How rheological properties affect fine-line screen printing of pastes: a combined rheological and high-speed video imaging study

Abstract

Fine-line screen printing is still a great challenge due to the lack of overall understanding between the rheological properties and screen printing process. Here, we prepared ZnO pastes including ethyl cellulose or Thixatrol Max as an additive introducing different physical mechanisms of structure and flow control. Yield stress, viscosity, and its recovery after high shear were obtained using rotational rheometry. Filament breakup was determined in uniaxial elongational tests. Pastes were printed using a commercial screen designed for Si-solar cell front-side metallization, and the process was monitored with high temporal and spatial resolution using a transparent glass substrate and a high-speed imaging setup. Length of the pre-injection zone ahead of the squeegee scales inversely with yield stress and length of the cling zone behind the squeegee is proportional to filament rupture strain, irrespective of used additive. Paste spreading observable at the busbar and fine-line intersection takes place within 100 ms, irrespective of sample composition, demonstrating that fine-line electrode width is determined within the pre-injection zone where the paste is under pressure. A simple flow model is proposed relating electrode width to the reciprocal product of yield stress and high shear viscosity consistent with experimental data including both types of pastes.