X-ray diffraction was used to measure the strain relaxation in passivated Al-0.5%Cu lines at 200°C after cooling directly from an anneal at the passivation deposition temperature. The strain decay is fit with a stress-assisted, thermally-activated relaxation model. A high-voltage scanning electron microscope was used to image the presence and growth of voids through the passivation. The time scale of the void growth is not the same as that of the hydrostatic relaxation, indicating that voiding is not solely responsible for the observed relaxation. The relaxation of the line is modeled using a von Mises yield criterion and allowing elastic compliance of the passivation. The magnitude of the calculated relaxation agrees with the measurements. It is suggested that a combination of voiding and passivation compliance is responsible for the measured hydrostatic strain relaxation.