Effect of isothermal ageing on the microstructure, shear behaviour and hardness of the Sn58Bi/SnAgCuBiNi/Cu solder joints

Abstract

The microstructure, shear behavior and hardness of the SnBi/SACBN/Cu solder joint before and after isothermal aging were investigated in comparison with the SnBi/Cu and SACBN/Cu solder joints. The experimental results indicated that the pre-soldered SACBN joint had a significant effect on the formation and growth of the β-Sn grains in the SnBi bulk solder. The brittleness of the SnBi/SACBN/Cu composite solder joint was also suppressed and its failure mode transformed from brittle failure to brittle-ductile failure after reflow. However, the shear strength and failure mode of the SnBi/SACBN/Cu composite solder joint became similar to those of the SnBi/Cu joints after 600 h isothermal aging. The shear strength of the three kinds of solder joints decreased after isothermal aging, but the SnBi/SACBN/Cu composite solder joint showed higher shear strength than SnBi/Cu did during aging. The shear strength of the composite solder joint was 67.1MPa after aging. Due to the diffusion of elements in the isothermal aging process, the microstructure of the composite solder joint was significantly coarsened after aging for 600 hours. This phenomenon further led to the decrease of the hardness and shear strength of the three kinds of solder joints.