Choosing an EMI gasket

Choosing the optimum conductive EMI gasket ensures good performance in EMC and environmental fields. In this article, Kemtrons David Wall highlights the key questions to ask. There may be tradeoffs but, in general, there will be a gasket to suit most applications

The first question to ask is at what stage during a products development should EMI gaskets be considered? EMI gasket mounting should be considered early in the design stage. Trying to retrofit a gasket to a flange that wasnt designed to take one will narrow the gasket options. In fact, it may be impossible, requiring a redesign. As a general rule, the more gasket land area available, the greater the options. Many mounting methods are available including: pressure sensitive adhesives; form in place; clip-on; slot mounting; mechanically captured and groove/channel mounting. The application generally dictates which method is used.
Compression force

For good gasket contact, a certain amount of compression must be applied, which will vary depending on gasket type and application. Ridged flanges with many fixings can compress hard gaskets but care should be taken to ensure flange bowing between fixings is avoided.

For cabinet doors requiring several meters of gasket strip, a very low compression force gasket will be required unless the cabinet is a robust construction and the door hydraulically operated. Conductive fabric-over-foam lip seals offer the lowest compression force as the gasket deflects. Beryllium copper fingers can also be 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 are the hardest to compress but offer a good dust and moisture seal.
Dust and moisture sealing

In addition to EMC requirements, many enclosures also need to meet an IP rating. Metallic gaskets, such as beryllium copper fingers and knitted wire mesh, do not provide dust and moisture protection so an additional rubber seal is required. These can often be incorporated in the EMI seal but this can increase the gaskets width which requires more land space. Conductive fabric over foam gaskets offer adequate dust protection but moisture can wick along the materials surface. Conductive elastomers (silicone or fluorosilicone filled conductive particles) provide excellent dust and water sealing to IP68.
Galvanic corrosion/material compatibility
When two different metals contact each other in the presence of moisture or salt spray, galvanic corrosion occurs. Thus, it is important to try and closely match the gasket and enclosure materials on the galvanic scale. For example, a nickel coated (plated or painted) enclosure in contact with a nickel coated graphite in silicone elastomer gasket is a perfect match. However, a silver or copper based gasket (more noble metal) in contact with aluminium (less noble metal) in a salt spray environment will cause rapid corrosion as the potential difference is so great.
Which frequencies?

Does the application need to shield magnetic and electric fields? Most military and aerospace applications require magnetic (H field) shielding, thus a gasket with a reasonably high metal content with high permeability is required. Magnetic fields induce currents in shields, thus 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 are required. Electric field (E field) and plane wave shielding is easier to control and all EMI gasket types will work to their own limitations.

Gaskets are prone to damage. Doors with knife edge closures that place sliding or shear forces on the gasket give excellent performance and, to some extent, are self-cleaning. However, when opened and closed many times they will abrade the gasket surface so metal type gaskets (such as beryllium copper or knitted wire mesh) would be best. However, all gaskets should be inspected periodically and replaced when necessary. Many gaskets are fit and forget and only see the light of day during maintenance or repair. However, it is easy to over compress a gasket during fitting, causing it to extrude out between the flanges and possibly break. This can be eliminated by including a compression stop.
EMI gasket cost is difficult to determine as it is application specific and installation time must be accounted for. Designing-in the gasket at an early stage and taking advice from an EMI gasket specialist will help achieve the lowest cost route. There are many different types/profiles/materials available but once you have identified the important characteristics the choice narrows down. The 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 required. Take advice from your EMI gasket supplier: it should be free.