Mechanical stress relaxation in passivateci metal lines generally has two components, shear relaxation at constant volume, changing the stress distribution, and volumetric relaxation changing the hydrostatic stress. In order to contribute to the understanding of the underlying mechanisms we have collected detailed data on relaxation of passivated Al- and AlSiCu-lines with different aspect ratios a (0.2 ≤ a ≤ 1) and different thicknesses. A bending beam technique was used to measure the stress of these lines during thermal cycling from RT to 450°C and isothermal relaxation at different temperatures between RT and 350°C. Main results are: i) During thermal cycling the stress at RT and the total stress amplitude increase with increasing aspect ratio a, while stress hysteresis formation increases with decreasing a. ii) During isothermal relaxation the relaxed stress is strongly dependent on temperature with maxima between 150 and 250°C. The maximum shifts to lower temperatures with decreasing aspect ratio. From the temperature dependence the effective activation energy of 0.6±0.3eV for AlSiCu can be determined, iii) Finite element calculation and the method of “eigenstrains” are used to estimate seperately volumetric (voiding) and shear plasticity in these lines. Clearly voiding dominates the isothermal stress relaxation in all measured samples.