Effect of electromigration induced joule heating and strain on microstructural recrystallization in eutectic SnPb flip chip solder joints

A refinement of the lamellar microstructure was observed in eutectic SnPb solder joints in electromigration. Electromigration has both atomic flux and electron flow. The latter generated joule heating and the former caused strain. The formation of nanoscale lamellar microstructure spent a large amount of interfacial energy, and we proposed that the driving force comes from the strain induced by electromigration under a high current density. Kinetically, refinement of the lamellar microstructure required fast atomic diffusion at a high homologous temperature. The joule heating mainly from the on-chip Al interconnect lines tremendously increased the temperature of solder bumps and enabled fast atomic diffusion. The strain induced by electromigration, when combined with a high homologous temperature, could lead to recrystallization in the sample to form the nanoscale lamellae.