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.
