Electromigration testing of an interconnect system comprised of copper metallization and a low-k dielectric material gives rise to bimodal lognormal statistics with early and late fails. When separated from one another, we observed failure modes characterized by a three-parameter lognormal distribution and the same threshold failure time. It is shown that the threshold failure time corresponds to damage (presumably voids) nucleation of the electromigration process. The dependence of the threshold failure time and the median time to failure on current density suggests that both void nucleation and void growth need to be considered for accurate modeling of the electromigration lifetime.