EMC's PCB design technology: comprehensive analysis of layering strategy, layout skills, and wiring rules

In addition to the selection of components and circuit design, a 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 reflux area as much as possible, allowing the reflux path to flow in the designed direction. The most common return current issues come from cracks in the reference plane, transforming the reference plane layer, and signals flowing through the connector. Jumping or decoupling capacitors may solve some problems, but it is necessary to consider the overall impedance of capacitors, vias, pads, and wiring.

This article will introduce EMC's PCB design technology from three aspects: PCB layering strategy, layout skills, and routing rules.

 

PCB layering strategy

The thickness, through-hole process, and number of layers in circuit board design are not the key to solving the problem. Excellent layered stacking is the key to ensuring the bypass and decoupling of power busbars, minimizing transient voltage on the power layer or ground plane, and shielding the electromagnetic fields of signals and power sources. From the perspective of signal routing, a good layering strategy should be to place all signal routing in one or several layers, which are adjacent to the power layer or grounding layer. For power sources, a good layering strategy should be to have the power layer adjacent to the ground plane and the distance between the power layer and the ground plane as small as possible, which is what we call a "layering" strategy. Next, we will discuss in detail the excellent PCB layering strategy.

 

The projection plane of the wiring layer should be within its return plane layer area. If the wiring layer is not within the projection area of its return plane layer, there will be signal lines outside the projection area during wiring, leading to "edge radiation" problems and an increase in the signal loop area, resulting in an increase in differential mode radiation.

2. Try to avoid setting adjacent wiring layers as much as possible. Because parallel signal routing on adjacent wiring layers can 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 circuits should be reduced.

3. Adjacent plane layers should avoid overlapping their projection planes. Because when the projection overlaps, the coupling capacitance between layers can lead to mutual coupling of noise between layers.

Multi layer 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, a multi-layer board design is generally required. When designing multi-layer boards, the following principles should be noted:

1. The critical wiring layer (including clock lines, buses, interface signal lines, RF lines, reset signal lines, chip selection signal lines, and various control signal lines) should be adjacent to the complete ground plane, preferably between the two ground planes, as shown in Figure 1. Key signal lines are generally strong radiation or extremely sensitive signal lines. Wiring them close to the ground plane can reduce the area of their signal circuit, reduce their radiation intensity, or improve their anti-interference ability.

Figure 1 Key wiring layer between two ground planes

2. The power supply plane should be inward contracted relative to its adjacent ground plane (recommended value of 5H to 20H). The contraction of the power plane relative to its return ground plane can effectively suppress the "edge radiation" problem, as shown in Figure 2.

Figure 2: The power supply plane should be inward contracted relative to its adjacent ground plane

In addition, the main working power plane of the single board (the most widely used power plane) should be adjacent to its ground plane to effectively reduce the circuit area of the power supply current, as shown in Figure 3.

Figure 3: The power plane should be adjacent to its ground plane

3. Is there no signal line ≥ 50MHz in the TOP and BOTTOM layers of the board. If there is, it is best to place the high-frequency signal between two plane layers to suppress its radiation to space.

Single layer and double layer board design

For the design of single-layer 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 running parallel to and adjacent to the power supply line to reduce the area of the power supply current circuit.

The key signal lines of the single-layer board should be arranged with "Guide Ground Lines" on both sides, as shown in Figure 4. The key signal lines of the double-layer board should have a large area of flooring on the projection plane, or the same treatment method as the single-layer board, and a "Guide Ground Line" should be designed, as shown in Figure 5. The "protective ground wire" on both sides of the critical signal line can reduce the area of the signal circuit and prevent crosstalk between the signal line and other signal lines.

 

Figure 4: Guide Ground Line on Both Sides of the Key Signal Line of a Single Layer Board

 

Figure 5 Large area paving on the ground projection plane of the key signal lines of the double-layer board

Overall, the layering of PCB boards can be designed according to the table below.

PCB layout skills

When designing PCB layouts, the design principle of placing them in a straight line along the signal flow direction should be fully followed, and efforts should be made to avoid back and forth wrapping, as shown in Figure 6. This can avoid direct signal coupling and affect signal quality. In addition, 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 placed in a straight line along the signal flow direction

If the interface on the single board is designed to be "clean", the filtering and isolation devices should be placed on the isolation strip between the "clean" and the working area. This can prevent filtering or isolation devices from coupling with each other through planar layers, weakening the effect. In addition, on a "clean" surface, no other devices can be placed except for filtering and protective devices.

When multiple module circuits are placed on the same PCB, digital circuits and analog circuits, as well as high-speed and low-speed circuits, should be arranged 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, in order to avoid high-frequency circuit noise radiating outward through the interface, the layout principle in Figure 7 should be followed.

 

Figure 7 Layout Principles for High, Medium, and Low Speed Circuits

3. The filtering circuit of the power input port on the circuit board should be placed close to the interface to prevent the lines that have already been filtered from being coupled again.

 

The filtering circuit of the power input port in Figure 8 should be placed close to the interface

4. The filtering, protection, and isolation devices 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 protective circuits at the interface, the principle of protection before filtering should be followed. Because the protective circuit is used for external overvoltage and overcurrent suppression, if the protective circuit is placed after the filtering circuit, the filtering circuit will be damaged by overvoltage and overcurrent. In addition, due to the weakening of filtering, isolation, or protection effects when the input and output lines of the circuit are coupled with each other, the layout should ensure that the input and output lines of the filtering circuit (filter), isolation, and protection circuit are not coupled with each other.

 

Figure 9: Filtering, protection, and isolation devices of the interface circuit placed close to the interface

5. Sensitive circuits or devices (such as reset circuits, etc.) should be kept at least 1000mil away from the edges of the single board, especially the edge of the single board interface.

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

7. Filter components need to be placed side by side to prevent the filtered circuit from being disturbed again.

8. Strong radiation devices such as crystals, crystal oscillators, relays, and switching power supplies should be kept at least 1000mil away from single board interface connectors. This can directly radiate interference outward or couple current on the outgoing cable to radiate outward.

PCB wiring rules

In addition to the selection of components and circuit design, good printed circuit board (PCB) wiring is also a very important factor in electromagnetic compatibility. Since PCB is an inherent component of the system, enhancing electromagnetic compatibility in PCB wiring will not bring additional costs to the final completion of the product. Anyone should remember that a poor PCB wiring can lead to more electromagnetic compatibility issues, rather than eliminating them. In many cases, even adding filters and components cannot solve these problems. In the end, the entire board had to be rewired. Therefore, developing good PCB wiring habits at the beginning is the most cost-effective way. Below, some common rules for PCB wiring and design strategies for power, ground, and signal lines will be introduced. Finally, based on these rules, improvement measures will be proposed for typical printed circuit board circuits of air conditioners.

1. Wiring separation

The function of wiring separation is to minimize crosstalk and noise coupling between adjacent lines within the same layer of PCB. The 3W specification states that all signals (clock, video, audio, reset, etc.) must be isolated between lines and edges 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 Line trace isolation

2. Protection and diversion lines

Setting up diversion and protection lines is a very effective method for isolating and protecting critical signals, such as system clock signals in a noisy environment. In Figure 21, the parallel or protective lines inside the PCB are arranged along the critical signal lines. The protective circuit not only isolates the coupling magnetic flux generated by other signal lines, but also isolates critical signals from their coupling with other signal lines. The difference between a shunt circuit and a protection circuit is that the shunt circuit does not need to be terminated (connected to ground), but both ends of the protection circuit must be connected to ground. In order to further reduce coupling, the protective circuit in multi-layer PCBs can add a path to ground every other section.

 

Figure 11 Diversion and Protection Lines

3. Power cord design

According to the current of the printed circuit board, try to increase the width of the power line as much as possible and reduce the loop resistance. At the same time, aligning the direction of power and ground wires with the direction of data transmission helps to enhance noise resistance. In single panel or double-sided boards, if the power cord runs very long, coupling capacitors should be added to the ground every 3000mil, with a capacitance value of 10uF+1000pF.

4. Ground wire design

The principles of ground wire design are:

(1) Digitally separate from analog. If there are both logical and linear circuits on the circuit board, they should be separated as much as possible. The grounding of low-frequency circuits should be connected in parallel with a single point as much as possible. If there are difficulties in actual wiring, it can be partially connected in series and then connected to the ground in parallel. High frequency circuits should be grounded in series with multiple points, and the ground wire should be short and grounded. Grid shaped large-area ground foil should be used around high-frequency components as much as possible.

(2) The grounding wire should be thickened as much as possible. If the grounding wire is made of very sewn lines, the grounding potential will change with the change of current, resulting in a decrease in noise resistance performance. Therefore, the grounding wire should be thickened so that it can pass through three times the allowable current on the printed circuit board. If possible, the grounding wire should be at least 2-3mm.

(3) The grounding wire forms a closed loop. A printed circuit board composed solely of digital circuits, with its grounding circuit arranged in a circular loop, can mostly improve its noise resistance.

5. Signal Line Design

For critical signal lines, if the board has an internal signal routing layer, clock and other critical signal lines are laid in the inner layer, and the preferred routing layer is given priority. In addition, key signal lines must not cross the dividing area, including reference plane gaps caused by through holes and solder pads, otherwise it will lead to an increase in the area of the signal circuit. And the key signal line should be at least 3H away from the edge of the reference plane (H is the height of the line from the reference plane) to suppress edge radiation effects.

For strong radiation signal lines such as clock lines, buses, RF lines, and sensitive signal lines such as reset signal lines, chip selection signal lines, and system control signals, they should be kept away from interface outgoing signal lines. To avoid coupling interference from strong radiation signal lines to outgoing signal lines and radiating outward; It also avoids the coupling of external interference brought in by signal lines outside the interface to sensitive signal lines, leading to system misoperation.

For differential signal lines, they should be run in the same layer, of equal length, and in parallel to maintain consistent impedance, and there should be no other runs between the differential lines. Because ensuring the common mode impedance of differential line pairs is equal can improve their anti-interference ability.

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 Improving Air Conditioners

Overall, the improvement of PCB design on EMC is to study the design scheme of the reflux path before wiring, which has the best chance of success and can achieve the goal of reducing EMI radiation. And before actually wiring, changing the wiring layer and so on does not require any cost, which is the cheapest way to improve EMC.