Electrospray Printed Silver Films for EMI Shielding of SiP Architectures

There is a growing demand for compartmental EMI shielding of system-in-package architectures. We report on the use of electrospray printing to form thin silver films on epoxy structures and industry-standard flip chip substrates. Electrospray is a low-cost additive manufacturing technology capable of printing conformal films onto complex surface geometries. We aim to replace board-level shielding, which is not optimized for space-constrained environments and inhibits the miniaturization of SiP architectures. Films are evaluated from 1 GHz - 10 GHz, with a minimum target of 50 dB of isolation.Electrospray printing has many advantages over traditional thin film manufacturing processes such as chemical vapor deposition (CVD), sputtering, and inkjet printing. In electrospray, strong electric potentials are used to atomize a liquid precursor solution into a spray of electrically charged droplets containing the print material. In flight, the carrier solvent partially evaporates, leaving behind particles of solute material that are deposited onto the target substrate. Over time, these particles aggregate to form thin, continuous, conformal films with highly controlled microstructure.We target two insulating substrates in this work: a patch antenna encapsulated in epoxy molding compound on a PCB board and a flip chip substrate. It is challenging to deposit material onto insulative targets using electrospray due to the accumulation of electric charge on the target. We have developed a low-cost method to increase deposition efficiency onto insulating targets through a stencil to impose a dielectric constant "mismatch." The dielectric properties of the stencil are carefully chosen to impose a slower rate of charge decay on the stencil compared to the target, forcing the stencil to focus material deposition onto the underlying, exposed insulator.