Conductive elastomers shield devices from EMI

Kemtron has developed a range of conductive Elastomers as gaskets and gasket strip for EMI shielding of electronic devices.

The base elastomer is silicone for normal environments and fluorosilicone for fuel and oil resistance; both offer a wide temperature range of -40 to 160C and up to 200C for some grades.

The conductive fillers available are silver-plated aluminium, silver-plated copper, pure nickel and nickel-coated graphite.

Other fillers, such as silver-plated nickel, are available to special order.

In general, the conductive elastomer will have a Shore A hardness of between 60 and 75, depending on grade, and still maintain good tensile strength - this will ensure the gasket deflects sufficiently when under compression giving a good EMI and environmental seal.

The electrical properties of the conductive elastomer are measured in volume resistivity (ohms-cm) and shielding effectiveness is stated in dB; however, the two do not directly correlate with each other.

The volume resistivity of silver-plated aluminium in silicone will be max 0.008 ohms-cm, giving a shielding effectiveness at 2GHz of 102dB, whereas nickel-coated graphite will have a volume resistivity more than 10 times greater but exhibits a similar shielding effectiveness.

Nickel-coated graphite in silicone is the most cost-effective conductive elastomer, giving excellent shielding characteristics, even though the material is much more resistive than precious metal-plated particle elastomers.

This high performance is due to the fact that the nickel graphite particles are very irregular in shape and have sharp edges: when the gasket is put under pressure by being compressed between two surfaces, the particles dig into the surface, giving very low contact resistance.

Graphite is also a good microwave absorber, thereby enhancing the shielding performance.

Nickel graphite in silicone costs approximately 60 per cent less than silver-plated aluminium in silicone and 70 per cent less than silver-plated copper in silicone.

Nickel graphite also offers high galvanic corrosion resistance compared with silver-plated particles when in contact with aluminium flanges.

Kemtron also deposits liquid conductive silicones directly to customers' hardware as a form-in-place gasket: this process can deposit very small gaskets on complex paths with narrow land.

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