A brief discussion on shielding technology (reposted)

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A brief discussion on shielding technology (reposted) [Copy link]

Shielding is the metal isolation between two spatial regions to control the induction and radiation of electric fields, magnetic fields and electromagnetic waves from one region to another. Specifically, it is to use a shield to surround the interference source of components, circuits, assemblies, cables or the entire system to prevent the interference electromagnetic field from spreading outward; use a shield to surround the receiving circuit, equipment or system to prevent them from being affected by the external electromagnetic field. Because the shield absorbs energy (eddy current loss), reflects energy (interface reflection of electromagnetic waves on the shield) and offsets energy (electromagnetic induction generates a reverse electromagnetic field on the shielding layer, which can offset part of the interference electromagnetic waves) from external interference electromagnetic waves such as wires, cables, components, circuits or systems and

internal electromagnetic waves, the shield has the function of reducing interference. (1) When the frequency of the interference electromagnetic field is high, the eddy current generated in the metal material with low resistivity is used to form an offsetting effect on the external electromagnetic waves, thereby achieving the shielding effect.

(2) When the frequency of the interference electromagnetic wave is low, high magnetic permeability materials should be used to confine the magnetic lines of force inside the shield to prevent them from spreading to the shielded space.

(3) In some cases, if good shielding effect is required for both high-frequency and low-frequency electromagnetic fields, different metal materials are often used to form a multi-layer shield.

Many people do not understand the principle of electromagnetic shielding and think that as long as a box is made of metal and then grounded, it can play the role of electromagnetic shielding. Under the guidance of this concept, the result is failure. Because electromagnetic shielding has nothing to do with whether the shield is grounded or not. There are only two factors that really affect the shielding effectiveness of the shield: one is that the entire surface of the shield must be conductive and continuous, and the other is that there must be no conductor that directly penetrates the shield. There are many conductive discontinuities on the shield, the most important of which is the non-conductive gaps formed at the junction of different parts of the shield. These non-conductive gaps produce electromagnetic leakage, just like fluid leaking from the gaps in the container. One way to solve this leakage is to fill the gaps with conductive elastic material to eliminate the non-conductive points. This is just like filling the gaps in the fluid container with rubber. This elastic conductive filling material is the electromagnetic sealing gasket.

In many documents, the electromagnetic shielding body is likened to a liquid sealed container. It seems that only when the gap is sealed to the extent of watertightness with conductive elastic materials can the electromagnetic wave leakage be prevented. In fact, this is not accurate. Because whether the gap or hole will leak electromagnetic waves depends on the size of the gap or hole relative to the wavelength of the electromagnetic wave. When the wavelength is much larger than the opening size, there will be no obvious leakage. Therefore, when the frequency of interference is high, the wavelength is short, and electromagnetic sealing gaskets are needed. Specifically, when the frequency of interference exceeds 10MHz, it is necessary to consider using electromagnetic sealing gaskets.

Any material that is elastic and has good conductivity can be used as electromagnetic sealing gaskets. Electromagnetic sealing gaskets manufactured according to this principle are:

Conductive rubber: metal particles accounting for 70% to 80% of the total weight are filled in silicone rubber, such as silver powder, copper powder, aluminum powder, silver-plated copper powder, silver-plated aluminum powder, silver-plated glass balls, etc. This material retains some of the good elastic properties of silicone rubber and has good conductivity.

Metal braided mesh: Beryllium copper wire, monel wire or stainless steel wire are woven into long tubular strips, and the appearance is very similar to the shielding layer of a shielded cable. However, its weaving method is different from that of the cable shielding layer. The cable shielding layer is woven with multiple wires, while this shielding pad is woven with one wire. To make a vivid analogy, it is like the sleeves of a sweater. In order to enhance the elasticity of the metal mesh, a rubber core is sometimes added to the mesh tube.

Finger-shaped reed: Reeds made of beryllium copper have good elasticity and conductivity. Conductivity and elasticity.

Multiple conductive rubber: It consists of two layers of rubber, the inner layer is ordinary silicone rubber, and the outer layer is conductive rubber. This material overcomes the shortcomings of poor elasticity of traditional conductive rubber and allows the elasticity of rubber to be fully reflected. Its principle is somewhat like a metal wire with a rubber core.

【Key points of design】: a. It is better to place the shielding plate close to the protected object, and the grounding of the shielding plate must be good! ! ! b. The shape of the shielding plate has a significant impact on the shielding effectiveness. A fully enclosed metal box is the best, but it is difficult to achieve in engineering! c. The material of the shielding plate is preferably a good conductor, but there is no requirement for thickness, as long as it has sufficient strength. 2 Magnetic field shielding Magnetic field shielding usually refers to the shielding of DC or low-frequency magnetic fields, and its effect is much worse than that of electric field shielding and electromagnetic field shielding. 【Shielding mechanism】: It mainly relies on the low magnetic resistance of high magnetic permeability materials to shunt the magnetic flux, greatly weakening the magnetic field inside the shielding body. 【Key points of design】: a. Select high magnetic permeability materials, such as Permalloy; b. Increase the thickness of the shielding body; all of the above are to reduce the magnetic resistance of the shielding body; c. The shielded object should not be placed close to the shielding body to minimize the magnetic flux passing through the shielded object; d. Pay attention to the structural design of the shielding body. Any seams, ventilation holes, etc. may increase the magnetic resistance of the shielding body, thereby reducing the shielding effect. e. For the shielding of strong magnetic fields, a double-layer magnetic shielding body structure can be used. For shielding external strong magnetic fields, the outer layer of the shielding body should be made of materials that are not easy to saturate, such as silicon steel; and the inner layer can be made of high magnetic permeability materials that are easy to reach saturation, such as Permalloy. On the contrary, if you want to shield the internal strong magnetic field, the order of arrangement of the materials should be reversed. When installing the inner and outer layers of the shielding body, pay attention to the insulation between them. When there is no grounding requirement, insulating materials can be used as support parts. If grounding is required, non-ferromagnetic materials (such as copper and aluminum) can be used as support parts. 3 Electromagnetic field shielding Electromagnetic field shielding is a measure to prevent the propagation of electromagnetic fields in space by using shielding bodies. 【Mechanism of electromagnetic field shielding】: a. When the electromagnetic wave reaches the surface of the shield, the incident wave is reflected due to the discontinuity of impedance at the interface between air and metal. This reflection does not require the shielding material to have a certain thickness, but only requires discontinuity at the interface; b. The energy that is not reflected by the surface and enters the shield is attenuated by the shielding material during the process of propagating forward in the body. This is also called absorption; c. When the remaining energy that has not yet attenuated in the shield reaches the other surface of the material, it encounters the interface of metal-air impedance discontinuity, which will form a second reflection and return to the shield. This reflection may have multiple reflections at the interface between the two metals. In short, the attenuation of electromagnetic waves by electromagnetic shielding is mainly based on the reflection and absorption of electromagnetic waves. 【Absorption loss】Different materials and different material thicknesses have different absorption effects on electromagnetic waves. It can be obtained based on the line diagram of material absorption loss. 【Reflection loss】It is divided into three categories: low impedance magnetic field, high impedance electric field, and plane wave field. The calculation method of the reflection loss nomogram for low impedance magnetic field and high impedance electric field is the same, which is related to the metal material, frequency and the distance from the radiation source to the shield. For plane waves, the wave impedance is a constant and has nothing to do with the distance from the radiation source to the shield. In the nomogram, you only need to connect the metal material and the frequency of interest to find the reflection loss value at this time. 4 Actual electromagnetic shielding body [Structural materials] a. Most of the materials suitable for the bottom plate and the casing are good conductors, such as copper, aluminum, etc., which can shield the electric field. The main shielding mechanism is reflection of the signal rather than absorption. b. Ferromagnetic materials are required for shielding the magnetic field, such as high permeability alloys and iron. The main shielding mechanism is absorption rather than reflection. c. In a strong electromagnetic environment, the material is required to shield both the electric field and the magnetic field, so a structurally sound ferromagnetic material is required. The shielding efficiency is directly affected by the thickness of the material and the quality of the overlap and grounding method. D. For plastic shells, the shielding layer is sprayed on the inner wall, or metal fibers are added during the autoclaving. The electrical discontinuity of the structure must be minimized to control the leakage radiation entering and exiting through the bottom plate and the casing. improve