Effect of Strain Rate and Temperature on Tensile Properties of Bi-Based Lead-Free Solder

Abstract

Tensile properties of three Bi-based lead-free solder which are pure Bi, Bi-1.0Ag-0.3Sn-0.03Ge (mass%), and Bi-2.5Ag (mass%) were investigated and compared with that of Pb-rich Pb-2.5Ag-2.5Sn (mass%) solder. Tensile strength of pure Bi is the minimum among solder investigated regardless of the temperature and strain state. Although tensile strength of Bi-based solder is lower than that of Pb-2.5Ag-2.5Sn at 25℃, those of Bi-1.0Ag-0.3Sn-0.03Ge and Bi-2.5Ag improve and become analogous and higher than that of Pb-2.5Ag-2.5Sn at a temperature of 125℃ or more. The effect of strain rate on elongation is negligible in solder investigated. Although elongations of Bi-based lead-free solder are lower than that of Pb-2.5Ag-2.5Sn at 25℃, they increase with increasing temperature. While the elongation of Pb-2.5Ag-2.5Sn relatively stable at approximately 20–30% regardless of temperature, elongations of Bi-1.0Ag-0.3Sn-0.03Ge and Bi-2.5Ag become a same level with that of Pb-2.5Ag-2.5Sn at 125℃ and 175℃. In particular, the ductility of pure Bi which is about 5% improves drastically at temperatures of 75℃ or more and the elongation rises to approximately 60%. From microstructure observation results, it was confirmed that the addition of small amount of Sn and Ge is effective to form fine microstructure. From fracture surface observation results, it was confirmed that brittle fracture occurs at 25℃ and the fracture mode changes to ductile fracture when the temperature increases and the ductility improves.