EMC W
ith increasingly complex RF environments in highly connected products that are required to meet the EMC directive, enclosure shielding is becoming ever more important and
complex as frequencies in the new generation of equipment are increasing. In the past, up to 18GHz was about the top end but 40GHz shielding is now required for many applications and with 5G, this can go even higher. Some RFI/EMI screening materials will not screen at these high frequencies so it is important to know the frequency requirement and environment of the enclosure before choosing shielding gaskets and components.
THE USE OF SHIELDING
RFI/EMI screening is a discipline that is of interest to both the electronic engineer and the enclosure design engineer. The electronic engineer must take EMC into account when designing PC boards by careful component and circuit layout, using PCB shielding cans as small Faraday cages over problem components, paying attention to wiring runs, use of filters and ferrites etc. This can eliminate the majority of EMI emissions. However, circuit design and layout may not be
sufficient to attenuate emissions to the required levels and shielding of the enclosure may also be necessary. This is where the enclosure design engineer is required to design the casing with sufficient thought to using EMI screening gaskets at joint interfaces. Although the brief may be to make the enclosure as small as possible, if this is not taken into consideration early on then this can result in an enclosure with no landed area on which to place an EMI screening gasket. This would require very expensive redesign or the use of more complex and expensive gasket materials and configurations. Enclosures can be made of any material provided it is electrically conductive and will form a Faraday cage. Metallic enclosures have this characteristic, however all plastic enclosures need to be made conductive by applying a suitable coating to the inner surface. This can be done using conductive paints, electroplating or vacuum metallisation. Screened enclosures are rarely a simple case of a closed box design and may have openings for optical displays, ventilation and cable entry. Examples of these areas that may have to be screened include: n Optical displays with the use of screened optical windows which can be made by using fine wire meshes laminated in glass or clear plastic, such as acrylic or polycarbonate, or conductive clear coatings on the substrate, such as indium tin oxide. n Enclosure ventilation can be screened using simple pierced or expanded metal in low performance applications or by the use of honeycomb ventilation panels made from aluminium or steel in higher performance applications. n Cable entry into a screened enclosure needs to have the cable screening bonded to the conductive surface of the enclosure through a screened cable entry gland or screened connector.
20 /// Testing & Test Houses /// June 2021 RFI
Sealed against
EMC shielding specialist Kemtron considers enclosure design and the important role it plays in creating an RFI seal to meet the increasing EMC requirements of electrical and electronic equipment
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