Abstract: The influence of thermal cycling on the microstructure and joint strength of Sn3.5Ag0.75Cu (SAC) and Sn63Pb37 (SnPb) solder joints was investigated. SAC and SnPb solder balls were soldered on 0.1 and 0.9 μm Au finished metallization, respectively. After 1000 thermal cycles between -40℃ and 125℃, a very thin intermetallic compound (IMC) layer containing Au, Sn, Ni, and Cu formed at the interface between SAC solder joints and underneath metallization with 0.1 μm Au finish, and (Au, Ni, Cu)Sn4 and a very thin Au-Sn-Ni-Cu IMC layer formed between SAC solder joints and underneath metallization with 0.9 μm Au finish. For SnPb solder joints with 0.1 μm Au finish, a thin (Ni, Cu, Au)3Sn4 IMC layer and a Pb-rich layer formed below and above the (Au, Ni)Sn4 IMC, respectively. Cu diffused through Ni layer and was involved into the IMC formation process. Similar interfacial microstructure was also found for SnPb solder joints with 0.9 μm Au finish. The results of shear test show that the shear strength of SAC solder joints is consistently higher than that of SnPb eutectic solder joints during thermal cycling.

Key words: Sn3.5Ag0.75Cu, Solder joint, Au finish, Intermetallic compound