Influences of low In alloying and aging on microstructure and plastic deformation behavior of Sn-58Bi solder

The SnBi eutectic solder shows high ductility at low strain rate, while plastic deformation of the Sn-rich phase in it is restrained due to the high content of the brittle Bi-rich phase and the fine interlocked lamellar structure, thus the impact toughness of the SnBi solder is poor. In this study, 1.5 wt% of In was added into the Sn-58Bi eutectic solder, and the influences of In alloying and aging on microstructure, deformation and fracture behaviors of the SnBi solder were deeply investigated. The results reveal that both the In alloying and aging can coarsen the SnBi solder and change the interlocked lamellar structure. The tensile elongation of the SnBiIn solder is much higher than that of the SnBi solder, and can be further improved after aging, with a little decrease in strength. The SnBi solder deforms mainly through grain boundary and phase boundary sliding, and fractures in a brittle mode, with cleavage of the Bi-rich phase and little plastic deformation in the Sn-rich phase. In contrast, serious deformation and fragmentation of the Sn-rich phase occurs in the SnBiIn solder even under a lower stress, especially for the aged SnBiIn solder, which not only due to coarsen of the microstructure and less discontinuous of the Bi-rich phase, but also soften of the Sn-rich phase. The impact toughness of the SnBiIn solder is improved, with fits with the increase in tensile elongation.