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44 SMT Magazine • April 2017 2. Solder Alloy Factor The voiding difference between different alloys was mainly attributed to the difference in wettability, with 63Sn/37Pb being the best, followed by 57Bi/42Sn/1Ag, and SAC305 being the poorest, as reported in an earlier work [9] . 3. Reflow Temperature Factor The relationship between reflow tempera- ture and voiding was a complicated one. This was caused by two changes induced by temper- ature. The first was the weight loss rate of flux, and the second was the flux activity. 4. Flux Quantity Factor The flux quantity effect showed two oppo- site trends for different alloys. This was attrib- uted to different reflow temperatures for differ- ent alloys. Conclusion By studying varying solder alloy types, flux quantity coated on the preforms, extent of oxi- dation on the Cu coupons, reflow peak temper- ature, and weight applied on the top of simulat- ed die, we can see the following voiding behav- ior in high-power die-attached soldering using preforms: • BLT: For SAC305, the voiding area % in- creased with decreasing BLT at first, then lev- eled off at a lower BLT, although the voiding volume decreased with decreasing BLT due to constrained lamellar solder flow. • Oxidation: Voiding was the highest for SAC305, followed by 57Bi/42Sn/1Ag, with 63Sn/37Pb being the lowest, and increased with the increasing oxidation of the Cu coupons. • Flux Quantity: As flux quantity increased, voiding increased for SAC305 and 63Sn/37Pb, but decreased for 57Bi/42Sn/1Ag, mainly due to the different temperature ranges at reflow. • Reflow Temperature: Voiding increased with increasing reflow temperature up to 170°C due to increasing vaporization, decreased with the further increase in the reflow temperature up to 210°C due to increasing flux activity, and increased again at a temperature beyond 210°C due to rapid flux outgassing. SMT References 1. Adamson, "Physical Chemistry of Surfac- es," 3rd ed., John Wiley & Sons, 1976. 2. Chingchen S. Chiu, Ning-Cheng Lee, Kim- bela Randle and Christopher Parrish, "Voiding in BGA at Solder Bumping Stage," ISHM, p.462- 471, Philadelphia, PA, Oct. 14-16, 1997. 3. Wanda Hance and Ning-Cheng Lee, "Void- ing Mechanisms in SMT," China Lake's 17th An- nual Electronics Manufacturing Seminar, 1993. 4. Wanda Hance and Ning-Cheng Lee, "Void- ing Mechanism in BGA Assembly, ISHM, p.24- 30, Los Angeles, CA, Oct. 24-26, 1995. 5. Ning-Cheng Lee, "Reflow soldering pro- cessing and troubleshooting SMT, BGA, CSP, and Flip Chip T echnologies", Newnes, pp.288, 2001. 6. Ning-Cheng Lee, "Optimizing Reflow Pro- file Via Defect Mechanisms Analysis", IPC Print- ed Circuits Expo 1998. 7. Y an Liu, Bill Manning, Benlih Huang, and Ning-Cheng Lee, "A Model Study of Profiling for Voiding Control at Lead-free Reflow Soldering", Nepcon Shanghai, China, April 11, 2005. 8. Ning-Cheng Lee, "Critical Parameters in Voiding Control At Reflow Soldering", Chip Scale Review, August-September 2005. 9. Benlih Huang and Ning-Cheng Lee, "Pros - pect of Lead Free Alternatives for Reflow Sol- dering", International Microelectronics Assem- bly and Packaging Society (IMAPS) Conference 1999, Chicago. Editor' s Note: This paper was originally pub - lished in the proceedings of SMTA International, 2016. Dr. Ning-Cheng Lee is the vice president of technology at Indium Corp. Dr. Arnab Dasgupta is a research chemist at Indium Corp. Elaina Zito has since returned to school to further her education. VOIDING CONTROL AT HIGH-POWER DIE-ATTACH PREFORM SOLDERING

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