ESD is what we often call Electro-Stac discharge. From the learned knowledge, we can know that static electricity is a natural phenomenon, which is usually generated by contact, friction, induction between electrical appliances, etc. It is characterized by long-term accumulation, high voltage (which can generate thousands or even tens of thousands of volts of static electricity), low electricity, low current and short acting time. For electronic products, if the ESD design is not well designed in PCB layout design, it often leads to unstable operation of electronic and electrical products.
ESD (electrostatic discharge) damages electronic products, including sudden damage and potential damage. Sudden damage means that the device is seriously damaged and its function is lost. This kind of damage can usually be found in the quality inspection in the production process, so it mainly brings the cost of rework and maintenance to the factory. The potential damage refers to the part of the device that has been damaged and its function has not been lost, and it can’t be found in the detection of the circuit board production process. However, during use, the product will become unstable, sometimes good or bad, thus posing greater harm to the product quality. Among the two kinds of damages, potential failure accounts for 90% and sudden failure accounts for only 10%. That is to say, 90% electrostatic damage can’t be detected, and it will only be found when users use it. Most of the problems of mobile phones, such as frequent crashes, automatic shutdown, poor voice quality, loud noise, good signal time difference and key errors, are related to electrostatic damage. Because of this, electrostatic discharge is considered as the biggest potential killer of electronic product quality, and electrostatic protection has become an important content of electronic product quality control. However, the differences in the stability of domestic and foreign brand mobile phones basically reflect their differences in electrostatic protection and antistatic design of products.
PCB, an electronic product, if there is no technical problem in R&D, once it breaks down, it is mostly related to ESD static electricity. As we all know, ESD static electricity is everywhere. As long as some tiny electronic components are broken down by static electricity, the whole production line will also face collapse. What kind of influence does static electricity have on electronic components?
The basic physical characteristics of static electricity are as follows: attraction or repulsion, and there is a potential difference with the earth, which will produce a discharge current. Effects of three characteristics on electronic components:
Electrostatic dust adsorption can reduce the insulation resistance of components, thus shortening the service life of components.
The heat generated by electrostatic discharge or current may cause potential damage to components.
The electromagnetic field generated by electrostatic discharge has a large amplitude and a wide frequency spectrum, which causes interference or even damage to electronic devices.
Electrostatic discharge damage, so that the components are damaged and can’t work.
It can be seen that the harm of ESD static electricity is incalculable, and enterprises should do a good job of comprehensive protection to prevent the losses caused by ESD static electricity.
1. Use multilayer PCB as much as possible.
Compared with double-sided PCB, the ground plane and power plane, as well as the closely arranged spacing between signal lines and ground lines, can reduce the common-mode impedance and inductive coupling to 1/10 to 1/100 times that of double-sided PCB. Try to close each signal layer to a power layer or a ground layer. For the high-density PCB with components on the top and bottom surfaces, short connecting wires and lots of filling places, the inner wire can be considered.
2. For double-sided PCB, closely intertwined power grid and ground grid should be adopted.
The power cord is close to the ground wire, between the vertical and horizontal lines or the filled area, and should be connected as much as possible. The grid size of one surface is less than or equal to 60mm, and if possible, the grid size should be less than 13mm.
3. Ensure that each circuit is as compact as possible.
4. Put all connectors aside as much as possible.
5. The same “isolation zone” should be set between the chassis ground and the circuit ground on each floor; If possible, keep the separation distance at 0.64mm.
6. During PCB assembly, do not coat any solder on the top or bottom pads.
Use screws with embedded washers to achieve the close contact between PCB and the metal chassis/shield or the support on the ground plane.
7. If possible, lead the power cord from the center of the card, and stay away from the area that is easily directly affected by ESD.
8. On all PCB layers under the connectors leading out of the chassis (easily hit by ESD directly), place wide chassis ground or polygonal filling ground, and connect them with holes at intervals of about 13mm.
9. Place mounting holes on the edge of the card, and the periphery of the mounting holes is connected to the chassis ground with solder-free top and bottom pads.
10. At the top and bottom of the card near the mounting hole, connect the chassis ground and circuit ground with a 1.27mm wide wire along the chassis ground every 100mm. Adjacent to these connection points, pads or mounting holes for mounting are placed between the chassis ground and the circuit ground. These ground connections can be cut with a blade to keep open circuit, or they can be connected by magnetic beads/high frequency capacitors.
11. If the circuit board will not be placed in the metal chassis or shielding device, solder resist cannot be applied to the ground wires of the top and bottom chassis of the circuit board, so that they can be used as discharge electrodes of ESD arc.
12. An annular ground should be set around the circuit in the following ways:
(1) In addition to the edge connector and the chassis ground, an annular ground passage is placed around the whole periphery.
(2) Ensure that the ring width of all layers is greater than 2.5mm.
(3) Connect the rings with holes every 13mm.
(4) Connect the ring ground with the common ground of the multilayer circuit.
(5) For double panels installed in metal chassis or shielding device, the ring should be connected with the circuit in common. Unshielded double-sided circuits should be connected to the chassis ground annularly, and the annular ground should not be coated with solder resist, so that the annular ground can act as an ESD discharge rod. At least a 0.5mm wide gap should be placed at a certain position on the annular ground (all layers), so as to avoid the formation of a large loop. The distance between the signal wiring and the ring ground should not be less than 0.5mm.
13. In areas that can be directly hit by ESD, a ground wire should be laid near each signal line.
14. the I/O circuit should be as close to the corresponding connector as possible.
15. Circuits that are susceptible to ESD should be placed in an area close to the center of the circuit, so that other circuits can provide some shielding effect for them.
16. Usually, a transient protector is placed at the receiving end. Use short and thick wires (length less than 5 times width, preferably less than 3 times width) to connect to the chassis ground. The signal wire and ground wire from the connector should be directly connected to the transient protector before being connected to other parts of the circuit.
17. Usually, series resistors and magnetic beads are placed at the receiving end. For cable drivers that are easily hit by ESD, you can also consider placing series resistors or magnetic beads at the driving end.
The layout is designed at the connector or within 25mm of the receiving circuit, and the filter capacitor should be placed.
(1) Connect to the chassis ground or receiving circuit ground with a short and thick wire (the length is less than 5 times the width, preferably less than 3 times the width).
(2) The signal line and the ground line are connected to the capacitor first and then to the receiving circuit.
18, layout design to ensure that the signal line is as short as possible.
19. When the length of the signal line is greater than 300mm, a ground wire must be laid in parallel.
20. Ensure that the loop area between the signal line and the corresponding loop is as small as possible. For long signal lines, the positions of signal lines and ground lines should be changed every few centimeters to reduce the loop area.
21. Drive signals from the center of the network into multiple receiving circuits.
22. If possible, the unused areas should be filled with land, and the filling areas of all layers should be connected at intervals of 60mm.
23. Ensure that the loop area between the power supply and the ground is as small as possible, and place a high-frequency capacitor near each power supply pin of the integrated circuit chip.
24. Place a high-frequency bypass capacitor within 80mm of each connector.
25. The reset line, interrupt signal line or edge trigger signal line cannot be arranged near the edge of PCB.
26, ensure that in any large landfill area (about more than 25mm&mes; 6mm) to be connected to the ground.
27. When the length of the opening on the power supply or ground plane exceeds 8mm, connect the two sides of the opening with a narrow line.
28. layout design connects the mounting holes with the circuit common ground or isolates them.
(1) When the metal bracket must be used together with the metal shielding device or the chassis, a zero ohm resistor should be used to realize the connection.
(2) Determine the size of the mounting hole to realize the reliable installation of the metal or plastic bracket. Large pads should be used on the top and bottom of the mounting hole, and solder resist should not be used on the bottom pad, and the bottom pad should not be welded by wave soldering.
29. Protected signal lines and unprotected signal lines cannot be arranged in parallel.
30. layout design should pay special attention to the wiring of reset, interrupt and control signal lines.
(1) High frequency filtering should be adopted.
(2) Keep away from input and output circuits.
(3) Stay away from the edge of the circuit board.
31. The PCB should be inserted into the chassis, and it should not be installed at the opening or the internal joint.
32. Pay attention to the wiring of signal lines under the magnetic beads, between pads and possibly touching the magnetic beads. Some magnetic beads have good conductivity, which may lead to unexpected conductive paths.
3. If there are several circuit boards to be installed in a chassis or motherboard, the circuit board that is most sensitive to static electricity should be placed in the middle.
