Ask SSP for Parker Chomerics CHO-SEAL® alternatives and offsets to GORE GS2100 and GORE GS5200 UL 94 V0 flame-retardant EMI shielding.
Electrically Conductive Silicones
EMI gaskets that are made from electrically conductive silicones provide environment sealing, thermal insulation, and shielding against electromagnetic interference (EMI). They support a variety of fabrication methods, are available in materials that meet tough standards, and can be supplied with adhesive backings for ease-of-installation.
EMI Gaskets vs. Other Elastomeric Gaskets
EMI gaskets seal the gaps between two mating surfaces. Like other types of environmental gaskets, they seal-out the external environment and seal-in to prevent leakage. What makes EMI gaskets different is that they also seal against conducted or radiated EMI that can interfere with circuits.
When this “noise” reaches the EMI gasket, the signals are absorbed and the resulting electrical current is sent to ground. Silicone is normally an electrical insulator rather than an electrical conductor, but the addition of metal or metal-coated particles, or metal wires or wire mesh, imparts the necessary electrical conductivity.
Silicones vs. Fluorosilicones for EMI Gasketing
Silicone also provides broad temperature resistance and is a thermal insulator that can resist thermal cycling between temperature extremes. Flurosilicone, a type of silicone with fluorine additions, is sometimes used as the base material instead. Fluorosilicone EMI gaskets cost more, but flurosilicone offers greater resistance to fuel, oil, and other chemicals.
Silicone and fluorosilicone EMI elastomers are available in the same form factors: sheets, rolls, extrusions, and ready-to-mold compounds.
Metals vs. Other Fill Materials
The particles in EMI silicones are made of pure silver, silver and another metal, or a metal and a non-metal.
- Pure silver offers excellent electrical conductivity, but it’s also more expensive.
- Bimetallic particles include nickel-aluminum, silver-aluminum, silver-copper, and silver-nickel.
- Nickel-graphite and silver-glass are examples of particles made of a metal and a non-metal material.
EMI Gasket Compression
Like other types of elastomeric gaskets, EMI gasketing is compressed by a percentage of its size. This compression forms a seal that physically fills the gap between two surfaces. When the compressive stresses are removed, the EMI gasket is supposed to return to its original thickness. If it does not, this irrevocable deformation (compression set) can leave a gap and cause seal failure.
Harder materials with higher durometers are more difficult to compress, but electrically conductive silicones are not excessively hard because of the addition of particles. In fact, EMI silicones are available in a range of hardnesses, including lower durometers for gaskets where there is less closure force.
Enclosure Sealing and Insulation
EMI gaskets that are made of electrically conductive silicones are installed within enclosures. Examples include electrical and telecommunications equipment, electronic and medical devices, robotic end-effectors, and flat-panel displays.
Some of these enclosures need to meet specific requirements for ingress protection (IP) against dust and water.
- North America: EMI gaskets are sometimes used within NEMA enclosures that need meet specific IP requirements from the National Electrical Manufacturers Association.
- Europe: IP ratings or codes are defined in IEC 60520. As with NEMA enclosures, some IP enclosures require EMI gaskets for protection against electromagnetic interference.
EMI Gasket Fabrication
EMI gaskets can be cut or molded. Products that are not fabricated as a single piece can be joined using cold bonding or hot splicing.
Cutting and Molding
- Flash cut EMI gaskets use CNC equipment to cut EMI elastomers into specific shapes without custom tooling. Flash cutting is ideal for rapid prototyping and short runs, and this gasket fabrication method produces extremely smooth edges with minimal loss of material. This digital manufacturing process works from your CAD file and supports flash cut EMI gaskets with a conductive PSA.
- Die cut EMI gaskets are cut into specific shapes and sizes from sheets or rolls of EMI elastomers. SSP makes its own dies in-house and fabricates die cut EMI gaskets with a high degree of uniformity, even across higher volumes. Precise dimensions, parts nesting, and punched holes with specific radii are readily achievable. SSP can apply conductive pressure-sensitive adhesive (PSA) tapes for peel-and-stick parts.
- Molded EMI gaskets are produced using compression molding, a process that places a preformed rubber material into the cavity of a heated mold. SSP makes its own molds in-house and produces compression-molded EMI gaskets using the same materials that we supply as sheets, rolls, and extrusions. For large picture-frame style gaskets, molding can increase material yields and reduce material waste.
Bonding and Splicing
- Cold bonding joins EMI materials by using an RTV silicone adhesive or a non-silicone glue. Using a non-conductive adhesive increases the risk of EMI leakage, but conductive glues are available. With non-silicone glues, acrylic adhesives lack the temperature resistance of silicones.
- Hot splicing joins EMI materials by applying heat and pressure. A splicing tool, or mold, is required, but hot spliced EMI gaskets have consistent properties throughout. They don’t have a “hard spot” like with a cold bonded EMI gasket either.
EMI Gasket Standards
EMI gasket standards include requirements for EMI shielding and environmental sealing. Some standards, such as NEMA and IP ratings, apply only to environmental sealing but may still affect EMI gaskets made of electrically conductive silicones. In addition to NEMA, the following organizations maintain standards that EMI gasket designers may need to meet.
- U.S. Department of Defense (DoD)
- Underwriters Laboratories (UL)
- ASTM International (ASTM)
Military Standards
For some military applications, EMI gaskets must use materials that meet MIL-DTL-83528 requirements. MIL-DTL-85528 is a detail specification from the DoD that establishes general requirements for electrically-conductive elastomeric shielding gaskets. MIL-DTL-83528 contains lettered sections, each of which contains requirements for the base elastomer, durometer, fill material, plane wave shielding effectiveness, and continuous use temperature. Because MIL-DTL-83528 only applies to fill materials that are pure silver or silver-coated, it does not encompass nickel-graphite filled silicones or wire-oriented silicones that contain Monel or aluminum mesh.
UL Standards
UL maintains two flammability standards that may apply to EMI gaskets: UL 94 V0 and UL 50-E. Neither standard is silicone-specific, and both apply to plastics. UL 94 V-0 is part of a larger standard, UL 94, that classifies materials according to how they burn in various orientations and part thicknesses. UL 50E is an IP standard against dust and water that applies to enclosures for electrical equipment that will be installed and used in non-hazardous locations.
ASTM Standards
For EMI gaskets that require resistance to galvanic corrosion, such as those used in marine environments, ASTM B117 may apply. Galvanic corrosion occurs when two dissimilar metals are immersed in a conductive solution, such as salt water, and are electrically connected. There are also electrically conductive silicones for EMI gaskets that need to meet ASTM E595 for outgassing, a problem in high vacuum environments, such as outer space, where released gases can condense upon and cloud optics.
EMI Filter vs. EMI Gasket
EMI shielding isn’t the only way to promote electromagnetic compatibility and to address compliance with EMC regulations and standards. Electronic designers can also use EMI suppression filters, which target a specific source of noise and control the flow of electromagnetic energy. Typically, EMI filters are used at the inputs and outputs of an electrical system since these are vulnerable points where gaps in EMI shielding may occur. EMI filters are also used at other specific circuit locations for targeted protection. In many if not most electronic designs, both EMI shielding and EMI filtering is used.
