Relaxation of thermally induced stresses in passivated line structures can lead to void formation, and its rate is strongly correlated with microstructural change in the lines. A bending beam technique was used to measure the thermal stress relaxation behavior of passivated Al (1 wt % Cu) line structures with 3, 1, and 0.5 mu m linewidths as a function of temperature up to 15 h. Our data indicate that stress relaxation in Al(Cu) lines depends on line geometry and temperature, and exhibits log (time) kinetics, which is consistent with a thermally activated dislocation glide mechanism. The observed behavior can be explained by a combined result of the driving force due to the effective shear stress and kinetics due to thermally activated mass transport. Scanning electron microscope and transmission electron microscope techniques were used to study stress-induced voiding as a function of aging time and temperature. A strong correlation between stress relaxation rate and void density was observed for 1 mu m wide lines, but not for 0.5 mu m wide lines. A lower void density of 0.5 mu m wide lines suggests that the bamboo grain structure may influence the mass transport and retard void formation.