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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. Use multi-layer PCB boards as much as possible. Compared with double-sided PCB boards, ground and power planes, as well as closely arranged signal line-ground line spacing can reduce common-mode impedance and inductive coupling, making it possible to achieve double-sided PCB boards. 1/10 to 1/100 of. 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.
For double-sided PCBs, use tightly interwoven power and ground grids. 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. Make sure every circuit is as compact as possible. Set aside all connectors as much as possible. If possible, run the power cables through the center of the card and away from areas that are directly affected by ESD. Place wide chassis grounds or polygon fills on all PCB layers below the connectors leading out of the chassis (which are prone to direct ESD hits) and connect them together with vias about 13mm apart. Place mounting holes on the edge of the card with solder-free top and bottom pads around the mounting holes to the chassis ground. Do not apply any solder to 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. Between the chassis ground and the circuit ground of each layer, set the same "isolation area"; if possible, keep the separation distance of 0.64mm. Connect the chassis ground and the circuit ground with a 1.27mm wide wire every 100mm along the chassis ground wire at the top and bottom layers of the card near the mounting holes. 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.
If the board will not be placed in a metal chassis or shield, do not apply solder resist to the top and bottom chassis grounds of the board so that they can act as discharge electrodes for the ESD arc. To set a ring ground around the circuit in the following way:
(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 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.
In the area that can be directly hit by ESD, a ground wire should be laid near each signal line. The I/O circuits should be placed as close as possible to the corresponding connectors. Circuits that are susceptible to ESD should be placed close to the center of the circuit so that other circuits can provide some shielding for them. Usually, a series resistor and magnetic bead are placed on the receiving end, and for those cable drivers that are easily hit by ESD, a series resistor or magnetic bead can also be considered on the driving end. Transient protectors are usually placed on 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. 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 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.
Make sure the signal wires 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 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. Signals are driven into multiple receive circuits from a central location in the network. To ensure the loop area between power and ground is as small as possible, 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, fill unused areas with the ground, connecting the fill grounds of all layers every 60mm distance. Make sure to connect to the ground at two opposite endpoints of any large ground fill area (approximately larger than 25mm x 6mm). When the length of the opening in the power or ground plane exceeds 8mm, use a narrow wire to connect the two sides of the opening. Reset lines interrupt signal lines or edge-triggered signal lines cannot be placed near the edge of the PCB. Connect the mounting holes to 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 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. Solder resist cannot be used on the bottom pads, and ensure that the bottom pads are not processed by wave soldering. welding.
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) High-frequency filtering should be used.
(2) Keep away from input and output circuits.
(3) Keep away from the edge of the circuit board.
The PCB board should be inserted into the chassis, not installed in the opening or the internal seam. Pay attention to the routing under the bead, between the pads, and signal lines that may touch the bead. Some magnetic beads conduct electricity quite well and may create unexpected conduction paths. If a case or motherboard is going to contain several circuit boards, the static-sensitive PCB board should be placed in the middle.
