The printed electronics market, according to market research firm IDTechEx, is slated to grow from $29.8 billion in 2013 to $73.69 billion in 2023 (Das, and Harrop, 2015). Printing transparent grids with a known printing process such as flexography is attractive to this market because transparent grids are part of the fast-growing touch screen market and flexography offers a low-cost/high-volume print platform that can handle a variety of substrates, high press speeds, roll-to-roll (R2R) printing, and the ability to handle different ink rheologies. However, flexography has print characteristics that can be problematic for printed electronics such as high image (feature) gain and the ability to hold fine images (features) in the relief printing plate.
This study used a "banded" anilox roll to investigate the optimum process parameters needed to produce transparent conductive films with the highest transparency and lowest resistance. The banded anilox roll, which has multiple volumes and cell counts separated into bands, was used to print with conductive nano-particle silver water-based ink, and a PET substrate on a commercial flexo narrow-web press. The process parameters also included types of mounting tape, anilox cell count/volume, plate imaging, and plate surface morphology. The printed transparent grid patterns were evaluated for conductivity, which was measured with a digital multimeter; and transparency, which was measured with a transmission densitometer.
The results of the study showed that when the photopolymer plates were imaged at a resolution of 8000dpi and with a "flat top dot" (1:1 file - plate) exposure technique, the flexo plate was able to hold a minimum of 6.35 microns line width. The 6.35 microns lines were successfully printed on the press to produce conductive lines. Types of mounting tape and different anilox cell volumes have an impact on measured resistance of the grid pattern. A high modulus, or "firm" mounting tape with a low screen count/high-volume anilox (800 cpi/2.85bcm) resulted in the lowest (<15 ohms) resistance. In addition, the introduction of plate surface texture during the platemaking lowered resistance. The grid pattern shapes of diamond/square and hexagon yielded the best transmissivity (above 92%).
The study proved that commercially available flexographic materials with optimized process parameters can improve the printability and performance of transparent grid patterns using conductive inks.