Microstructural evolution of Sn-rich Au–Sn/Ni flip-chip solder joints under high temperature storage testing conditions

In this study, we fabricated Sn-rich, Au–Sn flip-chip solder bumps by using a sequential electroplating method with Sn and Au. After reflowing, only a (Ni,Au)3Sn4 intermetallic compound (IMC) layer was formed at the Au–Sn solder/Ni interface. During aging for up to 1000 h at 150 °C, the solder matrix was transformed sequentially in the following order: β-Sn and η-phase, η-phase, η-phase and ε-phase. The microstructure variation resulted from the preferential consumption of more Sn atoms within the solder matrix during aging.