Twist Testing for Flexible Electronics

Electronic devices have found use in a variety of flexible form factors such as wearable health monitoring devices, fitness trackers integrated into garments, etc. These form factors expose the electronic components (printed traces, passives, active devices, etc.) to a wide range of mechanical strains that are not typically seen in traditional rigid devices. In fact, the utility of these devices depends on its ability to stretch, bend, fold, and/or twist. As such, there is a need for methods and standards for testing these devices under these mechanical loads. In our ongoing work, we have conducted extensive stretch and bend testing of conductors, co-planar waveguides, antennas, and other elements. Both monotonic and fatigue testing of some of these components have also been carried out. In this paper we describe the development of a test apparatus that can subject flexible electronic test coupons to controlled twists while monitoring in-situ the electrical resistance of the printed conductor. Unlike in stretch testing, in twist testing buckling of the samples is a concern, and thus, appropriate models have been used to ensure that the substrates are subjected to pure twisting without any apparent buckling of the sample. Samples were subjected to twist angles ranging from ± 30° to ± 135° and the resulting resistance was measured in-situ. Both polyimide and PET substrates with screen-printed silver conductors have been tested and the results have been presented for both monotonic and cyclic loadings. The thermal coefficient of resistance (TCR) of screen-printed silver ink has been measured to be 0.00233 per °C and accounted for in the twist resistance measurements to reduce the temperature effect on resistance during long-term cyclic twist testing. Cyclic twisting to ±135° or shear strains of ± 2.99 x 10-3 induced minimal changes in resistance (less than 1.5%) over 5000 cycles.