Conformal Silver Films for EMI Shielding of SiP Architectures

Abstract

Electrospray printing is an additive manufacturing technique where a precursor solution composed of a solute material in a compatible solvent is subjected to a high electric potential, generating a spray of charged, solvent-encapsulated particles. Due to the high surface area to volume ratio, the solvent rapidly evaporates to deposit dry or semi-dry solute material onto a target surface. Over time, a thin continuous film is formed. Material emitted by electrospray is highly charged and will follow the electric field lines established between the emitter and a grounded target. As a result, this process is best suited for deploying material onto (grounded) conductive surfaces and can be used to create conformal coatings, even on non-line-of-sight surfaces. In contrast, delivering material to insulating surfaces is more difficult due to the accumulation of charge on the target, inhibiting the deposition of the solute and preventing the formation of a continuous film. We report on a new approach to improve deposition on dielectric (insulating) targets by manipulating the electric field and distribution of charge in the vicinity of the target surface. Colloidal silver in triethylene glycol monomethyl ether is deployed to create a conformal coating on packages consisting of a (i) broadband radiator encapsulated in epoxy molding compound and (ii) a model silicon flip-chip assembly. We seek to replace board level shielding and provide electromagnetic interference protection for System-in-Package at frequencies from 100 MHz to 6 GHz, with a target of 60 dB of isolation at 1 GHz.