Conductive Gasket Design For Shielded Enclosures

A conductive gasket is the part that closes the gap between a shielded enclosure and its lid. The metal walls of the box carry the current, but the seam between two pieces of metal is not continuous, and at radio frequencies a discontinuity of a few millimetres behaves as a slot antenna. The gasket fills the seam with a compliant conductive material so that the enclosure behaves as one conductor.

This article covers how a gasket closes a seam, how much compression it needs, which materials are used, and how corrosion and mechanical support decide whether the shield still works after a year in the field.

How A Gasket Closes A Seam

Shielding effectiveness is limited by the longest dimension of the largest opening, not by the total open area, so a single long slot is far worse than many small holes with the same total area. The gasket works by breaking that slot into short segments and by conducting across it. Its own contact resistance, combined with the inductance of the path it offers, sets the impedance across the seam, and that impedance must be low compared with the impedance of the enclosure wall at the highest frequency of interest.

As frequency rises the current in the wall stops flowing through the bulk of the metal and concentrates in a thin layer at the surface, so the return path runs along the inside of the seam. That is why a gasket on the outside face of a flange is much less effective than one on the mating surface, and why a gasket must be positioned on the surface the current actually crosses. The underlying principles are set out under EMI suppression design principles.

Compression And Deflection Limits

A gasket only conducts where it is compressed, so the mechanical design has to guarantee a minimum compression across the whole seam after every tolerance has been applied. The stack includes the flange flatness, the thickness of the plating, the fastener torque and the stiffness of the two halves. A typical soft conductive elastomer needs twenty to thirty percent deflection to reach its rated contact resistance, and its force rises steeply beyond that, so the usable window between the minimum compression and the force the fasteners can hold is often narrow.

Over compression is the more common field failure. A gasket compressed past its limit takes a permanent set, and when the lid is removed and refitted the gasket no longer springs back to fill the gap. Where a closure will be opened repeatedly, a gasket with a defined compression stop, or a channel that limits deflection, is worth the extra machining. Fastener spacing matters for the same reason, because a gasket between two fasteners deflects the flange away from the gasket and the compression drops in the middle of the span.

Materials And Contact Resistance

Three families are in common use. Conductive elastomers are silicone loaded with silver, nickel, graphite or silver-plated glass particles; they are soft, conform well to a surface and can be moulded into complex shapes. Wire mesh and knitted mesh over an elastomer core are mechanically robust and take high compression, but they can abrade the plating they press against. Fabric over foam is light and inexpensive, and it is the usual choice for small consumer enclosures where the required attenuation is modest.

Contact resistance depends on the material, the plating it bears against and the surface finish of that plating. A rough, oxidised or painted flange raises the resistance of every contact point, and the problem is local: a single patch of poor contact along a long seam dominates the leakage. The mating surfaces are therefore specified as bare, plated and conductive, and a conductive finish over a non-conductive substrate is not acceptable because the current cannot spread into the wall beneath. Where a coating is used elsewhere on the board or housing, the guidance under conformal coating as board protection explains how thin films behave on conducting surfaces.

Galvanic Corrosion At The Joint

Two dissimilar metals in contact in the presence of moisture form a galvanic couple, and the less noble metal corrodes. Silver-filled gaskets against aluminium, and nickel-graphite gaskets against a tin-plated steel chassis, are both common couples that behave differently outdoors and in salt fog. The corrosion product is usually a poor conductor, so the seam resistance climbs over time and the shield quietly degrades.

The remedies are to keep the two metals close together in the galvanic series, to plate the less noble surface so that the couple is smaller, to keep the joint dry, and to avoid trapping electrolyte in the seam. Where the enclosure is sealed, a gasket that also serves as the environmental seal must be chosen so that its electrical and sealing functions do not conflict, and the seam should drain rather than collect water. Mechanical mounting details that keep the seam accessible and drainable belong with the general board outline and mounting design.

Mechanical Support Around The Seam

A gasket is a spring, and the enclosure has to react its load. Thin sheet metal flanges bow between fasteners, and moulded plastic covers creep under continuous load, so the compression measured on the first assembly is not the compression present after a year. Ribs behind the flange, a stiffer material, or a metal insert at each fastener keeps the load where it was designed to be.

The reference for the shield is the ground structure inside the product, and the return current has to reach that structure without crossing a long path. A gasket that ties the lid to a chassis which is itself floating does not shield anything, and a seam near a switching supply needs the same low impedance as the return path of the supply itself, as described under ground routing and power trace planning. Coordination between the mechanical and electrical teams at the layout stage is what keeps that path short.

Test And Verification

The seam is verified in two ways. A shielded room measurement with a radiating source inside the enclosure, or an antenna outside it, gives the attenuation of the whole assembly and shows whether the design meets its target. A point measurement of contact resistance across the seam, taken at several places along its length, shows whether a specific gasket is making contact at all, and it is fast enough to use on production units.

Both tests should be done after the environmental sequence, not before it, because compression set, corrosion and thermal cycling all change the joint. A unit that passes on the bench and fails after humidity and temperature cycling has a gasket problem rather than an electronics problem, and the fix is usually in the compression budget or in the material choice rather than in the circuit.

FAQ

Can a gasket be reused after the lid is opened? A soft elastomer that has been compressed to its rated deflection usually takes a set and will not seal as well the second time. Where service access is expected, a gasket with a compression stop or a replaceable strip is the safer design.

Does a gasket need to run around the whole perimeter? It needs to cross every seam that the high frequency current crosses. A partial gasket can be adequate where the mechanical joint is already continuous metal, but the analysis has to show that no long slot remains unbridged.

Why does shielding degrade after a few months outdoors? Galvanic corrosion between the gasket and the flange is the usual cause. The corrosion product is resistive, and the resistance of the seam rises with it, so the choice of metal pair and the sealing of the joint decide the long term behaviour.