electromagnetic compatibility (Electromagnetic Compatibility) refers to aElectronic equipmentIt will not interfere with or be affected by other equipment.Electromagnetic compatibility is the same as the safety we are familiar withproduct qualityOne of the most important indicators.Safety involves personal and property, while electromagnetic compatibility involves personal and environmental protection.
The interference of electronic components to the outside world is called EMI (Electromagnetic Interference);electromagnetic waveIt will interact with electronic components to generateInterference phenomenon, called EMS (Electromagnetic Susceptibility).For example, TVFluorescent screenThe common "snowflake" on indicates that the received signal is disturbed.
Electromagnetic shielding is used to weaken the electromagnetic shielding caused by certain sources in a certain area of spacefield strengthMeasures.In most cases,ShieldIt can be made of copper, aluminum, steel and other metals, but for constant andELFMagnetic field, also availableFerriteAnd other materials as shielding body.Within a system or between different systemsRegular meetingsproduceElectromagnetic noiseOr interferencesystem performance Deteriorate, so it is required that ①power lineorMagnetic line of forceLimited to a certain area; ②Make an area free from the influence of external power lines and magnetic force lines.
Chinese name
electromagnetic shielding
Interpretation
Measures to weaken the field strength caused by some sources in a certain area of space
electromagnetic compatibility (Electromagnetic Compatibility) is an abbreviation for EMC, which meansElectronic equipmentIt will not interfere with or be affected by other equipment.Electromagnetic compatibility is the same as the safety we are familiar withproduct qualityOne of the most important indicators.Safety involves personal and property, while electromagnetic compatibility involves personal and environmental protection.
The interference of electronic components to the outside world is called EMI (Electronic Interference);electromagnetic waveIt will interact with electronic components to generateInterference phenomenon, called EMS (Electromagnetic Susceptibility).For example, TVFluorescent screenThe common "snowflake" on indicates that the received signal is disturbed.
becauseShieldAbsorbs energy for external interference electromagnetic waves and internal electromagnetic waves from wires, cables, components, circuits or systems（Eddy current loss）, reflected energy (reflection of electromagnetic wave on the interface of the shield) and offset energy (electromagnetic inductionShielding layerReverse onelectromagnetic field, can counteract part of the interference electromagnetic wave), so the shield has the function of reducing interferenceWhen the frequency of the interfering electromagnetic field is high, the lowresistivityOfMetallic materialsThe eddy current generated in theOffsetting effectSo as to achieve the shielding effect. ⑵When the frequency of interfering electromagnetic wave is low, highPermeabilityTo makeMagnetic line of forceIt is confined inside the shield to prevent it from spreading to the shielded space. ⑶In some cases, if a good shielding effect is required for both high-frequency and low-frequency electromagnetic fields, different metal materials are often used to form a multi-layer shield.[1]
principle
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Many people do not understand the principle of electromagnetic shielding, and think that as long as a box is made of metal and then grounded, it can play the role of electromagnetic shielding.Under the guidance of this concept, the result is failure.Because electromagnetic shielding has nothing to do with whether the shield is grounded or not.There are only two factors that really affect the shielding effectiveness of the shield: one is that the entire shield surface must be conductive and continuous, and the other is that there must be no conductors that directly penetrate the shield.There is a lot of conduction on the shieldDiscontinuityThe most important type is the non-conductive gap formed at the junction of different parts of the shield.These non-conductive gaps are createdElectromagnetic leakage, as if the fluid would leak from the gap on the container.One way to solve this leakage is to fill the gap with conductive elastic materials to eliminate non-conductive points.This is just like filling the gap of the fluid container with rubber.This elastic conductionFiller materialIt's electromagnetismSealing gasket。In many literatures, electromagnetic shielding body is compared to a liquid sealed container. It seems that electromagnetic wave leakage can be prevented only when the gap is sealed to a watertight level with conductive elastic materials.In fact, this is not accurate.Because whether the gap or hole will leak electromagnetic waves depends on the size of the gap or hole relative to the electromagnetic wave length.When wavelengthMuch greater thanThe size of the opening will not cause obvious leakage.[1]
mechanism
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a、When the electromagnetic wave reaches the shield surface, due to the discontinuity of impedance on the interface between air and metalincident waveGenerated reflections.This reflection is not requiredShielding materialThere must be a certain thickness, as long as the discontinuity on the interface is solved;b、The energy that is not reflected off the surface but enters the shield is attenuated by the shielding material in the process of forward propagation in the body.It is also called absorption;c、When the residual energy that has not been attenuated in the shielding body is transferred to another surface of the material, it will form re reflection when encountering the interface of discontinuous metal air impedance and return to the shielding body again.This reflection may have multiple reflections at the interface of two metals.In a word, the attenuation of electromagnetic shield is mainly based on the reflection and absorption of electromagnetic waves.
Figure 1 Application of Electromagnetic Shielding Materials
Now there are many questions about product radiation andConducted emissionRestrictednational standardandinternational standard。Some also stipulate the minimum interferenceSensitivityrequirement.Generally, there are different standards for different types of electronic equipment.Although meeting these standards is necessary for a product to succeed in the market, meeting these standards is voluntary.
However, some countries give specifications rather than standards, so they need to sell products in these countries,Compliance with standardsIt is mandatory.Some specifications not only stipulate standards, but also give the authority to confiscate nonconforming products.[1]
According to the position of the interference source relative to the shield (inside or outside the shield), it can be divided into active shielding and passive shielding.If the shield is used to prevent interference fields from entering the shielded spaceShielding structureIt is called passive shielding.Several interference sources are inside the shield, which is used to prevent interference fields from leaking toExternal spaceIs called active shielding.Active shielding is not suitable for high frequencies, but for low frequencies.The passive shield is often used in situations where the shielding object is far away from the interference source, such asShield roomEtc.
Shield
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According to different shielding purposes, shielding can be divided into electrostatic shielding, magnetic shielding and electromagnetic shielding.
Electrostatic shield: It is made of diamagnetic materials (such as copper and aluminum) and connected to the ground.The role of the electrostatic shield is to terminate the electric field on the metal surface of the shield and transfer the charge to the ground.
Electromagnetic shield: mainly used to stopHigh frequency electromagnetic fieldThe interference field will form eddy current in the shielding body and reflect on the interface between the shielding body and the protected space, thus greatly weakening the interference field in the protected spacefield strengthValue, the shielding effect is achieved.Sometimes, in order to enhance the shielding effect, multi-layer shielding can also be used, and its outer layer is generallyconductivityHigh material to increase reflection, while the inner layer is made of high permeability material to increaseEddy current effect。
If there are holes or gaps on the shield, the shielding effect will be directly reduced.The higher the frequency, the more significant this phenomenon is.
In the space protection zone, the ratio of the field strength (E0 or H0) with shield and the field strength (E or H) without shield is called shielding coefficient.The smaller the shielding coefficient, the better the shielding effect.
The shielding effect can also be represented by shielding attenuation, which representsInterference field strengthAttenuation value through the shield.Shielding attenuation can be determined by
or
Seek - UnitsDecibel（dB）。The larger the shielding attenuation value, the better the shielding effect.
Scope of application
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Figure 2 Electromagnetic shielding
Notebook computer、GPS, ADSL, mobile phone, etc3C productsMetropolisHigh frequency electromagnetic waveInterference generationNoise, affecting communication quality.In addition, if the human body is exposed to strong electromagnetic fields for a long time, it may be vulnerablecancerpathological changes.Therefore, anti electromagnetic interference is a necessary and imperative process.Conductive paintEMI conductive paint spraying technology has highConductivityHigh electromagnetic shielding efficiency, simple spraying operation (just spray a thin layer of conductive paint inside the plastic shell as the surface painting operation), etc., widely used in communication products (mobile phones), computers (laptops), portable electronic productsConsumer Electronics、Network hardwareEMI shielding of (servers, etc.), medical instruments, household electronic products, aerospace and national defense electronic equipment.TF-801 conductive paint of Tengfei Metal Technology Co., Ltd. is applicable to all kinds ofPlastic productsShielding of（PC、PC+ABS, ABS, etc.).Spraying conductive paint solves the problem of metalShieldLimited by space constraints, operation and cost pressures, the conductive paint has attracted more and more attention and promotion because of its extremely simple spraying operation and metallization of plastics.Gradually replaced the previous stickersTinfoilCopper paper, the process of making metal shield.
shieldConductive paintIt is a kind of paint that can be used for spraying. After drying and forming a paint film, it can conduct electricity, thus shielding electromagnetic interference
Shielding is the metal isolation between two space areas to control the induction and radiation of electric field, magnetic field and electromagnetic wave from one area to another.Specifically, the shielding is used to connect the components, circuitsAssembly, cable or the whole systemInterference sourceSurround it to prevent the interference electromagnetic field from spreading outward;Enclose the receiving circuit, equipment or system with a shield to prevent them from being affected by external electromagnetic fields.
Conductive paint is used to conduct electricitymetal powder It is added to specific resin raw materials to make paint coatings that can be sprayed
The following specific types of products are widely used: those added with silver metal powder are called "TF-801 silver conductive paint";add tocopper powderTF-609 copper conductive paint;add tonickelThe powder is "TF-606 nickel conductive paint";Among them, "TF-801 silver copper conductive paint" is the most widely used, because it is much cheaper than TF-828 silver conductive paint and much better than TF-606 nickel conductive paint. Specific indicators can be referred to as follows: [1]
Copper conductive paint
Product No.: TF-801
Package specification: 20kg/barrel
Product Introduction
TF-801 is a silver copper conductive paint.It is directly sprayed inside the shell of electronic products, and the film can be well shielded after dryingelectromagnetic radiationAnti electromagnetic interference function.It is widely used in GPSDVD、DVB, medical devices and other electronic products.
TF-828 is a pure silver conductive paint.Directly sprayed inside the shell of electronic products, the paint film can play a good role in shielding electromagnetic radiation and resisting electromagnetic wave interference after drying.Extremely fine silver conductive paint film and excellent conductivity are suitable for high-end electronic products
Coating characteristics
Color: silver white
Specific gravity: 1.3 ± 0.05 (ASTMD1475-98)
Viscosity: thixotropic mixture
Solid content: 45 ± 1%
Theoretical coverage rate: 12mtwo/Kg (paint film thickness=13 microns)
technical parameter
Conductive effect: ≤ 0.015 ohm/cm2 (paint film thickness is not less than 13 microns)
Applied conductive parameters: 1 ohm/15 μ m film thickness/distance 10cm
Recommended film thickness: 15 microns (ASTMD4138-94)
Test conditions
The measured results shall be removedA multimeterThe resistance of itself and the conductor (i.e. the base price) is the conductive value of TF-828 paint film.
Guidelines
Use materials
ABS、HIPS、PS, PC and other plastic parts (if there are special materials, the company will prepare applicable conductive paint for customers)
Diluent: ordinary Tianna water (boiled oil-water)
Oil opening ratio: 1:0.5 (weight ratio)
Drying conditions:
The drying time of the meter is 15 minutes, and the drying time is 65 ± 5 ℃/30 ± 10 minutes
matters needing attention
Before use, the paint shall be fully stirred in the paint tank before use.The silver metal particles are evenly dispersed after stirring.
Product: Nickel conductive paint
Product No.: TF-606
Package specification: 4kg/can
Product introduction:
TF-606 is a nickel metal particle conductive coating.Directly sprayed inside the shell of electronic products, the paint film can play a very good role in shielding electromagnetic radiation, anti electromagnetic interference and demagnetization after drying.Because of its special degaussing function, it is widely used in the inside of the medical device shell.
Color: dark grey, black grey or silver grey
Specific gravity: 1.4 ± 0.05 (ASTMD1475-98)
Viscosity: thixotropic mixture
Solid content: 48 ± 1%
Theoretical coverage rate: 6-8mtwo/Kg (paint film thickness=25 microns)
technical parameter
Conductive effect: ≤ 0.25 ohm/cm2 (paint film thickness is not less than 25 μ m)
Applied conductive parameters: 2 Ω/25 μ m film thickness/distance 10cm
Recommended film thickness: 25-30 μ m (ASTMD4138-94) [1]
Test conditions
When measuring, the sprayed film must be completely cooled and dried.Should usedigital displayMeasure with a multimeter (generally, the gear of the multimeter should be adjusted to<200 Ω). The measured result should be the conductive value of TF-606 paint film after removing the resistance of the multimeter itself and the wire (i.e. the base mark).
Guidelines
Use materials
ABS, HIPS, PS, PC and other plastic parts
Diluent: ordinary Tianna water (boiled oil-water)
Oil opening ratio: 1:0.5 (weight ratio)
Drying conditions:
The drying time of the meter is 15 minutes, and the drying time is 65 ± 5 ℃/30 ± 10 minutes
matters needing attention
Before use, the paint shall be fully stirred in the paint tank before use.The nickel metal particles are evenly dispersed after stirring, and the sprayed paint film can achieve the conductivity.After the paint is diluted, it is better to mix it frequently for at least 5min to achieve the best conductive effect after coating;Diluted products should be used up as soon as possible to avoid long-term storage, because it is easy to precipitate after dilution, but it still does not affect the use characteristics after mixing.Protective measures: During use, ensure that the working environment is well ventilated to avoid long-termdirect contact Or inhalation, do not pour paint into the channel orsewerenvironment pollution.
