Estimation of Thermal Fatigue Resistances of Sn–Ag and Sn–Ag–Cu Lead-Free Solders Using Strain Rate Sensitivity Index

Abstract

A tensile test was conducted to evaluate thermal fatigue resistances of Sn–Ag and several Sn–Ag–Cu lead-free solder alloys. The test is based on the strain rate change method to obtain a strain rate sensitivity index, m. The m values were investigated at various strains during the tensile test until fracture. The plots of m and strain where m is measured showed a linear relationship. Therefore, the m value at zero strain, m₀, and the gradient of the fitting line, k, were obtained by extrapolation. Using m₀ and k values investigated, an estimation of the thermal fatigue resistance of the solder joint was attempted. The ranking of the reciprocals of m₀ and k in the solders is relatively in good agreement with that of the thermal fatigue resistance of the solder joints, and thus m₀ and k can be indexes to develop a new lead-free solder with the excellent thermal fatigue resistance. Moreover, the influences of the aging treatment on m₀, k and the microstructure of the solder were also investigated.