Shielding is the metal isolation between two space areas to control the induction and radiation of electric fields, magnetic fields and electromagnetic waves from one area to another. Specifically, the shielding body is used to surround the interference source of the components, circuits, assemblies, cables or the entire system to prevent the interference electromagnetic field from spreading outward; the shielding body is used to surround the receiving circuit, equipment or The systems are surrounded to protect them from the influence of external electromagnetic fields. Because the shielding body absorbs energy (eddy current loss) and reflects energy (the interface of electromagnetic waves on the shielding body) for external interference electromagnetic waves and internal electromagnetic waves from wires, cables, components, circuits or systems. Reflection) and energy cancellation (electromagnetic induction generates a reverse electromagnetic field on the shielding layer, which can offset part of the interference electromagnetic waves), so the shield has the function of weakening interference
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(1) When the frequency of the interfering electromagnetic field is high, the eddy current generated in the metal material with low resistivity is used to offset the external electromagnetic waves, thereby achieving a shielding effect.
(2) When the frequency of interfering electromagnetic waves is low, materials with high magnetic permeability should be used to limit the magnetic lines of force inside the shield and prevent them from spreading into the shielded space.
(3) In some cases, if it is required to have good shielding effect on both high-frequency and low-frequency electromagnetic fields, multi-layer shielding bodies are often composed of different metal materials.
Many people do not understand the principle of electromagnetic shielding and think that as long as they make a metal box and then ground the box, it can play the role of electromagnetic shielding. The result under this concept 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 shield surface must be conductive and continuous, and the other is that there cannot be any conductors that directly penetrate the shield. There are many conductive discontinuities on the shield. The most important type is the non-conductive gap formed at the junction of different parts of the shield. These non-conductive gaps produce electromagnetic leakage. PCB manufacturing is like fluid leaking from the gaps in the container. One way to solve this leakage is to fill the gaps with conductive elastomeric material to eliminate non-conductive points. This is like filling the gaps in a fluid container with rubber. This elastic conductive filling material is the electromagnetic sealing gasket.
In many literatures, the electromagnetic shield is compared to a liquid-sealed container. It seems that electromagnetic wave leakage can be prevented only when the gap is sealed to a watertight degree with conductive elastic materials. Actually this is not
exact. Because whether a 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, no obvious leakage will occur. Therefore, when the frequency of interference is higher, and the wavelength is shorter, an electromagnetic sealing gasket needs to be used. Specifically, when the frequency of interference exceeds 10MHz, the use of electromagnetic sealing gaskets should be considered.
Any material that is elastic and conducts electricity well can be used as an electromagnetic sealing gasket. The electromagnetic sealing gaskets manufactured according to this principle include:
Conductive rubber: Silicone rubber is filled with metal particles accounting for 70 to 80% of the total weight, such as silver powder, copper powder, aluminum powder, and silver-plated copper. powder, silver-plated aluminum powder, silver-plated glass balls, etc. This material retains part of the good elasticity of silicone rubber and has good electrical conductivity.
Metal braided mesh: Braided with beryllium copper wire, Monel wire or stainless steel wire into a long tubular strip, the shape is very similar to the shielding layer of a shielded cable. But its weaving method is different from that of the cable shield. The PCB cable shield is made of multiple wires, while this shielding liner is made of one wire. To use a vivid metaphor, 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 reed: Reed made of beryllium copper, which has good elasticity and conductivity. Conductivity and elasticity.
Multiple conductive rubber: It is composed 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 fully embodies the elasticity of rubber.
It works somewhat like a wire mesh strip with a rubber core.