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)^sub 2^ in the case of SnAgIn, and it formed Au(Sn,In)^sub 4^ and Au(In,Sn)^sub 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 Ni^sub 28^Sn^sub 55^In^sub 17^ in case of SnAgIn, and formed Ni^sub 3^(Sn,In)^sub 4^ in case of SnBiIn, at the solder joint interface. Under the same soldering conditions, the Ni^sub 3^(Sn,In)^sub 4^ layer in the SnBiIn/UBM is thicker than the Ni^sub 28^Sn^sub 55^In^sub 17^ 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. [PUBLICATION ABSTRACT]