The friction caused by daily activities such as walking and dressing will generate electrostatic voltages of different magnitudes, but its energy is very small and will not cause harm to the human body. However, for electronic components, this electrostatic energy cannot be ignored. of.
In a dry environment, the voltage of human body static electricity (ESD) can easily exceed 6~35Kv. When touching electronic equipment, PCB or components on PCB, the components or equipment will be damaged due to instantaneous electrostatic discharge. Interference, or even damage to equipment or components on the PCB.
The figure below roughly lists the electrostatic voltage generated by different behaviors:
1. How is static electricity generated?
Static electricity is a kind of electric energy, which exists on the surface of objects, and is a phenomenon produced when positive and negative charges are locally unbalanced, such as frictional electrification is a phenomenon of electrostatic discharge.
The problem of static electricity has always plagued many electronic products. The electromagnetic field generated by the discharge current has caused serious interference to the circuit through capacitive coupling, inductive coupling or space radiation coupling. Therefore, we must consider static electricity in the early stage of PCB design. Defense issues.
The following figure is a schematic diagram of static electricity generation:
2. How to design anti-static PCB
The principle of PCB layout is generally to place the ESD protection device as close as possible to the input terminal or connector, and the line distance between the ESD protection device and the protected line should be designed as short as possible. For PCB design, it is very important to set a certain isolation distance on the edge where electrostatic discharge is easy to occur. For rack products, the breakdown distance of electrostatic voltage per kilovolt is about 1mm. 3-5mm isolation area can resist 3-5Kv electrostatic voltage.
1. Low-speed board electrostatic protection design
1) The wiring needs to be horizontal and vertical, and the signal lines should be routed side by side as far as possible;
2) If space permits, the thicker the wiring, the better;
3) Carry out wiring according to the high-speed circuit design concept;
4) Avoid processing important signal lines, such as clocks, reset signals, etc., at the edge of the PCB;
5) The conductive loops of all PCB boards, including power and ground loops, should be as small as possible;
6) For single-sided or double-sided panels, in the absence of a power plane, a ground wire must be followed next to the power wiring;
7) Try to use multi-layer board wiring.
2. High-speed board electrostatic protection design
1) The wiring needs to have a good ground plane;
2) Keep sufficient spacing, such as filters, optocouplers, AC power lines and weak signal lines;
3) Long-distance wiring plus low-pass filter (C, electrostatic device, RC, LC);
4) Isolation (increase the shielding cover), avoid the protected wire and the unprotected wire running side by side;
5) Avoid using the same loop with other devices to connect to a common ground point.
3. General methods to prevent static electricity
1) Keep the integrity of the ground, increase the discharge area of the ground, spread copper evenly on the ground, keep the resistance value of the ground unchanged, and the ground planes in the horizontal state between each other are stable;
2) The periphery of the board is designed around the ground, and the data lines are surrounded by ground;
3) The more ground holes the better, and make each layer of ground tightly connected together;
4) Installing an optocoupler or transformer on the PCB, combined with dielectric isolation and shielding, can well suppress the impact of electrostatic discharge;
5) The GND of the PCB and the ground of the shell can be grounded at a single point to prevent the voltage generated by the electrostatic discharge current from being coupled into the circuit, but attention should be paid to the selection of the grounding point, and the grounding should be at the cable entrance;
6) If the PCB area permits, and the lap connection and electrostatic discharge channels of the whole system are good, a grounding protection ring can be set around the PCB, which can be treated with bare copper and connected with many vias.
3. Manufacturability inspection of anti-static design
Here we recommend a free domestic PCB and PCBA testing software: Huaqiu DFM, which is very professional in the field of manufacturability inspection, and its testing parameters can also be adjusted according to the actual needs of the designer, according to the requirements of different board factories. The process standard reminds the hidden dangers before production and gives the best modification suggestions. Although there is no corresponding inspection item for the PCB anti-static design performance mentioned above, you can check whether the anti-static design position in the PCB design diagram exists. manufacturability risk, etc.
The technical industry has a specialization, the problem of anti-static treatment, the engineers can take protective measures in the PCB routing layout and structural interface, and the problems of manufacturability and assembly can be solved by professional Huaqiu DFM software ! If necessary, you can visit the official website to download the experience.