In the design of PCB, the ESD resistance design of PCB can be realized by layering, proper layout and installation. In the design process, most of the design modifications can be limited to the increase or decrease of components by prediction. By adjusting PCB layout and wiring, ESD can be well prevented.
Static electricity from human body, environment and even electronic equipment can cause various damages to precision semiconductor chips, such as penetrating the thin insulation layer inside the components; damaging the gate of MOSFET and CMOS components; locking the trigger in CMOS devices; short circuit reverse biased PN junction; short circuit forward biased PN junction; melting the welding wire or aluminum wire inside the active device. In order to eliminate the interference and damage of electrostatic discharge (ESD) to electronic equipment, various technical means should be taken to prevent it.
In the design of PCB, the ESD resistance design of PCB can be realized by layering, proper layout and installation. In the design process, most of the design modifications can be limited to the increase or decrease of components by prediction. By adjusting PCB layout and wiring, ESD can be well prevented. Here are some common precautions.
Using 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 spacing, can reduce common mode impedance and inductive coupling, and make it reach 1 / 10 to 1 / 100 of double-sided PCB. Try to put each signal layer close to a power or ground layer. For high-density PCB with components on both the top and bottom surfaces, with a short connection line and a lot of filling ground, the inner layer line can be considered.
For dual sided PCB, the power supply and ground grid should be closely interwoven. The power cord should be close to the ground wire and should be connected as much as possible between the vertical and horizontal lines or the filled area. The grid size of one side shall be less than or equal to 60 mm and, if possible, less than 13 mm.
Make sure each circuit is as compact as possible.
Put all connectors aside as much as possible.
If possible, lead the power cord from the center of the card and away from areas that are susceptible to ESD.
On all PCB layers below the connector leading to the outside of the chassis (easy to be directly hit by ESD), place wide chassis floor or polygon filling ground, and connect them with vias at a distance of about 13mm.
The mounting holes are placed on the edge of the card, and the top and bottom pads of solder free flux are used around the mounting holes to connect to the chassis floor.
During PCB assembly
, do not apply any solder on the top or bottom pad. Screws with built-in washers are used to achieve close contact between the PCB and the metal chassis / shield or bracket on the ground floor.
The same “isolation area” shall be set between the chassis ground and the circuit ground of each layer; if possible, the spacing distance shall be 0.64mm.
At the top and bottom of the card near the mounting hole, connect the chassis ground and circuit ground with 1.27mm wide wire along the chassis ground wire every 100 mm. Adjacent to these connection points, a pad or mounting hole for installation is placed between the chassis ground and the circuit ground. These ground connections can be scratched with a blade to keep the open circuit, or with a bead / high frequency capacitor jumper.
If the circuit board will not be placed in the metal cabinet or shielding device, the ground wires of the top and bottom chassis of the circuit board shall not be coated with solder paste, so that they can be used as discharge electrodes for ESD arc.
A ring ground shall be provided around the circuit in the following ways:
(1) In addition to the edge connector and the chassis ground, a circular ground path is placed around the entire periphery.
(2) Make sure that the width of the annular floor is greater than 2.5mm.
(3) Through holes are used to connect the rings every 13mm.
(4) Connect the ring ground with the common ground of the multilayer circuit.
(5) For double-sided printed circuit boards installed in metal cases or shielding devices, the ring ground should be connected to the circuit in common. For unshielded double-sided circuits, the ring ground should be connected to the chassis ground, and solder resist should not be applied on the ring ground, so that the ring ground can act as the discharge rod of ESD. At least a 0.5mm wide gap should be placed at a certain position on the ring ground (all layers), so as to avoid forming a large loop. The distance between signal wiring and ring ground shall not be less than 0.5mm.