Fatigue Properties of Lead-free Doped Solder Joints

The reliability of an electronic assembly is limited by a fatigue failure of one of the interconnected solder joints. Fatigue properties of the common lead free solder joints have been studied for large bulk solder samples. However, the microstructure of actual size solder joints is completely different than the large solder samples, which can lead to different mechanical and fatigue properties. SnAgCu (SAC) based solder alloys are widely used in the electronic industry recently. To further enhance the reliability of solder alloys, SAC alloys with small amounts of additives such as Ni, Bi, Sb was developed. In this paper, the mechanical properties of individual doped SAC solder joints are studied. The experiment includes shear test and shear fatigue cycling test of individual doped SAC based solder spheres and the common SAC305 solder spheres with three surface finishes including OSP, ImAg, and ENIG on cupper pads. The effect of surface finish and shear strain rate on the shear strength was studied, and the effect of stress amplitude on fatigue life was analyzed as well. The results showed that solder joints with high Ag have superior fatigue properties. ImAg surface finish was demonstrated as a better surface finish in terms of shear strength.