Interfacial reaction and mechanical properties of eutectic Sn-0.7Cu/Ni BGA solder joints during isothermal long-term 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 between 70 and 170 °C for up to 100 days. After reflow, the IMC formed at the interface was (Cu,Ni)6Sn5. In addition, only this (Cu,Ni)6Sn5 layer was observed in the samples aged at temperatures between 70 and 150 °C. On the other hand, after isothermal aging at 170 °C for 50 days, the solder/Ni interface exhibited a duplex structure of (Cu,Ni)6Sn5 and (Ni,Cu)3Sn4. The time exponents (n) determined at temperatures below 150 °C were about 0.2. The determined n values are presumably related to a grain boundary diffusion-controlled mechanism. In the ball-shear tests, the shear strength significantly decreased after aging for initial 1 day, after which it remained nearly unchanged through further prolonged aging. In all samples, fracture occurred in the bulk solder.