PCB can be divided into single-sided, double-sided and multi-layered boards from the structure, and the design focus of different boards is also different. Let's take a look at the PCB layering strategy and the design principles of PCB multilayer boards.
From the signal routing point of view, the ideal layering strategy should be to place all signal traces on one or several layers next to the power or ground plane. For the power supply, the ideal layering strategy should be that the power layer is adjacent to the ground layer, and the distance between the power layer and the ground layer is as small as possible, which is what we call the "layering" strategy.
1. The layering strategy of PCB board is as follows:
1.1. The projection plane of the wiring layer should be in the area of the reflow plane layer. If the wiring layer is not in the projection area of its return plane layer, there will be signal lines outside the projection area during wiring, which will cause the problem of "edge radiation", and will also cause the signal loop area to increase, resulting in increased differential mode radiation. .
1.2. Try to avoid setting adjacent wiring layers. Because parallel signal traces on adjacent wiring layers will 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 loops should be reduced.
1.3. Adjacent plane layers should avoid overlapping of their projection planes. Because when the projections overlap, the coupling capacitance between the layers will cause the noise between the layers to couple to each other.
2. Design principles of PCB multilayer boards
When the clock frequency exceeds 5MHz, or the signal rise time is less than 5ns, in order to control the signal loop area well, it is generally necessary to use a multi-layer board design (high-speed PCBs are generally designed with multi-layer boards). When designing multi-layer boards, we should pay attention to the following principles:
2.1. The key wiring layer (the layer where the clock line, bus, interface signal line, radio frequency line, reset signal line, chip select signal line and various control signal lines are located) should be adjacent to the complete ground plane, preferably between the two ground planes . The key signal lines are generally strong radiation or extremely sensitive signal lines, and wiring close to the ground plane can reduce the area of the signal loop, reduce its radiation intensity or improve the anti-interference ability.
2.2. The power plane should be retracted relative to its adjacent ground plane (recommended value 5H ~ 20H). Retracting the power plane relative to its return ground plane can effectively suppress the "edge radiation" problem. In addition, the main working power plane of the board (the most widely used power plane) should be in close proximity to its ground plane to effectively reduce the loop area of the power supply current.
2.3. Whether there is no signal line ≥50MHz on the TOP and BOTTOM layers of the single board. If there is, it is better to walk the high-frequency signal between the two plane layers to suppress its radiation to space.
Knowledge expansion: The number of layers of a multi-layer PCB depends on the complexity of the circuit board. The number of layers and stacking scheme of a PCB design depends on the cost of hardware, the outgoing lines of high-density components, signal quality control, schematic signal definition, and PCB manufacturers. Processing capability baseline and other factors.
3. PCB single-layer board and double-layer board design
For the design of single-layer boards 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 adjacent to the power trace and parallel traces to reduce the area of the power supply current loop.
The "Guide Ground Line" should be placed on both sides of the key signal lines of the single-layer board. There should be a large area of ground on the projection plane of the key signal lines of the double-layer board, or a "Guide Ground Line" should be designed in the same way as the single-layer board. The "guard ground" on both sides of the key signal line can reduce the signal loop area on the one hand, and also prevent crosstalk between the signal line and other signal lines.
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