We have studied two kinds of solder reactions between eutectic SnPb and Cu. The first is wetting reaction above the melting point of the solder, and the second is solid state aging below the melting point of the solder. In wetting reaction, the intermetallic compound (IMC) formation has a scallop-type morphology. There are channels between the scallops. In solid state aging, the IMC formation has a layer-type morphology. There are no channels but grain boundaries between the IMC grains. Why scallops are stable in wetting reactions has been an unanswered question of fundamental interest. We have confirmed that the scallop-type morphology is stable in wetting reaction by re-wetting the layer-type IMC by molten eutectic SnPb solder. In less than 1 min, a layer-type Cu6Sn5 is transformed back to scallops by the molten solder at 200°C. In analyzing these reactions, we conclude that the scallop-type morphology is thermodynamically stable in wetting reaction, but the layer-type morphology is thermodynamically stable in solid state aging, due to minimization of interfacial and grain boundary energies.
- Flip chip technology
- Intermetallic compound formation
- Morphological stability
- Solder reaction