Electromagnetic compatibility EMC refers to the ability of an electronic system to work normally in a specified electromagnetic environment according to the design requirements. The electromagnetic interference suffered by the electronic system not only comes from the radiation of the electric field and the magnetic field, but also has the influence of the common impedance of the line, the connection between the wires and the circuit structure. When developing and designing circuits, we also hope that the designed printed circuit board is not affected by external interference as much as possible, and it also interferes with other electronic systems as little as possible. There are many factors that affect the anti-interference performance of printed boards, including the thickness of copper foil, the width and length of printed conductors and the crosstalk between adjacent conductors, the rationality of the layout of components in the board, and the common impedance of conductors, Electromagnetic fields generated by wires and components in space, etc.
The primary task of PCB anti-interference technology design printed board is to analyze the circuit and determine the key circuit. This is to identify which circuits are interference sources and which circuits are sensitive circuits, and figure out what paths the interference sources may take to interfere with sensitive circuits. In analog circuits, low-level analog circuits are often sensitive circuits, and power amplifiers are often sources of interference. When the working frequency is low, the interference source mainly interferes with the sensitive circuit through the barrel combination between the lines; when the working frequency is high, the interference source mainly interferes with the sensitive circuit through electromagnetic radiation. In digital circuits, high-speed repetitive signals, such as clock signals and bus signals, are rich in frequency components and are the biggest source of interference, often posing a threat to sensitive circuits. Reset circuit, interrupt circuit, etc. are sensitive circuits, which are easily disturbed by spike signals, so the digital circuit cannot work normally. The input/output circuit (1/0) is connected to the outside world and should also be paid special attention. If the UO circuit is close to the interference source such as the clock line, the unwanted high-frequency energy will be coupled to the input and output wires, and the noise on the wire will interfere with the sensitive circuit near the cable through radiation or conduction.
On the basis of fully analyzing the circuit and determining the key circuit, the circuit must also be properly arranged on the printed board. For example, for digital circuits, high-speed circuits (such as clock circuits, high-speed logic circuits, etc.), medium and low-speed logic circuits and UO circuits should be arranged in different areas, and the interference source and sensitive circuits should be separated as much as possible in space, so as to make the interference source The radiation interference to sensitive circuits is greatly reduced.
The purpose of the anti-interference design of the PCB board is to reduce the electromagnetic radiation of the PCB board and the crosstalk between the circuits on the PCB board. In addition, the ground wire design of the PCB also directly affects the common mode voltage radiation of the 1/0 cable. Therefore, the anti-interference design of the PCB is of great significance to reduce the electromagnetic information radiation of the system.
PCB layout design The density of printed circuit boards is getting higher and higher, and the quality of PCB design has a great impact on the ability to resist interference, so PCB layout plays an important role in design.
1. Layout requirements for special components:
1.1. The connection between high-frequency components should be as short as possible, and the electromagnetic interference between each other should be minimized; components susceptible to interference should not be too close; input and output components should be kept as far away as possible;
1.2. Some components have a high potential difference, so the distance between them should be increased to reduce common mode radiation. The layout of components with high voltage should pay special attention to the rationality of the layout;
1.3. The thermal element should be kept away from the heating element;
1.4. The decoupling capacitor should be close to the power pin of the chip;
1.5. The layout of adjustable components such as potentiometers, adjustable inductance coils, variable capacitors, and micro switches should be placed in positions that are easy to adjust as required;
1.6. The location occupied by the positioning hole of the printed board and the fixing bracket should be reserved.
2. Layout requirements for common components:
2.1. Place the devices of each functional circuit unit according to the circuit process, so that the signal flow direction is as consistent as possible;
2.2. Take the core components of each functional circuit as the center, and make layout around it. The components should be evenly and neatly arranged on the PCB, and the leads and connections between the components should be minimized and shortened;
2.3. For circuits that work at high frequencies, the interference between components should be considered. In general, the components should be arranged in parallel as much as possible to facilitate wiring;
2.4. The outplace-line of the PCB is generally not less than 80mil from the edge of the circuit board. The optimal shape of the circuit board is a rectangle. The aspect ratio is 3:2 or 4:30.
3. The wiring density of the PCB is getting larger and larger, so the wiring design of the PCB is particularly important.
3.1. The power line layer of the four-layer board should be as close as possible to the ground line layer to obtain the minimum power supply impedance. From top to bottom are: signal line, ground line, power line, signal line. Considering electromagnetic compatibility, the best six-layer board from top to bottom is: signal wire, ground wire, signal wire, power wire, ground wire, signal wire;
3.2. The clock line should be adjacent to the ground layer, the line width should be enlarged as much as possible, and the line width of each clock line should be the same;
3.3. High-speed digital signal lines and low-level analog signal lines are laid on the signal layer adjacent to the ground wire, and low-speed signal lines and high-level analog signal lines are laid on the farther layer;
3.4. The wiring of the input and output terminals should avoid the adjacent parallel as far as possible to avoid the occurrence of feedback vehicles;
3.5. The corner of the printed wire generally takes an obtuse angle of 135 degrees;
3.6. The line width of power lines and ground lines should be increased as much as possible, and the wiring width of devices with 0.5mm pin spacing should not be less than 12mil;
3.7. The general digital circuit signal line width is 8.il-10nul, and the spacing is 6mi1-8mil;
3.8. The lead of the deradiation capacitor should not be too long, especially the high-frequency bypass capacitor should not have a lead;
3.9. The digital ground and the analog ground on the mixed-signal circuit board are separated. If the wiring crosses the separation gap, the electromagnetic radiation and signal disturbance will increase sharply, resulting in electromagnetic compatibility problems. Therefore, PCB design generally adopts a unified layout and layout through digital circuits and analog circuits;
3.10. For some high-speed signals, differential pair wiring can be used to reduce electromagnetic radiation.
Jiban Technology is a manufacturer specializing in PCB boards, FPC boards and membrane switch panels. The main products are: PCB circuit board, FPC flexible circuit board, membrane switch, membrane panel, acrylic panel and other products, the company has a vigorous, professional and experienced technical, production and management team, with advanced printing The special production and testing equipment for boards has a standardized operation system, which makes the internal operation more accurate and efficient, and the external service is more systematic and timely, creating greater value for customers.
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