Discussion on electromagnetic shielding technology[Copy link]
In recent years, with the development of electromagnetic compatibility, electromagnetic shielding technology has been used more and more widely. In order to have a deeper understanding of electromagnetic shielding technology, we should conduct a more in-depth discussion on the performance and application of shielding materials, precautions for shielding technology, detection of shielding effectiveness, and shielding measures for special parts.
1 Technical principle of electromagnetic shielding Electromagnetic shielding is one of the main measures of electromagnetic compatibility technology. It is a measure to enclose the electromagnetic interference source with metal shielding materials so that its external electromagnetic field strength is lower than the allowable value; or to enclose the electromagnetic sensitive circuit with metal shielding materials so that its internal electromagnetic field strength is lower than the allowable value. 1.1 Electrostatic shielding Use a complete metal shielding body to surround the positively charged conductor. The inner side of the shielding body will induce negative charges equal to the charged conductor, and the outer side will have positive charges equal to the charged conductor. If the metal shielding body is grounded, the positive charges on the outer side will flow into the earth, and there will be no electric field on the outer side, that is, the electric field of the positively charged conductor is shielded in the metal shielding body. 1.2 Alternating electric field shielding In order to reduce the coupling interference voltage of the alternating electric field on the sensitive circuit, a metal shielding body with good conductivity can be set between the interference source and the sensitive circuit, and the metal shielding body can be grounded. The coupling interference voltage of the alternating electric field on the sensitive circuit depends on the product of the alternating electric field voltage, the coupling capacitance and the grounding resistance of the metal shielding body. As long as the metal shield is well grounded, the coupling interference voltage of the alternating electric field on the sensitive circuit can be made very small. Electric field shielding is mainly based on reflection, so the thickness of the shield does not need to be too large, and the structural strength is the main consideration. 1.3 Alternating magnetic field shielding Alternating magnetic field shielding is divided into high frequency and low frequency. Low-frequency magnetic field shielding uses high-permeability materials to form a low magnetic resistance path so that most of the magnetic field is concentrated in the shield. The higher the magnetic permeability of the shield, the greater the thickness, the smaller the magnetic resistance, and the better the magnetic field shielding effect. Of course, it must be coordinated with the weight of the equipment. High-frequency magnetic field shielding is achieved by using the reverse magnetic field of eddy currents generated by high-conductivity materials to offset the interfering magnetic field. 1.4 Alternating electromagnetic field shielding generally uses high-conductivity materials as shielding bodies, and the shielding body is grounded. It uses the shielding body to generate an eddy current magnetic field in the opposite direction under the action of the high-frequency magnetic field to offset the original magnetic field and weaken the interference of the high-frequency magnetic field, and the electric field shielding is achieved because the shielding body is grounded. The thickness of the shielding body does not need to be too large, and the skin depth and structural strength are the main considerations.
2 Shielding effectiveness calculation Shielding effectiveness (SE) is defined as the ratio of the electromagnetic field strength without shielding to the electromagnetic field strength after adding the shield at the same location in the electromagnetic field. It is usually expressed in decibels (dB). SE = A + R + B (1) Where: A is the absorption loss; R is the reflection loss; B is the multiple reflection loss. 2.1 Electromagnetic wave reflection loss Due to the different electromagnetic wave impedances of air and shielding metal, the incident electromagnetic wave is reflected. The electromagnetic wave impedance of air is different in different field sources and field areas, and is calculated as follows. The reflection loss R (dB) in the near field of the magnetic field source is R = 20log10 {[1.173 (μr/fσr)1/2/D] + 0.0535D (fσr/μr)1/2 + 0.354} (2) Where: μr is the relative magnetic permeability; σr is the relative electrical conductivity; f is the electromagnetic wave frequency (Hz); D is the distance from the radiation source to the shield (cm). The reflection loss R (dB) in the near field of the electric field source is R = 362-20log10 [(μrf3/σr)1/2D] (3) The reflection loss R (dB) in the far field of the electromagnetic field source is R = 168-10log10 (μrf/σr) (4) 2.2 Electromagnetic wave absorption loss When the electromagnetic wave entering the metal shield propagates in the shielding metal, absorption occurs due to attenuation. The absorption loss A (dB) is A = 0.1314d (μrfσr)1/2 (5) Where: d is the thickness of the shielding material (mm). 2.3 Multiple reflection loss The multiple reflection loss B (dB) of electromagnetic waves between shielding layers is B = 20log10 {1-〔(Zm-Zw)/(Zm+Zw)〕210-0.1A(cos0.23A-jsin0.23A)} (6) Where: Zm is the electromagnetic wave impedance of the shielding metal; Zw is the electromagnetic wave impedance of the air. When A>10dB, multiple reflection loss can generally be ignored. 2.4 Shielding effectiveness calculation example The shielding effectiveness (dB) calculation results of the shielding body (thickness 0.254mm) made of different materials with a distance of 30cm from the source are shown in Table 1. The boundary between the near field and the far field in Table 1 is λ/2π, and λ is the wavelength of the electromagnetic field. Table 1 Shielding effectiveness of shields made of different materials (thickness 0.254mm) at a distance of 30cm from the field source dB Frequency/Hz Copper Iron Aluminum Magnetic field Near field Electric field Near field Far field Magnetic field Near field Electric field Near field Far field Magnetic field Near field Electric field Near field Far field 60 3.46 3.22 1k 24.89 14.66 10k 44.92 212.73 128.73 51.50 217.50 134.00 150k 69.40 190.20 132.40 188.0 308.0 248.00 1M 97.60 185.40 141.60 391.0 479.0 435.00 88.00 176.0 — 15M 205.0 245.0 225.0 1102.0 1143.0 1123.0 174.0 215.0 — 100M 418.0 426.0 422.0 1425.0 1434.0 1430.0 342.0 350.0 —
3 Shielding considerations 3.1 Shielding integrity If the shield is incomplete, it will lead to electromagnetic field leakage. In particular, electromagnetic field shielding uses the shield to generate eddy current magnetic fields in the opposite direction under the action of high-frequency magnetic fields to offset the original magnetic field and weaken the interference of high-frequency magnetic fields. If the shield is incomplete, the effect of eddy current is reduced, that is, the shielding effect is greatly reduced. 3.2 Shielding effectiveness and application occasions of shielding materials The progress of electromagnetic shielding technology has promoted the continuous development of shielding materials, and is no longer limited to the single-layer metal plate mode, and the shielding effectiveness has also been continuously improved. When applying, special attention should be paid to the different shielding effectiveness and application occasions of different shielding materials. 3.2.1 Metal plate Electronic equipment uses metal plates as chassis, which are both durable and have electromagnetic shielding effects. Its electromagnetic shielding effectiveness is related to parameters such as the properties of the metal plate material, the properties of the electromagnetic field source, the distance between the electromagnetic field source and the metal plate, and the grounding condition of the shield. The performance of various metal shielding materials is shown in Table 2. Table 2 Performance of various metal shielding materials Metal shielding material relative to copper conductivity (σCu=5.8×107Ω/m) Relative magnetic permeability at f=150kHz Absorption loss at f=150kHz/(dB/m) Silver 1.05 1 52 Copper 1.00 1 51 Gold 0.70 1 42 Aluminum 0.61 1 40 Zinc 0.29 1 28 Brass 0.26 1 26 Cadmium 0.23 1 24 Nickel 0.20 1 23 Phosphor bronze 0.18 1 22 Iron 0.17 1000 650 Steel #45 0.10 1000 500 Permalloy 0.03 80000 2500 Stainless steel 0.02 1000 220 3.2.2 Shielding film Nowadays, many electronic devices use engineering plastics as chassis. Due to the good processing performance of engineering plastics, the chassis is not only beautiful in shape, but also low in cost and light in weight. However, engineering plastics have no electromagnetic protection performance. Shielding film is a process technology such as spraying, vacuum deposition, electroplating and pasting, which covers a layer of conductive film on the surface of engineering plastics and organic media, thereby playing the role of flat shielding. Generally, the thickness of the conductive film is less than 1/4 of the wavelength of electromagnetic waves propagating inside it. The shielding effectiveness achieved by several spraying processes is shown in Table 3. Table 3 Shielding effectiveness achieved by several spraying processes Spraying process thickness/μm resistance/(Ω/mm2) Shielding effectiveness/dB Zinc thermal spraying 25 4.0 50～60 Nickel-based coating 50 0.5～0.2 30～75 Silver-based coating 25 0.05～0.1 60～70 Copper-based coating 25 0.5 60～70 Graphite-based coating 25 7.5～20 20～40 Electroplating 0.75 0.1 85 Chemical plating 1.25 0.03 60～70 Vacuum deposition 1.25 5～10 50～70 Ionization plating 1.0 0.01 50 The shielding effectiveness of copper films of different thicknesses is shown in Table 4. 96 21.96 1M 2.90 109 －3.5 108 21.96 1G 92 79 0 171 Shielding effectiveness of copper film Thickness/μm Frequency/Hz ARB Shielding effectiveness/dB 0.105 1M 0.14 109 －47 62 0.105 1G 0.44 79 －17 62 1.25 1M 0.16 109 －26 83 1.25 1G 5.20 79 －0.6 84 2.196 1M 0.29 109 0.6 110 2.196 1G 9.20 79 0.6 90 21.96 1M 2.90 109 －3.5 108 21.96 1G 92 79 0 171 Transparent conductive material is a conductive film covering the surface of organic medium or glass, which makes it transparent and has a certain shielding effectiveness. The shielding effectiveness of conductive glass with different transmittance is shown in Table 5. Table 5 Shielding effectiveness of conductive glass with different transmittance Transmittance 1MHz 10MHz 100MHz 1000MHz 60% 94 72 46 21 65% 90 68 42 16 71% 84 62 36 11 75% 78 56 30 6 80% 74 52 28 4 3.2.3 Metal mesh When there is a need for ventilation, light transmission, water addition, measurement, etc., holes should be opened on the equipment shell. In order to improve the electromagnetic shielding effect of the equipment, metal mesh holes should be used for shielding. Or it can be used in the joints of the electronic equipment shell to provide effective electromagnetic shielding. The shielding effectiveness SE (dB) of the holes is related to the frequency of the electromagnetic wave, the size and number of the holes, and other parameters. To improve the shielding effectiveness of the holes, the following measures can be taken: 1) Cover the large-diameter holes with metal mesh, and make sure that the mesh is in good contact with the shielding body; 2) Change the large holes to small holes; 3) Use waveguide attenuator-type vents; 4) Cover the light-transmitting and measuring holes with shielding glass with metal mesh; 5) Pad the holes that need to be water- and air-tight with metal mesh containing rubber and other materials. The following introduces several commonly used metal mesh shielding materials. 3.2.3.1 All-metal mesh gasket All-metal mesh gasket is an elastic, conductive woven metal gasket wire mesh strip used in the joints of electronic equipment housings to provide effective electromagnetic shielding. When used, cast or machined housings use all-metal mesh gaskets with rectangular cross-sections, and sheet metal housings use all-metal mesh gaskets with circular cross-sections, with a compression of about 25% of the original height. The shielding effectiveness of all-metal mesh gaskets is shown in Table 6. Table 6 Shielding effectiveness of all-metal wire mesh gaskets dB Material Magnetic field Electric field Plane wave (100kHz) (10MHz) 1GHz 10GHz Silver-plated brass 80 135 105 95 Tin-plated copper-clad steel 80 130 105 95 Tin-plated phosphor bronze 80 130 110 100 Aluminum 60 130 90 80 Nickel-copper alloy 60 130 90 80 3.2.3.2 Environmentally sealed wire mesh gaskets Environmentally sealed wire mesh gaskets are made of woven wire mesh and rubber. In addition to providing effective electromagnetic shielding, environmentally sealed wire mesh gaskets can also provide effective environmental sealing. They can be used in fixed joints or movable joints of electronic equipment housings, such as door gaps. The general compression amount is about 25% of the original height. The shielding effectiveness of rubber core wire mesh gaskets is shown in Table 7. Table 7 Shielding effectiveness of wire mesh gasket with rubber core dB Material Magnetic field Electric field Plane wave (100kHz) (10MHz) 1GHz 10GHz Tin-coated copper steel 80 130 105 95 Tin-coated phosphor bronze 80 130 110 95 Nickel-copper alloy 60 125 90 80 3.2.3.3 Metal mesh shielding glass Metal mesh shielding glass is a metal mesh pressed between two layers of glass, which can not only provide effective electromagnetic shielding, but also provide effective light transmission. It can be used for observation windows of electronic equipment, such as meter heads, digital or image displays, etc. The shielding effectiveness of metal mesh shielding glass is shown in Table 8. Table 8 Shielding effectiveness of metal mesh shielding glass dB Material Magnetic field Electric field Plane wave (100kHz) (10MHz) 1GHz 10GHz Blackened copper wire mesh, 60% opening 55 120 60 40 Blackened copper wire mesh, 45% opening 55 120 80 50 3.2.3.4 Aluminum honeycomb ventilation panel Aluminum honeycomb ventilation panel is composed of aluminum honeycombs in an aluminum frame. The waveguide type honeycomb not only has electromagnetic shielding effectiveness, but also has high air flow. It can be used for ventilation windows of electronic equipment. The shielding effectiveness of aluminum honeycomb ventilation panel is shown in Table 9. Table 9 Shielding effectiveness of aluminum honeycomb ventilation panel dB Material Magnetic field Electric field Plane wave (100kHz) (10MHz) 1GHz 10GHz Single layer chromate plating 40 80 60 40 Single layer cadmium plating 75 125 105 85 Single layer tin plating 70 125 105 85 Single layer nickel plating 80 135 115 95 Multilayer chromate plating 65 110 95 85 3.2.4 Conductive fiber Conductive fiber is divided into the following 5 categories. 1) Conductive fabric is made by plating copper or nickel on chemical fiber fabric, which has flexible shielding performance against high frequency and microwave. 2) Conductive fabric and resin are compounded to make absorptive conductive fabric. Since the resin that can absorb electromagnetic waves is selected, the shielding performance is better. (3) Conductive fabric is made by mixing metal or carbon black fiber with chemical fiber with good conductivity. The above three kinds of conductive fabrics can be used as anti-static and anti-electromagnetic radiation work clothes, shielding curtains, tents, and protective covers, and their shielding effectiveness is generally 50-60dB. 4) Conductive paper made by mixing conductive fibers and wood pulp can be used as shielding packaging for sensitive integrated circuits, and its shielding effectiveness is generally 30-40dB. 5) Directed metal wire-filled silicone rubber made by synthesizing many independent metal wires into silicone rubber can provide effective electromagnetic shielding and environmental sealing, and is often used in non-fixed gaps, such as flange connections. Its shielding effectiveness is shown in Table 10. Table 10 Shielding effectiveness of oriented metal wire filled silicon solid rubber dB Material Magnetic field Electric field Plane wave (100kHz) (10MHz) 1GHz 10GHz Tin-plated phosphor bronze 75 130 110 100 Nickel-copper alloy 80 130 115 100 3.2.5 Conductive particles Conductive particle shielding materials are silver-plated glass particles, pure silver particles, carbon black particles, copper-plated silver particles, nickel-plated silver particles, aluminum-plated silver particles, and graphite-plated nickel particles mixed in silicon or fluorosilicone rubber, which can be extruded into various shapes for electromagnetic and water vapor sealing. Their shielding effectiveness is shown in Table 11. Table 11 Shielding effectiveness of conductive particle shielding materials dB Material Magnetic field Electric field Plane wave (100kHz) (10MHz) 1GHz 10GHz Glass silver-plated conductive rubber 811 65 130 100 90 Pure silver conductive rubber 856, 857 70 130 100 90 Carbon black conductive rubber 860 93 77 68 88 Copper silver-plated conductive rubber 871 75 120 115 110 Nickel silver-plated conductive rubber 891 75 120 110 100 Aluminum silver-plated conductive rubber 895 75 120 110 100 Graphite nickel-plated conductive rubber 750 100 100 100 85 3.2.6 Conductive adhesive Conductive adhesive is made by adding pure metal particles, such as silver, nickel, copper-plated silver, aluminum-plated silver, etc., to silicon and epoxy resin adhesives. It is used between various shielding materials to play the role of bonding, shielding and sealing. 3.2.7 Conductive coating Conductive coating is made by adding pure silver particles to polypropylene and polyurethane. It can be used for plastic housing shielding and equipment that requires flexible shielding. 3.2.8 Conductive foil Conductive foil is made of copper or aluminum tape with conductive polypropylene adhesive on one side. It can be used for shielding and sealing of electronic equipment joints, winding cable shielding, etc. Its shielding effectiveness is generally 55-60dB. 3.2.9 Beryllium copper reed Beryllium copper reed is an elastic shielding material that can be used for shielding of electronic equipment movable joints, such as doors and windows. Its shielding effectiveness is shown in Table 12. Table 12 Shielding effectiveness of beryllium copper reed dB Material Magnetic field Electric field Plane wave 100kHz 10MHz 1GHz Standard reed 110 100 90 Soft reed 95 85 75 3.2.10 Shielding composite board Shielding composite board is composed of metal foil, insulating substrate and pressure-sensitive adhesive, which can be used for shielding of printed circuits and electronic equipment. Its shielding effectiveness is generally 40-45dB. 3.2.11 Pure cotton polyester electromagnetic material Pure cotton polyester electromagnetic material is a material that is transparent and has electromagnetic shielding function by evenly distributing copper atoms in cotton polyester material. It can be used for video screen radiation protection, mobile phone microwave radiation protection, etc. Its shielding effectiveness is >50dB. 3.3 Good grounding of the shielding body The metal shielding body is well grounded. For electrostatic shielding, the induced charge outside the shielding body will flow into the ground, and there will be no induced electric field. For alternating electric field shielding, since the magnitude of the coupling interference voltage of the alternating electric field on the sensitive circuit depends on the product of the alternating electric field voltage, coupling capacitance and the grounding resistance of the metal shielding body, as long as the metal shielding body is well grounded, the coupling interference voltage of the alternating electric field on the sensitive circuit can be made very small. Therefore, if the metal shielding body is not well grounded, the shielding effect will be reduced.4 Conductive fibers Conductive fibers are divided into the following 5 categories. 1) Conductive cloth is made by plating copper or nickel on chemical fiber fabrics, which can have flexible shielding properties for high frequencies and microwaves. 2) Conductive cloth and resin are compounded to make absorbent conductive cloth. Since the resin that can absorb electromagnetic waves is selected, the shielding performance is better. (3) Conductive cloth is made by mixing metal or carbon black fibers with chemical fibers with good conductivity. The above 3 types of conductive fabrics can be used as anti-static and anti-electromagnetic radiation work clothes, shielding curtains, tents, and protective covers. The shielding effectiveness is generally 50-60dB. 4) Conductive paper is made by mixing conductive fibers and wood pulp. It can be used as shielding packaging for sensitive integrated circuits. The shielding effectiveness is generally 30-40dB. 5) Oriented metal wire-filled silicone rubber made by synthesizing many independent metal wires into silicone rubber can provide effective electromagnetic shielding and environmental sealing. It is often used for non-fixed gaps, such as flange connections. Its shielding effectiveness is shown in Table 10. Table 10 Shielding effectiveness of oriented metal wire filled silicon solid rubber dB Material Magnetic field Electric field Plane wave (100kHz) (10MHz) 1GHz 10GHz Tin-plated phosphor bronze 75 130 110 100 Nickel-copper alloy 80 130 115 100 3.2.5 Conductive particles Conductive particle shielding materials are silver-plated glass particles, pure silver particles, carbon black particles, copper-plated silver particles, nickel-plated silver particles, aluminum-plated silver particles, and graphite-plated nickel particles mixed in silicon or fluorosilicone rubber, which can be extruded into various shapes for electromagnetic and water vapor sealing. Their shielding effectiveness is shown in Table 11. Table 11 Shielding effectiveness of conductive particle shielding materials dB Material Magnetic field Electric field Plane wave (100kHz) (10MHz) 1GHz 10GHz Glass silver-plated conductive rubber 811 65 130 100 90 Pure silver conductive rubber 856, 857 70 130 100 90 Carbon black conductive rubber 860 93 77 68 88 Copper silver-plated conductive rubber 871 75 120 115 110 Nickel silver-plated conductive rubber 891 75 120 110 100 Aluminum silver-plated conductive rubber 895 75 120 110 100 Graphite nickel-plated conductive rubber 750 100 100 100 85 3.2.6 Conductive adhesive Conductive adhesive is made by adding pure metal particles, such as silver, nickel, copper-plated silver, aluminum-plated silver, etc., to silicon and epoxy resin adhesives. It is used between various shielding materials to play the role of bonding, shielding and sealing. 3.2.7 Conductive coating Conductive coating is made by adding pure silver particles to polypropylene and polyurethane. It can be used for plastic housing shielding and equipment that requires flexible shielding. 3.2.8 Conductive foil Conductive foil is made of copper or aluminum tape with conductive polypropylene adhesive on one side. It can be used for shielding and sealing of electronic equipment joints, winding cable shielding, etc. Its shielding effectiveness is generally 55-60dB. 3.2.9 Beryllium copper reed Beryllium copper reed is an elastic shielding material that can be used for shielding of electronic equipment movable joints, such as doors and windows. Its shielding effectiveness is shown in Table 12. Table 12 Shielding effectiveness of beryllium copper reed dB Material Magnetic field Electric field Plane wave 100kHz 10MHz 1GHz Standard reed 110 100 90 Soft reed 95 85 75 3.2.10 Shielding composite board Shielding composite board is composed of metal foil, insulating substrate and pressure-sensitive adhesive, which can be used for shielding of printed circuits and electronic equipment. Its shielding effectiveness is generally 40-45dB. 3.2.11 Pure cotton polyester electromagnetic material Pure cotton polyester electromagnetic material is a material that is transparent and has electromagnetic shielding function by evenly distributing copper atoms in cotton polyester material. It can be used for video screen radiation protection, mobile phone microwave radiation protection, etc. Its shielding effectiveness is >50dB. 3.3 Good grounding of the shielding body The metal shielding body is well grounded. For electrostatic shielding, the induced charge outside the shielding body will flow into the ground, and there will be no induced electric field. For alternating electric field shielding, since the magnitude of the coupling interference voltage of the alternating electric field on the sensitive circuit depends on the product of the alternating electric field voltage, coupling capacitance and the grounding resistance of the metal shielding body, as long as the metal shielding body is well grounded, the coupling interference voltage of the alternating electric field on the sensitive circuit can be made very small. Therefore, if the metal shielding body is not well grounded, the shielding effect will be reduced.4 Conductive fibers Conductive fibers are divided into the following 5 categories. 1) Conductive cloth is made by plating copper or nickel on chemical fiber fabrics, which can have flexible shielding properties for high frequencies and microwaves. 2) Conductive cloth and resin are compounded to make absorbent conductive cloth. Since the resin that can absorb electromagnetic waves is selected, the shielding performance is better. (3) Conductive cloth is made by mixing metal or carbon black fibers with chemical fibers with good conductivity. The above 3 types of conductive fabrics can be used as anti-static and anti-electromagnetic radiation work clothes, shielding curtains, tents, and protective covers. The shielding effectiveness is generally 50-60dB. 4) Conductive paper is made by mixing conductive fibers and wood pulp. It can be used as shielding packaging for sensitive integrated circuits. The shielding effectiveness is generally 30-40dB. 5) Oriented metal wire-filled silicone rubber made by synthesizing many independent metal wires into silicone rubber can provide effective electromagnetic shielding and environmental sealing. It is often used for non-fixed gaps, such as flange connections. Its shielding effectiveness is shown in Table 10. Table 10 Shielding effectiveness of oriented metal wire filled silicon solid rubber dB Material Magnetic field Electric field Plane wave (100kHz) (10MHz) 1GHz 10GHz Tin-plated phosphor bronze 75 130 110 100 Nickel-copper alloy 80 130 115 100 3.2.5 Conductive particles Conductive particle shielding materials are silver-plated glass particles, pure silver particles, carbon black particles, copper-plated silver particles, nickel-plated silver particles, aluminum-plated silver particles, and graphite-plated nickel particles mixed in silicon or fluorosilicone rubber, which can be extruded into various shapes for electromagnetic and water vapor sealing. Their shielding effectiveness is shown in Table 11. Table 11 Shielding effectiveness of conductive particle shielding materials dB Material Magnetic field Electric field Plane wave (100kHz) (10MHz) 1GHz 10GHz Glass silver-plated conductive rubber 811 65 130 100 90 Pure silver conductive rubber 856, 857 70 130 100 90 Carbon black conductive rubber 860 93 77 68 88 Copper silver-plated conductive rubber 871 75 120 115 110 Nickel silver-plated conductive rubber 891 75 120 110 100 Aluminum silver-plated conductive rubber 895 75 120 110 100 Graphite nickel-plated conductive rubber 750 100 100 100 85 3.2.6 Conductive adhesive Conductive adhesive is made by adding pure metal particles, such as silver, nickel, copper-plated silver, aluminum-plated silver, etc., to silicon and epoxy resin adhesives. It is used between various shielding materials to play the role of bonding, shielding and sealing. 3.2.7 Conductive coating Conductive coating is made by adding pure silver particles to polypropylene and polyurethane. It can be used for plastic housing shielding and equipment that requires flexible shielding. 3.2.8 Conductive foil Conductive foil is made of copper or aluminum tape with conductive polypropylene adhesive on one side. It can be used for shielding and sealing of electronic equipment joints, winding cable shielding, etc. Its shielding effectiveness is generally 55-60dB. 3.2.9 Beryllium copper reed Beryllium copper reed is an elastic shielding material that can be used for shielding of electronic equipment movable joints, such as doors and windows. Its shielding effectiveness is shown in Table 12. Table 12 Shielding effectiveness of beryllium copper reed dB Material Magnetic field Electric field Plane wave 100kHz 10MHz 1GHz Standard reed 110 100 90 Soft reed 95 85 75 3.2.10 Shielding composite board Shielding composite board is composed of metal foil, insulating substrate and pressure-sensitive adhesive, which can be used for shielding of printed circuits and electronic equipment. Its shielding effectiveness is generally 40-45dB. 3.2.11 Pure cotton polyester electromagnetic material Pure cotton polyester electromagnetic material is a material that is transparent and has electromagnetic shielding function by evenly distributing copper atoms in cotton polyester material. It can be used for video screen radiation protection, mobile phone microwave radiation protection, etc. Its shielding effectiveness is >50dB. 3.3 Good grounding of the shielding body The metal shielding body is well grounded. For electrostatic shielding, the induced charge outside the shielding body will flow into the ground, and there will be no induced electric field. For alternating electric field shielding, since the magnitude of the coupling interference voltage of the alternating electric field on the sensitive circuit depends on the product of the alternating electric field voltage, coupling capacitance and the grounding resistance of the metal shielding body, as long as the metal shielding body is well grounded, the coupling interference voltage of the alternating electric field on the sensitive circuit can be made very small. Therefore, if the metal shielding body is not well grounded, the shielding effect will be reduced.
3.4 Special shielding measures for special parts 3.4.1 Joint treatment At the joints of the shielding body, gaps are formed due to uneven and unclean bonding surfaces, poor welding quality, gaps between fastening screws, etc., resulting in reduced shielding effect of the shielding body. For fixed joints, continuous welding is best used. Before welding, non-conductive materials on the surface to be welded should be cleaned. All shell discontinuities should be overlapped as much as possible. For non-fixed joints, conductive pads should be used and pressed to improve the electromagnetic sealing effect of the joints. Commonly used conductive pad materials include metal braids, rubber containing metal wires, etc. For movable joints, such as on door frames, elastic finger springs are used to improve the electromagnetic shielding effect of the joints. The fixing methods of conductive pads include groove positioning, adhesive fixing, and rib fastening. To improve the shielding effectiveness of the gap, the following measures can be taken: 1) Increase the thickness of the metal plate, which can be achieved by increasing the side length; 2) Reduce the width of the joint surface gap, which can be achieved by improving the machining accuracy of the joint surface, welding or integral casting; 3) Add conductive pads, commonly used conductive pads include woven metal mesh, soft metal, comb-shaped reeds, conductive rubber, etc.; 4) Apply conductive paint on the joints, commonly used conductive paints include conductive glue, conductive grease, etc.; 5) Adjust the spacing between fastening nails to be less than λ/20 (λ is the wavelength of the electromagnetic field); 3.4.2 Eyelet shielding When there is a need for ventilation, lighting, watering, measurement, etc., in order to improve the electromagnetic shielding effect of the equipment, eyelet shielding should be used. The effect of eyelet shielding is related to parameters such as the frequency of the electromagnetic wave, the size and number of the eyelets. 3.4.3 Braided shielding Because the cable needs to be active and bent, its shielding is in the form of a braided belt. The shielding effect of the braided belt increases with the increase of the braiding density and decreases with the increase of the frequency of the electromagnetic wave. Generally, the shielding layer of the cable is woven with non-magnetic metal wire, which can achieve electric field shielding. If magnetic field shielding is required, the shielding layer of the cable should be woven with magnetic metal wire. 3.4.4 Honeycomb shielding When the ventilation and shielding requirements of the equipment are high, the honeycomb shielding has a better effect. Honeycomb shielding is formed by welding many parallel hexagonal metal tubes together. Each of the metal tubes acts as a waveguide attenuator, and the wind pressure drop of ventilation is not large. The electromagnetic shielding effect of the honeycomb depends on the attenuation characteristics of the waveguide, that is, it is related to the geometric dimensions of the waveguide. 3.4.5 Panel hole shielding When the equipment needs to install a meter, data or graphic display, the panel hole should be shielded to ensure the integrity of the shielding. The best method for panel hole shielding is to set a shielding cover behind the meter or display. The shielding cover is connected to the metal panel through a conductive gasket, and a through-hole capacitor is set through the inlet and outlet wires of the shielding cover. 3.4.6 Electrical connector shielding The selected shielded electrical connector should have enough pins for each shielding layer in the cable to be terminated at the electrical connector head. To ensure the integrity of the shield, the outer shielding layer of the cable and the electrical connector should be connected to the ground all around the cable, preferably by welding; the electrical connector seat should maintain good electrical connection with the metal shell of the equipment through the conductive pad; the electrical connector head should also maintain good electrical connection with the electrical connector seat. 3.4.7 Multi-layer shielding When the effect of single-layer shielding does not meet the requirements, multi-layer shielding can be used. In particular, for shielding with a wide frequency band, using several materials with high conductivity and magnetic permeability to form a multi-layer shield can achieve good shielding effects on both high-frequency electric fields and low-frequency magnetic fields. 3.4.8 Shielding of printed circuit boards 1) Set up a wire shield between the electromagnetic interference source and the receiving circuit sensitive to electromagnetic interference, and connect it to the reference potential of the circuit board. 2) Keep as much coating layer between the conductive lines as possible and connect it to the reference potential of the circuit board. 3) Set ground wires on the three peripheries of the printed circuit board (except the electrical connector side). 4) Set up shielding covers for the electromagnetic interference source and the receiving circuit sensitive to electromagnetic interference, and connect them to the reference potential of the circuit board. 5) Set up a shielding plate between the printed circuit boards and connect it to the reference potential of the circuit board. 4 Shielding effectiveness test After the shielding body is completed, the shielding effectiveness needs to be tested. 4.1 Shielding effectiveness test equipment Shielding effectiveness test equipment includes variable frequency signal source, RF amplifier, transmitting antenna, electromagnetic field receiving antenna, attenuator, measurement receiver, and data recorder. 4.2 Shielding effectiveness test method 1) Locate the measurement point; 2) Calibrate the test equipment; 3) Measure the environmental level H when there is no transmission; 4) Measure the electromagnetic field strength W received from the transmitter at the measurement point when there is no shielding; 5) Measure the electromagnetic field strength Y received from the transmitter at the measurement point when there is shielding. 4.3 Shielding effectiveness SE test analysis The shielding effectiveness SE calculation formula is SE=20log10[(W-H)/(Y-H)] (7) After calculation, compare the shielding effectiveness SE with the design requirements to see whether the design requirements are met, whether the safety margin meets the requirements, and whether there is over-design. If the requirements are not met, the reasons should be analyzed in detail and improved until the requirements are met. If there is an over-design, the reasons should also be analyzed in detail and improved in the future design.
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