Nucleation behaviors of the intermetallic compounds at the initial interfacial reaction between the liquid Sn3.0Ag0.5Cu solder and Ni substrate during reflow

The soldering of eutectic Sn3.0Ag0.5Cu solder on an electroplating Ni is carried out at 250 °C for 5 s followed by rapid quench with liquid nitrogen. During soldering, metastable NiSn compound formed in contact with Ni layer because Ni dissolves into the liquid solder to react with Sn. The formation of Sn–Cu–Ni ternary amorphous diffusion zone is observed between NiSn nucleation zone and solder. In this ternary amorphous diffusion zone, different types of structures are formed such as base structure, atomic clusters, short-range order structure formations and nucleation of nanocrystalline NiSn and Cu6Sn5 compounds. The long-range order structures of nano cells consist of the short-range order and base structures. The atoms within an atomic cluster which is composed of several base structures tend to arrange to form the short-range order. The atomic clusters and short-range order may be the intermediate state during the nucleation of intermetallic compunds.