Comparison of Sn2.8Ag20In and Sn10Bi10In solders for intermediate-step soldering

We chose Sn−2.8Ag−20In and Sn−10Bi−10In (numbers are in weight percentages unless specified otherwise) as Pb-free solder materials for intermediate-step soldering. We then investigated how the two solders reacted with the under bump metallurgy (UBM) of Au/Ni (Au: 1.5 μm and Ni: 3 μm) at 210°C, 220°C, 230°C, and 240°C for up to 4 min. All, of the Au UBM was dissolved into the solder matrix as soon as the interfacial reaction started. The reaction formed Au(In, Sn)2 in the case of SnAgIn, and it formed Au(Sn, In)4 and Au(In, Sn)2 in the case of SnBiIn. The formation mechanism of the intermetallic phases is explained thermodynamically. The exposed Ni layer reacted with the solder and formed Ni28Sn55In17 in case of SnAgIn, and formed Ni3(Sn, In)4 in case of SnBiIn, at the solder joint interface. Under the same soldering conditions, the Ni3(Sn,In)4 layer in the SnBiIn/UBM is thicker than the Ni28Sn55In17 layer in the SnAgIn/UBM. Because of the thicker intermetallic compound layer, the SnBiIn solder joint has weaker shear strength than the SnAgIn solder joint.