Some important knowledge about electromagnetic compatibility shielding design

Do you understand the shielding design of electromagnetic compatibility? EMC (electromagnetic compatibility) is a capability of equipment. It requires that the equipment can perform its functions normally in its electromagnetic environment without affecting its normal operation due to environmental interference. The EMC performance of a product is directly related to the product's working stability and environmental adaptability. EMC design is an indispensable and important part of communication product design. One of the more critical aspects of EMC design is the shielding design of the relevant equipment. Shielding can effectively suppress electromagnetic interference propagating through space. There are two purposes of shielding: one is to limit internal radiated electromagnetic energy from crossing a certain area; the other is to prevent external radiation from entering a certain area. Existing shielding measures are implemented based on these two purposes.

At present, the shielding system has been recognized by more and more users, and its good performance in electromagnetic compatibility is also being recognized by more and more people. The shielding system has good electromagnetic compatibility and can provide a safe, high-speed, and stable information transmission channel. Shielding System has a complete shielding and grounding system, providing the most complete and comprehensive cables, components and end-to-end fully shielded solutions to meet the increasing needs of today's networks. For shielding systems, grounding is a crucial process. Only correct and effective grounding can reflect the superiority and value of shielding.

1. Application of shielding system

1. Information security: Improving the security of information transmission and ensuring that sensitive data is not leaked is the most important reason for choosing a shielding system. With the popularization of network informatization, the importance of information security has been increasingly valued by the majority of users, and preventing information leakage has become a crucial issue.

2. High-speed network: Compared with 100M and 1000M networks, high-speed networks are more sensitive to external interference due to more complex coding and other reasons, and communication is more susceptible to external interference. For the shielding system, the shielding layer shields not only external electromagnetic interference, but also the interference between cables. The impact of the shielded wiring system on ANEXT has innate technical advantages, so the shielding system can operate at a relatively high speed. More stable and reliable.

3. Special transmission environment: Special installation environments require protection from external electromagnetic interference. For example, there are strong radio frequency sources such as radio stations and TV stations near buildings, or near large power plants, or in various industrial environments. There is clear presence of strong electromagnetic interference from continuous or intermittent operation in these environments. Before the implementation of the cabling system, although the impact of these interferences on network operation is difficult to quantitatively analyze, the use of shielding systems can better ensure the normal operation of network communications.

2. Screen design

1. Shielded room: It can provide a low-level and stable environment, which brings great benefits to the improvement of measurement accuracy, measurement reliability and repeatability. However, because the interference signal generated by the equipment under test in the shielded room produces irregular diffuse reflection through the 6 surfaces of the shielded room, the performance is more serious especially in the radiation emission measurement and radiation sensitivity measurement, resulting in huge errors in the shielded room. .

2. Mixed wave chamber: Place a large rotating metal reflector in a cuboid shielded room to achieve a uniform field environment. Therefore, the electromagnetic field at a certain point in the wave chamber is the vector sum of reflected waves from all directions at this point. In addition, the rotation of the fan blades changes the path length of the electromagnetic wave reaching this point and the number of reflections of the wave. Statistically speaking, the field in the cavity will not have an obvious resonant field structure, and ultimately a statistically uniform, polarized, and accessible field environment will be obtained in the test area of ​​the cavity.

3. Horizontal electromagnetic chamber: It consists of 3 parts: the main section, the transition section, and the rectangular coaxial connecting line. The horizontal electromagnetic chamber converts the input energy into a uniform magnetic field, thereby eliminating the uneven length that may be caused by the use of antennas. question. At the same time, the GTEM chamber floor partition and the gradient waveguide wall are matched and connected at the terminal, thus eliminating the reflection and resonance phenomena inherent in other equipment.

3. Application of shielding

1. Shielding of cabinet (or shielding box):

The actual electromagnetic shielding body is not a fully enclosed shielding body, that is, it is not electrically continuous and uniform. In the actual structural design of the chassis and shielding box, there are usually the introduction and exit of power lines and control lines. There are also openings for operation keys and display screens on the panel, and there are ventilation holes on the rear panel, etc., so in actual practice The chassis is electrically discontinuous, and an electrically discontinuous chassis will reduce its shielding effectiveness. The following are some basic practices in chassis design:

(1) Structural materials ① Most materials suitable for base plates and casings are good conductors, such as copper, aluminum, etc., which can shield electric fields. The main shielding mechanism is reflection rather than absorption; ② Ferromagnetic materials are required to shield magnetic fields. , such as high permeability alloys and iron. The main shielding mechanism is absorption rather than reflection; ③ In a strong electromagnetic field environment, materials are required to shield both electric and magnetic fields, so structurally sound ferromagnetic materials are required. The shielding efficiency is directly affected by the thickness of the material and the quality of the overlapping and grounding methods; ④ For plastic shells, spray a shielding layer on the inner wall, or incorporate metal fibers during auto-molding.

(2) Gap: ① Every gap and discontinuity between the base plate and the casing should be overlapped as well as possible. The worst electrical overlap plays a decisive role in the shielding effectiveness of the shell; ② Ensure metal-to-metal contact at the joints to prevent leakage and radiation of electromagnetic energy.

(3) Penetration and openings: ① Pay attention to the degree to which the overall shielding effectiveness is reduced due to the cable passing through the casing. When a typical unfiltered wire passes through the shield, the shielding effectiveness is reduced by more than 30dB. ② When the power cord enters the case, it should all pass through the filter box. It is best for the input end of the filter to penetrate outside the shielded casing; if the filter structure is not suitable for passing through the casing, a compartment should be set up for the filter where the power cord enters the casing. ③ When signal lines and control lines enter or exit the casing, they must pass through appropriate filters. Multi-core connectors (sockets) with filter pins are suitable for use in this situation.

(4) Overlap: ① Use the same metal for overlap as much as possible. Ensure that the DC resistance of the overlapping connection is not greater than 2.5 milliohms; ② When overlapping different metals, pay attention to the relative positions of various metals in the electrochemical sequence table. The potential difference should be as small as possible and appropriate anti-corrosion measures should be taken. When metals with widely different levels are overlapped, an intermediate-level metal washer needs to be placed between the two metal surfaces; ③ Trim the overlapping surface to obtain the maximum contact area; ④ Clean all mating surfaces before overlapping. To prevent oxidation, overlap the mating surfaces immediately after removing the protective layer; ⑤ For permanent lap joints, fusion welding or copper soldering should be used to connect all joints as much as possible.

Some joint surfaces. RF bonding should be given priority to permanent bonding.

2. Shielding of the plug-in box

When the operating frequency of a plug-in box is high and easily interferes with the work of other plug-in boxes, or when the work is very sensitive and susceptible to interference from other plug-in boxes, it is necessary to consider the shielding performance of the plug-in box. The shielding of the plug-in box is relatively complicated. Generally, the following methods can be used: (1) The panel adopts a metal panel, usually a U-shaped panel; (2) In order to ensure good overlap between panels, it is best to use metal reeds; (3) It is best to use metal reeds or conductive linings between the panel and the machine frame pads to ensure overlapping; (4) The overlapping of the machine frame must be well ensured; (5) The top and bottom of the plug-in box generally have ventilation and heat dissipation requirements, and porous sheets can be used; (6) In order to discharge static electricity on the panel in a timely manner, the minimum temperature on the panel must be It is good to have a metal pin, which has two functions: static discharge and positioning; there must be a positioning hole and a grounding spring at the corresponding pin in the machine frame.

3. Cable shielding: Since electromagnetic waves exist in every corner, a slight negligence may lead to serious losses. Therefore, the shielding of each object must be carefully planned. Among various objects, the most important thing is the shielding of cables. Braided metal mesh is most commonly used for cable shielding. Factors that affect its shielding effectiveness include: shielding material and thickness, the termination method of the shielding layer and the connectors used, and the wires. Standing wave ratio, the length and direction of the cable itself; in addition, in fact, the softness of the cable after adding a shielding layer should also be paid attention to. In addition, the shielding layer of the cable should be insulated because improper grounding will generate noise; at the same time, except for coaxial cables, the shielding layer should not be regarded as a signal return line. In addition, the shielding layer of cables used to transmit high-frequency signals should be grounded at both ends. In addition to the shielding layer of the cable, the joints should also be considered; at the same time, the shielding of the joints should be properly grounded, and its shielding efficiency should be no less than that of the cable shield.

4. Development direction of shielding materials

(1) Polymer conductive coating:

Polymer conductive paint is made by mixing metal powder, carbon powder, graphite and other conductive fillers with high molecular polymers such as epoxy resin, polyurethane, acrylic resin, etc., and then coating it on the plastic surface. Coating, low cost and other advantages are now becoming more and more popular. The main types of conductive coatings include silver-based, carbon-based, copper-based, nickel-based, etc. The chemical properties of silver are stable, the conductivity is good, and the shielding effect can reach more than 65dB.

(2) Composite shielding material:

Composite shielding materials are a new type of shielding material that is currently being researched. They are made of insulating synthetic resin, good conductive materials and additives. According to the type of filling material, it can be roughly divided into two categories: metal fiber composite materials and non-metal fiber composite materials.

(3) Conductive fabric:

Conductive fabrics are metal-plated on the surface of general textiles, or metal fibers are woven into textiles, so that they have good shielding performance of metal without losing the original flexibility of textiles. Its varieties mainly include carbon fiber and ordinary fiber blended fabrics, metal fiber non-woven fabrics, ordinary chemical fiber complexed copper fiber fabrics, etc. Due to its advantages such as convenience and light weight, conductive fabrics are now becoming a hot research topic.

(4)Foam material:

Foam materials generally include foam metal and foam plastic. Foamed metal is a composite material of metal and air. According to the shape of its internal bubbles, foamed metal can be divided into two types: independent bubble type where bubbles exist independently and continuous bubble type where bubbles are continuously distributed. Many metal materials such as carbon steel, stainless steel, aluminum, copper, lead, titanium, silver, nickel-based superalloys, etc. can be made into foamed metal; among them, foamed aluminum technology is the most mature and widely used. Due to its porous structure, the absorption loss and multiple reflections of electromagnetic waves inside the metal greatly increase the loss, so a very thin thickness can provide a good shielding effect.

5. There are still the following difficulties that need to be solved in electromagnetic shielding technology:

Reducing the thickness of the shielding material also reduces the shielding effectiveness of the material, which is in conflict with the structural design requirements for increased strength and thickness. Impedance matching is also a difficulty in the development of shielding absorbing materials. Ideally, the higher the electrical conductivity and magnetic permeability of a material, the better. However, it is difficult for a single-component material to have high dielectric constant and magnetic permeability at the same time: Low-frequency magnetic field shielding is still a difficult point in electromagnetic shielding. Currently, the commonly used methods are to use multiple shielding and stay away from field sources, but more effective measures require further research. Composite shielding materials and structures are difficult to process due to limitations in their design functions.

6. In short, shielding design is a very important part of EMC design.

The quality of shielding measures is directly related to the EMC effect of the entire system. However, since electromagnetic waves are everywhere, shielding is also a diverse, complex and changing problem. Only by paying more attention to every place that may be subject to electromagnetic interference and focusing on the subtleties can we achieve better shielding effects. The above is some knowledge about electromagnetic compatibility shielding design. I hope it can help you.