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

The addition of 0.5wt.% 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 mu m)/Ni(5 mu m) surface finish, which grow linearly during the aging at 75 and 100 degrees 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. (C) 2009 Elsevier B.V. All rights reserved.