Evolution of Microstructure Across Eutectic Sn-Bi Solder Joints Under Simultaneous Thermal Cycling and Current Stressing

Solder joints in practical service conditions are usually subjected to simultaneous current stressing and thermal cycling. At present, most studies focus on either thermal cycling or current stressing. Therefore, the existence of coupling between these two factors remains in doubt. In this study, experiments are conducted to reveal the effect(s) of current density magnitude on the thermomechanical fatigue (TMF) behavior of solder joints. At the early stage of coupling stressing, damage accumulation contributed to TMF at both high and low current densities. Fatigue micro-cracks readily nucleated and propagated along the boundary of Sn-rich and Bi-rich phases. Fatigue crack formation could be retarded through mass transport and Joule heating effects at an early stage. At later stages, the high current density led to electromigration (EM), which played an important role in the failure process by changing interfacial mechanics due to the mass transport. EM led to the final failure of solder joint, where fracture was located at the interface between the intermetallic compound and solder.