Electrospray Printing of Conformal Polyimide Films onto Complex Geometries

Electrospray printing is an additive manufacturing technique that utilizes a high electric potential to atomize a liquid suspension into a plume of droplets. In this work, the droplets consist of polyimide nanoparticles encapsulated in a volatile solvent, dimethylformamide, that evaporates in-flight due to the high surface area to volume ratio. The dry polymer nanoparticles are deposited onto a target, aggregating over time to build continuous films. Electrospray creates conformal coatings by targeting areas that are not directly within the line of sight of the emitter, making it an ideal process to coat complex geometries. We use electrospray to conformally coat 200 micrometer diameter copper wires and 20 micrometer diameter bond wires and pads. We report on how film thickness and coverage are governed by the print conditions, including spray time, delivery rate, and sprayed mass. We show that electrospray provides a uniform coating around the perimeter of the wires along the majority of the exposed length. Scanning electron microscopy is used to measure the film thickness and microstructure. A salt immersion assay showed that a bonded copper wire with an electrospray-printed polyimide coating provides 25 times better corrosion resistance compared to an uncoated wire.