Solid-state reactions between Sn-20.0 wt.%In-x wt.%Zn solders and Ag and Ni substrates

The Sn-20 wt.%In (Sn-20In) alloy is a promising base material for low-temperature Pb-free solders. Zn is usually added in solders to reduce the extent of undercooling during reflow, while Ag and Ni are commonly seen under bump metallurgy in contact with solder in electronic products. In this study, solid-state reactions at 150 °C between Zn-doped Sn-20In solders and Ag and Ni substrates are investigated. In Sn-20In-xZn/Ag couples, when the Zn-doping level is low (x ≤ 1.0), the reaction path is γ-InSn4/ζ-AgZn/ζ-(Ag,In)/ζ-AgZn/Ag, that the ζ-AgZn layer near the solder has non-uniform composition, the ζ-(Ag,In) layer has a porous microstructure, and the ζ-AgZn phase near the substrate is composed of small grains. When the Zn doping level is high (x ≥ 2.0), the reaction path becomes γ-InSn4/ε-AgZn3/γ-Ag5Zn8/ζ-AgZn/Ag, that all intermetallic compounds (IMCs) are planar and neither Sn nor In participate in the reactions. Although the reactions are very sensitive to Zn contents, the overall thicknesses of IMCs do not vary much with different Zn-doping levels. In Sn-20In-xZn/Ni couples, the planar Ni5Zn21 phase is the only reaction product with a very slow growth rate. The interfacial liquation in Sn-20In/Ni contacts can be fully mitigated with a minor Zn addition of 0.5 wt.%.