Electromigration lifetests performed on a variety of Al alloy films to study the form of the failure distribution are discussed. Sample sizes ranged from 35 to 120, allowing exploration near the 1% failure level. Results show that the lognormal rather than the logarithmic extreme value distribution holds where the grain size is smaller than the linewidth. Where the grain exceeds the linewidth, however, either distribution can be used to represent the data. >