Abnormal migration behavior and segregation mechanism of Bi atoms undergoing liquid–solid electromigration

The migration behavior of Bi atoms in Cu/Sn–58Bi/Cu solder interconnects undergoing liquid–solid electromigration (L–S EM) was in situ observed using synchrotron radiation real-time imaging technology. Bi atoms tend to migrate from the cathode toward the anode resulting in the linear growth of Bi-rich phase layer at the anode in both the heating and cooling stages but not in the dwelling stage. Since the temperature of Sn–Bi solder in the dwelling stage is higher than those in both the heating and cooling stages, the electrically driven migration effect diminishes with increasing temperature. As a result, a three-layer equilibrium distribution of Bi-rich, Sn–Bi and Sn-rich phases across the solder is observed in the dwelling stage of 140 °C, and a uniform Bi distribution across the solder is observed in the dwelling stage of a higher temperature of 170 °C, i.e., Bi atoms fully diffuse backward at such a high temperature. The abnormal migration behavior of Bi atoms is determined by the combined effect of chemical potential gradient-induced flux (Jchem) and EM-induced flux (Jem). The EM-induced flux (Jem) is determined by the effective charge number (Z*) of Bi atoms that is calculated as − 3.57 at 140 °C based on the growth kinetics of the Bi-rich layer. Furthermore, the Z* value of Bi atoms is calculated as − 3.04 by using a modified model for calculating Z* of liquid metals, which agrees well with the experimental value. The present work provides a reference for purifying refractory metals from alloys.