Interfacial Microstructure Evolution Between Sn-Zn Solders and Ag Substrate During Solid-State Annealing

In this study, solid-state interfacial reactions between Ag and Sn-Zn alloys with varying Zn content (0.1 wt.% to 9 wt.%) were investigated at 170°C. The reaction couples were prepared by electroplating Ag on the Sn-Zn alloy to avoid dissolution of Ag into the molten solder during soldering. The Zn content greatly influenced the reaction products and the interfacial microstructures. When the Zn content was less than 4 wt.%, Ag₃Sn and AgZn layers were simultaneously formed. Notably, Zn could actively diffuse through the Ag₃Sn layer and react with Ag to form the AgZn phase. With the proceeding reaction, small α-Ag particulates were produced within the AgZn phase. With 9 wt.% Zn, the dominant reactions formed Ag₅Zn₈ and AgZn layers. The interfacial microstructure evolved significantly with reaction time. Interface instability due to Zn depletion in the solder resulted in massive spalling of the Ag₅Zn₈ layer. The Ag₃Sn phase was then produced next to the AgZn layer. Moreover, another reaction couple, Sn-9 wt.%Zn/Sn(15 μm)/Ag, was prepared, in which fast interdiffusion between Zn and Ag across the Sn layer was demonstrated due to the strong chemical affinity of Zn.