Shielding methods for electromagnetic interference

Electromagnetic compatibility (EMC) refers to "the performance of a device, equipment or system that allows it to work normally in its own environment and does not cause strong electromagnetic interference to any other equipment in this environment (IEEE C63.12-1987)." For wireless transceivers, the use of non-continuous spectrum can partially achieve EMC performance, but many related examples also show that EMC is not always possible. For example, there is high-frequency interference between laptops and test equipment, between printers and desktop computers, and between cellular phones and medical instruments. We call this interference electromagnetic interference (EMI).

Sources of EMC Problems

All electrical and electronic equipment will have intermittent or continuous voltage and current changes when working, and sometimes the rate of change is quite fast, which will cause electromagnetic energy to be generated within different frequencies or between frequency bands, and the corresponding circuit will emit this energy into the surrounding environment.

There are two ways for EMI to leave or enter a circuit: radiation and conduction. Signal radiation leaks out through the seams, slots, openings or other gaps in the housing; while signal conduction leaves the housing by coupling to the power, signal and control lines and radiates freely in the open space, thus generating interference.

Many EMI suppression methods are achieved by combining shell shielding and gap shielding. Most of the time, the following simple principles can help achieve EMI shielding: reduce interference at the source; isolate the interference-generating circuits through shielding, filtering or grounding, and enhance the anti-interference ability of sensitive circuits. EMI suppression, isolation and low sensitivity should be the goals of all circuit designers, and these performances should be completed early in the design stage.

For design engineers, using shielding materials is an effective way to reduce EMI. A variety of enclosure shielding materials are now widely used, from metal cans, thin metal sheets and foils to spray coatings and platings (such as conductive paint and zinc wire spray) on conductive fabrics or tapes. Whether it is metal or plastic coated with a conductive layer, once the designer has determined the enclosure material, he can start to select the gasket.

Metal shielding efficiency

The shielding efficiency (SE) can be used to evaluate the suitability of the shielding cover. Its unit is decibel and the calculation formula is:

SE _dB =A+R+B

in

A: Absorption loss (dB)

R: Reflection loss (dB)

B: Correction factor (dB) (applicable to situations where there are multiple reflections in a thin shield)

A simple shielding cover can reduce the electromagnetic field strength to one tenth of the original value, that is, SE is equal to 20dB; while some occasions may require the field strength to be reduced to one hundred thousandth of the original value, that is, SE is equal to 100dB.

Absorption loss refers to the amount of energy loss when electromagnetic waves pass through the shielding cover. The absorption loss calculation formula is:

A _dB =1.314(f×σ×μ) ^1/2 ×t

in

f: frequency (MHz)

μ: magnetic permeability of copper

σ: electrical conductivity of copper

t: shielding cover thickness

The magnitude of the reflection loss (near field) depends on the nature of the source of the electromagnetic wave and the distance from the wave source. For a rod-shaped or linear transmitting antenna, the closer to the wave source, the higher the wave resistance, and then decreases as the distance from the wave source increases, but the plane wave resistance does not change (constantly 377).

On the contrary, if the wave source is a small coil, the magnetic field will be dominant at this time, and the closer to the wave source, the lower the wave resistance. The wave resistance increases with the distance from the wave source, but when the distance exceeds one-sixth of the wavelength, the wave resistance no longer changes and remains constant at 377.

The radiation loss varies with the ratio of the wave resistance to the shield impedance, so it depends not only on the type of wave but also on the distance between the shield and the wave source. This applies to small shielded equipment.

Near-field reflection loss can be calculated as follows:

R (electricity) _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)]

in

r: The distance between the wave source and the shield.

The last term of the SE formula is the correction factor B, which is calculated as follows:

B = 20lg [-exp (-2t/σ)]

This formula is only applicable to near magnetic field environments and when the absorption loss is less than 10dB. Since the shielding absorption efficiency is not high, the internal re-reflection will increase the energy passing through the other side of the shielding layer, so the correction factor is a negative number, indicating the decrease in shielding efficiency.

EMI Suppression Strategies

Only materials with high magnetic permeability such as metal and iron can achieve high shielding efficiency at extremely low frequencies. The magnetic permeability of these materials will decrease with increasing frequency. In addition, if the initial magnetic field is strong, the magnetic permeability will also decrease. In addition, the use of mechanical methods to make the shielding cover into a specified shape will also reduce the magnetic permeability. In summary, the selection of high magnetic permeability materials for shielding is very complicated, and solutions are usually sought from EMI shielding material suppliers and relevant consulting agencies.

In high-frequency electric fields, using a thin layer of metal as the outer shell or lining material can achieve good shielding effects, but the condition is that the shielding must be continuous and completely cover the sensitive parts without gaps or cracks (forming a Faraday cage). However, in practice, it is impossible to make a shield without seams and gaps. Since the shield is made in multiple parts, there will be gaps that need to be joined. In addition, holes are usually punched in the shield to install the connection with the plug-in card or assembly components.

The difficulty in designing a shield is that the apertures created during the manufacturing process are inevitable, and these apertures will be needed during the operation of the equipment. Manufacturing, panel wiring, vents, external monitoring windows, and panel mounting components all require holes in the shield, which greatly reduces the shielding performance. Although grooves and gaps are inevitable, it is beneficial to carefully consider the groove length related to the wavelength of the circuit operating frequency in the shield design.

The wavelength of an electromagnetic wave of any frequency is: Wavelength (λ) = speed of light (C) / frequency (Hz)

When the slot length is half the wavelength (cutoff frequency), the RF wave begins to attenuate at a rate of 20dB/10 (1/10 cutoff frequency) or 6dB/8 (1/2 cutoff frequency). Generally, the higher the RF transmission frequency, the more severe the attenuation is because its wavelength is shorter. When it comes to the highest frequencies, any harmonics that may appear must be considered, but in practice only the first and second harmonics need to be considered.

Once the frequency and intensity of the RF radiation in the shield are known, the maximum allowable gaps and grooves of the shield can be calculated. For example, if the radiation at 1GHz (wavelength is 300mm) needs to be attenuated by 26dB, a gap of 150mm will begin to attenuate, so when there is a gap less than 150mm, the 1GHz radiation will be attenuated. So for the 1GHz frequency, if 20dB attenuation is required, the gap should be less than 15 mm (1/10 of 150mm), when 26dB attenuation is required, the gap should be less than 7.5 mm (more than 1/2 of 15mm), and when 32dB attenuation is required, the gap should be less than 3.75 mm (more than 1/2 of 7.5mm).

This attenuation effect can be achieved by using suitable conductive pads to limit the gap size to a specified size.

Difficulties in shielding design

Since the seams reduce the conductivity of the shield, the shielding efficiency will also be reduced. It should be noted that the attenuation of radiation below the cutoff frequency depends only on the length-to-diameter ratio of the gap, for example, a length-to-diameter ratio of 3 will give 100 dB of attenuation. When perforations are required, the waveguide properties of the small holes in the thick shield can be used; another way to achieve a higher length-to-diameter ratio is to attach a small metal shield, such as a gasket of appropriate size. The above principles and their extension to the case of multiple seams form the basis for the design of porous shields.

Porous thin shielding layer: There are many examples of porous, such as ventilation holes on thin metal sheets, etc. When the holes are close together, the design must be carefully considered. The following is the shielding efficiency calculation formula for this case

SE = [20lg (f _c/o /σ)] - 10lg n

in

f _c/o : Cut-off frequency

n: number of holes

Note that this formula is only applicable when the hole spacing is smaller than the hole diameter, and can also be used to calculate the relative shielding efficiency of metal braided mesh.

Seams and joints: Electric welding, brazing or soldering are common ways to permanently fix thin sheets. The metal surface of the joint must be cleaned so that the joint can be completely filled with conductive metal. It is not recommended to fix with screws or rivets because the low-resistance contact state at the joint between the fasteners is not easy to maintain for a long time.

The function of conductive gasket is to reduce the slots, holes or gaps in the seams or joints so that RF radiation will not be emitted. EMI gasket is a conductive medium used to fill the gaps in the shielding cover and provide a continuous low impedance connection. Usually EMI gasket can provide a flexible connection between two conductors, so that the current on one conductor can be passed to another conductor.

The selection of sealing EMI gaskets can refer to the following performance parameters:

Shielding effectiveness in a specific frequency range

Installation method and sealing strength

Current compatibility with the enclosure and corrosion resistance to the external environment.

Operating temperature range

cost

Most commercially available gaskets provide adequate shielding performance to enable the device to meet EMC standards; the key is to properly design the gasket within the shielding enclosure.

Gasket system: An important factor to consider is compression, which can produce higher conductivity between the liner and the gasket. Poor conductivity between the liner and the gasket will reduce the shielding efficiency. In addition, if a piece is missing at the joint, a fine gap will appear, forming a slot antenna, and its radiation wavelength is about 4 times smaller than the gap length.

To ensure continuity, the gasket surface must be smooth, clean and treated to have good conductivity. These surfaces must be masked before bonding. It is also very important that the shielding gasket material has a continuous good adhesion to the gasket. The compressible nature of the conductive gasket can compensate for any irregularities in the gasket.

All gaskets have a minimum contact resistance for effective operation. Designers can increase the compression force on the gasket to reduce the contact resistance of multiple gaskets. Of course, this will increase the seal strength and make the shield more curved. Most gaskets work best when compressed to 30% to 70% of their original thickness. Therefore, within the recommended minimum contact area, the pressure between two facing dimples should be sufficient to ensure good conductivity between the gasket and the gasket.

On the other hand, the pressure on the gasket should not be so great that the gasket is in an abnormal compression state, which will cause gasket contact failure and possible electromagnetic leakage. The requirement for separation from the gasket is very important to control the gasket compression within the manufacturer's recommended range. This design needs to ensure that the gasket is stiff enough to avoid significant bending between the gasket fasteners. In some cases, additional fasteners may be required to prevent bending of the housing structure.

Compressibility is also an important characteristic in rotating joints, such as in doors or gates. If the gasket is easily compressed, the shielding performance will decrease with each rotation of the door, and the gasket will need a higher compression force to achieve the same shielding performance as a new gasket. In most cases this is unlikely to be possible, so a long-term EMI solution is needed.

If the shield or gasket is made of plastic coated with a conductive layer, adding an EMI gasket will not cause too many problems, but designers must consider that many gaskets will wear on the conductive surface, usually the plated surface of the metal gasket is more susceptible to wear. Over time, this wear will reduce the shielding effectiveness of the gasket joint and cause problems for subsequent manufacturers.

If the shield or gasket structure is metal, a liner can be added to cover the gasket surface before spraying the polishing material, which only requires conductive film and tape. If tape is used on both sides of the mating gasket, the EMI gasket can be fastened with mechanical fasteners, such as a "C-type" gasket with plastic rivets or pressure-sensitive adhesive (PSA). The gasket is installed on one side of the gasket to complete the EMI shielding.

Pads and accessories

There are many shielding and gasketing products available, including beryllium-copper connectors, metal mesh wire (with or without elastic core), metal mesh and oriented wire embedded in rubber, conductive rubber, and polyurethane foam gaskets with metal coatings. Most shielding material manufacturers can provide estimates of the SE that can be achieved with various gaskets, but remember that SE is a relative value and depends on the pores, gasket size, gasket compression ratio, and material composition. Gaskets come in a variety of shapes and can be used for various specific applications, including wear, sliding, and hinged applications. Many gaskets currently have adhesives or have fixing devices on the gasket, such as extrusion inserts, pin inserts, or barb devices.

Of the various types of gaskets, coated foam gaskets are one of the newest and most widely used products on the market. These gaskets are available in a variety of shapes and thicknesses greater than 0.5mm, or reduced to meet UL flame and environmental sealing standards. Another new type of gasket is the environmental/EMI hybrid gasket, which eliminates the need for a separate sealing material, thereby reducing the cost and complexity of the shield. These gaskets have an outer coating that is UV-stable and resistant to moisture, wind, and cleaning solvents, while the inner coating is metallized and highly conductive. Another recent innovation is a plastic clip on the EMI gasket, which is lighter, has a shorter assembly time, and is less expensive than traditional pressed metal gaskets, making it more attractive to the market.

in conclusion

Devices generally require shielding because of the inherent slots and gaps in the structure. The required shielding can be determined by some basic principles, but there is a difference between theory and reality. For example, the size and spacing of the pads at a certain frequency must also take into account the strength of the signal, as is the case when multiple processors are used in a device. Surface treatment and gasket design are key factors in maintaining long-term shielding to achieve EMC performance.