PCB Blog - Solve EMC PCB board design from layering, layout and routing

In addition to the selection of components and circuit design, good PCB board design is also a very important factor in electromagnetic compatibility. The key to PCB EMC design is to minimize the reflow area and allow the reflow path to flow in the designed direction. Common return current problems come from cracks in the reference plane, transforming the reference plane layer, and the signal flowing through the connector. Jumping capacitors or decoupling capacitors may solve some problems, but the overall impedance of capacitors, vias, pads, and traces must be considered. This lecture will introduce EMC's PCB board design technology from three aspects: PCB board layering strategy, layout skills and wiring rules.

1. PCB board layering strategy

The thickness, via process and the number of layers of the circuit board in the circuit board design are not the key to solving the problem. Excellent layered stacking is to ensure the bypass and decoupling of the power busbar, so that the transient voltage on the power plane or the ground plane is not affected. The key to shielding the electromagnetic fields of signals and power. From the signal routing point of view, a good layering strategy should be to place all signal traces on one or several layers next to power or ground planes. For power, 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, which is what we call the "layering" strategy. Below we will specifically talk about a good PCB layering strategy.

1) The projected plane of the wiring layer should be within its reflow plane layer area. If the wiring layer is not in the projection area of its return plane layer, there will be signal lines outside the projection area during wiring, which will cause the problem of "edge radiation", and will also cause the signal loop area to increase, resulting in increased differential mode radiation. .

2) Try to avoid the setting of 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 increased, and the layer spacing between the wiring layers and their signal loops should be reduced.

3) Adjacent plane layers should avoid overlapping of their projected planes. Because when the projections overlap, the coupling capacitance between the layers will cause the noise between the layers to couple to each other.

2. Multilayer board design:

When the clock frequency exceeds 5MHz, or the signal rise time is less than 5ns, in order to make the signal loop area well controlled, it is generally necessary to use a multi-layer board design. When designing multi-layer boards, the following principles should be paid attention to:

1) The key wiring layer (the layer where the clock line, bus, interface signal line, radio frequency 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 . The key signal lines are generally strong radiation or extremely sensitive signal lines, and wiring close to the ground plane can reduce the area of the signal loop, reduce its radiation intensity or improve the anti-interference ability.

2) The power plane should be retracted relative to its adjacent ground plane (recommended value 5H~20H). Retracting the power plane relative to its return ground plane can effectively suppress the "edge radiation" problem. In addition, the main working power plane of the board (which is widely used) should be in close proximity to its ground plane to effectively reduce the loop area of the power supply current.

3) Check whether there is no signal line ≥50MHz on the TOP and BOTTOM layers of the board. If so, route the high-frequency signal between the two plane layers to suppress its radiation to space.

3. Single-layer board and double-layer board design:

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 adjacent to the power trace and parallel traces to reduce the area of the power supply current loop. The "Guide Ground Line" should be placed on both sides of the key signal lines of the single-layer board. There should be a large area of ground on the projection plane of the key signal lines of the double-layer board, or a "Guide Ground Line" should be designed in the same way as the single-layer board. The "guard ground" on both sides of the key signal line can reduce the signal loop area on the one hand, and also prevent crosstalk between the signal line and other signal lines.

4. PCB board layout skills

When designing the layout of the PCB board, the design principle of placing in a straight line along the signal flow direction should be fully observed, and the back and forth looping should be avoided as much as possible. In this way, direct coupling of the signal can be avoided and the signal quality will be affected. 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:

1) If the interface is designed with "clean ground" on the board, the filtering and isolation devices should be placed on the isolation belt between the "clean ground" and the working ground. This prevents filtering or isolation devices from coupling to each other through the planar layer, which would impair the effect. Also, "cleanly", no other devices can be placed other than filtering and guarding devices.

2) When a variety of modular circuits are placed on the same PCB board, digital circuits and analog circuits, high-speed and low-speed circuits should be separated to avoid mutual interference between digital circuits, analog circuits, high-speed circuits and low-speed circuits. In addition, when there are high-speed, medium-speed and low-speed circuits on the circuit board at the same time, in order to avoid high-frequency circuit noise radiating outward through the interface.

3. The filter circuit of the power input port of the circuit board should be placed close to the interface to avoid re-coupling of the filtered line.

4. The filtering, protection and isolation devices of the interface circuit are placed close to the interface, 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 for external overvoltage and overcurrent suppression, 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 effect will be weakened. During layout, ensure that the input and output lines of the filter circuit (filter), isolation and protection circuit are not coupled to each other.

5) Sensitive circuits or devices (such as reset circuits, etc.) are at least 1000 mils away from each edge of the board, especially the edge of the board interface side.

6) Energy storage and high-frequency filter capacitors should be placed near the unit circuits or devices with large current changes (such as the input and output terminals of the power module, fans and relays) to reduce the loop area of the high-current loop.

7) The filter elements should be placed side by side to prevent the filtered circuit from being disturbed again.

8) Strong radiation devices such as crystals, crystal oscillators, relays, switching power supplies, etc. are at least 1000mil away from the single-board interface connector. In this way, the interference can be radiated directly to the outside, or the current can be coupled out on the outgoing cable to radiate outward.

5. PCB board wiring rules

In addition to component selection and circuit design, good printed circuit board (PCB) layout is also a very important factor in electromagnetic compatibility. Since the PCB board is an inherent component of the system, enhancing electromagnetic compatibility in the PCB board routing will not impose additional costs on the final product. Anyone should keep in mind that a poor PCB layout can cause more EMC problems than eliminate them, and in many cases, even adding filters and components doesn't solve them. Had to rewire the entire board. Therefore, developing good PCB board routing habits at the beginning is the way to save money. The following will introduce some general rules of PCB board wiring and the design strategies of power lines, ground lines and signal lines. According to these rules, improvement measures are proposed for typical printed circuit board circuits of air conditioners.

1) Wiring separation

The role of wiring separation is to couple crosstalk and noise between adjacent lines on 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 the magnetic coupling, the reference ground is placed near the key signal to isolate the coupling noise generated by other signal lines.

2) Protection and shunt circuit

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. Parallel or protection lines in the PCB are routed along the lines of critical signals. The guard line not only isolates the coupling flux generated by other signal lines, but also isolates critical signals from coupling with other signal lines. The difference between a shunt line and a protective line is that the shunt line does not have to be terminated (connected to ground), but both ends of the protective line must be connected to ground. In order to further reduce coupling, the protection lines in the multilayer PCB can be added with a path to ground every other section.

3) Power cord design

According to the size of the printed circuit board current, try to increase the width of the power line to reduce the loop resistance. At the same time, make the direction of the power line and the ground line consistent with the direction of data transmission, which will help to enhance the anti-noise capability. In single-panel or double-panel, if the power line is very long, decoupling capacitors should be added to ground every 3000mil, and the value of the capacitor should be 10uF+1000pF.

4) Ground wire design

The principles of ground wire design are:

a. Separate digital ground from analog ground. If there are logic circuits and linear circuits on the circuit board, they should be separated as much as possible. The ground of the low-frequency circuit should be grounded in parallel at a single point as far as possible. When the actual wiring is difficult, it can be partially connected in series and then grounded in parallel. The high-frequency circuit should be grounded at multiple points in series, the ground wire should be short and leased, and the large-area grid-shaped ground foil should be used around the high-frequency components as much as possible.

b. The ground wire should be as thick as possible. If the ground wire is very slender, the ground potential will change with the change of the current, which will reduce the anti-noise performance. Therefore, the ground wire should be thickened so that it can pass three times the allowable current on the printed board. If possible, the ground wire should be more than 2~3mm.

c. The ground wire forms a closed loop. For printed boards composed only of digital circuits, most of the grounding circuits are arranged in a loop, which can improve the anti-noise ability.

5) Signal line design

For key signal lines, if the board has an internal signal wiring layer, the key signal lines such as clocks are routed on the inner layer, and the wiring layer is preferred. In addition, the key signal lines must not be routed across the partition area, including the reference plane gap caused by vias and pads, otherwise the signal loop area will increase. And 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 interface outgoing signal lines. Therefore, the interference on the strong radiated signal line is prevented from being coupled to the outgoing signal line and radiated to the outside; it is also avoided that the external interference brought in by the outgoing signal line of the interface is coupled to the sensitive signal line, causing the system to malfunction. For differential signal lines, the same layer, equal length, and parallel lines should be run to keep the impedance consistent, and there should be no other lines between the differential lines. Because the common mode impedance of the differential pair is guaranteed 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. In general, if you study the design of the return path before wiring, you will have a chance of success and achieve the goal of reducing EMI radiation. Moreover, it is not necessary to spend any money to change the wiring layer before the actual wiring is performed. It is the practice of PCB board design to improve EMC.