When designing PCB, one of the most basic question to consider is to implement the requirements of the circuit functions need to how much a wiring layer, the ground plane and the power plane, and printed circuit board wiring layer, the ground plane and the power plane determination of the number of layers and the circuit function, signal integrity, EMI, EMC, manufacturing costs and other requirements.

For most designs, there are many conflicting requirements on PCB performance requirements, target cost, manufacturing technology, and system complexity. The laminated design of PCB is usually a compromise decision after considering various factors. High-speed digital circuits and whisker circuits are usually designed with multilayer boards.

Here are eight principles for cascading design:

1. Delamination

In a multilayer PCB, there are usually signal layer (S), power supply (P) plane and grounding (GND) plane. The power plane and GROUND plane are usually unsegmented solid planes that will provide a good low-impedance current return path for the current of adjacent signal lines.

Most of the signal layers are located between these power sources or ground reference plane layers, forming symmetric or asymmetric banded lines. The top and bottom layers of a multilayer PCB are usually used to place components and a small amount of wiring. The wiring of these signals should not be too long to reduce the direct radiation caused by wiring.

2. Determine the single power reference plane

The use of decoupling capacitors is an important measure to solve the power supply integrity. Decoupling capacitors can only be placed at the top and bottom of the PCB. The routing of decoupling capacitor, solder pad, and hole pass will seriously affect the effect of decoupling capacitor, which requires the design must consider that the routing of decoupling capacitor should be as short and wide as possible, and the wire connected to the hole should also be as short as possible. For example, in a high-speed digital circuit, it is possible to place the decoupling capacitor on the top layer of the PCB, assign layer 2 to the high-speed digital circuit (such as the processor) as the power layer, layer 3 as the signal layer, and layer 4 as the high-speed digital circuit ground.

In addition, it is necessary to ensure that the signal routing driven by the same high-speed digital device takes the same power layer as the reference plane, and this power layer is the power supply layer of the high-speed digital device.

3. Determine the multi-power reference plane

The multi-power reference plane will be split into several solid regions with different voltages. If the signal layer is adjacent to the multi-power layer, the signal current on the nearby signal layer will encounter an unsatisfactory return path, which will lead to gaps in the return path.

For high-speed digital signals, this unreasonable return path design can cause serious problems, so it is required that high-speed digital signal wiring should be away from the multi-power reference plane.

4. Determine multiple ground reference planes

 Multiple ground reference planes (grounding planes) can provide a good low-impedance current return path, which can reduce common-mode EMl. The ground plane and the power plane should be tightly coupled, and the signal layer should be tightly coupled to the adjacent reference plane. This can be achieved by reducing the thickness of the medium between layers.

5. Design wiring combination reasonably

The two layers spanned by a signal path are called a “wiring combination”. The best wiring combination is designed to avoid the return current flowing from one reference plane to another, but instead flows from one point (face) of one reference plane to another. In order to complete the complex wiring, the interlayer conversion of the wiring is inevitable. When the signal is converted between layers, the return current should be ensured to flow smoothly from one reference plane to another. In a design, it is reasonable to consider adjacent layers as a wiring combination.

If a signal path needs to span multiple layers, it is usually not a reasonable design to use it as a wiring combination, because a path through multiple layers is not patchy for return currents. Although the spring can be reduced by placing a decoupling capacitor near the through-hole or reducing the thickness of the medium between the reference planes, it is not a good design.

6. Setting wiring direction

When the wiring direction is set on the same signal layer, it should ensure that most wiring directions are consistent, and should be orthogonal to the wiring directions of adjacent signal layers. For example, the wiring direction of one signal layer can be set to the “Y-axis” direction, and the wiring direction of another adjacent signal layer can be set to the “X-axis” direction.

7. Adopted the even layer structure 

It can be found from the designed PCB lamination that the classical lamination design is almost all even layers, rather than odd layers, this phenomenon is caused by a variety of factors.

From the manufacturing process of printed circuit board, we can know that all the conductive layer in the circuit board is saved on the core layer, the material of the core layer is generally double-sided cladding board, when the full use of the core layer, the conductive layer of printed circuit board is even

Even layer printed circuit boards have cost advantages. Because of the absence of a layer of media and copper cladding, the cost of odd-numbered layers of PCB raw materials is slightly lower than the cost of even layers of PCB. However, the processing cost of ODd-layer PCB is obviously higher than that of even-layer PCB because the ODd-layer PCB needs to add a nonstandard laminated core layer bonding process on the basis of the core layer structure process. Compared with the common core layer structure, adding copper cladding outside the core layer structure will lead to lower production efficiency and longer production cycle. Prior to laminating, the outer core layer requires additional processing, which increases the risk of scratching and misetching the outer layer. The increased outer handling will significantly increase manufacturing costs.

When the inner and outer layers of the printed circuit board are cooled after the multi-layer circuit bonding process, the different lamination tension will produce different degrees of bending on the printed circuit board. And as the thickness of the board increases, the risk of bending a composite printed circuit board with two different structures increases. Odd-layer circuit boards are easy to bend, while even-layer printed circuit boards can avoid bending. 

If the printed circuit board is designed with an odd number of power layers and an even number of signal layers, the method of adding power layers can be adopted. Another simple method is to add a grounding layer in the middle of the stack without changing the other Settings. That is, the PCB is wired in an odd number of layers, and then a grounding layer is duplicated in the middle.

8.  Cost Consideration

In terms of manufacturing cost, multilayer circuit boards are definitely more expensive than single and double layer circuit boards with the same PCB area, and the more layers, the higher the cost. However, when considering the realization of circuit functions and circuit board miniaturization, to ensure signal integrity, EMl, EMC and other performance indicators, multi-layer circuit boards should be used as far as possible. Overall, the cost difference between multi-layer circuit boards and single-layer and two-layer circuit boards is not much higher than expected