Intermediate decomposition of metastable Cu5Zn8 phase in the soldered Sn–Ag–Zn/Cu interface

The structural evolution process of the soldered interface between Sn–3.7%Ag–0.9%Zn lead-free solder and Cu substrate with different soldering time (less than 10 min) was explored by microstructural observations. It is found that a metastable Cu5Sn8 intermetallic compounds (IMCs) layer was firstly separated out close to the Cu substrate, then a stable Cu6Sn5 layer formed above it, as a result of the decomposition of the metastable Cu5Sn8 layer. It is proved that the existence of an intermediate decomposition of metastable Cu5Sn8 layer in the evolution of the soldered interface slightly accelerates the diffusion of Sn atoms in the formed Cu6Sn5 IMCs layer.