（Figure 1 simple PCB loop）

（Fig. 2 PCB circuit composed of power line and ground wire ）

Instead of trying to find all possible loops, it is better to take the following steps to reduce the loop area: A, the power line and the ground wire should be close together to reduce the loop area between the power supply and the ground. Figure 3 illustrates several different ways to connect the power line to the ground wire.

（Fig. 3 reduction of loop area of power and ground）

B, multiple power supplies and ground wires should be connected to grid. Figures 4 and 5 illustrate this point: in this typical PCB design, one side of the PCB is perpendicular to the line, and the other side is the horizontal line (only the ground line is drawn).
As shown in Fig. 4, the typical ground wire structure makes the loop area very large, and can be added to the double sided plate to reduce the loop area, as shown in figure 5. The loop area of the grid is much smaller, which makes the inductive current very low, and the possibility of the problem is less. Insert at the bottom (or motherboard) PCB board PCB, there should be multiple ground and power line node, and in the length direction are uniformly arranged on the connector. This will be beneficial to reduce the loop area of the whole system.

（Figure 4 typical PCB ground wire structure）

（Fig. 5 ground wire mesh）

The above steps A and B can not only reduce the loop area between the power and ground, but also reduce the efficiency of the loop antenna. The steps below C and D will reduce the efficiency of the antenna and the signal line.
C and parallel wires must be placed together tightly, preferably with only one thick wire. Figure 6 illustrates this principle. That is to say, the ground plane should not have a large opening, because these openings, like parallel wires, are equivalent to loop antennas.

（Figure 6 shortening parallel paths）

The D and the signal line should be placed next to the ground wire. Arrange a ground wire next to each signal line. However, this may produce a lot of parallel ground wires. In order to avoid this problem, the ground plane or ground grid can be used instead of a single ground wire, as mentioned earlier. An example is shown in figure 7. Here, suppose, for some reason, the signal line can not move.

（Fig. 7 the signal line and the ground wire are close to the wiring）

The ground surface can be arranged on the opposite side of the signal line, as shown in figure 8. In fact, it‘s a good idea to fill the spare PCB part with ground wire.

（Fig. 8 hierarchical routing of signal line and ground or ground plane）

The longer power line or signal line between the E and the specially sensitive devices should be switched to the ground wire at regular intervals. The idea of switching is to move a wire from the top to the bottom, or from the left to the right, and the other leads to the opposite adjustment. Fig. 9 shows the equivalent effect of this method with the reduction of loop area: only minor loops exist after switching the wires.
F, the installation of high frequency bypass capacitor between the power line and ground wire. Because the impedance of the bypass capacitor is low in the lower frequency section of the ESD, the bypass capacitor can effectively reduce the loop area between the power supply and the ground. However, in the higher frequency segment of electrostatic discharge, because of the influence of parasitic inductance, even high-frequency capacitor, its role is very limited.
Of course, the closer the power line to the ground wire, the less obvious the effect of the filter capacitor. Because the loop area is small enough. Figures 10 and 11 illustrate this effect. Even though the bypass capacitor is installed next to each element, the circuit in Figure 10 still has a large loop area.

（Fig. 10 large loop area for installing bypass capacitor）

（Figure 11 the small loop area for installing bypass capacitors ）

The circuit shown in Figure 11, due to the power line and the ground next to put together the layout, which makes the loop area is greatly reduced. However, even if the power line is parallel to the ground wire, the longer conductor will lead to larger loop area.
Two. Make the length of wire as short as possible
The antenna has to be of high efficiency, and its length must be a large part of the wavelength. That is to say, longer wires will be beneficial to receive more frequency components generated by electrostatic discharge pulses, while shorter wires can only receive less frequency components. Therefore, the short wire receives and feeds the circuit less energy from the electromagnetic field generated by electrostatic discharge.
Making the wire as short as possible is a measure that is easier to achieve than the loop area as small as possible. Because it is not as easy to identify as the signal loop, the loop area as small as possible can not be seen immediately, and the length of the conductor is obvious. The design steps are as follows:
A) make all the components close together, and the PCB designer should not distract the components too much and take up more space;
(b) in the related element group, the elements that have a lot of interconnections between each other should be very close to each other. For example, the I/O device is as close as possible to the I/O connector;
(c) if possible, feed the power or signal from the center of the board instead of the edge of the board. As shown in Figure 12, the feed signal in the middle makes the connection of the most components shortest. When the circuit board is square, the effect is the most obvious,
When the circuit board is long and narrow, the effect is not obvious. But as long as possible, it should be done as much as possible. The proposed PCB design rule is mainly aimed at the field effect produced by electrostatic discharge current. But it is worth noting that the method to reduce the antenna efficiency described earlier, it also helps to prevent common mode noise into more trouble differential mode noise, Ninth common methods that begin to list in this chapter in the already mentioned. The reason for this effect is that all of the above steps are helpful to reduce the impedance difference of various PCB circuits. For example, the step D in rule 1 is especially useful, because this process makes the loop impedance of the signal line almost equal to that of the associated ground wire. Therefore, the common mode noise in the two paths is very close to the amplitude, and the differential mode noise is very small. In addition, PCB design can also take measures to reduce the problems caused by electrostatic field and charge injection. The following rules are related to this problem, and you‘ll find that there are several rules that are the same as the previous rules.
Three, as far as possible on the PCB using a complete ground surface (recommended multilayer)
As mentioned above, the ground plane helps to reduce the area of the loop, and also reduces the efficiency of the receiving antenna. As an important charge source, the ground plane can counteract the charge on the electrostatic discharge power source, which is helpful to reduce the problems caused by the electrostatic field. The PCB ground plane can also be used as the shield of the opposite signal line (of course, the greater the opening of the ground surface, the lower the shielding effectiveness). In addition, if the discharge occurs due to the PCB board ground plane is very big, very easy to charge into the ground plane, and not into the signal line. This will facilitate the protection of the components, because the charge can be released before the damage of the components is caused. (however, even if the charge released from the earth may damage the device, measures should be taken to avoid it)
Four. Strengthen the capacitance coupling between the power line and ground wire
The coupling between the power line and the ground wire is realized in two ways, which has already been mentioned before.
A, make the power line close to the ground wire, or use the multilayer PCB board. This will generate more parasitic capacitance between the power line and the ground wire.
B, connecting the high frequency bypass capacitor between the power line and the ground wire (the capacitance combination method can be applied to the low frequency and high static discharge situation). The coupling between the power line and ground wire will be helpful to reduce the charge injection problem. The voltage caused by the difference in charge between two objects depends on the capacitance between them (V=Q/C). If the charge of X coulomb is injected into the power line, the Y volt voltage will be generated between the power line and the ground wire. If the capacitance between the power line and the ground wire is doubled, the Coulomb charge of the X will only generate a voltage of Y/2 volts. Of course, the possibility of damage caused by this smaller voltage is also reduced. Five. In the discussion of electrostatic discharge effect of isolated electronic component and electrostatic discharge charge source, it has been pointed out that the charge injected into the electronic instrument can be solved by isolation. For PCB design, this means that the electronic instrument is separated from the possible charge source, and also isolated from the signal line where the connector port or the induction current tends to be concentrated. The following two steps can be used to isolate:
A, the PCB part of the electronic component that is far from the PCB line, will be exposed to ESD (e.g., where the operator can touch it directly).
B, an electronic component and a PCB line far away from any metal object exposed to electrostatic discharge (including screws, frames, connector housings, etc.). The latter requirement is less than the following design rules. Six, PCB on the chassis ground wire impedance is low, isolation is better, although the PCB trajectory on the solder layer is beneficial to isolate the PCB line, but the solder layer may lead to plug pinhole arc.
The best way to isolate the ground wire of A is to keep it away from the electronic instrument. In addition, if the impedance of the ground wire of the enclosure is very low, the electrostatic discharge current is easy to pass, and the arc will not occur. Of course, such a rapid charge release will produce a stronger field, but it is much better than the charge injected directly into the circuit through the arc.
The length of the ground wire of the B and the casing can not exceed four or five times of the width of the enclosure. The wider ground wire can only reduce its impedance (inductance) slightly, but narrower ground wires increase its impedance considerably. This length width ratio means that the ground wire must be very short, otherwise the width of the wire will be wide when the ground wire is increased. The priority of design rules: so far, the discussion of PCB design techniques to prevent ESD hazards has come to an end. Of course, sometimes these rules can‘t all be met. At this point, you have to consciously pick and choose something. This chapter begins with three potential electrostatic discharge hazards that can be used to determine the general order of electrostatic discharge problems. The following order is usually considered:
1, prevent charge injection into the system circuit, because this will cause damage to the circuit.
2. Prevent the problems caused by electrostatic discharge current field.
3. Prevent electrostatic field. Fortunately, most of these rules are compatible, and in the typical PCB design, all the problems can be well solved. Summary of PCB design guidelines: solutions for electrostatic discharge problems can be performed in accordance with the following twelve rules (in priority order):
1. The non insulated chassis ground wire of PCB must be at least 2.2 mm apart from other lines. This applies to all objects connected to the chassis, including the trajectory;
2. The length of the ground wire should not exceed five times the width of the enclosure;
3. Insulating the circuit with the operator can touch the PCB area or the metal object not to be separated at least more than 2 centimeters;
4, the power line and ground wire are placed side-by-side on the same layer of PCB, or placed in the adjacent two layers;
5. The ground plane and the ground wire must be grid shaped. In any direction, the vertical ground wire and the horizontal ground wire are connected at least once every 6 centimeters. Especially the double PCB plate, that is to say, the first layer PCB board can wire cloth level, while the second layer can be arranged perpendicular to the ground, must be at least every 6 cm by placing a through hole to connect the two (of course, in less than 6 cm. The connection is better than ground grid, ground plane better);
6, all signal lines must be in the ground plane edge or within 13 mm above the ground. The ground wire can be distributed either on the same layer as the signal line or on the layer adjacent to the signal line. If the length of the signal line reaches 30 centimeters or more, a ground wire must be placed beside it, and it is possible to place the earth wire above the signal line or on its adjacent surface;
7. The bypass capacitors connected between the power line and the ground wire can not be more than 8 centimeters apart (so that each integrated block may have multiple bypass capacitors connected);
8. The elements that connect more together depend on each other;
9. All components must be as close as possible to the I/O connector (note that the first third should be satisfied);
10. Fill all the spare parts of the PCB with the ground wire (should be noted to be connected every 6 centimeters to produce the ground wire grid);
11, if possible, feed the power line or signal line from the center of the edge of the PCB board, but not from a certain angle.
12. For a particularly sensitive and longer signal line (30 cm or longer), it should be switched to its ground wire at regular intervals. Note: these design rules must be applied to all PCB boards within the system (such as motherboards and boards inserted above). For example, when the application of article second, a ground wire length including the motherboard and the fender and the length of all the earth!