Electromigration effects upon the low-temperature Sn/Ni interfacial reactions

Sn/Ni interfacial reactions at 100 °C with and without the passage of electric currents were studied by using the Sn/Ni/Sn sandwich-type reaction couples. The Ni₃Sn₄ and metastable NiSn₃ phases were formed at both the Sn/Ni and Ni/Sn interfaces in the couples reacted at 100 °C without the passing through of electric currents. Metallographical analyses revealed that the metastable NiSn₃ phase nucleated and grew at the grain boundary, and the growth rate of the NiSn₃ phase was much faster than that of the Ni₃Sn₄ phase. For the couples with the passage of electric currents of 4 × 10³ A/cm² density, the Ni₃Sn₄ reaction layers were found at both interfaces as well. However, the NiSn₃ phase was found only at the Ni/Sn interface where the directions of electron flow and Ni diffusion were the same, and the NiSn₃ phase was not found at the Sn/Ni interface. The NiSn₃ phase formed at the Ni/Sn interface was found to nucleate and grow much faster than those without the passage of electric currents. It is likely that the electromigration effect enhances the movement of Ni atoms and accelerates the nucleation and growth of the NiSn₃ phase while at the Sn/Ni interface, where the directions of electron flow and Ni diffusion are opposite, electromigration effects retard the movement of Ni atoms and inhibit the nucleation of the NiSn₃ phase.