current density
as my understanding, electromigration will only present when a signal trace always has a one-way current flow, so only DC current counts, all AC current (either on top of the DC compnent or absolute AC) shall be ignored, that is what we did in our chip design.
think of the principle of electromigration, the collision of electron on the atom core moved the atom, so if electron are bumping from opposite direction, averagely the atom will not move, so that is why AC current shall never be counted, it is not like metal melt-down machanism which is related with average square of I.