PCB Technical - Several Skills of Circuit Board Design Anti-ESD

Static electricity from the human body, the environment, and even electronic equipment can cause various damages to precision semiconductor chips, such as penetrating the thin insulating layer inside the components; destroying the gates of MOSFET and CMOS components; and the triggers in CMOS devices are locked ; Short-circuit reverse-biased PN junction; Short-circuit forward-biased PN junction; Melt the welding wire or aluminum wire inside the active device. In order to eliminate electrostatic discharge (ESD) interference and damage to electronic equipment, a variety of technical measures need to be taken to prevent it.

When designing the PCB board, the anti-ESD design of the PCB can be achieved through layering, proper layout and installation. In the design process, the vast majority of design modifications can be limited to the addition or reduction of components through prediction. By adjusting the PCB layout and routing, ESD can be well prevented. The following are some common precautions.

1. Use multi-layer PCBs as much as possible. Compared with double-sided PCBs, the ground plane and power plane, as well as the tightly arranged signal line-ground spacing can reduce the common mode impedance and inductive coupling, so that it can reach the double-sided PCB. 1/10 to 1/100. Try to put each signal layer close to a power layer or ground layer. For high-density PCBs with components on the top and bottom surfaces, short connection lines, and many fills, you can consider using inner layer lines.

2. For double-sided PCBs, tightly interwoven power and ground grids should be used. The power line is close to the ground line, and as many connections as possible between the vertical and horizontal lines or the filled area. The grid size on one side is less than or equal to 60mm. If possible, the grid size should be less than 13mm.

3. Ensure that each circuit is as compact as possible.

4. Put all the connectors aside as much as possible.

5. If possible, introduce the power cord from the center of the card and keep it away from areas that are directly affected by ESD.

6. On all PCB layers below the connector that leads to the outside of the chassis (which is easy to be directly hit by ESD), place a wide chassis ground or polygonal fill ground, and use vias to connect them at intervals of about 13mm. Together.

7. Place mounting holes on the edge of the card, and connect the top and bottom pads with no solder resist around the mounting holes to the chassis ground.

8. When assembling the PCB, do not apply any solder on the top or bottom pads. Use screws with built-in washers to achieve close contact between the PCB and the metal chassis/shielding layer or the support on the ground plane.

9. Set the same "isolation zone" between the chassis ground and circuit ground on each layer; if possible, keep the separation distance 0.64mm.

10. On the top and bottom layers of the card near the mounting holes, connect the chassis ground and the circuit ground with a 1.27mm wide wire every 100mm along the chassis ground wire. Adjacent to these connection points, place pads or mounting holes for mounting between the chassis ground and the circuit ground. These ground connections can be cut with a blade to keep the circuit open, or jumper with magnetic beads/high-frequency capacitors.

11. If the circuit board will not be placed in a metal chassis or shielding device, solder resist should not be applied to the top and bottom chassis ground wires of the circuit board, so that they can be used as discharge electrodes for ESD arcs.

12. Set up a ring ground around the circuit in the following way:

(1) In addition to the edge connector and the chassis ground, a circular ground path is placed around the entire periphery.

(2) Ensure that the annular ground 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 panels installed in metal cases or shielding devices, the ring ground should be connected to the common ground of the circuit. 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 an ESD discharge bar. Place at least one at a certain position on the ring ground (all layers) 0.5mm wide gap, so you can avoid 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 must be laid near each signal line.

14. The 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 them with a certain shielding effect.

16. Generally, series resistors and magnetic beads are placed on the receiving end. For those cable drivers that are easily hit by ESD, you can also consider placing series resistors or magnetic beads on the drive end.

17. A transient protector is usually placed at the receiving end. Use a short and thick wire (length less than 5 times the width, preferably less than 3 times the 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.

18. Place a filter capacitor at the connector or within 25mm from 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, preferably less than 3 times the width).

(2) The signal wire and ground wire are connected to the capacitor first and then to the receiving circuit.

19. Make sure that 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. Ensure that the loop area between the signal line and the corresponding loop is as small as possible. For long signal lines, the position of the signal line and the ground line must be exchanged every few centimeters to reduce the loop area.

22. Drive signals from the center of the network into multiple receiving circuits.

23. Ensure 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 integrated circuit chip.

24. Place a high-frequency bypass capacitor within 80mm of each connector.

25. Where possible, fill the unused area with land, and connect the filling grounds of all layers at intervals of 60mm.

26. Make sure to connect with the ground at the two opposite end positions of the arbitrarily large ground filling area (about greater than 25mm*6mm).

27. When the length of the opening on the power supply or ground plane exceeds 8mm, use a narrow line to connect the two sides of the opening.

28. The reset line, interrupt signal line or edge trigger signal line cannot be arranged close to the edge of the PCB.

29. Connect the mounting holes to the circuit common ground, or isolate them.

(1) When the metal bracket must be used with a metal shielding device or a chassis, a zero-ohm resistance should be used to realize the 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, and no solder resist should be used on the bottom pads, and ensure that the bottom pads do not use wave soldering technology. welding.

30. The protected signal line and the unprotected signal line cannot be arranged in parallel.

31. Pay special attention to the wiring of reset, interrupt and control signal lines.

(1) Use high frequency filtering.

(2) Keep away from input and output circuits.

(3) Keep away from the edge of the circuit board.

32. The PCB should be inserted into the chassis, not installed in the opening or internal seams.

33. Pay attention to the wiring under the magnetic beads, between the pads and the signal lines that may be in contact with the magnetic beads. Some magnetic beads have very good conductivity and may produce unexpected conductive paths.

34. If a chassis or main board is to be equipped with several circuit boards, the circuit board most sensitive to static electricity should be placed in the middle.

To sum up, in the process of PCB production, PCB factories will get twice the result with half the effort if they follow the above techniques.