The shield is a tool for shielding electronic signals.The function is to shield the influence of external electromagnetic waves on internal circuits and the external radiation of internal electromagnetic waves.
Chinese name
Shield
Foreign name
screening can；shield case
Role
Shielding electronic signals
Composition
Consisting of outrigger and cover
Electromagnetic pollution
Power frequency alternating magnetic field, radio frequency electromagnetic radiation, etc
The shield cover is composed of outriggers and cover body, and the outriggers are movably connected with the cover body;The hood is spherical crown.
This part is mainly used in mobile phones, GPS and other fields to prevent electromagnetic interference (EMI) and shield components on PCB and LCM.
The shielding cover is generally made of 0.2mm thick stainless steel and white copper, of which white copper is a metal shielding material easy to tin.useSMT patchThe design of suction cup shall be considered.
Material application
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The shielding frame generally adopts Cu-C7521-H [general material], Cu-C7521-OH [soft material, for deep drawing] (nickel white copper, copper nickel zinc alloy),NickelSilver），t=0.2，0.3mm；The shielding cover is generally made of stainless steel SUS304R-1/2H [bending processing], SUS304R-1/4H [for deep drawing], t=0.15, 0.2mm, tinned steel strip (tinplate), etc;
Foreign white copper and tinplate can be used for welding on PCB, and it is recommended to use foreign white copper, mainly because foreign white copper is better in welding, heat dissipation and steam.
Design considerations
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The moving space of the tray where the shield cover is placed is too large, and it is easy to swing when pasting, so that the material can not be absorbed. The material must be placed in the tray, with a moving space of about 1.0mm. The moving space is too large, which causes the material to swing, and the material may not be taken up if it is too small.
The size of the reclaiming point of the shield cover should be appropriate. The reclaiming point should be in the middle of the material as far as possible. The size of the reclaiming point should be Φ 6.0mm. The larger the reclaiming point, the higher the stability and efficiency of the patch.
Magnetic shielding material
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Magnetic shielding material Material used for shielding magnetic field.There are three kinds of magnetic shielding: magnetostatic shielding, low-frequency electromagnetic shielding and high-frequency electromagnetic shielding.In practice, different shielding materials are selected according to different situations.
Magnetostatic shield: In order to concentrate the stray magnetic field on the shield shell, the shield should have the highest magnetic permeability.All high permeabilitySoft magnetic material, such as electromagnetic pure iron, ductile ironPermalloy, silicon steelSoft ferriteIn principle, they can be selected.The design shall be selected according to the requirements of magnetic screen, price, shell strength, processing performance, etc. after comprehensive consideration.
Low frequency electromagnetic shielding: In addition to shielding magnetostatic field, it also needs to shield the changing electromagnetic field.Such as power frequency electromagnetic field.In addition to high permeability, high conductivity is also required.The ideal material is permalloy. Considering the price and processing factors, electromagnetic pure iron is suitable for large shielding cover.When the nickel content in permalloy is higher than 40%, the permeability and conductivity are good.In addition, the aluminum bearingIron aluminum alloyIt is also a commonly used magnetic screen material.
High frequency electromagnetic shielding: For the main purpose of shielding electromagnetic waves, the principle of shielding is Lenz's Law. The induced electromagnetic field in the shield is used to offset external electromagnetic interference. Good conductors should be used to make the shield, such as aluminum and copper[1]。
Shielding effect
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Shielding is a device mode in electrotechnics.In order to prevent the interference of external electric field, magnetic field or electromagnetic field on internal equipment or to avoid the impact of electromagnetic field of equipment on the outside, the equipment is placed in a closed or nearly closed metal shell or metal mesh, which is calledShield。In structural chemistry, the term shielding is used to name the following effects.The repulsion of the inner electrons to the outer electrons in the atom reduces the effective attraction of the outer electrons in the atomic nucleus, which is equivalent to that the inner electrons "shield" part of the nuclear charge, so it is called the shielding effect.The shielding effect between electrons can be approximately expressed by the shielding coefficient[2]。
Prevention and control methods
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It refers to scientific and technical methods to prevent or protect electromagnetic radiation interference and injury.Artificial electromagnetic pollution sources include pulse, discharge, power frequency alternating magnetic field, radio frequency electromagnetic radiation, etc.The natural electromagnetic pollution is mainly the interference of lightning, volcanic eruption, earthquake and solar magnetic storm on short wave communication.The protection principle is to install electromagnetic shielding devices in the way of electromagnetic transmission, so as to reduce harmful electromagnetic fields to the allowable range.The shielding device is a closed shell made of metal materials. According to different objects and requirements, protective covers with different structures are used: ① For interference protection of small instruments or devices, copper aluminum compact shells are usedShield。②Low frequency electromagnetic interference, using iron or beryllium molybdenum alloy, etcmagnetic material Make the shell. ③For protection against interference of precision electromagnetic measurement at low temperature, superconducting material shield is used. ④For protection of large unit or control room, copper or steel plate is used to make hexahedral shielding room. ⑤Personal protection, with copper wire orMicrowave absorbing materialWork clothes, helmets and eye masks made of shielding.The comprehensive prevention and control measures and countermeasures mainly include: ① reasonable industrial layout to keep electromagnetic pollution sources away from dense residential areas. ②Improve electrical equipment to reduce pollution to the surrounding environment. ③Implement remote control and telemetry to reduce the chance for workers to contact high intensity electric field and prevent the body from being injured.The basic function of electromagnetic pollution prevention is to improve the stability and reliability of electronic instruments and electrical equipment in the environment of strong electromagnetic interference, and reduce the effect and harm of high intensity electromagnetic radiation on human body[3]。
