Damage Accumulation in Printed Interconnects on Flex Under Combinations of Bending and Tension with Different Amplitudes

The life of a flexible hybrid electronics (FHE) product under realistic use conditions may often be limited by fatigue of the interconnects between components. This is especially true for printed traces on flex. Even if the product is only intended for use for a short time, so that accelerated testing is not required, common test protocols tend to miss critical interactions between sequential strain amplitudes, strain rates and dwells, and/or between different loading modes. Indications are that this may lead the `worst-case' life to be less than anticipated by an order of magnitude or more. A comprehensive ongoing study is characterizing the behavior of different screen printed inks on TPU, PET and Kapton with and without encapsulants, as well as laser and thermally sintered aerosol jet printed nano-Ag and nano-Cu traces on Upilex and Kapton. While many details are unique to a particular combination of trace and substrate a generic picture is emerging.