PCB Technical - PCB layer design and emc discussion

In the process of high-speed circuit board design, electromagnetic compatibility design is an important and difficult point. This paper discusses how to reduce the electromagnetic interference caused by conduction coupling and radiation coupling and improve the electromagnetic compatibility by reducing the propagation path of coupling source from the aspects of layer number design and layer layout.

The introduction

Many reliability and stability problems of electronic products are caused by poor electromagnetic compatibility design. Common problems are signal distortion, signal noise is too large, signal instability in the process of work, the system is easy to crash, the system is susceptible to environmental interference, poor anti-interference ability, etc. Emc design is a very complex technology, from design to electromagnetism and other aspects of knowledge. This paper discusses some experiential skills of layer design and layer layout to provide some reference for electronic engineers.

Layer configuration

PCB board layer mainly includes power layer, layer and signal layer, layer number is the sum of the number of each layer. In the design process, the step is to coordinate and classify all sources and places, 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 ground planes should be corresponding. Various special signals, such as clock high and frequency signals, need separate design layer, and need to increase the ground plane to shield special signals, in order to improve electromagnetic compatibility. Of course, cost is also a factor to consider, and a balance should be found between electromagnetic compatibility and cost during the design process.

The first consideration in the design of power supply layer is the type and quantity of power supply. If there is only one power supply, consider a single power supply layer. In the case of high demand for power supply, multiple power supply layers can 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. If there is crossover, multiple power supply layers must be designed.

The design of signal layers takes into account the characteristics of all signals. Stratification and shielding of special signals are limited problems. Generally, it is designed with design software first and then modified according to specific details. Both signal density and the integrity of a particular signal must be considered in layer design. For special information, the ground plane must be designed as a shielding layer when necessary.

In general, it is not recommended to design single or double panels for reasons other than pure cost. Because single panel and double panel although the processing is simple and low cost, but in the case of high signal density and complex signal structure, such as high-speed digital circuit or analog mixed circuit, because single panel does not have a special reference grounding layer, the loop area increases, the radiation enhancement. Due to the lack of effective shielding, the anti-jamming ability of the system is also reduced.

PCB board layer layout design

After determining the signal and layer, the layout of each layer also needs to be designed scientifically.

The layout of PCB board design follows the following principles:

(1) Adjacent the power supply layer plane to the corresponding ground plane. The purpose of this design is to form coupling capacitance, and in conjunction with the decoupling capacitance on the PCB board, reduce the impedance of the power plane, while achieving a wider filtering effect.

(2) The selection of reference layer is very important. Theoretically, both the power supply layer and the ground plane can be used as the reference layer, but the ground plane can generally be grounded, so the shielding effect is much better than the power supply layer. Therefore, the ground plane is generally preferred as the reference plane.

(3) Key signals of two adjacent layers cannot cross the segmentation area. Otherwise, a larger signal loop will be formed, resulting in strong radiation and coupling.

(4) To maintain the integrity of the ground plane, wiring on the ground plane is not allowed. If the signal line density is too high, wiring on the edge of the power layer can be considered.

(5) in the high-speed signal, signal, high frequency signal and other key signals below the design of ground layer, so that the path of the signal loop is the shortest, the radiation is the smallest.

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

General principles to follow in the layout design of multi-layer printed boards:

(1) The power supply layer plane should be close to the grounding plane, and designed under the grounding plane.

(2) Wiring layer should be designed adjacent to the whole metal plane.

(3) digital signal and analog signal to have isolation design, first of all to avoid digital signal and analog signal in the same layer, if can not avoid, can use analog signal and digital signal area wiring, with grooves and other ways to isolate the analog signal area and digital signal area. The same is true for analog and digital power supplies. Especially the digital power supply, the radiation is very large, must be isolated and screened.

(4) The printed lines in the middle layer form a planar waveguide, and the microstrip lines in the surface layer form different transmission characteristics.

(5) Clock circuit and high frequency circuit is the main source of interference and radiation, must be arranged separately, away from the sensitive circuit.

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

Conclusion

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