In today's society, with the development and popularization of electronic products, electromagnetic radiation hazards are becoming more and more serious in life, and electromagnetic shielding products have entered people's sight. The pursuit of shielding effectiveness is also a rating of product performance.
The principle of electromagnetic shielding is to use the shielding body to reflect, absorb and guide the electromagnetic energy flow, which is closely related to the charges, currents and polarization induced on the surface of the shielding structure and inside the shielding body. Electromagnetic shielding refers to the shielding of the electric field (E field), and it usually has more types of shielding materials available.
1. Conductive elastomer lining material (conductive rubber)
Each conductive rubber is made of silicone, silicone fluoride, EPDM or carbon fluoride-silicon fluoride and other adhesives and pure silver, silver-plated copper, silver-plated aluminum, silver-plated nickel, silver-plated glass, and silver-plated It is composed of conductive fillers such as lead or carbon particles.
Since these materials contain silver, packaging and storage conditions should be similar to other silver-containing components, and they should be stored in plastic plates, such as polyester or polyethylene, away from sulfur-containing materials. Standard shapes are: solid O-shaped strips, hollow O-shaped strips, solid D-shaped strips, hollow D-shaped strips, U-row strips, rectangular strips, hollow rectangular strips, hollow P-shaped strips, channel strips, and molded conductive rubber moldings, Molded D-rings/O-rings, various flanges, I/O gaskets.
Features: It can reach 90 dB-120dB in the range of 20M-20GHz, and it can even reach more than 120dB for pure silver particles. Can play the role of shielding and environmental sealing, easy to install, suitable for communications, medical, military, aviation and other occasions.
2. EMI conductive foam lining
Conductive foam is to wrap a conductive braided sleeve on a foam core made of polyurethane or EPDM. The conductive braided sleeve is usually made of silver-plated nickel nylon, aluminum foil or Monel wire (nickel copper alloy) Ferrex (tinned It is composed of copper-clad steel wire) and has good electrical conductivity. Meet the flame retardant grade (UL94-V0), with good elasticity and flexibility and other mechanical properties. Conductive foam liner has good shielding performance. When encountering electric waves, it will reflect and absorb according to the nature of the object, provide excellent shielding effect, and has a very high cost performance. It is currently the latest and also applied The broadest shielding material.
Average shielding performance: 90dB
Surface resistance: 0.07/m2 or more
Operating temperature: -40～70°C
Permanent compression deformation: 3%～10%, better can achieve 0% deformation, 10,000 times abrasion test
Features: low pressure, very good shielding effect, shielding performance over 90dB, elasticity, light weight, low surface contact resistance, wide selection of cross-sections, UL-class fire protection, and can be fixed by conductive PSA.
3. Metal EMI lining, clip lining and hook material beryllium copper reed
These varieties include woven wire mesh, wire mesh wound on elastomer core, oriented metal wire in elastomer and metal wire screen, which usually provide 60dB-100dB attenuation in the range of 20MHz-1GHz. It can be fixed in the groove of the box by conductive adhesive or mechanically.
It can also be oriented embedded in silicone by Monel or aluminum wire, which can not only provide EMI shielding, but also can be used as rainwater, water leakage, dust and ventilation seals, and the temperature range is -55°C-+260°C.
The clip-on EMI lining material is made of carbon-filled silicone strips or Monel wire on the silicone tape or Ferrex wire on the neoprene rubber to provide EMI shielding. The lining material is fixed on the anticorrosive stainless steel spring clip, which is easy to install. . No holes or fasteners are required.
The beryllium copper reed is a finger-shaped reed made of a special alloy of beryllium copper, which combines high-level EMI shielding effect and elastic hook rubbing with small sealing force characteristics. The high-performance parameters of beryllium copper: high tensile strength, good corrosion resistance and good electrical conductivity, making it an ideal EMI shielding material used in a wide frequency range, and can be used in a wide range of electronics with EMI/RFI or ESD problems. In the device. There are different finished products of bright copper, bright nickel and bright tin, which are installed on different metal surfaces to reduce electrochemical noise and electrical corrosion between different metals.
Spira metal spiral tube can be installed in trough type, and the metal tube can also be filled with rubber or silicone core.
Four, conductive compounding agent
Including conductive adhesives and conductive paints. Conductive adhesives are generally made of metal fillers such as silver particles or silver-plated copper particles that are doped into epoxy resin, silicone, and flexible polyisoethylene through a special process. It has the unique comprehensive properties of high shear strength, low thermal expansion coefficient, pure ions, good thermal and electrical conductivity. Typical applications include bonding EMI shielding channels, windows or light-transmitting conductive screen films on permanent seams of shielding. .
Conductive adhesive is an adhesive with certain conductive properties after curing or drying. It usually uses matrix resin and conductive fillers, namely conductive particles, as the main components. The conductive particles are bonded together by the bonding of the matrix resin to form a conductive material. The path realizes the conductive connection of the adhered material. Since the matrix resin of the conductive adhesive is an adhesive, an appropriate curing temperature can be selected for bonding. At the same time, due to the rapid development of the miniaturization and miniaturization of electronic components and the high density and high integration of printed circuit boards, The conductive adhesive can be made into a slurry to achieve high line resolution. Moreover, the conductive adhesive has a simple process, easy operation, and can improve production efficiency. Therefore, the conductive adhesive is an ideal choice for replacing lead-tin soldering and realizing conductive connection.
(1) Conductive adhesive is used for microelectronics assembly, including the connection of thin wires and printed circuits, electroplating bottom plates, metal layers of ceramic adherends, and metal chassis, bonding wires and bases, bonding components and planes passing through the printed circuits Hole, bonding waveguide tuning and hole repair.
(2) Conductive adhesive is used to replace spot welding where the welding temperature exceeds the withstand capability of forming an oxide film due to welding. As a substitute for tin-lead solder, conductive adhesives are mainly used in telephone and mobile communication systems; broadcasting, television, and computer industries; automotive industry; medical equipment; solving electromagnetic compatibility (EMC) and other aspects.
(3) Another application of conductive adhesive is to bond electrode sheets and magnet crystals in ferroelectric devices. Conductive adhesives can replace soldering flux and crystals that tend to deposit due to soldering temperature. The bonding of battery terminals is another use of conductive adhesives when the soldering temperature is unfavorable.
(4) The conductive adhesive can form a joint with sufficient strength, so it can be used as a structural adhesive.
(5) The application of conductive adhesive at home and abroad-Weifeng conductive adhesive is a rising star in the domestic market. The conductive adhesive used in some high-end areas in the domestic market is mainly imported: Ablistick and 3M in the United States have almost occupied all IC and LED fields , Japan’s Sumitomo and Taiwan’s Yihua are also involved in these areas. Japan's Three-Bond company controls the application of conductive glue in the entire quartz crystal resonator. Domestic conductive adhesives are mainly used in some middle and low-end products, and the market in this area is mainly occupied by the Institute of Metal Research.
Conductive paint is generally sprayed on various plastic casings such as ABS, Noryl, PVC, etc., which can provide EMI shielding, anti-static protection, corona shielding and surface grounding. Fillers are divided into silver, silver-coated copper, copper, nickel, etc., adhesives Usually epoxy resin, polyurethane, etc., the conductivity can reach 0.015 ohm/sq, which can provide 60-80dB shielding for the case in the range of 30MHz-1GHz.
Five, absorbing materials
Absorbing materials are generally made by mixing alloy powder with polymer resins through various processes and calendering them into flexible sheet materials, which are converted into heat, potential energy and other forms of energy through mechanisms such as hysteresis loss, dielectric loss, and resistance loss. Achieve the effect of shielding and absorbing electromagnetic waves.
In addition, other commonly used electromagnetic shielding materials include metal conductive shielding tape, shielding vent plate, VIP shielding air filter, EMI shielding glass window, shielding translucent screen film for LED video window, and professional flat cable EMI shielding Sleeve, shielding braided wire mesh belt for ordinary cables, etc.