ENCLOSURES
EMC AND ELECTRONIC ENCLOSURE DESIGN
Kemtron explores some design considerations on a range of EMC gaskets and components for enclosures and PCB design
FI/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 be necessary. This is where the enclosure design engineer is required to design the enclosure with sufficient thought to using EMI screening gaskets at joint interfaces. Although his 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 a EMI screening gasket. This would require very expensive redesign or the use of more
R 44 MAY 2021 | ELECTRONICS TODAY
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 conductive 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. All these areas may have to be screened:
• Optical displays with the use of a screened optical window 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
• Enclosure ventilation can be screened using simple pierced or expanded metal in low
performance applications or with the use of honeycomb ventilation panels made of aluminium or steel in higher performance
applications • 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
Flange/gasket considerations The design requirements for a gasket joint in a screened enclosure are primarily that there must be good electrical conductivity between opposing flanges through the gasket. Poor conductivity or high resistance between the flange and the gasket will result in poor shielding and if in areas there is no contact with the gasket this can result in a gap which could possibly act as a slot antenna making things worse, it is therefore important to consider fixing pitch and gasket compression forces to ensure a good continuous seal.
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