EMI gaskets are specialized components designed to provide Electromagnetic Interference (EMI) shielding in electronic devices or equipment. These gaskets are used to physically close/seal gaps or openings in the equipment enclosure to prevent EMI from entering or leaking out of the electronic devices. They are often installed in areas where electromagnetic radiation might leak out, such as seams, enclosure doors, joints, or openings in electronic enclosures. 

It is very common to see the EMI gaskets used in conjunction with shielding enclosures of the electronic device to enhance shielding effectiveness. The gasket serves as a radiation barrier to block or reduce unwanted electromagnetic radiation signals, thereby protecting electrical and electronic devices and systems from EMI. 

Figure: EMI gaskets

EMI gaskets contribute significantly to the electromagnetic compatibility (EMC) of electronic devices and systems. An EMC-compatible device operates satisfactorily in its intended electromagnetic environment without emitting harmful EMI that could disrupt nearby devices. 

Note: EMI gaskets are also referred to as RFI gaskets when used for blocking radio frequency signals or radio frequency interference. Often, the terms EMI (electromagnetic interference) and RFI (radio frequency interference) are used interchangeably. 

EMI/RFI gaskets are typically made from conductive materials such as metal (e.g., copper, aluminum, stainless steel) or conductive elastomers (e.g., silicone or fluorosilicone loaded with conductive particles). The other materials used for making the gaskets include silver-coated aluminum, Nickel-graphite, and conductive foam materials. Ideally, an EMI gasket should be made from the same or compatible material used for the EMI shielding enclosure to ensure efficient shielding. 

The EMI/RFI gaskets come in various sizes and shapes such as strips, sheets, O-rings, rectangular, square, D-shape, J-shape, P-shape, U-shape, clip-on, and custom shapes and sizes. They are used for EMI shielding purposes in several applications, including commercial, industrial, automotive, and military electronics. 

How does an EMI gasket work?

Any openings or gaps in the electronic enclosure will allow electromagnetic fields to radiate through them unless current continuity can be maintained across gaps. The EMI/RFI gaskets are used to physically close/seal gaps or openings in the equipment enclosure. The gaskets function by establishing a conductive pathway along the seams and openings of electronic enclosures. This pathway facilitates smooth current flow and forms a Faraday cage around the enclosed electronics. When the electromagnetic interference signal encounters the EMI gasket, the signals are canceled out (or negated), and the resulting electrical current is safely directed to the ground. Thereby, the gaskets protect electrical and electronic devices and systems from EMI.

Note: A Faraday cage is a metal enclosure or shield used to block EMI or RFI. The cage is grounded for correct use. When the EMI/RFI signals encounter the Faraday cage, the free electrons in the metal structure will redistribute, resulting in current flow in the metal enclosure. The current flow induces an electric field in the outside of the metal enclosure, which is opposite to the direction of the electric field of the incoming EM wave.   So both fields cancel out, and due to that, there is zero net field inside the Faraday cage. The current in the metal enclosure is directed safely to the ground. This phenomenon protects the electronic device within the cage from external EMI/RFI as well as prevents the leak out of EMI/RFI from the enclosed device. 

The EMI gaskets fill/close the gaps in the shielding enclosure of the device and form a Faraday cage effect, thereby preventing external EMI entering into the device as well as EMI leaking out from the device. As a result, the electronics housed within the enclosure are protected from EMI/RFI.

Figure: Faraday Cage

What are the types of EMI gaskets? 

EMI/RFI shielding gaskets are available in various types, each designed to provide effective shielding against EMI/RFI. Let's discuss the main types of EMI/RFI shielding gaskets and their applications. 

Metal Gaskets: Metal gaskets are typically made up of metals such as copper, aluminum, or stainless steel. They have high conductivity and mechanical strength, making them well-suited for applications requiring robust EMI/RFI shielding performance.

Conductive Elastomer Gaskets: These EMI gaskets are made from elastomeric materials like silicone rubber or fluorosilicone rubber loaded with conductive particles like silver, nickel, or carbon. Conductive elastomer gaskets are electrically conductive and flexible. They can provide excellent compression, conductive contact, broadband shielding, and moisture sealing. These gaskets can offer protection from EMI, electrostatic discharge (ESD), radio frequencies, and the environment. 

The conductive elastomer gaskets are often used in electronic enclosures, connectors, and other applications requiring effective EMI/RFI shielding. Particularly, Silicone EMI Gaskets are used in most EMI shielding applications since they are resistant to fire, water, ozone, aging, chemicals, and fungal growth. These gaskets are ideal for industrial, commercial, and residential applications.

Fingerstock Gaskets: These gaskets consist of a series of spring-like fingers made from conductive metals such as stainless steel or beryllium copper. The spring-like metal fingers offer consistent pressure and conductive contact between mating surfaces. When compressed between mating surfaces, the fingers create a continuous conductive path, providing effective EMI/RFI shielding. The Fingerstock EMI Gaskets are often used in shielded doors, connectors, and other applications that involve frequent access or repeated opening and closing. 

Figure: Fingerstock EMI Gaskets

Knitted wire mesh gaskets: 

A knitted wire mesh gasket is made from knitted wire mesh, which consists of interlocking wire loops forming a mesh structure. These gaskets are commonly made from Tin Plated Copper Clad Steel (SN-FE-CU), Stainless Steel, Silver-Clad Copper, Tinned Copper, or Monel wire. They come in various shapes, including round profiles round cores with fin, rectangular profiles, and double round cores with fin.

Figure: Knitted wire mesh gaskets

Knitted wire mesh gaskets provide a cost-effective EMI shielding solution against both magnetic and electrical fields, including electromagnetic pulse (EMP). These gaskets are available in various constructions, such as completely made of knitted metal mesh or made from knitted metal mesh over an elastomer core. These gaskets are ideal for use in applications that require good EMI/RFI shielding along with ventilation. For example, they are ideal to seal enclosure lids and doors, interface gaskets for EMI-shielded vent panels, removable cover plates, and EMI/RFI-shielded windows.

Fabric-over-Foam (FOF) gaskets: 

Fabric-over-Foam (FOF) gaskets consist of metalized fabric wrapped around a soft foam core. These gaskets are flexible and are ideal for applications that require high conductivity and good shielding performance with low compression force.  The conductive fabric layer ensures a reliable electrical connection between mating surfaces, while the foam substrate provides cushioning and compression properties. Conductive fabric-over-foam EMI/RFI gaskets are commonly used in applications requiring both shielding effectiveness and flexibility, such as portable electronic devices, computer peripherals, telecommunication, or other equipment.

Conductive foam gaskets

Conductive foam gaskets are made from conductive foam materials. The conductive foam materials are the foam materials (e.g., polyurethane foam) loaded with conductive particles or coated with conductive coatings (e.g., copper and nickel). These EMI gaskets are well-suited for applications that need excellent conductivity and low compression force. They are available with UL 94 V-Rated and RoHS compliant. The conductive foam gaskets are ideal for computer, router, and telecom I/O shielding, as well as in aerospace equipment. They come to market in standard thicknesses of .040”, .059”, .087”, and .134”.

Figure: Conductive foam gasket

Conductive Adhesive Tapes: 

Conductive adhesive tapes consist of a layer of conductive material with an adhesive backing. These tapes are not traditional gaskets. The conductive adhesive tapes are employed to seal gaps and seams between components and the enclosure of electronic devices. Particularly, they are useful in applications where effective conduction, grounding, or shielding is essential. These tapes provide a simple and efficient solution for EMI/RFI shielding, especially in tight spaces. Conductive adhesive tapes find widespread use in various applications, including consumer electronics, communication equipment, and automotive electronics.

Figure: Conductive adhesive tape (Copper based)

How to select an EMI gasket?

The following are the key parameters that need to be considered while selecting the EMI/RFI gasket.

Frequency Range (Hz): It is the frequency range over which the EMI/RFI gasket provides shielding effectiveness (dB). EMI gaskets are available with different frequency ranges and shielding effectiveness (dB). 

Shielding Effectiveness (dB): This refers to the ability of the gasket to attenuate/block electromagnetic interference (EMI). While selecting the gasket, ensure that the shielding effectiveness (dB) meets the application's requirements. 

Material Compatibility: It is necessary to consider the compatibility of the gasket material with the mating surfaces and environmental conditions (e.g., temperature, humidity, chemicals).

Compression Force: It is the required compression force to achieve proper sealing and EMI shielding. It is measured in Ibs/in or  Kg/m. EMI shielding with lower compression force is desirable.

Compression Set: It is the EMI/RFI gasket's ability to return to its original thickness after being subjected to a compressive force over a given time period.  Selecting the gasket with a lower compression set is desirable to ensure reliable and long-lasting performance. For example, the gasket with a 15% compression set will regain 85% of its initial thickness after being subjected to the compression force.

Thickness: Represents the thickness of the gasket in mm. Thicker gaskets may provide better shielding effectiveness but could require higher compression forces. 

Temperature Range: It represents the operating temperature range of the gasket, typically measured in degrees Fahrenheit (°F) or degrees Celsius (°C). Within this temperature range, the gasket can operate effectively without experiencing degradation in performance. 

Corrosion: Select the gasket that is not susceptible to corrosion. 

Environmental Sealing: In addition to EMI shielding, consider whether the EMI gasket can provide environmental sealing against dust, moisture, or other contaminants.

Chemical Resistance: If the application involves exposure to chemicals or fluids, select a gasket material that is resistant to those substances to prevent deterioration over time.

Cost: Evaluate the cost of the EMI/RFI gasket relative to its performance and suitability for the application.

Customization Options: Some applications may need custom shapes, sizes, or configurations. Choose the gasket with customization options provided by the manufacturer to meet specific application requirements.

Each type of EMI gasket is designed to provide specific shielding requirements for an application. When selecting the EMI/RFI shielding gasket, one should consider factors such as the shielding level (dB) needed, environmental conditions, application, and material compatibility with the mating surfaces.