Reliability of PCB Solder Joints Assembled with SACm™ Solder Paste

Abstract

The solder alloy SACm™0510 has been reported to be a superior alloy when used as BGA balls, exhibiting not only an outstanding drop test performance when compared to SAC105, but also as having high thermal fatigue reliability when compared to high Ag SAC solders. In this study, SACm0510 solder was evaluated as a solder paste. The voiding behavior of SGA solder joints was comparable for SACm0510, SAC105, and SC305. When evaluating SGA assemblies on a customized drop test, SACm0510 outperformed SAC105 considerably, which in turn was much better than SAC305. The drop test performance was found to improve upon thermal aging at 150°C, and the difference between the alloys reduced significantly. This was explained by the speculated grain coarsening which resulted in a softened solder joint, and consequently, a shift of fracture mode from brittle failure toward ductile failure. This model was supported by the observation of the fractured surface moving away from the interface upon thermal aging. The improvement in drop test performance upon thermal aging can be further explained by the large solder joint size of the SGA employed in this study, where the bulk property of solder weighed more than a small solder joint. When the assembled chip resistors were evaluated with a −55°C/+125°C TCT test, no failure was observed after 369 cycles for all three alloys. SAC305 appeared to be the best in maintaining the integrity of the interfacial IMC layer. SACm0510 showed a few crack lines, but less than that of SAC105. SACm0510 solder paste was found to be very compatible with BGAs with SAC305 solder joints, and no abnormal microstructure was observed after thermal aging at 150°C for 1000 hours.