Joule heating refers to the increase in temperature of a conductor as a result of resistance to the electrical current flowing through it. At an atomic level, Joule-heating is the result of moving electrons colliding with atoms in a conductor, whereupon momentum is transferred to the atoms, increasing their kinetic energy in the form of heat.
Heat arising from Joule heating of the conductor causes the atoms to vibrate further from their ideal lattice positions, thereby increasing the electron scattering events. This is manifested as an increase in resistance, and acceleration in the process of electromigration.
High current density → Large number of scattered electrons → More electromigration
Temperature rises are normally small in ICs because the silicon substrate acts as a good heat sink.