It is a phenomenon that causes physical breakage due to current density, thus reducing the life of the part. It is mainly because of electrons moving fast which takes away some physical material. As some material is taken off, interconnect becomes narrower, which increases current density and makes the failure faster. Electromigration is severe at the output terminals of gates. This is mainly because the current is uni-directional (P transistor to N transistor) at the output terminal. The current flow from VDD through P transistor charges the interconnect cap. When the interconnect cap is discharged, current flow goes from interconnect to VSS through N transistor. Either way, it is uni-directional at the gate output. In interconnects, the phenomenon is referred to as AC electromigration or self-heat. The bottom line is that you need to control the current density (I/area), because it is a reliability issue.