As the technology of flexible electronics has remarkably advanced, the long-term reliability of flexible devices has attracted much attention, as it is an important factor for such devices in reaching real commercial viability. To guarantee the bending fatigue lifetime, the exact evaluation of bending strain and the change in electrical resistance is required. In this study, we investigated the bending strains of Cu thin films on flexible polyimide substrates with different thicknesses using monolayer and bilayer bending models and monitored the electrical resistance of the metal electrode during a bending fatigue test. For a thin metal electrode, the bending strain and fatigue lifetime were similar regardless of substrate thickness, but for a thick metal film, the fatigue lifetime was changed by different bending strains in the metal electrode according to substrate thickness. To obtain the exact bending strain distribution, we conducted a finite-element simulation and compared the bending strains of thin and thick metal structures. For thick metal electrodes, the real bending strain obtained from a bilayer model or simulation showed values much different from those from a simple monolayer model. This study can provide useful guidelines for developing highly reliable flexible electronics.

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In past decades, stretchable conductors have been investigated for a wide range of applications, and the operation strain range of such devices varies by application. To commercialize stretchable devices, it is necessary to optimize the deformation of stretchable electrodes based on a given device elongation range. Therefore, we investigated the deformation mechanics of a silver nanowire (AgNW) electrode on an elastomeric substrate depending on its junction treatment method. At low-strain (< 15%), a thermally annealed AgNW electrode showed more stable resistance than a laser-welded AgNW electrode. Conversely, at high strain (> 20%), the thermally annealed AgNW electrode rapidly increased in resistance, while the laser-welded AgNW electrode showed lower resistivity change. By in situ surface analysis and a repetitive tensile test, we observed that the thermally annealed AgNW electrode shows less cracking at low strain but the laser-welded AgNW electrode exhibits fracturing of individual nanowires at low strain. Furthermore, at high strain, laser-welded AgNWs could slide to reduce stress during elongation, resulting in a smaller change in resistance compared to that of thermally annealed AgNW electrode. These results indicate that optimization of adhesion is necessary to fabricate stretchable devices based on deformation range.