Effects of latent damage of recrystallization on lead free solder joints

Recrystallization behavior and microstructure evolution during liquid-liquid thermal shock of lead free solder alloys have been investigated in this study. SAC305 (Sn-3.0Ag-0.5Cu) solder alloy was used as the base solder alloy in which 5 different pitch sizes of ball grid array (BGA) were cycled in liquid-liquid thermal shock with (0/100 C) profile and almost zero dwell time. The results show that recrystallization takes place in all BGA assemblies regardless of pitch size, but at different times. However, the larger the pitch sizes the sooner recrystallization will take place. This partially due to strain magnitude difference between central and outer joints. Thus larger pitch size coupons were subjected to higher strain magnitude, especially corner joints and hence recrystallization takes place on these coupons earlier. Moreover, it was found that cracks usually start and extend along the recrystallized regions.