Without a doubt, the first level of EMC shielding is PCB design, however, despite best efforts, the enclosure will usually need some form of EMC shielding in the seams and openings such as doors, windows, ventilation and cable entry points. It is therefore imperative that EMC for enclosure design is considered in the early stages, since it is difficult to retrofit EMC gaskets and components.
Consideration should be given to the end-use environment, as dust and water sealing can be incorporated into the EMC gasket. Galvanic compatibility between materials is another important factor in a salt laden atmosphere.
Think big
Large enclosures, from 19in racks and cabinets up to shipping container sized communication cabins, need large gaskets to take up the tolerances that occur over long lengths such as doors. The width of the gasket should be no less than its height, so it is vital to calculate the land area for the gasket to sit on.
In these applications, there are a number of gasket types available. Conductive fabric over foam is ideal for commercial applications, being both soft and flexible, but it offers no environmental seal. Where magnetic shielding is required, knitted wire mesh or combination gaskets are best, and can be incorporated with environmental dust and water seals. Beryllium copper fingers are good where high performance is required, such as shielded room doors, but can be high maintenance due to finger breakage.
Small and mighty
Smaller enclosures, for instrumentation and communication equipment, require smaller profile gaskets. The most cost effective EMC and dust/moisture seal is an electrically conductive elastomer O ring positioned in a groove. Profiles down to one millimetre diameter section are possible or, where softer gaskets are required, a tubular section can be used.
Where there is insufficient land for a groove, the gasket can be directly applied to the surface with form in place conductive elastomers. Paste is applied through a syringe by an XYZ robot, which means complex paths can be achieved. Conductive elastomers are silicones or fluorosilicones filled with conductive particles such as silver plated copper, silver plated aluminium or nickel and coated graphite. Shielding effectiveness is not directly correlated to electrical conductivity and nickel coated graphite offers excellent performance for between half and one third the price of silver based fillers.
Other electrically conductive, flat sheet gasket types include oriented wires in silicone, available in solid or sponge silicone with either monel or aluminium wires. This gasket type is mounted directly to the flange. The wires provide aggressive contact with the mating surface, ensuring low contact resistance and good EMC shielding.