job:LAY00 page:1 colour:1 black–text SMT Soldering Handbook job:LAY00 page:2 colour:1 black–text mmmm job:LAY00 page:3 colour:1 black–text SMT Soldering Handbook Rudolf Strauss, Dr.Ing., FIM job:LAY00 page:4 colour:1 black–text Newnes An imprint of Butterworth-Heinemann Linacre House, Jordan Hill, Oxford OX2 8DP 225 Wildwood Ave, Woburn, MA 01801-2041 A division of Reed Educational and Professional Publishing Ltd A member of the Reed Elsevier plc group oxford boston johannesburg melbourne new delhi singapore First published 1994 as Surface Mount Technology Second edition 1998 © Rudolf Strauss 1994, 1998 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 Strauss, Rudolf Surface Mount Technology I. Title 621.3815 ISBN 0 7506 35894 Library of Congress Cataloguing in Publication Data Strauss, Rudolf. Surface mount technology Rudolf Strauss. p. cm. Includes bibliographical references and index. ISBN 0 7506 1862 0 1. Surface mount technology. 2. Printed circuits–Design and construction. I. Title. TK7870.15.S63 621.381531–dc20 93–44368 CIP Typeset by Vision Typesetting, Manchester Printed and bound in Great Britain by Biddles Ltd of Guildford and King’s Lynn job:LAY00 page:5 colour:1 black–text Contents Preface to the first edition xii Preface to the second edition xiv Glossary xvi 1 Why SMDs? 1 References 5 2 The SMD family 6 2.1 Shapes, sizes and construction 6 2.1.1 Melfs and chips 6 2.1.2 LCCCs 8 2.1.3 SOs and PLCCs 9 2.2 High-pincount components 9 2.2.1 TABs 10 2.2.2 Flip-chips and BGAs 11 2.3 Multichip modules 13 2.4 The solderable surfaces of SMDs 14 2.4.1 Melfs and chips 14 2.4.2 Components with legs 15 2.5 SMD shapes and wavesoldering behaviour 16 2.6 The popcorn effect 16 2.7 References 18 3 Soldering 20 3.1 The nature of soldering and of the soldered joint 20 3.1.1 The roles of solder, flux and heat 20 job:LAY00 page:6 colour:1 black–text 3.1.2 Soldering methods 21 3.1.3 Soldering success 24 3.2 The solder 24 3.2.1 Constituents, melting behaviour and mechanical properties 24 3.2.2 Composition of solders for use in electronics 27 3.2.3 Lead-free solders 29 3.2.4 Solder impurities 31 3.3 The soldered joint 36 3.3.1 Soldering as a surface reaction between a molten and a solid metal 36 3.3.2 Structure and characteristics of the soldered joint 37 3.3.3 Mechanical properties of soldered joints 40 3.3.4 Soldering on surfaces other than copper 41 3.3.5 Long-term behaviour of soldered joints 43 3.3.6 Long-term reliability of soldered joints 44 3.4 The flux 45 3.4.1 Tasks and action of the soldering flux 45 3.4.2 Wetting and interfacial tension 47 3.4.3 Properties required in a flux 48 3.4.4 Rosin fluxes 50 3.4.5 Low-solids and no-clean fluxes 52 3.4.6 Watersoluble fluxes 53 3.4.7 Solvents used in fluxes 54 3.4.8 Flux standards 54 3.4.9 Testing soldering fluxes 57 3.5 Soldering heat 60 3.5.1 Heat requirements and heat flow 60 3.5.2 Heating options 61 3.6 Solderability 63 3.6.1 Wetting and dewetting 63 3.6.2 Capillarity and its effects 65 3.6.3 Capillarity and joint configuration 65 3.6.4 The importance of solderability 69 3.6.5 Oxide layers 69 3.6.6 Solderability-enhancing surface coatings 71 3.6.7 Leaching effect of molten solder 73 3.6.8 Measuring solderability 74 3.7 References 81 vi Contents job:LAY00 page:7 colour:1 black–text 4 Wavesoldering 84 4.1 The wave concept 84 4.1.1 Wavesoldering before SMDs 84 4.1.2 Wavesoldering after SMDs 85 4.2 Applying the flux 87 4.2.1 Types of fluxer 87 4.2.2 Monitoring and controlling flux quality 96 4.3 Preheating the board 98 4.3.1 Heat requirements 98 4.3.2 Heat emitters and their characteristics 100 4.3.3 Temperature control 102 4.4 The solderwave 103 4.4.1 Construction of the soldering unit 103 4.4.2 Thermal role of the solderwave 104 4.4.3 Interaction between molten solder and the circuit board 105 4.4.4 Chipwaves 110 4.4.5 Formation and control of dross 114 4.5 Wavesoldering in an oxygen-free atmosphere 118 4.5.1 Origins and development 118 4.5.2 Wavesoldering in nitrogen 119 4.6 Board conveyor systems 126 4.6.1 Functional requirements 126 4.6.2 Board-handling systems 126 4.7 Wavesoldering practice 130 4.7.1 Operating parameters and their role 130 4.7.2 Choosing and monitoring operating parameters 131 4.7.3 Optimizing machine parameters 136 4.7.4 Machine maintenance 137 4.7.5 Check-analysis of the solderbath 138 4.7.6 Dealing with dross 138 4.7.7 Hygiene and safety 139 4.8 The role of adhesives in wavesoldering 141 4.8.1 Demands on the adhesive and the glued joint 141 4.8.2 Storage and handling behaviour of adhesives 141 4.8.3 Applying the adhesive 143 4.8.4 Curing the adhesive joint 145 4.8.5 The glass transition temperature 146 4.9 References 147 5Reflowsoldering 148 5.1 The reflow concept 148 Contents vii job:LAY00 page:8 colour:1 black–text 5.1.1 SMDs and reflowsoldering 148 5.1.2 Reflowsoldering versus wavesoldering 154 5.2 Solder paste 158 5.2.1 Operational requirements 158 5.2.2 Standard specifications 159 5.2.3 Solderpowder 160 5.2.4 The flux and its residue 164 5.2.5 Printing and dispensing properties 165 5.2.6 The solderball test 167 5.3 Putting the solder paste on the board 169 5.3.1 Single-spot dispensing 169 5.3.2 Stencilling and screen printing 171 5.3.3 Depots of solid solder 178 5.4 Vapourphase soldering 179 5.4.1 The basic concept 179 5.4.2 Vapourphase working fluids 180 5.4.3 The physics of vapourphase soldering 180 5.4.4 Vapourphase soldering equipment 184 5.4.5 ‘New-generation’ vapourphase soldering systems 186 5.5 Infrared soldering 189 5.5.1 Working principle 189 5.5.2 The physics of heat transfer by radiation 191 5.5.3 The physics of heat transfer by convection 200 5.5.4 Operation of infrared ovens 201 5.5.5 Oven design 203 5.5.6 Infrared soldering in a controlled atmosphere 208 5.6 Reflowsoldering with hot air or gas 209 5.6.1 Convection versus radiation 209 5.6.2 The physics of convection reflowsoldering 210 5.6.3 Convection reflow ovens 210 5.6.4 Development potential of convection reflowsoldering 212 5.6.5 Convection soldering of single components 214 5.7 Laser soldering 217 5.7.1 How a laser works 218 5.7.2 Nd: YAG and CO lasers 218 5.7.3 Laser soldering in practice 220 5.7.4 Laser-soldering equipment 221 5.8 Impulse soldering 223 5.8.1 Operating principle 223 5.8.2 The solder depot 224 viii Contents job:LAY00 page:9 colour:1 black–text 5.8.3 The thermode and its heating cycle 225 5.8.4 Impulse-soldering equipment 227 5.9 SMD soldering methods – A survey 228 5.10 References 232 6 The circuit board 234 6.1 The beginnings 234 6.2 SMD-specific demands on a circuit board 234 6.3 Thermal management 236 6.3.1 Thermal expansion mismatch 237 6.3.2 Effects of temperature differences between components and board 237 6.4 Solderable surfaces 238 6.4.1 Galvanic coatings 239 6.4.2 Hot tinning 239 6.4.3 Organic coatings 241 6.4.4 Flat solder depots 242 6.5 The soldermask 244 6.6 Layout 244 6.6.1 Layout for wavesoldering 244 6.6.2 Layout for reflowsoldering 247 6.7 References 249 7 Component placement 250 7.1 The task 250 7.2 Reliability of placement 251 7.3 Placement options 253 7.3.1 Fully manual placement 253 7.3.2 Semi-automatic placement 255 7.3.3 Fully automatic sequential systems 256 7.3.4 Simultaneous placement systems 257 7.4 The practice of automatic component placement 258 7.4.1 The range of choice 258 7.4.2 Classes of placement machines 259 7.5 Reference 260 8 Cleaning after soldering 261 8.1 Basic considerations 261 8.1.1 Reasons for cleaning 263 8.1.2 Designing for cleanability 266 8.1.3 What must be removed? 267 8.2 The theory of cleaning 269 Contents ix job:LAY00 page:10 colour:1 black–text 8.2.1 The physics of cleaning 269 8.2.2 The chemistry of cleaning 274 8.3 The practice of solvent cleaning 276 8.3.1 Organic solvents 277 8.3.2 Solvent-cleaning installations 282 8.3.3 In-line cleaning plants 288 8.3.4 Halogenated solvents: safety and health 291 8.3.5 The three environmental threats 293 8.3.6 Restrictions on solvent usage 297 8.3.7 Non-flammable organic solvents with reduced environmental risks 298 8.3.8 Flammable solvents 299 8.4 Cleaning with water 302 8.4.1 Chemical and physical aspects 302 8.4.2 Water quality 303 8.4.3 Water recycling and effluent problems 305 8.4.4 Removal of residue from watersoluble fluxes 306 8.4.5 Removal of residue from resinous fluxes 306 8.4.6 Water washing installations 307 8.5 Semi-aqueous cleaning 310 8.5.1 The concept 310 8.5.2 The cleaning solvents 311 8.5.3 Semi-aqueous washing installations for water-immiscible solvents 313 8.5.4 Semi-aqueous washing installations for water-miscible solvents 315 8.6 Testing for cleanliness 318 8.6.1 The meaning of cleanliness 318 8.6.2 Measuring ionic contamination (MIL test) 319 8.6.3 Measuring surface insulation resistance (SIR) 321 8.7 The future of cleaning and of fluxing 322 8.8 References 323 9 Quality control and inspection 325 9.1 The meaning of ‘quality’ 325 9.1.1 Product quality and product reliability 325 9.1.2 Classification according to reliability requirements 326 9.2 Soldering success and soldering perfection 327 9.2.1 Soldering success and soldering faults 327 9.2.2 Soldering perfection and soldering imperfections 330 9.3 Practical examples of soldering faults 332 9.4 The ideal and the imperfect joint 332 x Contents [...]... sticking through it Surface- mounted resistors stuck to circuit boards had been described in 1952, but the first mention of a device with its terminals in contact with conductors on a circuit board occurs in a British patent in 1960 In the mid sixties, the growth of hybrid technology provided the incentive to design surface- mounted devices which had no connecting wires Thick-film circuitry, carried... holes drilled in a board Surface- mounting technology (SMT) began to take over At the same time, components became flatter, and approached the two-dimensionality of the boards on which they sit The designers of soldering equipment and of SMDs had started to work together TABs are a typical example of the benefits of this cooperation Today, a number of driving forces, which are pushing SMT further forward,... will always have this last meaning in this book COB Chip-on-board: a bare chip, glued to a board and connected to its circuitry by wirebonding CSP Chip-size package: an SMD with a plastic or ceramic body which is not much larger than the chip which it contains DCA Direct chip attach (an alternative name for flip chip) DIL ‘Dual-in-line’: a through-mounted device (TMD) containing an integrated circuit... telephone cards), which at present have a standard thickness of 0.825 mm/33 mil 2.2.2 Flip-chips and BGAs TABs have severe limitations: they cannot be soldered to a board by wave- or reflowsoldering, the two dominating in-line assembly techniques for circuit boards, but are designed for impulse soldering (Section 5.8), a one-by-one operation using dedicated equipment, which interrupts the continuity of a production... ends of the gull-wing leads, the bane of fine-pitch components, is no longer a problem Accuracy of placement is much less critical than with fine-pitch components, because of their wide pitch and their ability to self-align during the reflow-process: if misplaced by even half a pitch, the surface tension of the molten job:LAY02 page:9 colour:1 black–text The SMD family 13 Figure 2.3 Ball-grid array solder... Packaging, Hybrid Circuits, No 6, pp 9–13 2 BPA Ltd (1989) Surface Mount Technology, A Critical Analysis, BPA Ltd, Dorking RH4 1DF, UK, pp 146–147 3 Anon (19934) Surface Mount Technology (Germany), p 68 4 Vardaman, E J (1992) New TAB Developments and Applications, Proc Nepcon West, pp 590–594 5 Vardaman, E J (1996) Worldwide Packaging Roadmaps Soft Soldering, Research and Practice 1996, Proc Academic Colloquium... the form of a standard cine-film, from super-8 up to 35 mm or more (Figure 2.2) These tapes are robust, and lend themselves to automatic placement and soldering on purpose-designed equipment, which by now has become well developed and widely available The inner ends of the leads are soldered to the pads of the IC by the manufacturer of the TAB by a solder-bump technique (inner-lead bonding, ILB) The outer... circuit board A ‘metal electrode face-bonded’ component: a resistor or a diode, encased in a cylindrical ceramic body with metallized solderable ends Printed circuit board Plastic leaded chip carrier: a CC with a body made of plastic, with J-shaped legs on all four sides Quad flatpack: a plastic body containing an IC, with gull-wing legs on all four sides A surface- mounted device ‘Small outline’: an SMD,... on the ‘flip-chip’, introduced in the 1970s by IBM for in-house use Since the mid-1990s, the use of flip-chips and BGAs, both of which carry solder bumps, has grown rapidly (Figure 2.3) With a flip-chip, the array of solder bumps sits on the underside of the ceramic job:LAY02 page:8 colour:1 black–text 12 The SMD family body or die of a bare chip Not being covered by a plastic package, a flip-chip must... between the ceramic body of the chip and the board laminate The underfill also locks in the flux residues left after soldering, which must therefore be strictly non-corrosive and non-conductive Finally, once cured it makes corrective removal and resoldering of a flip-chip impossible Flip-chips are mainly used where component height is at a premium, as for example in ‘smart cards’ Where height is less . wires or soldering lugs. They worked with electric soldering irons and solderwire with a rosin-flux core. Soldering quality on the whole was excellent, because every operator was his (or her). reflowsoldering are comprehensively treated. Features of the circuit board and of component placement are considered as far as they are relevant to the soldering process. Cleaning after soldering is treated. circuit boards by solder- ing. Temperatures are given in degrees Centigrade and Fahrenheit; as a rule, operating temperatures are rounded up or down to the nearest round figure, unless they relate