Solid-state reactions between Ni and Sn-Ag-Cu solders with different Cu concentrations

It had been reported that, during the reflow of the Sn-Ag-Cu solders over the Ni-bearing surface finishes, a slight variation in Cu concentration produced different reaction products at the interface. In this study, we extended our earlier efforts to investigate whether this strong Cu concentration dependency also existed for the solid-state aging reaction between the Sn-Ag-Cu solders and Ni. Specifically, five Sn-3.9Ag-xCu solders (x = 0.2, 0.4, 0.5, 0.6, and 0.8) were reacted with Ni at 180 °C. It was found that the strong Cu concentration dependency disappeared after the solid-state aging at high temperatures for sufficient periods of time. For all the Cu concentrations studied, the same intermetallic compounds, a layer of (Cu1 − yNiy)6Sn5 and a layer of (Ni1 − xCux)3Sn4, formed at the interface after aging. This study showed that the initial difference in the intermetallic compounds right after reflow could be aged out at high temperatures. The growth mechanisms for (Cu1 − yNiy)6Sn5 and (Ni1 − xCux)3Sn4 were different, and were pointed out in this study.