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

Sn/Ni interfacial reactions at 100 degreesC 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 degreesC 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 x 10(3) A/cm(2) 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.