Evaluation of Electromechanical Performance of A Flexible Hybrid Electronics Temperature Monitor

One advantage of flexible hybrid electronics (FHE) compared to traditional rigid electronic circuits is their ability to maintain electrical functionality despite repeated mechanical bending and twisting deformations. This characteristic allows conventional electronic devices to be re-engineered into FHE format for use in new applications. Conductor flexibility in these FHE circuits is enabled, in part, by the use of printable metal-polymer composite inks that remain flexible after curing. In this regard, FHE electronics are especially well suited for electronic sensing applications requiring that circuits bend and flex, including on-body wearable electronics for health monitoring. A typical FHE device consists not only of a flexible substrate and flexible conductor circuits, but is also comprised of small, rigid IC-chips and rigid or semi-rigid passive components. These are connected by rigid or semi-rigid conductive glues to the conductive circuit traces on the flexible substrates. However, under flexing and bending stresses, these rigid components bonded to the circuits can result in localized regions of higher strain concentrations. We term these "hinge" regions because of the rapid rate of deformation that occurs there. These occur, in order of increasing strain rates: where the FHE circuit transitions from flexible substrate alone to flexible substrate+conductor circuit; where the flexible substrate transitions from flexible substrate to flexible substrate+conductor circuit+rigid components; and again, where the flexible substrate+conductor circuit transitions to flexible substrate+conductor circuit+rigid component. These high strain or "hinge" areas are often the locus of bending fatigue related failures of metal circuits due to the delamination and cracking of conductors as they transition across a hinge. In this study, a flexible, wearable, FHE temperature monitoring device is fabricated and the detrimental effects of flex-cyclic bending on its electrotechnical performance is investigated. Here, the conductive traces and component bonding pads were fabricated by pattern-dispensing and then curing a conductive silver-filled organic precursor ink. The surface-mounted devices (SMD) were then bonded to silver pads using a silver-filled isotopically conductive epoxy (ICE) adhesive. Flex/bend stress tests (tension and compression) were performed 50/100/150 times on each sample, and after each 50 cycles, the functionality of the sample device was evaluated. In addition, the ability of the temperature-sensing FHE device to continue to measure the constant temperature maintained during testing was followed. The relationship between crack formation in conductor traces versus ICE bonds, and the electrical performance of the system as a function of bending, were both investigated.