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When designing a PCB, one of the most basic issues to consider is how many wiring planes, ground planes, and power planes are needed to achieve the functions required by the circuit, and the number of planes of the printed circuit board wiring planes, ground planes and power planes is determined. All these are related to circuit function, signal integrity, EMI, EMC, manufacturing cost, etc. requirements.

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

Here are 8 rules you should notice when designing PCB stack up.

1. PCB signal, power, and ground plane

There is a signal plane, power plane, and ground plane in multiple-layers PCBs. The power plane and ground plane usually have no split solid plane, and they will provide a good and low impedance electric current returning path for adjacent signal traces.

Most signal planes are in between power planes or ground reference plane planes, forming symmetrical stripline or asymmetrical stripline. In multiple-layer PCBs, the top plane and the bottom one are usually used for placing components and few traces, requiring the signal traces not so long as to reduce direct radiation generated itself.

2. Defining Single Power Supply Reference Plane

The use of decoupling capacitors is an important measure to address power integrity. Decoupling capacitors should only be placed on the top and bottom planes of the PCB. The traces, pads, and vias of the decoupling capacitors will seriously affect the effect of the decoupling capacitors. This requires that the traces connected to the decoupling capacitors should be as short and wide as possible, and the wires connected to the vias should also be as short as possible. For example, in a high-speed digital circuit, decoupling capacitors can be placed on the top plane of the PCB, with plane 2 assigned to the high-speed digital circuit (such as a processor) as the power plane, plane 3 as the signal plane, and plane 4 as the Set to high-speed digital circuit ground.

In addition, try to ensure that the signal traces driven by the same high-speed digital device use the same power plane as the reference plane, and this power plane is the power supply plane of the high-speed digital device.

3. Defining the Multiple Power Supplies Reference Plane

The multi-supply reference plane will be divided into several solid areas with different voltages. If the signal plane is in close proximity to the multi-power supply plane, the signal current on the adjacent signal plane will encounter an unsatisfactory return path, causing gaps in the return path.

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

4. Defining More Ground Reference Planes

Multiple ground reference planes (ground planes) can provide a good low-impedance current return path that reduces common-mode EMl. Ground and power planes should be tightly coupled, and signal planes should be tightly coupled with adjacent reference planes. This can be achieved by reducing the thickness of the dielectric between planes.

5. Reasonably Design Wiring Combo

The two planes spanned by a signal path are called a "wiring combo". The best wiring combo design is to avoid return current flow from one reference plane to another reference plane, but rather to flow from one point (surface) of one reference plane to another point (surface). In order to complete complex wiring, the interplane conversion of traces is inevitable. When transitioning between signal planes, ensure that the return current can flow smoothly from one reference plane to the other. In a design, it is reasonable to use adjacent planes as a wiring combo.

If a signal path needs to span multiple planes, it is usually not a reasonable design to combine it as wiring, because a path through multiple planes is not clear for return currents. Although the ground bounce can be reduced by placing decoupling capacitors near the vias or reducing the thickness of the dielectric between the reference planes, it is not a good design.

6. Setting up the Current Distribution Direction

On the same signal plane, the direction of most wiring should be consistent, and it should be orthogonal to the wiring direction of adjacent signal planes. For example, the wiring direction of one signal plane may be set to the "Y-axis" direction, and the wiring direction of another adjacent signal plane may be set to the "X-axis" direction.

7. Use even-numbered layers

From the designed PCB stack-up, it can be found that the classic stack-up design is almost all even-numbered layers instead of odd-numbered layers. This phenomenon is caused by a variety of factors.

It can be known from the manufacturing process of the printed circuit board that all the conductive planes in the circuit board are saved on the core plane, and the material of the core plane is generally a double-sided cladding board. When the core plane is fully utilized, the conductive plane of the printed circuit board The number is even.

Even-layer printed circuit boards have cost advantages. Due to one less layer of dielectric and copper cladding, the cost of raw materials for odd-layer printed circuit boards is slightly lower than that of even-layer printed circuit boards. However, because the odd-numbered layer printed circuit board needs to add a non-standard laminated core plane bonding process based on the core plane structure process, the processing cost of the odd-numbered layer printed circuit board is significantly higher than that of the even-numbered layer printed circuit board. Compared with the ordinary core plane structure, adding copper cladding outside the core layer structure will lead to a decrease in production efficiency and a longer production cycle. The outer core planes require additional processing prior to lamination bonding, which increases the risk of scratching and mismatching of the outer planes. The added outer plane treatment will substantially increase the manufacturing cost.

When the inner and outer planes of the printed circuit board are cooled after the multiple layer circuit bonding process, different lamination tensions will cause the printed circuit board to bend to different degrees. And as the thickness of the board increases, the risk of bending a composite printed circuit board with two different structures increases. The odd-numbered circuit board is easy to bend, and the even-numbered printed circuit board can avoid bending of the circuit board.

When designing, if there is an odd number of layers of stacking, the following methods can be used to increase the number of layers.

If the power supply plane of the design printed circuit board is even and the signal plane is odd, the method of increasing the signal plane can be used. The added signal plane does not lead to an increase in cost, but can reduce processing time and improve printed circuit board quality.

If you design a printed circuit board with an odd number of power planes and an even number of signal planes, you can use the method of adding power planes. Another simple method is to add a ground plane in the middle of the stack up without changing other settings, that is, first route the printed circuit board in odd layers, and then duplicate a ground plane in the middle.

In microwave circuits and mixed-dielectric (dielectrics with different dielectric constants) circuits, a blank signal plane can be added near the center of the printed circuit board stack-up to minimize stack-up imbalance.

8. Cost Consideration

In terms of manufacturing cost, with the same PCB area, the cost of multiple-layer circuit boards is definitely higher than that of single-plane and double-layer circuit boards, and the more layers, the higher the cost. However, when considering factors such as realizing circuit function and circuit board miniaturization, ensuring signal integrity, EMI, EMC, and other performance indicators, multiple-layer circuit boards should be used as much as possible. In comprehensive evaluation, the cost difference between multiple-layer circuit boards and single-plane circuit boards is not much higher than expected.