Do you understand the anti-static ESD function in PCB design?

Do you know what the anti-static ESD function is in PCB design? Static electricity from the human body, the environment and even inside electronic equipment can cause various damages to precision semiconductor chips, such as penetrating the thin insulating layer inside the component; damaging MOSFET and CMOS Gates of components; flip-flop lock-ups in CMOS devices; short-circuiting reverse-biased PN junctions; short-circuiting forward-biased PN junctions; melting soldering wires or aluminum wires inside active devices. In order to eliminate the interference and damage to electronic equipment caused by electrostatic discharge (ESD), a variety of technical means need to be taken to prevent it.

In the design of PCB boards, PCB anti-ESD design can be achieved through layering, proper layout, routing and installation. During the design process, prediction can limit most design modifications to adding or subtracting components. By adjusting the PCB layout and wiring, you can prevent ESD well. Let’s learn with the editor!

Use multi-layer PCB as much as possible. Compared with double-sided PCB, ground plane and power plane, as well as closely arranged signal line-ground line spacing can reduce common mode impedance and inductive coupling to 1/2 of double-sided PCB. 10 to 1/100. Try to place each signal layer as close as possible to a power layer or ground layer. For high-density PCBs that have components on both the top and bottom surfaces, very short connecting traces, and lots of ground fill, consider using inner-layer traces.

For double-sided PCBs, use a tightly interwoven power and ground grid. Place the power wires next to the ground wires, with as many connections as possible between vertical and horizontal wires or padded areas. The grid size on one side should be less than or equal to 60mm. If possible, the grid size should be less than 13mm. Ensure that each circuit is as compact as possible.

Keep all connectors as far away as possible. If possible, route power cables from the center of the card and away from areas prone to direct ESD exposure. Place wide chassis grounds or polygon fill grounds on all PCB layers below the connectors that lead out of the chassis (which are susceptible to direct ESD hits) and connect them together with vias every approximately 13mm.

Place mounting holes on the edges of the card, with top and bottom solder mask-free pads surrounding the mounting holes connected to the chassis ground. When PCB assembly, do not apply any solder to the top or bottom pads. Use screws with inline washers to achieve tight contact of the PCB with the metal chassis/shield or bracket on the ground plane.

The same "isolation zone" should be set up between the chassis ground and circuit ground on each layer; if possible, keep the separation distance at 0.64mm. On the top and bottom layers of the card close to the mounting holes, the chassis ground should be separated every 100mm. Connect the chassis ground and circuit ground together with a 1.27mm wide line. Adjacent to these connection points, place pads or mounting holes for mounting between chassis ground and circuit ground. These ground connections can be slit with a razor blade to keep them open, or jumpered with ferrite beads/high frequency capacitors.

If the circuit board will not be placed in a metal chassis or shielding device, do not apply solder mask to the top and bottom chassis ground wires of the circuit board so that they can act as discharge electrodes for the ESD arc.

Set up a ring ground around the circuit:

(1) Except for the edge connector and the chassis ground, place a circular ground path around the entire periphery.

(2) Ensure that the annular width of all layers is greater than 2.5mm.

(3) Connect the annular ground with via holes every 13mm.

(4) Connect the ring ground to the common ground of the multi-layer circuit.

(5) For double panels installed in metal chassis 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. The ring ground should not be coated with solder resist so that the ring ground can act as an ESD discharge rod. Place at least one somewhere on the ring ground (all layers). 0.5mm wide gap to avoid forming a large loop. The distance between the signal wiring and the ring ground should not be less than 0.5mm. In areas that can be directly hit by ESD, a ground wire must be laid near each signal line.

(6) The I/O circuit should be as close as possible to the corresponding connector. 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.

(7) It is common to place resistors and magnetic beads in series at the receiving end. For cable drivers that are easily hit by ESD, you can also consider placing resistors or magnetic beads in series at the driving end.

(8) 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 wires and ground wires coming out of the connector should be connected directly to the transient protector before being connected to other parts of the circuit.

Filter capacitors 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 receiving circuit ground (the length is 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.

(3) Make sure the signal line is as short as possible.

(4) When the length of the signal line is greater than 300mm, a ground wire must be laid in parallel.

(5) 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 the signal line and the ground wire should be exchanged every few centimeters to reduce the loop area.

(6) Drive signals from the center of the network into multiple receiving circuits. Make sure the loop area between power and ground is as small as possible and place a high-frequency capacitor close to each power pin of the integrated circuit chip.

(7) Place a high-frequency bypass capacitor within 80mm of each connector. Where possible, unused areas are to be filled with ground, connecting all layers of fill at 60mm intervals. Make sure that any large ground fill area (approximately larger than 25mm x 6mm) is connected to the ground at the two opposite end points.

(8) When the length of the opening on the power or ground plane exceeds 8mm, use a narrow wire to connect both sides of the opening. Reset lines, interrupt signal lines or edge trigger signal lines cannot be placed close to the edge of the PCB.

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

(1) When the metal bracket must be used with a metal shielding device or chassis, a zero-ohm resistor must be used to connect it.

(2) Determine the size of the mounting holes to achieve reliable installation of metal or plastic brackets. Use large pads on the top and bottom layers of the mounting holes. Do not use solder resist on the bottom pads, and ensure that the bottom pads do not use wave soldering processes. welding.

(3) Protected signal lines and unprotected signal lines cannot be arranged in parallel.

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

(1) Use high-frequency filtering.

(2) Keep away from input and output circuits.

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

(4) The PCB should be inserted into the chassis and should not be installed in openings or internal seams.

(5) 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 conductive paths.

(6) If a chassis or motherboard is to contain several circuit boards, the circuit board that is most sensitive to static electricity should be placed in the center. The above is the anti-static ESD function in PCB design. I hope it can help you.