Interfacial Microstructure Evolution and High-Speed Ball Shear Fracture of SAC305/Cu-20wt%Zn Solder Joints Under Isothermal Aging

The interfacial microstructure evolution and high-speed ball shear reliability of SAC305 (Sn-3.0Ag-0.5Cu)/Cu-20wt%Zn solder joints were systematically investigated after isothermal aging at 180 °C for up to 250 h. SAC305/electroless nickel immersion gold (ENIG) joints were used as a comparative reference. Unlike prior studies that benchmarked Cu-Zn against bare Cu, this work directly compares the two systems, establishing ENIG as the industrially relevant reference. Cu6(Sn,Zn)5 was identified as the dominant intermetallic compound (IMC) phase at the SAC305/Cu-Zn interface by SEM/EDS analysis. At the SAC305/ENIG interface, (Cu,Ni)6Sn5 formed as the dominant IMC phase, accompanied by a P-rich layer at the (Cu,Ni)6Sn5/Ni(P) boundary. The IMC thickness of SAC305/Cu-Zn joints increased from approximately 2.50 μm in the as-reflowed condition to 3.41 μm at 250 h, consistently exceeding that of SAC305/ENIG joints (1.94–2.25 μm) throughout aging. Despite this, the high-speed ball shear strength of SAC305/Cu-Zn joints was equivalent or superior to that of SAC305/ENIG joints at all aging durations. Fractographic analysis confirmed that the P-rich layer in ENIG joints acted as a preferential crack propagation path under impact loading, driving the brittle fracture ratio to approximately 75% at 250 h—compared to approximately 47% in SAC305/Cu-Zn joints. These results demonstrate that Cu-Zn electroplated from a neutral pyrophosphate-based bath constitutes a highly reliable wetting layer, offering impact reliability equivalent or superior to that of the conventional ENIG surface finish.