EMI Gaskets

Stockwell Elastomerics manufactures custom EMI shielding gaskets, RFI gaskets and other EMI management gaskets from various conductive rubber/conductive silicone EMI shielding materials to specification.

Stockwell Elastomerics also makes combination silicone sponge – wire mesh EMI shielding gaskets.

What are EMI Gaskets?

EMI gaskets that are made of electrically conductive elastomers such as silicone or fluorosilicone 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 conductive adhesive backings for ease-of-installation. There are many types of EMI shielding materials, but electrically conductive silicones provide gasket designers with performance and versatility.

Specifically, EMI gaskets seal the gap between two mating surfaces. Like other types of environmental gaskets, they seal out the external environment or 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 electromagnetic interference “noise” reaches the EMI gasket, the signals are negated 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 imparts the necessary electrical conductivity.

EMI Gasket Manufacturing

Stockwell Elastomerics offers manufacturing for all of these types of EMI/RFI shielding gaskets:

• Waterjet Cut EMI Gasket
• Molded EMI Gasket
• Wire Mesh / Sponge EMI Gasket
• Conductive Adhesive Backed EMI Gasket
• RFI Gasket

More about how EMI gaskets are made is provided below.

• Conductive Rubber EMI Gaskets
• Combination EMI Gaskets
• Electrically Conductive Foam for Shielding and Grounding

EMI / RFI Management Requirements

The ever growing wireless and electronics markets have increased the stakes for managing electromagnetic interference (EMI) and radio frequency interference (RFI). This holds true for consumer, commercial/industrial markets as well as military applications. Managing EMI is required of electronic device manufactures under Title 47 of the Federal Code of Regulation and Mil-DTL-461. These are some leading EMI standards and their applications:

• Title 47 CFR Part 15 – Radio frequency (RF) devices
• Title 47 CFR Part 18 – Industrial, scientific & medical equipment
• Title 47 CFR Part 68 – Telecommunications
• Mil-STD-461 – Control of EMI for subsystems and equipment
• Mil-DTL-83528 – (Related) gasket material for EMI/RFI

EMI Management with Gaskets

Radiant EMI is often emitted by unintended antennas, meaning that internal wires and components can act as emitting antennas. A common practice to manage EMI radiation and EMI susceptibility is to completely encase electronics in a conductive enclosure. This usually entails a housing, lid, and a conductive gasket; it’s critical that the lid and enclosure have complete contact all the way around, hence a conductive gasket. Stockwell Elastomerics manufactures EMI gaskets for Class A (typically industrial), Class B (typically residential), and military applications. EMI gaskets manufactured by Stockwell Elastomerics regularly perform EMI attenuation as well as environmental sealing (rain, dust, wash down, etc.)

Combination Silicone Sponge and Wire Mesh EMI Shielding Gaskets

Stockwell Elastomerics manufactures custom combination EMI gaskets from expanded silicone or fluorosilicone sponge and have knitted wire mesh bonded to the edge (see picture at right).  Combination gaskets are usually thicker and softer than conductive rubber EMI gaskets.  This thicker and softer quality allows for more compression under lower clamping pressure, providing enclosure sealing with wider tolerance stacks.

Combination EMI gaskets are often used for large NEMA enclosures or other enclosures with limited clamping forces or sheet metal skins. Since silicone sponge or foam is very compliant under low clamping force, they “absorb” mating variation (flatness tolerances) between sealing faces creating a water seal for outdoor environments; the knitted mesh electrically bonds the metal faces. Note: bare metal or conductive chem-films are required at the mesh point of contact to facilitate a conductive path.

Combination EMI / RFI gaskets are most commonly manufactured in 0.125″ (3.18mm) and 0.25″ (6.4mm) though other thickness options are available upon request. Knitted Monel wire mesh is used for its corrosion resistance.

Combination EMI Gaskets Using UL Rated Silicone Foam

UL 50E and UL 94V-0 rated silicone foams are often used for the water or dust sealing function of the combination gasket. While the combination EMI gasket configuration has not been tested by UL, the silicone foams have been. This third party test adds confidence that a third party, pre-qualified gasket material is used.

Silicone foam has excellent UV and ozone resistance and long term rebound (compression set resistance), making it a good choice for outdoor EMI enclosure gaskets. Closed cell silicone foams are used when water sealing is needed, open cell silicone foams are used for dust sealing applications.

Examples

• Closed cell silicone foam combination gasket – for NEMA Type 3 or NEMA Type 4 EMI enclosure, where water sealing and EMI / RFI shielding are needed
• Open cell silicone foam combination gasket – for EMI vents or EMI filters used on power supply enclosures, where dust sealing, UL 94V-0 and EMI shielding are required

Combination EMI Gaskets Using Silicone Sponge

Closed cell silicone sponge materials are better suited for rugged enclosures or enclosures where the gasket is not protected from a jet blast (IP66 or NEMA 4). The gum based silicone sponge materials are tougher and more closed cell than liquid based silicone foams, so they are specified more often for rugged or outdoor EMI combinations gaskets.

Combination EMI / RFI gaskets using silicone sponge are available in soft, medium and firm grades that are widely used for military enclosures and remote location applications where environmental conditions can vary greatly, from sub-freezing temperatures to arid desert conditions. Knitted Monel wire mesh is bonded to the opposite side of the environmental conditions (typically the interior of the window gasket), protecting the metal mesh from the elements.

Combination EMI Gaskets Using Fluorosilicone Sponge

Military and Aerospace applications may require chemical resistance in addition to EMI shielding and environmental sealing; for these applications Stockwell Elastomerics manufactures EMI combination gaskets using R10490 closed cell fluorosilicone sponge. Fluorosilicone shares many of the same properties as silicone, such as high temperature and low temperature resistance, but with enhanced chemical resistance. Fluorosilicone is typically only specified where chemical resistance is needed due to the price premium.

Adhesive Backings for Combination EMI Gaskets

• 256M – General purpose, double coat (film supported) acrylic pressure sensitive adhesive (PSA) is widely used. The PET support layer gives the gasket X and Y axis dimensional stability.
• 9485 – High bond and strong adhesion. 3M 9485 offers high temperature resistance and is well suited for low surface energy such as powder coated paints.
• F9473 – High bond VHB acrylic adhesive is Stockwell Elastomericss’ preferred adhesive for fluorosilicone EMI combination gaskets due to enhanced fluid resistance.

Compression Stops or Compression Limiters for EMI Gaskets

Stockwell Elastomerics offers combination EMI / RFI gaskets with compression stops, sometimes called compression limiters, to protect the gasket from being crushed under torque. Since gasket materials relax they are very difficult to set torques, the use of compression stops allow assemblers to use torque specifications since fasteners will seat on the metal hard stop.

Combination EMI Gaskets for Large NEMA Enclosures

Large EMI gaskets for electrical enclosures and cabinets are often addressed using a corrosion resistant Monel wire mesh bonded to a closed cell silicone sponge. The wire mesh / sponge gasket allows for greater tolerances between sealing faces. The sponge and wire mesh offer forgiving, low closure forces to prevent sheet metal bowing between latches or fasteners.

• Closed Cell Silicone – Common thicknesses are .125″ and .250″. Closed cell silicone sponge R10470-M and medium density silicone foam HT-800 are most frequently specified for the environmental gaskets. Gaskets are normally supplied with a high strength pressure sensitive acrylic adhesive.
• Monel Mesh – .125″ x .125″ wide and .250″ x .125″ wide Monel mesh is bonded to the inside or outside gasket edge to provide a relatively low cost and reliable EMI shielding gasket with long term weather sealing. The Monel mesh is constructed from woven .0045″ wire that meets QQ-N-281.
• Aluminum mesh (AMS 4182) or tin plated copper clad steel wire (ASTM B520) are available on special order.

How are EMI Gaskets Made?

EMI silicone gaskets can be fabricated using waterjet cutting, die cutting, or compression molding.

• Waterjet cutting is a tool-less process that is ideal for prototypes and low-to-medium volumes. Because there is no tooling to wait for or pay for, water jetting supports fast, cost-effective iterations before higher volume production begins. Sheets, continuous rolls, and extrusions can be converted to final part shapes.
• Die cutting requires metal tools, or dies, and this adds cost and time to projects. For high-volume production, however, die cutting is cost-effective, especially with flat gaskets converted from sheets or rolls. Like waterjet cutting, die cutting supports parts nesting to layout cutting patterns that minimize material waste.
• Compression molding produces EMI gaskets from ready-to-mold compounds instead of sheets, rolls, or extrusions. Tooling is required, but the EMI gaskets are molded as a single piece. For picture window” style gaskets with an empty area in the middle, compression molding can maximize material yields since there is no unused center to discard. Complex shapes can be molded to avoid PCB components, create retention features, or provide a shape factor for lower closure force.

EMI gaskets that are not fabricated as a single piece can be joined using cold bonding or hot 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.

What’s the Difference between EMI Gaskets and RFI Gaskets?

EMI (electromagnetic interference) and RFI (radio frequency interference) are often used interchangeably. Both are electrical disturbances/electromagnetic emissions that can cause problems in some equipment. The simple difference between EMI and RFI is that EMI includes any electrical frequency, while RFI is limited to the radio frequency spectrum, which is a subset of the entire electrical noise spectrum. So EMI gaskets and RFI gaskets basically do the same function.