PCB Blog - How to prevent ESD when designing PCB board

How to prevent ESD when designing PCB board

Static electricity from the human body, the environment and even the interior of electronic equipment can cause various damages to precision semiconductor chips, such as penetrating the thin insulating layer inside the components; ; Short-circuit reverse-biased PN junctions; Short-circuit forward-biased PN junctions; Melt bonding wires or aluminum wires inside active devices. In order to eliminate the interference and damage to electronic equipment caused by electrostatic discharge (ESD), various technical measures need to be taken to prevent it. In the design of the PCB board, the anti-ESD design of the PCB board can be realized by layering, proper layout and installation. During the design process, most design modifications can be limited to adding or removing components through prediction. By adjusting the layout of the PCB board, ESD can be well prevented. Here are some common precautions.

1. Use multi-layer PCB board as much as possible. Compared with double-sided PCB board, the ground plane and power plane, as well as the closely arranged signal line-ground line spacing can reduce the common mode impedance and inductive coupling, so that it can be double-sided. 1/10 to 1/100 of the PCB board. Try to place each signal layer as close to a power or ground layer as possible. For high-density PCBs with components on both the top and bottom surfaces, with very short interconnects, and many ground fills, consider using inner layers.
2. For double-sided PCB board, tightly interwoven power and ground grids should be used. The power wires are placed close to the ground wire, with as many connections as possible between vertical and horizontal wires or padding. The grid size on one side is less than or equal to 60mm, if possible, the grid size should be less than 13mm.
3. Make sure that each circuit is as compact as possible.
4. Put all connectors aside as much as possible.
5. If possible, lead the power cable through the center of the card and away from areas that are directly affected by ESD.
6. On all PCB layers below the connectors leading out of the chassis (which are easy to be hit directly by ESD), place wide chassis grounds or polygon-filled grounds and connect them with vias at intervals of about 13mm together.
7. Place mounting holes on the edge of the card, with solder-free top and bottom pads around the mounting holes to the chassis ground.
8. Do not apply any solder on the top or bottom pads during PCB assembly. Use screws with built-in washers to make tight contact between the PCB board and the metal chassis/shield or bracket on the ground plane.
9. Set the same "isolation zone" between the chassis ground and the circuit ground of each layer; if possible, keep the separation distance at 0.64mm.
10. The top and bottom layers of the card are close to the mounting holes. Connect the chassis ground and the circuit ground with a 1.27mm wide line every 100mm along the chassis ground wire. Adjacent to these connection points, place pads or mounting holes for mounting between chassis ground and circuit ground. These ground connections can be diced with a blade to keep them open, or jumpered with ferrite beads/high frequency capacitors.

11. If the circuit boards will not be placed in a metal case or shielding device, do not apply solder resist on the ground wire of the top and bottom case of the circuit board, so that they can be used as the discharge electrode of the ESD arc.
12. Set a ring ground around the circuit in the following manner:
(1) In addition to the edge connector and the chassis ground, put an annular ground path around the entire periphery.
(2) Ensure that the annular width of all layers is greater than 2.5mm.
(3) Connect annularly with via holes every 13mm.
(4) Connect the ring ground to the common ground of the multilayer circuit.
(5) For double-sided panels installed in metal cabinets or shielding devices, the ring ground should be connected to the circuit common ground. For unshielded double-sided circuits, the ring ground should be connected to the chassis ground. Solder resist should not be applied to the ring ground, so that the ring ground can act as a discharge bar for ESD. Place at least one place on the ring ground (all layers). 0.5mm wide gap, this avoids forming a large loop. The distance between the signal wiring and the ring ground should not be less than 0.5mm.
13. In the area 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 as possible to the corresponding connector.
15. For circuits that are susceptible to ESD, they should be placed in an area close to the center of the circuit, so that other circuits can provide them with a certain shielding effect.
16. Usually, resistors and magnetic beads are placed in series at the receiving end. For those cable drivers that are easily hit by ESD, a series of resistors or magnetic beads can also be considered at the driving end.
17. A transient protector is usually placed at the receiving end. Use a short, thick wire (less than 5 times the width and less than 3 times the width) to connect to the chassis ground. The signal and ground wires coming out of the connector should be connected directly to the transient protector before connecting to the rest of the circuit.
18. A filter capacitor should be placed at the connector or within 25mm of the receiving circuit.
(1) Use a short and thick wire to connect to the chassis ground or the receiving circuit ground (the length is less than 5 times the width and less than 3 times the width).
(2) The signal wire and the ground wire are first connected to the capacitor and then to the receiving circuit.
19. Make sure the signal line is as short as possible.

20. When the length of the signal wire is greater than 300mm, a ground wire must be laid in parallel.
21. Make sure 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 need to be changed every few centimeters to reduce the loop area.
22. Drive signals from the center of the network into multiple receiver circuits.
23. Make sure that the loop area between the power supply and the ground is as small as possible, and place a high frequency capacitor close to each power supply pin of the IC chip.
24. Place a high frequency bypass capacitor within 80mm of each connector.
25. Where possible, fill in unused areas with ground, connecting the fill ground of all layers every 60mm.
26. Make sure to connect to ground at two opposite end points of any large ground fill area (approximately larger than 25mm x 6mm).
27. When the length of the opening on the power supply or ground plane exceeds 8mm, use a narrow wire to connect the two sides of the opening.
28. The reset line, interrupt signal line or edge-triggered signal line cannot be placed near the edge of the PCB board.
29. Connect the mounting holes with the circuit common, or isolate them.
(1) When the metal bracket must be used with a metal shielding device or a chassis, a zero-ohm resistor should be used for connection.
(2) Determine the size of the mounting hole to achieve reliable installation of metal or plastic brackets. Use large pads on the top and bottom layers of the mounting holes. Solder resist cannot be used on the bottom pads, and ensure that the bottom pads are not processed by wave soldering. welding.

30. The protected signal lines and the unprotected signal lines cannot be arranged in parallel.
31. Pay special attention to the wiring of reset, interrupt and control signal lines.
(1) High-frequency filtering should be used.
(2) Keep away from input and output circuits.
(3) Keep away from the edge of the circuit board.
32. The PCB board should be inserted into the chassis, and should not be installed at the opening or the internal seam.
33. Pay attention to the wiring under the magnetic beads, between the pads and the signal lines that may come into contact with the magnetic beads. Some magnetic beads conduct electricity quite well and may create unexpected conduction paths.
34. If several circuit boards are to be installed in a case or mainboard, the printed circuit boards that are sensitive to static electricity should be placed in the middle.