On the Brückner Pilot line PET films have been stretched simultaneously at different MD stretching ratios and different annealing temperatures. The thermal expansion of the BOPET film samples parallel to MD and TD and also the refractive indexes of some selected samples have been analyzed. The advantages of Brückner’s LISIM® technology in order to optimize the co-efficient of thermal expansion of BOPET films used as substrates for printed electronics applications has been shown.