What do PCB laminated design need to pay attention to?

In the design of PCB, one of the most basic issues to consider is how many wiring layers, grounding plane and power plane are needed to achieve the required function of the circuit, and the determination of the wiring layer, grounding plane and power plane of the printed circuit board is related to the circuit function, signal integrity, EMI, EMC, manufacturing cost and other requirements.

For most designs, there are many conflicting requirements for PCB performance requirements, target cost, manufacturing techniques, and complexity of the system, and the PCB laminated design is usually a compromise decision after considering various factors. High - speed digital circuits and beam circuits are usually multilayer design.

Here are eight principles of cascading design to be aware of.

stratification

A multilayer PCB usually contains a signal layer (S), a power supply (P) plane and a ground (GND) plane. The power plane and the ground plane are usually undivided solid planes that will provide a good low-impedance current return path for the current of adjacent signal routes.

Most of the signal layers are located between these power or ground reference plane layers, forming symmetrical or asymmetric strip lines. The top and bottom layers of multilayer PCB are usually used for placing components and a small amount of wiring. These signal wiring requirements should not be too long to reduce the direct radiation generated by the wiring.

Determine the single power reference plane

The use of decoupling capacitors is an important measure to solve the integrity of power supply. Decoupling capacitors can only be placed on the top and bottom layers of the PCB. The routing, pad and hole of the decoupling capacitor will seriously affect the effect of the decoupling capacitor, which requires that the design must consider the connection of the 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, decoupling capacitors can be placed on the top layer of the PCB, layer 2 assigned 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.

Determine the multi-power reference plane

The multiple power reference planes will be divided into several solid regions with different voltages. If the signal layer is close to the multi-power layer, the signal current in the signal layer near it will encounter an unsatisfactory return path, resulting in gaps in the return path.

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

Determine multiple ground reference planes

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

Rational design of wiring combination

The two layers that a signal path crosses are called a "wiring combination". The best wiring combination design is to avoid the flow of return current from one reference plane to another reference plane, but from one point (face) of one reference plane to another point (face). In order to complete complex wiring, interlayer switching is inevitable. When switching between signal layers, ensure that the return current can 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 passing through multiple layers is not unobstructed for return current. Although the ground elastic can be reduced by placing decoupling capacitors near the holes or reducing the thickness of the medium between the reference planes, it is not a good design.

Set routing direction

In the same signal layer, the direction of most wiring should be ensured to be consistent, and at the same time should be orthogonal to the direction of wiring in adjacent signal layers. For example, one signal layer can be wired in a "Y-axis" direction and another adjacent signal layer can be wired in an "X-axis" direction.

Adopt even layer structure

From the PCB board lamination design, it can be found that the classic 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 the printed circuit board, we can know that all the conductive layers in the circuit board are saved on the core layer. The material of the core layer is generally double-sided cladding board. When the core layer is fully utilized, the number of conductive layers of the printed circuit board is even.

Even layer PCB has cost advantage. As a result of less a layer of media and copper coating, so the cost of the odd layer of printed circuit board raw materials is slightly lower than the cost of the even layer of printed circuit board. However, the processing cost of odd-numbered PCB is obviously higher than that of even-numbered PCB because it needs to add non-standard laminated core layer bonding technology on the basis of the core layer structure technology. Compared with the conventional core structure, adding copper coating to the core structure will lead to decreased production efficiency and prolonged production cycle. The outer core layer requires additional processing before laminating, which increases the risk of scratches and misetched outer layers. The added outer processing will significantly increase manufacturing costs.

When the PCB circuit board is in the multi-layer circuit bonding process, the inner and outer layers are cooled, the different lamination tension will cause different degrees of bending on the PCB. And as the thickness of the board increases, the risk of bending becomes greater for composite printed boards with two different structures. Odd layer circuit boards are easy to bend, even layer printed circuit boards can avoid circuit board bending.

If you have an odd number of layers at design time, you can increase the number of layers by doing the following.

If the PCB is designed with an even number of power layers and an odd number of signal layers, the method of adding signal layers can be adopted. The addition of signal layer will not lead to cost increase, but can shorten the processing time and improve the quality of PCB.

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 used. Another simple method is to add a connection layer in the middle of the stack without changing the other Settings, that is, to route an odd number of layers of printed circuit boards, and then replicate a connection layer in the middle.

In microwave circuits and mixed media (media with different permittivity) circuits, a blank signal layer can be added near the center of the PCB stack to minimize stack unbalance.

Cost consideration

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