Cross-Interaction between Ni and Cu across Sn Layers with Different Thickness

The Ni/solder/Cu material sequence is one of the most common material sequences in the solder joints of electronic packages. In this study, the Ni/Sn/Cu ternary diffusion couples were used to investigate the solder volume effect on the cross-interaction between Ni and Cu. Experimentally, a pure Sn layer with the thickness of 100–400 μm was electroplated over Cu foils. A pure Ni layer (20 μm) was then deposited over the as-deposited Sn surface. The diffusion couples were aged at 160°C for different periods of time. With this technique, the diffusion couples were assembled without experiencing any high temperature process, such as reflow, which would have accelerated the interaction and caused difficulties in analysis. This study revealed that the cross-interaction could occur in as short as 30 min. A detailed atomic flux analysis showed that the Cu flux through the Sn layer was about 25–40 times higher than the Ni flux. Moreover, it was found that (Cu_1−x Ni _x )₆Sn₅ on the Ni side reduced the consumption rate of the Ni layer, and the cross-interaction also reduced the Cu₃Sn thickness on the Cu side.