Effect of Low Bi Content on Reliability of Sn-Bi Alloy Joints Before and After Thermal Aging

Abstract

Sn-58Bi, an eutectic alloy, has been explored for use as a low-temperature lead-free solder alloy. However, the properties of Sn-Bi alloys as well as those of their joints need to be improved significantly so that these alloys can be applicable for practical use. In particular, two drawbacks need to be addressed: the intrinsic brittleness of Bi and the microstructure coarsening of these alloys during aging. In this study, Sn-Bi-Zn (SBZ) and SBZ-In (SBZI) alloys with low Bi contents were examined to elucidate the effects of the addition of Zn and In to the Sn-45Bi alloy on the interface and shear strengths of Cu/Cu joints before and after aging. In the case of the SBZ/Cu and SBZI/Cu joints, Bi coarsening was not observed either near or at the interfaces of the Cu/Cu joints. The shear strengths of the SBZ and SBZI joints remained unchanged after aging for 1008 h, suggesting that the SBZI alloy demonstrated the highest long-term reliability among all the joints examined.