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Conductive filler types

These materials are based around a number of conductive fillers, but in practice there are a mainstay of three or four types that are most commonly used. They are summarised below

Silver-plated aluminium – the best conductive filler in terms of galvanic corrosion compatibility with aluminium alloy components/enclosures. Also so has very good high temperature performance and ageing characteristics

Nickel coated graphite – comparatively low cost, excellent high temperature resistance, very good electrical/shielding performance – particularly on surfaces with poor contact resistance e.g. stainless steel, zinc, heavy chromate finishes

Silver-plated copper – excellent conductivity, low contact resistance, good current handling for EMP type events

Carbon – low cost, ideally suited to electrostatic discharge applications. Can be used for some less demanding EMI shielding applications

Silver-plated nickel – good conductivity, excellent long-term high temperature performance

Silver-plated glass – general purpose, good high temperature performance

Pure silver – comparatively high cost, has a few niche applications

Design considerations

As with most electrically conductive EMI shielding gasket materials contact surfaces must have low electrical contact resistance in order to achieve maximum gasket performance.

Large area flat gaskets require considerable closure pressures. Enclosure flanges must be accurately machined and sufficiently rigid so as to apply force uniformly.

With a particular reference to the last point ensure that the design compression for any gasket i.e. gasket deflection is sufficient to accommodate tolerances in worse case conditions

Gasket sections mounted in channels (e.g. extrusions) generally require significantly less compression force but the channel must have a cross-sectional area somewhat greater than the gasket section itself

Assess the potential for galvanic corrosion as part of the design process. The gasket contains metallic particles that will almost certainly have a different electrode potential to the enclosure material. As a result if the gasket seam is exposed to moisture, then current will flow between the two causing the accelerated corrosion of the enclosure. The enclosure usually becomes the ‘sacrificial anode’. This effect can be controlled or even virtually eliminated by the careful selection of gasket materials and by incorporating certain design features

Ideally some form of gasket compression limitation should normally be incorporated into enclosure designs. This ensures that gaskets and, in some cases, the enclosure are protected from over-compression. It also enables fasteners to be tightened to their recommended working torque. This ensures they remain under tension, maintaining pressure on the gasket and firmly securing the assembly.

...and there's more

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