The Effect of Carbon Ink Rheology on Ink Separation Mechanisms in Screen-Printing

Abstract

Screen-printable carbon-based inks are available in a range of carbon morphologies and concentrations, resulting in various rheological profiles. There are challenges in obtaining a good print when high loading and elasticity functional inks are used, with a trade-off often required between functionality and printability. There is a limited understanding of how ink rheology influences the ink deposition mechanism during screen-printing, which then affects the print topography and therefore electrical performance. High speed imaging was used with a screen-printing simulation apparatus to investigate the effect of viscosity of a graphite and carbon-black ink at various levels of solvent dilution on the deposition mechanisms occurring during screen-printing. With little dilution, the greater relative volume of carbon in the ink resulted in a greater tendency towards elastic behavior than at higher dilutions. During the screen-printing process this led to the ink splitting into filaments while remaining in contact with both the mesh and substrate simultaneously over a greater horizonal length. The location of separating filaments corresponded with localized film thickness increases in the print, which led to a higher surface roughness (Sz). This method could be used to make appropriate adjustments to ink rheology to overcome print defects related to poor ink separation.

The influence of carbon morphologies and concentrations on the rheology and electrical performance of screen-printed carbon pastes

“…Therefore, this filament formation may be due to the elastic response of the ink exceeding a particular threshold for that morphology. Increases in the elastic response of the ink corresponding to the formation of filamentation and resultant reductions in print quality were also observed by the authors in previous studies [26,27].…”

“…This resulted in a greater elastic response and more filaments forming during separation. In the dilution study on a commercial carbon ink [26], the more elastic undiluted ink and 2.5 wt.% dilution were found to form filaments during ink separation while the less elastic higher dilutions did not. This was a result of reductions in the interparticle distances, leading to fewer particle to particle and particle to polymer interactions that elastically store energy with greater dilutions.…”

“…The study assessing the effect of diluting a commercial carbon ink on the separation mechanisms occurring during screen-printing found that changes in the rheological profile of the carbon-based ink had quantifiable effects on the ink separation mechanisms during screen printing. These separation mechanisms were found to be related to the print quality produced (surface roughness and presence of print defects such as mesh marking) and resultant print conductivity [26].…”

“…Different filler (carbon) concentrations also alter the rheological profile of the ink, which can be quantifiably related to the quality of the print produced [23,24]. High speed imaging techniques have been used by Xu and Willenbacher [25] and the authors [26] to identify how the rheological profiles of screen printing inks influence the inks deposition and separation mechanism during screen printing. Xu and Willenbacher's [25] study on fine line screenprinted ZnO pastes imaged the print process from directly below a glass substrate, looking up at the print.…”

“…This identified the influence of ink rheology on the amount of paste spread at the edge of the features, due to the ink slumping after it has been deposited which can affect line resolution. Previous studies by the authors conducted high speed imaging from the side of a custom-made screen-printing apparatus, enabling the ink deposition and separation mechanisms to be visualised for a range of parameter settings and dilutions of a commercial carbon ink containing graphite and carbon black [26,27]. The study assessing the effect of diluting a commercial carbon ink on the separation mechanisms occurring during screen-printing found that changes in the rheological profile of the carbon-based ink had quantifiable effects on the ink separation mechanisms during screen printing.…”