EMI / RFI shielding is material that attenuates the energy of electromagnetic waves that pass through it. The purpose of EMI / RFI shielding is usually to allow electronic devices to achieve EMC, so they can operate legally and efficiently; but increasingly it is used to protect people from possible negative health effects of electromagnetic radiation. Whatever the application, EMI / RFI shielding works by being a conductor.
When an electromagnetic wave runs into a conductive material, part of the wave will be reflected, part will be absorbed, and part will be transmitted. If the material is highly conductive, then the reflected and absorbed components will be exponentially larger than the transmitted component. In fact, the decibel scale that is used to measure shielding effectiveness is an exponential scale.
Shielding materials need not be solid. It is even acceptable to have holes in the surface of the shielding enclosure, if the diameter of the holes is less than the wavelength being shielded. The classic example of such shielding is the Faraday Cage.
While holes in shielding may be fine, seams are not. Electromagnetic waves are 2-dimensional, like pieces of paper. They will not fit through holes in the lid of a box, but they may easily slip through the seam between the top and bottom of the box if it is not properly sealed. In electromagnetic shielding terms “properly sealed” means the top and the bottom of the enclosure are in continual electrical contact with each other. Sometimes this requires conductive gaskets, but sometimes a well-designed enclosure can do without them.
In the past, metal enclosures were often used for electronics, and these inherently provided excellent EMI / RFI shielding. But most modern enclosures are plastic, offering no intrinsic protection. Conductive tapes are available for shielding plastic enclosures, but one of the most efficient, effective, and economical alternatives is conductive paint. Acrylic conductive paint is mostly used for this application. Its easy to use 1-part solvent borne system dries quickly and bonds strongly to plastics.
For architectural applications mesh can be applied under siding, but again, conductive paint is an excellent alternative. Water-based conductive paint is well suited to drywall and can effectively block WiFi and 5G signals. Unlike wire mesh, it can easily be painted over with common architectural paints, making it much easier to hide.
At a board-level the interference between components comprises more EMI than RFI, so shielding paints for these applications tend to be filled with silver, which is better then other pigments for high energy shielding. Components are often shielded with metal caps, but with miniaturization there is not always room. A thin film of conductive paint can fit in tight places, and sometimes can come to the rescue.
In EMI / RFI shielding applications involving electronics, it is important to note that shielding materials are conductive and will therefore short out electrical circuits if put in direct contact. Shielding paints for electronic enclosures are applied to the inner surface of the enclosure. Shielding paints used at a board level must have insulative materials between them and any circuits or they will short out the device. Conformal coatings are often suitable this role.