High temperature reliability and interfacial reaction of eutectic Sn–0.7Cu/Ni solder joints during isothermal aging

The interfacial reactions and growth kinetics of intermetallic compound (IMC) layers formed between Sn–0.7Cu (wt.%) solder and Au/Ni/Cu substrate were investigated at aging temperatures of 185 and 200 °C for aging times of up to 60 days. After reflow, the IMC formed at the interface was (Cu, Ni)6Sn5. After aging at 185 °C for 3 days and at 200 °C for 1 day, two IMCs of (Cu, Ni)6Sn5 and (Ni, Cu)3Sn4 were observed. The growth of the (Ni, Cu)3Sn4 IMC consumed the (Cu, Ni)6Sn5 IMC at an aging temperature of 200 °C due to the restriction of supply of Cu atoms from the solder to interface. After aging at 200 °C for 60 days, the Ni layer of the substrate was completely consumed in many parts of the sample, at which point a Cu3Sn IMC was formed. In the ball shear test, the shear strength decreased with increasing aging temperature and time. Until the aging at 185 °C for 15 days and at 200 °C for 3 days, fractures occurred in the bulk solder. After prolonged aging treatment, fractures partially occurred at the (Cu, Ni)6Sn5 + Au/solder interface for aging at 185 °C and at the (Ni, Cu)3Sn4/Ni interface for aging at 200 °C, respectively. Consequently, thick IMC layer and thermal loading history significantly affected the integrity of the Sn–0.7Cu/Ni BGA joints.