Effect of Ag addition on the intermetallic compound and joint strength between Sn-Zn-Bi lead free solder and copper substrate

This study investigated the effect of Ag addition on the formation of intermetallic compounds and joint strength of the Sn-Zn-Bi based alloys under liquid state aging. The intermetallic compounds were formed by reacting Sn-8Zn-3Bi and (Sn-8Zn-3Bi)-1Ag lead free solders on copper substrate. Scanning electron microscope (SEM) was used to see the morphology of the phases and energy dispersive x-ray (EDX) was used to estimate the elemental compositions of the phases. It was found that for Sn-8Zn-3Bi solder reacting with Cu substrate, the Cu5Zn8 intermetallic was formed. On the other hand, when 1% Ag was added into the solder, the Cu3Sn and Cu6Sn5 IMC were formed. The morphology of the Cu5Zn8 intermetallic was flat whereas for Cu3Sn and Cu6Sn5 were rather scallop. The addition of Ag into the Sn-Zn-Bi solder increases the shear strength of the solder joint. It is believed the scallop morphology of Cu-Sn contribute to strengthening the (Sn-8Zn-3Bi)-1Ag/Cu joints.