When designing pcb, it is necessary to pay attention to such a basic situation, that is, how many wiring layers, grounding planes and power planes are needed to achieve the required functions of the circuit, and the establishment of the layers of wiring layers, grounding planes and power planes of PCB is related to the requirements of basic circuit functions, signal integrity, EMI, EMC and manufacturing costs.
Compared with most designs, there are many conflicting requirements for the key factors such as performance requirements, cost, manufacturing technology and system complexity of PCB. Generally speaking, the laminated design of PCB is decided by compromise after considering all the key factors. Generally speaking, high-speed circuits and RF circuits are designed with multilayer boards.
layer
Multi-layer PCB usually contains all the signal, power plane and ground plane. Generally speaking, the power plane and the ground plane are solid planes without division. They will provide a good low-impedance current return path for the current of adjacent signal traces.
Most of the signal layers are located between these power supply or ground reference plane layers, forming symmetric striplines or asymmetric striplines. Generally speaking, the top and bottom layers of PCB should be used to prevent components and a small amount of wiring. The wiring requirements of such signals should not be too long to reduce the direct radiation caused by wiring.
Determine the single supply reference plane.
Using safe decoupling capacitor is a crucial measure to deal with the integrity of power supply. Decoupling capacitors can only be stored on the top and bottom layers of PCB.
The traces, pads and vias of decoupling capacitors will seriously affect the effect of decoupling capacitors. Therefore, in the design, it is necessary to give full consideration to the wiring connecting the decoupling capacitor, which should be as short and wide as possible, and the wiring connecting to the through hole should be as short as possible.
Determine the multi-power reference plane.
The multi-power reference plane will be divided into several physical areas with different voltages. If the signal layer is adjacent to the multi-power supply layer, the signal current on the signal layer nearby may encounter an unsatisfactory return path, resulting in a gap in the return path.
Compared with high-speed digital signals, these unreasonable return path designs may cause serious problems, so the wiring of high-speed digital signals needs to be far away from the multi-power reference plane.
Determining a plurality of ground reference planes
Many grounding reference planes can create a good low impedance current return path, which can greatly reduce common-mode EMI.
The ground plane and the power plane should be closely coupled, and the signal layer should also be closely coupled with the adjacent reference plane. Reduce the dielectric thickness between layers to achieve this purpose.
Reasonably design wiring combination
The two levels spanned by a signal path are a kind of [wiring combination]. The most suitable wiring combination design is to avoid the return current from one reference plane to another. But from one point (face) of a reference plane to another point (face).
In order to realize complex wiring, it is inevitable to switch between layers. During the signal layer transition, it is necessary to ensure that the return current can smoothly flow from one reference plane to another.