Design method of antistatic discharge in PCB with new technique

- Oct 20, 2018-

In the design of PCB board, the anti-esd design of PCB can be realized by layering, proper layout wiring and installation.By adjusting the PCB layout and wiring, it can prevent ESD well.* multi-layer PCB should be used as much as possible. Compared to double-sided PCB, the ground plane and power supply plane, as well as the tightly arranged signal line-ground spacing, can reduce the common mode impedance and inductive coupling to 1/10 to 1/100 of double-sided PCB.For both top and bottom surfaces there are components with very short connections.

Static electricity from inside the human body, environment and even electronic equipment can cause various damages to the precision semiconductor chips, such as penetrating the thin insulation layer inside the components.The grid electrode of MOSFET and CMOS components is damaged.The trigger in CMOS device is locked;PN junction of short circuit antibias;The PN junction of the forward offset of the short circuit;Melting the welding line or aluminum wire inside the active device.In order to eliminate the interference and damage of electrostatic discharge (ESD) to electronic equipment, a variety of technical measures are needed to prevent it.

In the design of PCB board, the anti-esd design of PCB can be realized by layering, proper layout wiring and installation.In the design process, the majority of design modifications can be limited to adding or subtracting components by prediction.By adjusting the PCB layout and wiring, it can prevent ESD well.Here are some common precautions.

* multi-layer PCB should be used as much as possible. Compared to double-sided PCB, the ground plane and power supply plane, as well as the tightly arranged signal line-ground spacing, can reduce the common mode impedance and inductive coupling to 1/10 to 1/100 of double-sided PCB.Try to keep each signal layer close to a power or ground layer.For high-density PCBS with components on top and bottom surfaces, with very short connections, and many filling locations, consider using the inner layer.

* for double-sided PCBS, use a tightly interwoven power supply and ground grid.The power cord is close to the ground, and should be connected as much as possible between vertical and horizontal lines or filling areas.The grid size of one side should be less than or equal to 60mm. If possible, the grid size should be less than 13mm.

* make sure each circuit is as compact as possible.

* set all connectors aside as much as possible.

* if possible, bring the power cord from the center of the card and away from areas prone to direct ESD effects.

* on all PCB layers below the connector leading to the outside of the case (easy to be hit directly by ESD), place a wide chassis or polygon fill and connect them with holes at approximately 13mm intervals.

* place the mounting hole on the edge of the card, and connect the top and bottom soldering pads around the mounting hole to the chassis.

* do not apply any solder to the top or bottom solder pan during PCB assembly.Close contact of the PCB with a metal case/shield or grounding surface mount is achieved by using a screw with an embedded washer.

* the same "isolation zone" shall be set up between the case floor and circuit ground of each layer;If possible, keep the distance to 0.64mm.

* at the top and bottom of the card close to the mounting hole, connect the chassis and circuit with a 1.27mm wide wire along the casing floor at intervals of 100mm.Adjacent to these connection points, place a pad or mounting hole between the case floor and the circuit floor for mounting.These ground connections can be cut open with a blade to keep open, or they can be skipped with a magnetic bead/high frequency capacitor.

* if the circuit board is not going to be placed in a metal case or shielding device, the soldering flux cannot be applied on the top and bottom surface of the circuit board, so that they can be used as discharge electrodes for the ESD arc.

* place a loop around the circuit in the following manner:

(1) put a circular ground path around the entire periphery, except the edge connector and the chassis.

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

(3) connect the ring with holes at intervals of 13mm.

(4) connect the loop to the common ground of the multilayer circuit.

(5) for a double panel mounted in a metal case or shield, the loop shall be connected to the common circuit.Unshielded double-sided circuits should be connected to the chassis, with no resistance flux applied to the annular ground, so that the annular ground can act as a discharge rod for the ESD, and a gap of at least 0.5mm wide is placed at some point on the annular ground (all layers) to avoid forming a large loop.The distance from the annular ground shall not be less than 0.5mm.

* in areas that can be directly hit by ESD, a ground line should be placed near each signal line.

*I/O circuits should be as close to the corresponding connector as possible.

* the ESD sensitive circuits should be located close to the center of the circuit so that other circuits can provide some shielding for them.

* normally put series resistance and magnetic beads at the receiving end. For cable drivers that are vulnerable to ESD, it is also advisable to consider placing series resistance or magnetic beads at the driving end.

* usually a transient protector is placed on the receiving end.Connect to the chassis with a short thick line (less than 5 widths in length, preferably less than 3 widths).The signal line and ground wire coming out of the connector are connected directly to the transient protector before the other parts of the circuit are connected.

* place the filter capacitance at the connector or within 25mm of the receiving circuit.

(1) connect to the chassis or receiving circuit with short and thick wires (the length is less than 5 widths, preferably less than 3 widths).

(2) signal line and ground line shall be connected to capacitance before connected to receiving circuit.

* make sure the signal lines are as short as possible.

* when the length of signal line is greater than 300mm, make sure to arrange a ground line in parallel.

* make sure the loop area between the signal line and the corresponding loop is as small as possible.Change the position of the signal line and ground line every few centimeters to reduce the area of the loop.

* drive the signal from the center of the network into multiple receiving circuits.

* make sure the loop area between the power supply and the ground is as small as possible, placing a high frequency capacitance close to each power pin of the IC.

* place a high-frequency by-pass capacitance within 80mm of each connector.

* where possible, fill the unused area, connecting the filling areas of all layers at 60mm intervals.

* make sure to connect to the ground at any two opposite ends of the fill area (approximately larger than 25mm x 6mm).

* when the opening length on the power supply or ground plane exceeds 8mm, connect the two sides of the opening with a narrow line.

* reset line, interrupt signal line or edge trigger signal line should not be placed near the edge of PCB.

* connect the mounting holes to the circuit common or isolate them.

(1) when the metal bracket must be used with the metal shield device or chassis, it shall be connected with a zero ohm resistance.

(2) determine the size of the mounting holes to realize the reliable installation of metal or plastic supports. Large welding pads should be used on the top and bottom of the mounting holes. Resistance welding agents should not be used on the bottom welding plates, and ensure that the bottom welding plates do not use the wave soldering process.

* do not align protected signal lines with unprotected signal lines.

Special attention should be paid to reset, interrupt and control the wiring of the signal lines.

(1) high frequency filtering should be adopted.

(2) stay away from input and output circuits.

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

* the PCB should be inserted into the case and not installed in the open position or inside joint.

Note the wiring under the bead, between the pad and the signal line that may touch the bead.Some magnetic beads conduct electricity fairly well and may create unexpected conductive paths.

* if a case or motherboard is to have several circuit boards built in, put the most electrostatic sensitive circuit board in the middle.