Electromigration-induced Pb segregation in eutectic Sn–Pb molten solder

Preferential Pb electromigration (EM) in eutectic Sn–Pb molten solder was characterized by examining Pb segregation at the anode of the solder on a thin stripe of Cu metallization. The eutectic Sn–Pb molten solder stressed at current densities of 103−104 A/cm2 was found to form columnar Pb-rich phase (α) at the anode side and Pb deficiency at the cathode side. The columnar α-phase grew in the direction parallel to the electron flow, and its growth rate increased proportionally with the current density applied through the molten solder. By measuring the growth rate of the columnar α-phase, we calculated the Pb effective charge number of EM in the eutectic Sn–Pb molten solder to be 2.7, which is one order of magnitude lower than the reported value of 47 for self-electromigration in bulk Pb. The mechanism for the EM-induced Pb segregation in Sn–Pb solder was also explored.