The Influence of Current Direction on the Cu-Ni Cross-Interaction in Cu/Sn/Ni Diffusion Couples

Abstract

The influence of current direction on the Cu-Ni cross-interaction in the Cu/Sn/Ni joint configuration was investigated in this study. During current stressing, an electric current towards or away from the Ni-side of Cu/Sn/Ni was imposed at 150°C. It was observed that the (Cu,Ni)₆Sn₅ ternary compound was the dominant reaction product at both interfaces, and its growth at the Ni-side strongly depended upon the direction and magnitude of the electron flow. When the electron flow was towards the Ni-side, more Cu was found to be driven to the Ni-side, resulting in an increase in the thickness of (Cu,Ni)₆Sn₅. This is due to the chemical-potential-induced Cu flux ($J_{\rm{chem}}^{\rm{Cu}}$) that was enhanced by the electromigration ($J_{\rm{em}}^{\rm{Cu}}$). In the case of electron flow away from the Ni-side, the supply of Cu to the Ni-side was retarded due to the fact that $J_{\rm{em}}^{\rm{Cu}}$ was in the opposite direction to $J_{\rm{chem}}^{\rm{Cu}} .$ The results of this study revealed that the Ni-side (Cu,Ni)₆Sn₅ thickness remained almost unchanged under current stressing of 10⁴ A/cm² at 150°C, which suggests the inward Cu flux is approximately equal to the outward flux, i.e., $J_{\rm{chem}}^{\rm{Cu}} \approx J_{\rm{em}}^{\rm{Cu}} .$