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46 PCB007 MAGAZINE I AUGUST 2018 Mechanism" of solder joints arising from board- component interactions under complex loading conditions. The system level effects can be com - plex and included: • Effect of PCB glass style and pad size • Influence of PCB mounting conditions • Thermo-mechanical effects on constrained assemblies • Vibration at elevated temperatures- vibration fatigue • Mitigating system level effects-like underfill By using test vehicles, microsections, Weibull charts and comparisons to FEA and model cor- relations, Dr. Serebreni made a compelling pre- sentation about the usefulness of model pre- dictions from simulations. The third talk of the day was by David Pinsky of Raytheon on "Tin Whis- ker Mitigation by SMT Re- flow." Tim covered the work Raytheon has been doing to prevent tin whis- ker growth by using the SMT assembly process to create a "self-miti- gation" condition where all exposed tin is cov- ered with tin-lead. The DOE matrix for the study was composed of four experimental fac- tors (Table 4): • Each of the seven assembly locations were provided with a kit containing eight bare boards (four types, two replicants) and all the components necessary to populate them. Each assembler chose process conditions as they would deem appropri- ate to achieve compliance with J-STD-001, Class 3 requirements – HASL finish/large pads – HASL finish/small pads – OSP finish/large pads – OSP finish/small pads • Components used: 0603 chip, TQFP100-14 mm, SSOP28-5.3 mm, A-TQFP44-10 mm-.8 mm, SO14G-3.8 mm, SO14G-3.8 mm, SSOP28-5.3 mm, DO-216AA, QFP44-0.8 mm, LQFP32-7 mm-0.8 mm, SO14G-3.8 mm, SOT223, LQFP48-7 mm-0.5 mm, LQFP100-14 mm-0.5 mm & 0612 chip • Assembly process details (Table 5) The analysis was complex and thorough. The summary of the results indicated: • Pad finish was not significant • Pad size was significant but had a very minor effect • Soldering process was very significant to self-mitigation • Package geometry is the most significant factor • Some components were nearly certain to self-mitigate under all process conditions (0603 chips, SO14Gs) Table 4: The four experimental factors in Raytheon's Tin Whisker Mitigation reflow study. David Pinsky Maxim Serebreni