Electromagnetic interference (EMI) shielding refers to the electromagnetic radiation barrier made of conductive or magnetic material. The EMI shielding serves as a radiation barrier, preventing radiation through the shield, thereby protecting equipment and people from harmful microwave and radiowave radiation. The materials used for EMI shields include metals, carbons, ceramics, conducting polymers, cement, hybrids, and composites (material combinations).
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Figure: Board level EMI shielding
The EMI/EM wave shielding can be applied to an electronic device as an enclosure to isolate the device from the surrounding environment, as well as to cables to isolate wires from the external environment. The EMI shielding not only protects equipment from external electromagnetic radiation (EMI) but also prevents the leak out of internally generated radiated EMI from reaching the surrounding environment, thereby ensuring electromagnetic compatibility (EMC) of the device. The shielding can ensure a device can operate properly without disturbance from the external environment as well as without disturbing any other nearby devices.
EMI shields are available and utilized in various forms, including solid enclosures, wire mesh, screens, O-rings, EMI gaskets, cable shields, shielding taps, and coatings.
Note: EMI shielding and magnetic shielding are distinct concepts. Magnetic shielding specifically pertains to the protection against magnetic fields, typically occurring at low frequencies, such as 60 Hz (the U.S. utility frequency).
Understanding EMI shielding mechanism:
The EMI shield or material attenuates or blocks electromagnetic radiation by means of three shielding mechanisms: reflection, absorption, and multiple internal reflections. The reflection loss is the primary shielding mechanism, which occurs due to impedance differences between the air and material boundary. The absorption is the secondary mechanism, which occurs due to losses in the material (like ohmic and polarization loss). Multiple internal reflections occur due to reflections at various surfaces or interfaces in the shield, leading to EM wave attenuation/loss.
Figure: Understanding EMI shielding mechanisms: Reflection, Absorption, and Internal reflections
The total shielding effectiveness (SET) of a material or shield is the sum of all losses due to these three shielding mechanisms. Shielding effectiveness (SE) means how well a shield or material blocks electromagnetic interference (EMI). The shielding effectiveness is measured in decibel unit (dB).
The reflection loss depends on the ratio σr / µr, whereas the absorption loss is the function of product σrµr. Here, σr &#; is the electrical conductivity of a material relative to copper, and µr &#; is the relative magnetic permeability. Hence, high conductivity materials such as silver, copper, gold, and aluminum are excellent for electromagnetic wave reflection, thereby blocking the EM wave radiation. High magnetic permeability materials such as mumetal and superpermalloy are excellent for EM wave absorption, thereby attenuating EMI. The absorption loss or shielding effectiveness due to absorption (SEA) increases proportionally with the thickness (t) of the EMI shield. Here, f is the frequency of EM wave radiation.
Table 1 provides information about the electrical conductivity (σr) related to copper and relative magnetic permeability (µr) for selected materials.
According to the skin effect, electromagnetic radiation at high frequencies penetrates only the near-surface region of an electrical conductor. The electric field of a plane wave penetrating a conductor drops exponentially with increasing the depth into the conductor. The depth at which the electric field value drops to 1/e of its incident value is called skin depth, which is given below.
Commonly used materials for Electromagnetic interference shielding:
As previously discussed, materials for Electromagnetic interference shielding include metals, carbons, ceramics, conducting polymers, cement, hybrids, and composites (material combinations).
EMI shielding materials are classified as either structural or functional materials. The functional EMI shielding materials can be integrated into a device like a mobile to provide EMI shielding. Metals and carbons are important functional materials. The EMI shielding structural materials (e.g., Carbon-fiber composites) can do dual purposes, i.e., both electromagnetic shielding and structural support. Hence, they are called multifunctional structural shielding materials, used in applications (like aerospace or civil engineering) that require strength as well as EMI protection.
The following sections discuss some commonly used EMI shielding materials and their applications.
Metal-based EMI shielding materials:
Metals are most commonly utilized as EMI shielding materials because of their properties, such as electrical conductivity, magnetic permeability, strength, and ductility. Some commonly used metals for EMI shielding include brass, copper, aluminum, silver, nickel, steel, and tin. Metals can provide excellent EM wave shielding performance. However, some drawbacks of metals include heavy weight, poor mechanical flexibility, susceptibility to corrosion, and limited tunability of shielding property.
Consideration of metal&#;s properties and cost is important while selecting a metal. For example, high-conductivity metals such as silver, copper, brass, and aluminum are suitable for EM wave reflection, while high permeability materials such as Mu-metal, Permalloy, and Supermalloy are suitable for EM wave absorption. The high conductivity materials (e.g., copper) attenuate EM waves by reflecting the electric field component of EM waves. The higher permeability materials (e.g., Mu-metal) attenuate EM waves by absorbing the magnetic field component of EM.
Silver has higher conductivity with good corrosion resistance, making it suitable for providing efficient shielding than copper and aluminum. But silver is more costly than other metals. To balance both cost and shielding efficiency, copper and aluminum materials are mostly used for EMI shielding. Aluminum has 60 % conductivity as compared to copper. Also, note that aluminum has high oxidation properties and corrodes more quickly than other metals.
Figure: Metal-based EMI shielding materials examples
The metal-based EMI shielding can be formed in various designs, including a thin layer of metal, sheet metal, metal screen, and metal foam. Also, Metallic ink and Foil shielding tape are available for EMI shielding purposes.
The metallic ink is made up of a carrier material loaded with appropriate metal, usually copper or nickel, in the form of very fine particles. This ink is ideal for use in electronic goods housed in plastic enclosures, where the ink is sprayed onto the enclosure. Once dry, it produces a continuous conductive metal layer, which can be connected to the equipment&#;s chassis ground, thus providing EM wave shielding.
The foil shielding tape consists of conductive metal like copper or silver with an adhesive. It is used to cover a device for providing EMI shielding. Similar to any other tape, EMI shielding tape can be effortlessly cut, shaped, and configured to accommodate devices of any size without adding extra weight, making it an ideal EMI/EM wave shielding solution. It is cost-effective and provides EMI shielding without producing any waste. These tapes are ideal for providing EMI shielding for electronics and cable wrap applications.
Figure: Copper foil EMI shielding tape
Carbon:
Carbon allotropes are forms of carbon. Some examples of carbon allotropes include exfoliated graphite, graphene, carbon fibers (CFs), and carbon nanotubes (CNTs). These materials are used as filler materials for EMI-shield composites due to their intrinsic strength and conductivity. Other examples of carbon materials are carbon quantum dots and foam-structured carbon materials.
Exfoliated graphite (flexible graphite) has high thermal conductivity and chemical inertness along with a low thermal expansion coefficient. Also, the flexible graphite has an EMI SE of around 130 dB. These properties make the Exfoliated graphite suitable for use in EMI shielding gaskets and in microelectronics fabrication.
Figure: Carbon Nanotube
Graphene, carbon nanotubes, and carbon fibers (CFs) are utilized as filler materials due to their high aspect ratio. These materials are typically embedded in polymers, cement, ceramics, and metals to form rigid structures. Metal-coated carbon fibers have been reported to exhibit greater efficiency in EMI shielding compared to uncoated carbon fibers. Also, compared to uncoated CF, Nickel coating of CF provides higher conductivity and magnetic behavior. The nickel-coated CF can exhibit SE of 87 dB (1&#;2 GHz) at a filler volume fraction of 7 vol %. For high-frequency shielding applications, graphene and carbon nanotubes are mostly preferred.
The foam-structured carbon materials belong to a type of carbon material with a 3D architecture. The foam-structured carbon materials have some attractive benefits, such as good flexibility, low density, highly ordered conductive network, high specific surface area, and good chemical stability. These benefits have made the foam-structured carbon materials a good candidate for EM wave shielding applications as compared to other carbon materials like carbon quantum dots, graphene, carbon nanotubes, and carbon fiber.
Polymers and Their Composites/Hybrids
Conventional polymeric materials are nonconductive materials and mainly transparent to electromagnetic wave radiation. But, Intrinsically Conducting Polymers (ICPs) are the special polymers that can conduct electricity, making them ideal for EM wave shielding materials. The ICPs can be produced by modification of existing nonconductive polymers. The electrical conductivity property of ICPs can be modified through doping or de-doping.
Examples of ICPs include polyacetylene (PA), polyindole (PIn), polypyrrole (PPy), polyaniline (PANI), and their copolymers. As compared to other conducting polymers, polyaniline (PAn) and polypyrrole (PPY) are mainly used in EMI shielding applications due to their excellent electrical conductivity. Polymer materials feature excellent plasticity and ease of process. However, conducting polymers still face challenges regarding mechanical and chemical stability, and also they are costly. The ICPs are mostly used as supplementary components (binding materials) in the structural design of nanocomposites/composites for EMI shielding to enhance shielding effectiveness (SE).
Polymer composites reinforced with carbon nanotubes, carbon black, graphene, and graphite are widely used for EMI shielding and other applications such as lithium-ion batteries, sensors, and solar cells.
Silicone:
Silicone is a nonconductive material, but it can be used for EMI shielding by adding conductive filler materials (metals) to make it conductive. Commonly used conductive filler materials include silver, silver-aluminum, silver-copper, silver-glass, nickel-aluminium and nickel-graphite. Most EMI shield silicone contains nickel-graphite content. The nickel-graphite silicones meet the shielding effectiveness requirements of the MIL-DTL- standard, which mandates a minimum shielding effectiveness of 100 dB across RF frequencies ranging from 20 to 10,000 Hz.
Silicone is a flexible material so that it can be cut and shaped to fit any type of EMI shielding. Also, this material is widely used because it is resistant to water and sunlight and remains stable across a wide temperature range. These properties have made this material an ideal solution to use in hot and cold environments such as aerospace. Also, silicone is widely used in EMI gaskets as a base material and is made conductive by adding filler materials such as silver, aluminum, copper, and nickel. The silicone EMI gaskets are often suitable for use in connector ports inside electronics or on circuit boards.
Other EMI gasket base materials include Fluorosilicone, Ethylene Propylene Diene Monomer (EPDM), Foam and Fabric Over Foam, and Beryllium Copper.
Ceramic materials:
Ceramic materials have some key properties that include high strength, high fracture toughness, excellent wear resistance, antistatic and high thermal as well as chemical stability. However, its electrical conductivity is lower as compared to metals or carbon.
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The Ceramic composite for EM wave shielding is formed by adding conductive filler material with a ceramic matrix phase (i.e., with a ceramic material). Ceramic composites are electrically conductive and can provide good shielding characteristics even in harsh environments. Some special ceramic materials, such as MXene ceramics, sulfide ceramics, and metal-organic frameworks, offer excellent EMI/EM wave shielding behavior. MXene-based materials typically provide SE ~ 24 dB to ~ 70 dB within frequencies of 8.2&#;12.4 GHz.
Also, some ceramic magnetic materials like ferrite (Fe3O4, also called magnetite) and nickel ferrite (NiFe2O4) provide EM wave shielding by means absorption mechanism.
Cement-Based EMI Shielding Materials:
Cement-based EMI Shielding Materials are the materials suitable for structural/Building EMI shielding applications. Cement is marginally conducting and has poor EMI protective characteristics. So the conductive filler material is added to cement paste to form cement-based composites. The cement-based composites/materials are electrically conductive and provide good shielding effectiveness. The conductive filler typically added to the cement can be either a carbon material (such as graphite or carbon fiber), a conductive polymer, or a metal material. The cement-based EMI materials/composites provide good strength, mechanical and thermal durability, high load-bearing capacity, and resistance to chemical attack, making them ideal for structural/Building EMI shielding applications.
Fabric:
Figure: Copper Based EMI fabric
EMI fabric is designed for EMI protection. It looks like traditional fabric and has physical properties like conventional fabric, for example, flexibility. The EMI fabric consists of nylon or polyester substrate interwoven with metal fibers. The flexibility of EMI fabric makes it possible to use in a wide variety of conditions and applications (Curtains, Tents, etc.). EMI fabric can only provide a moderate level of EMI protection, so it is ideal for conditions where a moderate level of EMI protection is required. A limitation of EMI fabric is that under certain conditions, the metal content within the fabric may tend to get surface corrosion. The EMI fabrics are very easy to cut and sew and are ideal for providing EMI Shielding for Microwave Signals, Phones, Smart Meters, cell towers, Security Systems, etc.
Other examples of EM wave shielding materials:
Other examples of EM wave shielding materials include Nickel-Silver, Phosphor Bronze, Pre-Tin Plated Steel, monolayer graphene ... etc. As human civilization progresses in nanotechnology, new types of electromagnetic interference (EMI) shields are continually being discovered and combined toward the improvement of EM wave shielding and the introduction of new materials.
Table 2 provides information about some EMI/EM wave shielding materials and their frequency range and shielding effectiveness (dB)
Material
Frequency range
Shielding Effectiveness (dB)
Thickness (mm)
Flexible graphite
1&#;2 GHz
130
Continuous carbon fiber
0.3 MHz
124
Laminated epoxy carbon fiber
~ 9 GHz
&#; 62
Acrylonitrile-butadiene-styrene/stainless steel fibers
8-12 GHz
11
Nickel coated carbon fiber/PC/ABS composites
1 GHz
47
Polyurethane/Polyaniline
S (1.9 &#; 3.9 GHz) and X (7 &#; 13 GHz) band
26.7 and 15.5
0.62, 1.26 and 1.9
Silver nanoparticles coated hollow carbon sphere/epoxy foam
8-12 GHz
60.2
1.5
Polyester fabric/polypyrrole
50 MHz-1.5 GHz
36
Each EMI shielding material has its own benefits and is suitable for specific applications. The choice of material for electromagnetic interference (EMI) shielding depends on 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 requirements.
Note: In most cases, the materials utilized to protect devices from EMI signals also effectively shield against radio frequency interference (RFI) signals.
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