Soldering in Electronics Assembly Soldering in Electronics Assembly Mike Judd and Keith Brindley UNEWNES Newnes An imprint of Butterworth-Heinemann Ltd Linacre House, Jordan Hill, Oxford OX2 8DP rJ5 PART OF REED INTERNATIONAL BOOKS OXFORD LONDON BOSTON MUNICH NEW DELHI SINGAPORE SYDNEY TOKYO TORONTO WELLINGTON First published 1992 © Mike Judd and Keith Brindley 1992 All rights reserved. No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1P 9HE. Applications for the copyright holder's written permission to reproduce any part of this publication should be addressed to the publishers. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 0 7506 0589 8 Printed and bound in Great Britain by Thomson Litho Ltd, East Kilbride, Scotland Preface Electronics assembly is about how components are soldered onto a printed circuit board. Soldering, as a principle in electronics asembly, is straightforward and simple — two metals surfaces (a component termination and a connecting pad on a printed circuit board track) are joined by the metallic bonds created by molten solder between them. Soldered joints are supported by the solder when it solidifies, and solder allows electrical contact between metals in the joints. These things cannot change. On the other hand, components are changing — they are smaller, they have greater numbers of leads, they have different mounting requirements. Printed circuit boards reflect this — they too are smaller, with narrower connecting tracks and different characteristics. Consequently, as components and their printed circuit boards change so must the ways they are soldered together change. Many estimates of how electronics assembly will change over the coming years have been made. While differing marginally in specific details, they all point to the fact we are in the middle of a colossal movement away from conventional through-hole assemblies using leaded components, towards sur- face mounted assemblies with only leadless components. BPA (Technology & Management) of the UK, for example, has shown how — in 1987 — less than 20% of European and North American assemblies featured any leadless surface mounted components at all. Figure 1 shows this together with figures for 1992 and 1997. Currently some 40% of assemblies feature surface mounted components. In 1997, however, we can expect over 60% of assemblies to be so. In terms of ■jf^y-fewM'^ 13 loot Rest of world 2 i* 90 ^ South-east Asia 80 H USA & 70 111 Europe 60 |_ tiJ Japan 50 40 |_ 30 L 20 L Figure 1 Estimates of electronics assemblies worldwide which 10 feature leadless components of any type, in terms of percentages of totals (BPA, 1992) 1987 1992 1997 X PREFACE sheer numbers of surface mounted assemblies, Figure 2 shows trends over the current ten years. Interestingly, values for Japan in both Figure 1 and Figure 2 are significantly higher than either Europe or North America. 1800 Japan „ 1600 o g 1400 Rest of world si 1200 USA 1000 Europe 800 South-east Asia 600 400 Figure 2 Estimates of electronics assemblies worldwide featuring 1987 1992 1997 leadless components, in terms of total numbers (BPA) Yet this is only half the story. While many of yesterday's and today's assemblies are mixed ie, they feature both leaded and leadless components, tomorrow's assemblies will tend to be totally surface mounted, with very-few leaded components at all. Figure 3 shows trends of three common types of assemblies from 1987 to 1997. While mixed technology types are in a process of reducing in numbers, pure surface mounted assemblies are increasing. By the turn of the century we might reasonably expect there to be only minimal through-hole assembly manufacture to speak of at all. iL _ 1S 5 01 □ 1987 <4m □ 1992 S40 9 ■ 1997 # 30 ||| III 20 III 10 111 111 Type II — leaded Type III — leaded components alone ccoommppoonneennttss oonn tloopp,, components on top, Figure 3 Estimates of electronics leadless components on leadless components on assemblies worldwide of three bottom top and bottom main asembly types, in terms of percentages of type totals (BPA) PREFACE xi This trend towards surface mounted technology is just the first step in miniaturization of products. Surface mounted components themselves are smaller than their leaded counterparts. Ways in which surface mounted assemblies can be manufactured allow surface mounted components to be positioned closer together. Accurate automatic assembly of surface mounted components means components with greater lead-counts may be used. Greater lead-counts mean narrower connecting tracks must be used. Narrower tracks mean that components can be more closely spaced — and so the cycle goes on! Products like personal computers which five years ago took up three-quarters of your desktop now sit on your lap, yet feature the same computing power. In five years time... Finally, as products become smaller they stretch the boundaries of the very technologies used to manufacture them. Smaller components with greater lead- counts make soldering to printed circuit boards more and more difficult. New soldering techniques and adaptations of older techniques are constantly in development. Our job in writing this book is to show all this. While looking at the principle of soldering, which has remained fixed since man first used molten metal to join two other metals (first known use of solder is Roman pipework, where sections of lead pipe were joined by melting their ends together), we also show how the techniques to fulfill the soldering process can change and, indeed, are changing with the requirements placed by components and printed circuit board assemblies on them. More than anything else these days (and in the future), state of component and printed circuit board technologies means soldering has to be clean and precise. In the distant past, when components were huge and printed circuit board tracks were wide, soldering was often a case of throwing sufficient molten solder at a board to make an adequate joint. Joints were initially made by hand. This is no longer the case. With minute component terminations and tracks, too much solder can be devastating. On the other hand, too little solder will not make a joint at all. Even if economic considerations were not important, joints can no longer be made by hand because hand soldering cannot guarantee adequate results with such small joints. Control of soldering systems has to be precise and closely monitored. Soldering and, inevitably, its control must be performed by machines. While it's impossible to summarize all we have to say, in this short preface, it is possible to make a list of points which our book, hopefully, spotlights. Overleaf we present the 10 rules of machine soldering. Acknowledgements We'd like to thank a number of people who have helped us in major ways, as we have researched and written this book. In no particular order of priority, gratitude goes to: # William Down; Electrovert USA # Don Elliott; Electrovert, Canada # Peter Grundy; Siemens, UK # Karen Moore-Watts, DEK Printing Machines, UK # Alan Keyte, Motorola, UK # Andrew Nicholson, Alpha Metals, UK # Fred Thorns, Fry's Metals, UK # Phil Fulker; Hollis Europe # Norman Hodson; Dynapert UK # Richard Hart; ex Multicore Solders # Colin Lea; National Physical Laboratory, UK # Alan Roberts and Claude Legault # many people within the Electovert group worldwide. Finally, special thanks go to Bob Willis of Electronic Presentation Services in the UK, for help and guidance throughout. This book is dedicated to a respected man, considered by many to be a guru in electronics soldering. Paul Bud, Technical Director of Electrovert is sadly missed. He was a kind man and a gentleman, always willing to listen and pass on his wide knowledge of soldering, communicating in many languages. He was an inspiration to us all. In his memory, we hope the book proves helpful to all who read it. , ,l ,, „" '.!M.'.,''i:...Mj!. ''i,viI'i'V!'i•K.'i+I!.".'«.!'. Mtf hukn 10 rules of machine soldering 1 machine soldering, correctly controlled, produces highest quality joints at lowest cost 2 machine soldering is a process and, like all processes, produces consistent results if properly controlled 3 touch-up of faulty soldered joints is costly, unreliable and unneces- sary 4 anything that reduces solder joint defects is cost effective 5 design, handling, assembly and maintenance are all parts of the soldering process and must be properly controlled 6 solderability of printed circuit boards and components accounts for 60% of all faulty soldered joints 7 never use parts which fail solderability testing: the ultimate cost is too high 8 soldering problems are solved by process control — not more inspec- tion and touch-up 9 everyone concerned with the soldering process must be formally and properly trained 10 zero-defect soldering is the lowest cost soldering. 1 Soldering process This chapter could be seen as just another introduction to soldering — it does, after all, explain in a fairly basic manner all important aspects of soldering in electronics assemblies. In this light many people may be tempted to skip it. However, this would be a mistake as it also explains philosophy behind the complete book. Further, it discusses main features and illustrates fundamental premises upon which we have based our text. Finally it also serves as a guide indicating where relevant and important information is to be found within the rest of the book. We strongly advise this chapter is looked at bearing all this in mind. Soldering, in principle though not in practice, is a reasonably straightforward process, used in the electronics industry to bond components together, forming one or more electrical connections. From this description, it's easy to see that soldering serves two functions: # mechanical support — holding components of an assembly together # electrical support — forming required electrical connections within a circuit. Most components in an assembly use the mechanical support of soldered joints alone to give adequate fixing into the assembly. A few isolated components (notably, larger, heavier components) may require additional mechanical sup- port, in the form of straps, nuts, bolts and so on. Where possible, however, such large components should be designed out (that is, care should be taken when designing a circuit not to use such components) of an assembly to keep extra procedures and cost to a minimum. On the other hand, all components may use solder as electrical support to form required electrical connections. No other method has yet been devised to take the place of solder in all assemblies to the same level of performance, cost and ease of operation. Time on its side Solder in one form or another has been around for a long time. The Romans are known to have used solder to form joints in their plumbing systems and, indeed, the word plumbing refers to the use of lead (from the Latin plumbum meaning lead) as a jointing compound. Nowadays, of course, pure lead plumbing is no longer considered, instead solder — which is an alloy of mostly tin and lead — is used. It's interesting to note that certain countries are already in the process of altering legislation to prevent lead being used in any plumbing where drinking water is present. 2 SOLDERING PROCESS Solder has been adopted by the electronics industry as the best method of making joints within assemblies and, for many years, this jointing process was undertaken manually — hand soldering. Inevitably hand soldering is a slow, laborious, time-consuming and hence expensive process as each component must be soldered into position to the printed circuit board individually. Quality and repeatability of joints depend almost totally on the individual operator. This clearly makes cost of large-scale electronics assembly production uneconomical and unsatisfactory. Generally, therefore, hand soldering of electronics assemblies is undertaken only in development and prototype stages, although there remains a situation in which small-volume production of electronics assemblies is economical. Hand soldering is used regularly in rework stages of manufacture, where assemblies need to be partially disassembled for repair and service purposes. Because of this situation we need to consider hand soldering and so it is discussed in Chapter 5. For the last 40 years or so, various methods of automating soldering processes have been developed. It's easy to see how vitally important automated soldering of assemblies has become, by remembering the space programme of getting a man on the moon could not have been achieved without it. Paul Bud, to whom this book is dedicated, related an interesting fact about NASA printed circuit boards used in spacecraft. Had NASA's assemblies been hand soldered, joints would have held more solder with the result that assemblies Component Component lead Solder fillet •.lV^Vl* «.*■.*«.*;■.*■. V, Printed circuit board base material ■'ft! Figure 1.1 Soldered joint formed n between a component lead and Copper track Plated through-hole 111 , .. copper track on a plated through- hole printed circuit board TIME ON ITS SIDE 3 would have been 10% heavier. So many assemblies are used in spacecraft that with this additional weight, the three astronauts (totalling, say, 250 kg) could not have been carried! Until just recently, however — say, the last ten years' — soldering of really only one type of electronics assembly was in the main undertaken. This assembly type, a typical joint of which is illustrated in Figure 1.1, is commonly called the through-hole printed circuit board, simply because the components feature leads which are inserted through holes in the printed circuit board. Copper track interconnections exist between holes which make up the circuit of an assembly and component leads are soldered to the copper, so that mechanical and electrical support is provided with each solder joint. Note that, in this ideal joint, solder has been drawn inside the hole during the soldering operation — this occurs by capillary action. Solder between the copper track and the component lead is called the fillet. Emergence of a different type of electronics assembly — surface mounted assemblies or SMAs — however, is altering the face of soldering processes used. So much is surface mount technology (SMT) changing soldering that, at present, somewhere around 50% of all electronics assemblies use at least a few surface mounted components (SMCs — also sometimes called surface mounted devices or SMDs). In 1980 the figure was, to all practical purposes outside of Japan, zero. This change in assembly technology has only been possible with developments bringing a parallel change in soldering technology. Changes in component technology, too, are pushing many companies along the surface mount route — a growing number of components simply cannot be obtained in through-hole forms and are only available as surface mounted components. Figure 1.2 shows a typical surface mounted assembly joint, in which a surface mounted component is soldered to a printed circuit board. Difference between this and the through-hole assembly joint of Figure 1.1 is immediately apparent — Surface mounted Solder Terminal component fillet Figure 1.2 Soldered joints formed Copper track Printed circuit board between component terminals and base material copper track on a surface mount assembly printed circuit board