Effect of Al content on the formation of intermetallic compounds in Sn–Ag–Zn lead-free solder

Abstract

The effect of addition of Al, up to 1 wt.%, on the formation of intermetallic compounds in the microstructure of Sn–3.7%Ag–0.9%Zn lead-free solder was investigated. The typical microstructure of Sn–Ag–Zn solder is composed of the β-Sn phase and mixed granules which contain the intermetallic compounds (IMCs) of Ag3Sn and AgZn. After alloying with 0.5 wt.% Al, the microstructure of the explored solder evolves into a mixture of the bulk Agl IMCs and β-Sn phase, and most of the bulk Ag2Al IMCs distribute on and around the grain boundaries. The addition of 1 wt.% Al into the Sn–Ag–Zn solder brings many granules and bars of the Ag2Al IMCs, while the amount of the bulk Ag2Al IMCs decrease greatly. The above observation suggests that the bulk Ag2Al IMCs is replaced by the granule-like Ag2Al IMCs appearing along the grain boundaries. Since the grain size of the solder alloyed with 0.5%Al is relatively small as compared to the one alloyed with 1 wt.% Al, the growth of the Ag2Al IMCs was prompted through the feasible diffusion channels along the grain boundaries. Thus, the addition of Al plays an important role on the morphology of the Ag2Al IMCs in the final microstructure of the explored Sn–Ag–Zn solder.