UNDERSTANDING THE ELECTRICAL RESPONSE TO APPLIED STRAIN OF POLYMER SUPPORTED SCREEN PRINTED INKS
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Cahn, Gabriel N.
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Abstract
Flexible hybrid electronics (FHE) is a classification of assemblies that describes rigid component islands bridged by interconnects printed onto compliant polymer substrates. They boast continued performance integrity in cases that require repeated elongation, including repeated stretching. The last 20 years have demonstrated exponential growth in the application of such designs in industries such as healthcare, energy harvesting, and smart home systems, driving the need for high volume, low cost, manufacturing approaches. Screen printing composite materials such as polymers with conductive particle inclusions offers electrical function with stretchability, but the performance limits of these materials have yet to be fully explored. This dissertation investigates the evolution of electrical performance of silver-filled polymer inks subjected to uniaxial strain.
This work begins by exploring conductivity of two inks with similar flake volume fractions of ~50% that are screen-printed with a single pass (thickness: 10 m) onto three different polymer substrates. The normalized resistance increases more rapidly with applied strain for the flexible ink (5025 with an acrylic binder), and has three times greater resistance at 35% strain when compared to the stretchable ink (PE 874 with polyurethane binder). While resistance increase is qualitatively consistent with percolation theory, in situ strain map analysis and post-mortem fractography reveal drastic differences in the root causes of the inks’ electrical behavior. Both inks form strain localization bands with similar spacing. For the flexible ink (5025), strain localization is accompanied by local necking and silver flake area fraction reduction. For the stretchable ink (PE 874), strain localization is associated with surface cracking initiated by pre-existing voids, with minimal changes in the silver flake area fraction. A model incorporating strain localization through an evolving Gaussian distribution of flakes with applied strain more closely accounts for the 5025 ink’s normalized resistance increase compared to models that assume uniform strain and a uniform flake distribution. Overall, local necking and reduction of the flake area fraction appear to be more detrimental to the resistance than the formation of surface cracks.
Expanding upon the discovery of surface cracks in PE 874, an investigation is made into the origins of electrical performance degradation under uniaxial stretching of a silver filled polyurethane ink (DuPont PE 874) screen printed onto a thermoplastic polyurethane (TPU) substrate. The ink develops surface ruptures at strains of only a few percent, yet remains conductive through continued elongation. It exhibits an increasing sensitivity to surface damage beyond 10% applied strain, εapp, as the trace width, w, is reduced from 2 to 0.1 mm. This lowers the εapp threshold 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 εapp, 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 deposition thickness by as much as 50% for w = 0.1 mm.
Fatigue response in filled polymers has so far remained largely unexplored, and is essential prior to using in health monitoring applications. PE 874 printed onto TE11C is evaluated under high-strain cycling. In-situ techniques, including 4-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/R0=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, εm, 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.
Finally, this dissertation presents modifications to traditional single-pass print processes in an effort to achieve different geometric and architectural outcomes. Existing models for predicting deposition thicknesses fail to capture the complexities of the array of process parameters. Void removal in PE 874 through vacuum, sonication, and calendering operations proved ineffective, suggesting a decrease in volume loading of silver may prevent a porous architecture.
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Date
2021-07-27
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Dissertation