Influence of Zn additions on the interfacial reaction and microstructure of Sn37Pb/Cu solder joints

Abstract

The effects of Zn (5 and 10 wt%) additions into Sn37Pb solder and isothermal solid state aging on the interfacial reactions between Sn37Pb–xZn solders and Cu substrates were investigated in this study. It was found that the addition of Zn changed the types and morphologies of interfacial IMC layers during reflowing and thereafter under aging condition. During reflowing, the planar-type Cu5Zn8 compound was the interfacial IMC for Sn37Pb–xZn (5 and 10 wt%) solder, while the scallop-type Cu6Sn5 was the interfacial IMC for Sn37Pb solder. After aging, the final interfacial structure for Sn37Pb–5Zn solder was solder/Cu5Zn8/Cu6(Sn,Zn)5/Cu, while solder/Cu6Sn5/Cu3Sn/Cu for Sn37Pb solder and solder/Cu5Zn8/Cu for Sn37Pb–10Zn solder, respectively. The Kirkendall voids disappeared with Zn addition into Sn37Pb solder. For the Sn37Pb–5Zn/Cu solder joint, the thickness of Cu6(Sn,Zn)5 layer increased, while the thickness of Cu5Zn8 layer decreased with aging time extended to 360 h due to the decomposition of the Cu5Zn8 IMC layer by diffusing Cu and Zn atoms into nether IMC layer, combining Sn atoms diffused from solder matrix to form Cu6(Sn,Zn)5 IMCs. Furthermore, the growth of Cu6Sn5 and Cu3Sn layers for Sn37Pb/Cu solder joint and the total IMC layer at the interface of Sn37Pb–xZn (x = 0, 5, and 10 wt%) solder with Cu substrate followed the diffusion control mechanism. Compared to the Sn37Pb–5Zn/Cu solder joint, higher Zn concentration depressed the growth of Cu5Zn8 layer for Sn37Pb–10Zn solder. In the end, refining effect on IMC grains was found by the addition of Zn into Sn37Pb solder and the 10 wt% Zn-doping significantly refined the interfacial IMC grains.