The effective charge number and the critical current density of electromigration in eutetic SnPb and Pb Free $(SnAg_{3.8}Cu_{0.7)$ flip chip solder bumps are studied. The effective charge number of electromigration in eutectic SnPb solder is obtained as 34 and the critical current density is $j=0.169{ imes}({delta}_{sigma}/{delta}_x});A/cm^2,;where;({delta}_{sigma}/{delta}_x})$ is the electromigration-induced compressive stress gradient along the length of the line. While the effect of electromigration in Pb free solder is much smaller than that in eutectic SnPb, the product of diffusivity and effective charge number $DZ^{ast}$ has been assumed as $6.62{ imes}10^{-11}$. The critical length for electromigration are also discussed.