The microstructure and the strength property of Sn-3.5 mass%Ag and Sn-2.5 m
ass%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 o
n 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 us
ed. 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.