Effect of Use Parameters on Fatigue-Life of Flexible Substrates under Bending Loads

Abstract

Flexible electronics have been emerging into the wearable application and their reliability in terms of failure mechanisms and lifetime plays a vital role in its survival in the application industry. Reliability of Flex-PCB under different human body working parameters are not been studied. Proper test standards and reliability study are not been done until now to provide confidence that flexible electronics will perform as required in the wearable application as compared to the Rigid-PCB. In this paper, a reliability study has been done based on the parameters - bend angle, the geometry of trace, trace width, pad size, aging, stress state(tension/compression) and sweat. The amount of strain on the traces is measured experimentally for quantification. The reliability study is done in order to compare the fatigue life with the human body data measurement at different joint angles. Many of the joints in a human body are V -shaped like elbow bend, knee bend etc. due to which this paper addresses the v-bend motion. The multi-layer connectivity reliability is also studied in this paper by considering different pad-to-trace ratio. The damage location of the trace is determined with the help of an optical microscope. The effect of cyclic bending under the exposure of human body conditions is been studied and analyzed. The failure of the traces is determined as per the IPC standards. The Flex-PCB are fabricated with subtractive itching method. The flexible substrate used is polyimide and traces are of copper (Cu).