What kind of PCB layout design can effectively prevent ESD?

 Abstract: ESD literally means electrostatic discharge. To put it simply, static electricity is a natural phenomenon, usually generated through contact, friction, induction between electrical appliances, etc. It is characterized by long-term accumulation, high voltage (can generate thousands of volts or even tens of thousands of volts of static electricity), and low electricity. , Small current and short action time. For electronic products, if the PCB ESD static electricity design is not well designed in the PCB layout design, it will cause unstable operation of electronic and electrical products.

Table of Contents

1. Basic introduction to ESD

2. Basic physical properties of static electricity

3. PCB layout design considerations

 

1. Basic introduction to ESD

There are two types of damage and damage caused by ESD (electrostatic discharge) to electronic products: sudden damage and potential damage. The so-called sudden damage refers to the severe damage of the device and loss of function. This kind of damage can usually be found in the quality inspection during the production process, so the main cost to the factory is the cost of rework and repair. The potential damage refers to the part of the device is damaged, the function has not been lost, and can not be found in the inspection of the circuit board production process, but the product will become unstable during use, and the quality of the product will be affected. Greater harm.

Of these two types of damage, potential failures accounted for 90%, and sudden failures accounted for only 10%. That is to say, 90% of static electricity damage cannot be detected, and will only be discovered when the user uses it. Most of the problems such as frequent crashes, automatic shutdown, poor voice quality, loud noise, good signal time, jet lag and key errors in mobile phones are related to static electricity damage. Because of this, electrostatic discharge is considered to be the biggest potential killer of electronic product quality and electrostatic protection has also become an important content of electronic product quality control. The differences in the stability of domestic and foreign brands of mobile phones basically reflect their differences in electrostatic protection and product anti-static design.

If there are no technical problems in the development of electronic products such as PCB boards, once a failure occurs, it is mostly related to PCB ESD static electricity. As we all know, ESD static electricity is everywhere. For some tiny electronic components, as long as they are broken down by static electricity, the entire production line will also face collapse. So what kind of effect does static electricity have on electronic components?

 

 

2. Basic physical properties of static electricity

The basic physical properties of static electricity are the following three types: attraction or repulsion, and there is a potential difference with the earth, which will generate a discharge current. The impact of these three characteristics on electronic components:

① The electrostatic adsorption of dust reduces the insulation resistance of the components and shortens the life of the components.

② The heat generated by electrostatic discharge or current may cause potential damage to the components.

③ The electromagnetic field generated by electrostatic discharge has a large amplitude and a very wide frequency spectrum, which can cause interference or even damage to electronic devices.

④ Electrostatic discharge destroys so that the components are damaged and cannot work.

It can be seen that the hazard of ESD static electricity is immeasurable, and companies should do comprehensive protection work to prevent losses caused by PCB ESD static electricity hazards.

 

3. PCB layout design considerations

PCB layout design should use multi-layer PCB as much as possible. Compared with double-sided PCB, the ground plane and power plane, as well as the tightly arranged signal line-ground line spacing can reduce common mode impedance and inductive coupling, and make it double-sided 1/10 to 1/100 of PCB. Try to put each signal layer close to a power layer or ground layer as much as possible. For high-density PCBs with components on the top and bottom surfaces, short connection lines, and many fills, you can consider using inner layer lines.

For double-sided PCBs, tightly intertwined power and ground grids should be used. The power cords are close to the ground, and as many connections as possible between the vertical and horizontal lines or filled areas are required. 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 that each circuit is as compact as possible, and put all the connectors aside as much as possible, between the chassis ground and the circuit ground on each layer. In between, set the same “isolation zone”; if possible, keep the separation distance 0.64mm. When doing PCB assembly, do not apply any solder on the top or bottom pads. Use screws with built-in washers to achieve a close contact between the PCB and the metal chassis/shielding layer or the bracket on the grounding surface. If possible, lead the power cord into the center of the card and keep it away from areas that are directly affected by PCB ESD static electricity. On all PCB layers below the connector that leads to the outside of the chassis (which is easy to be directly hit by ESD), place a wide chassis ground or a polygonal fill ground and connect them together with via holes at a distance of about 13mm. Place mounting holes on the edge of the card, connect the top and bottom pads with no solder resist around the mounting holes to the chassis ground, and close the top and bottom layers of the card to the mounting holes, and connect the chassis ground to the chassis ground every 100mm. The circuit grounds are connected together with 1.27mm wide wires. Adjacent to these connection points, place pads or mounting holes for mounting between the chassis ground and the circuit ground. These ground connections can be cut with a blade to keep the circuit open, or jumper with magnetic beads/high-frequency capacitors. If the circuit board will not be placed in a metal chassis or shielding device, solder resist should not be applied to the top and bottom chassis ground wires of the circuit board, so that they can be used as discharge electrodes for ESD arcs.

When PCB layout design, set a ring ground around the circuit in the following way:

① Except for the edge connector and the chassis ground put a circular ground path around the entire periphery.

② Ensure that the ring ground width of all layers is greater than 2.5mm.

③ Connect annularly 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 cases or shielding devices, the ring ground should be connected to the common ground of the circuit. 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 PCB ESD static electricity discharge bar. Place at least one at a certain position on the ring ground (all layers) 0.5mm wide gap, so you can avoid forming a large loop. The distance between the signal wiring and the ring ground should not be less than 0.5mm.

In areas that can be directly hit by ESD, a ground wire must be placed near each signal line. The 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 a certain shielding effect. Usually, a transient protector is 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) to connect to the chassis ground. The signal wire and ground wire from the connector should be directly connected to the transient protector before being connected to other parts of the circuit. Generally, series resistors and magnetic beads are placed on the receiving end. For cable drivers that are easily hit by ESD, you can also consider placing series resistors or magnetic beads on the drive end.

The PCB layout design at the connector or within 25mm from the receiving circuit, and the filter capacitor should be placed:

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, preferably less than 3 times the width).

② The signal wire and ground wire are connected to the capacitor first and then to the receiving circuit.

When designing the PCB layout, ensure that the signal line is as short as possible. When the length of the signal line is greater than 300mm, a ground line must be laid in parallel to ensure that the loop area between the signal line and the corresponding loop is as small as possible. For long signal lines, the position of the signal line and the ground line must be exchanged every few centimeters to reduce the loop area, and the signal is driven from the center of the network into multiple receiving circuits. Where possible, fill the unused area with land, connect the filling grounds of all layers every 60mm to ensure that the loop area between the power supply and the ground is as small as possible and close to each power tube of the integrated circuit chip. Place a high-frequency capacitor at the foot. Place a high-frequency bypass capacitor within 80mm of each connector. The reset line, interrupt signal line, or edge trigger signal line cannot be arranged close to the edge of the PCB. Make sure to connect with the ground at the two opposite end positions of the arbitrarily large ground filling area (about greater than 25mm&mes;6mm). When the length of the opening on the power supply or ground plane exceeds 8mm, use a narrow line to connect the two sides of the opening.

The PCB layout design connects the mounting holes with the circuit common ground, or separates them:

① When the metal bracket must be used with a metal shielding device or a chassis, a zero-ohm resistance should be used to realize 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 and no solder resist can be used on the bottom pads, and ensure that the bottom pads are not soldered by wave soldering. It is not possible to arrange protected signal lines and unprotected signal lines in parallel.

PCB layout design should pay special attention to the layout of reset, interrupt and control signal lines:

① High-frequency filtering should be used.

② Keep away from the input and output circuits.

③ Keep away from the edge of the circuit board.

The PCB should be inserted into the chassis, not installed in the opening or internal seams. Pay attention to the wiring under the magnetic beads, between the pads, and the signal lines that may contact the magnetic beads. Some magnetic beads have very good conductivity and may produce unexpected conductive paths. If a chassis or motherboard is to be equipped with several circuit boards, the circuit board that is most sensitive to static electricity should be placed in the middle.