Adaptive Curvature Flexure Test to Assess Flexible Electronic Systems

Flexible electronic systems are being explored for a variety of applications during which they may be stretched, bent, twisted, and folded a number of times and over different durations, etc. An understanding of electrical and mechanical behavior of flexible electronic systems under deformation is necessary for its design and reliability assessment. In this paper, we explore the use of an adaptive curvature flexure test to understand the strain and the electrical resistance variation in printed conductors. The adaptive curvature flexure test is a free-form test in which the flexible substrate is subjected to different radii of curvature through moving parallel plates. Printed silver conductors on flexible substrates - polyimide (PI) and polyethylene terephthalate (PET) of about 125 μm thickness are subjected to bending between two parallel plates from a gap of 70 mm to a gap of 5 mm. The conductor can be either on the tensile or compressive side of the sample during the experiment. The electrical resistance of the printed silver conductor is monitored throughout the experiment, and it is seen that the electrical resistance of the conductor under tension during bending increases, while the resistance of the conductor under compression remains nearly constant as the gap between the parallel plates is reduced. A finite-element simulation is carried out to investigate the strain variation in the printed silver conductor and the substrate. It is seen that the maximum normal strain in the conductor is about 0.03 when the gap between parallel plates is 5 mm, and the electrical resistance over the entire conductor increases by about 8% when the conductor is under tension during bending. This resistance continues to increase with cyclic bending with the conductor under tension. Through in-situ SEM imaging, it is seen that damage occurs in the conductor during the first cycle, and continues to grow with cyclic bending.