Electromechanical characterization of flexible screen-printed tracks

Abstract

To reduce the environmental impact of flexible printed electronics the most used material combination—silver-based ink on a plastic substrate—needs to be replaced. Carbon-based inks and paper substrates have the potential to do this, but their electromechanical reliability needs to be evaluated. This paper presents a systematic study of cyclic bending reliability for carbon-based inks on plastic substrates and silver-based inks on plastic and paper substrates, using a 9 mm compressive bending radius. Failure was defined as a 70% increase in resistance from the initial value, and carbon-based inks showed promising results as their resistance did not reach the failure criteria even after 30 000 bent cycles. Silver-based inks on plastic substrates failed by around 200 cycles as evaluated by the Weibull’s cumulative distribution function, but silver on a regenerated cellulose film lasted almost 30 000 cycles. However, for paper substrates, bending reliability showed significant variation. Therefore, silver-based inks on both substrate types were subjected to further characterization to study potential correlations among substrate surface roughness, substrate thickness, and substrate type. For paper substrates, a strong correlation was observed between surface roughness and the number of cycles to failure. Sustainable packing material failed around 40 cycles, likely due to the uneven distribution of silver, which contributed to failures. Smooth regenerated cellulose film on the other hand lasted almost 30 000 cycles. With plastic substrates no strong correlation was found as even the smoothest substrates failed relatively quickly. This study also demonstrates that low overall thickness of the substrate enhances bending reliability, especially with paper substrates, as the strain is decreased. Overall carbon-based inks and paper substrates show promising results in replacing the mainstream materials, but the lower conductivity of carbon compared to silver, and the effect of surface roughness on the reliability of paper substrates needs to be considered.