Connectors and enclosures must both be shielded against EMI. To provide for this, Kemtron manufacture 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 +160degC, up to +200degC 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 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 [cm] and shielding effectiveness stated [db]. The volume resistivity of silver plated aluminium in silicone will be max 0.008cm giving a shielding effectiveness at 2GHz of 102db whereas nickel coated graphite will have a volume resistivity more than 10 times greater but exhibit 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 excellent performance results from 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 percent less than silver plated aluminium in silicone and 70 percent 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.