FLANGE/GASKET DESIGN AND 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. 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.

SURFACE MOUNTED GASKETS Surface mounted gaskets, such as electrically conductive elastomeric gaskets, knitted wire mesh conductive fabric over foam and beryllium copper finger strips, aim to compress the gasket between 10 per cent – the minimum with a solid conductive elastomer style of gasket – and up to 50 per cent – with 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 gasket or made as an integral part of the flange; their height being 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 is particularly suited to gasketing complex, multi-compartment, labyrinth machined enclosures. Bead sizes can be as small as 0.5mm wide. Electrically conductive and non-conductive beads are available and can be placed in tandem to give EMI and environmental sealing.

❱❱ From left, below: elastomeric gaskets can be used with grooved surfaces so the compressible material can fill the groove and form a seal; the huge variety of electronic enclosures has resulted in sealing products in a vast array of forms and designs including (inset) honeycomb products for providing EMC protection through vents and apertures; and more complex sealing profiles can be provided with protection using form-in- place gaskets to provide both RFI screening and a certain amount of environmental protection

GASKETS IN GROOVES For typical electronic enclosures, 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 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. Therefore the groove needs to be the same volume as the gasket to let the 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.

CORROSION 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 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. T&TH

June 2021 /// Testing & Test Houses /// 21

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