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The effect of ambient atmosphere on the stability of Al-Cu bonds aged at 150 to 300°C was studied. Data show that atmosphere plays a major role in determining the failure mode of wires bonded and aged under otherwise identical conditions. Bonds aged in air, nitrogen, and vacuum all experienced an initial decrease in pull strength due to annealing of the wire. The mechanical strength of vacuum and nitrogen aged samples continued to degrade with time. A shift in the primary failure mode from wire breaks to bond lifts was noted. The pull strengths of air-aged samples remained stable through 1600 h. The dominant failure mode was wire breaks. Al-Cu intermetallic phases were formed under all conditions. Surface diffusion was suggested by the presence of a groove at the bond perimeter. With time, the groove propagated along the metallic Cu/Cu intermetallic interface, undercutting the bond. The formation of copper oxides in air-aged samples retarded this process. An effective activation energy of 0.45 eV was calculated for the Al-Cu failure mechanism in vacuum.