- Sources of EMC Problems
- Metal Shielding Efficiency
- EMI Suppression Strategies
- Shielding Design Difficulties
- Pads and Accessories
- Conclusion
Electromagnetic Compatibility (EMC) refers to "the performance of a device, piece of equipment, or a system, which allows it to function properly in its own environment". enable it to function properly in its own environment without causing strong electromagnetic interference with any other equipment in that environment (IEEE C63.12-1987)." For wireless transceivers, EMC performance can be partially achieved by using a non-contiguous spectrum, but there are many examples where EMC is not always possible. For example, there is high-frequency interference between laptop computers and test equipment, between printers and desktop computers, and between cellular phones and medical instruments, which we call electromagnetic interference (EMI).
EMC Problem Sources
All electrical and electronic equipment operates with intermittent or continuous voltage and current variations, sometimes at a fairly rapid rate, which results in the generation of electromagnetic energy at different frequencies or within a band, and the corresponding circuitry emits this energy into the surrounding environment.
EMI has two ways of leaving or entering a circuit: radiation and conduction. Signal radiation leaks out through seams, slots, openings or other gaps in the enclosure; while signal conduction leaves the enclosure by coupling to power, signal and control lines, which radiate freely in open space, creating interference.
Much of the EMI suppression is achieved using a combination of enclosure shielding and gap shielding, and most of the time the following simple principles can help to achieve EMI shielding: Reduce interference at the source; isolate interference-generating circuits by shielding, filtering, or grounding; and enhance the immunity of sensitive circuits, etc. EMI suppression, isolation, and hypoallergenics should be the goal of all circuit designers. These properties should be accomplished early in the design phase.
For design engineers, the use of shielding materials is an effective way to reduce EMI. Today there are a variety of enclosure shielding materials are widely used, from metal cans, thin metal sheets and foil strips to conductive fabric or tape spray coating and plating (such as conductive paint and zinc wire coating, etc.). Whether it's metal or plastic coated with a conductive layer, once the designer has decided on the enclosure material, he or she can begin the process of selecting a liner.
Metal shielding efficiency
The suitability of a shield can be evaluated in terms of shielding efficiency (SE), which is measured in decibels and is calculated as
SEdB=A+R+B
Where A: Absorption Loss (dB) R: Reflection Loss (dB) B: Correction Factor (dB) (applies to the presence of multiple reflections in a thin shield)
A simple shield will make the resulting electromagnetic field strength down to one-tenth of the initial, that is, SE is equal to 20dB; and some occasions may require the field strength down to one hundred thousandth of the initial, that is, SE to be equal to 100dB.
Absorption loss refers to the number of electromagnetic waves through the shielding of the amount of energy loss, the absorption loss is calculated as
AdB=1.314(f×σ×μ)1/2×t
Where f: frequency (MHz) μ: copper's permeability σ: copper's conductivity t: shield thickness
The size of the reflection loss (near-field) depends on the nature of the source of the electromagnetic wave generation as well as the distance from the wave source. For a rod or linear transmitting antenna, the closer the wave source, the higher the wave resistance, and then decreases as the distance from the source increases, but the plane wave resistance is unchanged (constant 377).
On the contrary, if the wave source is a small coil, then this time will be dominated by the magnetic field, the closer to the source the lower the wave resistance. The wave resistance increases with distance from the source, but when the distance exceeds one-sixth of the wavelength, the wave resistance no longer changes and is constant at 377.
Reflection loss varies with the ratio of wave resistance to shield impedance, so it depends not only on the type of wave but also on the distance between the shield and the source. This applies to small, shielded devices.
Near-field reflection loss can be calculated according to the following formula
R(electrical)dB=321.8-(20×lg r)-(30×lg f)-[10×lg(μ/σ)] R(magnetic)dB=14.6+(20×lg r)+(10×lg f)+[10×lg(μ/σ)]
Where r: distance between the wave source and the shielding distance.
The last term in the SE equation is the correction factor B, which is calculated as
B=20lg[-exp(-2t/σ)]
This equation is only applicable to the near-field environment and the absorption loss is less than 10dB. Since the shielding is not efficiently absorbed, and its internal re-reflection increases the energy passing through the other side of the shielding, the correction factor is a negative number, indicating a decrease in shielding efficiency.
EMI suppression strategies
Only materials with high magnetic permeability, such as metals and iron, can achieve high shielding efficiency at very low frequencies. The permeability of these materials decreases as the frequency increases, and if the initial magnetic field is strong, the permeability also decreases, and if the shield is mechanically shaped to the required shape, the permeability also decreases. In summary, the selection of highly conductive materials for shielding is complex, and solutions are often sought from suppliers of EMI shielding materials as well as from consulting organizations.
In high-frequency electric fields, the use of thin layers of metal as a shell or lining material can achieve good shielding, but the condition is that the shielding must be continuous, and the sensitive part of the complete cover, no gaps or slits (forming a Faraday cage). In practice, however, it is not possible to create a shield without seams or gaps, as the shield is fabricated in multiple parts, so there are seams to join, and often holes have to be punched in the shield to allow for adhesion to add-on cards or assembly components.
The difficulty in designing a shield is that holes are inevitably created during the manufacturing process and will be needed during the operation of the equipment. Fabrication, panel inline, vents, external monitoring windows, and panel adhesion assemblies all require holes in the shield, which significantly reduces shielding performance. Although grooves and gaps are unavoidable, it pays to give careful consideration to the length of the grooves as they relate to the wavelength of the frequency at which the circuit operates in the shield design.
The wavelength of any frequency electromagnetic wave is: wavelength (λ)=speed of light (C)/frequency (Hz)
When the length of the gap for the wavelength (cutoff frequency) of half the length of the RF wave began to 20dB/10 octave (1/10 cutoff frequency) or 6dB/8 octave (1/2 cutoff frequency) rate of attenuation. Typically the higher the RF transmitter frequency the more severe the attenuation is because its wavelength is shorter. When it comes to the highest frequencies, it is important to consider any harmonics that may be present, although in practice only the first and second harmonics need to be considered.
Once the frequency and intensity of RF radiation within the shield is known, the maximum allowable gaps and grooves in the shield can be calculated. For example, if you need to 1GHz (wavelength of 300mm) radiation attenuation of 26dB, then 150mm gap will begin to produce attenuation, so when there is less than 150mm gap, 1GHz radiation will be attenuated. So for 1GHz frequency, if need to attenuate 20dB, the gap should be less than 15 mm (150mm 1/10), need to attenuate 26dB, the gap should be less than 7.5 mm (15mm 1/2 or more), need to attenuate 32dB, the gap should be less than 3.75 mm (7.5mm 1/2 or more).
A suitable conductive liner can be used to make the gap size limited to the specified dimensions, so as to achieve this attenuation effect.
Shielding Design Difficulties
Shielding efficiency is also reduced because seams cause the shield conductivity to drop. Note that the attenuation of radiation below the cutoff frequency depends only on the length-to-diameter ratio of the seam, e.g., 100 dB attenuation can be obtained with a length-to-diameter ratio of 3. When perforation is required, the waveguide properties of the small holes in the top of a thick shield can be utilized; another way to achieve a higher length-to-diameter ratio is to attach a small metal shield, such as a suitably sized liner. The above principles and their generalization in the case of multiple slits form the basis for the design of porous shields.
Thin porous shields: There are many examples of porous shields, such as ventilation holes in thin metal sheets, etc., and careful consideration must be given to the design when the holes are closely spaced. The following is the shielding efficiency formula in such cases
SE=[20lg (fc/o/σ)]-10lg n where fc/o: cutoff frequency n: number of holes
Note that this formula only applies to the hole spacing is less than the hole diameter of the case, but also can be used for calculating the shielding efficiency associated with the woven metal mesh.
Seams and joints: Electro-welding, brazing or soldering are common methods of permanent fixing between sheets. The metal surfaces of the joints must be cleaned so that the joints can be completely filled with conductive metal. Screws or rivets are not recommended because the low resistance of the contact between fasteners is not easy to maintain for a long time.
The purpose of a conductive liner is to minimize slots, holes, or gaps in seams or joints so that RF radiation does not escape.
EMI liners are conductive media used to fill gaps in the shield and provide a continuous low impedance contact. Typically an EMI liner provides a flexible connection between two conductors, allowing current flow from one conductor to the other.
The following performance parameters can be used for the selection of a sealed hole EMI liner: ? Shielding efficiency for a specific frequency range ? Adhesion method and sealing strength ? Current compatibility with the outer shield and corrosion resistance to the external environment. ? Operating temperature range ? Cost
Most commercially available gaskets have sufficient shielding performance to enable equipment to meet EMC standards; the key is to design the gasket correctly within the shielded enclosure.
Gasket systems: An important factor to consider is compression, which creates a high level of conductivity between the gasket and the shim. Poor conductivity between the liner and spacer reduces shielding efficiency, and a missing piece of the joint creates a slotted antenna that radiates at a wavelength about four times smaller than the length of the slit.
To ensure that the conductivity of the first to ensure that the gasket surface smooth, clean and by the necessary treatment to have good conductivity, these surfaces must be covered before joining; in addition to shielding liner material for such gaskets have a consistently good adhesion is also very important. The compressible nature of the conductive backing compensates for any irregularities in the gasket.
All gaskets have a minimum contact resistance for effective operation, and designers can increase the compression of the gasket to reduce the contact resistance of multiple gaskets, which of course increases the sealing strength and can make the shield more curved. Most gaskets work better when compressed to 30% to 70% of their original thickness. Thus within the recommended minimum contact surface, the pressure between two opposing dimples should be sufficient to ensure good electrical conductivity between the liner and the gasket.
On the other hand, the pressure on the gasket should not be so great as to place the gasket in abnormal compression, which would result in failure of the gasket contact and possible electromagnetic leakage. The requirement for separation from the gasket is important to keep the gasket compression within the manufacturer's recommendations, and this design needs to ensure that the gasket is sufficiently stiff to avoid large bends between the gasket fasteners. In some cases, additional fasteners may be required to prevent bending of the enclosure structure.
Compressibility is also an important characteristic of rotating joints, such as in locations like doors or inserts. If the liner is easily compressed, the shielding performance will degrade with each rotation of the door, and the liner will need to be compressed more to achieve the same shielding performance as a new liner. This is unlikely to be possible in most cases, so a long-term EMI solution is needed.
Adding an EMI liner won't be much of a problem if the shield or spacer is made of plastic coated with a conductive layer, but designers have to consider that many liners wear out on their conductive surfaces, and often the plated surfaces of metal liners are more susceptible to wear. Growing this wear over time can reduce the shielding efficiency of the pad joints and cause problems for the manufacturer behind them.
If the shield or gasket structure is metal, then a liner can be added to wrap the gasket surface before spraying the polished material, using only conductive film and tape rolls. If tape is used on both sides of the spliced gasket, the EMI liner can be fastened with mechanical fixtures, such as a "Type C" liner with plastic rivets or pressure-sensitive adhesive (PSA). The liner adheres to one side of the gasket to complete the EMI shielding.
Padding and accessories
There is a wide range of shielding and padding products available today, including beryllium-copper connectors, metal mesh wires (with or without elastic cores), metal mesh and directional wires embedded in rubber, conductive rubber, and urethane foam pads with metallic coatings. Most shielding material manufacturers can provide an estimate of the SE that can be achieved with various types of liners, but it is important to remember that SE is a relative value that also depends on porosity, liner size, liner compression ratio, and material composition. Liners come in a variety of shapes and can be used in a variety of specific applications, including abrasive, sliding, and hinged applications. Many of today's liners are available with adhesive or with fixing devices right on top of the liner, such as extrusion inserts, leg inserts, or barbed devices.
Of the various types of gaskets, coated foam gaskets are one of the newest and most versatile on the market. These gaskets are available in a variety of shapes and thicknesses greater than 0.5mm, and can be reduced to meet UL flammability and environmental sealing standards. There is also a new type of liner, the hybrid environmental/EMI liner, which eliminates the need for a separate sealing material, thus reducing the cost and complexity of the shield. The outer cladding of these gaskets is UV stabilized and resistant to moisture, wind and cleaning solvents, while the inner coating is metallized and highly conductive. Another recent innovation is the inclusion of a plastic clip on the EMI liner, which has become more marketable due to its lighter weight, shorter assembly time, and lower cost compared to traditional pressed metal liners.
Conclusion
Devices generally need to be shielded because there are slots and gaps in the structure itself. The required shielding can be determined by some basic principles, but there are differences between theory and reality. For example, when calculating the size and spacing of pads at a given frequency, the strength of the signal must also be taken into account, as is the case when multiple processors are used in a single device. Surface finish and gasket design are key factors in maintaining long-term shielding for EMC performance.
Electromagnetic wave-absorbing materials utilize the phenomenon of increased loss in soft magnetic ferrites at high frequencies to achieve the purpose of absorbing electromagnetic waves. In the practical application of engineering, in addition to the requirements of the material in a very wide (from RF to microwave) frequency range has a high electromagnetic wave energy absorption rate, but also requires the material mechanical strength, thin coating, light weight, temperature and humidity, radiation and corrosion resistance. Currently practical materials are Ni-Zn ferrite and hexagonal crystal system ferrite and so on. In engineering applications, in order to improve the absorption rate of electromagnetic waves and extend the absorption frequency range, more made of metal doped short fibers and organic polymer materials complex. In addition, in order to overcome the early absorption materials parasitic on the surface of the radar target thus increasing the weight of the shortcomings of the recent foreign countries have also developed a new type of absorption materials and engineering plastics composite and new structural type of load absorbing materials, can be used for aircraft engine fairing, has been in the U.S. F-111 fighters on the operation of tens of thousands of hours. North America Rockwell also for the jet engine air intake developed into a complex honeycomb structure type absorber, has a high absorption rate and mechanical strength. The shape of the absorber material Sharp split shape. Microwave darkroom using the absorber is often made into a splintered shape, it is in the foam doped with carbon powder and then wrapped in a layer of high-strength foam as a protective layer, so that even if the absorber is subjected to external collision will not be damaged. However, the frequency is reduced (wavelength growth), the length of the absorber is also greatly increased, the ordinary spiked shaped absorber has an approximate relationship between L / λ ≈ 1, so in 100MHz, the spiked length of up to 3cm; in 60MHz, the spiked length of up to 5cm, which is not only difficult to realize the process, but also the microwave darkroom space available for effective and greatly reduced. Single-layer plate shape. The earliest developed foreign absorber is a single-layer flat plate shape, later made of the absorber are directly attached to the metal shielding layer, its thickness is thin, light weight, but the working frequency range is narrower. Double-layer or multi-layer plate shape. This kind of absorber can work in a wide range of operating frequencies, and can be made into any shape. Such as Japan's NEC will be ferrite and metal short fiber uniformly dispersed in the appropriate organic polymer resin made of composite materials, the working band can be as wide as 40-50% or so. The disadvantage is that the thickness is large, complex process, high cost. Coating shape. On the surface of the aircraft can only be used in the form of coating absorbing materials, in order to broaden the frequency band, are generally used in composite materials coating. Such as lithium cadmium ferrite coating thickness of 2.5-5m m, in the centimeter band can be attenuated 8.5dB; spinel ferrite coating thickness of 2.5m m, in the 9GHz can be attenuated 24dB; ferrite plus neoprene coating thickness of 1.7-2.5m m, in the 5-10GHz attenuation up to 30dB or so. Structure shape. Absorbent material doped into engineering plastics so that it has both absorption characteristics and load capacity, which is a direction of the development of absorbent materials. In recent years, in order to further improve the performance of the absorbing material, foreign countries have also developed a complex shape by the combination of several shapes of the absorber. Such as Japan's microwave darkroom made of this type of absorber, its performance is: 136MHz, 25dB; 300MHz, 30dB; 500MHz, 40dB; 1-40GHz, 45dB. Engineering applications of absorbing materials in the increasingly important stealth and electromagnetic compatibility (EMC) technology, electromagnetic wave absorbing materials, the role and status of very prominent, has become a modern electronic military. In the increasingly important stealth and electromagnetic compatibility (EMC) technology, the role and status of electromagnetic wave absorbing materials is very prominent, has become a modern military electronic countermeasures treasure and "secret weapon". Its engineering applications include: Stealth technology. In the aircraft, missiles, tanks, ships, warehouses and other equipment and military facilities coated with absorbing materials, you can absorb the reconnaissance waves, attenuation of reflected signals, thus breaking through the enemy radar defense zone, which is a powerful means of anti-radar reconnaissance to reduce the weapons system suffered infrared guided missiles and laser weapons attack a method. In addition, electromagnetic wave-absorbing materials can also be used to conceal landing lights and other airport navigation equipment and other ground equipment, ship masts, decks, submarine periscope mounts and ventilation ducts and other equipment. Improve the performance of the aircraft. False signals generated by the aircraft fuselage on the electromagnetic wave reflection may lead to highly sensitive airborne radar false interception or false tracking; an aircraft or a ship on several radars working at the same time, radar transceiver antennae crosstalk between the sometimes very serious, the aircraft or the ship's own jammer will also interfere with the self-borne radar or communications equipment ....... In order to reduce such interference, foreign countries often apply absorbent materials excellent magnetic shielding to improve the performance of radar or communications equipment. Such as in the radar or communication equipment fuselage, antenna and all around the interference on the coating of absorbing materials, can make them more sensitive, more accurate to find enemy targets; in the radar parabolic antenna openings around the wall coated with absorbing materials, can reduce the sub-flap on the main flap of the interference and increase the role of the transmitter antenna distance, the receiving antenna is to play a role in reducing the false target reflection of the interference effect; in the application of the absorption of the satellite communication system The application of absorbing materials in the satellite communication system will avoid the interference between the communication lines, improve the sensitivity of the satellite communication machine and the ground station, and thus improve the quality of communication. Safety Protection. Due to the application of high power radar, communication machine, microwave heating and other equipments, preventing electromagnetic radiation or leakage and protecting the health of the operators is a new and complicated subject, and absorbing materials can achieve this purpose. In addition, the current household appliances are generally electromagnetic radiation problems, through the rational use of absorbing materials and their components can also be effectively suppressed. Microwave darkroom. The space formed by the wall of the absorber decoration is called the microwave darkroom. In the dark room can form the equivalent of non-reflective free space (no noise zone), reflected back from the surrounding electromagnetic waves than direct electromagnetic energy is much smaller, and negligible. Microwave darkroom is mainly used for radar or communication antennas, missiles, aircraft, spaceships, satellites and other characteristics of the impedance and coupling measurements, astronauts with back-shoulder antenna direction map measurements, as well as the installation of spaceships, tests and adjustments, etc., which not only eliminates the outside world clutter interference and improve the measurement of accuracy and efficiency (indoor work can be 24 hours a day), but also to protect the secret.