PCB Tech - Electromagnetic compatibility of the circuit board design process

For PCB manufacturers, electromagnetic compatibility design is definitely a focus of the circuit board design process. This article will talk about how to improve electromagnetic compatibility from two aspects.

1. Introduction

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, system crashes, system susceptible to environmental interference, and poor anti-interference ability. Electromagnetic compatibility design is a very complex technology, from design to knowledge of electromagnetics and so on. This article discusses some empirical skills from layer design and layer layout to provide some references for electronic engineers.

Second, the configuration of the number of layers

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 coordinate and classify all sources and grounds, as well as various signals, and deploy and design on the basis of classification. In general, different power supplies should be divided into different layers, and different grounds should have corresponding ground planes. Various special signals, such as high clock and frequency signals, need to be designed separately, and a ground plane needs to be added to shield the special signals to improve electromagnetic compatibility. When cost is also one of the factors to be considered, a balance should be found between the electromagnetic compatibility and cost of the system during the design process.

The first consideration in the design of the power layer is the type and quantity of the power supply. If there is only one power supply, a single power layer can be considered. 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 layers, or you can divide different power supplies on the same power layer. The premise of the division is that there is no crossover between the power supplies. If there is a 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 is a priority. Under normal circumstances, it is first to use design software to design, and then modify according to specific details. Both the signal density and the integrity of the special signal must be the issues that must be considered in the layer design. For special information, the ground plane layer must be designed as a shielding layer when necessary.

Under normal circumstances, it is not recommended to design single or double panels unless the cost is purely considered. Although single-panel and double-panel processing are simple and cost-effective, but in the case of high signal density and complex signal structure, such as high-speed digital circuits or analog-to-digital hybrid circuits, the single-panel does not have a dedicated reference ground layer, which makes the loop The area increases and the radiation increases. Due to the lack of effective shielding, the system's anti-interference ability is also reduced.

Third, the layout design of the PCB layer

After determining the signal and layer, 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) Adjacent the power plane plane to the corresponding ground plane. The purpose of this design is to form a coupling capacitor and work with the decoupling capacitor on the PCB to reduce the impedance of the power plane while obtaining a wider filtering effect.

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

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

(4) To maintain the integrity of the ground plane, it is not possible to route the wires on the ground plane. If the signal wire density is too large, you can consider routing the wires on the edge of the power plane.

(5) Design a grounding layer below the key signals such as high-speed signals, test signals, high-frequency signals, etc., 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 entire system must be considered in the process of high-speed circuit design. Generally, 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 thickness of the medium than the ground plane. If you want to reduce the indentation of the power layer, the thickness of the medium must be as small as possible.

General principles to be followed in the layout design of multi-layer PCB:

(1) The power 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 must be designed to be isolated. First, avoid the digital signal and the analog signal on the same layer. If it is unavoidable, you can use the analog signal and the digital signal to divide the regional wiring, and use the slot to divide the analog signal area. Isolated from the digital signal area. The same is true for analog power supplies and digital power supplies. Especially the digital power supply has very large radiation, so it must be isolated and shielded.

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

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

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

Through the layer design and layer layout, the electromagnetic compatibility of the PCB can be greatly improved.

The layer number design mainly considers the power layer and ground layer, high-frequency signals, special signals, and sensitive signals.

The layer layout mainly considers various couplings, ground and power line layouts, clock and high-speed signal layouts, analog signals and digital information layouts.