A brief introduction to the design of the PCB anti-ESD method

In the design of the PCB board, the anti-ESD design of the PCB can be achieved through layering, proper layout wiring and installation. By adjusting the PCB layout wiring, can be good protection against ESD. As far as possible, the use of multilayer PCB, compared to double-sided PCB, the ground plane and power plane, as well as closely spaced signal lines – ground can reduce common mode impedance and inductive coupling, so that it reaches 1/10 to 1/100 of the double-sided PCB.
Static electricity from the human body, the environment and even inside the electronic equipment for sophisticated semiconductor chips can cause a variety of damage, such as penetrating the thin insulation layer inside the components; damaging MOSFETs and CMOS components of the gate; CMOS devices in the trigger lock-up; short-circuit reverse bias PN junction; short-circuit forward bias PN junction; melting active devices inside the solder wire or aluminum wire. In order to eliminate the electrostatic discharge (ESD) interference and damage to electronic equipment, a variety of technical means to prevent.
In the design of the PCB board, you can achieve the anti-ESD design of the PCB through layering, proper layout wiring and installation. In the design process, the majority of design modifications can be limited to the addition or subtraction of components through prediction. By adjusting the PCB layout wiring, can be well protected against ESD. Here are some common preventive measures.
As far as possible, the use of multi-layer PCBs, as opposed to double-sided PCBs, ground plane and power plane, as well as closely spaced signal lines – ground can reduce common-mode impedance and inductive coupling to 1 / 10 to 1 / 100 double-sided PCBs. For the top and bottom surface have components, with a very short connection line and many filler ground of high-density PCB, you can consider using the inner line.
For double-sided PCBs, use closely interwoven power and ground grids. The power line is close to the ground, between the vertical and horizontal lines or filler area, to connect as many as possible. The grid size on one side is less than or equal to 60 mm, and if possible, the grid size should be less than 13 mm. Make sure that each circuit is as compact as possible.
Keep all connectors on one side whenever possible.
If possible, bring the power cable through the center of the card and away from areas that are susceptible to direct ESD.
On all PCB layers below the connectors leading out of the chassis (vulnerable to direct ESD), place wide chassis grounds or polygon fill grounds and connect them together at approximately 13mm intervals with vias.
Place mounting holes on the edge of the card and connect the top and bottom pads around the mounting holes to the chassis ground with non-solder resist.
Do not apply any solder to the top or bottom pads during PCB assembly. Use screws with embedded washers to achieve tight contact between the PCB and the metal chassis/shield or bracket on the ground plane.
In each layer between the chassis ground and circuit ground, to set the same “isolation zone”; if possible, maintain a separation distance of 0.64mm. In the top and bottom of the card near the location of the mounting holes, every 100mm along the chassis ground and circuit ground with a 1.27mm wide wire connection together. Adjacent to these connection points, pads or mounting holes for mounting are placed between the chassis ground and the circuit ground. These ground connections can be scribed with a blade to maintain an open circuit or jumpered with a magnetic bead/high frequency capacitor.
If the board will not fit into a metal chassis or shield, the top and bottom chassis grounds on the board cannot be coated with solder resist so that they can act as a discharge electrode for ESD arcs.
To set up a ring around the circuit in the following way: (1) In addition to the edge connector and chassis ground, put a ring around the entire periphery of the ground path.
(2) Ensure that the width of the ring ground is greater than 2.5 mm for all layers.
(3) Connect the ring ground with overholes every 13mm.
(4) Connect the ring ground to the common ground of the multilayer circuit.
(5) For double-sided panels installed in metal enclosures or shielded devices, the ring ground should be connected to the circuit common ground. Unshielded double-sided circuits should then be connected to the ring ground to the chassis ground, the ring ground can not be coated with solder resist so that the ring ground can act as a discharge rod for ESD, at least a 0.5mm wide gap placed at a certain location on the ring ground (all layers), so as to avoid the formation of a large loop. The signal wiring should not be less than 0.5mm away from the ring ground. A ground wire should be laid near each signal line in the area that can be directly hit by ESD.
I/O circuits should be located as close as possible to the corresponding connectors.
For circuits 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 some shielding.
Resistors and beads in series are usually placed at the receiving end, while for those cable drivers that are susceptible to ESD, resistors or beads in series at the driver end may also be considered.
Transient protectors are 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) connected to the chassis ground. The signal and ground wires from the connector should be connected directly to the transient protector before the rest of the circuit can be connected.
Filter capacitors are to be placed at the connector or within 25mm of the receiving circuit.
(1) Use a short, thick wire to connect to chassis ground or receiver circuit ground (length less than 5 times the width, preferably less than 3 times the width).
(2) Connect the signal and ground lines to the capacitor first and then to the receiving circuit.
Be sure that the signal line is as short as possible.
When the length of the signal line is greater than 300mm, be sure to lay a ground line in parallel.
Make sure that the loop area between the signal line and the corresponding circuit is as small as possible. For long signal lines, switch the position of the signal and ground lines every few centimeters to reduce the loop area.
Drive signals from the center of the network into multiple receiver circuits.
Ensure that the loop area between power and ground is as small as possible by placing a high frequency capacitor close to each power pin of the IC chip.
Place a high-frequency bypass capacitor within 80 mm of each connector.
Where possible, ground fill unused areas by connecting all layers of the fill ground at 60mm intervals.
Ensure that the two opposite endpoint locations at any large ground fill area (approximately greater than 25mm x 6mm) are connected to ground.
When the length of the opening on the power or ground plane exceeds 8mm, connect both sides of the opening with a narrow wire.
The reset line, interrupt signal line or edge trigger signal line should not be arranged close to the edge of the PCB.
Connect the mounting holes together with the circuit commons or isolate them.
(1) When the metal bracket must be used with a metal shield or chassis, a zero-ohm resistor is used to achieve the connection.
(2) Determine the size of the mounting holes to achieve reliable installation of metal or plastic brackets. Large pads should be used on the top and bottom of the mounting holes, and no solder resist should be used on the bottom pads, and ensure that the bottom pads are not soldered using the wave soldering process.
The protected signal lines and unprotected signal lines cannot be arranged in parallel.
Special attention should be paid 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 board.
PCBs should be inserted into the chassis and not installed in open positions or internal seams.
Pay attention to the wiring of signal lines under the beads, between the pads, and that may come in contact with the beads. Some magnetic beads are quite conductive and may create unexpected conductive paths.
If a chassis or motherboard is to contain several circuit boards, the board that is most sensitive to static electricity should be placed in the very middle.