Electromigration in Sn–Pb solder strips as a function of alloy composition

Using thin film solder strips, we have investigated the electromigration of six different compositions of Sn–Pb solders at current density of ${10}^5 {\mathrm{A/cm}}^{\mathrm{2}}$ near ambient temperature. The six compositions are pure Sn, ${\mathrm{Sn}}_{\mathrm{80}}{\mathrm{Pb}}_{\mathrm{20}},$ ${\mathrm{Sn}}_{\mathrm{70}}{\mathrm{Pb}}_{\mathrm{30}},$ ${\mathrm{Sn}}_{\mathrm{62}}{\mathrm{Pb}}_{\mathrm{38}}$ (eutectic), ${\mathrm{Sn}}_{\mathrm{40}}{\mathrm{Pb}}_{\mathrm{60}},$ and ${\mathrm{Sn}}_{\mathrm{5}}{\mathrm{Pb}}_{\mathrm{95}}.$ The eutectic alloy, with the lowest melting point and a high density of lamella interfaces, was found to have the fastest hillock growth. As composition moving toward the two terminal phases, the hillock growth rate decreases; but it increases again in pure Sn. The interface between Sn and Pb, being the fastest kinetic path of mass transport, also serves as the place to initiate hillock and void formation.