Effect of La addition on the interfacial intermetallics and bonding strengths of Sn–58Bi solder joints with Au/Ni/Cu pads

Abstract

The addition of 0.5 wt.% La into a Sn–58Bi alloy leads to the formation of rare earth containing intermetallic plates in the solder matrix and a slight reduction of its eutectic point. After reflowing, Ni3Sn4 intermetallic compounds appear at the solder/pad interfaces of Sn–58Bi and Sn–58Bi–0.5La packages with Au(0.1 μm)/Ni(5 μm) surface finish, which grow linearly during the aging at 75 and 100 °C for various times ranging from 100 to 1000 h. It can also be observed that the interfacial intermetallics layers in Sn–58Bi–0.5La solder joints are much thinner than those in undoped Sn–58Bi. However, voids in the Sn–58Bi–0.5La solder balls resulted in lower bonding strengths of their solder joints as compared to those in Sn–58Bi packages. Ball shear tests with a shear rate of 0.4 mm/s caused the reflowed and aged solder joints of both packages to rupture through the solder balls with brittle characteristic, which remain unchanged as the shear rate increased to 2000 mm/s.