Electromigration effect upon the Sn–0.7 wt% Cu/Ni and Sn–3.5 wt% Ag/Ni interfacial reactions

Abstract

This study investigates the effect of electromigration upon the interfacial reactions between the promising lead-free solders, Sn–Cu and Sn–Ag, with Ni substrate. Sandwich-type reaction couples, Sn–0.7 wt% Cu/Ni/Sn–0.7 wt% Cu and Sn–3.5 wt% Ag/Ni/Sn–3.5 wt% Ag, were reacted at 160, 180, and 200 °C for various lengths of time with and without the passage of electric currents. Without passage of electric currents through the couples, only one intermetallic compound Ni3Sn4 with ∼7 at. % Cu solubility was found at both interfaces of the Sn–0.7 wt% Cu/Ni couples. With the passage of an electric current of 500 A/cm2 density, the Cu6Sn5 phase was formed at the solder/Ni interface besides the Ni3Sn4 phase. Similar to those without the passage of electric currents, only the Ni3Sn4 phase was found at the Ni/solder interface. Directions of movement of electrons, Sn, and Cu atoms are the same at the solder/Ni interface, and the growth rates of the intermetallic layers were enhanced. At the Ni/solder interface, the electrons flow in the opposite direction of the Sn and Cu movement, and the growth rates of the intermetallic layers were retarded. Only the Ni3Sn4 phase was formed from the Sn–3.5 wt% Ag/Ni interfacial reaction with and without the passage of electric currents. Similar to the Sn–0.7 wt% Cu/Ni system, the movement of electrons enhances or retards the growth rates of the intermetallic layers at the solder/Ni and Ni/solder interfaces, respectively. Calculation results show the apparent effective charge za* decreases in magnitude with raising temperatures, which indicates the electromigration effect becomes insignificant at higher temperatures.