PCB Blog - Discussion on PCB Board Design and Electromagnetic Compatibility

In the process of high-speed PCB board design, electromagnetic compatibility design is a key and difficult point. This paper discusses how to reduce the electromagnetic interference caused by conduction coupling and radiation coupling and improve electromagnetic compatibility from the aspects of layer design and layer layout. Many reliability and stability problems of electronic products are caused by the failure of electromagnetic compatibility design. Common problems include signal distortion, excessive signal noise, unstable signal during work, the system is prone to crash, the system is susceptible to environmental interference, and the anti-interference ability is poor. Electromagnetic compatibility design is a fairly complex technology, from design to knowledge of electromagnetics and so on.

Layer configuration

The layers of the PCB board mainly include the power layer, the ground layer and the signal layer, and the number of layers is the sum of the number of each layer. In the design process, the first step is to organize and classify all sources and grounds, as well as various signals, and deploy and design on the basis of classification. Under normal circumstances, different power supplies should be divided into different layers, and different grounds should also have corresponding ground planes. Various special signals, such as clock high and frequency signals, need to be designed separately, and a ground plane needs to be added to shield special signals to improve electromagnetic compatibility. Of course, cost is also one of the factors to be considered. In the design process, a balance point should be found between the electromagnetic compatibility of the system and the cost. The first consideration in the design of the power plane is the type and quantity of the power supply. If there is only one power supply, consider a single power plane. In the case of high power requirements, there can also be multiple power layers to supply power to devices of different layers. If there are multiple power supplies, you can consider designing multiple power supply layers, or you can divide different power supplies in the same power supply layer. The premise of segmentation is that there is no crossover between power supplies, and if there is crossover, multiple power supply layers must be designed. The design of the number of signal layers should take into account the characteristics of all signals. The layering and shielding of special signals are issues to be considered limited. Under normal circumstances, the design is first designed with the design software, and then modified according to the specific details. Both signal density and special signal integrity must be considered in layer design. For special information, be sure to design a ground plane layer as a shield if necessary. In general, single- or double-sided designs are not recommended if not purely for cost. Although single-sided and double-sided boards are simple to process and cost-effective, in the case of high signal density and complex signal structure, such as high-speed digital circuits or analog-digital hybrid circuits, since there is no special reference ground layer for single-sided boards, the loop is The area increases and the radiation increases. Due to the lack of effective shielding, the anti-jamming capability of the system is also reduced. The layout design of the PCB board layer, after the signals and layers are determined, the layout of each layer also needs to be scientifically designed.

The layout design of the middle layer of the PCB board design follows the following principles:

1) Place the power plane plane adjacent to the corresponding ground plane. The purpose of this design is to form a coupling capacitor and work together with the decoupling capacitor on the PCB to reduce the impedance of the power plane and obtain a wider filtering effect.

2) The choice of the reference layer is very important. In theory, both the power layer and the ground plane can be used as the reference layer, but the ground plane can generally be grounded, so that the shielding effect is much better than that of the power plane. Therefore, in general, the ground plane is preferred as the reference layer. reference plane.

3) The key signals of two adjacent layers cannot cross the partition. Otherwise, a large signal loop will be formed, resulting in strong radiation and coupling.

4) To maintain the integrity of the ground plane, no traces can be made on the ground plane. If the signal line density is too large, you can consider routing at the edge of the power plane.

5) The ground layer is designed under the high-speed signal, pilot signal, high-frequency signal and other key signals, so that the path of the signal loop is the shortest and the radiation is the smallest.

6) How to deal with the radiation of the power supply and the interference to the whole system must be considered in the high-speed circuit design process. In general, the area of the power plane should be smaller than the area of the ground plane, so that the ground plane can shield the power supply. Generally, the power plane is required to be indented by 2 times the dielectric thickness of the ground plane. If you want to reduce the indentation of the power plane, it is necessary to make the thickness of the dielectric as small as possible.

General principles to be followed in the layout design of multilayer printed boards:

1) The power plane plane should be close to the ground plane and designed below the ground plane.

2) The wiring layer should be designed to be adjacent to the entire metal plane.

3) The digital signal and the analog signal should have an isolation design. First of all, it is necessary to avoid the digital signal and the analog signal being on the same layer. If it cannot be avoided, the analog signal and the digital signal can be routed in different areas, and the analog signal area and the analog signal area can be separated by slotting and other methods. Digital signal area isolation. The same is true for analog power supplies and digital power supplies. Especially digital power, radiation is very large, must be isolated and shielded.

4) The printed lines in the middle layer form a planar waveguide, and the surface layer forms a microstrip line, and the transmission characteristics of the two are different.

5) Clock circuits and high-frequency circuits are the main sources of interference and radiation, and must be arranged separately and away from sensitive circuits.

6) The stray current and high-frequency radiation current contained in different layers are different, and cannot be treated equally when wiring.

The electromagnetic compatibility of the PCB board can be greatly improved by the number of layers design and the layout of the layers. The number of layers design mainly considers the power layer and ground layer, high-frequency signals, special signals, and sensitive signals. Layer layout mainly considers various coupling, ground and power line layout, clock and high-speed signal layout, analog signal and digital information layout.