How to resist ESD when PCB board is designed?
Static electricity from the inside of human body, environment and even electronic equipment can cause various kinds of damage to precise semiconductor chips, such as penetrating the thin insulating layer inside components; destroying the gate of MOSFET and CMOS components; trigger locking in CMOS devices; short-circuit reverse bias PN junction; short-circuit forward bias PN junction; melting active devices. Internal welding or aluminium wire. In order to eliminate the interference and destruction of electrostatic discharge (ESD) on electronic equipment, various technical measures should be taken to prevent it.
In the design of PCB board, ESD resistance design of PCB can be realized by layering, proper layout, wiring and installation. In the design process, most design modifications can be limited to add or subtract components by prediction. By adjusting PCB layout and wiring, ESD can be well prevented. Below are some common precautions I like to organize the network.
1. Using multi-layer PCB as far as possible can reduce common-mode impedance and inductive coupling to 1/10 to 1/100 of double-sided PCB compared with double-sided PCB. Make every signal layer as close as possible to a power or ground layer. For high-density PCBs with components on top and bottom surfaces, short connections and many fillings, the use of inner lines can be considered.
2. For double-sided PCBs, tightly interwoven power supply and ground grids should be used. The power cord is close to the ground wire, and should be connected as much as possible between the vertical and horizontal lines or filling areas. The grid size on one side is less than or equal to 60 mm, and if possible, the grid size should be less than 13 mm.
3. Ensure that each circuit is as compact as possible.
4. Put all connectors aside as far as possible.
5. If possible, introduce the power cord from the center of the card and away from the area that is susceptible to direct ESD.
6. On all PCB layers below connectors leading to the outside of the chassis (easy to be hit directly by ESD), place wide chassis floor or polygon filling floor, and connect them with through holes every 13 mm distance.
7. Place mounting holes on the edge of the clamp. Connect the top and bottom pads without flux around the mounting holes to the floor of the chassis.
8. When assembling PCB, do not coat any solder on the top or bottom pad. Screws with built-in gaskets are used to achieve close contact between PCB and metal chassis/shield or support on ground surface.
9. The same "isolation zone" should be set between the chassis ground and the circuit ground on each floor; if possible, the distance should be kept at 0.64 mm.
10. At the top and bottom of the card near the mounting hole, the cabinet ground and the circuit ground are connected with 1.27 mm wide wires every 100 mm along the cabinet ground line. Adjacent to these connection points, a pad or mounting hole for installation is placed between the cabinet ground and the circuit ground. These grounding connections can be cut with blades to keep the circuit open or to use magnetic beads/high frequency capacitors for jump-over.
11. If the circuit board is not placed in the metal chassis or shielding device, no flux can be applied on the top and bottom chassis ground wires of the circuit board, so they can be used as discharge electrodes of ESD arc.
12. Set a ring around the circuit in the following ways:
(1) In addition to the edge connector and the chassis floor, a circular path is placed around the whole periphery.
(2) Ensure that the annular width of all layers is greater than 2.5mm.
(3) Connect annularly with holes every 13mm.
(4) Connect the ring to the common connection of the multi-layer circuit.
(5) For double panels mounted in metal chassis or shielding devices, the ring should be connected with the circuit in common. The unshielded double-sided circuit should be connected annularly to the chassis floor, which can not be coated with solder resistance, so that the annular floor can act as an ESD discharge rod, and a gap of at least 0.5mm wide should be placed at a certain position on the annular ground (all layers), thus avoiding the formation of a large loop. The distance between signal wiring and annular ground should not be less than 0.5mm.
13. In the area that can be hit directly by ESD, a ground line should be laid near each signal line.
14. I/O circuit should be as close as possible to the corresponding connector.
15. Circuits that are susceptible to ESD should be placed near the center of the circuit so that other circuits can provide some shielding for them.
16. Series resistance and magnetic beads are usually placed at the receiving end. For cable drivers that are vulnerable to ESD hitting, series resistance or magnetic beads can also be considered at the driving end.
17. Transient protectors are usually placed at the receiving end. Connect the cabinet floor with short and thick lines (less than 5 times the width, preferably less than 3 times the width). The signal and ground wires coming out of the connector are connected directly to the transient protector before connecting to other parts of the circuit.
18. The filter capacitor should be placed at the connector or or within the range of 25mm from the receiving circuit.
(1) Connect to the chassis or receiving circuit with short and thick wires (less than 5 times the width, preferably less than 3 times the width).
(2) The signal line and ground line are connected to the capacitor first and then to the receiving circuit.
19. Make sure the signal line is as short as possible.
20. When the length of the signal line is longer than 300 mm, a ground line must be laid parallel.
21. Ensure that the area between the signal line and the corresponding circuit is as small as possible. For long signal lines, the position of signal lines and ground lines should be changed every few centimeters to reduce the area of ring road.
22. Driving signals from the central position of the network into multiple receiving circuits.
23. Ensure that the area of the loop between the power supply and the ground is as small as possible. Place a high frequency capacitor near each power pin of the integrated circuit chip.
24. Place a high frequency bypass capacitor within 80 mm of each connector.
25. Where possible, the unused areas should be filled with land, and the filling of all layers should be connected at 60 mm intervals.
26. Make sure to connect to the ground at two opposite endpoints in any large filling area (greater than 25 mm*6 mm).
27. When the length of the opening on the power supply or the ground exceeds 8 mm, the two sides of the opening should be connected by narrow wires.
28. Reset line, interrupt signal line or edge trigger signal line cannot be located near the edge of PCB.
29. Connect the mounting holes to the circuit communally or isolate them.
(1) When the metal bracket must be used together with the metal shielding device or chassis, a zero ohmic resistance is used to realize the connection.
(2) To ensure the reliable installation of metal or plastic brackets, large pads should be used on the top and bottom of the installation holes, and no flux should be used on the bottom pads, and the bottom pads should not be welded by wave soldering technology.
30. Protected and unprotected signal lines cannot be arranged in parallel.
31. Special attention should be paid to resetting, interrupting and wiring of control signal lines.
(1) High frequency filtering should be adopted.
(2) Far away from input and output circuits.
(3) Keep away from the edge of the circuit board.
32. PCB should be inserted into the chassis, not in the open position or in the internal seam.
33. Attention should be paid to the wiring of signal lines under magnetic beads, between welding pads and possibly contacting magnetic beads. Some beads have excellent conductivity and may have unexpected conduction paths.
34. If several circuit boards are to be built into a chassis or motherboard, the most electrostatic sensitive circuit boards should be placed in the middle.
