Mechanisms of thermal cycling damage in polycrystalline Sn-rich solder joints

Solder joints will experience elevated temperatures during operation due to resistive heating and ambient conditions. Device components have different thermal expansion characteristics, leading to straining of the solder joint and development of surface damage features. The anisotropic thermal expansion of βSn grains and discrepancy between the thermal expansion of intermetallic compounds causes degradation to be localized near grain boundaries and phase interfaces. Three experiments were conducted to investigate thermal effects on solder joint microstructure. Interrupted low temperature aging revealed the formation and movement of grain boundary ledges and uninterrupted thermal aging confirmed that the ledges formed during the cooling cycle of the experiment. Finally, high temperature thermal cycling demonstrated the acceleration of surface damage that accompanies an increase in peak temperature and cycling frequency. The orientation of the c-axis of the Sn matrix grains proved to be the critical feature controlling the direction of grain boundary ledge migration and type of surface deformation surrounding Cu₆Sn₅ intermetallic needles.