Let's talk about a few statements. 1. The commercial necessity of shielding; 2. The concept of shielding; 3. Larger (spacing) and rectangular (or irregular) shielding shapes between circuits and shields are better to avoid resonance; 4. Skin effect (difficult to block low frequencies) 5. Apertures (difficult to block high frequencies) 6. Low-frequency (magnetic field) shielding 7. Cut-off waveguide 8. Conductor gaskets (used for filling gaps) 9. Shielding of visible components (such as display skins, indicator lights, keyboards) 10. Shielding of ventilation (heat dissipation) holes 11. Shielding with painted or electroplated plastics 12. Non-metallic shielding 13. Installation of shielding rooms 14. Board-level electromagnetic shielding 1. The commercial necessity of shielding. An important concept proposed by the author: A project should consider shielding issues in the planning stage, so that the cost of shielding measures will be the lowest. If you wait until the problem is exposed to check for leaks and make up for the shortcomings, it often costs a lot. Shielding measures often increase costs and instrument weight. If it can be solved by other EMC methods, try to reduce shielding. (The implication is that shielding is the last resort) The following two points should be noted for PCB: 1. Keep the wires and components as close as possible to a large metal plate (this metal plate does not refer to the shielding body) 2. Keep the electrical components and lines as close to the ground layer as possible (reduce the electromagnetic interference of the interlayer signal, the ground layer can absorb some of the interference) In this way, even if shielding is required, the demand for shielding effectiveness (SE shiedling effectiveness) can be reduced. 2. The concept of shielding Shielding is equivalent to a filter, which is placed on the propagation path of the electromagnetic wave and forms a high impedance to a part of the frequency band. The larger the impedance ratio, the better the shielding effectiveness. For general metals, a thickness of 0.5mm can produce a good shielding effect for 1MHz electromagnetic waves, and a very good shielding effect for 100MHz. The problem is that thin metal shielding is not effective for waves below 1MHz or pores. This article focuses on this aspect. 3. Larger spacing and rectangular shielding are better (1) Larger spacing between circuits and shields can reduce mutual interference; (2) Rectangular (or irregular) shielding shapes can minimize frequency resonance; square shells are often prone to resonance; But in general, circuit boards are usually located inside the shield, and their components, circuits, etc. will change the expected resonant frequency point, so don't worry too much. 4. Skin effect364514[/attach] Skin depth is defined in engineering as the thickness from the surface to the current density dropping to 0.368 (i.e. 1/e) of the surface current density as skin depth or penetration depth Δ:364515[/attach] Where: μ - magnetic permeability of wire material; γ = 1/ρ - electrical conductivity of material; k - temperature coefficient of electrical conductivity (or resistivity) of material; Above: Skin effect depth of three metals at different frequencies (the higher the frequency, the shallower the depth, the more skin-like); from the perspective of conduction, the skin effect is expected to be deep, which means that the utilization rate of the wire is high; but for shielding, the skin depth is expected to be shallow, so that more electromagnetic frequency bands can be shielded with thinner metals; the skin depth of 50Hz is 5-15mm, which is difficult to shield... The metal used for shielding should have good electrical and magnetic conductivity, and the thickness is determined according to the skin depth generated by the lowest frequency of interference. Generally, 1mm low-carbon steel plate or 1μm galvanized layer can meet general applications. (This is also the reason why galvanizing is often seen on the chassis wall in practice) 5. Porosity If the entire shell of the shielding body is seamless and hole-free, then for 30MHz electromagnetic waves, it is not difficult to achieve a 100dB attenuation effect. The problem is that they are not seamless:
Opening a hole in a perfect shielding shell is equivalent to forming a half-wave resonant slot antenna. The relationship between shielding effectiveness SE and the maximum size d of the hole and the wavelength λ of the electromagnetic wave is as follows:
So for the 30MHz mentioned earlier, the wavelength is 10m, and assuming there is a USB port (diagonal aperture size is 10mm), the SE is 54dB. The larger d is, the smaller the SE is. The electromagnetic wave frequency bands we often use:
To achieve a 40dB SE, it is usually necessary to use conductor washers and spring fingers for sealing. Pay attention to the spacing between the internal components and the shielding cover, and the distance between the data bus and the openings and gaps. It should also be noted that when there is current in the shielding body and there are apertures and slits blocking the way in the direction of the current, forcing the current to detour, the apertures and slits will cause the apertures and slits to emit magnetic fields like antennas, and the voltage changes through the apertures and slits will generate magnetic fields. 6. Low-frequency magnetic field shielding uses alloy materials with high magnetic permeability (such as amorphous alloys and Permalloy) and is made into shielding covers according to certain specifications, which can greatly reduce the impact of magnetic fields. 7. Cut-off waveguide
8. Gaskets use good conductors and are used for filling gaps. They can withstand certain extrusion deformation, are corrosion-resistant, and are durable.
10. Shielding of ventilation holes The ventilation holes are made into two forms: (1) Metal mesh (similar to honeycomb aluminum plate) (2) (As of) waveguide 11. Painted or electroplated plastics are often used because molded plastics are beautiful and light. In this case, conductive materials are usually sprayed on the surface of the plastic cup. Because the thickness of the conductive layer cannot be too thick (micrometer level), the actual effect is not very good. For class II appliances (class II), it may also increase the possibility of electrostatic discharge (ESD). Class II appliances: These appliances use double insulation or reinforced insulation and have no grounding requirements. 12. Non-metallic shielding such as carbon fiber or conductive polymers (conductive plastics), but in any case their SE is not as good as metal. 13. Installation of shielding cover
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