Investigation on Creep Resistance of Lead-Free Bi-Based Solder Alloy with In Doping by Nanoindentation

Solder alloys are susceptible to creep deformation when subjected to prolonged stress during practical applications, with the creep resistance playing a crucial role in determining the life of alloy. In this study, In was introduced as a modification element in Bi-based solder alloys to enhance creep resistance. The addition of In resulted in a noticeable reduction in grain size and the formation of the strengthening phase Ag2In within the microstructure. Nanoindentation analysis revealed that while the elastic modulus exhibited no sensitivity to strain rate, the hardness showed a positive correlation with strain rate. The load-depth curves displayed displacement bursts, indicating the formation of dislocations at the initial stages of deformation. The stress exponents for pure Bi and the modified Bi-2Ag-0.5Cu-0.5In alloy were measured at 10.12 and 10.75, respectively, suggesting a dislocation slip mechanism. Furthermore, as the strain rate increased, the stress exponent of the Bi-2Ag-0.5Cu-1.5In alloy ranged from 2.5 to 13.78, leading to a transition in the creep mechanism from grain boundary slipping (GBS) to dislocation slipping. The enhanced elastic modulus, hardness, and resistance to creep in the alloy can be ascribed to the precipitation strengthening of Ag2In and the refined microstructure.