Release time：2021-10-08
With the development of communication technology, the integration of equipment is getting higher and higher, and the electromagnetic compatibility inside the system is also upgraded accordingly. In order to solve the problem of electromagnetic radiation leakage and interference, electromagnetic shielding materials are generally used for shielding. The upgrading of 5g base station core technology and the increase in the number of base stations have led to a substantial increase in the demand for electromagnetic shielding materials. At present, the widely used electromagnetic shielding devices include conductive plastic devices, conductive silica gel, metal shielding devices, conductive cloth pads, microwave absorbing devices, etc.
1、 Electromagnetic shielding principle
The principle of electromagnetic shielding is to reflect and absorb the electromagnetic wave from the interference source. Generally speaking, the material reflects and guides the electromagnetic energy flow, and internally generates the current and magnetic polarization field opposite to the source electromagnetic field, thereby weakening the radiation effect of the source electromagnetic field, which is closely related to the charge, current and polarization induced on the surface of the shielding structure and inside the shielding body.
Generally, the materials used for electromagnetic shielding have certain conductivity. In our impression, plastic is generally used as insulating material. How can conductive plastic be used for electromagnetic shielding? Let's have a brief look first.
2、 Introduction to conductive plastics
In fact, if plastic is processed to a certain extent, such as forming a composite system with island structure between plastic matrix and conductive filler, electrons can have the ability of particle to particle transition with the help of quantum mechanical tunneling effect, so as to improve the conductivity of plastic and become a "semiconductor" conductive plastic.
Generally speaking, the resistance of materials to current is characterized by surface resistivity. The smaller the surface resistivity, the stronger the conductivity. According to the different surface resistivity of materials, plastics can be divided into:
1. Insulation material: 10^12 ~ 10^15 ohm/sq
2. Antistatic material: 10^10 ~ 10^12 ohm/sq
3. Electrostatic dissipative material: 10^6 ~ 10^12 ohm/sq
4. Conductive material: ≤ 10^5 ohm/sq
5. Electromagnetic shielding material: 10^1 ~ 10^3 ohm/sq
Among them, conductive plastics with surface resistance less than or equal to 10^5 ohm/sq are conductive plastics, while conductive plastics with surface resistance between 10^1 and 10^3 ohm/s can generally be used as electromagnetic shielding materials. It has high conductivity, has certain absorption loss to electromagnetic wave, and can weaken the radiation effect of electromagnetic field. It is an important choice of electromagnetic shielding materials.
3、 Classification of conductive plastics
Conductive plastics are divided into two categories, among which conductive plastics made by physical methods are called composite conductive plastics, and conductive plastics made by chemical methods are called intrinsic (also known as structural) conductive plastics.
1、Intrinsic conductive plastic
It refers to the polymer itself or the plastic with conductivity after chemical modification, which can play the role of its own chemical structure to make it intrinsically conductive, and then increase its conductivity through chemical doping.
2、Composite conductive plastic
It refers to plastic with conductivity after physical modification, which is generally made by mixing conductive substances such as carbon black, carbon fiber, graphite, metal powder and metal fiber into the resin. At present, 90% of conductive plastics are composite materials, which are widely used.
Choosing different plastic substrates and combining different proportions of conductive materials will bring different electromagnetic shielding properties. As a shielding material, conductive plastic can adjust the shielding efficiency according to the thickness of the product, and its shielding efficiency is between 30~90db.
4、 Production technology of conductive plastics
Pc/abs, PC, ABS, PA, PPE and other resins are combined with nickel plated carbon fiber, stainless steel fiber and other conductive substances. Through the mixing of plastic and filler, through the plastic processing process, the conductivity is improved to a certain extent, and the great changes from insulator to semiconductor and then to conductor are realized, so that it has EMI shielding function and grounding function.
The conductive plastic adopts the injection molding process, which is one-time molding without complex secondary processes such as machining, electroplating, painting, coating, etc., which is conducive to greatly reducing the processing cost, and more complex structures can be achieved through injection molding. The density of conductive plastic (1.2~1.4g/cm3) is one quarter of that of common metals, even compared with aluminum, it is only half of that of aluminum. It can be processed into a thin-walled structure of about 1.0 mm, and the weight is reduced by 75%.
5、 Introduction to the application of conductive plastics
Conductive plastic has good electromagnetic shielding performance, low density, good toughness, low cost, easy processing, adjustable shielding performance and other advantages. It is a potential substitute for metal shielding materials. Conductive plastics are mainly used for EMI shielding shells in integrated circuits, automotive electronics, electrical and other fields, as well as semiconductor shielding materials used in medium and high voltage cables. They can maintain the inherent characteristics of plastics and have properties similar to metal conductivity.
1、Suspension structure of communication base station
The density of conductive plastics (1.2~1.4g/cm3) is one quarter of that of ordinary metals, and even compared with aluminum, it is only half of that of steel. Because of its low density, conductive plastic has certain application advantages in the suspension structure of the transmission tower of the communication base station. In addition, it is also a good substitute for the metal die-casting shell.
2、Power cover
At present, the power cover plate of the power supply system on the communication equipment mainly adopts aluminum die casting, but the main problems of the aluminum die casting are: heavy quality, high cost, difficult to process, non adjustable shielding efficiency, etc. Using conductive plastics instead of metal materials can avoid these problems and meet higher market demand.
3、Shielding cavity
At present, aluminum alloy die castings are mostly used in the shielding cavity of communication equipment, but the die castings have high density, easy corrosion, poor formability, and are plated with ordinary plastics. The coating is easy to fall off during long-term use, making the product function invalid; The conductive plastic integration scheme is adopted, which is convenient for assembly, good product reliability, shielding performance can meet the requirements, and the cost is also reduced.
The applications of conductive plastics in EMI also include communication false panels, connectors (optical fiber sheath, power sheath, etc.), computer display back shells, light guide boxes, and military materials.
