The electromigration failure mode and failure rate during accelerated electromigration testing is expected to be strongly affected by the mechanical stress state of Al lines, since tensile stress and compressive stress states favor void growth and hillock formations (extrusions), respectively. During electromigration testing, the mechanical stress state or evolution of mechanical stress of an interconnect is a function of current density and temperature, the two principal variables in electromigration testing. In our experiments, we have observed two different electromigration failure modes by varying the current density and temperatures where (i) the passivated Al lines tested at high current density and high temperatures failed by extrusion or hillock type failure and (ii) the interconnect lines tested at low current density and moderate temperature failed by voiding. A mechanical stress model which incorporates both the thermally generated stress and electromigration induced mechanical stress is invoked to explain the electromigration failure mode selection observed in our experiments.