A Brief Introduction to PCB Copy Shield Problem
Enclosing the components, circuits, assemblies, cables, or interference sources of the entire system with shields to prevent interference from the electromagnetic field from spreading out, enclosing the receiving circuit, equipment, or system to prevent them from being affected by external electromagnetic fields. Said the PCB copy of the shield. Choose what type of electromagnetic seal to use when considering four factors: shielding effectiveness requirements, with or without environmental sealing requirements, installation requirements, cost requirements.
Many people do not understand the principle of PCB Copy electromagnetic shielding, that as long as the use of metal to make a box, and then the box ground, you can play the role of electromagnetic shielding. Under the guidance of this concept the result is failure. Because, PCB Copy electromagnetic shielding and shielding ground or not and no relationship. 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 electrically continuous and the other can not have a conductor that penetrates the shield directly. There are many conductive discontinuities on the shield, and the most important one is the nonconductive gap formed by the different parts of the shield. These nonconductive gaps create an electromagnetic leak, as if the fluid would leak from the gap on the container. One way to solve this kind of leakage is to fill the gap with the conductive elastic material to eliminate the non-conductive point. It is like filling the gap in the fluid container. This elastic conductive filler is the electromagnetic seal.
In many of the literature, the electromagnetic shield is likened to a liquid sealed container, and it appears that it is only possible to prevent the electromagnetic wave from leaking when the gap is sealed with a conductive elastic material to the extent that the leak is not leaked. In fact it is not clear. Because the gap or hole will leak electromagnetic waves, depending on the gap or hole relative to the size of the wavelength of electromagnetic waves. When the wavelength is much larger than the opening size, and will not produce significant leakage. Therefore, when the frequency of interference is high, then the wavelength is shorter, you need to use electromagnetic seal gasket. Specifically, when the frequency of interference exceeds 10MHz, it is necessary to consider the use of electromagnetic seal gasket.
The PCB Copy has a reduced interference function.
(1) When the frequency of the electromagnetic field is high, the eddy current generated in the low-resistivity metal material forms a canceling effect on the external electromagnetic wave, thereby achieving the effect of shielding.
(2) When the frequency of interference with electromagnetic waves is low, to use high permeability of the material, so that the magnetic field lines within the shield to prevent the spread to the shielding space.
(3) In some cases, if the requirements of high frequency and low frequency electromagnetic field has a good shielding effect, often using different metal materials composed of shielded multi-layer body.
All materials that are flexible and electrically conductive can be used as an electromagnetic seal. In accordance with the principle of manufacturing the electromagnetic seal liner are:
Conductive rubber: in the silicone rubber filled with the total weight of 70 ~ 80% of the proportion of metal particles, such as silver, copper powder, aluminum powder, silver-plated copper powder, silver-plated aluminum powder, silver glass ball. This material retains a portion of silicone rubber with good elastic properties and has good electrical conductivity.
Metal braid: with beryllium copper wire, Monel silk or stainless steel wire woven into a tubular strip, looks like a shielded cable shield. But its weaving method is different from the cable shield, the cable shield is made of multiple lines, and this shielding pad is woven by a line. Playing a metaphor of the image, like a sweater sleeves. In order to enhance the elasticity of the metal mesh, sometimes in the network management by adding rubber core.
A reed made of beryllium copper, with good elasticity and electrical conductivity. Conductivity and elasticity.
Multi-conductive rubber: composed of two layers of rubber, the inner layer is ordinary silicone rubber, the outer layer is conductive rubber. This material overcomes the shortcomings of the traditional conductive rubber elasticity, so that the flexibility of rubber can be fully reflected. Its principle is like metal wire mesh with rubber core.