High temperature aging study of intermetallic compound formation of Sn–3.5Ag and Sn–4.0Ag–0.5Cu solders on electroless Ni(P) metallization

The Sn–3.5Ag and Sn–4.0Ag–0.5Cu solders on Au/electroless Ni(P) metallization exhibited different interfacial morphologies after high temperature storage (HTS) at 150 °C. Ni3Sn4 intermetallic compounds (IMCs) were found in the Sn–Ag system, while for the Sn–Ag–Cu system the IMCs consisted of two kinds of interfacial reactions. For the Sn–3.5Ag solder, the Ni3Sn4 IMC particles lost adhesion/contact to the electroless Ni(P) layer and clear gap was observed in the samples after high temperature storage (HTS) aging for 1000 h. In the Sn–4.0Ag–0.5Cu solder joint, both (Cu,Ni)6Sn5 and (Ni,Cu)3Sn4 compounds were observed after HTS aging. Since the difference in nucleation site and growth rate for kinds of IMCs, (Cu,Ni)6Sn5 was observed at top and (Ni,Cu)3Sn4 at bottom when the interfacial compound layer became thicker as a function of thermal aging. Some voids were found between the electroless Ni(P) interface and the Sn–Ag solder after 168 and 500 h of thermal aging, while the clear gap between the solder and the Ni layer existed after 1000 h aging. The formation mechanism for this gap could be the interconnection and growth of the voids. In the Sn–Ag–Cu system, voids were found inside the Sn–Ni–Cu ternary interfacial compounds after 500 and 1000 h. The formation mechanism for these voids was thought to be Kirkendall effect or etching process. The interfacial layer of Sn–Ag–Cu solder on electroless Ni(P) coating showed the better thermal stable than eutectic Sn–Ag solder.