Effect of 3 wt.% Bi in Sn-Zn solder on the interfacial reaction with the Au/Ni metallization in microelectronic packaging

The work presented in this paper focuses on the role of 3 wt.% Bi in the base Sn-9%Zn solder on the shear strengths and the interfacial reactions with Au/Ni/Cu pad metallization in ball grid array (BGA) applications. Sn-Zn based Pb-free solder alloys were kept in molten condition (240degC) on the Au/electrolytic Ni/Cu bond pads for different time periods ranging from 1 minute to 60 minutes. Cross-sectional studies of the interfaces were also conducted to correlate with the fracture surfaces. Sn-Zn-Bi solders showed better results in terms of shear strength on liquid state annealing than Sn-Zn solders. Two failure modes, ball cut and interfacial intermetallics/pad separation are assessed for the different solders and reflow times. The consumption of Ni in the Sn-Zn solder was larger than that in the Bi-containing solder. By the addition of 3% Bi in the eutectic Sn-Zn solder, the formation of Ni-Zn compound is reduced which in turn increase the reliability of the solder joint to the higher extent.