Precautions for PCB stack-up design for electronic engineers

When designing a PCB (printed circuit board), one of the most basic issues to consider is how many wiring layers, ground planes and power planes are needed to achieve the functions required by the circuit, and the wiring layers, ground planes and power planes of the printed circuit board The determination of the number of layers of the plane is related to the requirements of circuit function, signal integrity, EMI, EMC, manufacturing cost and so on. 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 RF circuits are often designed with multilayer boards.

Listed below are 8 principles to be aware of in cascading design

1. Layering

In a multi-layer PCB, there are usually signal layers (S), power (P) planes and ground (GND) planes. Power and ground planes are usually undivided solid planes that will provide a good, low-impedance current return path for current from adjacent signal traces. Signal layers are mostly located between these power or ground reference plane layers, forming either symmetrical striplines or asymmetrical striplines. The top and bottom layers of a multi-layer PCB are usually used to place components and a small number of traces. These signal traces are not required to be too long to reduce the direct radiation generated by the traces.

2. Determine the single-supply reference plane (power 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 layers 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 layer of the PCB, with layer 2 assigned to the high-speed digital circuit (such as a processor) as the power plane, layer 3 as the signal layer, and layer 4 as the Set up as a high-speed digital circuit board.

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

3. Determine the multi-power reference plane

The multi-supply reference plane will be divided into several solid areas with different voltages. If the signal layer is in close proximity to the multi-power supply layer, the signal current on the adjacent signal layer 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. Identify multiple ground reference planes (ground 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 layers should be tightly coupled with adjacent reference planes. This can be achieved by reducing the thickness of the dielectric between layers.

5. Reasonable design of wiring combination

The two layers spanned by a signal path are called a "routing combination". The best routing combination 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 interlayer conversion of traces is inevitable. When transitioning between signal layers, ensure that the return current can flow smoothly from one reference plane to the other. In a design, it is reasonable to use adjacent layers as a routing combination. If a signal path needs to span multiple layers, it is usually not a reasonable design to combine it as a routing, because a path through multiple layers 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. Set the wiring direction

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

7. Adopt an even-numbered layer structure

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

It can be known from the manufacturing process of the printed circuit board that all the conductive layers in the circuit board are on the core layer, and the material of the core layer is generally a double-sided cladding board. When the core layer is fully utilized, the conductive layer 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 layer bonding process on the basis of the core layer 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 layer 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 layers require additional processing prior to lamination bonding, which increases the risk of scratching and misetching of the outer layers. The added outer layer treatment will substantially increase the manufacturing cost.

When the inner and outer layers of the printed circuit board are cooled after the multi-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 the 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 layer of the design printed circuit board is even and the signal layer is odd, the method of increasing the signal layer can be used. The added signal layer 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 stackup 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 layer can be added near the center of the printed circuit board stackup to minimize stackup imbalance.

8. Cost Considerations

In terms of manufacturing cost, with the same PCB area, the cost of multi-layer circuit boards is definitely higher than that of single-layer 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, EMl, EMC and other performance indicators, multi-layer circuit boards should be used as much as possible. Comprehensive evaluation, the cost difference between multi-layer circuit boards and single-layer circuit boards is not much higher than expected.

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