Enhanced stress relaxation of Sn-3.8Ag-0.7Cu solder by electrical current

The stress relaxation responses of the Sn-3.8Ag-0.7Cu joints following exposure to electrical currents were examined to investigate the effect of clectromigration on the reliability of solder joints. It was found that the stress relaxation rate was enhanced for the Sn-3.8Ag-0.7Cu solder joints subjected to a current density of 2 × 10⁴ A/cm². Sn hillock formation was observed in situ on the surface of the solder joint and the increase of the hillock volume was obtained as a function of the current application time. Analysis of the vacancy flux indicated that the variations of the vacancy concentration with the clectromigration time from the calculations agreed with the growth kinetics of the hillocks observed in the experiments. By modeling the stress relaxation as a climb-assisted dislocation glide process, it is shown that the vacancy accumulation Induced by electromigration enhanced the dislocation climb rate, resulting in a large increase of the stress relaxation rate.