PCBA Tech - The PCB design technology of EMC is explained in detail from layering, layout and wiring

In addition to the selection of components and circuit design, good printed circuit board (PCB) design is also a very important factor in electromagnetic compatibility. The key of PCB EMC design is to reduce the backflow area as much as possible and make the backflow path flow in the design direction. Common problems with return current arise from cracks in the reference plane, shifting the reference plane layer, and signals flowing through the connector. Bonding or decoupling capacitors may solve some problems, but the overall impedance of the capacitor, the holes, the pads, and the wiring must be considered. This lecture will introduce EMC's PCB design technology from three aspects of PCB layering strategy, layout skills and wiring rules.

PCB layering strategy

In circuit board design, thickness, perforation process and layer number of circuit board are not the key to solve the proble

m. Good layering is the key to ensure the bypass and decoupling of power busbar, to make the transient voltage on the power supply layer or ground layer, and to shield the signal from the power supply electromagnetic field. In terms of signal routing, a good layering strategy is to place all signal routing in one or more layers, which are immediately adjacent to the power supply layer or ground layer. For the power supply, a good layering strategy should be that the power supply layer is adjacent to the ground layer and the distance between the power supply layer and the ground layer should be as small as possible. This is what we call the "layering" strategy. Let's talk more about good PCB layering strategies.

1. The projection plane of the wiring layer shall be within its reflow plane layer region. If the wiring layer is not in the ground projection area of its backflow plane layer, there will be signal lines outside the projected area during wiring, leading to the problem of "edge radiation". Moreover, it will also lead to the increase of signal loop area, resulting in the increase of differential mode radiation.

2. Try to avoid adjacent wiring layer Settings. Since parallel signal routing on adjacent wiring layers will lead to signal crosstalk, if adjacent wiring layers cannot be avoided, the spacing between the two wiring layers should be appropriately enlarged and the spacing between the wiring layer and its signal loop should be reduced.

3. Overlapping projection planes of adjacent plane layers should be avoided. Because when the projection overlaps, the coupling capacitance between layers will lead to the 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 make the signal loop area can be well controlled, it is generally necessary to use the multi-layer design. In the design of multi-layer board should pay attention to the following principles:

1. The critical wiring layer (the layer where clock cables, bus cables, interface signal cables, rf cables, reset signal cables, chip selection signal cables, and various control signal cables are located) should be adjacent to the complete ground plane, preferably between two planes. The key signal lines are generally strong radiation or extremely sensitive signal lines. Wiring close to the ground plane can reduce the signal loop area, reduce the radiation intensity or improve the anti-interference ability.

In addition, the single-board main working power plane (the power plane that is widely used) should be adjacent to its ground plane to effectively reduce the loop area of the power current.

3. Check whether the signal cable ≥50MHz exists at the TOP and BOTTOM layers of the board. If so, high-frequency signals are placed between two planar layers to suppress their radiation to space.

Single plate and double plate design:

For single-layer and double-layer panels, pay attention to the design of key signal cables and power cables. To reduce the area of the power supply current loop, a ground cable must be installed adjacent to and parallel to the power supply.

The "Guide Ground Line" should be placed on both sides of the key signal cables of the single-layer plate, as shown in Figure 4. There should be a large area of Ground laying on the projection plane of key signal Line of double-layer plate, or the same treatment method of single-layer plate, and the "Guide Ground Line" should be designed. The "guard ground cables" on both sides of key signal lines can reduce the signal loop area on the one hand and prevent crosstalk between signal lines and other signal lines on the other.

In general, PCB board layering can be designed according to the following table.

PCB Layout Tips

PCB layout design should fully comply with the design principle of placing along the straight line of signal flow direction, and avoid circling as far as possible. In this way, the direct coupling of signals can be avoided and the quality of signals can be affected. In addition, to prevent interference and coupling between circuits and electronic components, the placement of circuits and the layout of components should comply with the following principles:

1. If the interface "clean floor" is designed on the board, the filter and isolation devices should be placed on the isolation belt between the "clean floor" and the working ground. This prevents filtering or isolation devices from coupling with each other through the plane layer, weakening the effect. In addition, no other devices other than filters and protective devices may be placed on the "clean floor".

2. When multiple module circuits are placed on the same PCB, digital circuit and analog circuit, high speed circuit and low speed circuit should be separately arranged to avoid mutual interference between digital circuit, analog circuit, high speed circuit and low speed circuit. In addition, when there are high, medium and low speed circuits on the circuit board at the same time, the layout principle in Figure 7 should be followed to avoid high frequency circuit noise radiating out through the interface.

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

4. The filtering, protection and isolation devices of the interface circuit are placed close to the interface, as shown in FIG. 9, which can effectively realize the effects of protection, filtering and isolation. If the interface has both filtering and protection circuits, the principle of protection before filtering should be followed. Because the guard circuit is used for external overvoltage and overcurrent suppression, if the guard circuit is placed behind the filter circuit, the filter circuit will be damaged by overvoltage and overcurrent. In addition, because the coupling of input and output lines of the circuit weakens the filtering, isolation, or protection effect, the layout should ensure that the input and output lines of the filter circuit (filter), isolation, and protection circuit are not coupled with each other.

5. The sensitive circuit or device (such as reset circuit) is at least 1000mil away from the edge 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 variations (such as input and output terminals of power modules, fans and relays) to reduce the loop area of large current circuits.

7. The filters must be placed side by side to prevent the filtered circuit from being disturbed again.

8. The devices with strong radiation, such as crystals, crystal oscillators, relays, and switching power supplies, should be at least 1000mil away from the interface connectors on boards. In this way, the interference can be directly radiated outward or the current can be coupled on the outgoing cable to radiate outward.