Effect of cooling rate on microstructure and strength properties of tin-silver-copper solder ball bonding

The microstructure and the strength property of Sn-3.5 mass%Ag and Sn-2.5 mass%Ag-0.8 mass%Cu solder ball bonding formed at various cooling rates were investigated. The grain size of Sn phase and the lamellar space depended on the cooling rate Re. The reaction layer was thinner and the grain size of Sn phase coarsened when solder balls containing Cu were used. The reaction layer formed between Sn-3.5 mass%Ag and Cu pad was composed of (Ni-Au, Ag)(3)Sn-4. (Ni-Au, Ag)(3)Sn-2 and (Ni-Au, Ag)(3)Sn precipitates. (Cu-Ni, Au, Ag)(6)Sn-5 was formed in the bonding interface when the ball with Cu was used. The lamellar space in both balls became smaller at faster cooling rates and was proportional to Re(-1/2). The micro Vickers hardness of the solder ball with Cu decreased slightly at the slower cooling rates because of the grain size and lamella coarsening. The shear strength of joint using Sn-2.5 mass%Ag-0.8 mass%Cu was a little higher than that using Sn-3.5 mass%Ag at the slower cooling rate because of the fine lamellar structure.