Effect of Ni to the Cu substrate on the interfacial reaction with Sn-Cu solder

• Ni addition to Cu alloy affects the diffusion of Cu and Sn atoms, which in turn affects the growth of the Cu3Sn.
• A Ni-rich interfacial reaction layer ((Ni,Cu)3Sn) about 600 nm in thickness was formed at the Cu-0.2%Ni/Cu3Sn interface.
• Thermodynamic analysis revealed that a diffusion path was from the Cu3Sn to the Cu-0.2%Ni, by way of (Ni,Cu)3Sn.
• Calculation results are interpreted as the (Ni,Cu)3Sn mitigates the effect of Ni on the diffusion driving force.

Both the formation of voids and the growth of a Cu3Sn layer were suppressed during reactive diffusion between Sn-0.7%Cu solder and Ni added Cu substrates compared with between the solder and a Cu substrate without Ni. The effects of Ni on the formation of voids and of Cu3Sn were studied using microstructural, thermodynamic and kinetic approaches. Thermodynamic calculations were performed on the assumption where the addition of Ni affected the chemical potential and diffusion driving force at both the Cu-Ni/Cu3Sn and Cu3Sn/Cu6Sn5 interfaces. The results suggest that the elements (Cu and Sn) in the Cu3Sn layer are diffuse in opposite directions than normal after prolonged annealing. The results of calculations performed to assess the effects of Ni on elemental diffusion in the Cu3Sn layer were in agreement with experimental observations. In addition, calculations were performed for scenarios both with and without the presence of (Ni,Cu)3Sn to understand the role that the addition of Ni plays in decreasing the thickness of the Cu3Sn layer.

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