Hybrid Solder Joint for Low-Temperature Bonding Application

Low-temperature bonding has become a significant trend in advanced electronics packaging technology. A low-reflow-temperature process can effectively reduce the risk of warpage, thus greatly enhancing device reliability. SnBi eutectic solder is one of the best candidates for low-temperature assembly. In this study, first, a layer of pre-solder (Sn-0.3Ag-0.7 Cu, SAC0307) was reflowed; then, another layer of low-temperature solder (Sn-56Bi-1Ag-0.2Cu, SB102) was reflowed on top to create a hybrid solder joint. Small bismuth precipitates were uniformly distributed in the SAC0307 layer after long-term aging. Cross-sectional images were examined to analyze the kinetics between the SB102/SAC interdiffusion reaction. Shear tests and fracture surface analyses were undertaken to investigate the joint strength and the failure mode. The SAC0307 layer prevented the solder embrittlement by reducing the ratio of Bi-rich phases in the SB102 matrix and successfully transformed the mechanical fracture mode from brittle to ductile. The results demonstrated that the mechanical properties of the solder joint were greatly improved after long-term heat treatment.