Additive methods have emerged as a favorable alternative to conventional plate-n-etch processes that require hard tooling, largely due to their ramp-up speed and the benefits of soft tooling. Coinciding with the escalating demand for electronic miniaturization, flexible hybrid electronics(FHE) have seen accelerated development, finding applications in diverse sectors such as consumer electronics, automotive, healthcare, and transportation. The present generation of additively printed electronics inks used to realize conductive circuits are composed of nano-particles in volatile solvents such as isopropyl alcohol, xylene, methyl-ethyl-ketone, formic acid, ethyl alcohol, polyvinyl pyrrolidone (PVP), and sodium borohydride. Environmental, social, and geographic factors have become increasingly important for electronics manufacturing owing to high volumes and increased attention to sustainability. Biodegradable Substrates, water-based inks, and lower-temperature processable materials have emerged for achieving conductive circuits using additive processes. The performance and reliability of water-based ink-printed circuits, biodegradable substrates, and low-temperature processing with surface-mount devices are not well understood.
Previously, a number of aspects related to the use of water-based inks have been investigated, including the effect of solid content in aqueous-based inks [Faddoul12], particle morphologies for enhanced conductivity [Lai14], and solvent selection. The processes required for the realization of circuits and components attached on additively printed circuits fabricated with water-based inks have not been explored. Consumer electronics applications have further placed a focus on lower-temperature processing [Hu10]. Low substrates include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and paper.
In this study, two methods of aerosol-jet printing and gravure offset printing have been used for the fabrication of circuits with biodegradable substrates and low-temperature interconnects. The reference circuits include comparative designs using volatile organic compound-based inks.The gravure offset printing has been used to print switch mode buck battery charging circuits using water-based silver conductible ink. Aerosol-jet printing methods have been used to study signal processing circuits through the fabrication of differentiator circuits. Component attachment for the circuits has been performed using electrically conductive adhesive (ECA) and low-temperature solder (LTS). The charging circuits have been designed to supply differing output currents to study the effect of current on trace resistance evolution for long term accelerated life cycling. In addition, the output charging current of the circuit has been monitored to track its evolution correlated with line resistance. Furthermore, another angle to investigate the sustainability of printed electronics is their post-repair performance. To this end, the printed circuits were subjected to a repairability study wherein the attached components were manually removed; the component pads then underwent site redressal using conductible ink and the respective component attachment material (ECA/ LTS) which were dispensed using direct write (DIW) technique. This was followed by attachment of new components and subsequent curing. The performance of the repaired charging circuits was also tested and then compared with that of the pristine circuits. Finally, the switch mode buck charging circuits were also printed using non-sustainable silver ink, i.e. inks not using water as the primary solvent to compare their pre- and post-repair performance with the circuits printed using sustainable water-based ink.