EHD and Inkjet Printing in MicroLEDs: From Repair to Color Conversion

Electrohydrodynamic (EHD) Printing has long been known as a potentially superior printing technology compared to conventional inkjet. Back in the 80’s, EHD was explored for graphical applications by major players but has long since been abandoned. Beginning of the century, new activities have emerged mainly in relation to emerging industrial applications like printed electronics. The problems that keep the technology from becoming industrially relevant remain the same though. EHD printheads are not manufacturable with large nozzle counts, mainly because of cross-talk issues, unbearably high driving voltages as well as general long-term stability challenges.Those road-blockers come at a time when emerging materials and technologies in the display market have boosted the interest in EHD technology as an enabling technology. Particularly in microLED production EHD seems like a perfect fit because it works much better with solution-processed quantum dots than lithography does, but it comparison to regular inkjet printing it can also fulling stringent precision and quality criteria. While an increasing number of scientific works have promoted the EHD technology, Scrona’s has set its task to enable a conversion of these results into economic reality. At Scrona, first MEMS-based EHD printheads have been created that eliminate the problems that have kept EHD from industrial use-cases so far. Scrona’s new EHD printheads contain nozzles at a density that can be more than five times higher than that of modern piezo-driven inkjet heads and they can be individually operated with less than 100V which makes them compatible with massively scaled driving electronics. Specialized nozzle architectures not only keep liquid in place, but also strongly reduce clogging issues by a highly efficient recirculation flow. During the masterclass, some of the key success factors will be explained and examples will be given that demonstrate unseen performance in terms of printing resolution and droplet placement precision, with particular focus on microLED applications. While other additive manufacturing technologies operating at sub-5µm printing resolution generally require extremely precise control of printhead-substrate distance, Scrona will show how its printhead can place droplets with sub-micron precision even at distances of 1mm and more.