Reliability and failure mechanism of solder joints in thermal cycling tests

Previously crack propagation and joint failure in thermal cycling tests were correlated with recrystallization of Sn grains in SnAgCu (SAC) ball grid array (BGA) solder joints. Generally recrystallization of the Sn grains was observed to occur in the high strain region before solder joint failure. In an effort to better understand this failure mechanism in SnAgCu solder joints subjected to mild thermal cycling profiles, and in smaller solder joints that have interlaced Sn grain morphologies, both conventional (-40/125°C, 0/100°C) and mild (20/80°C) accelerated thermal cycling (ATC) tests were performed on various SAC solder joints. Correlations between microstructure and failure mechanism for solder joints on various BGA packages, chip scale packages (CSP), and quad-flat no-lead (QFN) packages were examined. The microstructure of samples was carefully analyzed; selected samples were removed from the chamber after different numbers of cycles in order to investigate the evolution of the SAC solder joint microstructure. Both recrystallization and intergranular crack growth were observed in these SAC solder joints after thermal cycling. Distinct coarsening of precipitates was observed in the recrystallized areas adjacent to cracks, consistent with strain enhanced coarsening. The 20/80°C reliability test results suggested that the failure mechanism of SAC assemblies is similar to that of conventional ATC profiles (0/100°C, -40/125°C) commonly performed in industry. After the same percentage of projected characteristic life, crack lengths were observed to be much smaller for interlaced twinning structures than for larger beach ball structures. This correlation of longer SAC solder joint lifetimes with interlaced Sn grain morphologies suggests that optimized control of Sn grain morphology in SAC solder joints may significantly enhance Pb free solder joint lifetime.