lee, n.-c. (2001) reflow soldering processes and troubleshootin

Reflow Soldering Processes and Troubleshooting: SMT, BGA, CSP and Flip Chip Technologies To my mother, Shu-shuen Chang, for her care and encouragement To my wife, Shen-chwen Lee, for her understanding and full support Reflow Soldering Processes and Troubleshooting: SMT, BGA, CSP and Flip Chip Technologies Ning-Cheng Lee BOSTON OXFORD AUCKLAND JOHANNESBURG MELBOURNE NEW DELHI Copyright  2002 by Newnes, an imprint of Butterworth-Heinemann All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher. Recognizing the importance of preserving what has been written, Butterworth-Heinemann prints its books on acid-free paper whenever possible. Butterworth-Heinemann supports the efforts of American Forests and the Global ReLeaf program in its compaign for the betterment of trees, forests, and our environment. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalog record for this book is available from the British Library The Publisher offers special discounts on bulk orders of this book. For information, please contact: Manager of Special Sales Butterworth-Heinemann 225 Wildwood Avenue Woburn, MA 01801 – 2041 Tel: 781-904-2500 Fax: 781-904-2620 For information on all Newnes publications available, contact our World Wide Web home page at: http://www.newnespress.com 10987654321 Typeset by Laser Words Private Limited, Chennai, India Printed in the United States of America Preface 1 Introduction to Surface Mount Technology 1.1 Surface mount technology 1.1.1 History and benefits 1.1.2 Surface mount components 1.1.3 Types of surface mount assembly technology 1.1.4 Surface mount soldering process 1.1.5 Advantages of solder paste technology in SMT 1.2 Surface mount technology trends 1.2.1 Technology driving force 1.2.2 Area array packages 1.3 Conclusion 2 Fundamentals of Solders and Soldering 2.1 Soldering theory 2.1.1 Spreading 2.1.2 Fluid flow 2.1.3 Dissolution of base metal 2.1.4 Intermetallics 2.2 Effect of elemental constituents on wetting 2.3 Phase diagram and soldering 2.4 Microstructure and soldering 2.4.1 Deformation mechanisms 2.4.2 Desirable solders and the soldering process 2.4.3 Effect of impurities on soldering 2.5 Conclusion Appendix 2.1 Effect of flux surface tension on the spread of molten solder 3 Solder Paste Technology 3.1 Fluxing reactions 3.1.1 Acid base reactions 3.1.2 Oxidation reduction reactions 3.1.3 Fluxes for reflow soldering 3.2 Flux chemistry 3.2.1 Resins 3.2.2 Activators 3.2.3 Solvents 3.2.4 Rheological additives 3.3 Solder powder 3.3.1 Atomization 3.3.2 Particle size and shape 3.4 Solder paste composition and manufacturing 3.5 Solder paste rheology 3.5.1 Rheology basics 3.5.2 Solder paste viscosity measurement 3.6 Solder paste rheology requirement 3.6.1 Effect of composition on rheology 3.7 Conclusion 4 Surface Mount Assembly Processes 4.1 Solder paste materials 4.1.1 Paste handling and storage 4.1.2 Paste deposition 4.2 Printer level consideration 4.2.1 Stencil 4.2.2 Squeegee 4.2.3 Printing and inspection process 4.3 Pick-and-place 4.4 Reflow 4.4.1 Infrared reflow 4.4.2 Vapor phase reflow 4.4.3 Forced convection reflow 4.4.4 In-line-conduction reflow 4.4.5 Hot-bar reflow 4.4.6 Laser reflow 4.5 Effect of reflow atmosphere on soldering 4.6 Special soldering considerations 4.6.1 Step soldering 4.6.2 Reflow-alloying 4.6.3 Paste-in-hole 4.7 Solder joint inspection 4.8 Cleaning 4.9 In-circuit-testing 4.10 Principle of troubleshooting reflow soldering 4.11 Conclusion 5 SMT Problems Prior to Reflow 5.1 Flux separation 5.2 Crusting 5.3 Paste hardening 5.4 Poor stencil life 5.5 Poor paste release from squeegee 5.6 Poor print thickness 5.7 Smear 5.8 Insufficiency 5.9 Needle clogging 5.10 Slump 5.11 Low tack 5.12 Short tack time 5.13 Conclusion 6 SMT Problems During Reflow 6.1 Cold joints 6.2 Nonwetting 6.3 Dewetting 6.4 Leaching 6.5 Intermetallics 6.5.1 General 6.5.2 Gold 6.6 Tombstoning 6.7 Skewing 6.8 Wicking 6.9 Bridging 6.10 Voiding 6.11 Opening 6.11.1 Pillowing 6.11.2 Other openings 6.11.3 Fillet lifting 6.11.4 Projected solder 6.12 Solder balling 6.13 Solder beading 6.14 Spattering 6.15 Conclusion 7 SMT Problems At the Post- reflow Stage 7.1 White residue 7.2 Charred residue 7.3 Poor probing contact 7.3.1 Flux residue content 7.3.2 Top-side flux spread 7.3.3 Bottom-side flux spread 7.3.4 Residue hardness 7.3.5 Reflow atmosphere 7.3.6 Metal content 7.3.7 Soft-residue versus low-residue 7.3.8 Soft-residue versus RMA residue 7.3.9 Multiple cycles probing testability 7.4 Surface insulation resistance or electrochemical migration failure 7.4.1 Surface insulation resistance (SIR) 7.4.2 Electrochemical migration (EM) 7.4.3 Effect of flux chemistry on IR values 7.4.4 Effect of soldering temperature 7.4.5 Effect of cleanliness of incoming parts 7.4.6 Effect of conformal coating/encapsulation 7.4.7 Effect of interaction between flux and solder mask 7.4.8 Effect of interaction between solder paste flux residue and wave flux 7.5 Delamination/voiding/non-curing of conformal coating/ encapsulants 7.5.1 Voiding 7.5.2 Delamination 7.5.3 Incomplete curing 7.6 Conclusion 8 Solder Bumping for Area Array Packages 8.1 Solder criteria 8.1.1 Alloys used in flip chip solder bumping and soldering 8.1.2 Alloys used in BGA and CSP solder bumping and soldering 8.1.3 Lead-free solders 8.2 Solder bumping and challenges 8.2.1 Build-up process 8.2.2 Liquid solder transfer process 8.2.3 Solid solder transfer processes 8.2.4 Solder paste bumping 8.3 Conclusion 9 BGA and CSP Assembly and Rework 9.1 Assembly process 9.1.1 General stencil design guideline 9.1.2 BGA/CSP placement 9.1.3 Reflow 9.1.4 Inspection 9.2 Rework 9.2.1 Process flow 9.2.2 Pre-baking 9.2.3 Component removal 9.2.4 Reflow equipment 9.2.5 Site preparation 9.2.6 Solder replenishment 9.2.7 Placement of component 9.2.8 Reflow of BGA and CSP 9.3 Challenges at assembly and rework stages 9.3.1 Starved solder joint 9.3.2 Poor self-alignment 9.3.3 Poor wetting 9.3.4 Voiding 9.3.5 Bridging 9.3.6 Open 9.3.7 Uneven joint height 9.3.8 Solder webbing 9.3.9 Solder balling 9.3.10 Popcorn and delamination 9.4 Conclusion 10 Flip Chip Reflow Attachment 10.1 Flip chip attachment 10.1.1 Conventional flip chip attachment 10.1.2 Snap cure 10.1.3 Epoxy flux 10.1.4 No-flow 10.1.5 SMT 10.1.6 Fluxless soldering 10.1.7 Wafer-applied underfill 10.1.8 Wafer level compressive-flow underfill (WLCFU) 10.2 Problems during flip chip reflow attachment 10.2.1 Misalignment 10.2.2 Poor wetting 10.2.3 Solder voiding 10.2.4 Underfill voiding 10.2.5 Bridging 10.2.6 Open 10.2.7 Underfill crack 10.2.8 Delamination 10.2.9 Filler segregation 10.2.10 Insufficient underfilling 10.3 Conclusion 11 Optimizing a Reflow Profile Via Defect Mechanisms Analysis 11.1 Flux reaction 11.1.1 Time/temperature requirement for the fluxing reaction 11.1.2 Fluxing contribution below the melting temperature 11.2 Peak temperature 11.2.1 Cold joint and poor wetting 11.2.2 Charring, delamination, and intermetallics 11.2.3 Leaching 11.3 Cooling stage 11.3.1 Intermetallics 11.3.2 Grain size 11.3.3 Internal stress-component cracking 11.3.4 Deformation of joints 11.3.5 Internal stress solder or pad detachment 11.4 Heating stage 11.4.1 Slumping and bridging 11.4.2 Solder beading 11.4.3 Wicking 11.4.4 Tombstoning and skewing 11.4.5 Solder balling 11.4.6 Poor wetting 11.4.7 Voiding 11.4.8 Opens 11.5 Timing considerations 11.5.1 Ramp-up stage 11.5.2 Soaking zone 11.5.3 Onset temperature of spike zone 11.6 Optimization of profile 11.6.1 Summary of desired profile feature 11.6.2 Engineering the optimized profile 11.7 Comparison with conventional profiles 11.7.1 Conventional profiles 11.7.2 Background of conventional profiles 11.7.3 Approach of conventional profiles 11.7.4 Compromise of conventional profiles 11.7.5 Earlier mass reflow technologies 11.7.6 Forced air convection reflow technology 11.7.7 Defect potential associated with conventional profiles 11.8 Discussion 11.8.1 Profiles for low temperature solder pastes 11.8.2 Profiles for high temperature solder pastes 11.8.3 Limited oxidation tolerance 11.8.4 Unevenly distributed high thermal mass systems 11.8.5 Nitrogen reflow atmosphere 11.8.6 Air flow rate 11.8.7 Adjustment of optimal profile 11.9 Implementing linear ramp-up profile 11.10 Conclusion 12 Lead-free Soldering 12.1 Initial activities 12.2 Recent activities 12.3 Impact of Japanese activities 12.4 US reactions 12.5 What is lead-free interconnect? 12.6 Criteria of lead-free solder 12.7 Viable lead-free alloys 12.7.1 Sn96.5/Ag3.5 12.7.2 Sn99.3/Cu0.7 12.7.3 Sn/Ag/Cu 12.7.4 Sn/Ag/Cu/X 12.7.5 Sn/Ag/Bi/X 12.7.6 Sn/Sb 12.7.7 Sn/Zn/X 12.7.8 Sn/Bi 12.8 Cost 12.9 PCB finishes 12.10 Components 12.11 Thermal damage 12.12 Other problems 12.13 Consortia activity 12.14 Opinions of consortia 12.15 The selections of pioneers 12.16 Possible path 12.17 Is lead-free safe? 12.18 Summary of lead-free adoption 12.19 Troubleshooting lead-free soldering 12.19.1 Compatibility with reflow process 12.19.2 Fillet lifting 12.19.3 Conductive anode filament 12.19.4 Grainy surface 12.19.5 Sn/Pb/Bi ternary low melting eutectic phase 12.20 Conclusion Index [...]... Surface mount soldering process 1.1.4.1 Wave soldering As mentioned above, the two major soldering processes involved in surface mount technology are wave soldering and reflow soldering Wave soldering, a type of flow soldering, has long been used in the through-hole technology era Typically, the PCBs with THCs inserted are prefluxed via a foam fluxer, then passed over a single laminar solder wave for soldering. .. CSP and flip chips Chapter 11 focuses on reflow profile optimization, since the profile is vital to reflow performance and often is easily controllable by manufacturers Chapter 12 summarizes the background and options of lead-free soldering It also discusses the defect types and mechanism of lead-free reflow processes This book emphasizes reflow process description and troubleshooting The solutions for troubleshooting... preheating prior to wave -soldering A typical wave -soldering thermal profile for SMCs soldering is shown in Figure 1.17 Use of dual-wave and proper 1.1.3.2 Type II Type II boards have both SMCs and THCs on one side of the board and chip components on the other side, as shown in Figure 1.12 Normally the SMCs are attached via reflow soldering, then followed by wave soldering the THCs and chip components, as... Reflow Soldering Processes and Troubleshooting pitch, and provide higher I/O density together with easier manufacturability, smaller package size, and higher speed BGA was the first array family to demonstrate the robustness of the surface mountable-area array package concept and now prevails in the industry As a logical consequence of need for miniaturization, and based on the success of BGA, CSP and. .. Steve Berry and Sandra Winkler, ‘‘Flip Chip Market Expanding to Meet Speed, Performance Demands’’, Chip Scale Review , 11/12, p 6 (1999) 31 L Smith, C Scanlan, and P O’Brien, ‘‘FCIP Delivers Flip Chip Benefits without DCA Complications’’, Advanced Packaging, pp 32–35, (August 1999) 2/19 Fundamentals of Solders and Soldering 2 Soldering uses molten filler metal to wet the surfaces of a joint and form metallurgical... brazing [1] Soldering is a vital interconnect technology involved in both level 1 (IC packaging) and level 2 (mounting of electronic components onto printed circuit boards) processes of the modern electronics industry Therefore, to achieve a high quality and high yield soldering process, it is essential to understand the fundamentals of solder and soldering compound layer [2] In this figure, fluid stands for... Technology 1/5 requires the use of both wave soldering and reflow soldering This complicates the assembly, test, and rework processes, and results in a need for more floor space or boards with heat sensitive components The assembly technology to be chosen depends on the board layout and whether there are through-hole components to be attached In general, the assembly processes can be categorized into three... the general design background and trends of electronic packaging and surface mount technology Chapters 2 and 3 provide the fundamentals of soldering and solder materials Chapter 4 describes the basics of reflow processes These four chapters serve as the fundamentals needed for analyzing soldering defects Chapters 5 through 7 discuss the defect types, defect mechanisms, and solutions for eliminating... 2.1 Soldering theory γSF = γLS + γLF × cos θ Although soldering has been carried out by humans for more than several thousand years, an understanding of this process was minimal until recently The soldering process can be depicted in Figure 2.1, and can be roughly divided into three stages: (1) spreading, (2) base metal dissolution, and (3) formation of an intermetallic In this relation, γSF stands... solder and Cu have grown as scallop-like grains into the molten solder Between the 2/24 Reflow Soldering Processes and Troubleshooting Figure 2.9 Cross-section of a coating 60Sn/40Pb on soft copper (marker = 20 µ) Two intermetallic compound layers Cu3 Sn and Cu6 Sn5 are produced [46] Figure 2.10 Formation of Cu6 Sn5 intermetallics between Sn96.2/Ag2.5/Cu0.8/Sb0.5 and Cu substrate Fundamentals of Solders and . understanding and full support Reflow Soldering Processes and Troubleshooting: SMT, BGA, CSP and Flip Chip Technologies Ning-Cheng Lee BOSTON OXFORD AUCKLAND JOHANNESBURG MELBOURNE NEW DELHI Copyright. Reflow Soldering Processes and Troubleshooting: SMT, BGA, CSP and Flip Chip Technologies To my mother, Shu-shuen Chang, for her care and encouragement To my wife, Shen-chwen Lee, for her understanding. density level, conventional THT is not only more expensive, it is also unmanufacturable. Additional ben- efits of SMT include a lower cost in the shipping and warehousing of components, and in