When you have an EMI problem that is difficult to solve with a board design, RFI shielding materials can be used to suppress noise leakage from the PCBA and the enclosure. This is not always the best path forward for every design or product, but these materials can help a product get past a round of EMI compliance testing and into an initial market release. These materials are available for mounting directly to a PCB, or it can be applied to elements in an enclosure to reduce noise leakage at higher frequencies.
RFI Shielding Material Options
There are several easy-to-use RFI shielding materials that can be used to block or absorb radiated EMI from a PCBA. These materials can be used to target very specific noise sources when the material is placed around a specific component, or these materials can provide noise suppression from the entire assembly when mounted on an enclosure. Here are some common RFI shielding material options.
The most common shielding option used by many designers is conductive surface-mount shielding materials, which are better-known as shielding cans. These shielding materials are designed to surround a component package and connect to a ground net. The ideal connection is to a set ground plane on a nearby layer so that the shielding can will totally surround a target component. The grounded shielding around the component acts like a Faraday cage that blocks radiated EMI up to relatively high frequencies.
Shielding cans on a PCB
Shielding components are not available or intended for every component in your PCB. The use of shielding cans is appropriate in cases where the component exhibits large signal swings and the noise can’t be reduced enough with nearby ground, such as in a switching regulator on a two layer PCB. Another case is in modules or SoCs built on an interposer; a shielding can could be used instead of encasing the component in epoxy as the can will provide noise suppression.
These elastomer compounds can absorb radiated emissions at high frequencies and they can be used to patch openings in enclosures. They are often used on gaps and cracks in an enclosure, at cable inputs/outputs, or to cover a large surface of an enclosure region to block directional radiation. They are also available as sheets that can also be molded into covers for cables.
Shielding elastomer available as a sheet. [Source: ID Group]
Although not normally used for shielding, and not considered a shielding compound, conformal coatings can also absorb strongly in the 100’s of MHz range and higher. Conformal coatings can be used as a shielding material on specific components if the coating is applied through a stencil mask. This is a more specialized coating process in PCBA manufacturing and not all manufacturers or assembly houses will offer this service.
Enclosures can also provide shielding when they are constructed from conductive materials (usually aluminum sheet metal). Shielding can also be incorporated into a plastic enclosure that has a much cleaner form factor than a simple sheet metal enclosure. Even if your design does not have noise problems, a shielded enclosure is very useful as it can provide a safety ground for electrostatic discharge (ESD). The gaps and openings in a shielded enclosure will determine the maximum frequency it can suppress; generally smaller gaps and openings will allow higher-frequency radiation to escape from the enclosure.
Shielded enclosure examples.
Shielding Tapes and Gaskets
Conductive copper tapes and gaskets can be used as shielding material whenever there is no room or budget for a shielding compound. These are conductive materials (copper or aluminum) that can be attached to the PCB or the enclosure to provide high shielding effectiveness against radiated emissions. These materials are very inexpensive and can be purchased from many industrial suppliers.
Roll of copper tape.
The Best Shielding is a Ground Plane
If you find your board has EMI problems and you are thinking of applying shielding materials in the PCB, it’s likely the noise is being generated or contributed from incorrect routing and/or lack of proper grounding. In particular, omitting a ground plane, and trying to create the lowest layer count board with routed ground, are two common reasons EMI will be generated from routed digital traces, layer transitions, and components with large signal swing rates.
Without a doubt, the best shielding your PCBA can have is a ground plane, ideally on a thin laminate very close to your component layers. In designs with more interconnects, or that have to source signals with large swings, a ground plane will provide the best defense against noise. Ground pour or planes will provide natural shielding between signals on different layers, as well as suppression of emitted radiation.
When you need to place shielding materials in your PCB layout and your enclosure, make sure you use OrCAD, the industry’s best PCB design and analysis software from Cadence. OrCAD users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.