Microelectronic devices require material systems combining multiple layers of material for proper operation. These inevitably have different properties, for example, the elastic modulus or the coefficient of thermal expansion. Permanently reoccurring Joule heating and successive cooling during the operation of such devices lead to high thermal stresses within the materials and even failure due to thermomechanical fatigue or delamination of layers. This is dependent on the internal stress state and the amount of plastic strain accumulated. Here, in situ thermomechanical cantilever bending experiments on a Si–WTi–Cu material system to investigate these internal stress states and their influence on deformation behavior using a novel experimental methodology are shown. During heating to (Formula presented.), the Cu layer undergoes partial plastic deformation, which may lead to the failure of a potential device using this material combination. To assess the internal stress and strain states based on the in situ observation, a model incorporating plastic deformation and known residual stresses of layers is proposed and verified by Finite Element Analysis.