Strength of bonding interface in lead-free Sn alloy solders

The effect of applying ultrasonic wave to soldering on the strength of the bonding interface in lead-free Sn–3.5%Ag, Sn–57%Bi and Sn–57%Bi–1%Ag solder joints has been investigated at various test temperatures and strain rates. The bonding strength of the solder joints depends on the test temperature and the strain rate. The bonding strength of the Sn–Bi solder joints near room temperature is much higher than that of the Sn–3.5Ag solder, but the ductility is lower. The relation between the maximum stress in the stress–strain curve and the strain rate is indicated by the same power law type equation as high temperature creep. Applying ultrasonic wave to soldering influences the microstructure near the bonding interface. As a result, the fine microstructure layer is formed near the interface between the solder and the copper and it increases the strength of the bonding interface in the solder joints.