Figure: Understanding Conducted EMI (propagate via power line/wire) and Radiated EMI (unwanted electromagnetic energy emissions, propagate via air)

Electromagnetic interference (EMI) refers to the disturbance to electrical and electronic devices caused by unwanted electromagnetic signal emissions from external sources. The external source can be man-made (e.g., nearby electrical devices) or natural source (e.g., lightning). When EMI affects the operation of radio communication systems, it is referred to as radio frequency interference (RFI). Therefore, EMI is sometimes also known as RFI. 

EMI/RFI signals emitted from external sources (e.g., a computer) can disturb or even damage nearby equipment (e.g., a TV) through conductive means (traveling via wires and cables) and/or radiated means (traveling through the air), see Figure. EMI can also pose potential risks to human health. EMI can be caused by switching circuits, SMPS, computer circuits, brush motors, arc welders, electronic devices, lightning discharges, solar flares, etc. EMI can be caused by switching circuits, SMPS, computer circuits, brush motors, arc welders, electronic devices, lightning discharges, solar flares, etc.

Understanding EMI shielding:       

EMI shielding refers to a protective shield or enclosure made of conductive or magnetic materials.  It encloses an object (e.g., an electronic device) or area and acts as a barrier against electromagnetic radiation. The shielding blocks or attenuates the energy of an incident radiation signal through three primary mechanisms: reflection, absorption, and multiple internal reflections. These mechanisms prevent EMI/RFI signals from entering or exiting the shielded space, thereby protecting equipment and people within and nearby the shielded enclosure from harmful radiated EMI/RFI. Typically, EMI shielding structure is grounded to enhance its effectiveness.  

Figure: Understanding EMI shielding and its shielding mechanism  

EMI shielding helps ensure that electrical and electronic equipment operates reliably without disturbing nearby devices in real-world environments, thereby enhancing the electromagnetic compatibility (EMC) of the equipment. The EMI/RFI shielding is used to protect electrical and electronic equipment from EMI/RFI across various fields, including household appliances, medical devices, communication systems, industrial equipment, and military applications.

Note: EMI shielding protects equipment from radiated EMI. EMI filters are employed to suppress conducted EMI on power and signal lines, protecting connected equipment from conducted disturbance.

Understanding shielding Effectiveness (SE):

Shielding effectiveness (SE) is a measure of how well a material or shielding enclosure blocks or attenuates EMI, typically expressed in decibels (dB). SE of the shielding structure depends on several factors, including the following:

• Incident radiation signal frequency
• Shielding material characteristics (such as thickness, permeability (µ_r), conductivity (σ_r), and impedance). 
• Distance from the EMI source (whether in the far field or near field)
• Characteristics of the interfering source (such as wave impedance, amplitude, frequency, and polarization).

Table 1 provides information about the electrical conductivity (σ_r) related to copper and relative magnetic permeability (µ_r) for selected materials. 

EMI shielding methods and shielding materials: 

Various EMI shielding methods and shielding materials are utilized to provide effective EMI/RFI protection for electrical and electronic devices in different applications. The shielding methods or techniques include:

• Conductive enclosures
• EMI shielded windows and shielding doors 
• EMI/RFI gaskets 
• EMI/RFI shielding mesh 
• EMI/RFI shielding foil 
• Conductive coatings and paints 
• EMI shielding tape, shielded cables 
• EMI shielding window film
• Magnetic shielding.

These shielding solutions can use various EMI shielding materials such as metals, carbon, conductive polymers, ceramics, hybrids, composites, and cement based EMI shielding material. The following sections provide more details about the shielding methods and shielding materials.

Understanding EMI shielding methods:

Conductive Enclosures: 

Conductive enclosures are made from materials such as brass, copper, aluminum, silver, nickel, and steel, all of which exhibit excellent electrical conductivity. This enclosure works based on the principle of a Faraday cage that protects enclosed objects (e.g., electronic equipment) or specific areas from radiated EMI/RFI. It also protects nearby equipment from EMI/RFI signals emitted by the devices within the enclosure. Conductive enclosures come in various sizes, ranging from small-scale enclosures for electronic devices, board-level EMI shielding, and RF test boxes to larger implementations like server rooms, EMI shielding rooms, and RF test chambers. 

Figure:   Few EMI shielding solutions or methods shown

EMI shielded windows and Shielding Doors: Designed for use in the architecture of shielded rooms, such as Faraday cages, anechoic chambers, MRI rooms, computer rooms, RF test boxes, and secure communication cabins. 

The shielded windows prevent electromagnetic waves from entering or exiting the shielded space while allowing clear visibility. This window design typically features a very fine woven wire mesh embedded between two glass sheets or clear optical plastic substrates, such as acrylic or polycarbonate (double-layer design). The EMI/RF shielding doors prevent electromagnetic radiation from entering or exiting a shielded environment, ensuring an effective shielding environment while allowing the user to access the shielded device or area when required. These doors are made from conductive materials such as copper, aluminum, and steel. The door material is the same as that used for shielding the room. 

EMI/RFI Gaskets: 

EMI/RFI gaskets are shielding components designed to physically close or seal gaps and openings in shielding enclosures, preventing electromagnetic waves from entering or escaping the shielded area. These gaskets are typically installed in areas prone to electromagnetic radiation leakage, such as seams, enclosure doors, joints, or openings in electronic enclosures and shielding rooms. 

EMI/RFI gaskets are made from various shielding materials such as copper, aluminum, stainless steel, or conductive elastomers like silicone or fluorosilicone embedded with conductive particles. Other materials commonly used for gasket fabrication include silver-coated aluminum, nickel-graphite, beryllium copper, and conductive foam. 

The gasket material is usually matched to the material of the EMI-shielded structure. EMI/RFI gaskets come in a wide range of sizes and shapes, including strips, sheets, rectangles, squares, D-shapes, O-rings, J-shapes, P-shapes, U-shapes, clip-on designs, as well as custom shapes and sizes. 

EMI/RFI Shielding Foil: 

EMI/RFI shielding foil is a thin, flexible sheet of conductive material, typically made of copper or aluminum. It is typically used to wrap or cover electronic components and cables to block RF signals. The shielding foil is commonly used in applications where a lightweight and flexible EMI shielding solution is required, such as in the construction of shielded rooms, around cables, or in board-level shielding.

EMI/RFI Shielding Mesh: 

EMI/RFI shielding mesh consists of a woven grid of conductive wires, typically constructed from materials like copper, aluminum, or stainless steel. This mesh is used in various applications, including EMI/RF-shielded windows, ventilation panels, EMI/RFI shielding in vehicles, and as a lining within walls and ceilings to create a Faraday cage. The mesh is effective at blocking EM radiation signals while allowing for air circulation or visibility.

Conductive Coatings and Paints:

Conductive paints or coatings are applied to surfaces to provide effective EMI shielding. These paints typically contain fine metallic particles, such as silver or copper, to ensure conductivity. They are commonly used on plastic enclosures or surfaces like building walls and ceilings, where traditional shielding methods may not be practical or feasible.

EMI shielding fabrics: 

EMI shielding fabrics, also known as RF shielding fabrics, are specialized textiles designed to block or attenuate EMI/RFI signals. They are fabricated from polyester, cotton, or nylon substrate interwoven with fine metal particles such as silver, copper, or stainless steel. These fabrics look similar to conventional fabrics and have identical physical properties, such as being lightweight, flexible, and easy to work with. EMI shielding fabrics are used in personal protective clothing, EMC tents, curtains, shielding pouches, electronic enclosures, and cable shielding.

EMI Shielding Tape: 

EMI shielding tape is made of conductive metal like copper or silver with an adhesive. It is commonly used to seal seams and gaps or wrap cables in electronic enclosures to provide a quick and flexible  EMI/RFI shielding solution. It can be easily cut, shaped, and configured to accommodate any device size without adding extra weight. The tape is ideal for applications where traditional enclosures or gaskets may not be suitable.

EMI Shielding Window Film: 

EMI shielding window film is designed to be applied to glass windows in buildings to provide EMI/RFI shielding while still allowing visible light to pass through. The film can be fabricated by laminating a conductive mesh, such as a copper mesh, onto a transparent plastic film like PET. Alternatively, it can be made by spraying a thin layer of metal, such as indium tin oxide (ITO), onto the surface of the PET film.

Shielded Cables: 

Shielded cables feature conductive shielding around the core conductors to minimize EMI. This shielding is typically achieved by wrapping the cable with thin metal foil or a woven mesh of metal wires. EMI shielding tape can also be used for additional protection. These cables are commonly used in data transmission, audio/video equipment, and industrial machinery.

Magnetic Shielding:

Magnetic shielding is a type of EMI shielding that utilizes magnetic materials, such as nickel or iron, to protect devices or equipment from external magnetic field interference. Magnetic shielding is commonly used in applications where magnetic fields are a concern, such as MRI machines and other medical equipment.

Overview of EMI shielding materials:

Metal-based EMI shielding materials: 

Metals such as copper, aluminum, nickel, steel, silver, brass, beryllium copper, and tin are the most commonly used materials for EMI shielding. High-conductivity metals like copper, silver, and aluminum attenuate or block electromagnetic radiation primarily by reflecting most of the electromagnetic energy. Metals are especially efficient in reflecting high-frequency electromagnetic waves, which makes them ideal for shielding applications involving radio frequency (RF) interference. However, metals have some drawbacks, such as heavy weight, susceptibility to corrosion, poor mechanical flexibility, and limited tunability of shielding properties. 

Metal-based shielding materials are widely used in various forms, including conductive enclosures or shielded rooms, shielded cables, EMI gaskets, EMI shielding doors and windows, EMI/RFI shielding fabrics, shielding tapes, shielding foils and meshes, and conductive paints.  

Carbon based EMI shielding materials:

Carbon allotropes such as exfoliated graphite, graphene, carbon fibers (CFs), and carbon nanotubes (CNTs) are the various forms of carbons. These materials are utilized as fillers in EMI shielding composites due to their intrinsic strength and conductivity. For example, Graphene, carbon nanotubes, and carbon fibers (CFs) materials are typically embedded in polymers, cement, ceramics, and metals to form rigid structures. The flexible graphite provides an EMI SE of around 130 dB and can be used in EMI shielding gaskets and microelectronics fabrication.  

Metal-coated carbon fibers have been reported to exhibit greater EMI shielding efficiency compared to uncoated carbon fibers. Also, as compared to uncoated CF, the Nickel coating of CF offers higher conductivity and magnetic behavior. The SE of nickel-coated CF is 87 dB (1–2 GHz) at a filler volume fraction of 7 vol %. For high-frequency shielding applications, graphene and carbon nanotubes are the most preferred EMI shielding materials. 

Other forms of carbon materials include carbon quantum dots and foam-structured carbon materials. Foam-structured carbon materials possess key properties such as flexibility, low density, highly ordered conductive network, high specific surface area, and good chemical stability. These characteristics make the foam-structured carbon materials a good candidate for EM wave shielding applications when compared to other carbon materials like graphene, carbon nanotubes, carbon quantum dots, and carbon fiber.

Ceramic materials:

Ceramic materials possess key properties such as high strength, excellent wear resistance, high fracture toughness, antistatic behavior, and exceptional thermal and chemical stability. However, their electrical conductivity is lower than that of metals or carbon. To improve conductivity, ceramic composite materials are developed by incorporating conductive filler materials into a ceramic matrix phase.

These ceramic composites are electrically conductive and can provide effective EMI shielding even in harsh environments. Examples of such materials used for EMI shielding include MXene ceramics, sulfide ceramics, and metal-organic frameworks. MXene-based materials SE generally provide ~ 24 dB to 70 dB within the frequency range of 8.2 to 12.4 GHz.

Additionally, some ceramic magnetic materials, such as nickel ferrite (NiFe₂O₄) and ferrite (Fe₃O₄, also known as magnetite), provide EMI shielding through absorption mechanisms.

Conductive silicones: 

Silicon is a nonconductive material that resists sunlight and water. But, conductive silicon can be made by adding conductive filler materials such as silver, aluminum, nickel, copper, silver-copper, silver-aluminum, silver-glass, nickel-aluminium, and nickel-graphite. Conductive silicones are employed for fabricating EMI shielding gaskets. The nickel-graphite silicone is cost-effective and meets the MIL-DTL-83528 standard shielding effectiveness requirements. This standard mandates a minimum shielding effectiveness of 100 dB across RF frequencies ranging from 20 to 10,000 Hz. The silicone EMI gaskets are mostly used in connector ports inside electronics or on circuit boards.

Other base materials used for manufacturing EMI gaskets include Fluorosilicone, Ethylene Propylene Diene Monomer (EPDM), Foam and Fabric Over Foam, and Beryllium Copper.

• Fluorosilicone addresses many problems that silicone gaskets may encounter in harsh conditions. It offers greater resistance to extreme environments and is commonly used in internal circuit board applications. Similar to silicone, fluorosilicone can be made conductive by adding fillers such as silver, aluminum, nickel, and copper. 
• EPDM is a synthetic rubber that has high resistant to caustic chemicals like acetones, hydrocarbons, and detergents, as well as environmental factors. However, it has a lower resistance to high temperatures as compared to silicone and fluorosilicone, as well as has a poor resistance to oil exposure.
• Beryllium copper is a high-performance metal that does not require filler materials like silver or nickel graphite. It has a low compression set, making it suitable for fabricating EMI gasket designs that involve repetitive opening and closing. Beryllium copper can offer EMI shielding across a wide range of signal frequencies.

Polymers and Their Composites/Hybrids

Polymers and Their Composites/Hybrids Conventional polymers are non-conductive and transparent to electromagnetic radiation. However, they can be made conductive through a process known as doping, which involves adding electron acceptors or donors. This results in conductive polymers such as polypyrrole (PPy), polyacetylene (PA), polyindole (PIn), polyaniline (PANI), and their copolymers. These polymers are also known as Intrinsically Conductive Polymers (ICP). They possess electrical conductivity properties, making them suitable for EMI shielding applications, such as electronic enclosures and shielding rooms. Among conductive polymers, polyaniline (PANI) and polypyrrole (PPY) are the most commonly used for EMI shielding due to their exceptional electrical conductivity.

Polymer composites reinforced with conductive materials like graphene, carbon nanotubes, carbon black, and graphite are widely utilized for EMI shielding and in other applications, including sensors, lithium-ion batteries, and solar cells.

Plastic materials in EMI shielding:

Plastics are non-conductive materials with resistivity in the range of 10¹⁵–10¹⁸ Ω·cm. However, they can be made conductive and suitable for EMI shielding purposes through the following methods:

• Conductive Coating: A conductive coating can be applied to plastics to enhance electrical conductivity and provide EMI shielding. This method is often used to create electronic enclosures that offer effective EMI protection.
• Compounding with Conductive Fillers: Plastics can be made conductive by incorporating materials such as carbon black, carbon fibers, stainless steel fibers, aluminum fibers, and nickel-coated graphite fibers.

Cement-Based EMI Shielding Materials: 

Cement-based EMI Shielding materials are formed by adding conductive filler such as graphite or carbon fiber, a conductive polymer, or a metal material to the cement. These materials are suitable for structural/Building EMI shielding applications. They have good strength, mechanical and thermal durability, high load-bearing capacity, and resistance to chemical attack.  

EMI Absorbers:

EMI absorbers are shielding components used to protect electronic equipment from electromagnetic interference (EMI) by absorbing electromagnetic waves. They are typically made up of flexible polymer resins loaded with soft metal flakes and ferrite sheets. Often used in anechoic chambers and RF absorbers. 

Magnetic shielding materials:

Metals with high magnetic permeability, such as nickel and iron, and specialized alloys like permalloy and mu-metal, are commonly used for magnetic field shielding. These materials attenuate external magnetic fields by absorbing and redirecting magnetic field lines, effectively preventing them from penetrating the shielded area. 

These magnetic materials are suitable for low-frequency magnetic shielding applications (e.g., transformers, and motors). Magnetic shielding is crucial in environments where magnetic interference needs to be minimized and is commonly used in devices like MRI machines, electronic enclosures, and various industrial systems.

Other examples of EMI shielding materials: 

Other examples of EMI wave shielding materials include Phosphor Bronze, Pre-Tin Plated Steel, Nickel-Silver, monolayer graphene ...  etc. Continued R&D and technology, such as nanotechnology, help to introduce new EMI shielding materials.

Table 2 provides information about a few EMI/EM wave shielding materials and their frequency range and shielding effectiveness (dB).

Every EMI/RFI shielding material possesses unique properties and benefits, making it suitable for particular applications. The choice of material for EMI/RFI shielding depends on several factors, such as the frequency range of the electromagnetic radiation, the level of shielding or shielding effectiveness (dB) required, environmental conditions, cost considerations, and application needs.