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 degrees C, and a uniform Bi distribution across the solder is observed in the dwelling stage of a higher temperature of 170 degrees 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 (J(chem)) and EM-induced flux (J(em)). The EM-induced flux (J(em)) is determined by the effective charge number (Z*) of Bi atoms that is calculated as - 3.57 at 140 degrees 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.