Polyimide Films Manufactured Using Partially Wet Electrospray Deposition

Abstract

In electrospray deposition (ESD), solute material is dispersed in a carrier solvent and atomized into a cloud of microdroplets under the influence of a strong electric potential. The volatility, ESD parameters, and environmental conditions determine the rate of solvent evaporation. Complete in-flight solvent evaporation produces dry particles of material that are deposited on the target. In contrast, when the evaporation is incomplete, some residual solvent is delivered with the material. We define these regimes as dry (complete evaporation) and partially wet (incomplete evaporation) ESD. Here, we compare the microstructure and functional characteristics of films deposited in both regimes. We selected polyimide as our solute material given its importance in electronic, health, and environmental applications. An electrospray printhead that provides a tunable solvent evaporation rate via co-flowing air is used to deploy the polyimide. The dry and partially wet regimes can be selected by altering the solution flow rate and the air co-flow rate. Particulate films are formed when the polyimide is delivered to the target dry (solvent-free). When solvent is delivered with the polyimide, a dense (partially wet) film is formed on the target. A numerical model of droplet evaporation was developed, which shows the dominant effects of solution flow rate and freestream vapor concentration on the ESD regime. The dry and partially wet films were compared for various functional characteristics including dielectric strength, optical characteristics, and corrosion resistance. For most metrics, the partially wet ESD films outperformed the dry ESD films. Compared to the dry ESD films, the partially wet films had far greater dielectric strength (up to 340 V/μm) and higher optical transparency. Both ESD regimes provided comparable corrosion resistance, which enhanced the reliability of metallic bond joints in an electronics package.