Electromigration of eutectic SnPb solder interconnects for flip chip technology

The electromigration of eutectic SnPb solder interconnects between a Si chip and a FR4 substrate was studied at 120 °C for up to 324 h with current stressing of 104 amp/cm2. Hillocks were observed at the anode and voids at the cathode. The dominant diffusing species was found to be Pb, confirmed by its accumulation at the anode. Diffusion markers were used to measure the electromigration flux and calculate the effective charge of atomic diffusion in the solder. Extensive microstructural evolution was also observed in the two-phase solder alloy that occurred by a ripening process.