EMI Gasket Design Considerations
In most new design situations the EMI gasket is also required to act as an environmental seal. The proper selection of the silicon elastomer to be used in the design of the gasket is critical and will often determine the effectiveness and life span of the gasket. Electrically conductive filler combined with an elastomer compound will in most cases provide very effective protection from moisture, chemicals, and various environmental contaminants that may possibly affect the performance of the overall design and application requirements of the final product.
National Power and Signal manufactures silicone elastomer gaskets conductive or non-conductive, the product is available in a wide range of shapes and sizes. For example, O-ring profiles in solid extrusion form is available from 0.0300” to 0.5000” hollow O-ring profiles are available with a wall thickness of no less than 0.02500”.
Also available are die cut gaskets in standard shapes and sizes such as wave guide gaskets and custom made die cut EMI gaskets ranging in size from 1.0000” square to gaskets used in large enclosure applications for room doors and bulkhead seals. For all die cut product National Power and Signal offers an electrically conductive pressure sensitive adhesive or PSA. In certain high volume OEM applications the conductive PSA has proven to be very effective during the assembly process.
Soft or sponge EMI elastomer gaskets are most commonly used in low closure force applications and are used for example in sheet metal enclosures having an overall closure force of approximately 5 to 15 psi. For these types of soft or highly compressible gasket applications compression stops are used. The compression stop can be designed in as part of the enclosure or can also be an integral part of the gasket.
In order to achieve an actual pressure seal above 15 psi a gasket contained in a milled groove is required and this type of gasket or seal is referred to as the O-ring. The deign of the machined groove must accommodate almost the entire OD of the O-ring cross section. The machined groove should be completely filled by the cross section of the compressed gasket and a machined surface finish in the groove must be at least 1 micron. This design method will allow for a pressure seal of up to 100 psi. It should be noted that solid gasket material does not compress but actually deflects closure force pressure, the average deflection for a solid profile is typically 5 to 15% and is always a function of the overall hardness of the gasket material and the shape of the profile.
National Power and Signal uses nickel graphite (Ni/g) as the conductive filer for all EMI gaskets. Although various other conductive fillers are available and may be specified by the customers application requirements nickel graphite has proven to be the most cost effective for commercial OEM shielding applications.