No abstract

Flexible hybrid electronics rely upon compliant interconnects in order to maintain performance integrity in cases that require repeated elongation, including repeated stretching. A class of such conductive interconnects are composites of polymer with conductive particles that can be stretched at high strains without circuit failure. However, their fatigue response has so far remained largely unexplored and is essential prior to using in health monitoring applications. In this research, a stretchable silver-filled conductor is evaluated under high-strain cycling. In-situ techniques, including four-point resistance measurement and laser profilometry, are used to correlate changes in electrical performance to the fatigue response. Surface crack formation is extensive upon stretching during the first loading cycle, forming a heavily interconnected crack network at higher strains that does not immediately result in open circuit failure. Resistance increase with cycling is attributed to a gradual deepening of these cracks until their depths approach the film thickness, eventually leading to electrical failure. Fatigue life, the number of cycles required to reach a predetermined electrical performance limit, is shown to be most influenced by the applied strain amplitude. Using a normalized resistance increase limit of R/R 0 = 500, it is found that 500 µm wide conductive lines endure 23 cycles at 35% strain amplitude, but this becomes over 500 cycles when the amplitude is dropped to 5%. Sensitivity to mean strain, ε m , is relevant to strain amplitudes below 15%. In this manner, a composite conductor was shown to exhibit crack evolution behavior distinctly different from homogeneous metallic films.

This work investigates the origins of electrical performance degradation under uniaxial stretching of a silver filled polyurethane ink (DuPont PE 874) screen printed onto a thermoplastic polyurethane substrate. The ink develops surface ruptures at strains of only a few percent yet remains conductive through continued elongation. We identify increasing sensitivity to surface damage beyond 10% applied strain, ɛapp, as the trace width, w, is reduced from 2 to 0.1 mm. This lowers the threshold strain for open circuit failure, from approximately 180% for w = 2 mm down to 25% for w = 0.1 mm. The damage progression remains largely consistent across trace widths: surface cracks coalesce to form longer channels, which grow perpendicular to the direction of elongation. These channels both deepen and widen with increasing ɛapp and some become laterally linked. The evolution of the network of interlinked channels is not width dependent, but a width effect manifests as a result of the channels constituting a larger fraction of specimen width for narrower traces. In addition, the narrower traces exhibit reduced cross sections due to an edge taper—an artifact of the screen printing process—which attenuates ink thickness by as much as 50% for w = 0.1 mm.