Choosing an EMI gasket
The correct choice of conductive EMI gasket will ensure good performance in both EMC and environmental fields. There are several questions you should ask yourself before choosing.
Q At what stage in design should you consider an EMI gasket?
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 or be unattainable and some re designing may be required. 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
Q 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.
Q Dust and moisture sealing
Many enclosures as well as having to meet EMC requirements may also need to meet an IP (ingress protection) rating. Metallic gaskets such as beryllium copper fingers and knitted wire mesh will not provide dust and moisture protection so an additional rubber seal will be required to provide this, these can often be incorporated as part of the EMI seal but can make the gasket wide requiring more land space for the gasket. Conductive fabric over foam gaskets will give adequate dust protection but moisture can wick along the surface of the material. Conductive elastomers (silicone or fluorosilicone filled conductive particles) will provide excellent dust and water sealing up to IP 68.
Q 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.
Q 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.
Q Which frequencies do you need to shield?
Do you need to shield in the magnetic as well as electric field? Most military and aerospace applications require magnetic (H field) shielding therefore a gasket with a reasonably high metal content that has a high permeability will be required. A magnetic field will induce a current in the shield therefore knitted wire mesh, beryllium copper fingers and metallic filled elastomers work well. For low frequencies around 10Khz the more metal content the better. For very low frequency H fields exotic materials such as Mumetal shields will be required. Electric field (E field) and plane wave shielding is easier to control and all EMI gasket types will work up to their own limitations.
Q Gasket maintenance
Gaskets are prone to damage and wear and tear and should be taken into account. A door with a knife edge closure that has a sliding or shear force on the gasket will give excellent performance and to some extent be self cleaning. But when opened and closed many times will abrade the gasket surface so metal type gaskets such as beryllium copper or knitted wire mesh would be the best choice however all gaskets should be inspected periodically and replaced if necessary. Many gaskets are fit and forget and only see the light of day during maintenance or repair however it is very easy to over compress a gasket during fitting which will cause it to extrude out between the two flanges and possibly break. This can be eliminated by providing a compression stop for the gasket or compression stops or collars can be fitted to many types of flat gaskets.
EMI gasket cost is difficult to determine as it has to be application specific and installation time should be taken into account. Designing in the gasket in the early stages and taking advice from an EMI gasket company will help you achieve the lowest cost route. There are many different types/profiles/materials available but once you have identified the important characteristics you require the choice gets narrowed down. Your gasket supplier will be able to advise if a simple conductive “O” ring in a groove or die cut flat gasket will suffice or a more complex fabricated or moulded gasket is needed. Take the advice of your EMI gaskets supplier- it should be free.