IR-sintering efficiency on inkjet-printed conductive structures on paper substrates

The use of fibre-based materials as substrates in printed electronics has been increasing, mainly due to its attractive characteristics, such as low-cost or wide availability. Additionally, paper enables recycling and it shows attractive features, such as high thermal stability, when compared to traditional polymer-based substrates (Tobjörk and Österbacka, 2011 [1]). Nevertheless, one of the drawbacks of using paper substrates is that the surface usually is very rough, typically, showing roughness values above 10 μm2. In most cases, printing structures that need to be highly uniform, without disruptions, require additional coating. Inkjet printing provides sharp detail reproduction and strict lines on printed structures. Sintering is required for drying the ink. Thermal sintering is the traditionally used method, but requires long periods of time and promotes the ageing of the paper due to a long exposure at high temperature. When printing conductive structures on paper alternative photonic sintering methods such as IR-sintering show some attractive characteristics. IR-sintering is compatible with roll-to-roll fabrication, providing low-cost, fast and localized sintering, which makes it suitable for fibre-based substrates (Tobjörk et al., 2012 [3]). This work has been carried out to study and compare the efficiency of thermal and IR sintering of conductive structures on different paper and polymer substrates. All substrates were printed using silver based ink, which was applied on the substrate surface by inkjet. Resistivity values of the printed structures were used to compare the performance on the substrates. IR-sintering showed the best results in terms of achieved conductivity of the printed lines when using short sintering time of no more than 10 min. The conductivity values of the inkjet-printed silver lines on Lumi silk substrate reached about 40% of the bulk silver value after IR-sintering, whereas with thermal-sintering this value only reached about 20% of the bulk silver value. IR-sintering improves the sintering process, increasing the conductivity of the printed structures and at the same time reducing significantly the sintering time. In the case of Lumi silk substrate, high conductivity was observed after only two minutes of sintering time when IR-oven was used. Both techniques can be used in a roll-to-roll mass manufacturing process, enabling the fabrication in large scale of flexible electronic devices, on paper substrates, without the need for extra steps, such as coatings.