The Effect of Bi on the Mechanical Properties of Aged SAC Solder Joint

Recent studies demonstrate significant reductions in mechanical properties of lead-free interconnection materials in harsh environments such as automotive and military applications. SAC alloys are subjected to substantial deterioration under high-temperature operating conditions. This problem is worsened with extended aging conditions and high-stress levels. After aging, the reliability of the solder joint is reduced due to the microstructure evolutions. Doping the solder joints with certain elements such as Bi and Sb leads to improve their mechanical properties. Three different solder alloy compositions are considered in this study, including SnAgCu (SAC305), SnAgCu-3Bi, and SnAgCu-6Bi. The solder joints are subjected to four levels of aging time at 150°C, including 0, 10, 100, 1000 hrs. The Instron 5948 Micro Tester is used to perform shear and micro-indentation for individual solder joints by using customized experimental setup. The ultimate shear stress of Bi alloys was notably higher than the ultimate shear strength of SAC 305 regardless of the aging time due to solid solution hardening and precipitate hardening. The creep rate of SAC305 is significantly higher than SAC-Bi alloys regardless of the aging time due to the solid solution phenomena of Bi in the Sn lattice. However, the aging effect on the SAC-Bi alloys is relatively marginal compared to SAC305. The goal of this research paper is to investigate the effect of adding Bi on the mechanical properties and the microstructure of SAC-based solder joints by considering aging times.