Scanning electron microscope in-situ investigation of fracture behavior in 96.5Sn3.5Ag lead-free solder

In-situ tensile tests of as-cast 96.5Sn3.5Ag eutectic solder were performed under the scanning electron microscope (SEM) using different strain rates at room temperature, and various crack initiation and propagation behavior was observed on the specimen surface. It was found that, due to the existence of Ag₃Sn intermetallic particles and the special microstructure of β-Sn phases in Sn3.5Ag solder, grain boundary sliding (GBS) was no longer the dominant mechanism for this Pb-free solder. In the lower strain rate regime, accompanied by partial intragranular cracks, intergranular fracture along the grain boundaries in Sn-Ag eutectic structure or along the interphase boundaries between Sn-rich dendrites and Sn-Ag eutectic phases occurred primarily for the Sn3.5Ag solder in the early tensile stage. However, significant plastic deformation was observed in large areas for the specimens tested at higher strain rates, and cracks propagated in a transgranular manner across the Sn dendrites and Sn-Ag eutectic structure.