Study of bending reliability and electrical properties of platinum lines on flexible polyimide substrates

We have experimentally studied the variation in electrical resistance of flexible platinum lines patterned on polyimide foil when they are subjected to circular bending constraints. The lines were patterned by means of standard photolithography and sputtering deposition. Two different photolithography masks were used for comparative evaluation: an un-expensive transparency mask and a standard chromium mask. Measurements of the temperature coefficient of resistance (TCR) and time stability of the resistance have been acquired for lines bent down to 1.25 mm radius of curvature on a customized bending setup, showing good reliability results. The robustness of the lines has been also assessed by registering their change in resistance while bending at different radii of curvature. The lines showed reliability issues for radii of curvature below 1.25 mm, presenting a resistance variation of 19% for transparency mask-fabricated lines and 9% for chromium mask-fabricated lines. The worse reliability performances of transparency mask lines, compared to the chromium mask ones, was found to be due to their imperfect edges, which promoted the formation and propagation of cracks during bending. The results of the experiments in this work permitted to compare the performances of flexible conductive lines with different geometry and fabricated with two different masks, establishing quantitative and qualitative bending limits for their appropriate operation in flexible electronics systems.