text: The typical current density at which electromigration occurs in Cu or Al interconnects is 10 to 10 A/cm. For solder joints (SnPb or SnAgCu lead-free) used in IC chips, however, electromigration occurs at much lower current densities, e.g. 10 A/cm.
  It causes a net atom transport along the direction of electron flow. The atoms accumulate at the anode, while voids are generated at the cathode and back stress is induced during electromigration. The typical failure of a solder joint due to electromigration will occur at the cathode side. Due to the current crowding effect, voids form first at the corners of the solder joint. Then the voids extend and join to cause a failure. Electromigration also influences formation of intermetallic compounds, as the migration rates are a function of atomic mass.