Solid-state interfacial reactions between Sn–3.5Ag–0.7Cu solder and electroless Ni-immersion Au substrate during high temperature storage test

The interfacial reactions between Sn–3.5Ag–0.7Cu solder and an electroless nickel-immersion gold (ENIG)-plated Cu substrate were investigated during aging at 200 °C for up to 1000 h. During aging, the Ni(P) layer was transformed partially into an Ni3P layer, and then the Ni3P layer was also transformed into an Ni–Sn–P ternary layer. After aging for 1000 h, Cu6Sn5 and Cu3Sn IMCs were formed below the Ni–Sn–P layer. During aging at 200 °C, the interfacial IMC was transformed sequentially into (Cu,Ni)6Sn5, (Ni,Cu)3Sn4 and finally (Cu,Ni)6Sn5.