Influence of mixed rare earth addition on the microstructure of the Sn-Cu-Ni solder and interfacial reaction of Cu/Sn-Cu-Ni/Cu joints

In recent years, the Sn-Cu-Ni lead-free solders have attracted considerable attention from electronic packaging manufactures, due to the advantage of low cost. In this study, La-Ce mixed rare earth (MRE) with three different amounts of 0.05, 0.10 and 0.25 % (wt.%) was added into the Sn-0.7Cu-0.05Ni alloy to form the Sn-0.7Cu-0.05Ni-xMRE solder, and then, the interfacial reaction and microstructure evolution of the interfacial intermetallic compounds (IMC) in Cu/Sn-0.7Cu-0.05Ni-xMRE/Cu micro-scale joints were investigated systematically. Results show that all the Ni content in the solder has been consumed in the formation of the interfacial IMC (Cu, Ni)₆Sn₅ during the liquid-solid interfacial reaction process, and the presence of Ni element has not been detected in the IMC phases which formed and grew during solidification process of the solder joint. All joints prepared by using Sn-0.7Cu-0.05Ni-xMRE solders show two common features, i.e., the well-distributed microstructure and non-presence of the MRE phase. Adding a trace amount of La-Ce MRE into the Sn-0.7Cu-0.05Ni based solder can obviously refine the microstructure of the solder and significantly influence the thickness of the interfacial IMC layers in the joint, a small amount addition (0.05 wt.%) of MRE in the Sn-0.7Cu-0.05Ni based solder can greatly suppress the growth of interfacial IMC in the solder joint.