Effects of trace amounts of rare earth additions on the microstructures and interfacial reactions of Sn57Bi1Ag/Cu solder joints

The effects of trace amounts of rare earth (RE) additions on the melting property and microstructural evolution of SnBiAg/Cu solder joints were studied by differential scanning calorimetric test and microstructural observation. The results indicated that with the increase of RE additions, the solidus temperature decreased and the mushy temperature zone increased slightly in SnBiAg-xRE (x = 0.25, 0.5, 0.75 and 1.0) solder alloys. The microstructures of the Bi-rich dendrites in SnBiAg-xRE solder alloys were refined by the additions of minor RE elements. However, too many RE elements added into solder matrices led to the formation of large RE(Bi,Sn)(3) intermetallic compound (IMC) which weakened the adsorption effect of RE elements on the Bi-rich dendrites. In addition, the thickness of the Cu6Sn5 IMC layers of SnBiAg-xRE/Cu solder joints were reduced remarkably due to the adsorption effect of RE elements at the interfaces of the Sn element and the Cu6Sn5 IMC layer.