Electrospray Printing of Polyimide Films for Electronics Packaging Applications

Electrospray printing utilizes a strong electric potential to atomize a precursor ink solution into a spray of electrically- charged microdroplets containing the print material. The carrier solvent rapidly evaporates while the droplets are in-flight, leaving behind dry material which can be delivered to a substrate to create a film. Here we report on electrospray printing of polyimide, a critical material in electronics packaging. Polyimide films were printed onto flat silicon substrates, as well as industry-standard 20-μm diameter bond wires and pads. The films printed onto silicon were analyzed for their dielectric breakdown strength and surface wettability. Thicker films were able to achieve breakdown potentials exceeding 1kV, however breakdown field strength was greater for thinner films resulting in a maximum sustained field strength of 127 V/μm. The printed films were also moderately hydrophobic, with an average contact angle of 110°. Polyimide films printed onto the bond wire and bond pads displayed a high degree of conformality. Printing times as low as 5 minutes produced conformal coatings around the 20-μm bond wires. Salt- water immersion and salt-spray testing were conducted to evaluate corrosion protection of the printed polyimide films. Wire bonds with a printed polyimide coating survived up to 10 times longer in a corrosive environment than those without a coating.