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ENCLOSURES


Surface mounted gaskets Surface mounted gaskets such as electrically conductive elastomeric gaskets, knitted wire mesh conductive fabric over foam, beryllium copper finger strips etc, aim to compress the gasket between 10% and 50%, 10% being the minimum with a solid conductive elastomer style of gasket and up to 50% with a hollow tubular or cellular styles. Some form of compression stop or limit is essential with surface mounted gaskets to eliminate the possibility of over compression. These compression stops can be built into many styles of gaskets or made as an integral part of the flange and their height should be equal to that of the maximum compressed height of the gasket. Conductive fabric over foam and beryllium copper fingers can be compressed further than mesh and elastomeric gaskets but lack the benefits of environmental sealing.


Very small land widths can be gasketed with a “form in place” conductive elastomer bead deposited directly to the gasket surface. This technology is particularly suited to gasketing complex multi compartment labyrinth machined enclosures. Bead sizes can be from 0.5mm wide, electrically conductive and non-conductive beads are available or in tandem to give EMI and environmental sealing.


Gaskets in grooves


For the typical electronic enclosure, groove mounted gaskets such as “O” rings are generally a better option than surface mounted gaskets as when the gasket is compressed in the groove the two mating flanges can come into contact with each other thereby enhancing the screening performance by improving low contact resistance, “O” rings in grooves are also a


MAY 2021 | ELECTRONICS TODAY 45


much more cost effective solution and the groove also acts as a compression stop, thus protecting the gasket.


Most types of gaskets can be fitted into grooves, however a solid conductive elastomer gasket when compression forces are applied cannot change volume and has to deflect, and therefore the groove needs to be the same volume or more as the gasket to let the gasket material fill the space provided for it. If the gasket overfills the groove, damage and gasket failure can occur when the mating flanges are closed together.


An EMC gasket that is placed between two conductive surfaces is generally of a different material to the mating surfaces. In certain conditions this can lead to severe


problems of bimetallic galvanic corrosion and degradation in shielding effectiveness. There are two ways of reducing the corrosion risk. One is to use a separate non- conductive environmental seal outboard of the EMC seal therefore isolating the joint from the environment. This would allow the use of materials that would otherwise be unsuitably matched. However limitations on flange widths and the increased cost of using two gaskets can make this impractical. A far better method is to try and match as near as possible the material of the gasket and the flanges thus reducing the electro potential difference between them.


Kemtron www.kemtron.co.uk


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