Electromigration (EM)
Beginner
Wires slowly wearing out, because the flowing electricity pushes metal atoms along.
Novice
Slow wear-out of a wire caused by the current itself: over years, flowing electrons push metal atoms along, leaving thin spots or gaps that can eventually break the wire. Checks make sure no wire carries more current than its width allows.
Expert
Current-driven migration of metal atoms that forms voids (and eventually opens) or extrusions. Checked against per-layer current-density limits (average, RMS and peak). Rule-based checks build on Black’s equation and the Blech short-length effect. Power wires carry mostly one-way current; signal and clock wires carry two-way current with partial recovery, so their limits differ. Temperature matters strongly.