On the formation of Cu-Ni-Sn ternary intermetallics in SMT solder joints

Formation of intermetallics for SnPb eutectic solder in a sandwich structure with one side copper pad/HASL finish and other side copper pad/NiAu finish was studied. The experimental results strongly indicate that the interface IMCs depends not only on the solder material and the metallization it reacted to but also on the metallization on the other side of the soldering interface. Copper can diffuse fast across the molten solder to react with nickel layer on the other side to form (CuNiSn) ternary alloy. Depending on the quantity of diffused copper, different types of IMCs are formed.