In the design of the pcb board, the anti- ESD design of the PCB can be realized through layering, proper layout and installation . By adjusting the PCB layout and wiring, ESD can be well prevented. *Use multi-layer PCB as much as possible. Compared with double-sided PCB, the ground plane and power plane, as well as the closely arranged signal line-ground spacing can reduce the common mode impedance and inductive coupling , making it 1 of double-sided PCB. /10 to 1/100. Both the top and bottom surfaces have components , with very short connecting wires.
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 through the thin insulating layer inside components; damaging gates of MOSFET and CMOS components; trigger lockup in CMOS devices ; Short circuit reverse biased PN junction; Short circuit forward biased PN junction; Melt solder wire or aluminum wire inside active device. In order to eliminate the interference and damage to electronic equipment caused by electrostatic discharge (ESD), it is necessary to adopt various technical means to prevent it.
In the design of the pcb board, the anti-ESD design of the PCB can be realized through layering, proper layout and installation. During the design process, prediction can limit most design modifications to adding or removing components. By adjusting the PCB layout and wiring, ESD can be well prevented. Here are some common precautions.
*Use multi-layer PCB as much as possible. Compared with double-sided PCB, the ground plane and power plane, as well as the closely arranged signal line-ground spacing can reduce the common mode impedance and inductive coupling, making it 1 of double-sided PCB. /10 to 1/100. 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, very short connection lines, and lots of ground fills, consider using inner-layer traces.
* For double-sided PCBs, tightly interweaved power and ground grids are used. Power wires are placed next to ground wires, and as many connections as possible are made 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.
*Make sure every circuit is as compact as possible.
*Keep all connectors out of the way if possible.
*If possible, route the power cord from the center of the card and away from areas that are directly exposed to ESD.
*On all PCB layers below the connectors leading out of the chassis (easy to be directly hit by ESD), place a wide chassis ground or polygon fill ground, and connect them together with vias at a distance of about 13mm .
*Place mounting holes on the edge of the card with solder mask-free top and bottom pads around the holes to chassis ground.
*During PCB assembly, do not apply any solder to the top or bottom pads. Use screws with built-in washers to achieve tight contact between the PCB and the standoff on the metal chassis/shield or ground plane.
* Between the chassis ground and the circuit ground of each layer, the same "isolation zone" should be set; if possible, keep the separation distance at 0.64mm.
*At 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 line along the chassis ground every 100mm. Adjacent to these connection points, pads or mounting holes for mounting are placed between chassis ground and circuit ground. These ground connections can be slashed with a blade to keep an open circuit, or jumpered with a ferrite bead/ high frequency capacitor .
*If the board will not be placed in a metal chassis or shielding, no solder resist should be applied to the top and bottom chassis grounds of the board so that they can act as discharge electrodes for the ESD arc.
*To place a ring ground around the circuit in the following way :
(1) In addition to the edge connector and the chassis ground, put a ring ground path around the entire periphery.
(2) Make sure that the annular ground width of all layers is greater than 2.5mm.
(3) Connect the ring grounds 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 chassis or shielding devices, the ring ground should be connected to the common ground of the circuit. Unshielded double-sided circuits should connect the ring ground to the chassis ground, and the ring ground should not be coated with solder resist, so that the ring ground can act as an ESD discharge rod, and at least one position on the ring ground (all layers) should be placed 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.
*In the area that can be directly hit by ESD, a ground wire should be placed near each signal line.
*I/O circuit should be as close as possible to the corresponding connector.
*For circuits that are susceptible to ESD, they should be placed near the center of the circuit so that other circuits can provide them with some shielding.
*Usually place series resistors and magnetic beads at the receiving end, and for those cable drivers that are vulnerable to ESD , you can also consider placing series resistors or magnetic beads at the driving end.
*Typically place a transient protector at the receiving end. Use a short and thick wire (less than 5 times the width, less than 3 times the width) to connect to the chassis ground. The signal and ground wires coming out of the connectors should go directly to the transient protector before going to the rest of the circuit.
*A filter capacitor should be placed at the connector or within 25mm from the receiving circuit .
(1) Use a short and thick line to connect to the chassis ground or the receiving circuit ground (the length is less than 5 times the width, less than 3 times the width).
(2) The signal wire and the ground wire are first connected to the capacitor and then connected to the receiving circuit.
*Make sure the signal lines are as short as possible.
*When the length of the signal line is greater than 300mm, a ground line must be laid in parallel.
* Make sure that the loop area between the signal line and the corresponding return is as small as possible. For long signal lines, the position of the signal line and the ground line should be exchanged every few centimeters to reduce the loop area.
* Drive signals from a central location in the network into multiple receive circuits.
*Ensure that 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 IC chip .
*Place a high frequency bypass capacitor within 80mm of each connector.
*Where possible, unused areas should be filled with ground, and the filled grounds of all layers should be connected every 60mm.
* Make sure to connect to ground at the two opposite ends of an arbitrarily large ground fill area (approximately greater than 25mm x 6mm).
*When the length of the opening on the power or ground plane exceeds 8mm, use a narrow line to connect the two sides of the opening.
* Reset lines, interrupt signal lines, or edge-triggered signal lines cannot be placed close to the edge of the PCB.
*Connect the mounting hole with the circuit common, or isolate them.
(1) When the metal bracket must be used 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 achieve reliable installation of metal or plastic brackets. Large pads should be used on the top and bottom layers of the mounting holes. Solder resist should not be used on the bottom pads, and ensure that the bottom pads do not use wave soldering. welding.
*Protected signal lines and unprotected signal lines cannot be arranged in parallel.
* 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.
*PCB should be inserted into the chassis, not installed in the opening or the internal seam.
*Pay attention to the wiring under the magnetic beads, between the pads and the signal lines that may touch the magnetic beads. Some ferrite beads conduct electricity quite well and may create unexpected conductive paths.
*If several circuit boards are to be installed in a case or motherboard, the circuit boards that are sensitive to static electricity should be placed in the middle.
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