Microelectronic devices require material systems combining multiple layers ofmaterial for proper operation. These inevitably have different properties, forexample, the elastic modulus or the coefficient of thermal expansion. Permanentlyreoccurring Joule heating and successive cooling during the operation of suchdevices lead to high thermal stresses within the materials and even failure due tothermomechanical fatigue or delamination of layers. This is dependent on theinternal stress state and the amount of plastic strain accumulated. Here, in situthermomechanical cantilever bending experiments on a Si–WTi–Cu materialsystem to investigate these internal stress states and their influence on defor-mation behavior using a novel experimental methodology are shown. Duringheating toTmax¼400°C, the Cu layer undergoes partial plastic deformation, whichmay lead to the failure of a potential device using this material combination. Toassess the internal stress and strain states based on the in situ observation, amodel incorporating plastic deformation and known residual stresses of layers isproposed and verified by Finite Element Analysis