signature and the base to emitter junction appears as a
zener diode signature. These signatures should be familiar to you based on the previous
discussion on diodes.

NPN Transistor

PNP Transistor

Typical signatures for a PNP transistor:

Base to Emitter

Base to Collector

Emitter to Collector

Typical signatures for a NPN transistor:

Base to Emitter

Base to Collector

Emitter to Collector

Transistors have polarity and the signatures will reverse if the component or test leads are
reversed. NPN and PNP transistors will display signatures that exhibit reversed polarity when
compared to each other. V-I test can be used to determine transistor type (bipolar, Darlington,
etc.), polarity (PNP or NPN), or pin configuration (base, emitter, collector) and also be used as
a basic curve tracer for matching transistor pairs.

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Chapter 5

Integrated Circuits
Integrated circuits, whether analog or digital, suffer degradation or failure due to the following
reasons:
▪
▪
▪
▪
▪
▪

Electrical overstress
Electrostatic discharge
Corrosion or aluminum metallization
Dendrite formations
Purple plagues
Ionic contamination

Except for catastrophic failures, most faulty ICs do not exhibit visible defects. Thus, V-I testing
becomes an important means of visually verifying the integrity of these integrated devices.151

Digital ICs
Most logic ICs contain multiple circuits of the same type within one package. These chips often
exhibit just several signatures despite having many pins, which can simplify diagnosis by
comparing signatures of similar pins. An example would be the 74LS245 octal transceivers
shown below:
VCC EN 1Y1 2A4 1Y2 2A3 1Y3 2A2 1Y4 2A1
20 19 18 17 16 15 14 13 12 11

1
2
3
4
5
6
7
8
9 10
DIR 1A1 2Y4 1A2 2Y3 1A3 2Y2 1A4 2Y1 GND
74LS245 Octal Transceivers
Four different pin types can be identified:
▪
▪
▪

Pins 2-9 and 11-18 are bidirectional i.e. connected to both input and output of a buffer.
Pins 1 and 19 are enable lines and inputs to AND gates.
Pin 10 is ground and pin 20 is VCC.

Realistically, though, V-I testing is limited to the input and output logics of integrated circuits. Failures that
are embedded deep within the core logic substrates may not manifest or be visually obvious with the signature
method. Still, almost 75% of IC failures do show visible anomaly signs.
151

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LEARNING THE ROPES

Signature Analysis

Each pin type will display a signature typical for that circuit and can be used for comparison
when testing similar pins. For example, the signatures of all bidirectional lines should have
similar signatures, as are the two enable lines. This same process can be applied to other
logic ICs such as the 7400 and 7404, etc. Let’s look at the 7404:
VCC 6A
14 13

6Y
12

5A
11

5Y
10

4A
9

1
1A

3
2A

4
2Y

5
3A

6
7
3Y GND

2
1Y

4Y
8

7404 Hex Inverters
Here are the three pin signatures measured with reference to GND:

Input Pin

Output Pin

Power Pin

And the same three pins with reference to VCC:

Input Pin

Output Pin

Ground Pin

Testing ICs with reference to VCC may show fault differences compared to a GND reference. A
good practice is to start with GND as the common reference and use VCC as a secondary option,
if the former shows no anomaly.
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Chapter 5

TTL used to be one of the primary logic families, although there are also non-TTL logic types
that perform similar logic functions but fabricated differently. The differences in logic types
are reflected in their V-I signatures. As an example, the 74HC14 and 74LS14 are both hex
inverters and have identical truth tables, but belonged to different logic families. The former
has faster switching speed due to its CMOS implementation, while the latter uses Schottky
transistors and consumes less power.

VCC
A

VCC

Y

RL

PMOS
A
0
1

Y
1
0

Truth-Table

A

Y

Y
NMOS

CMOS Logic

A

Rs

Schottky Transistor

For 74HC14:

Input Pin

Output Pin

Power Pin

Input Pin

Output Pin

Power Pin

For 74LS14:

As expected, the different internal constructions of these two inverter ICs will exhibit different
signatures for similar pins. It is important to make a distinction between these logic families
in order not to end up with a wrong diagnosis.
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LEARNING THE ROPES

Signature Analysis

Analog ICs
Operational amplifiers (op-amp) are the most common analog ICs in used today. Due to their
internal structure and circuit elements, each IC pin can exhibit a different analog signature,
so the best approach would be to compare pins of similar function if there is more than one
element per IC package, such as the LF412 dual op-amp shown below.

AOUT

1

8

V+

A-IN

2

7

BOUT

A+IN

3

6

B-IN

V-

4

5

B+IN

LF412 Pinout
Of course, the two op-amps may not be configured in the same way on a PCB since they may
be used for different purposes and functions. This is what makes analog ICs more difficult to
diagnose compared to digital ICs.

Non-inverting Input

Inverting Input

Resistive Fault

Sometimes, it might even be necessary to remove the chip and check it off-board. In such
cases, the output pin of an op-amp can be used as common reference while making a
signature comparison between the inverting and non-inverting input pins. This method works
well provided the op-amp is isolated from power and ground.

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Chapter 5

V-I Testers
There are basically two broad categories of V-I testers available in the market——benchtop and
portable USB models. Benchtop models are more expensive compared to the portable USB
ones, but that’s because they are capable of measuring and comparing more than one pair
of test points at a time, with additional functions such as switched power supply outputs and
in-circuit component testing for ICs and discrete devices.152

PFL780 Functional and V-I Test Panels
In my former work center, I had the opportunity to work on a number of these benchtop V-I
testers, namely the Diagnosys Pinpoint, Qmax Test QT-22, Polar Instruments PFL-780, and the
ABI System 8 Analogue Test Station (ATS). Most are decent in performance in terms of their
V-I test capability. The price difference, though, can vary quite a bit due to their built-in
functionalities.153
Some portable USB models such as the FADOS7F1 and FADOS9F1 do come with extra functions such as a
single power source, IR temperature sensing, PC oscilloscope and digital/analog outputs, the last two of which
make use of the same dual V-I test channels.
152

153

Personally, I like both the PFL780 and the ABI System 8 ATS for their ease of use and overall performance.

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Signature Analysis

We did not have any of the portable USB V-I testers for obvious reasons——they are dedicated
V-I tester and nothing more. For those with budget constraints, the FADOS7F1/9F1 from ProT
Ar-Ge, and the UCE-CT220S from uCore Electronics are good options:

FADOS7F1

UCE-CT220S

UCE-CT220S Advanced Test Mode Panel
My preference is the UCE-CT220S which is limited to just the V-I test function and thus priced
lower than the FADOS7F1.154 The test software operates in two modes——basic and advanced
(see top figure). The basic mode allows comparison using two PCBs (the good PCB is used as
reference). Comparisons can be made within a user-defined tolerance and the results are
visually and audibly (sound can be turned off if not required) indicated by the program. In the
advanced mode, a test database can be created by learning and recording the V-I signatures
of a good PCB, which can then be used to test a faulty PCB of the same made in the future
without needing to reference a good PCB.

154

As of this writing, the FADOS7F1 costs $2,500 per unit, while the UCE-CT220S is priced at $549.

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Chapter 5

Summary
Signature analysis (V-I test) employs AC signals over a range of fixed frequencies to display
and analyze the current versus voltage characteristics of the devices being tested. For simple
combinations of passive components like resistors, capacitors and inductors, the relative
amplitude and phase of the voltage and current provides a measure of the lumped impedance
of the network. The measured results can then be reduced to a simple equivalent RLC series
or parallel network.
The test frequency can be adjusted to accommodate the component values to be tested. For
example, a low enough frequency could be chosen such that the reactive part of the network
can be neglected and just the resistive part measured. At low frequencies inductors can be
considered as shorts and capacitors as opens, and vice versa at high frequencies.
Faulty PCBs with gross failures (short or open circuit) can be quickly detected with V-I test
without complex test analysis. Even though the curves may not be easy to decipher, there is
no need to understand them to use this diagnostic method. By simply comparing the curves
of an unknown board with a similar known good board, faults can often be identified without
schematic diagram or PCB documentation.

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A TCM practitioner views the human body as a holistic and interconnected entity. Internal
organs are interrelated and complement each other. In this sense, it is quite different from
Western medicinal practice that segregates the body into different isolated parts to be treated
separately by specialists.
Knowledge of the meridians is useful in diagnosis. Meridians are channels through which the
‘qi’ or life-energy flows throughout the body. It's a complex subject but for simplicity sake let's
limit the meridians to just 14 channels——four governing general body functions and ten
corresponding to the organs. Moles, spots, warts and discoloration along the meridians may
indicate problems in the corresponding organs. A good way to discover which organ may be
weak is to apply pressure to its meridian points.
Alternatively, and more commonly, a TCM practitioner will
feel the pulses of a patient as a first attempt to determine
the root problem. And while Western medicine recognizes
only one pulse, TCM recognizes three on each wrist which
can be taken on the surface or by pressing deeply using
three fingers. The pulses and their corresponding organs
are listed as follows:
Right Wrist
1
2
3

Left Wrist

Deep
Surface

Lungs
Large Intestine

Deep
Surface

Heart
Small Intestine

Deep

Spleen/Pancreas

Deep

Liver

Surface

Stomach

Surface

Gall Bladder

Deep

Heart Governor

Deep

Kidneys

Surface

Triple Warmer

Surface

Bladder

Similarly, a PCB has its vital and peripheral organs——components that are interconnected via
external and internal traces——electrical meridians. When powered up and operating, signals
flow through these traces in the form of electrical pulses, whether analog or digital. This
chapter discusses the clip-n-test approach, much like a TCM physician would feel around the
patient’s pulses and meridians, to diagnose the source of a failure.
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Chapter 6

In-Circuit Benchtop Testers
The term in-circuit test (ICT) is often associated with bed-of-nail type fixtures used in highvolume production testing. In this chapter, however, we are referring to a genre of benchtop
testers with the capability to perform component and board level testing in-circuit of a PCB.
Examples include the Diagnosys Pinpoint Alpha, Qmax QT-200, Polar Instruments PFL-780,
and the ABI System 8 Diagnostic Tools.
These benchtop testers have many features in common, most notably V-I signature analyzer,
digital and analog test channels, power supplies, and waveform generator. Below are three of
the more popular models:

Pinpoint Alpha

Qmax QT200NXg

ABI System 8 Diagnostic Tools
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LEARNING THE ROPES

Clip-n-Test

ABI System 8 Diagnostic Tools155
The System 8 range of diagnostic tools is made up of modules which can be configured to suit
a variety of test applications. These modules require a PC to work with the System 8 Premier
software and can be integrated into a tower bay with PCI interface. Alternatively, they can be
fitted in an external case with a USB interface.

Advanced Matrix Scanner (AMS)
This module is capable of acquiring V-I signature
of discrete components or integrated circuits
under power off conditions. Signatures can be
analyzed or compared against a reference to
detect faults as well as short and open circuits.
Each AMS module has 64 scanning V-I channels and 4 single V-I channels operating with
variable parameters (frequency, voltage, impedance, pulse outputs).

Advanced Test Module (ATM)
This module can test and diagnose all digital ICs
from all logic families, including TTL, CMOS, LVTTL
and ECL. The module offers power on and power
off tests, either in or out of circuit. Configurable
up to 2048 digital test channels.

Analogue IC Tester (AICT)
This module allows in-circuit functional testing of
analogue ICs and discrete components. All
common analogue devices can be tested as they
are configured on the PCB. The AICT also
includes a fully configurable V-I tester equipped
with a pulse generator to test gate-activated
devices.

Board Fault Locator (BFL)
The BFL is aimed at testing TTL/CMOS digital ICs.
With 64 test channels, it offers functional testing
(in-circuit and out-of-circuit), connections and
voltage tests, as well as V-I analysis and thermal
test.
For our discussion, I will be using the System 8 Diagnostic Tools from ABI Electronics since they provided a
very good sample test report upon my request.
155

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Chapter 6

Multiple Instrument Station 4 (MIS4)
The MIS4 is eight instruments in one module. It
offers a range of virtual instruments——digital
oscilloscope, multimeter, arbitrary waveform
generator, frequency counter, programmable
universal I/O channels, and four independent
auxiliary power supplies.

Programmable Power Supply (PPS)
The PPS provides the necessary supply voltages
to the unit under test. The three output channels
are variable in voltage, offering overvoltage and
current limit protections, as well as sensing for
accuracy.

ABI System 8 Test Types
The modularity and versatility of the ABI System 8 diagnostic tools allow for configurations to
perform the following type of tests:
▪

IC Component Tests
Digital IC testing with 64 test channels, 4 bus disable outputs, and 5V @ 5A power supply.
Includes logic function (truth-table), voltage, connections, thermal and V-I tests.
Functions include logic tracing, EPROM verification, IC identification, all with adjustable
logic thresholds. Auto clip positioning and circuit compensation. Upgradable to 256
channels or used for live comparison with two BFL modules.
Analogue IC testing with 24 channels plus 3 discrete. Library driven tests available for
op amps, comparators, optocouplers, transistors, diodes and special function devices.
Includes functional, connections and voltage tests. Auto clip positioning and circuit
compensation.

▪

V-I Signature Analysis
Digital circuit analysis with 64 test channels, optimized for digital components and
variable voltage range. Upgradable to 256 channels.
Analogue circuit analysis with 24 channels plus two independent probes. Variable
frequency, impedance, voltage and waveforms. 2 adjustable pulse outputs. Automatic
calibration. V-I, V-T and I-T displays. Optional out-of-circuit adapter is available.
Matrix 24 channels with rotating reference. Multi-plot display with single waveform
zoom. Mean percentage comparison for each pin with audible and visual indication.

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Clip-n-Test

▪

Test Vector Generation
64 test channels. Graphically programmable sequences for inputs, outputs and bidirectional channels. Responses can be learnt, vectors can be saved, loaded and
compared.

▪

Short Locator
Three resistance ranges. Audible and visual indication of proximity to short. Audible
continuity checker.

▪

Test Instrumentation
1. Floating Digital Multimeter
Two auto-ranging channels. DC and AC voltage measurements up to 400V, current
measurements up to 2A, and resistance measurement up to 20MΩ. Statistics for
minimum, maximum and average readings. Calculator for data processing and
logging.
2. Universal I/O
Four analogue and digital channels. Analogue channels can output and measure
voltages from –9V to +9V, as well as sinking and sourcing currents up to 20mA.
Digital channels can output and read back TTL compatible logic levels.
3. Auxiliary Power Supply
5V output at 0.5A, +9V output at 100mA and –9V output at 100mA. Current monitoring
on all three outputs.
4. Variable Power Supply
2.5V to 6V variable logic supply with over voltage protection. Variable positive and
negative supplies to 24V with variable current up to 1A.

In the next section, we will do a case study that uses the various diagnostic tools of the ABI
System 8 benchtop tester, so it would be necessary to refer to the preceding pages to know
which of the modules we are using at different stages of the repair process. For simplicity, I
will refer to their acronyms as follows:
▪
▪
▪
▪
▪
▪

AICT
AMS
ATM
BFL
MIS4
PPS

Analogue In-Circuit Tester
Advanced Matrix Scanner
Advanced Test Module
Board Fault Locator
Multiple Instrument Station
Programmable Power Supply

Let's get started…

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Case Study: Tenta CPCI SCOM-0800
For our example, we will look at the Tenta CompactPCI SCOM-080X general-purpose serial
communication card with standard RS232 and RS485 ports:

Tenta SCOM-0800 Serial Communication Card
Prior to testing any PCB, it is important to analyze and have a basic idea of its functionalities
so as to know where to begin. This will enable the test engineer to be more proficient in the
job and after the repair, to gain a better understanding and improve on the repair process of
the PCB in the future.
The SCOM-0800 card can be divided into three regions based on its component layout and
circuit functions:
A. In the right region there is a 160-pin TQFP chip, a 16PCI954 which works together with
a 20MHz oscillator and a serial EEPROM to provide PCI functions for the board;
B. In the left region contains the most components, mainly digital ICs which are buffers
and communication transceivers;
C. In the bottom region is a DC/DC converter module which provides a 5V analog to 5V
digital supply conversion, mainly for isolating the digital component supply (5VD) from
the input analog supply (5VA). Any fault condition on these two voltages are indicated
by the two LEDs on the side panel. (see above figure)

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Segregating the PCB into regions and clusters
Generally, if there are no visible component faults detected during initial inspection, the next
step would be to check on the power rails, which will also pave the way for us to ensure there
is no power fault before proceeding to test the components. We can use the MIS4 multimeter
function or the ATM short locator to find out which components the 5VA and 5VD are supplying
power to. In our case:
Input
Output
Cluster a
5VA – AGND
Capacitor
Capacitor
Clusters b
Both 5VA and 5VD156
Clusters c
5VD – DGND
U1 (ZUS-1R5-0505) is a single supply DC/DC converter
which can accept an input voltage between 4V to 9V and
produces a stable output of 5V at 300mA capacity. We can
access the module's leads by flipping the PCB to the
solder side, or simply making use of the filter capacitors
as indicated on the right figure.
156

Components of this nature are termed coupling elements.

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Before turning on the power, we need to ascertain that U1 is
intact and not faulty. For that, we can make use of the AICT's
dual channel V-I function to check U1's input (Vin) and output
(Vout) without powering up the board. From the V-I curves we
see a 17% deviation in the pattern, indicating a difference in
loading between the input and output but no short circuit. We
could simply use a DMM to measure between U1's input and
output, and then each with respect to GND individually. But
that will require three measurements instead of just one
using the V-I method, and that without the benefit of a visual
comparison of their electrical characteristics.

Vout

17%

Vin

Based on U1's specifications, we can vary the input voltage between 4V to 9V and the output
should remain at 5V. But subjecting this PCB to 9V will definitely damage the components
connected to the 5VA–AGND rail. Instead, we will input a range from 4V to 5V using the PPS
module. As expected, the output stayed at 5.06V which shows that U1 is OK. Now that we
covered region C, let's look at the other two regions in detail:

Region A
1. Twelve jumpers, in groups of three, are used to
configure four communication channels to either
RS232 or RS422. The current jumper positions are
for RS232 operation.
2. A 20MHz crystal oscillator provides the clock signal
for the PCI interface chip U3. It can be tested using
the MIS4 module's frequency counter and digital
oscilloscope functions.
3. Serial EEPROM U5 stores the code that configures
the PCI interface chip U3 to the programmed UART
functions.
4. U3 is an integrated quad UART and PCI interface
chip. Analyzing this 160-pin TQFP IC is not easy but
we can start from the simpler parts like the limiting
resistors (10Ω) surrounding it which are more likely
to fail since these are data communication paths
to the driver/receiver chips. For this, we can use the
V-I function of the AICT or AMS for comparison.
5. F1 is an SMD fuse with a rating of 2A.

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Region B

SP232

SP232

26LS31 26LS32

FCT244

FCT244

26LS31
MOCD207

HCT14

FCT244

FCT244

TLP2631

FCT244

HCT14

FCT244

FCT393

HCT14

HCT14

This region contains digital communication ICs, which includes RS232 chips, signal coupling
devices, logic buffers, etc. The optocouplers operate on the 5VA and 5VD so it's important to
make sure they are grounded at the power supply side for proper test results. Here are the
breakdowns of the various components:
▪

SP232AC – Enhanced dual RS232 line drivers/receivers. Two of these make a quad pairs
for the SCOM-0800 card.

▪

DS26LS31/32 – Quad differential line drivers/receivers for RS422 communication.

▪

MOCD207 – Dual channel optocouplers that operate on two different 5V power.

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▪
▪
▪
▪

74HC14 – Hex Schmitt trigger inverters.
TLP2631 – Dual photocouplers with isolated line receivers.
74HCT244 – Octal tri-state buffers/line drivers.
74FCT393 – Dual 4-bit decade/binary counters.

Consolidating all the electrical characteristics of the above regions, we thus determined that
the following System 8 modules are needed:
▪
▪
▪
▪

PPS
MIS4
AICT (or AMS scanning V-I)
ATM

These modules work together with ABI's Premier software which defines their working models
that uses standard test procedures that can be recorded for future reference.

PCB Test Process
The System 8 Diagnostic Tool console containing the above modules must first be connected
to a PC or laptop running the Premier software via an USB cable. Next, we will look at testing
the SCOM-800 component parts, supplemented with captured test screen illustrations and the
actual test situations observed.

Region B
We will begin with region B.
SP232AC (U32 and U33) 157

Modules: ATM and PPS

First, the SCOM-800 card must be powered up using the PPS module. Then, using the digital
component test function of the ATM module, we proceed to test the two SP232AC ICs found in
region B of the board. This is accomplished by means of a 16-pin DIP IC clip attached to one
of the ATM's main I/O connectors (figure 1).
The specific in-circuit test for SP232AC can be found in the ATM's software library (figure 2), of
which four types of run mode can be selected (in this case, single loop). As can be seen from
the enlarged pinout diagram (figure 3), each pin of the IC under test displays the voltage
measured and the logic state (HIGH, LOW) it is currently in. The power (VCC) and ground (GND)
pins are denoted with SHV+ and SHV- to indicate that they are directly connected to the power
source.
157

Refer overleaf diagram to correlate with the test narrative that follows.

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Static voltage and logic level measurements
According to the datasheet's electrical diagram, the SP232AC has voltage doubling and inverter
circuits , so it's not surprising to measure voltages higher than the supply voltage at the V+
and V- pins. The standard voltage range falls between 6V and 9V and has direct relationship
with the voltages measured at the T1OUT and T2OUT signal pins. These voltage readings thus
reflect the correct signal transmission levels of the RS232 operating protocol.
To test the logic functions of the SP232AC chips, we need to make use of the ATM's test vector
generator capability.

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Graphical test vector generator
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The left figures show the digital test vectors employed for the dynamic working mode of the
SP232AC, comprising a pair of transmitter gates and a pair of receiver gates. T1IN, T2IN, R1OUT
and R2OUT are logic level signals (TTL/CMOS), whereas T1OUT, T2OUT, R1IN and R2IN are RS232
level signals. Because these signal voltages are different, we need to test them separately.
By default, two sets of test patterns were saved in the library for the SP232AC device, and can
be used immediately to test these chips functionality. Additional test vectors can be created
by the user, or existing test patterns can be edited and saved, if needed.

In the first phase of testing, we obtained the working voltages of the SP232AC as well as the
static logic states. After applying the +5V power, the voltages at pins +V and -V can be verified
if they fall between 6V and 9V, which is in line with the device specifications. Test results
showed no abnormal phenomenon found.
In the second phase, we verified the dynamic function of the SP232AC. For the transmitter side,
logic level signals at the inputs resulted in inverted RS232 level signals at the output terminal.
Similarly. the RS232 level signals at the receiver inputs are also inverted as logic level signals
at the outputs. The SP232AC devices are therefore working normally.
During testing of the SP232AC chips, we discovered that it had to undergo two separate stages
due to the IC's unique working characteristics. As a transmission interface device, it has to
work with two different signal levels, so it is necessary not only to test its operating condition
but also whether it is working within the allowable specifications. Thankfully, the rest of the
components in region B is no more complicated than this IC.
Next up, we will look at testing the RS422 devices.

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DS26LS31 (U17 and U31) / DS26LS32 (U24)

Modules: ATM and PPS

The DS26LS31 is a quad differential line driver chip designed for digital data transmission over
balanced lines. This IC meets all the requirements of EIA Standard RS422 to provide unipolar
differential drive to twisted pair parallel wire transmission lines.

The graphical display pinout shows the static voltages present on each pin of the device. L1
indicates that both pins 1 and 7 are joined, while OPCT denotes pins 10 and 14 are in open
circuit condition.

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The DS26LS32 is a quad differential line receiver chip that complements the DS26LS31. It also
satisfies the EIA Standard RS422 requirements.

Input pins with the IPMH connotation in the graphical display pinout implies these pins are in
the high unstable logic states according to the logic level definition setting of the software.
In-circuit test results of the RS422 chips show that they are working normally, denoted by the
tick marks on the top right of the user interface panel. The results section at the bottom left
also show the kind of tests that passed, namely truth-table (logic), connections and voltage
comparisons. (see overleaf figures)

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Logic trace data displays
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74HCT14 (U20, U22, U27 and U30)

Modules: ATM and PPS

These are hex Schmitt trigger inverter chips with three pairs of connected gates indicated by
L1, L2 and L3 in the graphical display pinout. This is not unusual in communication designs
since Schmitt triggers are used to clean up noisy signals and sharpen slow edges.

One of the ICs (U30) exhibited unstable conditions on pin 12 which was unable to achieve logic
high (OPCT HIGH) in the static voltage check stage, leading to incorrect logic state detected
during the in-circuit functional test stage. (see overleaf figures)
It is still too early to conclude that U30 is the cause of failure for the SCOM-0800 board. The
proper practice for clip-n-learn method is to verify every device manually, then tally the findings
and decide the next step to take.158

Unlike automated testing (next chapter) which calls out a fault list of possible failed components, the onus is
on the engineer doing the manual test approach to determine which component is faulty or suspect. Either case,
it may not be always conclusive, though the problem area is thus narrowed down for further investigation.
158

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Anomalies and failures detected (pointed by arrows)
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74FCT244 (U8-U9, U14-16 and U28)

Modules: ATM and PPS

These are octal buffers/line drivers with tri-state enable ICs commonly used in controlling the
flow of data buses.

The graphical display pinout indicates a few conditions present when testing these ICs in static
voltage mode:
▪
▪
▪

IPMH
L1 FLOT
CFLT

Input Mid-High (unstable logic state)
Pins linked and floated
Pins experiencing conflict condition

While this is expected of tri-state bus control devices with outputs tied together, U16 did exhibit
abnormal test results and are flagged as failures (red crosses instead of green ticks). Output
pin 12 could not reach the required logic voltage level (MDHI HIGH) during the static voltage
check stage, and hence is a suspect. (see overleaf figures)

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Anomalies and failures detected (pointed by arrows)
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74LS393 (U6)

Modules: ATM and PPS

This IC is a dual 4-bit binary counters, each counter having its own clear control and clock
that triggers output changes on the negative edges.

LOW HIGH and HIGH LOW in the graphical display pinout have the following meaning:
▪
▪

LOW indicates the pin has several logic low transitions
HIGH indicates the pin has several logic high transitions

These activities can be seen in the timing diagram (see overleaf). While counter A appeared
normal with all the falling edges of its clock activity, counter B's QB, QC and QD outputs were all
offset by half a clock and its QA output remained low for nine of the clocks. This is consistent
with the LOW indication on pin 11 in the graphical display pinout above, pointing to counter B
as the culprit.159 This chip would be the first component to be replaced.

The possibility of poor contact or open in the test clip cable is unlikely since these will usually be detected
and flagged out as OPCT instead of LOW.
159

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Timing diagram provides clear fault indications

TLP2631 (U19, U21, U23, U26 and U29)

Modules: ATM and PPS

These devices are dual photocouplers, each comprising a pair of light-emitting diodes (LEDs)
operating on a corresponding pair of photodetectors. From the electrical diagram, we see that
it is different from the usual optocoupler chips in that its outputs have additional logic buffers
that drive open-collector transistors.
This component part is not found in the ATM's test library so a generic 8-pin dual-in-line IC has
been selected with the designation DIL 8 as shown. From the static voltage checks we
determined that the device operates from a VCC supply and provides output voltages in excess
of 4V. Both anode inputs (pins 1 and 4) are also linked, which means we must drive them as
a single input instead of separately.
In the test pattern editor, we set both cathodes to low and toggle both anodes with the same
vectors. Once the logic test patterns are set, we can then power up and test these devices.
From the test results, we confirmed that all five photocouplers are working correctly.

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Logic trace data display
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MOCD207 (U7 and U10-13)

Modules: AICT

We come to the last group of components in region B——the dual optocouplers devices. As
shown in the electrical diagram, these ICs are purely analogue in characteristics. This is the
reason we choose the AICT over the ATM module to perform parametric testing which does not
require power source application.

In the Analysis section, we can find the measured parameter
values, being:
▪
▪

CTR
Vled

Current Transfer Ratio (in percentage)
Forward voltage value of the input photodiode

In the graphical display pinout, linked pins 6 and 8 have I>>
indications which imply there is low impedance state causing
high current flowing through these pins. This is perfectly normal
in optocoupler testing and the devices being in-circuit, can be
affected by other components that are connected to these pins.
However, we can still know from the test results that the
electrical characteristics are those of an optocoupler.
Let's now move on to region A.

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Region A
As mentioned previously, there are three components in this region namely, an oscillator, a
serial EEPROM, and a general purpose PCI interface chip. The EEPROM is socketed so it can be
removed and its content read out on a device programmer for checksum comparison. The PCI
interface chip is not easy to remove nor test so we will only do some peripheral test around
its surrounding pins.
Oscillator (U2)

Modules: MIS4

The DSO function of the MIS4 module is used to measure the output signal parameters of the
oscillator, as shown below.160

160

This is an earlier version of the MIS user interface. Newer version has a different presentation.

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OX16PCI954 (U3)

Modules: AICT / AMS

As for the PCI interface chip, we see that its surrounding has many SMD resistors which can
be measured using a DMM. Since these resistors are connected to the pins of the big IC, we
also want to be able to check the pin impedance as well. The V-I function of the AICT or AMS
would be suitable for this purpose. Moreover, we can use the clip-type SMD detection probe
when measuring the SMD resistor values, as shown:

With the source impedance set at 100 ohms, this row of 10-ohm SMD resistors produced a
near vertical line on the V-I curve, as expected. The purely resistive nature also suggest that
one end of these resistors is in the open state, possibly extending to the connector pins of the
PCI interface bus.

For another group of SMD resistors, there are connecting elements at both ends; the V-I curve
showing a turning point and ellipse can be attributed to the internal impedance of the IC
component. These curves can be learned and saved for future comparison tests.

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A summary of the findings:
▪

Most of the devices in region B (logic, RS232/RS422 ICs and optocouplers) are tested
and found to be normal. Three ICs, however, exhibited anomalies——U30 (74HCT14), U16
(74FCT244), and U6 (74LS393). Further actions are required to ascertain which of these
devices might be faulty.

▪

Limited tests were conducted for devices in region A, due to the complexity of the main
component (U3). Oscillator (U2) is easily verified using the MIS4's DSO function, while
the serial EEPROM (U5) is checked using a device programmer for correct checksum of
its data content.

In conclusion, the SCOM-0800 is basically an interface circuit board that converts RS232/RS422
into PCI communication signals. Its internal power supply is isolated by a 5V power converter
into two sets of 5V supplies, the purpose being to improve noise immunity of signals. During
preliminary visual inspection, no abnormal components were found. There was also no smell
or sound detected when powering up the PCB. The clip-n-test method focuses on how to test
digital components operating from two different power supplies, as well as the optocoupler
devices. The components in region B accounts for 90% of the PCB, and it is relatively easy to
confirm whether their functions are normal. Based on experience, testing a PCB of this nature
takes approximately three hours for the first repair. Subsequent repairs will require half the
time or less with familiarity and past test records.
When it comes to PCB repair work, engineers should explore different diagnostic approaches
to improve their efficiency and effectiveness in finding the failure causes. At present, most
engineers rely only on one or two methods in their daily repair work. If there is no good circuit
board or schematic diagram to refer, the task will be even more challenging. Therefore, the
test tools and experience of the engineer becomes important. The System 8 circuit board
benchtop tester designed by ABI provide many test options that engineers need.

Acknowledgement:
Special thanks to Wesly Lo of ABI Electronics Ltd for providing this case study to demonstrate
some of the test capabilities of the System 8 range of products.

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Summary
The clip-n-test breed of benchtop PCB diagnostic tools is a more affordable and space-saving
alternative to the automated test equipment (ATE) which we will discuss in the next chapter.
This test method is given prior treatment to give reader a better appreciation of its manual
interactive approach, in contrast to the automated and time-saving method offered by the ATE
approach.161

To be fair, this statement precludes the amount of time and effort involved in building the interface fixture as
well as developing and debugging the test program that tests a PCB. It applies only from the operator's point of
view as opposed to the clip-n-test method which requires the user to have certain level of familiarity and
acquaintance with PCB diagnostics.
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Western medical authorities view TCM as an inexact science, primarily because the process
and accuracy of diagnosis is based on the experience of the practitioner rather than a defined
set of repeatable procedures. Not that TCM has no underlying principles but because it treats
the human body as a holistic entity of interrelated functions, making it difficult to quantify and
define how a cure should be prescribed.
Understandably, those who see TCM as a viable alternative to Western medicine would seek
to give it a scientific makeover——by automating the diagnosis process. For example, tongue
somatotopy in TCM studies allow segmentation and feature extraction to be incorporated into
some form of automated diagnosis by means of image processing, coupled with advanced
statistical and mathematical models to extract the needed pathological parameters to classify
a patient’s condition.

Tongue Somatotopy

Automated Tongue Diagnosis System

Since the shape, size, surface and coloration of tongue varies from person to person, robust
and accurate algorithms are required to deal with these structural difficulties. A number of
papers have been written by researchers on how to circumvent such anomalies and converge
on a good correlation between image analysis results and the patient’s physiological as well
as pathological conditions. Certainly, there is still much work to be done to overcome these
obstacles and limitations.
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Another area of interest is pulse diagnosis, an important diagnostic technique which requires
sensitivity and specialized skills. The subtle variations of the three-point palpations on both
wrists are usually identified through continuous practice and accumulated experience——a
unique human perception that is difficult to replicate with sensors and the data interpreted
by computational machines.

Pulse taking in TCM162

Bi-Sensing Pulse Diagnosis Instrument (BSPDI)
Three-dimensional pulse mapping method to mimic a TCM practitioner’s fingertip sensations
has been proposed, but pulse depth qualities in relation to different patients’ pathological
features present no small challenges. Even so, big strides are constantly made to bridge the
gap between human and machine efficacy, and the day may come when automated diagnosis
becomes a reality, just like automated testing for PCBs.

The front position corresponds to ‘heaven’ and reflects the conditions from the head to the chest; the middle
position corresponds to ‘man’ and reflects from the diaphragm to the umbilicus; the rear position corresponds
to ‘earth’ and reflects from the umbilicus to the feet.
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Automated Testing

Automated Test Equipment (ATE)163
In the beginning of this book, I mentioned several test machines or platforms that I worked on
during my military and commercial career years. Some readers might have heard or even
operated these equipment but not many, I suppose, are as privileged. Those who are already
familiar with the concept can choose to skip this section. For the rest, I hope it would be an
eye-opening and educational tour.164
So what exactly is an automated test equipment or ATE?
As the name implies, an ATE is a computerized platform that automates manual electronic
testing processes with minimal human interaction. Many PCBs today are tested by these
systems to ensure compliance of performance, functionalities, and safety for operation and
field deployment.165 A typical ATE system is made up of three major components——hardware,
software, and the test interface. These components are usually consolidated into an all-in-one
test system which can vary greatly in terms of size and portability——from small, compact and
mobile test stations to multiple-bay, stationary test towers resembling data center server
cabinets.

Mobile ATE

Test Tower ATE

Also known as automatic test equipment, though I prefer ‘automated’ over ‘automatic’ for the fact that test
engineers are still needed to debug the test programs generated by these machines as well as write customized
test routines for components not found in the system’s test databases.
163

I won’t go into too much technical details as that would be overbearingly painful to write. Instead, I will give a
succinct introduction to the design concept of different ATEs and then showcase a few examples of what a test
program set (TPS) looks like to wrap up the chapter.
164

Of course, ATEs are not limited to testing PCBs but include integrated circuits (ICs) and electronic products
like mobile phones, storage devices (HDDs and SSDs), systems and modules, etc.
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Hardware
Depending on the type of platform, there are several hardware aspects to the overall design
of an ATE:
▪

Mechanical Structure
This is the physical structure that houses all the other ATE hardware and provides the
mechanical support needed for transportation, operation and maintenance of the
whole system.
Traditionally, ATEs employ rack and stack housing method because test equipment
generally come in 19–inch widths with one or two-unit height variation. Test tower
configurations with two or more bays were common to accommodate these bulky test
instruments, with sliders attached on both sides for ease of access when performing
maintenance or troubleshooting.
As technology improved, these monolith test towers slowly gave way to trolley-type test
bays with roller wheels for mobile deployment. While the rack and stack method is still
present, now each rack may comprise several instruments in the form of cards instead
of the usual bulk casing of a single equipment unit.
Some ATE manufacturers, however, came up with their own customized platforms, not
for novelty reasons but out of necessity. In-circuit testers are known to house all the
test resources in compartments beneath their tabletop frameworks.166

▪

Host Controller
Without exception, all ATEs operate from a host controller, which can be a computer
running a commercial or customized test executive program, or an embedded single
board computer operating a dedicated test menu with specific test procedures. It is
not uncommon for legacy ATEs to use archaic processing machines like the PDP-11
and the HP1000 mini-computers.167 Modern ATEs, on the other hand, tend to gravitate
towards today’s powerful PC-based platforms with graphical interfaces like Windows
and Linux-based GUI such as GNOME and KDE.168

The Factron S700 series and Teradyne Spectrum 8800 series of testers are good examples. GenRad used
to be a major player in the in-circuit tester market but had since been acquired by Teradyne. Another well-known
name in the game is Agilent (formerly HP).
166

The PDP-11 runs mainly on a few flavors of DEC’s OSes, whereas the HP1000 operates on HP’s RTE OSes
which are usually text-based. When workstations become popular, ATEs designed with these machines would
run on the UNIX OS.
167

I did encounter a legacy ATE which is built around a 486SX CPU running on DOS 6.22 using text-based menu,
and another with an even older 386 CPU and 387 co-processor operating on Windows 3.11 which is really just
a graphical program running on top of MS-DOS. Both are industrial PCs with multiple peripheral slots to allow
different interface boards to control various instrumentations. These are machines built before the turn of the
millennium when DOS was still the dominant operating system.
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Automated Testing

▪

Interface Bus Protocols
When Hewlett-Packard (HP) was king of the hill in the test equipment industry, they
came out with a communication protocol standard for all their products——the HPIB
bus which was widely adopted and later known as the General Purpose Interface Bus
(GPIB). It became the de facto parallel interface standard for test instruments under
the IEEE-488 specifications. Since then, instrument control interfaces or buses have
undergone tremendous changes and spun a wide array of other protocols, layers and
APIs, depicted by the chart below:

LXI

VXI-11
Ethernet

TCP/IP
HiSLIP

IEEE-488.2
Syntax

VICP
SCPI
USB

USBTMC

GPIB

IEEE-488.1

RS-232

RS-232

RS-422

RS-422

Physical

Universal API

VISA
What this means is that besides the physical implementation of interface bus protocols
there are also efforts to streamline and standardize the communication processes of
these protocols.169
The LXI consortium oversees standards related to LAN/Ethernet communications, while VISA is really just a
communication API to enable bus independent communication for different interface protocols.
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Suffice it to say that all these protocols are created with one thing in mind——to enable
host controller access to various test instrumentation to realize the test specifications
for which an ATE is designed for. Most of the time, the low-level communication stuff
is taken care of by the API’s instrument drivers, so the operator need only to focus on
the test processes and the engineer responsible for writing and maintaining the test
program uses a high-level test language to control the test resources of the ATE he is
trained on.170
▪

Test Resources
Test resources can refer to the physical test instruments as a collective whole, as well
as the functionalities that are made available for the operation of the ATE. This implies
that every test instrument present on an ATE may not necessarily be used to its full
potential. Often, the designer of the ATE has to consider the overall test specifications
of the machine he is building, and then decide on a suitable equipment that meet at
least the minimum requirement. Most of the time, though, he has to settle for a few
that have more capabilities and functions than is needed.
That said, any test engineer working on an ATE must be familiar with the machine——
what it is able to do and what its limits are. It will determine to what extent he is able
to test a PCB and how much fault coverage the test program will offer.171
From a test engineer’s perspective, test resources can also refer to the development
software tools that enable him to write test programs that control the instrumentation
of the ATE. The tools are not limited to just the codes that execute the test processes
but related software components that allow him to check for syntax errors, compile the
source into machine code for faster execution,172 control execution steps of the test
program for debugging, and generating test report of a selective or end-to-end test run.
Powerful diagnostic tools may also be incorporated to guide operator where to probe
for fault isolation when a failure is encountered.

▪

Test Interface
The final component in the hardware aspect of an ATE is its test interface. This is such
an important part of the ATE that it warrants a separate treatment, but is mentioned
here for completion.

I have to qualify this as a general statement because test engineers sometimes do need to go down to the
API and machine level to access certain functions that are not available at the test language level. Provision is
usually made in test language syntax to execute machine level codes for this reason.
170

Fault coverage is determined by various factors, including the type of electrical tests performed and the
number of pins that can be tested.
171

Some ATEs use interpreters to execute test programs line by line instead of compiling them. This allows on
the fly modification of the source code without needing to compile the whole test program, though the flip side
is slower run-time.
172

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Software
If hardware is the body of an ATE, then software is its soul and personality. And just as different
people exhibit different personalities, so the make and purpose of each type of ATE will have
its own set of supporting software tools to manage the test resources and drive the test
program development process.
Legacy ATEs like the CAT-IIID and RADCOM from formerly Grumman Aerospace173 basically
have no software development tool apart from its ATLAS interpreter-compiler that runs from
the HP’s RTE-IV operating system, configured to recognize the test resources specific to each
of the test stations. There’s no syntax highlighting to warn you of errors, automatic completion
to make coding a breeze, or even a decent full-screen text editor to begin with. For those
working on these machines, the only programming environment is a line editor with all the
archaic prompt commands of bygone years to work on.174
Production test ATEs that double up as diagnostic testers fared much better. The development
software tools come in two flavors——text-based and graphical-based. Examples of text-based
SDT are the Factron S700 testers while the Teradyne Spectrum 8800 series sport the more
familiar Windows environment.

DEC Terminals

MicroVAX Server

To circumvent the text-based Factron’s limitations and free up the testers for debugging and
repair, Schlumberger improvised a time-sharing networking solution by creating a program
management system (PMS) that runs on a MicroVAX server to allow multiple test engineers to
work on their test programs simultaneously via DEC terminals. When a test program is ready
for debugging, the test engineer would then upload his or her program package to the specific
tester to perform the task.
173

It merged with Northrop Corporation in 1994 to form Northrop Grumman.

Despite this shortcoming, the CAT-IIID and RADCOM are impressive machines that rivalled many ATEs in
terms of test resources and interfaces that allow them to run over a hundred test programs that virtually covered
all the E-2C’s avionics (radar, communications, navigations, displays, power, etc.).
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A screenshot of what working in the PMS which emulates the Factron test environment looks
like:

On the tester, you can simply type ‘net u1.8’ to obtain
the connectivity of the node. In the PMS environment, a
‘net’ command has to be issued first before entering the
node ‘u1.8’ at the ‘NET>’ prompt.

Iterations in the debugging process is inevitable since a PCB in-circuit test program contains
many sections, beginning from the simple passive and discrete (shorts, resistors, capacitors,
diodes, etc.) components to the more complex integrated circuits (digital and analog).175

A test fixture had to be built and a working PCB available to perform program debugging. At the peak strength
of the work center, we had over 30 engineers developing test programs for the three Factron testers, which
necessitated time slot assignments based on project priority as well as delivery deadline.
175

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The Windows-based Teradyne testers are easier to work with because of their graphical Test
Development Environment (TDE) and Microsoft network sharing scheme.

Each test engineer can work on his or her project at their individual PCs and when ready for
program debugging, simply port the project folder via the shared network to the specific tester
and get on with the task.176
Modular ATEs like the National Instruments PXI-based
test systems have their own custom development tools
like LabVIEW (Laboratory Virtual Instrument Engineering
Workbench) with a dataflow programming language ‘G’
that is integrated to a whole library of virtual instrument
panels that facilitate real-time control and interactions
with NI’s range of plug-and-play instruments. Such test
systems perform PCB-level functional testing in contrast
to component-level in-circuit testers.

This is just a simplified scenario to give readers a quick glimpse; the Teradyne test platforms are capable of
more complex test processes that are too involved to be treated in this book.
176

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Test Interface
This is the portal through which an ATE interacts with the PCB or UUT (unit under test) as it is
often referred to. The type of tests performed on a PCB will in some measure determine how
an ATE’s test resources are configured to interface with the subject being tested. Logically,
the same kind of test resources are grouped together to facilitate proper fixture wiring and
minimize crosstalk that may affect test signal integrity. One good example is the RADCOM’s
Multiple Matrix Switch (MMS) interface panel shown below:

MMS Interface Panel Organization
There are a total of 528 interface pins divided into two major sections177 and grouped based
on their functions, such as power (direct and switched), ground, digital I/O and references,
universal (analog) I/O,178 relays, synchro/resolver, etc. Multi-bay type ATEs usually have more
than one interface panel, and in the case of the RADCOM, it has a separate dedicated RF
interface unit and an auxiliary interface panel.
177

Out of these 528 interface pins, 11 are unused spare pins.

178

For routing instruments like the DMM, signal analyzer and function generator to the UUT.

180

LEARNING THE ROPES

Automated Testing

An example of a test fixture that interfaces to the RADCOM is shown below:

Patch panel of test fixture
(wirings not shown)

Exploded view of a RADCOM test fixture
In general, an ATE test interface may contain some or all of the following elements:
▪

Power (direct/switched)
All PCBs require power to operate in order for the ATE to test them. These can be AC,
DC, or both, depending on what is being tested. AC power is usually targeted towards
avionics electronics which run on standard aircraft generator power of 115Vac, 400Hz.
26Vac may be included for synchro/resolver reference and operation if the ATE tests
these functions. DC power is mandatory and are usually programmable in terms of
voltage and current outputs. These can be directly applied or via switched channels for
protection reasons.

▪

Ground
Ground provides a common reference for both the ATE instruments and PCB circuits
from which signals and measurements are based. PCBs with mixed signal designs may

PCB Diagnostics

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Chapter 7

have separate digital and analog grounds but when interfaced to an ATE, will be
connected together at the test fixture side.
▪

Digital Input/Output
Modern PCBs are increasingly more digital than analog due to the greater processing
power, better accuracy, higher noise immunity, and lower power consumption digital
technology affords. Testing these devices require digital inputs (stimulus) to exercise
their logical functions and digital outputs (response) for evaluation of the extracted
data. Digital input signals can be programmed to work with different logic families, as
are the thresholds for measuring the digital outputs. The amount of digital input data
that can be injected and captured per digital channel is dependent on the memory size
available and the type of test patterns involved.179

▪

Analog Input/Output
We live in an analog world so it’s an indispensable part of PCB design, however small
a role it may play. Analog power and signals are generated as well as measured by the
parameters that defined them, which can include amplitude,180 frequency, phase shift
and power. Examples of analog instruments to provide stimulus and measurement are
DC power supply, multimeter, function generator, signal analyzer, oscilloscope, signal
analyzer, etc.181 Some test interfaces have fixed allocations for specific instruments
(e.g., Factron) while others employ switching matrix for flexibility of signal routing (e.g.,
RADCOM’s multiple matrix switch). The former’s limitation can still be circumvented by
using relay channels available at the test interface, though it means additional test
resources are thus taken up at the cost of flexibility.

▪

RF Input/Output
PCBs that handle signals in the radio frequency range require special test instruments
such as RF signal generator, power meter, spectrum analyzer. Even the test interface
has to cater for the effects of insertion loss, signal attenuation, and VSWR, which are
not as critical in normal analog signals. The RADCOM has a separate RF Interface Unit
(RFIU) that incorporates a power plane concept, an automatic path loss compensation
technique that removes the need for complex calibration procedure.182

For example, a clock signal requires only a single channel with two bits repeated across a certain frequency,
whereas a four-bit gray code pattern will need four channels and 16 bits per channel. Arbitrary data patterns will
take up more memory to store and evaluate.
179

The amplitude parameter alone can encompass values such as peak-to-peak, average, root-mean-square,
and DC offset, to name a few.
180

To ensure reliability and accuracy, these instruments usually require periodic maintenance and calibration to
be certified as fit for use.
181

Identical cable and switch path losses are provisioned such that the RF signals generated at the RFIU output
connectors will have the same power level as that measured by the power meter sensor. This is an interesting
field of study in the RF test and measurement discipline.
182

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Automated Testing

▪

Relays and Switches
As mentioned, these are extra test resources that provide additional flexibility to the
available test instruments and even external test equipment that may be connected
via the test fixture. They can also route static and dynamic test signals to emulate a
PCB’s operating conditions.183

▪

Miscellaneous
Some ATEs may allow extra test resources to be incrementally added to extend their
test capabilities. These include loads, attenuators, and filters, which can be passive or
active, fixed or programmable. Actuators and sensors are also common add-ons to an
ATE’s test arsenal.

When talking about ATEs, the following related elements must be discussed:
▪
▪
▪
▪
▪

Test Program Set (TPS)
Test Fixture and Panel
Interface Cables
Test Program
Test Program Set Document (TPSD)

Test Program Set (TPS)
A test program set comprises the hardware, software and documentation aspects that enable
a PCB or electronic module to be tested on a target ATE. The hardware refers to the fixture or
panel that mates with the ATE’s test interface, and may come with additional interface cables
for connectivity or additional accessibility. The software is the test program to be executed by
the ATE to test a PCB’s functionalities. The documentation contains the necessary information
to enable an operator to carry out the testing.

Test Fixture and Panel
Depending on the ATE type, test fixtures can come as a bed-of-nails (BON) like those found in
in-circuit testers (ICT) that lay flat on a testbed, or panel-type modules that are lever-engaged
to a vertical receiver on tower type functional testers. For BON fixtures, the PCBs to be tested
are simply positioned on top of the nails that make contact with the solder pads. In the case
of panel modules, the PCBs are usually interfaced via their primary connectors using mating
connectors mounted on the panels. Whether it’s BON or mating connector, the ATE’s test
resources are routed from the testbed or receiver inside the test fixture or panel, sometimes
with additional signal conditioning and control circuits built into them as necessary.

Static test signals can include DC voltages and loads, while dynamic test signals can come from external
signal sources or even routing a PCB’s output back to its input for closed loop feedback effect.
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Interface Cables
Some PCBs may have more than one connector, though often the primary connector is what
interfaces the PCBs to the systems they operate in while peripheral connectors, if any, are
provided more as a means for debugging and diagnostic purposes by their PCB designers. In
view of this, it is not surprising that test engineers would take advantage of these additional
access points to improve their test program fault coverages.184 As far as electronic modules
are concerned, these do not normally have edge connectors but circular type connectors, in
which case interface cables would be indispensable.

Test Program
A test program can be as simple as a single executable file or as complex as containing many
overlaying modules with options for an end-to-end run or a single test selection. Preliminary
checks include signature ID tests,185 safe-to-turn on tests, warm-up time, etc. are performed
before the main tests are carried out. For ATEs which support graphical display, test programs
may use colors to denote the progress and status of each consecutive test. Provisions must
also be made to allow single-stepping, repeating a test, or interrupting the test program at a
convenient point, if required. Printing of test results is also mandatory for keeping records as
well as deciphering the measurement values for possible discrepancies.186

Test Program Set Document (TPSD)
This is the operational handbook for the test program set (TPS) that an operator would refer
to when testing a PCB or electronic module on a specific ATE. The content of this document
may include some or all of the following elements:
▪
▪
▪
▪
▪
▪
▪
▪
▪
▪

Preliminary Pages (title, authorization, distribution list, amendment record, etc.)
Introduction——what this TPS is about and the PCB it tests
Equipment and Tools Requirement
Safety (general and specific)
Test Instructions
PCB Documentation (schematic, layout, part list, etc.)
Test Strategy (description and flowcharts)
Interface Devices (fixture, adapters, cables, etc.)
Quality Control Document
Program Listings (source codes)

Of course, test programs can be written to guide operators in probing specific test points on PCBs when a
failure is encountered during main program run and branched out to a related diagnostic path.
184

185

To ensure the correct test fixture and PCB are hooked up for testing.

This is necessary if after replacing a suspected faulty part called out by the test program and the same test
still fails. No test program is perfect, especially if the tests involved are complex in nature.
186

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Automated Testing

Flying Probe Testers (FPT)
Any discussion on the ATE topic is not complete without mentioning flying probe testers, or
FPT in short. This breed of ATE uses a measurement method much like in-circuit testers but
utilizes a small number of fixed and movable probes for contacting instead of a bed-of-nail
(BON) fixture. The power supply nets are usually connected to the fixed probes while the rest
of the circuit points are contacted in sequence via probes that traverse in X-Y coordinates
direction.

A Flying Probe Tester

Testing in progress

What is the difference between an FPT and an ICT? A comparison of the pros and cons of
these two genre of testers will give you a better idea:
FPT
Pros:
▪
▪
▪
▪

ICT
Pros:

Lower setup cost (no fixture required
so no modification if PCB changes)
Faster development cycle
No need for test pads in high-density
PCB designs
Can test off-angled leads and pads of
various component packages

▪
▪
▪
▪

PCB Diagnostics

Onboard programming, in-circuit and
functional testing can be realized on
one test fixture.
Suitable for mass production testing
due to fast test speed
Can accommodate thousands of test
points at any one time
Comprehensive component testing is
achievable
185

Chapter 7

FPT
Cons:
▪
▪

ICT
Cons:

Needle probes have limited lifespan
and require periodic replacement
Unsuitable for mass production test
due to low test speed

▪
▪
▪
▪

If PCB layout changes new test fixture
must be built
Test pads must be provided to access
hidden BGA pins
High cost of test fixtures
Unable to access off-angled leads

It really boils down to one major difference——test fixture with static bed of nails (ICT) versus
fixtureless dynamic flying probes (FPT).
FPTs are usually employed in passive analogue measurements (resistance, capacitance and
inductance). This allows the operator to check for correct mounting and in some cases, the
functionality of most discrete components and integrated circuit orientations. The system can
be partially extended to include other test methods like boundary scan, optical inspection
(AOI), functional tests or thermal analysis to achieve a higher fault coverage.
FPTs are mainly used in the following three areas:
▪

Bare board check
Continuity and isolation tests are usually carried out by FPTs fitted with around 20
probes for high throughput and operating at voltages greater than 100V.

▪

Manufacturing defect test
These FPTs are equipped with at least four probes, often on both sides of the
assembled PCB and slightly angled on the component side for better access. For higher
coverage, it is often necessary to supplement with additional test methods such as AOI
and JTAG.

▪

PCB repair
Equipped with just a scan head and 1-2 probes, the bare minimalist approach is aimed
at detecting fault at a node level by means of impedance comparison against a golden
board, which is very similar to benchtop V-I testers.

In short, FPTs are well-suited for low-volume, high-mix manufacturing defect analysis (MDA). Incircuit testers (ICTs) do the job at the cost of the fixture and fabrication time, but with the
added advantage of more thorough testing and shorter test times.187
Let’s now look at two ATEs for a better appreciation of their workings.
187

A more in-depth look into FPT can be found in my book, PCB-RE: Tools & Techniques chapter 5.

186

LEARNING THE ROPES

Automated Testing

Example 1: The RADCOM WSTS
The RADCOM is a vital part of the US Navy’s repair bay assets.188 It is capable of servicing 75%
of the E2C’s avionics system with the remaining 25% covered by the primarily digital CAT-IIID.
And it’s no wonder when you look at the RADCOM’s configuration:

The RADCOM Weapon System Test Station (WSTS) is made up of four racks——a computer bay,
a display monitor bay, a digital/analog interface bay, and an RF interface bay. Together, these
provide all the necessary test resources for UUT testing. Communication between the host
computer, an HP1000, and test equipment is via the computer’s I/O backplane where the
interface cards, mainly IEEE-488 bus interface, are located. Each bay has its own blower unit
to keep the test equipment cool during operation.
A US aircraft carrier with the E2C surveillance planes is equipped with at least one of these test stations to
service the E2C’s weapon replaceable assemblies (WRAs) in its avionics workshop. Back in 1985, a RADCOM
tester cost the Singapore government almost $800K a piece (and we bought two of these!).
188

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Chapter 7

The test language adopted by the RADCOM is ATLAS,189 a high-level, descriptive test-oriented
language which is very much English-like. A typical ATLAS program is made up of two types of
statements: non-signal and signal-oriented, and is structured into two sections comprising a
preamble which precedes the procedural section. Non-signal statements deal with definition
of variables and subroutines, program flow control, operator input, test results output, and
computational expressions. Signal-oriented statements are either source or sensor types that
define actions, signal descriptions, and input/output connections.
The RADCOM has three distinct interfaces for interconnecting its test instruments to the UUT —
—a Multiple Matrix Switch (MMS),190 an RF Interface Unit (RFIU), and an Auxiliary Interface Panel
(AIP). Here is a photo shot of the test station in action:

RADCOM operator testing a PCB
Based on the UUT part number, the operator will refer to a test program instruction (TPI) and
retrieve the necessary accessories (fixture, interface cables, test program disc), then set up
the test connections, load the test program and run it.

Initially it stood for Abbreviated Test Language for Avionics Systems, but due to its widespread popularity the
‘Avionics’ term has since been changed to ‘All’.
189

This is the primary interface by which a test fixture (or Panel ID) is mated to. The other two interfaces are for
cable assemblies mating only.
190

188

LEARNING THE ROPES

Automated Testing

A typical test setup diagram in the TPI would look like this:
RADCOM

UUT

P2

P3

AIP

J1

P1
W1*

MMS
J1

P2

P1

J2

J4

P1

J2

P2

J3

P3

J4

P4
W1

J5

J3

P1

PANEL ID

P2
W2

RFIU

W1*

J13

P1

J2

P2

W3

P1

CONNECT TO P2 OR P3
OF UUT (DIAGNOSTIC)

COMMON POWER CABLE (P/N: 123SAV53640 -1)

Prior to the test setup, the Panel ID (fixture) would have to undergo a self-test to verify that it
is in working condition. So a RADCOM test program set will come with two executive programs
——a self-test (usually named PANEL.TP) and a UUT-test.191 A program startup procedure would
go by the following steps:
▪
▪
▪
▪
▪

Insert the test program disc into the drive set the RUN/STOP switch to RUN.
When DRIVE READY light is lit, press the spacebar on the keyboard to obtain the system
prompt, then type: 30> ON,FMGR
At the file manager prompt, invoke the ATLAS compiler: :ATLAS
To run self-test, at the ATLAS prompt, type: !RUN PANEL
To run UUT test, type: !RUN <filename>

Due to the limitation imposed by the RTE-IV B operating system, the filename can only have a maximum of
six letters with a two-letter extension.
191

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Chapter 7

A test operation will generally follow the flowchart below:192
START

NO

PERFORM
SELF-TEST

YES
RUN
PANEL TEST

SELF-TEST
PASSED?

NO

YES
RUN
UUT TEST

SELF-TEST
PASSED?

NO

FAULT
ISOLATION

YES

YES

TEST UUT
AGAIN?
NO

YES

TEST UUT
AGAIN?
NO

END

This is different from the test strategy flowcharts for both the panel self-test and the UUT test, which would
span several pages each and contain the main program flow as well as the diagnostic flow sections.
192

190

LEARNING THE ROPES

Automated Testing

This is how an ATLAS program looks like (partial listing):

C

C

000000 BEGIN,ATLAS PROGRAM $
* * * * * * * * * * * *
PREAMBLE SECTION * * * * * * * * * * * *
$
000100 DECLARE,INTEGER,STORE,'PO','U','L','USER','TGRP',
'EXIT','BRKP' $
10 DECLARE,DECIMAL,STORE,'DATA' $
20 DECLARE,MSGCHAR,LIST ,'UNITS'(4),5 CHAR $
30 DECLARE,MSGCHAR,STORE,'PRI',17 CHAR,'P1',17 CHAR,
'SEC',17 CHAR,'P2',17 CHAR,
'STATNO',6 CHAR $
:
000200 FILL,'UNITS'(1),C'OHMS',C'VDC',C'VRMS',C'DBM' $
10 FILL,'PRI',C' '$
20 FILL,'SEC',C' '$
30 FILL,'USER',0 $
:
000300 DEFINE,'HEADER',MESSAGE,
STATNO
MEASURED VALUE
UPPER LIMIT
LOWER LIMIT
UNIT
--------------------------------------------------------------------$
05 DEFINE,'FORMAT',MESSAGE,
(7)
(14)
(11)
(11)
(4) $
:

* * * * * * * * * * * * PROCEDURAL SECTION * * * * * * * * * * * *
$
E090000 ERASE,DISPLAY $
05 DISPLAY,ERASE $
10 DISPLAY,MESSAGE,
*** O P E R A T O R
A C T I O N ***
VERIFY THAT PANEL ID P/N: 9501-10000-001
IS PROPERLY ENGAGED TO MMS INTERFACE AND
UUT P/N: 659-5723-001 IS SETUP AS SHOWN
IN FIGURE 4.1 OF ASSOCIATED TPS DOCUMENT
M-P0085-TM001.
$

ENTER "GO" TO CONTINUE

:
99 PANEL ID SIGNATURE TESTS $
100000 MEASURE,(RESISTANCE),IMPEDANCE-DMM1,CNX HI XU53
05 DISCONNECT,IMPEDANCE-DMM1,CNX HI XU53 LO XU59 $
10 FILL,'STATNO',C'100000' $
15 PERFORM,'EVAL',51700,42300,1 $ R16=47K
20 GOTO,STEP 500000 IF NOGO $
:
500000 FILL,'PRI',C'R16' $
05 DISPLAY,MESSAGE,('SIGRES','PRI') $
10 PRINT ,MESSAGE,('SIGRES','PRI') $
15 FINISH $
:
999999 TERMINATE,ATLAS PROGRAM $
C

PCB Diagnostics

LO XU59

$

191

Chapter 7

And this is how a test printout with failure looks like:193
******************************************
***
***
***
TEST PROGRAM
***
***
FOR
***
***
ARC-182 POWER SUPPLY A5
***
***
***
******************************************
PRINT OPTION SELECTED
ALL

DATA

TEST GROUP 01 : SIGNATURE TESTS
STATNO
MEASURED VALUE
UPPER LIMIT
LOWER LIMIT
UNIT
-------------------------------------------------------------------100000
47470.2
51700.
42300.
OHMS
101000
67559.6
74800.
61200.
OHMS
102000
6.88
8
6
OHMS
103000
19.711
21.34
17.46
OHMS
104000
19.317
21.34
17.46
OHMS
105000
7.12799
8.03
6.57
OHMS
TEST GROUP 02 : SAFE-TO-TURN ON TESTS
STATNO
MEASURED VALUE
UPPER LIMIT
LOWER LIMIT
UNIT
-------------------------------------------------------------------110000
1.99223E+10
9999
1.00000E+06
OHMS
111000
1.99230E+10
9999
1.00000E+06
OHMS
112000
1.99254E+10
9999
1.00000E+06
OHMS
113000
198205.
220000.
180000.
OHMS
114000
2.55902E+06
9999
10
OHMS
115000
821.301
9999
500
OHMS
116000
3698.46
9999
500
OHMS
117000
934.954
9999
500
OHMS
TEST GROUP 03 : POWER ON TESTS
STATNO
MEASURED VALUE
UPPER LIMIT
LOWER LIMIT
UNIT
-------------------------------------------------------------------120000
84.0503
85
77
VDC
120500
.970182
.5
-9999
VRMS
END OF PROGRAM
UUT FAULTY
ARC-182 POWER SUPPLY A5
PART NMBR : 659-5723-001
TEST FAILED AT STATNO : 120500
FAULTS IN

193

A1C5,A1C6
A2C4

This failure detected high ripples at the +450Vdc output which is attributed to the list of filter capacitors.

192

LEARNING THE ROPES

Automated Testing

Example 2: The Factron S730 In-Circuit Tester
The Factron S700 series of in-circuit testers is based on a high-speed, purpose-designed host
computer which controls the various instrumentation, test channels and switching facilities
for a unified test approach.

In-circuit test channels are equipped with appropriate logic drivers/sensors and analog/digital
changeover switches to provide hybrid capabilities that allow individual test probes to be used
for both analog and digital testing, thereby reducing the number of channels needed.
A cross-section of how the bed-of-nails and test
fixture internal wiring looks like is shown on the
right figure.
For digital testing, a choice of programmable
logic driver/sensor types and timing control are
available, operating at speeds up to 10MHz and
capable of testing PCBs using system-generated
or user-supplied pattern sequences.
For analog testing, total flexibility of channel
selection is provided by the analog measurement
system. Under control of the test program, the
test resources can be routed via the hybrid incircuit test channels (ITCs) or to the switched
signal channels (SSCs).

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Chapter 7

Test Head
All system test facilities are brought up to a 5236-pin test head: the desired facilities are then
connected to the UUT via a suitably wired bed-of-nails (BON) test fixture. The test head is
subdivided into fields of related pin functions, to a standard layout:
112

97 96

65

64

33 32

1

SPARE

A

SSF 2

PTF 2

ITF 1

PTF 1

Q

SPARE

AA

SSF 3

ITF 2

GFF

SSF 1

QQ
112

97 96

In-Circuit Test Field

65

64

33 32

1

(ITF)

In–circuit test channels are available in modules of 128 channels (16 real channels each
multiplexed by 8) of the following types: Standard (slow) and Enhanced (fast),
Performance Test Field (PTF)
This field is meant for functional testing and is empty for in-circuit configuration.
Signal Switching Field

(SSF)

Signal switching fields cater for the analog switching channels. Each channel can be used
as a force and sense pair or as two entirely independent lines designated F and S. These
channels are provided by analog switching modules equipped with dry–reed relays.
General Facility Fields

(GFF)

There are four divisions in the general facility fields:
▪

GFF1 Up to ten programmable UUT power supplies of various types

▪

GFF2 Test support facilities:
–
–
–
–
–

194

Trigger Ports for trigger input/output, measurement and routing purposes
Analog in–circuit highways P, Q and R
FLO–TRACER clamp voltage references, user–supplied
Analog ground pins
Digital Volt–Ohm Meter (DVOM) access points
LEARNING THE ROPES

Automated Testing

▪

GFF3 Further switching facilities provided on
– Analog Routing Modules (ARMs)
– Free Relay Modules (FRMs)

▪

GFF4 IEEE-488 bus–driven instruments
64

33
AA

GFF

QQ
GFF4

GFF3

GFF2

GFF1

Test Software
The S700 system has a rich set of test-related software to support the creation of an in-circuit
test program integrated within its test program development process, which comprises four
main elements:
▪

CAPITAL
CAPITAL is an integrated package of software for carrying out the entire range of
operations necessary to generate and validate in-circuit test programs, and the test
fixtures with which they are used.

▪

MEDIATOR
MEDIATOR is the powerful application-oriented high-level test language which is used in the
S700 system.

▪

Test Program Preparation
In order for CAPITAL to generate the test program and fixture wire list, the following files
are required: board description (.bd), component source (.cs), target system (.ts), and
image (.im).

▪

Component Libraries
A collection of ready-compiled component descriptions available within the system.

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195

Chapter 7

Test Philosophy
The automatic test program generator CAPITAL will, on receipt of a user-supplied board
description for the PCB, access the relevant library-based component descriptions and incircuit test routines (ICTRs) to generate:
▪
▪
▪

a software description of the PCB
a comprehensive test program, and
a test fixture wire list

for use on the system, or on a different S700 ATE, based on the appropriate target system
configuration details given.
When running the test program from start to end, the system will first discharge the PCB of
any residual electrical charges, and then carry out a pin check sequence to ensure that the
test points are all making contact with the board under test. It then performs a test sequence
as follows:
▪

Unpowered PCB tests:
1. Jumpers, shorts and continuity checks.
2. Discrete device measurement of component values against tolerances specified in
the components list. Passive components such as resistors,194 capacitors,195 and
inductors are checked. Guarding techniques are employed, if necessary, to nullify
the effects of parallel networks.
3. Semiconductor orientation checks on forward and reverse junction bias for diodes,
transistors, and MOSFETs.

▪

Powered PCB tests:
Functional testing of digital ICs common logic families are performed at pattern speeds
appropriate to the component type and the system options supplied. Automatic
guarding techniques, applied by CAPITAL, are also employed to nullify the effects of
onboard signals that would otherwise interfere with the tests. Back-driving is used to
overcome wrong states on input pins, but back-driven components are protected by
means of an automatic time-out facility which restricts the duration of buffer drive
current.196

Including resistor networks and potentiometers. For resistor networks, the number of tests correspond to the
number of elements in a package. For potentiometers, the set values are checked.
194

If the amount of capacitors present is huge or there are many large value capacitors, interim discharge may
be necessary between each group of tests.
195

Guarding is a common technique for in-circuit testing to minimize the influence of interconnected networks
and unwanted signal activities from adjacent components.
196

196

LEARNING THE ROPES

Automated Testing

A simple in-circuit test program generated by CAPITAL and debugged by an operator is listed
as follows:
INCLUDE
INCLUDE
INCLUDE
INCLUDE
INCLUDE
INCLUDE
INCLUDE
INCLUDE
INCLUDE
INCLUDE

SUBROUTINE
SUBROUTINE
SUBROUTINE
SUBROUTINE
SUBROUTINE
SUBROUTINE
SUBROUTINE
SUBROUTINE
SUBROUTINE
SUBROUTINE

7400_R;
74245_R;
74373_R;
IDT71256_R;
DISCHARGE_CAPACITORS_A;
NAIL_CONTACT;
ORIENTATION_TEST;
SHORTS_TEST_D;
CONTINUITY_TEST_D;
BUS_TEST;

SUBROUTINE CAPACITOR_DISCHARGE;
IC_TEST DCHARGE BETWEEN C1,C2,C3,C4,C5,C6,C7,C8,C9,
C10,C11,C12,C13,C14
USING DISCHARGE_CAPACITORS_A;
END SUBROUTINE;
VACUUM ON;
WAIT FOR 500 MSECS;
FAIL SUMMARY;
\* ::::::::::::::::::: CONTACT TEST ::::::::::::::::::: *\
BLOCK 5 CAPACITOR_DISCHARGE; END BLOCK;
BLOCK 10
SRS(DIC_A)[L 2.000V,H 2.000V];
SRD(DIC_A)[L 0.000V,H 4.000V];
IC_TEST PINCHK BETWEEN !5/!122,!124/!125,!127/!140,
!142/!143,!145,!148/!161,!171/!183,!185/!186,!188
LATCH_HI !1/!4 USING NAIL_CONTACT;
END BLOCK;
\* ::::::::::::::::::: SHORTS TEST :::::::::::::::::::: *\
BLOCK 15
IC_TEST SHORTS BETWEEN !1/!27,!29/!33,!35/!104,
!106/!121,!123/!142,!145,!148/!150,!152,!157/!158,
!161,!171/!180 USING SHORTS_TEST_D;
END BLOCK;
\* ::::::::::: REVERSE LEAKAGE CURRENT TEST ::::::::::: *\
BLOCK 20
IC_TEST CR1 BETWEEN CR1.AN,CR1.CA
USING MIVDC VALUE: 100.0U,SOURCE:-1.000,PTOL: 0.0;
END BLOCK;
BLOCK 25
IC_TEST CR2 BETWEEN CR2.AN,CR2.CA GUARD VCC,GND
USING MIVDC VALUE: 100.0U,SOURCE:-1.000,PTOL: 0.0;
END BLOCK;
BLOCK 30
IC_TEST Q1 BETWEEN Q1.CO,Q1.EM,Q1.BA
USING MIVDC VALUE: 100.0U,PTOL: 0.0,SOURCE:-1.000;
END BLOCK;

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197

Chapter 7
BLOCK 35
IC_TEST Q1 BETWEEN
USING MIVDC VALUE:
END BLOCK;
BLOCK 40
IC_TEST Q1 BETWEEN
USING MIVDC VALUE:
END BLOCK;

Q1.BA,Q1.EM GUARD R2.AA
100.0U,PTOL: 0.0,SOURCE: 1.000;
Q1.CO,Q1.BA
100.0U,PTOL: 0.0,SOURCE:-1.000;

\* :::::::::::::::::: RESISTORS TEST :::::::::::::::::: *\
BLOCK 45
IC_TEST R4 BETWEEN R4.BB,R4.AA
USING MRV VALUE: 14.00 ,PTOL: 5.000%,NTOL:
END BLOCK;
BLOCK 50
IC_TEST R2 BETWEEN R2.BB,R2.AA
USING MRV VALUE: 100.0 ,PTOL: 5.000%,NTOL:
END BLOCK;
BLOCK 55
IC_TEST R1 BETWEEN R1.AA,R1.BB
USING MRV VALUE: 200.0 ,PTOL: 5.000%,NTOL:
END BLOCK;
BLOCK 60
IC_TEST R3 BETWEEN R3.BB,R3.AA
USING MRV VALUE: 200.0 ,PTOL: 5.000%,NTOL:
END BLOCK;

5.000%;

5.000%;

5.000%;

5.000%;

\* ::::::::::::::::: CAPACITORS TEST ::::::::::::::::: *\
BLOCK 65
IC_TEST C14 BETWEEN C14.AA,C14.BB
USING MC VALUE: 100.0N,PTOL: 10.00%,NTOL: 10.00%;
END BLOCK;
BLOCK 70
IC_TEST C9,C10,C11,C12,C13 BETWEEN C9.BB,C9.AA
GUARD R2.BB
USING MC VALUE: 500.0N,PTOL: 12.00%,NTOL: 10.00%;
END BLOCK;
BLOCK 75
IC_TEST C6,C7,C8 BETWEEN C6.BB,C6.AA
USING MC VALUE: 3.000U,PTOL: 10.00%,NTOL: 10.00%;
END BLOCK;
BLOCK 80
IC_TEST C3,C4,C5 BETWEEN C3.BB,C3.AA GUARD R4.AA
USING MC VALUE: 6.60U,PTOL: 10.00%,NTOL: 10.00%;
END BLOCK;
BLOCK 85
IC_TEST C1,C2 BETWEEN C1.BB,C1.AA GUARD R1.AA
USING MC VALUE: 15.90U,PTOL: 10.00%,NTOL: 10.00%;
END BLOCK;
\* :::::::::::::: FORWARD VOLTAGE TEST ::::::::::::::: *\
BLOCK 90
IC_TEST CR1 BETWEEN CR1.CA,CR1.AN
USING MVIDC VALUE: 0.525,SOURCE:-10.0MI,TOL: 30.00%;
END BLOCK;

198

LEARNING THE ROPES

Automated Testing
BLOCK 95
IC_TEST CR2 BETWEEN CR2.CA,CR2.AN
USING MVIDC VALUE: 0.750,SOURCE:-10.0MI,TOL: 30.00%;
END BLOCK;
BLOCK 100
IC_TEST Q1 BETWEEN Q1.SO,Q1.DR GUARD Q8.BA,Q6.DR
USING MVIDC VALUE: 0.750,SOURCE: 50.0MI,TOL: 30.00%,
DELAY: 50.00MI;
END BLOCK;
BLOCK 105
SRD(DIC_B)[L0.0V,H0.5V];
IC_TEST ORIENT_A BETWEEN !5/!33,!35/!109,!114/!123,
!127/!135,!145,!150,!152/!161,!171/!180
LATCH_HI !2/!4 LATCH_LO !1 USING ORIENTATION_TEST;
END BLOCK;
BLOCK 110
SU(1)[ 5.000V @ 2.000A ON];
IF PROBING THEN PROBE ON; END IF;
BLOCK 115
SRS(DIC_A)[L 0.800V,H 2.000V];
SRD(DIC_A)[L 0.000V,H 4.000V];
KNA 1025/1088,1089/1152,1153/1216,1217/1280,
1281/1344,1345/1408,1409/1472;
\* :::::::::::::::::::::: BUS TEST ::::::::::::::::::::: *\
BLOCK 120
IC_TEST 'D0' LATCH_HI U4.22
USING BUS_TEST STATE:"Z",CURRENT:5.0MI,WIDTH:1;
END BLOCK;
BLOCK 125
IC_TEST 'D1' LATCH_HI U4.22
USING BUS_TEST STATE:"Z",CURRENT:5.0MI,WIDTH:1;
END BLOCK;
BLOCK 130
IC_TEST 'D2' LATCH_HI U4.22
USING BUS_TEST STATE:"Z",CURRENT:5.0MI,WIDTH:1;
END BLOCK;
BLOCK 135
IC_TEST 'D3' LATCH_HI U4.22
USING BUS_TEST STATE:"Z",CURRENT:5.0MI,WIDTH:1;
END BLOCK;
BLOCK 140
IC_TEST 'D4' LATCH_HI U4.22
USING BUS_TEST STATE:"Z",CURRENT:5.0MI,WIDTH:1;
END BLOCK;
BLOCK 145
IC_TEST 'D5' LATCH_HI U4.22
USING BUS_TEST STATE:"Z",CURRENT:5.0MI,WIDTH:1;
END BLOCK;
BLOCK 150
IC_TEST 'D6' LATCH_HI U4.22
USING BUS_TEST STATE:"Z",CURRENT:5.0MI,WIDTH:1;
END BLOCK;
BLOCK 155
IC_TEST 'D7' LATCH_HI U4.22
USING BUS_TEST STATE:"Z",CURRENT:5.0MI,WIDTH:1;
END BLOCK;

PCB Diagnostics

199

Chapter 7
\* :::::::::: DIGITAL INTEGRATED CIRCUIT TEST :::::::::: *\
BLOCK 160
IC_TEST
END BLOCK;
BLOCK 165
IC_TEST
END BLOCK;
BLOCK 170
IC_TEST
END BLOCK;
BLOCK 175
IC_TEST
END BLOCK;
END BLOCK;
END BLOCK;

U1 USING 7400_R;
U2 USING 74245_R;
U3 USING 74373_R;
U4 USING IDT71256_R;

TERMINATE;

The program statements are quite straightforward and logical to interpret so I will not provide
any explanation. However, I want to draw attention to two in-circuit test routines, one for a
7400 and another, an IDT71256 chip:

ICTR 1: Testing a 7400 Logic Chip
In-circuit test routines (or ICTRs) form the heart of an in-circuit test program. Besides defining
to the ATE which in-circuit test channels are used as input (drive), output (sense) or bus
(bidirectional), an ICTR also tells the ATE what test vectors (patterns) to use and the type of
timing parameters to employ to exercise the component under test.197
Consider a typical 7400, a quad 2-input NAND gates IC:
VCC 4A 4B 4Y 3A 3B 3Y
14 13 12 11 10 9 8

1 2 3 4 5 6 7
1A 1B 1Y 2A 2B 2Y GND

A
0
0
1
1

B
0
1
0
1

Y
1
1
1
0

Truth Table

Pinout and Truth-table
Unlike the unpowered tests which utilizes analog test routines to perform checks on discrete components,
all ICTRs require power to verify the functionality of the integrated circuit under test.
197

200

LEARNING THE ROPES

Automated Testing

An ICTR implementation for the 7400 is listed below:198
SUBROUTINE 7400_P;
DECLARE PRIORITY 49.0;
SELECT DIC 1/6,8/13;
TIME_UNIT 50 N SECS;
EVENT_FRAME EF1 DURATION 20 EP1 16 TO 18;
SIGNAL_TIMING * TRANSFER ON START NO PULSE
WINDOW MEASURE ON EP1;
FAST_SUBROUTINE NAND_1;
IN EF1; ()
(I1,2,O3,M3,)
(L1,L2,H3, X)
(L1,H2,H3, X)
(H1,L2,H3, X)
(H1,H2,L3, X)
(O3,N3,)
END FAST_SUBROUTINE;

Line 1:
Line 2:
Line 3:
Line 4:
Line 4:
Line 5:

Define inputs 1,2 and output 3. Monitor 3.
Set 1=L, 2=L expect 3=H Measure
Set 1=L, 2=H expect 3=H Measure
Set 1=H, 2=L expect 3=H Measure
Set 1=H, 2=H expect 3=L. Measure
Neglect output 3

FAST_SUBROUTINE NAND_2;
IN EF1; ()
(I4,5,O6,M6,)
(L4,L5,H6, X)
(L4,H5,H6, X)
(H4,L5,H6, X)
(H4,H5,L6, X)
(O6,N6,)
END FAST_SUBROUTINE;
FAST_SUBROUTINE NAND_3;
IN EF1; ()
(I10,9,O8,M8,)
(L10,L9,H8, X)
(L10,H9,H8, X)
(H10,L9,H8, X)
(H10,H9,L8, X)
(O8,N8,)
END FAST_SUBROUTINE;
FAST_SUBROUTINE NAND_4;
IN EF1; ()
(I13,12,O11,M11,)
(L13,L12,H11, X)
(L13,H12,H11, X)
(H13,L12,H11, X)
(H13,H12,L11, X)
(O11,N11,)
END FAST_SUBROUTINE;
DIC_BLOCK 100 RUN NAND_1; END DIC_BLOCK;
DIC_BLOCK 200 RUN NAND_2; END DIC_BLOCK;
DIC_BLOCK 300 RUN NAND_3; END DIC_BLOCK;
Note that there can be more than one way of writing the test routine for a particular IC, depending on how
you want to test it. In this instance, the test routine uses the Accelerator function of the Series 700 Tester (as
denoted by the signal timing definitions and fast subroutine syntax).
198

PCB Diagnostics

201

Chapter 7
DIC_BLOCK 400 RUN NAND_4; END DIC_BLOCK;
END SUBROUTINE;

ICTR 2: Testing an IDT71256 Static RAM
A more complex but common ICTR example involves testing a memory chip. The method is
known as a five pass RAM test which includes:
▪
▪
▪
▪
▪

Walking ones and zeros on the address lines
Output enable and tri-state tests
Address decoding logic test (random data)
Address location tests (write/read 55H)
Address location tests (write/read AAH)

A RAM contains an address decoding logic (ADL) to decode the binary address into a physical
location in the memory array for storage or retrieval of data. To ensure that the ADL decodes
correctly, every location of the memory array should contain a unique signature (or data). But
given the limited size of the data (1, 4, 8 or 16 bits), the number of possible combinations (2,
16, 256, or 65536) would seem insufficient to provide a unique signature per location. This is
why different algorithms were developed and exhaustive testing of memory ICs require a
substantial amount of time.
SUBROUTINE IDT71256_P;
DECLARE PRIORITY 75.0;
DECLARE SIGNAL 'A14' :
'A12' :
'A7' :
'A6' :
'A5' :
'A4' :
'A3' :
'A2' :
'A1' :
'A0' :
'D0' :
'D1' :
'D2' :
'D3' :
'D4' :
'D5' :
'D6' :
'D7' :
'_CS' :
'A10' :
'_OE' :
'A11' :
'A9' :
'A8' :
'A13' :
'_WE' :

202

( 1),
( 2),
( 3),
( 4),
( 5),
( 6),
( 7),
( 8),
( 9),
(10),
(11),
(12),
(13),
(15),
(16),
(17),
(18),
(19),
(20),
(21),
(22),
(23),
(24),
(25),
(26),
(27),

A14
A12
A7
A6
A5
A4
A3
A2
A1
A0
D0
D1
D2
GND

1
2
3
4
5
6
7
8
9
10
11
12
13
14

28
27
26
25
24
23
22
21
20
19
18
17
16
15

VCC
_WE
A13
A8
A9
A11
_OE
A10
_CS
D7
D6
D5
D4
D3

IDT71256 Pinout

LEARNING THE ROPES

Automated Testing
BUS

'ADDR' : ('A14','A13', 'A12','A11', 'A10',
'A9','A8', 'A7','A6', 'A5','A4',
'A3','A2','A1','A0'),
'DATA' : ('D7','D6', 'D5','D4', 'D3','D2',
'D1','D0');
SELECT DIC 1/13,15/27;
TIME_UNIT 50 N SECS;
EVENT_FRAME EF1 DURATION 20 EP1 16 TO 18;
SIGNAL_TIMING * TRANSFER ON START NO PULSE
WINDOW MEASURE ON EP1;
FAST_SUBROUTINE WALKING_1;
IN EF1; ()
(IL'_CS',IH'_OE',IH'_WE',)
(IL'ADDR',)
(I'DATA',PATTERN #55 ON 'DATA';)
(L'_WE',)(H'_WE',)
(PATTERN #AA ON 'DATA';)
(PATTERN &100000000000000 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &010000000000000 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &001000000000000 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &000100000000000 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &000010000000000 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &000001000000000 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &000000100000000 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &000000010000000 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &000000001000000 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &000000000100000 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &000000000010000 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &000000000001000 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &000000000000100 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &000000000000010 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &000000000000001 ON 'ADDR';)(L'_WE',)(H'_WE',)
(O'DATA',N'DATA',)(IL'ADDR',)
(IL'_OE',)
(O'DATA',M'DATA',)(PATTERN #55 ON 'DATA'; X)
(IH'_OE',)
(I'DATA',PATTERN #AA ON 'DATA';)
(L'_WE',)(H'_WE',)
(O'DATA',N'DATA',)
(IL'_OE',)
(O'DATA',M'DATA',)(PATTERN #AA ON 'DATA'; X)
(O'DATA',N'DATA',)
()
END FAST_SUBROUTINE;
FAST_SUBROUTINE WALKING_0;
IN EF1; ()
(IL'_CS',IH'_OE',IH'_WE',)
(IH'ADDR',)
(I'DATA',PATTERN #AA ON 'DATA';)
(L'_WE',)(H'_WE',)
(PATTERN #55 ON 'DATA';)
(PATTERN &011111111111111 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &101111111111111 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &110111111111111 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &111011111111111 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &111101111111111 ON 'ADDR';)(L'_WE',)(H'_WE',)

PCB Diagnostics

203

Chapter 7
(PATTERN &111110111111111 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &111111011111111 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &111111101111111 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &111111110111111 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &111111111011111 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &111111111101111 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &111111111110111 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &111111111111011 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &111111111111101 ON 'ADDR';)(L'_WE',)(H'_WE',)
(PATTERN &111111111111110 ON 'ADDR';)(L'_WE',)(H'_WE',)
(O'DATA',N'DATA',)(IH'ADDR',)
(IL'_OE',)
(O'DATA',M'DATA',)(PATTERN #AA ON 'DATA'; X)
(IL'_OE',)
(I'DATA',PATTERN #55 ON 'DATA';)
(L'_WE',)(H'_WE',)
(O'DATA',N'DATA',)
(IL'_OE',)
(O'DATA',M'DATA',)(PATTERN #55 ON 'DATA'; X)
(O'DATA',N'DATA',)
()
END FAST_SUBROUTINE;
FAST_SUBROUTINE OUTPUT_TS;
IN EF1; ()
(IL'_CS',IH'_OE','_WE',)(IL'ADDR',)
(IL'DATA',)(L'_WE',)(H'_WE',)
(IL'_OE',)()()(OL'DATA',M'DATA', X)
(IH'_OE',)()()(OH'DATA',M'DATA', X)
(IH'_CS',)
(IL'_OE',)()()(OH'DATA',M'DATA', X)
(IH'_OE',)()()(OH'DATA',M'DATA', X)
()
END FAST_SUBROUTINE;
FAST_SUBROUTINE RAMTEST_1;
IN EF1; ()
(IL'_CS',IH'_OE',IH'_WE',)
(I'ADDR','DATA',) ()()()()
INITIALISE RANDOM;
()()()()()()()()()
COUNT LENGTH 32768 DO
(PATTERN RANDOM ON 'DATA';)
(PATTERN COUNT ON 'ADDR';)(L'_WE',)(H'_WE',)
END COUNT; ()
(L'_OE',)
(O'DATA',M'DATA',)()()()()
INITIALISE RANDOM;
()()()()()()()()()
COUNT LENGTH 32768 DO
(PATTERN RANDOM ON 'DATA';)
(PATTERN COUNT ON 'ADDR'; X)
END COUNT; ()
(O'DATA',N'DATA',)
()
END FAST_SUBROUTINE;

204

LEARNING THE ROPES

Automated Testing
FAST_SUBROUTINE RAMTEST_2;
IN EF1; ()
(IL'_CS',IH'_OE',IH'_WE',)
(I'ADDR','DATA',)
(PATTERN #55 ON 'DATA';)
()()()()()()()()()
COUNT LENGTH 32768 DO
(PATTERN COUNT FROM #0000 ON 'ADDR';)
(L'_WE',)(H'_WE',)
END COUNT; ()
(L'_OE',)
(O'DATA',M'DATA',)
()()()()()()()()()
COUNT LENGTH 32768 DO
(PATTERN COUNT FROM #0000 ON 'ADDR'; X)
END COUNT; ()
()
END FAST_SUBROUTINE;
FAST_SUBROUTINE RAMTEST_3;
IN EF1; ()
(IL'_CS',IH'_OE',IH'_WE',)
(I'ADDR','DATA',)
(PATTERN #AA ON 'DATA';)
()()()()()()()()()
COUNT LENGTH 32768 DO
(PATTERN COUNT FROM #0000 ON 'ADDR';)
(L'_WE',)(H'_WE',)
END COUNT; ()
(L'_OE',)
(O'DATA',M'DATA',)
()()()()()()()()()
COUNT LENGTH 32768 DO
(PATTERN COUNT FROM #0000 ON 'ADDR'; X)
END COUNT; ()
(O'DATA',N'DATA',)
()
END FAST_SUBROUTINE;
DIC_BLOCK 100 RUN WALKING_1; END DIC_BLOCK;
DIC_BLOCK 200 RUN WALKING_0; END DIC_BLOCK;
KNC 'DATA';

Apply pull-ups on DATA for tristate test.

DIC_BLOCK 300 RUN OUTPUT_EN; END DIC_BLOCK;
KND 'DATA';

Remove pull-ups on DATA bus.

DIC_BLOCK 400 RUN RAMTEST_1; END DIC_BLOCK;
DIC_BLOCK 500 RUN RAMTEST_2; END DIC_BLOCK;
DIC_BLOCK 600 RUN RAMTEST_3; END DIC_BLOCK;
END SUBROUTINE;

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Chapter 7

Summary
This chapter gives a general overview of automated test equipment and uses two platform
examples to help readers grasp the basic concept involved in operating and developing test
programs on these machines. However, to be proficient in any kind of ATE, a former training
(theory and practical) conducted by an ATE vendor certified trainer is required.

206

LEARNING THE ROPES

It is said that there are three major schools of TCM in ancient China——Huangdi, Bianque and
Baishi. Huangdi excels in all kinds of external surgical procedures;199 Baishi is famous for its
dissection skills and was responsible for the discovery of the body’s meridians and acupoints.
Bianque is the most mysterious of the three, an accomplished master of this art of diagnosis
is deemed to possess penetrating vision on the human body condition, much like a CT scan
machine. Sounds incredulous?
The Annuls of Ancient Chinese History (Shiji) recorded a person by
the name Qin YueRen who lived in the Seven Warring States period
and was well-versed in the art of the Bianque diagnosis. Once, he
passed by the state of Qi and was given a special audience before
the warlord. He noticed that the warlord didn’t look well and
remarked, “Your skin colors are not well. Better get treated before
it gets worse.” The warlord did not take his words seriously and
replied, “I feel alright, it’s no big deal.” After a couple of days, the
physician went to see the warlord out of concern, and after some
observations said to him, “Your ailment has reached the blood, do
not delay treatment please.” The warlord wasn’t very happy and
simply ignore his advice. A few days later, the physician came over
and found that the warlord’s condition has worsen, so he urged,
“Your gastrointestinal functions are in bad shape, you may lose your
life if you refuse treatment.” The warlord became very upset and dismissed the physician in a
fit of anger. On his final visit before he left the Qi state, he took a look at the warlord and this
time he departed without a word. The warlord was surprised by the physician’s behavior and
sent someone to inquire. The physician sighed and said, “When the sickness is at the skin
color level, herbal medicine can quickly treat it. When the sickness enters the bloodstream,
acupuncture can still treat it. When the sickness reaches the gastrointestinal region, tincture
concoction can possibly treat it. But now that the sickness has reached the marrow, there is
little I can do so there’s no point giving any more advice.” True enough, the warlord became
very ill and when he tried to send for the physician, he was nowhere to be found. Within a few
days, the warlord was dead.
This anecdote sets the tone for our final treatment on PCB repair——the use of thermal imaging
to diagnose failures.
199

There are two branches of study in the Huangdi school, namely WaiJing (external) and NeiJing (internal).

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Infrared Vision
Heat is a form of infrared radiation (IR). It can be felt but is invisible to the human eye. IR is
emitted by all objects at temperatures above absolute zero; the amount of thermal radiation
increases with temperature.
shorter

10-12
Gamma rays

10 6

longer

10-10

10-8

X-rays

10 4

10-6
UV

10 2

Infrared

1

higher

10-4

10-2

Wavelength (meters)

10-2
Microwaves

10-4

Radiowaves

10-6

lower

Visible

10 2

1

10-8
Energy (electronvolts)

The Electromagnetic Spectrum
The only way to see thermal energy is with the help of an IR camera which depicts thermal
variations across a target area using a visual image, a process known as thermal imaging. An
IR camera’s construction is similar to a digital camera, except that the lens materials used
are of different wavelengths compared to optics used in visible light cameras.200

FOV

Lens

Filter

Detector

Amplifier and
Signal Processing

Thermal (IR) Camera Concept
The field-of-vision (FOV) is determined by the distance between the object and the lens. IR
energy emitted by the object is captured by the lens, filtered through a spectral band and
converted by a detector into an electrical signal, which is then amplified and processed into
thermographic data for display in real time.
Materials that are transparent to IR are often opaque to visible light. Nonetheless, IR has the same properties
as visible light in terms of reflection, refraction and transmission, so the optics for thermal cameras are designed
in a fashion similar to those found in visual cameras.
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IR Detection and Calibration
Thermography is a kind of imaging technique employed with an IR camera that is calibrated
to display temperature values across an object or area. In this respect, thermography allows
non-contact measurements of an object’s temperature. An IR camera's construction is similar
to a digital video camera. However, instead of a charge coupled device which a digital camera
uses, the IR camera's detector is a focal plane array (FPA) of micrometer size pixels made up
of various materials sensitive to IR wavelengths.201
Most IR detectors have response curves that are narrower than
the full IR range. Therefore, a detector must be selected with
the appropriate IR range response that fits a user’s application.
Besides wavelength response, other important characteristics
include sensitivity, the ease of making the focal plane array
with micrometer size pixels, and the amount of cooling needed,
if any. Commercial FPA detectors come in two forms: thermal
and quantum. Majority of IR cameras use the microbolometer
(thermal) type detector which does not require cooling. This
allows compact camera designs that are relatively low in cost.
The flip side is lower sensitivity, broader response curve, and
slower response time.
IR cameras are calibrated at the factory. Some IR cameras
include a built-in blackbody to allow quick calibration check,202
and these checks should be done periodically to ascertain the
validity of measurements. Whether the software is on-board
the camera or runs from an external PC, the calibration process
is oriented toward thermographic imaging and temperature
measurements. This software provides a post-measurement
capability to further modify atmospheric conditions, spectral
responsivity, atmospheric transmission changes, internal and
external filters, and other important criteria as needed.
In addition, an IR camera’s software and firmware provide other user inputs that refine the
accuracy of temperature measurements. One of the most important functions is nonuniformity correction (NUC) of the detector FPA. This type of correction is needed due to the
fact that each individual detector in the camera’s FPA has a slightly different gain and zero
offset. To create a useful thermographic image, the different gains and offsets must be
corrected to a normalized value.
IR covers a portion of the electromagnetic spectrum from 900 to 14,000 nanometers (0.9–14µm) in terms
of wavelength, which is longer than visible light.
201

A perfect blackbody is a reference object which has no reflected or transmitted radiation. In simple terms,
thermography depicts how hot an object is, whereas radiometry determines how much energy it is giving off.
202

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Types of IR Cameras
There is much confusion on the type of IR cameras that can be used for failure detection and
analysis of PCBs. Generally, there are two main categories in the related industry:
▪
▪

Thermal imaging camera
Infrared thermometer (thermal imager gun)

Both are used for non-contact temperature measurement in a wide variety of applications and
work on the same principle——detecting infrared radiation and translating it into a visual image
or numerical reading. A thermal camera, however, has several advantages compared to an IR
thermometer.

Infrared handheld thermal imager gun
An IR thermometer, also known as a spot pyrometer or a temperature gun, gives you a single
reading——the temperature measurement of a single spot on your target. Because it works
according to the same physical principle as a thermal camera, an IR thermometer is likened
to a thermal camera with only one pixel. Though useful for quick inspections, it is not suitable
for PCB failure detection and analysis.
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Thermal Imaging

A thermal imaging camera, on the other hand, provides temperature readings for every pixel
of the entire thermal image and allows visualization of an entire PCB in thermographic display.
Moreover, a thermal imaging camera has a much larger distance-to-size (D:S) ratio compared
to an IR thermometer.203
Power Connector
Fixing Screw
RJ45 Connector

Host Enclosure

Infrared Camera

Visible Light Camera

Bracket
Heat Insulation Pad

Base

Thermal imaging camera
Unlike the limited thermal display screen of an IR thermometer, the thermal imaging camera
has a better detector resolution which can be further extrapolated by the application software
that compliments the system.204 Thermal imaging cameras sold by renown suppliers like FLIR
and InfraTec don’t come cheap. Thankfully, there are affordable alternatives such as the
QianLi LC-IR02 thermal imaging camera (figure above) which uses a double interlaced camera
spectral positioning technology and comes with a feature-rich graphic visualization software
(see overleaf).

An IR thermometer, for instance, might be able to measure a 1 cm target at a distance of between 10-50 cm.
Most thermal imaging cameras can accurately measure the temperature of a target of this size (1 cm) from
several meters away.
203

Most handheld IR thermometers have an LCD display screen of 32 x 32 resolution, whereas thermal imaging
camera with an IR detector can exhibit much higher pixel resolutions and after processing by the application
software, can achieve HD display resolutions.
204

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Temperature display area

Image adjustment (fixed) menu

Image adjustment common
function menu

SuperCam Real-time image acquisition
Technical Specifications
Infrared Detector Parameters:
Detector type: uncooled vanadium oxide
(VOX) detector
Detector resolution: 160 × 120
Wavelength range: 7.5–13.5μm
Field of view: 61.8 ° × 49.5 °
Visible Light Camera Parameter:
Resolution: 1920 × 1080
Interface and Display Function:
Display mode: thermal visualization
Video output: RJ45205
Photo storage format: JPEG

Physical Features:
Base size: 18 x 18cm
The height of the set: 17cm
Working area: 16.5 x 12.5mm (mat area)
Weight: 2.8 kg (with handle)
Power Supply:
Operating voltage: 12V DC
Power consumption: 2.2W (max 3W)
Temperature Measurement Function:
Temperature range: –10°C to 150°C
Accuracy: ± 5°C or ± 5%

Communication between the host computer and the camera takes place via a network cable with an RJ45
connector. It is possible to display the camera image in a local LAN on any number of computers at the same
time. This means that the setup can also be used for classroom teaching purposes.
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Real thermal imaging technology and dual-light fusion algorithm can display the temperature
and outline of the PCB in real time. Flexibility to zoom in on the PCB details makes locating
tiny components quick and easy.

Calibrating the IR Camera
Like most electronic measuring devices (e.g., multimeter), a brand new infrared camera is set
to factory standards. Over time, however, wear and tear such as electronic component aging
can cause calibration shifts and impact the reliability of thermal radiation measurements. This
may not be a major concern if you’re just doing quantitative inspections such as determining
hot and cold regions of a particular area, instead of qualitative measurements to ascertain
the component that is causing failure on a PCB.
The human body has an ideal temperature of 36.9°C and normal range from 36.1°C to 37.2°C;
anything above or below is considered an abnormal condition. Similarly, components have
different operating temperature ranges, depending on their manufacturing grades and
specifications:206
Grade
Commercial
Industrial
Military/Aerospace

Operating Temp
0°C to 70°C
–40°C to

85°C

–55°C to 125°C

These temperature ranges are the allowable limits guaranteed by manufacturers for their
products to operate in, though it may not necessarily be the kind of operating temperatures
that the components will heat up to in optimum conditions. This is why PCB designers have to
ensure adequate heat dissipation to prolong the useful lifespan of their hardware, as well as
prevent erratic behaviors that usually happen when components overheat.
Calibrating a thermal camera, therefore, is the process of correlating what the camera sees
(infrared radiation) with known temperatures, so that the camera can accurately measure the
radiation it detects. Calibration is performed under controlled conditions with a large number
of blackbody reference sources. 207 These blackbody reference sources are arranged in a
semi-circle and set to different known temperatures, and then the thermal camera (connected
to a robotic arm) is pointed at each reference source one by one. The signal value at each
temperature is captured by calibration software, and each pair of signal and temperature
values are plotted along a curve, the equation of which is based on a physics model. This data
is then loaded into the camera, calibrating it to ensure it meets accuracy specifications.
Nevertheless, manufacturers are known to define their own temperature grades so it's important to pay close
attention to the actual datasheet specifications.
206

Blackbodies are physical bodies with very high emissivity, meaning they radiate and absorb almost all
electromagnetic radiation. Blackbodies in a calibration lab are certified and traceable to international standards.
207

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Thermal Profiling a PCB
When talking about thermal profiling PCBs, electronic manufacturers associate it with creating
a repeatable thermal process that meets the specifications required by the solder paste and
components in a reflow soldering setting.208
For PCB diagnostics, however, we’re talking about creating a thermal image profile of a PCB
operating in normal, working conditions. That is to say for any given PCB, it is expected to
exhibit a reasonable temperature distribution across its whole surface under a normal room
(or repair laboratory) environment. That’s where the thermal camera comes in.
Technically, though, it is not a straightforward affair by just powering up a PCB and capturing
a thermal image profile to be stored and use for comparison against a malfunctioning one
later. We need to realize that a PCB does not work in isolation but is part of a system in which
it resides and serves its intended purpose. So when a faulty PCB is pulled out of a system and
sent to a repair center, it is like a fish that has left its natural habitat to be dissected on an
operating table for examination.
Ideally, a PCB’s thermal profile should cover two aspects——idle and operating modes. Take
the Raspberry Pi 3 B+ for example:

Raspberry Pi 3 B+ Single Board Computer
It is popular among the Raspberry Pi models due to an efficient processor and an improved
design in its power circuitry compared to its predecessors. At idle, the board draws a mere
1.91W; when running a synthetic workload, it increases to just 5.77W.

Also known as an oven recipe, it spells out the need to meet component and solder requirements for a specific
PCB assembly.
208

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Thermal Imaging

The thermal profiles of the Raspberry Pi 3 B+ in these two modes provide a visual indication
of the heat distributions expected of a working unit:

Thermal Profile in Idle Mode

Thermal Profile in Operating Mode
At idle, the system-on-chip (SoC) is relatively cool (39.8°C) while the combined USB and
Ethernet controller to the middle-right is a noticeable hot spot (45.3°C). At load, measured
after one minute of a CPU-intensive synthetic workload, the SoC surpassed the controller to
become the hottest component (58.1°C).

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Keeping a profile record of the Raspberry Pi 3 B+ operating in idle and operational modes will
thus provide a basic reference to allow future diagnosis of faulty units.209 We need to realize,
however, that not all failures will result in faulty components overheating and exhibiting higher
than normal temperatures. Sometimes, the failure could be attributed to a broken power link
within the component package, causing the device to be rendered inoperable. In such cases,
the failed device would show up as a cooler color region instead.
On the other hand, those in the mobile phone repair business will tell you that you don’t need
any thermal profile data to start using a thermal imaging camera for repair work. To a certain
extent this statement is true, especially when you’re talking about a dead phone that could
be caused by a short circuit due to failed filter capacitors, a common occurrence from the
constant abuses mobile phones are subjected to these days. Before we jump in to look at an
interesting example, there is one more thing in the equation that we need to consider:

Power Sources and Cables
It’s obvious that some kind of DC power source is required to power up a PCB or a dead mobile
phone in order to utilize a thermal imaging camera for detecting failed components onboard.
But it’s not that straightforward when it comes to the choice of power cables for connecting
the DC supply to the target subject to be checked. Take the mobile phone repair power cables
for instance. Currently, there are many different types available on the market, two of which
are shown below:

Power cable for iOS and Android

Power cable for Android only

Again, it is important to have periodic checks and calibration to be performed if necessary in order for the
saved profile data to be valid for comparison.
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Thermal Imaging

These power boot cables, as they are usually called, come with a pair of banana plugs at one
end that can be connected to the output terminals of a benchtop DC power supply. At the
other end, however, are a bunch of cables with different types of connectors, either catered
for just Android phone models or both iOS and Android. These power cables may provide
simple direct connections from the power supply to the phone, or go through some kind of
power control and regulating module for added safety.
The question, though, is which type of power cable is suitable for thermal imaging diagnosis?
According to an iPhone service repair guy in Malaysia, certain power cables with these ‘power
blocks’ tend to hamper the process of thermal fault detection because they introduce voltage
drops. He demonstrated in a video that when the DC supply is set to 2V, the output of the
cable registered zero voltage, and this condition persisted until he increased the supply to 5V
at which point the cable managed a 4V output with an inherent 1V loss.210

At 4V, the faulty component exhibited a
brightness that flooded its surrounding
area, obscuring the thermal image and
prevented pinpoint detection.

Applying this level of voltage to a shorted iPhone logic board, the thermal image captured by
his Seek Compact IR Camera is anything but useless (see top figure).

Spot on!

However, using a direct power cable with no voltage loss in between,
he only needed to apply the minimum short voltage of 2.1V to obtain a
clear thermal image of the faulty component’s location. Once that
failed device (a filter capacitor) is removed, the iPhone could then boot
up without problem.
The drawback of a direct power cable alone is its lack of support for
battery data to fool the iPhone into thinking that a battery is present
and preventing it from rebooting. In the end, he recommended a power
cable that comes with this feature to allow a more thorough diagnosis
to be carried out.

210

You can watch his video at: https://www.youtube.com/watch?v=MJTBVtYMyNA

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Talking about mobile phone repair, a whole new
industry has come about to provide support and
servicing. In fact, some manufacturers of thermal
imaging cameras also came up with additional
resources, like the benchtop DC power supplies
shown on the right that is designed specifically for
mobile phone repair.
Unlike a normal benchtop DC power supply which
provide digital readouts for voltage and current
settings alone, these breed of DC power sources
have additional analog meters to provide quick
visual indications of current surge when powering
up mobile phone circuit boards with short circuit
faults. The usable range is therefore limited to just
enough for mobile phone repair (15V @ 5A).
Alternatively, if you already have a benchtop power supply, you may want to consider getting
a set of the following cable set for mobile phone repair, which comes with four USB ports and
a separate interface port for plugging in an analog ammeter:

Power cable set for mobile phone repair
This is a neater way of organizing all your test cables instead of having a whole bunch dangling
around while you carry out diagnosis and repair work, which can be cumbersome and untidy
on the workspace.

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Thermal Imaging

Example 1: Water Damaged iPhone SE211
There’s nothing more frustrating than to drop your mobile phone into a toilet bowl first thing
in the morning. Unfortunately, such occurrences are common, next to dropping your phone on
the hard pavement. But while the latter may just crack the display screen which can be easily
replaced, a waterlogged mobile phone could mean kissing goodbye to all your valuable data
and photos. And this was what happened to one Apple iPhone SE user.
Having visited several phone repair shops and being told that her phone was beyond repair
each time after it was examined and some rectification works performed, she had almost lost
hope when her boyfriend came across one particular shop——iPhone Service. The experienced
repair man not only restored her phone back to life so she could retrieve all her precious
memories, he even made a video and described the whole process.212
Here is his account:
The customer came into our shop with a dead iPhone SE, claiming that she had brought it to
many repair shops with no success. The reason given was the same——the motherboard is as
good as dead. We did not want to dismiss her words too soon, but told her that any logic board
can be repaired and the data saved as long as the CPU and memory are still intact.

Indicator

CPU

Memory

iPhone SE motherboard
211

Beginning with iPhone 13, Apple mobile phone users no longer had to worry about this issue anymore.

212

You can watch his video at: https://www.youtube.com/watch?v=CKDf-8LBIfk

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Most iPhones are equipped with liquid contact indicators (LCI) that can be observed without
opening up the device. The color is normally white or silver but when the LCI contacts water
or liquid containing water, it will turn full red. For the iPhone SE, there is also one located on
the motherboard (see figure above).
After opening up the phone and removing the motherboard for examination, I proceeded to
plug in the power to check current consumption. It registered only 80mA at the battery voltage
of 3.9V which indicated no CPU activity. The next thing to try is to check which component was
drawing power and preventing the motherboard from working. For this, I enlisted the help of
a handheld thermal imaging camera which had been modified for closeup scan.

The problematic component quickly showed up on the thermal scan to be a display IC which
exhibited a higher temperature (32°C) than its surrounding, though at this temperature it is
not possible to feel the difference since our fingers are at a much higher temperature than
that. Removing the chip revealed corrosion marks on the I2C solder balls area which caused
the stuck at boot up failure mentioned earlier.

TI 65730A0P Display IC (BGA)
After replacing the faulty IC, the iPhone SE could now boot up normally and we have a very
happy and satisfied customer!
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Thermal Imaging

Example 2: iPhone 7 Battery Drain Problem
Another common problem experienced by mobile phone users is abnormally fast battery drain
which shortens the usable time of the device. This time, the repair man is given an iPhone 7
exhibiting this symptom.
First, he removed the motherboard from the phone and hooked it up to his trusty power supply
with analog meters (see arrow).

20mA leakage

3.8V applied

Note 1:
Applying pressure on the button
power key pin on J4504 with a
probe lead caused the current
to momentarily spike indicating
a possible problem in the power
rail of the boot up circuit.

detected

Immediately he noted that there was a 20mA current leak upon application of the 3.8V battery
voltage without turning on the phone. This points to some low resistance path that might be
causing the battery drain (Note 1). Based on his experience, the repair man zoomed in on the
NAND chip area on the reverse side of the motherboard (see overleaf) where several
decoupling capacitors are located to check for possible short. After removing the adhesive
layer, he found that one of the capacitors for the PP3V0_NAND registered a short. There are
several capacitors on this power rail so he needed to use thermal imaging to find out which
one is overheating to determine the faulty capacitor.
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So he applied the battery voltage and pressed the power button while scanning the area with
his thermal camera. However, this approach did not allow the power IC or even the boost IC
to heat up long enough to get a good indication.

2G secure
element IC

NAND

Overheating
area

He then switched to using a fully charged battery instead of the direct power cable, while doing
away with the need to constantly press the power button. As a result, there was prolonged
heating that narrowed the suspected area to just two capacitors (C1713 and C1721). To further
isolate the culprit, he unplugged the battery, applied some solder flux on these capacitors,
then plugged back the battery and let the heat did the work. It was C1721.
After removing the shorted capacitor, he reconnected the direct power cable to check if the
current leak is resolved. It did not. Obviously there was some other component failure that
was causing the battery drain. Again, experience played an important part here. The repair
man suspected that it has to do with the 2G secure element IC chip213 (see figure). Thermal
imaging could still be used but only if it was combined with another method——the blinking
technique.
213

Denoted as ICEFALL chip IC with the part number 77359-8 (Intel version) and designation SE2_RF.

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Basically, what this technique does is varying the voltage from zero to battery voltage and then
back to zero again, repeatedly. The problem IC will usually exhibit a glow at the battery voltage
and fades off at zero.

IC fades at zero

IC glows at 4V

This was what happened to the 2G IC chip. With the chip removed, the current now stays at
zero with battery voltage applied and the phone turned off. The repair man figured that since
mobile network these days no longer uses 2G, he could safely leave the iPhone 7 without the
chip and the user would probably not notice any difference in performance.214

Acknowledgement:
Special thanks to Mr. Lim from iPhone Services, Malaysia for his kind permission to use his
YouTube video snapshots for illustration. Besides repairing Apple iPhones, he also conducts
training classes and workshops on how to repair iPhones in which he not only imparts basic
knowledge but real-world diagnostic skillsets as well. Readers interested in iPhone repairs can
check out more of his videos on YouTube.

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You can watch his video at: https://www.youtube.com/watch?v=5SPbz0jQMNk

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Summary
There are significant limitations to being able to characterize a PCB and even specific circuits
on it. Test points and surface mounted components with external leads are primarily used for
manual probing with an oscilloscope or multimeter.215 PCB’s connectors are sometimes used
to gain access to specific circuit sections, but the increasing use of rigid-flex reduces physical
interconnects, resulting in time-consuming probing as well as accessibility.
Measuring the impedance of the voltage rails is usually the first step in debugging a failure. If
the impedance measures less than 10 ohms, there is almost certainly a problem. But where
do you go from there? There could be dozens of parts all on the same PCB and lifting pins on
every device or removing entire components is a time-consuming endeavor, not to mention
the difficulty in soldering them back. This is where the benefits of a thermal imaging camera
come into play.

Normal image

Thermal image

Thermal imaging cameras can rapidly diagnose faulty PCBs and identify failed components
which manifest as hot spots and elevated subcircuit temperatures. Small and affordable, they
complement the use of multimeters and oscilloscopes.

To probe surface mounted components, they must have accessible leads which means ball or land grid array
components are excluded.
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The art of PCB diagnostics is not limited to the techniques discussed so far. To achieve better
efficiency and comprehensiveness in fault coverage, PCB manufacturing and repair industries
are always innovating new ways to catch defects before their products leave the assembly
house, or improve their capabilities to quickly rectify failures in the field to reduce operational
downtime.
In this chapter, we will look briefly at several other diagnostic techniques, namely boundary
scan, X-ray and automated optical inspection to give this topic a more complete treatment. A
comparison of these test methods against the in-circuit technique is depicted in the following
chart:

X-Ray
- Insufficient
- Excess
- Cold Solder
- Marginal Joints
- Voids

In-Circuit
Polarity

- Missing
- Gross Shorts
- Lifted Leads
- Bent Leads

- Extra Part
- Bridging
- Tombstone
- Misaligned

- Shorts
- Open
- Misoriented
- Wrong Parts

AOI

- Dead Part
- Bad Part
- Short/Open on PCB
- Functional Fault
- In-system Programming
- At-speed Memory Test
- At-speed Interconnect
- Gate-level Diagnosis
- Fault Insertion

Boundary Scan

Not surprising, there are overlaps in detecting certain faults due to similarities in the design
and functional capabilities of these tools. However, the equipment cost and skills required to
operate each of these tools vary greatly and are dependent on the purpose they are built to
accomplish.
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Boundary Scan Test216
Boundary scan test (BST) is a test technique defined by the Joint Test Action Group (JTAG)
under the IEEE-1149.x standard to test ICs and interconnects on PCBs, using just four wires
as its test interface.217 It is extremely versatile where physical access to individual component
is difficult or impossible, but requires IC designers to provision additional test logic into their
chips and also PCB designers to adhere to the JTAG guidelines when using these BST-enabled
components. Nowadays, all FPGAs, most 64-bit processors, and many other microprocessors
and Ethernet PHYs already incorporate boundary scan cells.

Basic JTAG Chip Architecture
This test methodology, which was developed in the 90s for the Intel 80486 microprocessor, is a standard
technique today that manufacturers and engineers use to program, debug and test almost all embedded devices
and systems.
216

217

An optional fifth wire or pin (TRST) is sometimes added to provide reset capability.

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Other Techniques

JTAG Chip Architecture
The IEEE-1149.x JTAG standard defines how IC scan logic must behave to achieve interoperability among components, systems, and test tools. ICs consist of logic cells, or boundary
scan cells, between the system logic and the signal pins or balls that connect the IC to the
PCB. Each cell provides specific test capabilities——some cells can be used as input, others
as output, while some are bidirectional.
The boundary scan cells within a device are connected together to form a shift register, which
is accessed through a serial test data input (TDI) and test data output (TDO) interface. The
TMS and TCK signals control an internal state machine that allows the boundary scan
functionality to be controlled.
The cells are added at the chip’s boundary——between the IC’s core logic and the I/O pins or
boundary hence the name. This establishes a serial test data path running through the entire
chip. In test mode, the cells control the status of the output pin while reading that of the input
pin, enabling testing of the chip and board interconnections. These cells appear transparent
during normal operation of the chip.
For a component to comply with the boundary scan JTAG standard, it must include:
▪
▪
▪
▪
▪

A boundary-scan cell for each I/O pin.
A scan path where all boundary cells are serially connected.
A Test Access Port (TAP) interface and controller to handle the boundary scan signals.
An additional 4-5 pins for the JTAG signals.
A boundary scan description language (BSDL) file provided by the chip vendor.218

There are two types of registers associated with boundary scan. Each compliant device has
one instruction register and two or more data registers. The instruction register holds the
current instruction which defines to which of the data registers signals should be passed.
The three primary data registers are:
▪

Boundary Scan Register (BSR). This is the main data register which is used to move
data to and from the I/O pins of a device.

▪

Bypass Register. This is a single-bit register that passes information from TDI to TDO
and allows other devices in a circuit to be tested with minimal overhead.

▪

Device ID Register. This optional register contains the IDCODE and revision number for
the device. This information allows the device to be linked to its boundary scan
description language (BSDL) file and contains the details of the boundary scan
configuration for the device.

This file describes the boundary scan behavior, package information, and capabilities for the component,
implementation process, instructions, scan-cells available, design warnings, etc.
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Test Access Port (TAP) Controller
A JTAG compliant chip has a Test Access Port (TAP) controller that comprises the following
signals and the logic that connects and controls the device:
▪
▪
▪
▪
▪

TDI
TDO
TMS
TCK
TRST

serial input pin for the instructions, test, and programming data
serial out pin for the instructions, test, and programming data
input for the signal that manages the TAP controller state machine
clock signal input pin for the boundary scan circuitry
reset signal (optional)

The TAP controller comprises a 16-state finite state machine (see overleaf). These states are
controlled by the test clock (TCK) and test mode select (TMS) signals. The JTAG interface
provides a means to connect external test software to the inbuilt TAP controller.

TAP Controller State Machine Diagram

228

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Other Techniques

The IEEE-1149.x standard defines a set of instructions that must be available for a device to
be considered compliant. These instructions are:219
▪

IDCODE. This instruction causes the TDI and TDO to be connected to the IDCODE register.

▪

BYPASS. This instruction causes the TDI and TDO to be connected via a single-bit passthrough register, and allow testing of other devices in the JTAG chain without any
unnecessary overhead.

▪

EXTEST. This instruction causes the TDI and TDO to be connected to the BSR. The
device’s pin states are sampled with the ‘Capture DR’ state and new values are shifted
into the BSR with the ‘Shift DR’ state; these values are then applied to the pins of the
device using the ‘Update DR’ state.

▪

SAMPLE/PRELOAD. This instruction causes the TDI and TDO to be connected to the BSR.
However, the device is left in its normal functional mode. During this instruction, the
BSR can be accessed by a data scan operation to take a sample of the functional data
entering and leaving the device. It is also used to preload test data into the BSR prior
to loading an EXTEST instruction.

How BST Works

JTAG daisy-chain of multiple devices

INTEST is an optional instruction which causes the TDI and TDO to be connected to the boundary scan register
(BSR). While the EXTEST instruction allows the user to set and read pin states, the INTEST instruction relates to
the core-logic signals of a device.
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Boundary scan technique enables configuration of the BSR in two primary test modes——an
internal test mode (INTEST) that tests the core logic of the chip, and an external test mode
(EXTEST) that checks the interconnection between ICs on the PCB. JTAG compliant ICs on a
PCB are usually daisy-chained together such that the TDO of one chip connects to the TDI of
another chip. Test vectors are then streamed through the first IC on the chain and emerged
from the last IC. Such an arrangement helps to verify the continuity of the interconnections
between various components.220
Benefits of boundary scan testing include, but not limited to, the following;
▪

Ability to test ICs and PCBs with limited or no access to internal connections and pins.

▪

Reduces PCB testing efforts, costs and time without compromising on quality. Offers
reusable test patterns, better test coverage, and shorter time-to-market.

▪

Provides a low cost debugging and in-circuit programming of CPLDs, serial EEPROMs,
Flash, on-chip memory, etc.221

▪

Reduces the risk of physically damaging the PCB, pins or creating shorts associated
with the use of mechanical probes.

▪

Improves production and field testing at the board level while eliminating the need for
other costly test procedures and equipment.

▪

Performance and overall signal integrity of the board can be improved because the
designer doesn't need to add tracks to test points.

Despite the benefits, the boundary scan technique has some drawbacks:
▪

Requires additional IC floorspace for the boundary scan circuitry. Also, poor board
layout and terminations could degrade the signal integrity of the JTAG signals.

▪

The JTAG interface, while useful for testing or reprogramming ICs and PCBs, can be
exploited for hacking connected devices such as the IoT. This backdoor entry access
poses a security threat that may compromise data, cause a malfunction or even bring
down a system.

Notwithstanding, the increasing use of boundary scan technology for interconnect testing and
in-system programming have seen various hardware and software companies developing a
wide range of JTAG test and ISP tools. Examples include ABI Electronics, Corelis, Teradyne,
Acculogic, Göpel Electronic, Asset InterTech, Intellitech, Flynn Systems, and XJTAG.

More complex designs may utilize additional circuitry or a dedicated JTAG bridge to selectively configures a
scan chain that contains multiple devices, or even multiple sub-assemblies.
220

Many TAP interfaces employ signals in addition to those required by the JTAG standard. For example, on-chip
debugging applications may include signals for asynchronous halt and reset, while in-system programming
applications may increase programming speed by taking advantage of additional pins for time-critical function
such as toggling the write enable signal or polling a ready/busy signal.
221

230

LEARNING THE ROPES

Other Techniques

Boundary Scan Description File (BSDL)
The boundary scan description language is based on the syntax and grammar of VHDL and is
used to describe how JTAG is implemented in a particular device. JTAG tools use information
in a BSDL file to work out how to access a device in the JTAG chain.
The following elements can be found in a BSDL file:
▪
▪
▪
▪
▪
▪
▪
▪
▪

Entity Description: Statements naming the device or a section of its functionality.
Generic Parameter: A value such as a package type. The value may come from outside
the current entity.
Port Description: Describes the nature of the pins on the device (input, output,
bidirectional, linkage).
Use Statements: References external definitions (such as IEEE-1149.x).
Pin Mapping(s): Maps logical signals in the device to physical pins.
Scan Port Identification: Defines the pins used on the device to access the JTAG
capabilities.
Instruction Register Description: The signals used for accessing JTAG device modes.
Register Access Description: Which register is placed between TDI and TDO for each
JTAG instruction.
Boundary Register Description: List of the boundary scan cells and their functionality

ALL manufacturers of JTAG compliant ICs provide BSDL files (see samples below):222
Actel
AMD
Analog Devices
Agilent Technologies
Altera
AMD
Atmel
Cirrus Logic
Cypress
Fairchild
Freescale (formally Motorola)
Fujitsu
IDT

Infineon
Intel
Intersil
ISSI
Lattice Semiconductors
LSI
Maxim
Micron
Mosel Vitelic
National Semiconductor
NEC
OKI
Philips

Phytec
PMC Sierra
QuickLogic
Renesas
Samsung
Silicon Laboratories
STMicroelectronics
Texas Instruments
Toshiba
Xilinx
Zarlink

The main elements and sample extracts of a BSDL file is shown overleaf:

These files are available for download from their respective manufacturer’s websites. You can find a huge
collection at the BSDL Files Library for JTAG website (https://bsdl.info). Note that in some cases, you may need
to sign an NDA before you can obtain these files.
222

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Main Elements

232

Extract from Custom Cell-Type Package

IDCODE Structure

Chapter 9

LEARNING THE ROPES

Other Techniques

When boundary scan tests are run, serial data sequences are clocked into the TDI pin of a
JTAG device. The JTAG control signal, TMS, allows these sequences to step the device’s state
machine between different stable states and to scan data through the chain; a serial vector
format (SVF) file is one way to represent these sequences in ASCII text. It allows a file to be
created that is agnostic of the tools that will use it, making it a suitable format for files that
will be used to program a device, or for when data needs to be transferred between different
vendors’ tools.
SVF sequences are purely sequential and lack support for conditional statements. It is
therefore not possible to create loops, and familiar constructs such as those using FOR
statements are not supported, often leading to large file sizes. SVF files can, however, be a
good medium for device programming.
An alternative format is Standard Test and Programming Language (STAPL), which is the
standardized version of the earlier JAM™ language created by Altera. STAPL permits loops and
other flow control methods, frequently making its file sizes smaller and its execution time
shorter. It also supports the polling of devices for their status and therefore does not require
the fixed delays often associated with SVF file sequences. An SVF consists of a sequence of
commands as shown below:
!Begin Test Program
TRST OFF;
ENDIR IDLE;
ENDDR IDLE;
HIR 8 TDI (00);
HDR 16 TDI (FFFF) TDO (FFFF) MASK (FFFF);
TIR 16 TDI (0000);
TDR 8 TDI (12);
SIR 8 TDI (41);
SDR 32 TDI (ABCD1234) TDO (11112222);
STATE DRPAUSE;
RUNTEST 100 TCK ENDSTATE IRPAUSE;
!End Test Program;

!Disable Test Reset line
!End IR scans in IDLE
!End DR scans in IDLE
!8-bit IR header
!16-bit DR header
!16-bit IR trailer
!8-bit DR trailer
!8-bit IR scan
!32-bit DR scan
!Go to stable state DRPAUSE
!RUNBIST for 100 TCKs

Because a BSDL file is the medium for describing how an IC’s boundary scan architecture is
implemented, it is essential to obtain that file if boundary scan testing is to be performed. It
contains information such as the size of the instruction register, the binary codes that equate
to the different instructions, which of the optional commands are supported, and details of
each cell used in the boundary register.
SVF and STAPL files, in comparison, are not needed to run a boundary scan test, but are ways
to list the state machine transitions and bit patterns that make up a particular test or function.
Because boundary scan can be used to program devices, tools provided by IC manufacturers
can often generate these files as a means of in-circuit programming.
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Example: Testing an 80486DX2 CPU
As mentioned in an earlier footnote, Intel developed the boundary scan test methodology for
its i486DX2 microprocessor back in the 1990s, and this technique has since been widely
adopted by the industry as the JTAG standard. Before going into the JTAG hardware and
software tools in use today, it would be beneficial to take a look at how an actual test code is
implemented.
Back in 1996, while I was developing an in-circuit test program for a CPU board based on the
80486DX2 chip, I was deliberating on the approach to test this Intel processor. I could take the
arduous route of writing assembly codes to exercise the IC on a machine-level; but after going
through the data book223 I was heartened to know there was a simpler and quicker way ——
boundary scan. It was quite new to me at that time, and rightly so since Intel was the first to
come up with this novel test method.

i486DX2 Microprocessor
After poring through the details and getting a better understanding, I decided on the following
test blocks:
▪
▪
▪
▪
▪

CPU Reset
Boundary Scan Bypass Register
Boundary Scan Device Identification Register
Boundary Scan External Test
CPU Built-In Self-Test

The test codes are written using the Factron S700 in-circuit tester’s Mediator language which
uses a high-level test programming syntax. In Chapter 7, I gave a short description for a 7400
chip’s in-circuit test routine, so please refer to it to refresh your memory before diving into the
code segments that follow.

223

Appendix F contains a section of the 80486DX data book related to this topic.

234

LEARNING THE ROPES

Other Techniques

CPU reset test code:
FAST_SUBROUTINE RESET_80486;
IN EF1;
(IL'HOLD','AHOLD',IH'_BOFF',)(IH'_FLUSH','_A20M',)
REPEAT 10 TIMES DO
(IL'CLK',)
Assert RESET signal (active high)
(IH'RESET',)
REPEAT 50 TIMES DO
(H'CLK',)(L'CLK',)
END REPEAT;
*ADS should go high before removing RESET
(O'_ADS',M'_ADS',H'_ADS', X)
(IL'RESET',)
signal (active low)
REPEAT 450 TIMES DO
(H'CLK',)(L'CLK',)
Check for *ADS signal to go low
(TL'_ADS',)
IF MATCH THEN () GOTO 10;
END IF; ()
END REPEAT;
T(10)
(O'_ADS',N'_ADS',)
ADDRHI defines the high address lines from
(O 'ADDRHI','ADDRLO', 'A3','A2',
A18 to A25.
M 'ADDRHI','ADDRLO', 'A3','A2',)
(TH'ADDRHI','ADDRLO',TL'A3','A2',)
ADDRLO defines the low address lines from
IF MATCH THEN () GOTO 20;
A4 to A17.
END IF; ()
(O'ADDRHI','ADDRLO', 'A3','A2',
N'ADDRHI','ADDRLO', 'A3','A2', )
END REPEAT;
T(20)
(H'ADDRHI','ADDRLO',L'A3','A2', X)
(O'ADDRHI','ADDRLO', 'A3','A2',
N'ADDRHI','ADDRLO', 'A3','A2', )
END FAST_SUBROUTINE;

Reset224 Timing Cycle
224

RESET is an asynchronous input. t20 must be met only to guarantee recognition on a specific clock edge.

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Boundary scan BYPASS register test code:
FAST_SUBROUTINE JTAG_BYPASS;
IN EF1;
(I'TMS','TCK','TDI',)
(H'TMS',)
REPEAT 5 TIMES DO
(L'TCK',)(H'TCK',)
END REPEAT;
(L'TMS',)(L'TCK',)(H'TCK',)
(H'TMS',)(L'TCK',)(H'TCK',)
(H'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)
(H'TDI',)(L'TCK',)(H'TCK',)
(H'TDI',)(L'TCK',)(H'TCK',)
(H'TDI',)(L'TCK',)(H'TCK',)
(H'TDI',)(L'TCK',)(H'TCK',)
(H'TMS',)(L'TCK',)(H'TCK',)
(H'TMS',)(L'TCK',)(H'TCK',)
(H'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)
(O'TDO',M'TDO',)
(L'TDI',)(L'TCK',)(H'TCK',)
(H'TDI',)(L'TCK',)(H'TCK',)(L'TDO',
(L'TDI',)(L'TCK',)(H'TCK',)(H'TDO',
(H'TDI',)(L'TCK',)(H'TCK',)(L'TDO',
(L'TCK',)(H'TCK',)(H'TDO',
(O'TDO',N'TDO',)
END FAST_SUBROUTINE;

Test logic reset

Apply BYPASS instruction code = 1111

X)
X)
X)
X)

Test for BYPASS condition with alternating
0s and 1s

Device identification (IDCODE) register test code:
FAST_SUBROUTINE JTAG_IDCODE;
IN EF1;
(I'TMS','TCK','TDI',)
(H'TMS',)
REPEAT 5 TIMES DO
(L'TCK',)(H'TCK',)
END REPEAT;
(L'TMS',)(L'TCK',)(H'TCK',)
(H'TMS',)(L'TCK',)(H'TCK',)
(H'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)
(L'TDI',)(L'TCK',)(H'TCK',)
(L'TDI',)(L'TCK',)(H'TCK',)
(H'TDI',)(L'TCK',)(H'TCK',)
(L'TDI',)(L'TCK',)(H'TCK',)
(H'TMS',)(L'TCK',)(H'TCK',)
(H'TMS',)(L'TCK',)(H'TCK',)
(H'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)

236

Test logic reset

Apply IDCODE instruction code = 0010

LEARNING THE ROPES

Other Techniques
(O'TDO',M'TDO',)
(L'TCK',)(H'TCK',)(H'TDO',
(L'TCK',)(H'TCK',)(H'TDO',
(L'TCK',)(H'TCK',)(L'TDO',
(L'TCK',)(H'TCK',)(L'TDO',
(L'TCK',)(H'TCK',)(H'TDO',
(L'TCK',)(H'TCK',)(L'TDO',
(L'TCK',)(H'TCK',)(L'TDO',
(L'TCK',)(H'TCK',)(L'TDO',
(L'TCK',)(H'TCK',)(L'TDO',
(L'TCK',)(H'TCK',)(L'TDO',
(L'TCK',)(H'TCK',)(L'TDO',
(L'TCK',)(H'TCK',)(L'TDO',
(L'TCK',)(H'TCK',)(H'TDO',
(L'TCK',)(H'TCK',)(L'TDO',
(L'TCK',)(H'TCK',)(H'TDO',
(L'TCK',)(H'TCK',)(L'TDO',
(L'TCK',)(H'TCK',)(L'TDO',
(L'TCK',)(H'TCK',)(L'TDO',
(L'TCK',)(H'TCK',)(L'TDO',
(L'TCK',)(H'TCK',)(H'TDO',
(L'TCK',)(H'TCK',)(L'TDO',
(L'TCK',)(H'TCK',)(H'TDO',
(L'TCK',)(H'TCK',)(L'TDO',
(L'TCK',)(H'TCK',)(L'TDO',
(L'TCK',)(H'TCK',)(L'TDO',
(L'TCK',)(H'TCK',)(L'TDO',
(L'TCK',)(H'TCK',)(L'TDO',
(L'TCK',)(H'TCK',)(L'TDO',
(O'TDO',N'TDO',)
END FAST_SUBROUTINE;

X)
X)
X)
X)
X)
X)
X)
X)
X)
X)
X)
X)
X)
X)
X)
X)
X)
X)
X)
X)
X)
X)
X)
X)
X)
X)
X)
X)

First bit of IDCODE
MFG ID Intel = 009H

Model = 00101

Family = 0100

Intel architecture type = 000001

VCC SUPPLY = 0 (+5V)

Boundary scan EXTEST register test code:
The boundary scan register contains a cell for each pin, as well as cells for control of high/low
and 3-state pins. The following is the bit order for the CPU’s boundary scan register:
TDI 
WRCTL ABUSCTL BUSCTL MISCCTL ADS# BLAST# PLOCK# LOCK# PCHK# BRDY# BOFF# BS16#
BS8# RDY# KEN# HOLD AHOLD CLK HLDA WR# BREQ BEO# BE1 # BE2# BE3# MIO# DC# PWT
PCD EADS# A20M# RESET FLUSH# INTR NMI UP# FERR# IGNNE#
D31 D30 D29 D28 D27 D26 D25 D24 DP3 D23 D22 D21 D20 D19 D18 D17 D16 DP2 D15 D14
D13 D12 D11 D10 D9 D8 DP1 D7 D6 D5 D4 D3 D2 D1 D0 DP0
A31 A30 A29 A28 A27 A26 A25 A24 A23 A22 A21 A20 A19 A18 A17 A16 A15 A14 A13 A12
A11 A10 A9 A8 A7 A6 RESERVED A5 A4 A3 A2
 TDO

Note: ‘RESERVED’ corresponds to no connect ‘NC’ signals on the CPU.

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All the *CTL cells are control cells that are used to select the direction of bidirectional pins or
3-state output pins. If ‘1’ is loaded into the control cell (*CTL), the associated pin(s} are 3stated or selected as input. The following lists the control cells and their corresponding pins:
▪
▪
▪
▪

WRCTL controls the D31-0 and DP3-0 pins.
ABUSCTL controls the A31-A2 pins.
BUSCTL controls the ADS#, BLAST#, PLOCK#, LOCK#, WR#, BEO#, BE1#, BE2#, BE3#, MIO#,
DC#, PWT, and PCD pins.
MISCCTL controls the PCHK#, HLDA, BREQ, and FERR# pins.
FAST_SUBROUTINE JTAG_EXTEST;
IN EF1;
(I'TMS','TCK','TDI',)
(H'TMS',)
REPEAT 5 TIMES DO
(L'TCK',)(H'TCK',)
END REPEAT;
(L'TMS',)(L'TCK',)(H'TCK',)
(H'TMS',)(L'TCK',)(H'TCK',)
(H'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)
(L'TDI',)(L'TCK',)(H'TCK',)
(L'TDI',)(L'TCK',)(H'TCK',)
(L'TDI',)(L'TCK',)(H'TCK',)
(L'TDI',)(L'TCK',)(H'TCK',)
(H'TMS',)(L'TCK',)(H'TCK',)
(H'TMS',)(L'TCK',)(H'TCK',)
(H'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)

Test logic reset

Apply EXTEST instruction code = 0000

Exit UPDATE-DR State
Enter SHIFT-DR State

\* ALL BI-DIRECTIONAL PINS PROGRAMMED TO OUTPUT MODE
*\
\* LOGIC 0 : A2-A31,D/C,MI/O,BE0-BE3,W/R,LOCK,BLAST,ADS *\
\* LOGIC 1 : D0-D31,DP0-DP3
*\
(L'TDI',)
REPEAT 31 TIMES DO
(L'TCK',)(H'TCK',)
END REPEAT;
(H'TDI',)
REPEAT 50 TIMES DO
(L'TCK',)(H'TCK',)
END REPEAT;
(L'TDI',)
REPEAT 24 TIMES DO
(L'TCK',)(H'TCK',)
END REPEAT;
(L'TDI',)(L'TCK',)(H'TCK',)
(L'TDI',)(L'TCK',)(H'TCK',)
(L'TDI',)(L'TCK',)(H'TCK',)
(L'TDI',)(L'TCK',)(H'TCK',)

238

\*
\*
\*
\*

MISCCTL
BUSCTL
ABUSCTL
WRTL

=
=
=
=

0
0
0
0

*\
*\
*\
*\

LEARNING THE ROPES

Other Techniques
\* ENTER RUN-TEST/IDLE STATE AFTER UPDATING DR LATCH *\
(H'TMS',)(L'TCK',)(H'TCK',)
\*PARALLEL OUTPUTS *\
(H'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)
(O'ADDRHI','ADDRLO','A3','A2',
M'ADDRHI','ADDRLO','A3','A2',
L'ADDRHI','ADDRLO','A3','A2', X)
(O'ADDRHI','ADDRLO','A3','A2',
N'ADDRHI','ADDRLO','A3','A2', )
(O'DATAHI','DATALO','DP03',
M'DATAHI','DATALO','DP03',
H'DATAHI','DATALO','DP03', X)
(O'DATAHI','DATALO','DP03',
N'DATAHI','DATALO','DP03', )
(O'D_C','M_IO','BE03','W_R','_LOCK','_BLAST','_ADS',
M'D_C','M_IO','BE03','W_R','_LOCK','_BLAST','_ADS',
L'D_C','M_IO','BE03','W_R','_LOCK','_BLAST','_ADS', X)
(O'D_C','M_IO','BE03','W_R','_LOCK','_BLAST','_ADS',
N'D_C','M_IO','BE03','W_R','_LOCK','_BLAST','_ADS', )
(IL'_IGNNE','_EADS','_FLUSH','_BS16','_A20M','_BS8',
'_BOFF','_KEN','_RDY','_BRDY',)
(IH'INTR','AHOLD','NMI','CLK','RESET','HOLD',)
(H'TMS',)(L'TCK',)(H'TCK',)
CAPTURE/SHIFT-DR STATE
(L'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)
REPEAT 68 TIMES DO
\* SKIP 68 OUTPUT PINS *\
(L'TCK',)(H'TCK',)
END REPEAT;
\* MONITOR ONLY INPUT PINS *\
(O'TDO',M'TDO',)
(L'TCK',)(H'TCK',)(L'TDO', X) \* IGNNE# *\
(L'TCK',)(H'TCK',)
(L'TCK',)(H'TCK',)
(L'TCK',)(H'TCK',)
(L'TCK',)(H'TCK',)
(L'TCK',)(H'TCK',)(H'TDO', X) \* NMI
*\
(L'TCK',)(H'TCK',)(H'TDO', X) \* INTR *\
(L'TCK',)(H'TCK',)(L'TDO', X) \* FLUSH# *\
(L'TCK',)(H'TCK',)(H'TDO', X) \* RESET *\
(L'TCK',)(H'TCK',)(L'TDO', X) \* A20M# *\
(L'TCK',)(H'TCK',)(L'TDO', X) \* EADS# *\
REPEAT 11 TIMES DO
(L'TCK',)(H'TCK',)
END REPEAT;
(L'TCK',)(H'TCK',)\* (H'TDO', X) CLK
*\
(L'TCK',)(H'TCK',)
(L'TCK',)(H'TCK',)(H'TDO', X) \* AHOLD *\
(L'TCK',)(H'TCK',)(H'TDO', X) \* HOLD *\
(L'TCK',)(H'TCK',)(L'TDO', X) \* KEN# *\
(L'TCK',)(H'TCK',)(L'TDO', X) \* RDY# *\
(L'TCK',)(H'TCK',)(L'TDO', X) \* BS8# *\
(L'TCK',)(H'TCK',)(L'TDO', X) \* BS16# *\
(L'TCK',)(H'TCK',)(L'TDO', X) \* BOFF# *\
(L'TCK',)(H'TCK',)(L'TDO', X) \* BRDY# *\

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Chapter 9
REPEAT 9 TIMES DO
(L'TCK',)(H'TCK',)
END REPEAT;
(O'TDO',N'TDO',)
\* ALL BI-DIRECTIONAL PINS PROGRAMMED TO OUTPUT MODE
\* LOGIC 1 : A2-A31,D/C,MI/O,BE0-BE3,W/R,LOCK,BLAST,ADS
\* LOGIC 0 : D0-D31,DP0-DP3
\* STILL IN SHIFT-DR STATE
(H'TDI',)
REPEAT 31 TIMES DO
(L'TCK',)(H'TCK',)
END REPEAT;
(L'TDI',)
REPEAT 50 TIMES DO
(L'TCK',)(H'TCK',)
END REPEAT;
(H'TDI',)
REPEAT 24 TIMES DO
(L'TCK',)(H'TCK',)
END REPEAT;
(L'TDI',)(L'TCK',)(H'TCK',)
(L'TDI',)(L'TCK',)(H'TCK',)
(L'TDI',)(L'TCK',)(H'TCK',)
(L'TDI',)(L'TCK',)(H'TCK',)

\*
\*
\*
\*

MISCCTL
BUSCTL
ABUSCTL
WRTL

=
=
=
=

0
0
0
0

*\
*\
*\
*\

*\
*\
*\
*\

\* ENTER RUN-TEST/IDLE STATE AFTER UPDATING DR LATCH *\
(H'TMS',)(L'TCK',)(H'TCK',)
\* PARALLEL OUTPUTS *\
(H'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)
(O'ADDRHI','ADDRLO','A3','A2',
M'ADDRHI','ADDRLO','A3','A2',
H'ADDRHI','ADDRLO','A3','A2', X)
(O'ADDRHI','ADDRLO','A3','A2',
N'ADDRHI','ADDRLO','A3','A2', )
(O'DATAHI','DATALO','DP03',
M'DATAHI','DATALO','DP03',
L'DATAHI','DATALO','DP03', X)
(O'DATAHI','DATALO','DP03',
N'DATAHI','DATALO','DP03', )
(O'D_C','M_IO','BE03','W_R','_LOCK','_BLAST',
M'D_C','M_IO','BE03','W_R','_LOCK','_BLAST',
H'D_C','M_IO','BE03','W_R','_LOCK','_BLAST', X)
(O'D_C','M_IO','BE03','W_R','_LOCK','_BLAST',
N'D_C','M_IO','BE03','W_R','_LOCK','_BLAST', )
(IH'_IGNNE','_EADS','_FLUSH','_BS16','_A20M','_BS8',
'_BOFF','_KEN','_RDY','_BRDY',)
(IL'INTR','AHOLD','NMI','CLK','RESET','HOLD',)
(H'TMS',)(L'TCK',)(H'TCK',)
CAPTURE/SHIFT-DR STATE
(L'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)
REPEAT 68 TIMES DO

240

\* SKIP 68 OUTPUT PINS *\

LEARNING THE ROPES

Other Techniques
(L'TCK',)(H'TCK',)
END REPEAT;
\* MONITOR ONLY INPUT PINS *\
(O'TDO',M'TDO',)
(L'TCK',)(H'TCK',)(H'TDO', X) \* IGNNE# *\
(L'TCK',)(H'TCK',)
(L'TCK',)(H'TCK',)
(L'TCK',)(H'TCK',)
(L'TCK',)(H'TCK',)
(L'TCK',)(H'TCK',)(L'TDO', X) \* NMI
*\
(L'TCK',)(H'TCK',)(L'TDO', X) \* INTR *\
(L'TCK',)(H'TCK',)(H'TDO', X) \* FLUSH# *\
(L'TCK',)(H'TCK',)(L'TDO', X) \* RESET *\
(L'TCK',)(H'TCK',)(H'TDO', X) \* A20M# *\
(L'TCK',)(H'TCK',)(H'TDO', X) \* EADS# *\
REPEAT 11 TIMES DO
(L'TCK',)(H'TCK',)
END REPEAT;
(L'TCK',)(H'TCK',)(L'TDO', X) \* CLK
*\
(L'TCK',)(H'TCK',)
(L'TCK',)(H'TCK',)(L'TDO', X) \* AHOLD *\
(L'TCK',)(H'TCK',)(L'TDO', X) \* HOLD *\
(L'TCK',)(H'TCK',)(H'TDO', X) \* KEN# *\
(L'TCK',)(H'TCK',)(H'TDO', X) \* RDY# *\
(L'TCK',)(H'TCK',)(H'TDO', X) \* BS8# *\
(L'TCK',)(H'TCK',)(H'TDO', X) \* BS16# *\
(L'TCK',)(H'TCK',)(H'TDO', X) \* BOFF# *\
(L'TCK',)(H'TCK',)(H'TDO', X) \* BRDY# *\
REPEAT 9 TIMES DO
(L'TCK',)(H'TCK',)
END REPEAT;
(O'TDO',N'TDO',)
\* ALL BI-DIRECTIONAL PINS PROGRAMMED TO INPUT MODE
\* STILL IN SHIFT-DR STATE
REPEAT 105 TIMES DO
\* BYPASS ALL SIGNAL CELLS
(L'TCK',)(H'TCK',)
END REPEAT;
(H'TDI',)(L'TCK',)(H'TCK',)
\* MISCCTL = 1
(H'TDI',)(L'TCK',)(H'TCK',)
\* BUSCCTL = 1
(H'TDI',)(L'TCK',)(H'TCK',)
\* ABUSCCTL = 1
(H'TDI',)(L'TCK',)(H'TCK',)
\* WRTL
= 1

*\
*\
*\

\* LOGIC 0 : A4-A31
\* LOGIC 1 : D0-D31,DP0-DP3
\* 3-STATE : ADS,BLAST,LOCK,WR,BE0-BE3,MIO,DC

*\
*\
*\

(H'TMS',)(L'TCK',)(H'TCK',)
(H'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)

*\
*\
*\
*\

RUN TEST/IDLE STATE

(IL'ADDRHI','ADDRLO',)
(IH'DATAHI','DATALO','DP03',)
(H'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)

CAPTURE/SHIFT-DR STATE

(O'TDO',M'TDO',)

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(L'TCK',)(H'TCK',)
(L'TCK',)(H'TCK',)
(L'TCK',)(H'TCK',)(L'TDO', X)
(L'TCK',)(H'TCK',)(L'TDO', X)
(L'TCK',)(H'TCK',)

\* A4
\* A5

*\
*\

REPEAT 20 TIMES DO
(L'TCK',)(H'TCK',)(L'TDO', X)
\* A6-A25 *\
END REPEAT;
()
REPEAT 6 TIMES DO
(L'TCK',)(H'TCK',)
\* SKIP UNTESTED SIGNALS *\
END REPEAT;
()
REPEAT 36 TIMES DO
(L'TCK',)(H'TCK',)(H'TDO', X)
\* D0-D31,DP0-DP3 *\
END REPEAT;
(O'TDO',N'TDO',)
\* LOGIC 1 : A4-A31
\* LOGIC 0 : D0-D31,DP0-DP3
\* 3-STATE : ADS,BLAST,LOCK,WR,BE0-BE3,MIO,DC
(H'TMS',)(L'TCK',)(H'TCK',)
(H'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)

*\
*\
*\

RUN TEST/IDLE STATE

(IH'ADDRHI','ADDRLO',)
(IL'DATAHI','DATALO','DP03',)
(H'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)

CAPTURE/SHIFT-DR STATE

(O'TDO',M'TDO',)
(L'TCK',)(H'TCK',)
(L'TCK',)(H'TCK',)
(L'TCK',)(H'TCK',)(H'TDO', X)
(L'TCK',)(H'TCK',)(H'TDO', X)
(L'TCK',)(H'TCK',)

\* A4
\* A5

*\
*\

REPEAT 20 TIMES DO
(L'TCK',)(H'TCK',)(H'TDO', X)
\* A6-A25 *\
END REPEAT;
()
REPEAT 6 TIMES DO
(L'TCK',)(H'TCK',)
\* SKIP UNTESTED SIGNALS *\
END REPEAT;
()
REPEAT 36 TIMES DO
(L'TCK',)(H'TCK',)(L'TDO', X)
\* D0-D31,DP0-DP3 *\
END REPEAT;
(O'TDO',N'TDO',)
END FAST_SUBROUTINE;

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CPU built-in self-test (BIST) test code:
FAST_SUBROUTINE JTAG_RUNBST;
IN EF1;
(IL'HOLD','AHOLD',IH'_BOFF',)(IH'_FLUSH','_A20M',)
(I'CLK',ELP'CLK',)
(IH'RESET',)
Reset CPU prior to running BIST
REPEAT 150 TIMES DO ()
END REPEAT;
(O'_ADS',M'_ADS',)
(IL'RESET',)
REPEAT 450 TIMES DO
(TL'_ADS',)()
IF MATCH THEN (DLP'CLK',) GOTO 30;
END IF; ()
END REPEAT;
T(30)
(O'_ADS',N'_ADS',)
(O'HOLD',N'HOLD',)
(I'TMS','TCK','TDI',)
Test logic reset
(H'TMS',)
REPEAT 5 TIMES DO
(L'TCK',)(H'TCK',)
END REPEAT;
(L'TMS',)(L'TCK',)(H'TCK',)
(H'TMS',)(L'TCK',)(H'TCK',)
(H'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)
Apply RUNBIST instruction code = 1000
(L'TDI',)(L'TCK',)(H'TCK',)
(L'TDI',)(L'TCK',)(H'TCK',)
(L'TDI',)(L'TCK',)(H'TCK',)
(H'TDI',)(L'TCK',)(H'TCK',)
(H'TMS',)(L'TCK',)(H'TCK',)
RUN TEST/IDLE STATE
(H'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)
(O'TDO',M'TDO',)
(L'TCK',)(H'TCK',)(H'TDO', X)
Check RUNBIST register = 1
REPEAT 1200 TIMES DO
()
REPEAT 998 TIMES DO
(ELP'CLK',)
Apply 1.2 million clock cycles
END REPEAT;
()
END REPEAT; ()
(DLP'CLK',)
CAPTURE/SHIFT-DR STATE
(H'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)
(L'TMS',)(L'TCK',)(H'TCK',)
(L'TCK',)(H'TCK',)(L'TDO', X)
Check RUNBIST register = 0 (Pass)
(O'TDO',N'TDO',)
END FAST_SUBROUTINE;

Let’s now look at some advanced JTAG hardware and software tools that isolate users from
such complexity by providing easy-to-use tools with intuitive graphical interfaces.
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XJTAG
XJTAG is a world leading supplier of boundary-scan hardware and software tools. The company
supplies a wide range of high-performance JTAG boundary scan controllers and test extension
hardware products that work with all XJTAG software, offering a fully integrated development
environment for rapid test generation, real-time debug, production test execution as well as
in-system programming.

XJLink2 USB JTAG Controller

XJAnalyser JTAG Software Tool (copyright © XJTAG)
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When used with XJAnalyser, a powerful tool for real-time circuit visualization and debugging,
the JTAG tools provide user a graphical view of JTAG chains, giving complete control on a pinby-pin basis of both the pin states (driven as an output or tri-stated as an input) and the pin
values (high or low when driven). When connected to a JTAG-enabled PCB, the XJAnalyser
automatically investigates the JTAG chain, identifies the correct BSDL files from its library, 225
and is all set to allow you to control and read individual pins. Another of the XJTAG tools,
XJDeveloper, allows extensive boundary scan board tests to be created and run.

Example: The XJDemo Board

XJDemo Board (copyright © XJTAG)
The XJDemo is a fully populated demonstration board that comes supplied with the following
components:






Intel MAX V 5M40Z CPLD
Kinetis K22F Cortex-M4 processor
9 LEDs
Pushbutton
8M-bit SPI NOR Flash







RS485 transceivers
8 channel serial ADC
3-axis linear Accelerometer
8 MHz Oscillator
Standard logic devices226

Like any JTAG tools, XJAnalyser doesn't come with a pre-loaded library, so the user needs to download the
BSDL files for their board first.
225

226

4 channel 3-state buffer, 4-bit fanout buffer, NOR gates and inverter.

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 2K-bit serial EEPROM
 4M-bit static RAM

 Debug links (simulate open/short circuits)
 JTAG connector (XJLink/PXI)

A test run with the demo board connected to the JTAG controller, which in turn is connected
via USB to a PC or laptop running XJAnalyser, yields the following results:
NAME
Check Chain – using profile 'All Chains'
Connection Test - using profile 'All Chains'
SRAM Tests - using profile 'All Chains'
SPI Flash Tests - using profile 'All Chains'
EEPROM Tests - using profile 'All Chains'
Oscillator Tests - using profile 'All Chains'
Accelerometer Tests - using profile 'All Chains'
ADC Tests - using profile 'All Chains'
RS485 Tests - using profile 'All Chains'
TOTAL TIME

RESULT
Passed
Passed
Passed
Passed
Passed
Passed
Passed
Passed
Passed

TIME
0.293
0.053
0.016
0.019
0.017
0.126
0.189
0.984
4.061
5.762

A breakdown of each test follows:
After selecting the 'All Chains' profile, XJAnalyser first checks the integrity of the JTAG chain to
ensure that it is good before executing the rest of the tests.
Selecting profile 'All Chains'...
Checking the integrity of the JTAG chain.
CheckChain passed - ran in profile All Chains
Next, an interconnect test is carried out to check the JTAG devices digital pins and other parts
of the circuit for soldering faults. If there is a short, a stuck-high or stuck-low, this test will find
it. If there is enough access to nets, it will also check for open circuits. XTJAG software tool
generates this test automatically from the supplied circuit data, which forms the bulk of the
test coverage. It's also very fast, taking just a mere fraction of a second to complete.
Performing standard Connection Test...
Generating Connection Test data...
Performing Logic Tests (phase 1)...
Performing Logic Tests (phase 2)...
Analysing Connection Test results...
Analysing Logic Tests (phase 1)...
Analysing Logic Tests (phase 2)...
Running additional external hardware interaction tests...
CONNTEST passed – ran in profile All Chains

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Of course, a PCB may contain many non-JTAG devices, such as memory ICs. The demo board
has three different types of memory devices——an SRAM, a SPI Flash, and an I2C EEPROM. They
are tested without requiring complex coding since these are standard tests found in the builtin library.

Testing
Testing
Testing
Testing
Testing
Testing
U6.Test

SRAM U6...
address and data bus...
chip enable...
nOE...
nBLE...
nBHE...
passed - ran in profile All Chains

Testing SPI Flash U3...
Testing device IDs...
Winbond device found
Device family – W25
Device size – 8 Mbit
All ID values as expected.
U3.ReadIDTest passed - ran in profile All Chains
Testing I2C EEPROM U5...
Testing I2C interface of U5...
Device successfully acknowledged I2C address 0xA0
Testing read/write functionality...
U5.Test passed - ran in profile All Chains
What about clock generator devices, such as the 8 MHz oscillator? Well,
boundary scan isn't fast enough to measure its frequency but there is a
test in the library that can check the clock line by monitoring the net and
counts transitions. If it detects a handful of samples, it knows the clock
line is active and so must be working. From the results, we see that the
test also checks the oscillator's enable line.

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Testing Oscillator X1...
Testing with device enabled...
Oscillator test took 33 ms
Oscillator output pin 3 detected 137 transitions
Testing with device disabled...
Oscillator test took 66 ms
Oscillator output pin 3 detected 0 transitions
Test from MCU passed - ran in profile All Chains
What else is there on the demo board that needs to be tested? There is an accelerometer, an
ADC, and a pair of RS485 transceivers. The XJTAG library has the tests for all of them, isn't that
cool?

Testing Accelerometer U12 (I2C Mode)...
Testing I2C interface of U12...
Device successfully acknowledged I2C address 0x3A
Checking device ID reading 1 byte starting at address 0x0F...
Device ID read correctly at address 0x0F: 0x41
Running accelerometer self-test with positive polarity...
X-Axis Baseline = -35 mg; Self-test = 350 mg; Change = 385
Y-Axis Baseline = -22 mg; Self-test = 391 mg; Change = 413
Z-Axis Baseline = 992 mg; Self-test = 1303mg; Change = 311
Running accelerometer self-test with negative polarity...
X-Axis Baseline = -35 mg; Self-test =-474 mg; Change =-439
Y-Axis Baseline = -22 mg; Self-test =-439 mg; Change =-417
Z-Axis Baseline = 992 mg; Self-test = 684 mg; Change =-308
Testing interrupt pins...
U12.Test passed - ran in profile All Chains

mg;
mg;
mg;
mg;
mg;
mg;

Testing I2C interface of U11...
Device successfully acknowledged I2C address 0x90
U11.I2C_CheckPresent passed - ran in profile All Chains
Testing U11 ADC Voltages...
Testing I2C interface of U11...
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Device successfully acknowledged I2C address 0x90
Testing channel 0...
Channel 0: Read 2274 mV (scaled to 3280 mV) (value OK)
Testing channel 1...
Channel 1: Read 1803 mV (value OK)
Testing channel 2...
Channel 2: Read 2284 mV (scaled to 3295 mV) (value OK)
ADC Channel Test passed - ran in profile All Chains
Testing RS485 Device U4...
Performing RS485 local loopback test...
U4.Test passed - ran in profile All Chains
Testing RS485 Device 74...
Performing RS485 local loopback test...
U7.Test passed - ran in profile All Chains
Performing RS485 External Loopback Tests...
Testing data transfer from U4 to U7...
Testing data transfer from U7 to U4...
U4andU7LoopbackTest passed - ran in profile All Chains
>>>> PASSED <<<<
When it comes to the accelerometer, the program uses the in-built self-test routine. As for the
ADC, the analog voltages from its three channel inputs are read. For the RS485 transceivers a
physical loopback link is connected to the demo board's connector to allow the pair U4 and U7
to talk to each other.
The advantage of boundary scan testing isn't just limited to doing away with building test jig
or fixture, which can be time-consuming and error prone. You don't need any firmware running
on a board to use boundary scan——it doesn't even have to boot. All the above tests are in fact
performed with the CPLD and MCU unprogrammed. Once the board is tested OK, the firmware
can then be programmed.

Acknowledgement:
The above example is adapted from XJTAG's tutorial video227 with modified transcript to suit
the content of this chapter. All images supplied and related to XJTAG are copyright of XJTAG.
Used with permission. (Note: XJTAG products are professional tools aimed at qualified
engineers in industrial companies.)

227

https://www.xjtag.com/?videos=jtag-testing-with-xjtag-boundary-scan

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Automated Optical Inspection (AOI)
Automated optical inspection is a process employed in PCB manufacturing to detect issues
such as poor soldering, missing, misoriented, or wrong components, etc.228 Both 2D and 3D
imaging systems are used to facilitate inspection. A single camera or imaging sensor may be
used to provide an overhead view of the circuit board. Additional cameras allow for the capture
of 3D images. Many systems use a combination of axial and angled light sources as well as
light of several colors to enhance the contrast between objects and the background.
Defects detectable through 2D imaging include missing and wrong components, offset and
skewed placements, polarity reversals, excess or insufficient solder, and bridging. 3D imaging
detects package coplanarity, lifted leads, absence and presence of components, tombstoning
and billboarding, and the presence of foreign material. Soldered connections are measured
for their geometry and then determined through algorithmic computations whether they have
sufficient or too much, too little solder.

Machine inspection for PCB faults
AOI is used at different stages of manufacturing such as bare-board inspection, solder-inspection, and preand post-reflow inspections, the last stage being a common point of installation because many defects are often
discovered here.
228

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Other Techniques

AOI Lenses
Some AOI systems have cameras mounted on X-Y motion systems to allow imaging of large
circuit boards like the inline systems. Others use a fixed image sensor while the PCB is mobile
which is common in bench-type systems. Still, some machines employ a hybrid combination
of camera and board motion, such as the dual-side inspection type which employs both topdown and bottom-up cameras.
Lenses must be selected to match the chosen camera and the optical performance required.
Conventional cameras use wide angle or telephoto lenses (a zoom lens is a particular type of
telephoto lens). For general photography image distortions would not be an issue, but for
optical inspection this can pose a serious problem.229 To eliminate distortion and allow for the
stitching of images telecentric lenses are used. These lenses have a limited field of view (FOV)
to ensure parallel light from the object (i.e., the board under inspection) is parallel when it
reaches the sensor of the camera.

Camera

Camera

Sensor

Sensor

Conventional
Lens

Low object is
obstructed by
large object

Telecentric
Lens

Low object is
visible and
unobstructed

PCB
FOV

PCB
FOV

Conventional versus Telecentric Lens

For conventional lenses, tall components will appear magnified and areas of the PCB will be shadowed by
these components, resulting in adjacent images unable to be properly merged (stitched) into one whole image
for inspection and comparison.
229

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Inspection Methods
There are two methods of processing captured images by an AOI system to determine if a PCB
passes inspection:

Image-based System
By comparing these images with those of one or more known good boards, the placement of
components, the integrity of soldering, and other measures of quality can be determined. In
this method, a library of good and defective boards begins to accumulate against which the
image processor compares the test board in an attempt to match a pattern. The quicker a
matched pattern is found, the faster the inspection can be completed. If the system cannot
find a pattern match, then the board will be rejected and a report issued. Thus, the more
images accumulated which the system can index through, the lower the likelihood of false
rejection. Having many images to look through can slow inspection speed. As components
can vary somewhat in sizes and colors, oftentimes a statistical process is employed to provide
a window of acceptability for a given board design.

Misaligned component detected

Algorithm-based System
A second method is based on algorithms that search for patterns in an image, for example
the outline of a component package. This is a powerful processing technique as the captured
image does not need to be compared to a large library of images, and minor differences in
color between identical components can be safely ignored instead of being flagged out as a
false failure. Algorithmic processing, though, is not as intuitive as image-based processing.

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Pattern algorithm verification

AOI Programming
Inspection programming is usually performed offline so as not
to interfere with production. An important part of the
programming sequence is post-inspection review where the
programmer follows along with the inspection routine to affirm
calls made by the automation. False flags and escapes are
considered as measures of system reliability. The review step
in the process is important as the programmer can adjust the
level of pass/fail based on the criticality of a particular PCB
assembly.
Experience with real-world manual inspection is a big benefit
for would-be AOI system programmers. Learning to program
an image-based AOI is considered easier as results are
achieved almost immediately. Algorithm-based machines are
considered more difficult to program at first but once learned,
the programming is more efficient and less time-consuming.

PCB Diagnostics

Convert Gerber file to
KY PCB format
PCB Fiducial Learning

Image Grabbing (Scan)
Import Component
Refernce Designators

Library Manager and
Package Registration
Setting Up
Inspection Conditions

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Inspection Process
The first stage in the generation of an inspection program is to load images of the board under
test and its associated data and line up data information with board images. Models are then
constructed for each component to be inspected, which is either carried out on the inspection
system or offline.
A model is a pictorial representation of a specific component combined with its associated
attributes. For example, a chip resistor is a rectangular box with solder points at each end and
a value on its body. The displayed image should resemble the actual component. Different
components will require different models. For similar components only one model will be
needed and be associated through the parts list to all other similar components on that PCB.
However, it may be necessary to create alternative models for the same type of component.
Again considering a chip resistor, a 10K value can exist as either a size 1002 or 103, in this
case if both devices are present on the same board it will be necessary to create alternative
models.
Solder areas are also created during the preparation of a model. This can either be done on
screen or if Gerber data is available for pad layout, it can be imported when board data is
loaded and the solder areas will then automatically be set. Associated with each model is a
set of test parameters. While these are initially set high, in practice it may be necessary to
reduce the values due to component quality or board layout. However, care must be exercised
when reducing pass scores so that faulty devices or poor solder are not invariably accepted.

An inspection program is generated while component models are developed. When all the
models are generated, the settings can be checked by inspecting the board used to generate
the necessary models. If all the models are correct the board will pass. The inspection program
can then be optimized by inspecting a small quantity of new boards (3–4 should be sufficient)
and settings adjusted to compensate for component quality and visual differences.230
Pass scores should not be reduced to such an extent that faults are disguised; it is better to accept some
false calls and clear them at the fault review stage.
230

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While the position and value test performed on the above component are correct, the solder
test is suspect. The displayed result indicates insufficient solder on some legs (thicker lines
on pins 1, 2, 4, 5, and 16).
A report is generated for every PCB that contains faults detailing the component identification,
position and the type of fault.

Challenges in AOI
While there have been great developments in the capabilities of AOI machines over the years,
many challenges still persist:
▪
▪
▪
▪
▪
▪

Programming of AOI parameters is complicated and cumbersome.
Misjudgment is still a common occurrence because discrete components can exhibit
similar physical attributes yet possess different values.
Polarity (orientation) check accuracy can be affected by similar devices with different
character font labeling.
Solder joint problems cannot be checked for components with leads that are not visible
(BGA, CSP, flip-chip, etc.)
Processing speed of AOI machines is generally slow. Those using scanning method are
faster but also more prone to misjudgment.
Difficulty in detecting shielding cover and shielding points.

Automated optical inspection works well in PCBs with clearly visible solder joints. However,
many PCBs today employ surface mounted integrated circuits where the solder joints are not
visible. Such cases require extended support with automated X-ray inspection (AXI) equipment
that can check solder joints underneath the components.

PCB Diagnostics

255

Chapter 9

X-Ray Inspection
Automated X-ray inspection (AXI) is a technology based on the same principles as automated
optical inspection (AOI). It uses X-rays as its source instead of visible light to automatically
inspect PCB features which are typically hidden from view.

Photo image

X-ray image

X-ray inspection is widely used in areas such as medical, industrial control, and aerospace for
controlling the quality of circuit board assemblies and to analyze defects in hidden solder
joints. Materials absorb X-ray proportional to their atomic weight and the rate of absorption
differs depending on density, atomic number and thickness.231 Optimal use of this technique
requires both good X-ray equipment and a trained operator.
There are two primary types of X-ray inspection system for surface mounted components with
hidden solder joints,232 namely the 2D and 3D transmission systems. The 2D type system
generates X-rays at a single point which passes through the PCBA; the process triggers an
image on the electronic detector and produces a digital format picture after it completes the
analysis. This technique is used for single-sided boards and assemblies where accuracy is of
utmost importance.233

Generally speaking, materials made of heavier elements absorb more radiation and are easily imaged, while
those made of lighter elements are more transparent to X-ray.
231

232

These include chip size packages (CSP) and ball grid array (BGA) ICs.

233

Medical practices also use this principle to inspect the condition of fractured bones.

256

LEARNING THE ROPES

Other Techniques

24-pin QFN chip

Passed solder joints

Failed solder joints

Comparison of Inspection Systems
Although AXI and AOI have the same working principles and play similar roles in PCB assembly
production lines, the type of defects they can detect differs to some extent. The following table
shows a comparison between AOI, AXI and ICT in terms of their coverages:

PCB Diagnostics

257

Chapter 9

Summary
As chip and circuit complexity and density continue to increase, it poses greater challenge to
direct electrical test access and increases the need for built-in-test capability within devices.
Higher signal speeds are also prohibiting external probing access while increasing the need
for both structural and parametric testing during manufacturing, since design margins are
constantly tightened to squeeze out every bit of performance capability.
In-circuit test technique using bed-of-nail probing access will continue to be used to perform
structural assembly testing with precise diagnosis for structural defects, but will slowly decline
in usage due to the aforementioned pressures on probing access. There is thus an increased
need for complex devices to incorporate built-in self-test (BIST) and advanced programming
capabilities, such as in CPUs, FPGAs and CPLDs, to extend testing capability.
What does this imply for PCB diagnostics? One thing is certain——we will need to rely on more
than just one tool or technique to craft the right test strategy.

258

LEARNING THE ROPES

Other Techniques

PCB Diagnostic Flowchart
1

Visual Inspection
2

5

Standard multimeter
measurements

No

Can PCB be powered
up safely?

Yes

Power up PCB
6

No

Is reference PCB
available?

Check voltages

Yes

Look for similar
designs on PCB
for comparison

7

Replace components
as necessary
3

Check clock
signals
8

V-I checks on all
discrete components
4

Check HMI
devices
9

V-I checks on all
integrated circuits

Perform digital
IC tests
10

Perform analog
IC tests

If a reference PCB is not available for a full board
comparison, the alternative is to check for similar
component or sub-circuit designs on the PCB to do
V-I signature check. An example of a component
would be a 74F244 3-state octal buffer line drivers
IC with eight similar inputs and outputs, and two
output enable (OE) signals that can be used for V-I
comparison.

PCB Diagnostics

11

Perform custom
device tests
12

JTAG tests

259

Chapter 9

Notes:
1. Check for signs of damage on components (burnt, discoloration, leak, etc.) and PCB
(crack, broken or corroded traces, etc.).
2. Check for shorts on power rails, capacitors and relays (stuck contacts). Check for open
on fuses, resistors, inductors and transistors. Resistor values can be checked but may
not reflect true reading in-circuit.
3. Discrete components that exhibit distinct V-I signatures include resistors, capacitors,
inductors, and semiconductors (diodes, transistors, MOSFETs, etc.).
4. Integrated circuits can be tested using individual V-I pin-pair comparisons but it is both
tedious and time consuming. Some benchtop testers provide matrix V-I test capability
that allow clipping of the IC component and scanning all the pins at one go.
5. Two type of checks can be performed upon powering up a PCB: current consumption
(power supply source readout) and heat dissipation (by feeling around components).
It is important to set the current limit before turning on the power.
6. Voltage checks include regulator and converter outputs (DC), transformer windings
(AC), test points and IC power pins, etc. DMM and DSO can be used to measure DC
average, AC rms values, as well as voltage ripples.
7. Clock signals from crystal oscillators, PLLs, multivibrator circuits, etc. DSO is the best
option to check the waveform characteristics and parameters. Use the correct probes
to avoid capacitive loading and interference distortions.
8. Human-machine interfaces (HMI) include switches, indicators (LEDs, lamps, etc.) and
displays (LCD, 7-segment, matrix, etc.). Check for difference in brightness, dead pixels
and blackout or corrupted display.
9. Digital IC tests for standard logic devices (gates, counters, registers, flip-flops, etc.)
are usually pre-written in the tester's library. Testing these devices in-circuit requires
proper guarding to prevent interference and possible backdriving damage.
10. Analog IC tests are quite limited in most cases, especially for op-amps and ADC/DAC
devices. Static tests such as voltage and saturation (on-off) are conducted rather than
parametric tests due to limited tester resources.
11. Custom device tests include relays, ADCs/DACs, memory and programmable logic
devices (checksum and VHDL). Note that some memory or logic devices are securitybit protected and renders checksum test invalid.
12. JTAG or boundary-scan test is applicable for PCBs that are compliant to the IEEE-1149
standard only.

260

LEARNING THE ROPES

There is never enough information when you need it.

1.
2.
3.
4.
5.
6.
7.
8.

Resistor Color Codes
Capacitor Color Codes
SMD Resistor Alphanumeric Codes
Capacitor Alphanumeric Codes
Radial-Lead Capacitors
Solid Electrolytic Capacitors
Diode Color Codes
SMD Diode Marking Diagrams

PCB Diagnostics

A–2
A–3
A–4
A–5
A–6
A–6
A–7
A–8

A–1

Appendix A

1. Resistor Color Codes

A–2

APPENDICES

Tables & References

2. Capacitor Color Codes

PCB Diagnostics

A–3

Appendix A

3. SMD Resistor Alphanumeric Codes

Size Code
Inches
0402
0603
0805
1206
1210
1812

A–4

Metric
1005
1508
2012
3216
3225
4532

Dimensions
Inches
.04 x .02
.06 x .03
.08 x .05
.12 x .06
.12 x .10
.18 x .12

Millimetres
1.0 x 0.5
1.5 x 0.8
2.0 x 1.2
3.2 x 1.6
3.2 x 2.5
4.5 x 3.2

APPENDICES

Tables & References

4. Capacitor Alphanumeric Codes
Code
100
150
220
330
470
101
121
131
151
181
221
331
471
561
681
751
821
102
152
202
222
332

pF
10
15
22
33
47
100
120
130
150
180
220
330
470
560
680
750
820
1000
1500
2000
2200
3300

nF
0.01
0.015
0.022
0.033
0.047
0.1
0.12
0.13
0.15
0.18
0.22
0.33
0.47
0.56
0.68
0.75
0.82
1.0
1.5
2.0
2.2
3.3

uF
0.00001
0.000015
0.000022
0.000033
0.000047
0.0001
0.00012
0.00013
0.00015
0.00018
0.00022
0.00033
0.00047
0.00056
0.00068
0.00075
0.00082
0.001
0.0015
0.002
0.0022
0.0033

Code
472
502
562
682
103
153
223
333
473
683
104
154
204
224
334
474
684
105
155
205
225
335

pF
4700
5000
5600
6800
10000
15000
22000
33000
47000
68000
100000
150000
200000
220000
330000
470000
680000
1000000
1500000
2000000
2200000
3300000

nF
4.7
5.0
5.6
6.8
10
15
22
33
47
68
100
150
200
220
330
470
680
1000
1500
2000
2200
3300

uF
0.0047
0.005
0.0056
0.0068
0.01
0.015
0.022
0.033
0.047
0.068
0.1
0.15
0.2
0.22
0.33
0.47
0.68
1.0
1.5
2.0
2.2
3.3

Polarity (+)
Picofarad Code
Rated Voltage
Manufactured Date

PCB Diagnostics

A–5

Appendix A

5. Radial-Lead Capacitors
MAX OPERATING VOLTAGE
CODE
MAX VOLTAGE
1H
050V
2A
100V
2T
150V
2D
200V
2E
250V
2G
400V
2J
630V
CAPACITANCE CONVERSION VALUES
uF
nF
pF
0.000001
0.001
1
0.00001
0.01
10
0.0001
0.1
100
0.001
1
1,000
0.01
10
10,000
0.1
100
100,000
1
1,000
1,000,000
10
10,000
10,000,000
100
100,000
100,000,000

TOLERANCE
CODE
PERCENTAGE
B
±0.1pF
C
±0.25pF
D
±0.5pF
F
±1%
G
±2%
H
±3%
J
±5%
K
±10%
M
±20%
Z
+80%,-20%

6. Solid Electrolytic Capacitors

A–6

APPENDICES

Tables & References

7. Diode Color Codes

COLOR
BLK
BRN
RED
ORG
YEL
GRN
BLU
VIO
GRY
WHT

PCB Diagnostics

1st
0
1
2
3
4
5
6
7
8
9

2nd
0
1
2
3
4
5
6
7
8
9

3rd
0
1
2
3
4
5
6
7
8
9

4th
0
1
2
3
4
5
6
7
8
9

Suffix
A
B
C
D
E
F
G
H
J

A–7

Appendix A

8. SMD Diode Marking Diagrams

A–8

APPENDICES

The four most common causes of failure in electronic circuits are:
▪
▪
▪
▪

Component defects
Environmental factors
Design, specifications and quality
Aging and degradation

Components exhibit certain similarities and differences in how they fail due to their inherent
design and electrical characteristics. Failures caused by manufacturing defects are usually
captured during the quality check process where sentry tests are carried out. Field deployed
failures are often attributed to operating environment as well as design issues and end of life
of the product.
Below are analysis of common causes found in component failures:

Resistors
▪

Open circuit caused by thermal overstress due to high current flow leading to excessive
heat dissipation greater than the device's specified wattage.

▪

Open circuit due to mechanical stress leading to fracture at the lead-body junction.

▪

Value degradation due to electrical (power surges and cycling) and environmental
(humidity, temperature, etc.) stresses.

Capacitors
▪

Rupture of oxide film in electrolytic capacitors caused by high electric field.

▪

Leakage of electrolyte in electrolytic capacitors due to high operating temperature or
sealant degradation.

▪

Short circuit due to moisture ingress in voids between the leads and body.

▪

Dielectric degradation due to exposure to humidity, high temperature, aging.

▪

Excessive derating of applied voltage resulting in electrolytic capacitors developing
voltage memory at lower operating voltages even though the ratings may be higher.

PCB Diagnostics

B–1

Appendix B

▪

Insulation resistance degradation due to wear and tear.

▪

Electrodes corrosion due to chemical action caused by contaminants and moisture.

▪

Polarity reversal in electrolytic capacitors.

▪

Electrolyte drying up caused by high operating temperatures.

▪

Dielectric breakdown due to application of high voltage beyond the rating.

Inductors
▪

Open circuit of coil wire due to thermal overstress.

▪

Shorting of adjacent turns where insulation has been damaged due to manufacturing
defect or breakdown.

▪

Nicks and kinks in the coil wire.

▪

Oxidation of coil wire caused by moisture ingression leading to corrosion.

Transformers
▪

Open circuit in primary and secondary windings due to excessive thermal stress.

▪

High levels of parasitic such as leakage inductance, inter-winding capacitance due to
poor design and manufacturing technique.

▪

Short circuit between primary and secondary due to faulty isolation or low dielectric
withstanding voltage.

▪

High levels of copper and eddy current losses leading to high heat dissipation within
the transformer and impacts adjacent components.

▪

Corona discharge between adjacent turns or windings. To prevent this, encapsulation
or impregnation of the transformer should be properly applied.

Relays

B–2

▪

Damaged or welded contacts due to induced arcing.

▪

Corroded contacts caused by moisture, flux, or cleaning agents due to poor or leaky
sealing.

▪

Melted contacts due to electrical overstress (EOS).

▪

Damaged coil due to EOS.

▪

Damaged plastic casing due to high temperature from soldering or internally generated
heat caused by EOS.

APPENDICES

Common Failures

Semiconductors234
▪

Moisture ingression, flux contamination during soldering and high humidity storage
condition due to seal integrity problem.

▪

Cracks in packaging or die due to mechanical stress, rapid thermal expansion, etc.

▪

Chip-to-substrate attachment failure leading to voids and thermal stress problems.

▪

Bond wire snapping due to EOS.

▪

Deformation of bond wires due to improper bonding.

▪

Cracks at the bond wire and solder pad junction.

▪

Metallization damage due to EOS, electrostatic discharge (ESD), or corrosion.

▪

Electromigration of metal along the direction of current flow.

▪

Oxide layer breakdown due to impurities, ESD damage, pinhole effect caused by poor
etching processes.

▪

Crystallization defects in the bulk semiconductor material.

▪

Design and fabrication faults, misalignment of layers, geometric defects, etc.

▪

Leakage at the p-n junction.

▪

Deviation from normal electrical characteristics.

Printed Circuit Boards
▪

Discoloration due to high temperature soldering, or excessive heat dissipation caused
by components on the board.

▪

Delamination or disintegration235 of PCB layers due to high operating temperature.

▪

Warping due to high temperature or poor board design due to insufficient thickness of
the laminate, faulty layout and inappropriate component mountings.

▪

Electrostatic discharge damage due to improper packaging, storage and handling.

Despite improved quality control during manufacturing, semiconductor devices still pose a higher percentage
failure compared to discrete passive components due to their more delicate nature. This is why they are often
segregated into different grades after undergoing post-production testing. The cheaper batches are usually less
reliable——you get what you paid for.
234

The choice of material for multi-layer PCB is critically important. We came across boards made from low grade
materials that started to disintegrate after five years of operation. Whenever such PCBs are subjected to normal
soldering without adequate preheating, the internal linkages would give way and become open. Some of the
PCBs were so fragile that after many attempts at repairing the breakages it became futile and the boards had to
be classified as BER (beyond economical repair).
235

PCB Diagnostics

B–3

Appendix B

Failure and Stress Distributions
Based on the above analysis, we can correlate the percentile of failure and stress distributions
depicted in the following two pie charts:

Failure type and stress distributions
We see that capacitors and semiconductors are the most vulnerable electronic components,
next to the PCB. On the other hand, temperature is the major attribution to environmental
stress that impacts electronic failures, since electrical circuits are dependent upon power to
function which inadvertently generates heat as the by-product of current flow.

B–4

APPENDICES

Conformal coating applies a thin film of protective chemical substance or membrane over a
PCB or electronic module that conforms to its contours and components. It acts as a layer of
insulation against moisture, dust, heat, fungus, and corrosion, etc. PCBs that are conformal
coated usually exhibit a glossy shine on its component and solder sides and glows under UV
light.
There are generally five types of conformal coating materials in use in industry and military
applications:
Properties
Chemical
Resistance
Poor

Humidity
Resistance
High

Rework

Usage

Acrylic (AR)

Surface
Adhesion
Acceptable

Easy

High

Epoxy (ER)

Good

Excellent

Acceptable

Difficult

Seldom

Parylene (XY)

Excellent

Excellent

Excellent

Impossible

Rarely

Urethane (UR)

Good

High

Acceptable

Difficult

High

Silicone (SR)

Poor

Low

Excellent

Possible

Moderate

Material

These are resin-based and the chemical composition determines the characteristics of the
conformal coating. The choice of conformal coating depends on the operational requirements
of the board and components. PCB manufacturers employ six methods to apply conformal
coatings: spraying——manual and automated, regional coating, dipping, brushing, and vapor
deposition.

Acrylic

Epoxy

Polyurethane
PCB Diagnostics

Parylene

Silicone
C–1

Appendix C

To perform PCB-RE, removal of conformal coating on a PCB is necessary. Depending on the
type of coating, four techniques can be applied:
▪
▪

Chemical
Thermal

▪
▪

Mechanical
Abrasive

Chemical
This used to be the most popular technique for the removal of conformal coatings without
adversely affecting the board or its components. However, there is no one perfect solvent
for all applications, and in some cases no solvent will be suitable at all. When choosing a
solvent for the removal of a particular conformal coating, you should examine the following
criteria:
1. Does it quickly and completely remove the coating?
2. Does it selectively remove the coating while not damaging or adversely affecting
the substrate and other components or devices?
3. Is it safe to work with?
4. Is it environmentally acceptable?
Thermal
This method is the least recommended technique since most conformal coatings require
very high temperature or long heating times. This in turn can cause discoloration, leave
residues, and adversely affect solders, PCB materials or its components. Thermal removal
can lift surface mount pads and damage temperature–sensitive components. Caution
must also be exercised because some coatings emit toxic vapors that are hazardous.
Mechanical
Mechanical removal methods include cutting, picking, sanding or filing the area of coating
to be removed. However, conformal coatings are hardy and abrasion–resistant making the
risk of damage to the board quite high.
Abrasive
Controlled sandblasting is gaining popularity as an alternative to the chemical method of
conformal coating removal. Three factors to consider are ESD voltages, cutting media and
air pressure. Sandblasting machines tend to generate high static voltages that inherently
damage sensitive PCB components. This problem is resolved by ionizing the air stream
and the chamber environment, which effectively reduces ESD voltages to a ±10V safety
range. Cutting media will depend on the type of coating though the three most common
are aluminum oxide, biological (wheat starch, walnut shell, etc.), and sodium bicarbonate.
Air pressure must be carefully monitored together with the time duration to ensure no overcutting happens, which may damage the internals of a PCB.

C–2

APPENDICES

Conformal Coatings

Most conformal coated PCBs I worked on so far use some form of acrylic resin as the medium
which is quite easily removed with solvents like the HumiSeal 1080, a VOC-compliant, nonozone depleting chemical. There are rare instances where the coating is epoxy or polyurethane
in which case I resorted to running a fine-pitch file gently over the tips of the component legs
to lightly scrap off the coating to expose the conductive solder, and then brushing off the
flakes using an anti-static brush.
Another consideration is deciding whether to strip only the solder side, or both the component
and solder sides. This is applicable if the PCB is through-hole since the component legs can
be accessed from the solder side. Stripping just the solder side has the advantage that you
only need to re-coat that side after you're done with your job. The flip-side is it's a tedious and
time-consuming process.

PCB Diagnostics

C–3

Appendix C

Personal Protection Equipment (PPE)
VOC236 chemicals used for conformal coating removal are toxic and hazardous if inhaled over
extensive periods of time. Even the normal alcohol used for cleaning PCB after soldering work
can get you high and induce withdrawal syndromes. There's also the possibility of spillage
when handling these chemical solvents, so some form of personal protection is required. For
breathing equipment, a face mask respirator (half or full-face) is recommended;237 for body
protection, hand gloves and apron are the essentials.

Face mask respirators: Half-face (left) and Full-face (right)

Re-apply Conformal Coating
Once the reverse engineering work is completed, it will be necessary to recoat the PCB, in part or total, depending on how extensive the removal was
in the first place. My preference is the HumiSeal 1B31 aerosol spray, a fastdrying acrylic which is quick and easy to apply by layers for fully stripped
PCBs. For partially stripped or component legs with coating scrapped off, I
use cotton buds dipped in acrylic solution to lightly touch up the affected
areas. Of course, you'll need to give time for the compound to spread even
and cure.

236

Volatile organic compound

237

It is advisable to carry out such work in a room with proper ventilation and air filtering exhaust.

C–4

APPENDICES

In the course of doing PCB repairs, it is inevitable that you will encounter counterfeit parts that
not only frustrate your repair efforts, but introduce additional problems to the PCB under
servicing. The ongoing problem of counterfeit electronic components not only pose serious
threat to the supply chain but also cause severe quality and safety issues as they are likely to
malfunction and impact product performance, which can be dangerous if the products are
developed for the aerospace, medical and defense industries.
To avoid being a victim, here are six quick steps you can follow to detect if your component is
authentic or counterfeit:

1. Inspect packaging thoroughly
Every manufactured component will always be accompanied with its specific datasheet.
This document is usually made available by the manufacturer and provides important
information about the component.

Component package label
PCB Diagnostics

D–1

Appendix D

When you receive the components, check for any incorrect spelling or information on the
package label. Details such as manufacturer, part number, serial number, etc., should
match the corresponding information provided in the datasheet.

2. Check for moisture sensitive packaging
Electronic components should be packed in
anti-static bags. In addition, genuine parts
such as ICs that are sensitive to moisture
should also come with a moisture absorbing
packet (silica gel) and a humidity indicator
card. Counterfeit parts do not usually come
with any humidity indicator card or forged
cards are enclosed instead. If a component
requires a moisture absorbing packet or a
humidity indicator card, this will be clearly
spelt out in the datasheet.

3. Verify markings on the component top surface
The top surface of a component will usually contain information such as logos, part
number (in short or full form), production location, date code, etc.238 This information can
be used to trace the component back to the manufacturer to verify its authenticity. You
can also verify via the authorized supplier.

If space allows, most of the information will be displayed on top. However, if a component is limited in size, a
code is usually printed instead. You can refer to the datasheet to verify the code format, location and what it
stands for.
238

D–2

APPENDICES

Counterfeit Parts

4. Inspect the date code
The date code of a component is a 4-digit code that corresponds to its production date.
The datasheet will provide detailed instructions on how to read the date code. The code
is usually found in two formats: YYWW or WWYY (WW=the week number of the year; and
YY = the last two digits of the year).

For example: The component above has the date code 1815 (YYWW), which indicates the
part was manufactured on the 15th week in the year 2018.
Date codes found on counterfeit parts are usually a combination of wrong numbers or
are set in the future. For example:
▪

Date code: 9058 (90 = the year 1990, 58 = the week number of the year). This
part was manufactured in the year 1990 on the 58th week of the year. This is
likely to be a counterfeit part as there are only 52 weeks in a year.

▪

Date code: 2340 (24 = the year 2023, 40 = the week number of the year). This
part was manufactured in the year 2023 on the 40th week of the year. This is also
a counterfeit part as the production date is set in the future.239

5. Check for blacktopping
Blacktopping is a technique in which a thin layer of asphalt or bitumen (a sticky, black
and highly viscous liquid) is applied on the top surface of a component to cover any details
such as the original manufacturer part number. The surface is then re-printed with false
information and then resold in the market.

239

The 40th week of 2023 starts from October 2 and at the time of this writing, we are on week 4 (January 22).

PCB Diagnostics

D–3

Appendix D

An easy way to determine if a component has been altered is to scrub the component’s
top surface with acetone, a solvent that is made up of three parts mineral spirits and onepart alcohol. Once you have secured the component on a jig, apply a little acetone on the
top layer and then scrub the surface with a brush or a cotton swab. If the component is
counterfeit, any false information (printed on the top) along with the blacktopping will be
removed, revealing the original component details.

6. Check component size and pin specifications
Check the size of the component by
measuring the length, width and height and
compare these to the information provided
in the datasheet. If the measurements do
not match or if there are variances across
the same batch of parts received, then
further detailed investigation will be
required (which can be performed with an XRay).
In addition, check the alignment of the pins
to make sure they are evenly spaced from
each other, especially the distance between
pins, if necessary. Specifications can also
be found in the datasheet.
Check the exposed metal of the component
pins as well. If the component is authentic,
the exposed metal will be clean and free
from oxidation. The pins must be uniformly
shaped and should be free from any marks
on its surface. The pins should be silver in color but with a little dim finish. Pins on
counterfeit components are often bright and glossy in appearance.

D–4

APPENDICES

This appendix contains extracted pages from the Intel486 DX Microprocessor Databook dated
July 1992 related to the boundary scan architecture of the chip. The content and trademarks
are copyright of Intel Corporation and are provided 'as is' for reference only.

PCB Diagnostics

E–1

Appendix E

E–2

APPENDICES

486DX2 Databook (Partial)

PCB Diagnostics

E–3

Appendix E

E–4

APPENDICES

486DX2 Databook (Partial)

PCB Diagnostics

E–5

Appendix E

E–6

APPENDICES

486DX2 Databook (Partial)

PCB Diagnostics

E–7

Appendix E

E–8

APPENDICES

486DX2 Databook (Partial)

PCB Diagnostics

E–9

Appendix E

E–10

APPENDICES

486DX2 Databook (Partial)

PCB Diagnostics

E–11

Appendix E

E–12

APPENDICES

–A–

Active Component
A device that requires an external source of power to operate upon its inputs. Examples
of active devices are transistors, rectifiers, diodes, amplifiers, oscillators, mechanical
relays, etc.

Analog Circuit
An electrical circuit that provides a continuous quantitative output as a response from
its inputs, which can be digital, analog, or mixed signal.

ATLAS
Abbreviated Test Language for All Systems. Originally developed by Aeronautical Radio,
Incorporated (ARINC) and standardized under ANSI/IEEE-Std-416 and released on
December 22,1983. Its purpose was to serve as a standard programming language for
testing and maintenance of electronic systems for military and commercial aerospace
applications. The language was designed to be platform-independent.

Automated Optical Inspection (AOI)
Automatic laser or video inspection of traces and pads on the surface of inner layer cores
or outer layer panels. The machine uses CAM data to verify copper feature positioning,
size and shape. Instrumental in locating open traces, missing features or shorts.

Automated Test Equipment (ATE)
An equipment that automatically tests and analyzes functional parameters to evaluate
performance of the tested electronic devices (UUT).

Automated X-ray Inspection (AXI)
A technology based on the same principles as AOI but uses X-rays as its source instead
of visible light to automatically inspect features which are typically hidden from view. It
is used on PCBs containing components with leads that are not accessible and visible,
such as BGAs and CSPs.

PCB Diagnostics

F–1

Appendix F

–B–

Ball Grid Array (BGA)
A flip-chip type of package in which the internal die terminals form a grid style array, and
are in contact with solder balls ( solder bumps ), which carry the electrical connection to
the outside of the package.

Bare Board
A finished printed circuit board (PCB) that has no components mounted yet.

Bed of Nails (BON)
A test fixture consisting of a frame and a holder that houses a field of spring loaded pins
that make electrical contact between a planar test object (such as a PCB) and interfaces
to the test resources of a tester via its test bed or panel.

Bill of Materials (BOM)
A list of components included on a PCB assembly that includes reference designators
and descriptions for the components used to uniquely identify each part.

Boundary Scan Test (BST)
Also known as JTAG (named after the Joint Test Action Group which codified it). A test
technique that utilizes the IEEE-1149 standard to exercise the functionality embedded
within certain components of a PCB which conforms to the daisy-chain scheme.

Built-In Self-Test
An electrical testing method that enables a device to test its own functionalities with
specific hardware built into its original design purpose.

–C–
Chip-Scale Package (CSP)
A type of integrated circuit package in which The die may be mounted on an interposer
upon which pads or balls are formed (e.g., flip-chip BGA), or the pads may be etched or
printed directly onto the silicon wafer, resulting in a package very close to the size of the
silicon die (e.g., wafer-level package).

Conformal Coating
An insulating and protective coating that conforms to the configuration of the object
coated and is applied on the completed board assembly, to protect against corrosive or
harsh operating environmental elements.

F–2

APPENDICES

Glossary

Continuity
An uninterrupted path for the flow of electrical current in a circuit.

–D–
Date Code
Marking of products to indicate their date of manufacture. ACI standard is WWYY (weekweek-year-year).

Die Bonding
The attachment of an IC chip to a substrate.

Dual-in-line Package (DIP)
The most common through-hole IC package with two parallel rows of pins extending
perpendicularly out of a rectangular plastic housing. The overall dimensions of a DIP
package depend on its pin count.

–F–
Fiducial Mark
A printed board feature that is created in the same process as the conductive pattern to
provide a common measurable point for component mounting with respect to a land
pattern or several land patterns.

Fine Pitch
Refers to chip packages with lead pitches below 50 mils. It is more commonly referred
to surface-mount components with a lead pitch of 25 mils or less.

Flying Probe Test (FPT)
A type of electrical test machine that uses probes mounted on mechanical arms to locate
and touch the pads and component leads on the board. The probes move quickly across
the board verifying electrical characteristics of one or multiple nets.

FR-4
The most commonly used PCB board material. 'FR' stands for 'Flame Retardant' and '4'
means woven glass reinforced epoxy resin.

Functional Test (FCT)
A test technique in which the functionalities and electrical performance of the PCB is
checked. The entire PCB assembly is tested rather than individual components.
PCB Diagnostics

F–3

Appendix F

–G–
Ground
A common reference point for electrical circuits that provides current returns, shielding
or heat sinking. The three most common types are earth ground, chassis ground, and
signal ground. Two additional types of grounds are floating ground (isolates electronic
circuits from the mains) and virtual ground (a steady reference potential in operational
amplifiers).

–I–
In-Circuit Test (ICT)
A test technique that involves measuring and testing all the components individually on
the PCB. The tests are usually grouped under power off and power up checks. This test
method helps identify defects such as open or short circuits, missing, misoriented, wrong
or faulty components.

–K–
Known Good Board (KGB)
Also known as a golden board. Refers to a PCB or assembly that is verified to be free of
defects and used as a test standard.

–L–
Layout Diagram
A mechanical diagram of a PCB that indicates the nomenclature, orientation and location
of its components.

–N–
Net
A collection of terminals all of which are, or must be, connected electrically. Also known
as signal.

Netlist
List of names of symbols or parts and their connection points which are logically
connected in each net of a circuit.

–O–
Open: Open Circuit
An unwanted break in the continuity of an electrical circuit which prevents current from
flowing.

F–4

APPENDICES

Glossary

–P–
Plastic Leaded Chip Carrier (PLCC)
A fine-pitch SMT package that is rectangular or square with J- leads on all four sides.

Plated Through Hole (PTH)
A hole with the plated copper on its sides to provide electrical connections between
conductive patterns at the levels of a printed circuit board.

–Q–
Quad Flat Pack (QFP)
A fine-pitch SMT package that is rectangular or square with gull-wing shaped leads on
all four sides.

–R–
RoHS
Restriction of Hazardous Substances. A European legislation to strictly curtail the use of
cadmium, hexavalent chromium, and lead in all products from automobiles to consumer
electronics.

–S–
Schematic Diagram
A wiring diagram which shows by means of graphic symbols the electrical connections
and functions of a specific circuit arrangement. Device symbols can conform to ANSI,
IEC, or DIN standard.

Short: Short Circuit
An abnormal connection of relatively low resistance between two points of a circuit. The
result is excessive and often damaging current between these points.

Small Outline Integrated Circuit (SOIC)
An integrated circuit with two parallel rows of pins in surface mount package, similar to
that of the dual-in-line package. The overall dimensions of a SOIC package depend on
its pin count.

–T–
Test Fixture
Also known as a test jig. A device that interfaces between a test equipment and the unit
under test, which can either be straight-wired or contain additional power sources and
signal conditioning circuits.
PCB Diagnostics

F–5

Appendix F

Test Program Set (TPS)
A collection of hardware, software and documentation used for testing a PCB assembly
or electronic module. The hardware basically comprises a test fixture, interface cables
and add-on devices (attenuator, terminator, etc.), if necessary. The software is the test
program that controls specific test resources to verify the functionalities of the unit under
test, and may include a self-test for the hardware. Documentation comprises operator
instructions on setting up and running the test programs, and may include additional
information related to the unit under test (schematic diagram, parts list, etc.).

–U–
UUT
Unit Under Test. A PCB or electronic module that is the subject of automated or manually
operated testing.

–V–
V-I Signature Test
Also known as analogue signature test. A power-off test technique in which a currentlimited AC sinewave is applied across two points of an electronic component or circuit.
The resulting current/voltage waveform is shown as a signature display using vertical
deflection for current and horizontal deflection for voltage. This unique analog signature
represents the overall health of the part being analyzed. By comparing signatures of a
known good circuit board to those of suspect boards, faulty nets and components can
be quickly identified.

F–6

APPENDICES

About The Author
NG KENG TIONG is an engineer turned writer with a passion to
share his knowledge and experience of over 30 years in
electronics in the field of PCB-RE, testing and repair. He formerly
worked as a Principal Engineer at Singapore Technologies (ST)
Electronics Limited, a subsidiary of ST Engineering. Upon
graduation from the Singapore Polytechnics, he signed up with
the Republic of Singapore Air Force (RSAF) as an aircraft
technician and worked in the E-2C Hawkeye repair bay,
servicing the aircraft's avionics using automated test systems
(CAT-IIID and RADCOM) and in-house test equipment.
Upon invitation, he left the RSAF after his first contract and joined the home-grown
defense industry, writing test programs and doing PCB diagnostics on Factron S700
series testers. He then went on to work on other test platforms such as the Teradyne
Spectrum 8800 series, the Westest DATS/2000 test station, and some special-to-typeequipment (STTE) of similar nature. He also has experience in logic simulation using
the HHB Systems CADAT software and CATS-10000 hardware modeler, as well as
Teradyne's LASAR simulator.
Through the course of his work, he encountered many printed circuit boards and
electronic modules without schematic diagrams or documentation. That started him
on the journey of doing PCB reverse engineering, in part or total, to perform the
necessary troubleshooting for repair. Over time, he has refined the skill into an art and
re-produced it into a series of books (see overleaf).