Study of the Effects of Zn Content on the Interfacial Reactions Between Sn-Zn Solders and Ni Substrates at 250°C

The interfacial reactions of Ni with Sn-Zn alloys with 1 wt.% to 9 wt.% Zn at 250 degrees C were examined. The Zn content greatly affected the intermetallic compounds formed and microstructural evolution. A continuous Ni(5)Zn(21) layer was formed for the Sn-Zn/Ni couples with a Zn content higher than 5 wt.%. A stable reaction layer existed at the interface and grew thicker with time. When decreasing to 3 wt.% Zn, two thin reaction layers of Ni(5)Zn(21) and (Ni,Zn)(3)Sn(4) were simultaneously observed initially, and then an extremely large faceted Ni(5)Zn(21) phase was formed near the boundary between the Ni(5)Zn(21) layer and the solder. Furthermore, when the Zn content was lower than 2 wt.%, the dominant phase changed to (Ni,Zn)(3)Sn(4). The Zn concentration of the solder gradually decreased with reaction, and thus the interfacial stability was reduced. Subsequently, a large amount of (Ni,Zn)(3)Sn(4) grains were dispersed into the molten solder, and finally the reaction product at the interface changed to Ni(3)Sn(4).