A tool for simulation of electromigration and electromigration-induced damage, MIT/EmSim, has been used to investigate interconnect reliability, focusing on transitions in failure mechanisms associated with void nucleation, growth, and growth saturation. Conventional scaling of electromigration test results assume that the median time to electromigration-induced failure scales with the current density j to the power −n. The effects of transitions in failure mechanisms have been studied by characterizing the apparent current density exponent. When failure is limited by void nucleation, n=2 scaling is observed, and when failure requires substantial void growth, n=1 scaling is observed. When lines end at diffusion barriers such as W or liner-filled vias, void growth saturates in short lines at low current densities, and, depending on the failure criterion, lines under these conditions can be ‘immortal’. As growth saturates, apparent current density exponents increase above 2. Failure mechanism maps can be constructed to illustrate the failure mechanisms and scaling behavior as a function of line length and current density. Failure maps can be used in accurately scaling test results to service conditions, to suggest layout strategies for optimized circuit reliability, and to assess the reliability of new interconnect materials and structures.