In addition to component selection and circuit design, good printed circuit board (PCB) design is also a very important factor in electromagnetic compatibility. The key to PCB EMC design is to minimize the return flow area and allow the return flow path to flow in the designed direction. The most common return current issues arise from cracks in the reference plane, shifting reference plane layers, and signals flowing through connectors. Jump capacitors or decoupling capacitors may solve some problems, but the overall impedance of the capacitors, vias, pads, and traces must be considered.
This article will introduce EMC’s PCB design technology from three aspects: PCB layering strategy, layout techniques and wiring rules.

PCB layering strategy

In circuit board design, the thickness, via process and number of layers of the circuit board are not the key to solving the problem. Excellent layer stacking is to ensure the bypass and decoupling of the power bus and minimize the transient voltage on the power layer or ground layer. And the key to shielding the electromagnetic fields of signals and power supplies. From the perspective of signal routing, a good layering strategy should be to place all signal routing on one or several layers, with these layers next to the power layer or ground layer. For power supply, a good layering strategy should be that the power layer is adjacent to the ground layer, and the distance between the power layer and the ground layer is as small as possible. This is what we call the "layering" strategy. Below we will talk specifically about excellent PCB layering strategies.

1. The projected plane of the wiring layer should be within the area of its reflow plane layer. If the wiring layer is not within the projection area of its reflow plane layer, there will be signal lines outside the projection area during wiring, causing "edge radiation" problems, and it will also increase the area of the signal loop, resulting in increased differential mode radiation. .
2. Try to avoid setting adjacent wiring layers. Because parallel signal traces on adjacent wiring layers will cause signal crosstalk, if adjacent wiring layers cannot be avoided, the layer spacing between the two wiring layers should be appropriately widened and the layer spacing between the wiring layer and its signal loop should be narrowed.
3. Adjacent plane layers should avoid overlapping of their projection planes. Because when projections overlap, the coupling capacitance between layers will cause the noise between the layers to couple to each other.

Multilayer board design

When the clock frequency exceeds 5MHz, or the signal rise time is less than 5ns, in order to control the signal loop area well, it is generally necessary to use a multi-layer board design. When designing multi-layer boards, you should pay attention to the following principles:
1. The key wiring layer (the layer where the clock line, bus, interface signal line, RF line, reset signal line, chip select signal line and various control signal lines are located) should be adjacent to the complete ground plane, preferably between the two ground planes, such as As shown in Figure 1. Key signal lines are generally strong radiation or extremely sensitive signal lines. Routing close to the ground plane can reduce the signal loop area, reduce its radiation intensity or improve its anti-interference ability.

Figure 1 The key wiring layer is between the two ground planes
2. The power plane should be retracted relative to its adjacent ground plane (recommended value is 5H~20H). Shrinking the power plane relative to its return ground plane can effectively suppress the "edge radiation" problem, as shown in Figure 2.

Figure 2 Power planes should be set back relative to their adjacent ground planes
In addition, the main working power supply plane of a single board (the most widely used power plane) should be immediately adjacent to its ground plane to effectively reduce the loop area of the power supply current, as shown in Figure 3.

Figure 3 The power plane should be immediately adjacent to its ground plane
3. Check whether there are no signal lines of ≥50MHz on the TOP and BOTTOM layers of the board. If so, it is best to route the high-frequency signal between two plane layers to suppress its radiation into space.

Single and double layer board designs

For the design of single-layer boards and double-layer boards, the main attention should be paid to the design of key signal lines and power lines. There must be a ground wire close to and parallel to the power supply wiring to reduce the power supply current loop area.
"Guide Ground Line" should be laid on both sides of the key signal lines of the single-layer board, as shown in Figure 4. The projection plane of the key signal lines of the double-layer board should be covered with a large area, or the "Guide Ground Line" should be designed in the same way as the single-layer board, as shown in Figure 5. "Guarding ground wires" on both sides of key signal lines can reduce the area of signal loops and prevent crosstalk between signal lines and other signal lines.

Figure 4 "Guide Ground Line" is laid out on both sides of the key signal lines of the single-layer board

Figure 5 The key signal lines of the double-layer board are laid on a large area on the ground projection plane.
In general, the layering of PCB boards can be designed according to the following table.

PCB layout tips

When designing the PCB layout, the design principle of straight line placement along the signal flow direction should be fully followed and try to avoid going back and forth, as shown in Figure 6. This can avoid direct signal coupling and affect signal quality. In addition, in order to prevent mutual interference and coupling between circuits and electronic components, the placement of circuits and the layout of components should follow the following principles:

Figure 6 Circuit modules are placed in a straight line along the signal flow direction
1. If the interface "clean ground" is designed on the board, the filtering and isolation components should be placed on the isolation belt between the "clean ground" and the working ground. This prevents filtering or isolation components from coupling to each other through the planar layer and weakening the effect. In addition, no other devices can be placed on the "clean ground" except filtering and protection devices.
2. When multiple module circuits are placed on the same PCB, digital circuits and analog circuits, high-speed and low-speed circuits should be laid out separately to avoid mutual interference between digital circuits, analog circuits, high-speed circuits and low-speed circuits. In addition, when there are high, medium and low-speed circuits on the circuit board at the same time, in order to prevent high-frequency circuit noise from radiating outward through the interface, the layout principles in Figure 7 should be followed.

Figure 7 High, medium and low speed circuit layout principles
3. The filter circuit of the power input port of the circuit board should be placed close to the interface to prevent the filtered lines from being coupled again.

Figure 8 The filter circuit of the power input port should be placed close to the interface
4． The filtering, protection and isolation components of the interface circuit are placed close to the interface, as shown in Figure 9, which can effectively achieve the effects of protection, filtering and isolation. If there are both filtering and protection circuits at the interface, the principle of protection first and then filtering should be followed. Because the protection circuit is used to suppress external overvoltage and overcurrent, if the protection circuit is placed after the filter circuit, the filter circuit will be damaged by overvoltage and overcurrent. In addition, since the input and output lines of the circuit are coupled to each other, the filtering, isolation, or protection effects will be weakened. When laying out, ensure that the input and output lines of the filter circuit (filter), isolation, and protection circuits do not couple with each other.

Figure 9 The filtering, protection and isolation components of the interface circuit are placed close to the interface
5. Sensitive circuits or devices (such as reset circuits, etc.) should be kept at least 1000 mil away from the edges of the board, especially the edge of the interface side of the board.
6． Energy storage and high-frequency filter capacitors should be placed near unit circuits or devices with large current changes (such as the input and output terminals of power modules, fans and relays) to reduce the loop area of large current loops.
7. Filter components need to be placed side by side to prevent the filtered circuit from being interfered with again.
8. Strong radiation devices such as crystals, crystal oscillators, relays, and switching power supplies should be kept at least 1000 mil away from the single-board interface connector. This can radiate the interference directly outwards or couple the current on the outgoing cable to radiate outwards.

PCB routing rules

In addition to component selection and circuit design, good printed circuit board (PCB) wiring is also a very important factor in electromagnetic compatibility. Since the PCB is an inherent component of the system, enhancing electromagnetic compatibility in PCB routing will not bring additional costs to the final completion of the product. Anyone should remember that a poor PCB layout can cause more EMC problems than eliminate them. In many cases, even adding filters and components will not solve them. In the end, the entire board had to be rewired. Therefore, developing good PCB routing habits at the beginning is the most cost-effective way. Some general rules for PCB wiring and design strategies for power lines, ground lines, and signal lines will be introduced below. Finally, based on these rules, improvement measures are proposed for the typical printed circuit board circuit of the air conditioner.
1. Wiring separation
The role of wiring separation is to minimize crosstalk and noise coupling between adjacent lines within the same layer of the PCB. The 3W specification states that all signals (clock, video, audio, reset, etc.) must be isolated from line to line and edge to edge as shown in Figure 10. In order to further reduce magnetic coupling, the reference ground is placed near key signals to isolate coupling noise generated on other signal lines.

Figure 10 Trace isolation
2. Protection and shunt lines
Setting up shunt and protection lines is a very effective way to isolate and protect critical signals, such as system clock signals, in a noisy environment. In Figure 21, the parallel or protection lines in the PCB are laid along the lines of key signals. Guard lines not only isolate coupling flux generated by other signal lines, but also isolate critical signals from coupling to other signal lines. The difference between a shunt line and a protected line is that the shunt line does not have to be terminated (connected to ground), but both ends of the protected line must be connected to ground. In order to further reduce coupling, the protection circuit in the multi-layer PCB can be added to the ground at every other section.

Figure 11 Shunt and protection lines
3. Power cord design
According to the size of the printed circuit board current, try to make the power line width as thick as possible to reduce the loop resistance. At the same time, make the direction of the power line and ground wire consistent with the direction of data transmission, which will help enhance the anti-noise capability. In a single panel or double panel, if the power line trace is very long, a decoupling capacitor should be added to the ground every 3000 mil. The capacitor value is 10uF + 1000pF.
4． Ground wire design
The principles of ground wire design are:
(1) Separate digital ground and analog ground. If there are both logic circuits and linear circuits on the circuit board, they should be kept as separate as possible. The ground of the low-frequency circuit should be grounded in parallel at a single point as much as possible. If there are difficulties in actual wiring, it can be partially connected in series and then grounded in parallel. High-frequency circuits should be grounded at multiple points in series. The ground wire should be short and flat. Try to use a large-area grid-shaped ground foil around high-frequency components.
(2) The grounding wire should be as thick as possible. If the ground wire is very thin, the ground potential will change with the change of current, which will reduce the noise immunity performance. Therefore, the ground wire should be thickened so that it can pass three times the allowable current of the printed board. If possible, the ground wire should be above 2~3mm.
(3) The ground wire forms a closed loop. For printed boards that are only composed of digital circuits, most of the ground circuits arranged in group loops can improve noise immunity.
5. Signal line design
For key signal lines, if the board has an internal signal wiring layer, key signal lines such as clocks are laid on the inner layer, and the wiring layer is given priority. In addition, key signal lines must not be routed across partitioned areas, including reference plane gaps caused by vias and pads, otherwise the area of the signal loop will increase. Moreover, the key signal line should be ≥3H from the edge of the reference plane (H is the height of the line from the reference plane) to suppress the edge radiation effect.
For strong radiation signal lines such as clock lines, bus lines, and radio frequency lines, and sensitive signal lines such as reset signal lines, chip select signal lines, and system control signals, they should be kept away from the outgoing signal lines of the interface. This prevents interference on strong radiation signal lines from being coupled to outgoing signal lines and radiating outward; it also prevents external interference brought in by outgoing signal lines from the interface from coupling into sensitive signal lines, causing system misoperation.
Differential signal lines should be on the same layer, of equal length, and run in parallel to keep the impedance consistent, and there should be no other routing between differential lines. Because the common mode impedance of the differential line pair is ensured to be equal, its anti-interference ability can be improved.
According to the above wiring rules, the typical printed circuit board circuit of the air conditioner is improved and optimized, as shown in Figure 12.

Figure 12 Typical printed circuit board circuit for improved air conditioner
Generally speaking, the improvement of EMC in PCB design is: before wiring, study the design of the return path first, and you will have the best chance of success and achieve the goal of reducing EMI radiation. Moreover, it does not cost anything to change the wiring layer before actually wiring. It is the cheapest way to improve EMC.