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Choosing an EMI gasket
30 June 2015
EMI gasket mounting should be considered early on in the design stage, trying to retrofit a gasket on a flange that wasn’t designed to take one will narrow your gasket options.
As a general rule the more land area available to fit a gasket will give greater options. Many mounting methods are available some of which are, pressure sensitive adhesives, form in place, clip on, slot mounting, mechanically captured and groove/channel mounting. The application will generally dictate which method is used i.e. door seals, access panel seals etc
Compression forces
To get good contact with the gasket a certain amount of compression needs to be applied, this will vary depending on the gasket type and the application. Ridged flanges with many fixings can compress hard gaskets but care should be taken to ensure bowing of the flange between fixings does not occur.
For cabinet doors requiring several meters of gasket strip a very low compression force gasket will be required unless the cabinet is of robust construction and the door is hydraulically operated. Conductive fabric over foam lip seals offer the lowest compression force as the gasket deflects. Beryllium copper fingers can also be very easy to compress but can suffer damage through snagging and broken fingers. Sponge or hollow tube type gaskets such as knitted wire over a rubber core offer medium forces and solid silicone rubber seals loaded with conductive particles will be the hardest to compress but of course would offer a good dust and moisture seal.
Galvanic corrosion/ material compatibility
When two different metals are in contact with each other in the presence of moisture or more so in salt spray, galvanic corrosion will occur. It is therefore important to try and match as closely as possible the conductive gasket metal to the enclosure metal on the galvanic scale. For example a nickel coated (plated or painted) enclosure in contact with a nickel coated graphite in silicone elastomer gasket will be a perfect match, however at the other end of the scale a silver or copper based gasket (more noble metal) in contact with aluminium (less noble metal) in a salt spray environment corrosion will rapidly take place causing a breakdown in the conducive joint as the potential difference between them is great.
Shielding effectiveness
Conductive gaskets can offer shielding effectiveness up to 120db, however this is under perfect testing conditions. High contact resistance between the gasket and enclosure can negate any shielding effectiveness of the gasket. High resistive passivation finishes or oxides on gasket mating surfaces will cause the gasket to fail as there will be little or no conductive path between the gasket and mating surface. This is particularly a problem with smooth surfaced gaskets such as conductive fabric over foam as only small amounts of pressure can be applied. Aggressive rough finish gaskets such as wire mesh will penetrate the resistive finish but these may not be suitable for the application. It is therefore important that particular attention is paid to the mating surface cleanliness and that it has a good conductive finish.
For further information please contact Kemtron Ltd
on Tel: +44 (0) 1376 348115 Fax: +44 (0) 1376 345885 or visit the web site www.kemtron.co.uk
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