Electromigration reliability of different metallization systems (Al-2%Si, Al-Cu/TiW, Cu, and TiN/Al-2%Si/TiN) and structures (Al-via and W-plug) under bidirectional current stress has been studied in a wide frequency range (from mHz to 200 MHz). The experimental results show that at low frequency, the damage healing factor and lifetime under ac stress increases with frequency. At very high-frequency regions, the pure ac lifetime was found to be determined by the thermal process (caused by asymmetrical geometry, etc.) instead of electromigration. All the observations are in agreement with an average current model.