Development of Test Protocols for the Flexible Substrates in Wearable Applications

Abstract

Flexible electronics in wearable applications may be subjected to flexing, bending, stretching in addition to exposure to temperature and humidity. Presently, there is a general lack of test protocols for the reliability assessment and survivability assurance of the flexible substrates. Flexing and bending in operation may be accrued under stresses of daily motion. Flexible substrates often use serpentine patterns to accommodate high stretch in the neighborhood of 25-100 percent. Flexible copper traces subjected to cyclic mechanical bending result in stretching of the outside layers and simultaneously compressing the inner layers. Cyclic bending may also result in formation of wrinkles on the flexible substrate causing delamination. Meaningful accelerated test protocols and acceleration transforms relating test-performance to operational reliability are needed. In this paper, a new test protocol has been developed for replicating the stresses of daily motion for flexible substrates. The conventional-fabricated flexible circuits have been studied. Test coupons have been designed to include the common trace geometries encountered in flexible electronics applications. The effect of cyclic mechanical bending, exposure to human body temperature has been studied and analyzed using Flex-PCBs. Flex-PCBs have been subjected to cyclic bending and the failure modes were analyzed as a function of bend radius, bend angle and number of cycles to failure. Extremely tight bend radius and bend angle larger than 90° have been studied. Further, in order to monitor the resistance of copper traces and develop life prediction model, Data Acquisition unit has been used. This prognostic health monitoring technique captures the increase in resistance of copper traces with the growth in fatigue due to cyclic mechanical bending. The study addresses the need for life prediction models of flexible copper traces on flexible polyimide substrate.