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 Ni 3 Sn 4 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/cm 2 density, the Cu 6 Sn 5 phase was formed at the solder/Ni interface besides the Ni 3 Sn 4 phase. Similar to those without the passage of electric currents, only the Ni 3 Sn 4 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 Ni 3 Sn 4 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 z* a decreases in magnitude with raising temperatures, which indicates the electromigration effect becomes insignificant at higher temperatures. © 2001 American Institute of Physics.