Using Kelvin test structures, the via reliability with respect to electromigration failure of tungsten and aluminum vias under DC, pulse-DC and AC stressing have been studied. The results indicate that although W-plug vias can eliminate the step coverage problem, this metallization system is not ideal because the Al/W contact presents an undesirable flux divergence location for electromigration. Al vias are more reliable than W-plug vias with respect to electromigration failure. The via lifetimes under bidirectional stressing current were found to be orders of magnitude longer than DC lifetimes under the same stressing current density for both W and Al vias. The unidirectional 50% duty-factor pulse-DC lifetime was found to be twice the DC lifetime at the low-frequency region (<200 Hz) and 4-5 times the DC lifetime at the high-frequency region (>10 kHz), in agreement with the vacancy relaxation model.