Circuits Assembly - January 2008 - (Page 32)

Cover Story Table 11. Detailed Results from Phase 1 Screening Flux Solids IPC J-STD-004 Designation Impact Weight <85% Barrel Fill 5 Solder Bridges 4 Solder Balls 2 SMD Skips 2 Residue A B C D E F G H I J K L M 4.50% 3.70% 3.50% 7.00% 4.25% 6.00% 2.90% 19.50% 22.40% 15.00% 15.00% 6.20% 2.40% ORL0 ORL0 ORL1 ROL1 ROM0 ROL0 ORL0 ORH1 ORH1 ROM1 ORL1 ORL0 ROL0 3420 2828 2980 2759 2871 3400 3395 2712 2882 2605 2626 2599 3450 4640 353 568 725 280 616 545 784 338 298 289 367 352 904 7648 15 16 16 4 7 12 15 0 0 6 1 16 8 16 39 46 29 30 37 35 56 22 35 35 31 21 37 4096 Total Defects Possible Solder Skips on Glued Bottom-Side WaveSoldered SMDs Preheat effect. Higher preheat increased solder skips when using a VOC-free OANC flux, while lowering the incidence of skips with low VOC LRNC and VOC OAWS fluxes. This again was likely due to the sublimation of flux solids during preheat. In general, preheat was a less significant factor. Conveyor speed effect. As expected, solder skips were more numerous at higher conveyor speeds, but only with the no-clean fluxes, and not to a significant extent. Flux selection for reduced solder skips. VOC-free OANC fluxes had the best results for solder skips. LRNC fluxes had very few skips. Higher preheat was worse for VOCfree OANC than VOC OAWS flux. By far, the OAWS fluxes generated more solder skips. Solder Balls Preheat and conveyor speed were not a factor in the presence or absence of solder balls with the organic acid fluxes. When using low-residue LRNC flux, there appeared to be fewer solder balls at higher conveyor speed. Flux selection was the only critical factor. OAWS flux by its nature had zero solder balls due to washing of these boards. All boards with VOC-free OANC flux had solder balls. There were also significant differences between fluxes within each category. Conclusions As indicated, there are tradeoffs in characteristics within fluxes of the same type and among different flux type categories. For Pb-free soldering 0.125"-thick OSP boards, the following conclusions can be drawn: 1. For increased Pb-free barrel fill, the water-soluble 32 Circuits Assembly JANUARY 2008 fluxes are the best to use, followed by low-VOC no-clean fluxes, followed by VOC-free 1 Lower is better no-clean fluxes. Flux Cat. Phase 1 2. For reduced Pb-free sol24 1.072 OANC der bridging for bottom-side 28 1.200 OANC wave-soldered SMDs, water24 1.228 OANC soluble fluxes are best, fol52 0.783 LRNC lowed by low-VOC no-clean 46 1.075 LRNC fluxes and VOC-free no-clean 48 1.169 LRNC fluxes. 46 1.461 LRNC 3. For reduced Pb-free sol8 0.617 OAWS der balls on the bottom side 8 0.670 OAWS of the wave-soldered board, 60 0.773 HRNC water-soluble fluxes are 48 0.767 HRNC best because solder balls are 24 0.918 LVRE removed by washing, followed 42 1.326 LVRE by low-VOC no-clean fluxes 64 and VOC-free no-clean fluxes. 4. For reduced Pb-free solder skips on the board bottom-side, VOC-free no-clean fluxes are best, followed by low-VOC no-clean fluxes and water-soluble fluxes. The data presented indicate much more work to do to improve Pb-free wave soldering results for organic acid VOC-free and low-residue rosin no-clean fluxes. Future work should concentrate on the development of VOC-free and low-residue rosin no-clean fluxes for lead-free soldering, especially for boards with thicknesses of greater than 0.100". Also, pin probability evaluations need to be conducted more if low-residue, rosin no-clean fluxes are used, as the solids content may affect pin probe results. ■ Weighted Score References 1. J-STD-004A, Requirements for Soldering Fluxes, January 2004. 2. IPC-A-610D, Acceptability of Electronic Assemblies, February 2005. Acknowledgments The authors would like to acknowledge the flux suppliers for proving their wave soldering fluxes for this evaluation. They would also like to acknowledge their internal company colleagues who helped to support much of this work. Ed.: This article was originally published at SMTA International in October 2007 and is used with permission. The Appendix is online at circuitsassembly.com/cms/content/view/5895. Douglas Watson is process development engineer, Jasbir Bath is lead engineer and Pan Wei Chih is process engineer at Flextronics International (flextronics.com); douglas.watson@flextronics.com. circuitsassembly.com http://circuitsassembly.com/cms/content/view/5895 http://flextronics.com http://circuitsassembly.com

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Circuits Assembly - January 2008 Contents Caveat Lector Industry News Market Watch Talking Heads Focus on Business On the Forefront Screen Printing Better Manufacturing Walking on Water Flux Selection for Lead-Free Wave Soldering An A-to-Z Guide to X-Ray Inspection, Part II Evolution in Action Research Priorities for the Electronics Industry Selective Soldering Process Doctor Pb-Free Lessons Learned Materials World Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - January 2008

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