Flexible substrates (polymers) for plastic electronic products are far less stable to environmental factors, like heat and moisture, than currently used non-flexible substrates (silicon). This introduces problems during the lithography process of these products. This study presents a thermo-mechanical model of PEN foil; an organic material often used as flexible substrate for plastic electronics. The model is validated with experimental foil behavior during a loading experiment and with the experimental bending behavior in a foil on-carrier situation. These kind of models are useful as they can be used to predict and to optimize lithographic process steps. ©2009 IEEE.