Conductive elastomers are for RFI/EMI shielding

20 January 2009

Kemtron’s elastomers aid shielding in various applications.

The base Elastomer is silicone for normal environments and Fluorosilicone for fuel and oil resistance. Both offer a temperature range of -40°C to +160°C and up to +200°C 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.

The manufacture of a conductive Elastomer is a balance of conductive particle loading and distribution throughout the silicone base. The distribution must be sufficient to ensure that the particles are in contact with each other to ensure a good conductive path through the Elastomer, but the loading must not be so great to cause the silicone to lose its elastomeric properties.

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 will deflect sufficiently when under compression, giving a good emi and environmental seal.

The electrical properties of the conductive Elastomer are measured in volume resistivity, and the volume resistivity of silver plated aluminium in silicone will be max 0.008Ωcm; giving a shielding effectiveness at 2Ghz of 102db. 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 performance can be accredited to the fact that the nickel graphite particles are very irregular in shape and have sharp edges. When the gasket is put under pressure and 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% less than silver plated aluminium in silicone and 70% less than silver plated copper in silicone. Nickel graphite also offers excellent galvanic corrosion resistance over silver plated particles when in contact with aluminium flanges.

Kemtron’s process capability is extrusion, moulding and conductive vulcanised jointing. Many extrusion profiles are available including cords, tubes, hollow D, solid D, and custom profiles are available. Extrusions are supplied in continuous lengths, cut to length or made into large or small diameter O rings which are produced by joining extruded profiles with the same conductive compound as the base material. The join is heat vulcanised ensuring a strong conductive joint. Mouldings include sheet stock, O rings, custom shapes and gaskets. Sheets can be die cut to customers’ design and a conductive PSA can be applied to aid assembly. Kemtron also deposit liquid conductive silicones directly to customers’ hardware as a form in place gasket.

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