Bimodal electromigration-induced open failure time distributions are reported for both single-level and double-level interconnect structures due to inherent microstructural variations. For flat interconnect lines, the bimodality arises when some lines in a test population have only bamboo boundaries, and other lines contain some triple junctions as well as bamboo boundaries. For via structures, two failure mechanisms result if some of the vias are not in contact with grain boundaries in the underlying metal, and others are. For both lines and vias, a large apparent deviation in failure times, σ, is a signal of a possible bimodal distribution, which can also be confirmed by microscopic examinations, resistance measurements, or an investigation of the metal microstructure. Insufficient testing can result in an incorrect asumption of a monomodal failure distribution, leading to either optimistic or pessimistic reliability predictions, depending on the size of the test sample population and the degree to which the test population is representative of the actual overall population.