RF screen coating for moulded packages

EDN Europe, 18 Feb 2008

MicroShield is a screening technology that integrates RF screening on the surface of a moulded package, which might contain a single chip or a multi-chip module. RF Micro Devices developed the process to replace metal screening cans that designers must often mount on printed circuit boards either to shield the enclosed circuitry from external interfering signals, or to prevent RF interference from leaking out. MicroShield takes the form of a sprayed-on conductive coating that makes conductive contact with a “skirt” contact band that runs around the bottom edge of the moulded package. When the assembly process solders the package to the PCB, in the usual way, the “skirt” contacts a grounded ring that surrounds the package site. In this way, a five-side screen surrounds the contents of the moulded package; a ground-plane under the package, with vias for connections, will complete the six-sided screen. At the time of introduction RFMD was not quoting figures for screening effectiveness in terms of dB, but a spokesman indicated that results were competitive with metal cans while reducing height and volume for RF circuit sections by 25 to 50%. RFMD will employ the process on its own product lines as an option, and will also license it to other manufacturers: they can apply it any over-moulded package technology.