EMI shielding and shielding materials
EMI shielding can make products quickly and effectively comply with EMC regulations. When the frequency is below 10MHz, electromagnetic waves are mostly conducted in the form of conduction, while higher frequency electromagnetic waves are mostly in the form of radiation. New materials such as single-layer solid shielding material, multi-layer solid shielding material, double shielding or more than double shielding can be used in the design. For low-frequency electromagnetic waves, a thick shielding layer is required. It is most suitable to use high-permeability materials or magnetic materials, such as nickel-copper alloys, to obtain the maximum absorption loss. For high-frequency electromagnetic waves, metal shielding materials can be used.
Conductive shielding material
New shielding structural materials and shielding methods:
New shielding structure and commonly used materials. The shielding body composed of aluminum, steel, and copper has a large reflection loss to electromagnetic waves, so it is only suitable for electrical shielding. Iron and high-permeability alloy bodies have great absorption loss to magnetic field waves, so they are suitable for use in magnetic shielding environments. If conditions permit, stainless steel can be used to make electromagnetic shielding enclosures with high reliability. When the equipment is under mechanical stress, the anti-tilting corners help the chassis maintain the integrity of the mechanical properties and shielding effectiveness.
When choosing the most effective electromagnetic shielding gaskets for commercial electronic equipment such as communications, computers, automation, and medical treatment, the following three types of gaskets can usually be considered: conductive rubber, conductive cloth, and beryllium copper finger springs. The popular new types of shielding materials include conductive plastics, activated conductive coating plastics, foamed aluminum, foamed nickel, ultra-microcrystalline nanocrystalline alloys, nickel-based/cobalt-based amorphous alloys, permalloy foil tapes, and so on.
Multiple shielding: The principle of multiple shielding is that the shielding layers cannot be connected together. They should be separated by air or filled with other media, otherwise the multi-layer shielding function will be lost, and the materials of each layer of shielding body should not be the same. In addition to considering the permeability, the saturation level must also be considered. Sometimes due to the need to double or even more layers of electromagnetic shielding of the system/sub-system. The internal electromagnetic environment of some system equipment is very harsh, so that the requirements for the shielding effectiveness of the shell are also very high.
The biggest source of interference in general equipment is the oscillation circuit, which should be enclosed with an auxiliary sub-shield and then installed in the main shield of the system. There is only one necessary connection point between these sub-shields and the main screen box, outer shield/other sub-shields, and other places must be separated and cannot be connected. The surface resistance of conductive glass generally available on the market is 10W/square, and its shielding effectiveness is closest to that of metal shielding nets. But the higher the frequency, the greater the difference between the two. In the radio frequency range, especially at frequencies above 30MHz, the shielding performance of conductive glass is much lower than that of metal mesh.
However, from an aesthetic point of view, conductive glass is better than shielding nets. Therefore, which type of viewing window to use as system equipment depends on the shielding requirements and actual conditions. The shielding effectiveness of conductive glass is lower than that of metal shielding net. Shielding of the connector: In the shielding design of the connector, it should be noted that the shielding ring of the connector should be grounded before the pin is connected, and when it is disconnected, the pin should be disconnected before the connector shell is disconnected; The pins should be female or concave to avoid contact with other parts of the connector housing.
Shielded cables and special connectors: The metal wire braid has the most opportunities to be used on shielded cables due to its softness and strong practicability. Generally, the shielding effectiveness increases as the density of the braid increases, and decreases as the frequency increases. Attention should be paid to the use of shielded cables in the system and sub-system equipment, but not outside the equipment to avoid unnecessary electromagnetic coupling and crosstalk. Multi-point grounding can be used for the cable, that is, the two ends of the cable shielding layer are grounded, or you can consider adding a grounding connector in the middle to facilitate grounding. The audio cable can only be grounded at one end, and the shielded wire layer must not be grounded. It must be insulated to the ground. Of course, this line is not suitable for using a shielded cable with other signal lines. For cables that are both audio and video, twisted wires should be used, and both ends of the shield should be grounded.
Other shielding materials:
● Conductive rubber: The rubber is mixed with conductive particles, so that this composite material has both the elasticity of rubber and the conductivity of metal. However, due to the incorporation of conductive particles of more than 75% by weight in the rubber, the structure of the rubber is destroyed, and the characteristics of good elasticity and high tensile strength of pure rubber are no longer available.
● Double conductive rubber: It is not mixed with conductive particles in all parts of the rubber. The advantage obtained is that it not only maintains the elasticity of the rubber to the maximum, but also ensures the conductivity. It is a new type of shielding material. Metal braided mesh cover: a hollow mesh cover woven with metal wires. This material has elasticity and conductivity.
● Rubber core braided mesh sleeve: a metal braided mesh sleeve with rubber as the core. This material has good elasticity and pressure resistance due to the rubber as the core.
● Spiral tube gasket: a spiral tube made of stainless steel, beryllium copper or tin-plated beryllium copper, which has good elasticity and conductivity.
● Finger-shaped reed: elastic reed material made of beryllium copper