Clarification of Cu rich phase in SAC/FeNi solder joint

In this study, interfacial reaction of SAC/FeNi solder joints is investigated, and the Cu-rich IMC was clarified according to scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations. At SAC/27Fe73Ni interface, Cu rich IMC is rod-like dispersing in the solder. On the other hand, at the interface of SAC/55Fe45Ni solder joints, Cu rich IMC has an island-shape. From the EDS analysis, the Cu rich IMC is possibly Cu6Sn5 with Fe and Ni solid solutionized. However, the morphology of this Cu rich IMC differs from Cu6Sn5 formed in the same condition of SAC/Cu solder joint greatly. In order to identify this Cu rich IMC precisely, electron diffraction analysis was carried out using TEM on the cross section sample of solder joints. Through sample tilting, a series of selected area electron diffraction (SEAD) patterns of the Cu rich IMC were obtained, which can be indexed with η-Cu6Sn5 structure exactly. Therefore, the Cu-rich IMC formed at the interface of SAC/FeNi solder joints was clarified as η-(Cu,Ni)6Sn5 with hexagonal crystal structure.