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 charge, current 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.
1. Conductive elastomer lining material (conductive rubber)
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-ring/O-ring, various flanges, I/O gasket.
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 or Monel wire (nickel copper alloy) Ferrex (tinned It is composed of copper-clad steel wire) and has good conductivity. It meets the flame retardant grade (UL94-V0) and has 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 application The broadest shielding material.
Average shielding performance: 90dB
Surface resistance: above 0.07/m2
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. The temperature range is -55°C-+260°C.
The clip-on EMI lining 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 is fixed on the anticorrosive stainless steel spring clip for easy installation. . No holes or fasteners are required.
Beryllium copper reed is a finger-shaped reed made of special alloy beryllium copper, which combines high-grade 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 by doping metal fillers such as silver particles or silver-plated copper particles 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 to permanent seams of shielding .
(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) The 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; radio, television, and computer industries; automotive industry; medical equipment; 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-the rising star of Weifeng conductive adhesive 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 the entire 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 Ω/sq, which can provide 60-80dB shielding for the chassis 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 and other forms of energy through mechanisms such as hysteresis loss, dielectric loss, and resistance loss. To 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 transparent screen film for LED video window, and professional flat cable EMI shielding Sleeve, shielding braided wire mesh belt for ordinary cables, etc.