Printing of conducting inks on paper

Paper has many attractions as a substrate for the manufacture of flexible electronics, particularly the comparative ease of recycling and recovering the constituents of the inks. However, it is not as smooth as the polymer substrates and also does not normally have sufficient barrier properties for sensitive reactive inks. A paper has been created that has sufficient smoothness for printing of electronics. As a first stage of the evaluation of this coated stock, a series of trials have been undertaken where appropriate images have been offset printed using a silver conducting ink.This paper presents an analysis of the images in terms appropriate for the manufacture of electronics. The surface roughness, edge straightness, defects (including shorts), the resistance of the lines and line gaps were measured using optical methods, white light ineterferrometry and resistance meters. These are evaluated through two print runs to provide consistency data as required for the manufacture of electronics. In the second print run, an overprint was used to increase the film thickness and hence potentially reduce the resistance of the conductors. The specified gap were improved compared to screen printing. Through a 2000 print run the variation in printed edge (largely a result of substrate roughness) was significant and reduced the printed feature size as “shorts” between adjacent tracks could not be guaranteed over the test length. Small deviations were seen in the gap width were evident between the print direction and cross sheet direction. The printed film thickness was difficult to measure using the white light interferrometry was estimated at approximately 1 micron. Measurement of track resistance confirmed the continuity between adjacent tracks and also implied that film thickness increased through the print run. Double printing of the ink layer led to an increase in track resistance, which was contrary to that expected. Possible reasons for this include the removal of ink from the first printed layer by the second ink application, the predominant transfer of the high resistance base at the second ink application and a lower transfer of the second ink film which is effectively insulated from the first ink film. The results are encouraging and show the potential of paper as a substrate for volume manufacture of disposable electronics. Prototype devices have already been produced from the sheets printed in the trial