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One of the challenges in printed electronics is the capacity to print a high resolution electrode. However, it is difficult to gain control over fidelity of microscale line-width of printed patterns especially in roll-to-roll (R2R) gravure printing process. Here, we report a simple solution based on the wettability of ink on the substrate to prevent a widening effect of printed patterns thereby enhancing the precision. The widening effect was found to be affected by intrinsic (ink wetting behavior, cell geometry) and extrinsic (nip pressure, printing speed) conditions. Analysis was conducted to figure out an effect of surface tension of ink and surface energy of substrate on the printed pattern width via contact angle (θ IS ). For a given cell volume, the width of printed patterns decreased with increasing contact angle. The experimental method was used to determine the optimal extrinsic condition and unstable region border. Finally, we identified three ink setting regions of widening (θ IS θ w IS,cr ), un-widening (θ w IS,cr <θ IS <θ in IS,cr ) and unstable region (θ IS θ in IS,cr ). The experimental data showed a good agreement with expected results based on the established analytical approach. This result could be used as an important practical guideline to be applied in R2R gravure printing process with high resolution.

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A hybrid process including roll-to-roll (R2R) gravure printing, via-hole printing, and electroless plating was investigated for the creation of a double-side flexible printed circuit board (FPCB). A R2R gravure process with an Ag seed layer that includes front-and back-side printing with a polyimide film at the center was investigated. The gravure-printed Ag pattern was laser drilled for high accuracy. A via hole was filled with a low-viscosity Ag ink using the drop-casting method. In addition, an electroless Cu plating process was performed to increase the conductivity of the printed circuit. The interconnection performance was confirmed from the resistance values obtained using a feed-through test and microscopic images. To evaluate the reliability of the FPCB, a cyclic bending motion test was performed; stable electrical performance was observed even after 400,000 cycles. The results obtained in this study suggest that the proposed hybrid process for double-side FPCBs is viable for a mass production system.

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