The interfacial microstructure and Kirkendall voids in In-48Sn/Cu solder joint

The interfacial microstructure and Kirkendall voids between eutectic SnIn and polycrystalline Cu substrate was investigated after reflowing and long-term solid-state aging process. The results indicated that two crystal-structure IMC species: Cu(In, Sn)₂ phase existing at the solder side and Cu₂(In, Sn) phase locating at the Cu side, among which Cu₂(In, Sn) phase with two different morphologies, the coarse-grained and fine-grained Cu₂(In, Sn) sublayers formed at the interface after reflowing. During aging, Cu(In, Sn)₂ phase transformed into Cu₂(In, Sn). At the interface between coarse- and fine-grained Cu₂(In, Sn) sublayers, there exist formed Kirkendall voids after reflowing and solid-state aging, which may degrade the mechanical property of the solder joints.