CRC Electronic Components and Technology User Manual

Electronic Components and Technology

THIRD EDITION

TUTORIAL GUIDES IN ELECTRONIC ENGINEERING

Series editors

Professor G. G. Bloodworth, University of York Professor A. P. Dorey, University of Lancaster Professor J. K. Fidler, University of Northumbria

This series is aimed at rst- and second-year undergraduate courses. Each text is complete in itself, although linked with others in the series. Where possible, the trend toward a “systems” approach is acknowledged, but classical fundamental areas of study have not been excluded. Worked examples feature prominently and indicate, where appropriate, a number of approaches to the same problem.

A format providing marginal notes has been adopted to allow the authors to include ideas and material to support the main text. These notes include references to standard mainstream texts and commentary on the applicability of solution methods, aimed particularly at covering points normally found difcult. Graded problems are provided at the end of each chapter, with answers at the end of the book.

Electronic Components and Technology

THIR D ED ITIO N

Stephen Sangwine

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Preface to the Second Edition vii

Preface

Acknowledgments

Author

1 Introduction 1

2 Interconnection technology 5

Jointing Discrete wiring Cables Connectors Printed circuits Printed circuit assembly Rework and repair Case study: A temperature controller Summary 29 Problems 30

3 Integrated circuits 31

Review of semiconductor theory Integrated-circuit fabrication Semiconductor packaging Handling of semiconductor devices Custom integrated circuits Summary 50 Problems 51

xiii

11 12 15

18 24 25 26

33 35 43 45 46

4 Power sources and power supplies 53

Energy sources Batteries Power supplies Summary 74 Problems 74

5 Passive electronic components 77

Passive component characteristics Resistors Capacitors Inductors Summary 92 Problems 93

54 55 60

77 83 87

6 Instruments and measurement 95

Quantities to be measured Voltage and current measurement Frequency and time measurement Waveforms — The oscilloscope Summary 109 Problems 110

7 Heat management 111

Heat transfer Thermal resistance Heat sinking Forced cooling Advanced heat-removal techniques Summary 121 Problems 122

8 Parasitic electrical and electromagnetic effects 123

Parasitic circuit elements Distributed-parameter circuits Electromagnetic interference Applications studies Summary 149 Problems 150

96 98

101

104

112 113 114 119

120

123 128 133 142

9 Reliability and maintainability 153

Failure The “bathtub” curve Measures of reliability and maintainability High-reliability systems Maintenance Summary 168 Problems 168

10 Environmental factors and testing 171

Environmental factors Type testing Electronic production testing Summary 184 Problems 185

11 Safety 187

Electric shock Other safety hazards Design for safety Summary 198

References 201

Answers to problems

154 155 156 162 165

171 178

180

188 194 195

203

Index

205

Preface to the Second Edition

This book is intended to support Engineering Applications studies in electronic engineering and related subjects such as computer engineering and communications engineering at rst- and second-year undergraduate level. Engineering Applications, abbreviated as EA, is a term rst used in the report of the Finniston inquiry into the future of engineering in the United Kingdom. Finniston used the terms EA1 and EA2 to refer to the rst and second elements of a four-stage training in Engineering Applications, to be taken as part of a rst-degree course in engineering. Later, the Engineering Council, established as a result of the Finniston report, expressed the concept of EA1 as

An introduction to good engineering practice and the properties, behaviour, fabrication and use of relevant materials, systems and components.

and EA2 as

Application of scientic and engineering principles to the solution of practical problems of engineering systems and processes.

Although EA studies should be integrated into the fabric of a degree course, there is a need to draw out elements of practice to provide emphasis. It is intended that this book should be used as a source, complementing other texts, for such studies.

In the context of electronics, product design is an activity that begins,

by and large, with components rather than materials. This is not to say that a study of materials is not relevant as a part of EA1, but as there are many existing texts covering the subject, materials has been excluded from this book in favour of more coverage of components.

The book begins with an introduction to electronic interconnection technology including wiring, connectors, soldering and other jointing techniques, and printed circuits. Chapter 3 is devoted to the very important technology of integrated circuits, concentrating on their fabrication, packaging, and handling. Components is taken to include power supplies, as in many applications a power supply unit is bought-in as a subsystem. The main characteristics of power supplies and batteries are covered in Chapter 4. Passive electronic components are introduced in Chapter 5, and with them the book begins to include a major theme developed in Chapters 7 and 8: the parasitic effect. This includes the nonideal properties of passive components introduced in Chapter 5, heat and its management in Chapter 7, and parasitic electromagnetic effects in Chapter 8. EA1 is essentially practically oriented and will include

These quotations are taken

These quotations are taken from Standards and routes to

from Standards and routes to

registration (second edition),

otherwise known as SARTOR,

otherwise known as SARTOR, published by the Engineering

published by the Engineering Council in January 1990. They

Council in January 1990. They are reproduced here with the

are reproduced here with the permission of the Engineering

permission of the Engineering Council, United Kingdom.

Council, United Kingdom.

vii

laboratory-based work, including the use of tools and instruments. A new chapter has been added to the second edition to add to the utility of the book in supporting EA1 studies and laboratory activities. Thus, Chapter 6 introduces the instruments and measurements used in electronics and related subjects. Chapter 9 reviews good engineering practice in relation to reliability and maintainability, two important aspects of design which, unfortunately, are often overlooked by electronic circuit designers. Chapter 10 introduces environmental inuences on electronic products and the subject of testing both for environmental endurance and in production. The nal chapter in the book introduces safety.

The book assumes that the reader has taken the rst one or two terms of a degree course, although some of the earlier material could be studied sooner. Extensive cross-references to more specialized texts have been given in the marginal notes and the bibliography, including, where appropriate, references to other texts in the Tutorial Guides Series. These have been updated for the second edition to include later books in the series where appropriate, the latest editions of technical standards, new editions of books previously listed, and some additional books published since 1986. Other major revisions in the second edition include updated information in Chapter 3 to take into account changes in IC technology since 1986, changes to the nal chapter to take into account new legislation, and some new illustrations.

Part of the aim of this book is to inform the reader about components, technology, and applications, but it is also intended to create an awareness of the problems of electronic engineering in practice. I hope that the readers of this book will be encouraged to tackle these problems and go on to become competent and professional electronics engineers.

Safety Note

The material in Chapter 11 is of course at an introductory level only, and readers are cautioned that professional competence in safe electrical design cannot be achieved merely by studying the contents of this chapter.

viii

Preface

Since the publication of the second edition of this book in 1994, some very signicant changes have occurred in technology, particularly the further miniaturization of electronic products, and the steady and quite dramatic increases in the speed of computers. In electronics, surfacemount technology has become almost universal and vastly better batteries have been developed for laptop computers and portable phones. Technically, however, electronics has not changed in revolutionary ways. When revising the book for this third edition, it was a surprise to discover that components and technologies featured in the second edition were still commercially available. Nevertheless, the revisions needed after 12 years were extensive, but they did not require the text to be restructured. Examples of the areas that needed updating are: the introduction of lead-free solders and digital oscilloscopes, and new types of batteries. The bibliography has been brought up to date, and all references to technical standards, European Union directives, and the like have been checked and, where necessary, updated.

The book has now been in print for 19 years. The previous editions were published in the United Kingdom and largely written for a British audience. In revising the book for this third edition, the opportunity has been taken to make the book more usable elsewhere in the Englishspeaking world, by small changes in terminology and vocabulary, and by reference to international standards, rather than British Standards, where applicable.

This new edition was prepared electronically, which should make it easier to update at reprinting if the publishers wish to do so. Therefore, please contact me with any corrections or suggested amendments at S.Sangwine@IEEE.org.

Stephen J. Sangwine

Colchester, United Kingdom

Acknowledgments

The rst edition of this book developed from a lecture course that I rst presented in 1985 as a part of new EA material introduced into engineering courses at the University of Reading. I would like to thank my former colleague Peter Atkinson for his early suggestion that a book could be written and for his help and encouragement while I wrote the book and subsequently. I would also like to thank S. C. Dunn, former chief scientist at British Aerospace, who made many helpful suggestions at an early stage, including the theme of parasitic effects. The following people contributed advice, criticism, or technical background during the preparation of the rst edition, and I wish to acknowledge their help: Susan Partridge of General Electric Company (GEC) Hurst Research Centre for reading the rst draft of Chapter 3; Alistair Sharp of Eurotherm Ltd. for help with the case study in Chapter 2; John Barron of Tectonic Products, Wokingham, for help with the subject of printed circuits; John Terry of the Health and Safety Executive and Ken Clark, deputy director at Baseefa, for both help and criticism of Chapter 10 (now Chapter 11); Dr. George Bandurek at Mars Electronics at Winnersh for commenting on Chapter 8 (now Chapter 9); and Martin Thurlow of the Electromagnetic Engineering Group, British Aerospace, and David Hunter of the Army Weapons Division, British Aerospace, for assisting with illustrations and background. I am also grateful to Carole Hankins for typing the manuscript and to my wife, Elizabeth Shirley, for her support over many months of writing and revising. Professor A. P. Dorey was consultant editor for this book and made many helpful comments on drafts of the manuscript.

During revision for the second edition, the following people contributed advice and I wish to acknowledge their help: Trevor Clarkson of King’s College for suggesting the addition of measurement; John Whitehouse for suggestions in Chapter 8; and John Terry (again) for reading the former Chapter 10 and suggesting numerous updates, which I have incorporated into Chapter 11. I would also like to thank Charles Preston for helping with the illustrations, particularly for Chapter 6. Professor A. P. Dorey was, once again, consultant editor and made helpful comments on drafts of the new material.

During revision for the third edition, the following people assisted with illustrations and I wish to acknowledge their help: Mark Johnson of Megger Limited; Dr. Ursula Kattner of National Institute of Standards and Technology (NIST) for help with lead-free solders; Stephen Head and Dave Bremner of Eurotherm Ltd.; Marcus Brain of Technisher Überwachungsverein (TÜV) Product Services; Steve Smith of Schaffner Limited; R. J. Sullivan of Aavid Thermalloy; Jeff Weir, Naomi Mitchell, and Cole Reif of National Semiconductor; Paul Bennett of Bulgin Components; Greg Macdonald of Amphenol Canada and Gilles Dupre

of Amphenol-Socapex, France; Natasha Moore of BEAB–ASTA (the British Electrotechnical Approvals Board and Association of Short-Circuit Test Authorities); Yuko Takahashi of Fujitsu Limited; and Gary Silcott of Texas Instruments.

Several companies and organizations have supplied illustrations or granted permission for me to use their copyright material, and they are acknowledged in the text. Permission to reproduce extracts from BS-EN 61340-5-1: 2001 is granted by the British Standards Institution (BSI). British Standards can be obtained from BSI Customer Service, 389 Chiswick High Road, London W4 4AL, United Kingdom; Telephone: +44 (0)20 8996 9001. The author thanks the International Electrotechnical Commission (IEC) for permission to reproduce information from its International Technical Specication IEC 60479-1, Fourth Edition (2005) and from its International Standard IEC 61340-5-1, First Edition (1998). All such extracts are copyright of IEC, Geneva, Switzerland. All rights reserved. Further information on the IEC is available from www. iec.ch. IEC has no responsibility for the placement and context in which the extracts and contents are reproduced by the author, and IEC is not in any way responsible for the other content or accuracy therein.

xii

Author

Stephen J. Sangwine was born in London, United Kingdom in 1956.

He has a B.Sc. degree in electronic engineering from the University of Southampton, United Kingdom (1979), and a Ph.D. degree from the University of Reading, United Kingdom (1991). He is currently a senior lecturer with the Department of Electronic Systems Engineering at the University of Essex, Colchester, United Kingdom. From 1985 to 2000, he was a lecturer with the Department of Engineering at the University of Reading, where he wrote the rst and second editions of Electronic Components and Technology. From 1979 to 1984, he worked in the civilian nuclear power industry at the United Kingdom Atomic Energy Authority’s Harwell Laboratory, designing radiological monitoring instruments, including one of the very earliest applications of complementary metal-oxide semiconductor (CMOS) microprocessors to a pocket-sized instrument.

As well as authoring Electronic Components and Technology, Dr. Sangwine coedited The Colour Image Processing Handbook (Chapman & Hall, 1998), and he has also authored or coauthored more than 70 papers, the majority in the eld of image processing.

His principal research interest is in linear vector ltering and transforms of vector signals and images, especially using hypercomplex algebras, on which he collaborates with researchers in the United States and France. In 2005, he was a chercheur invité (visiting researcher), Centre National de la Recherche Scientique (CNRS), at the Laboratoire des Images et des Signaux, Grenoble, France, for 7 months, with nancial support from the Royal Academy of Engineering, United Kingdom.

Dr. Sangwine has been a senior member of the Institute of Electrical and Electronics Engineers (IEEE) since 1990.

xiii

Introduction

Modern electronic engineering products are found in a wide range of applications environments from the oor of the deep ocean (submarine cable repeaters) to geostationary orbit (microwave transceivers on board communications satellites), from the factory oor (industrial process controllers and numerically controlled machine tools) to the ofce (computers and printers). They can be found in the home (audio and video systems and microwave ovens), in schools (computers and pocket calculators), in hospitals (computerized tomography [CT] and magnetic resonance imaging [MRI] scanners, bedside monitors), inside the human body (heart pacemakers), and inside road vehicles (electronic ignition and engine management, antilock braking). Electronic products can also be found in the pocket (portable phones, personal audio, and video players). These products may be mass produced by the million, or they may be one-off special systems. They may be intended to last for decades, or they may be designed deliberately for a fairly short life. They should all be t for their intended purpose and be of signicant use to their users.

All electronic products depend on the physical and electrical properties of insulating, conducting, and especially semiconducting materi-

als, but by and large, the designer of an electronic product works with components and technologies, such as integrated circuit (IC) tech-

nology, rather than with basic materials. A critical aspect of product design is the interconnection of components, and for this reason this book starts with a chapter covering the technology of interconnection. The technology of interconnecting electronic components, circuits, and subsystems was, until the publication of this book, often neglected in electronic engineering texts at degree level. It is true that the detailed layout of a printed circuit board (PCB) is not a task likely to be undertaken by a graduate engineer unless the PCB is to carry high-frequency or high-speed circuitry. Nevertheless, a PCB has electrical properties and its design, together with the choice of components to go on it, can have a signicant effect on the performance, the cost of production, the production yield, and the reliability and maintainability of the assembled board, and quite likely the product of which it is a part. Jointing techniques, especially soldering, are of tremendous importance in electronic engineering, and solder is an engineering material that should be specied as carefully as a mechanical engineer species structural steel: what type of solder is best suited to a particular application? In many cases, just “solder” will not do.

The third chapter deals with IC technology. Only a few engineers are involved in high-volume IC design, but a more signicant number design or use semicustom ICs. Consequently, the treatment in this book is not for the IC specialist: it is aimed at the much larger group of electronics engineers who will be using ICs or designing a gate-array or standardcell IC of their own.

Competent electronics engineers need a good understanding of the components and subsystems from which their designs will be constructed and the instruments needed to test and characterize prototypes. They must be aware of not only the ideal behaviour of components, subsystems, and instruments, but also their performance limitations. The next three chapters, therefore, cover power sources and power supplies (an important class of electronic subsystem), passive electronic components, and instruments and measurement. To understand the performance limitations of components, an engineer must appreciate how the components are fabricated. To understand the performance limitations of power supplies and instruments, an engineer must appreciate the principles on which they operate. Chapters 5 and 6 cover these topics as well as provide factual information for reference.

The study of electronic components introduces the third major theme of this book: the parasitic effect. Real electronic components and circuits, as opposed to ideal ones, possess parasitic properties that are incidental to their intended properties. A wire-wound resistor, for example, is also inductive and has an impedance that varies with frequency. Heat is produced in signicant quantity in some electronic systems, and positive design measures often have to be taken to remove it. Electromagnetic energy can radiate from electronic circuits and couple into other circuits, causing faulty operation. A chapter has been devoted to this and other parasitic electromagnetic effects. This book does not attempt to cover all possible parasitic effects: to do so would be impossible even in a much larger book and would serve little useful purpose. Electronics engineers must learn to expect parasitic effects and try to take them into account when designing electronic products.

So far this introduction has dealt with matters that affect design and performance in ways that are important at the beginning of the life of a product. Without an understanding of components, technology, and parasitic effects, the design engineer will not be able to design good products that meet the required level of performance at the required cost. Many electronic products, however, will have a life that lasts far longer than the designer’s interest in the design. It is during the operating life of a product that long-term effects become important. Components and materials age: they deteriorate physically and chemically, and ultimately they fail. The study of these problems and of the prediction of product life is known as reliability. Not surprisingly, the reliability of a product can be inuenced by its design, for better or for worse, and if a product is capable of being repaired, the ease and expense with which it can be restored to working order can also be affected by decisions taken at the design stage. Reliability can also be inuenced by a product’s operating environment. Did the designers consider the effects of temperature, humidity, corrosion, and dust? Is there some unknown environmental factor that will doom their product to early failure? As with parasitic effects, after introducing some of the many environmental hazards to electronic equipment, this book leaves the readers to consider what the problems of their products’ environment might be.

Lastly, this introduction has dealt with the electronic product itself: will it work and continue to work for long enough? Will it succumb to environmental stress? Engineers must also look at their designs from

another viewpoint: will they do anyone, or the environment, any harm? All design engineers, including those working in electronic engineering, have a professional duty to consider safety when designing products, and in many countries a statutory (that is, legal) duty also. The nal chapter introduces the subject of safety in electronic engineering.

Interconnection technology

Objectives

To emphasize the importance of interconnection in electronic

□

product design. To discuss jointing technology, especially soldering and solderless

□

wire-wrapping. To outline the main types of discrete wiring and cabling.

□

To describe the technology of printed circuits.

□

To give an introduction to rework techniques.

□

To present a short case study illustrating the importance of

□

interconnection in industrial product design.

All except the smallest of electronic systems are built up from subsystems or subassemblies that are in turn built from electronic components such as resistors, capacitors, transistors, integrated circuits (ICs), displays, and switches. A desktop personal computer, for example, is likely to be built from a power supply subsystem, a main circuit board, and a number of peripheral subsystems such as a CD/DVD drive and plugin memory modules. Small self-contained electronic products such as pocket calculators and portable phones are often built directly from components with no identiable subsystems.

From the lowest level of component up to the system level, the constituent parts of an electronic system have to be interconnected electrically. The lowest level in the hierarchy of interconnection is the electrical joint. From the very earliest days of electronics, long before the invention of the transistor and integrated circuit, soldering has been an important technique for making electrical joints. Hand soldering is still used in prototype work, repair work, and, to a much lesser extent, production. Not all electrical joints in an electronic system need to be soldered: the technology of solderless wire-wrapping is well established in digital electronics, for both prototype and production wiring, and joints can also be made by insulation displacement, welding, or crimping. Components and subsystems are interconnected by wiring that can be in the form of either discrete wires and cables or printed circuits. A short case study at the end of this chapter illustrates the trend in electronic engineering over the last 20 years towards printed circuit interconnection wherever possible, avoiding discrete wiring because of the high cost of assembling and inspecting individual wires.

The printed circuit board, or PCB, is tremendously important in almost all applications areas of modern electronics. Not only does it provide a cheaply mass-produced means of interconnecting hundreds or thousands of individual components, but it also provides a mechanical mounting for the components.

In the very early days of

In the very early days of electronics when thermionic

electronics when thermionic (vacuum tube) valves were used,

(vacuum tube) valves were used, interconnection with discrete

interconnection with discrete wiring was normal — but this

wiring was normal — but this was soon superceded with the

was soon superceded with the advent of transistors by the

advent of transistors by the introduction of printed circuit

introduction of printed circuit boards.

boards.

Copper Copper

Before soldering

Air gap

Solder

After soldering

Figure 2.1 Diagrammatic representation of a soldered joint (not to scale).

Jointing

Several techniques are used in electronic engineering for making electrical joints. The most important of these is soldering, which is used mainly for attaching and jointing components to PCBs, but it is also widely used for jointing in cable connectors. Another important technology is solderless wire-wrapping, which nds application in prototype wiring for logic circuits and in production wiring of backplanes interconnecting PCBs. Welding is used in some specialized electronic applications and is a very important jointing technique in integrated circuit manufacture. Finally, in applications where soldering or welding cannot be used, mechanical crimping can make sound electrical joints.

Soldering

Solder is a low-melting-point alloy of tin and other metals, used for making electrical and mechanical joints between metals. Soldering does not melt the surfaces of the metals being joined, but adheres by dissolving into the solid surface. Figure 2.1 shows an idealized cross-section of a soldered joint illustrating this point. Soldered joints can be made by hand using an electrically heated soldering iron or by a mass-soldering process in which all the joints on a PCB are made in one automated operation. Both techniques are important, and they are described in detail below and in a later section of this chapter.

In the twentieth century, solders used in electronics were almost always alloys of tin and lead. From July 2006, the European Union (EU) has required lead (and many other toxic substances) to be eliminated from electronic and other products, and therefore lead-free solders are now used in place of tin–lead solders. Because of the historical importance of tin–lead solder in electronics, we start with its properties before progressing to the characteristics of lead-free solders.

Commercial tin–lead solders were available with several different proportions of tin to lead and with traces of other metals to enhance their properties. Tin and lead are soft metals with melting points of 232°C and 327°C respectively. Alloys of these metals generally start to melt at a temperature of 183°C, which is lower than the melting temperature

100

0:100 20:80 40:60

Tin:lead ratio

(by weight)

Liquid

Plastic

19%

tin

63:37 97%

tin

Solid

60:40 80:20 100:0

232

200

300

327

400

183

°C

Figure 2.2 Phase diagram for tin–lead solder alloys (simplied).

of either pure metal. Figure 2.2 shows a simplied phase diagram for tin–lead alloys. The ratio (by weight) of tin to lead is plotted horizontally with 100% lead on the left and 100% tin on the right. The vertical scale represents temperature. In the top region of the diagram, above the line extending from the melting point of lead at 327°C across to the melting point of tin at 232°C via the point at a tin:lead ratio of 63:37 and a temperature of 183°C, the alloys are liquid. The bottom region of the diagram represents the ranges of temperature and tin:lead ratio over which the alloys are solid. The two triangular regions represent temperatures and compositions where the alloys are in a plastic state consisting partly of solid and partly of liquid. The alloy with a tin:lead ratio of 63:37 is the only one that changes sharply from solid to liquid at a single temperature. This alloy is known as a eutectic alloy. It is fully liquid at the lowest possible temperature for a tin–lead alloy.

For general electronic jointing, a 60:40 solder was used, which is fully molten at about 188°C. 40:60 solder, which is fully molten at about 234°C, was also readily available for applications where a higher melting point was needed.

The most common lead-free solder for use in electronics is an alloy of tin (Sn), silver (Ag), and copper (Cu) in proportions of about 95 to 96% tin, 3 to 4% silver, and 0.5 to 1% copper. Alloys of this composition have a melting point of around 215 to 218°C, which is somewhat higher than the melting point of a 60:40 tin:lead solder. Figure 2.3 shows part of the phase diagram for the tin–silver–copper alloys (the full phase diagram takes the form of an equilateral triangle, but since a practical alloy for electronic soldering has 95 to 96% tin, only one corner of the full diagram is shown). Temperature is shown by contour lines, since to plot them would require a third dimension, out of the page. The higher melting point of lead-free solders means that more care has to be taken to control soldering processes, since the higher temperatures could more easily cause thermal damage to electronic components.

Four requirements must be met if a good soldered joint is to be made, and an understanding of these is essential in order to develop skill at hand soldering. First, the surfaces to be joined must be solderable. Not

Phase diagrams in general and

Phase diagrams in general and the tin–lead phase diagram

the tin–lead phase diagram in particular are discussed by

in particular are discussed by Anderson et al. (1990).

Anderson et al. (1990).

The proportion of tin (by weight)

The proportion of tin (by weight) is conventionally stated rst.

is conventionally stated rst.

PCBs sometimes incorporate

PCBs sometimes incorporate a solderability test pad in an

a solderability test pad in an unused corner of the board,

unused corner of the board, so that the solderability of the

so that the solderability of the board or a batch of boards can

board or a batch of boards can be checked before component

be checked before component assembly.

assembly.

Mercury is toxic. Do not try this

Metal

terminal

Globular solder with no “wetting”

of surfaces

Wire

Correctly “wetted” solder joint

1 mm (typ.)

Solder wire with

integral ﬂux cores

Metal

terminal

Globular solder with no “wetting”

of surfaces

Wire

Correctly “wetted” solder joint

1 mm (typ.)

Solder wire with

integral ﬂux cores

0.5 1.0 1.5 2.0 2.5 3.0

226

222

320

(Sn)

Ag3Sn

Mass % Ag

Mass % Cu

Mercury is toxic. Do not try this experiment.

experiment.

Figure 2.3 Partial phase diagram for tin–silver–copper lead-free solder alloys. (Courtesy of Dr. Ursula Kattner, National Institute of Standards and Technology [NIST], United States.)

all metals are readily soldered without special techniques. Aluminium, for example, is an extremely reactive metal that rapidly oxidizes on exposure to air to form a passivating oxide layer that prevents solder from alloying with the underlying metal. It is possible to demonstrate the reactive nature of aluminium by removing the oxide layer with a little mercury rubbed onto the aluminium surface. The freshly exposed surface reacts rapidly with moisture in the air, and the metal becomes hot. Gold is very easily soldered because the metal does not oxidize. Copper and brass oxidize readily, but the oxide layer can be removed easily, so that these metals are solderable. The second requirement for a good soldered joint is cleanliness: the surfaces to be joined must be free of grease, dust, corrosion products, and excessively thick layers of oxide. In most electronics applications, the surfaces to be soldered will have been plated with gold or a solder alloy during manufacture in order to provide a readily solderable surface. Coating a metal with solder is known as tinning, and protects the metal from oxidation. Cleaning is not usually needed, therefore, unless a component or PCB has been stored for a long time or becomes contaminated with grease or dirt. The third requirement is that any layer of oxide on the surfaces must be removed during soldering and prevented from regrowing until the molten solder has alloyed with or “wetted” the surface. This is achieved by a ux that chemically removes the oxide layer and reduces surface tension, allowing molten solder to ow easily over the surfaces to be joined.

For hand soldering, solder wire with integral cores of ux is used. The uxes commonly used for electronic work are rosin based and chemically mild, leaving a noncorrosive residue. More powerful uxes based on acids may be needed on less solderable metals, but must be thoroughly cleaned off afterwards because they leave a corrosive residue. The nal requirement for a good soldered joint is heat. Both surfaces

to be joined must be heated above the solidication temperature of the

Wire and terminal

cut into each other

at pin corners

One or two turns

of insulation

provides strain

relief

About seven turns of bare

wire evenly and

closely spaced

solder, otherwise the solder will chill on contact and will fail to ow evenly and alloy with the surfaces. When soldering by hand, heat transfer from the soldering iron to the joint is improved if there is a little molten solder on the tip of the soldering iron. The joint should be heated with the iron, and the solder wire applied to the joint (not the iron). The iron should not be removed until the solder has owed into the joint. It is very important that the joint is not disturbed until the solder has fully solidied, otherwise a high-resistance (dry) joint will result from mechanical discontinuities in the solder.

Wire-wrap jointing

Soldering is a good technique for mass jointing on PCBs, but it has several disadvantages for discrete wiring joints. An alternative jointing technique exists for logic circuits and low-frequency applications known as solderless wire-wrap. Special wire and terminal pins are used for wire-wrap jointing, and special hand-operated or electrically powered tools are required. Various types of IC sockets and connectors, including PCB edge connectors, are made with wire-wrap terminal pins. Wire-wrap interconnection is used for prototype and production wiring of logic boards and for production wiring of backplanes for interconnecting a rack of PCB subunits. It is a faster technique than soldering, requires no heat, produces no fumes, and can easily interconnect terminals spaced as little as 2.5 mm apart. Figure 2.4 illustrates a typical joint. The terminal pin is typically 0.6 to 0.7 mm square and 15 to 20 mm long. The wire is solid and about 0.25 mm in diameter. The joint consists of about seven turns of bare wire and one to two turns of insulated wire, wound tightly around the terminal pin. At each corner of the terminal pin, the wire and the pin cut into each other, making a metalto-metal connection, which improves with age through diffusion of the two metals. The wire is under tension and slightly twists the square pin. The insulated turns of wire act as a strain relief at what would otherwise be a weak point of the joint.

Wire-wrapping was rst

Wire-wrapping was rst developed at Bell Telephone

developed at Bell Telephone Laboratories, New Jersey, United

Laboratories, New Jersey, United States, circa 1950.

States, circa 1950.

Figure 2.4 A wire-wrap joint.

Oﬀset hole for wire

Centre hole to ﬁt

terminal pin

End view of a wire-wrap bit

Oﬀset hole for wire

Centre hole to ﬁt

terminal pin

End view of a wire-wrap bit

Tracy Kidder’s classic book

Wrong – “Daisy-chained” connections

Right

Tracy Kidder’s classic book

The Soul of a New Machine,

rst published in 1981, tells the

rst published in 1981, tells the story of a group of computer

story of a group of computer engineers working on a 1970s

engineers working on a 1970s minicomputer and testing a

minicomputer and testing a prototype machine wired with

prototype machine wired with wire-wrap technology.

wire-wrap technology.

Figure 2.5 Right and wrong ways of wire-wrapping a group of terminals.

The tools required for making wire-wrap joints are more expensive than those needed for making soldered joints, but the time saved in production work soon covers the cost of the tools. The wire is stripped using a tool that ensures that the correct length of insulation is removed. The stripped end of the wire is then inserted into the offset hole of a wrapping bit, which may be part of a hand-operated or powered tool. The centre hole of the wrapping bit is then slipped over the terminal pin, and the tool is rotated evenly to wrap the wire around the pin. Most wire-wrap terminal pins have enough length for up to three joints.

One of the very signicant advantages of wire-wrapping for prototype wiring is the ease and speed with which joints can be unwrapped to allow circuit modications. A special tool is required to unpick a joint, and any joints further out along the pin have to be unpicked rst. For this reason, when a group of pins are to be connected, each wire should be at the same level on both pins connected. Figure 2.5 shows the right and wrong ways of connecting a series of pins. In the “daisy-chained” arrangement, a change to one wire can often require many other wires to be unpicked and remade. Because of the cutting action of the wire on the corners of the terminal pins, there is a limit to the number of times that a pin can be rewired. The small number of modications likely to be needed to build a prototype are not likely to cause problems with poor joints, but continual reuse of wire-wrap IC sockets could be more trouble than the sockets are worth. The wire removed from an unpicked joint is not reusable, so in nearly all cases, both ends of a wire have to be unpicked. The unpicked wire should be carefully removed from the circuit and thrown away, taking care that fragments of stripped wire do not fall back into the circuit. An intermittent short circuit caused by a piece of loose wire can take a long time to diagnose.

One nal point about wire-wrapped connections is that there is little point in trying to arrange the wires into tidy bundles: direct point-topoint wiring can be easier to inspect and check, and minimizes problems with crosstalk.

Insulation displacement

Insulation displacement is a mechanical jointing technique in which an unstripped insulated wire is pushed between the sharp edges of a forked

terminal. The wire is thus held mechanically and the insulation is displaced by the cutting edges of the terminal, making an electrical joint. The two sides of the terminal are pushed slightly apart by the insertion of the wire and thus exert spring pressure on the conducting core of the wire. The technique is widely used for connecting discrete wires in the cabling of domestic and commercial telephone sockets, because it is fast and does not require soldering or power tools.

Welding

Welding is a jointing technique where two metal surfaces are placed in intimate contact and then fused together by melting both surfaces. Heat can be applied by a ame, thermal conduction, or electric heating. There are limited applications of welding in electronics, except in the bonding of ICs to their packages. External connections from bonding pads on an IC are made by attaching ne gold wires using either ultrasonic welding or thermocompression bonding, as described in the next chapter.

Crimping

Insulation displacement jointing

Insulation displacement jointing is also used in some types of

is also used in some types of cable connector, as discussed

cable connector, as discussed later in this chapter.

later in this chapter.

A fth jointing technique used in electronics (and much more extensively in electrical engineering for heavier currents) is crimping. A crimped connection is made by crushing a special terminal onto a wire of the correct size using a purpose-made tool. The wire is gripped mechanically by the crushed terminal. Electrical contact depends on the mechanical integrity of the joint. Unlike a wire-wrapped or soldered joint, the electrical connection is not gas tight and can therefore be prone to corrosion. Crimping is an especially useful technique for jointing unsolderable wires such as aluminium, and for rapid wiring assembly in production.

Discrete wiring

Modern electronic product designers tend to avoid using discrete wiring in favour of printed-circuit interconnection because of the high cost of hand jointing and the likelihood of errors. Some wiring is nearly always needed, however, especially in larger systems.

A wire is a single or multistranded conductor with or without insu-

lation, whereas a cable is a collection of wires or conductors bound together, possibly with some overall insulation or other protection. Equipment wire used in electronics is normally made of copper or tinned copper with either polyvinyl chloride (PVC) or polytetrauoroethylene (PTFE) insulation. PVC-covered wire can be used at up to 70°C. Above this temperature, PTFE-covered wire must be used. The size of a wire can be stated either as a cross-sectional area (in mm2) or as the number of strands followed by the diameter of each strand. Thus “7/0.2” represents seven strands of 0.2 mm diameter. IEC 60228 species standard wire cross-sections in mm2.

For many electronic purposes, the voltage and current ratings of the lightest equipment wire are far greater than the requirements of the circuit (Table 2.1). A 7/0.2 PVC-covered wire, for example, is rated at

1.4 A (this is fairly small wire). This rating is conservative (the wire

Crimping is very widely used

Crimping is very widely used in the automotive industry for

in the automotive industry for rapid assembly of vehicle wiring

rapid assembly of vehicle wiring harnesses.

harnesses.

You may still nd wire sizes

You may still nd wire sizes quoted in British Standard Wire

quoted in British Standard Wire Gauge (SWG) or American Wire

Gauge (SWG) or American Wire Gauge (AWG). You will need a

Gauge (AWG). You will need a table of wire gauges to nd out

table of wire gauges to nd out the cross-sectional area.

the cross-sectional area.

Table 2.1 Current ratings of copper equipment wire

Construction Cross-section area (mm2) Current rating (A)

(a) PVC insulated

7/0.2 0.22 1.4 16/0.2 0.5 3.0 24/0.2 0.75 4.5 32/0.2 1.0 6.0

(b) PTFE insulated

7/0.15 0.12 3.5 7/0.2 0.22 6.0 19/0.16 0.38 9.0

could carry a greater current without overheating) to allow for the possibility of several wires being bundled together in a conned space. A 7/0.2 PTFE-covered wire is rated at 6A because the insulation can withstand higher temperatures caused by self-heating. As the following worked example shows, however, there will be a considerable voltage drop along wire of this cross-section carrying 6A.

Worked Example 2.1

Calculate the voltage drop along 500 mm of 7/0.2 PTFE-insulated copper equipment wire carrying a current of 6A. The resistivity of copper is 1.7 × 10–8 Ωm.

Solution

The cross-sectional area, A, of the wire is 7π × (0.1)2 mm2 or

0.22 mm2. The resistance of a length, l, resistivity, ρ is

Hence the resistance of the 500 mm length is

1.7 × 10

–8

× (0.5)/0.22 × 10–6, or about 40 mΩ

From Ohm’s law, the voltage drop is about 240 mV. If this wire is used to connect a power supply to a load only half a metre away, the voltage at the load will be 0.48 V less than the voltage at the terminals of the power supply (there will be a drop of 0.24 V in each conductor) if the full-rated current of the wire is drawn.

This solution has ignored any change in resistance due to self-heating

of the wire that would increase the voltage drop.

Cables

Cables are used mainly for signal and data transmission and for interconnecting subsystems within an electronic system. Table 2.2 summa rizes the main types of cable used in electronic engineering, and these

Table 2.2 Common cable types

Type Construction Applications Screened One or more wires with an

overall metal braid or helical screen and insulation.

Twisted pair Two wires insulated and

twisted together, covered with overall sheath, and possibly screened.

Coaxial One solid or stranded

conductor surrounded by dielectric, metal braid, and outer insulation.

Twin feeder Two conductors laid

parallel about 10 mm apart, insulated, and separated by a web of plastic.

Ribbon 10 to 50 stranded

conductors laid parallel and coplanar, covered and separated by insulation.

Low-power signal

transmission at up to audio frequencies.

Signal transmission at up

to 100 MHz.

Signal transmission at up

to 1 GHz.

Radio receiver antenna

downleads.

Parallel logic

interconnection in microprocessor and computer systems.

are illustrated in Figure 2.6. Multicore cables can combine these types within one cable for special applications. Each type of cable has its own range of uses and its own characteristic parameters.

There are only a few applications in electronic engineering for cables of the type known as ex. These consist of several insulated wires with overall insulation, such as three-core mains ex used to connect mainspowered equipment to a mains outlet (this is one of the few applications). The reason for this is that cables are designed to carry electromagnetic signals and the cable must either exclude unwanted signals present in the surroundings (interference) or else prevent energy escaping from the cable (and causing interference elsewhere). A screened cable is intended to prevent pickup of unwanted signals. The wire or wires within the cable carry the signal (typically from a microphone or other transducer) and are surrounded by a metal screen wound helically or woven from bare wires in the form of a braid. The screening is effective only against electric elds and high-impedance electromagnetic elds (with a strong electric component). Magnetic elds and low-impedance electromagnetic elds (with a strong magnetic component) cannot easily be screened against. The effect of a magnetic eld on a cable can be reduced, however, by twisting a pair of wires together. This means that currents induced in the wires by a changing magnetic eld tend to cancel because each twist of the wires reverses the polarity of the wires relative to the eld. A twisted-pair cable can be used to transmit

Chapter 8 discusses

Chapter 8 discusses electromagnetic effects in

electromagnetic effects in greater detail. Carter (1992)

greater detail. Carter (1992) discussed electrostatic and

discussed electrostatic and magnetic screening.

magnetic screening.

Outer insulation

(a)

Stranded screen

(b)

(c)

(d)

(e)

Braided outer

conductor

Inner conductor

Conductors

Dielectric

The theory of transmission lines

The theory of transmission lines is covered by Carter (1992).

is covered by Carter (1992).

Figure 2.6 Common cable types: (a) screened, (b) twisted pair, (c) coaxial, (d) twin feeder, and (e) ribbon (insulation displacement).

frequencies of up to 100 MHz, but energy will be radiated at megahertz frequencies and a coaxial cable should properly be used for the higher frequencies. Signicant applications of twisted-pair cables for high-frequency signals include 100 megabit/s (Mbit/s) Ethernet, and digital sub scriber line (DSL)/asymmetrical digital subscriber line (ADSL) (which is carried over twisted-pair telephone cables). Coaxial cables will carry signals down to zero frequency, but their main use is for transmission of radio frequency (r.f.) signals at up to 1 GHz.

Any cable that is longer than the wavelength of the signals being carried must be regarded as a transmission line. Cables designed to carry signals of frequency higher than audio frequencies therefore have characteristics which include transmission-line parameters. The two most important characteristics are the characteristic impedance, Z0, which is typically 50 to 150 Ω and the attenuation, α, which is usually stated in dB per metre, 100 m, or km (the frequency must also be given, since α depends on frequency). The characteristic impedance is independent of the length of a cable. Three other parameters commonly stated for a cable are the capacitance per metre (typically < 100 pF), the maximum working voltage, and the operating temperature range.

Ribbon cables are widely used in microprocessor and computer systems to carry parallel logic signals over distances as long as 5 to 10 m, but typically much shorter. Their main advantage over conventional multicore cables is that they can be mass terminated: a connector can

Table 2.3 Common types of cable connectors

Type Construction Features/applications IEC 60320 C14 Three-pole male body

with female contacts, female sockets with male contacts.

BNC 50/75 Ω

Coaxial bayonet. Instruments, general

10A rating mains

connector, sockets available with integral lters.

screened and coaxial connector.

“D” type 9- to 50-way connectors

with contacts in two or three rows available for soldering, wirewrapping, ribbon cabling, and PCB mounting.

RJ11/RJ45 Plastic shell with 4 or

8 sprung-nger contacts and latch.

Widely used multipole

connectors, 25-way version used for “RS-232” data transmission, 9-way for serial ports on computers.

Telephones, local area

networks (LANs, or Ethernet).

be tted to the cable in one operation taking less than a minute. If a ribbon cable is carrying high-speed digital signals, each conductor should operate as a transmission line. For this reason it is common practice to earth alternate conductors. If an even number of signals, n, is to be carried, as is normally the case, the cable should have 2n + 1 conductors so that all signal conductors have an earth on both sides. This ensures that the characteristic impedances of all the signal conductors are equal.

Connectors

Connectors, or plugs and sockets, are used in electronic products to make electrical connections that can be easily disconnected and reconnected. They are used to connect external cables to equipment and are also tted internally to allow subassemblies (such as PCBs) to be disconnected and removed easily for repair. One can therefore classify connector types into two main categories: cable connectors, suitable for making external connections to equipment; and wiring connectors, designed for use internally.

Table 2.3 lists a few common types of cable connectors and is limited to widely used, standardized designs. Some of these are illustrated in Figure 2.7. There are many types of proprietary connector, especially for use with multicore cable. Most cable connector types are keyed or polarized mechanically in some way so that they can be connected in one position only. Connectors designed to be attached to the end of a cable are called free connectors and incorporate some form of strain relief to grip the cable mechanically so that tension in the cable is not transmitted to the electrical joints inside the connector. Fixed connectors are designed for mounting on a panel or PCB.

+ 200 hidden pages

CRC Electronic Components and Technology User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual

Contents

Preface to the Second Edition

Preface

Acknowledgments

Author