The time-dependent temperature distribution induced by electric-current heating in a double edge-cracked, unpassivated thin aluminum or gold film interconnect lines is monitored using a high-resolution infrared imaging system. A pure aluminum or gold film of 0.2 micron thickness is deposited by high-vacuum evaporation coating and patterned into test structures of varying widths. The operative mechanisms of mass transport are assessed in view of the monitored temperature profile. The pre-cracked aluminum film showed fine crack growth toward the positive electrode, which originated from the initial crack tips. The crack-tip temperature was close to melting, during propagation. A hot spot was formed afterward between the two elongated cracks and led to failure. The crack growth generated a backward mass flow toward the negative electrode. The gold film showed a different pattern where the original cracks propagated toward each other with a slight tilt toward the negative electrode. The tip temperature was lower than the melting temperature.