When designing a PCB (printed circuit board), one of the most basic questions to consider is how many wiring layers, ground planes and power planes are required to achieve the functions required by the circuit, while the wiring layers, ground planes and power supply of the printed circuit board The determination of the number of plane layers 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 factors such as PCB performance requirements, target cost, manufacturing technology, and system complexity. The stack design of PCB is usually determined after considering various factors. High-speed digital circuits and radio frequency circuits usually use multi-layer board design.
1. Layered
In a multi-layer PCB, the signal layer (S), power (P) plane, and ground (GND) plane are usually included. The power plane and ground plane are usually undivided physical planes, which will provide a good low-impedance current return path for the current of adjacent signal traces. The signal layer is mostly located between these power or ground reference plane layers, forming a symmetric stripline or an asymmetric stripline. The top and bottom layers of a multilayer PCB are usually used to place components and a small number of traces. These signal traces must not be too long to reduce direct radiation generated by the traces.
2. Determine the single power reference plane (power plane)
The use of decoupling capacitors is an important measure to solve power integrity. Decoupling capacitors can only be placed on the top and bottom layers of the PCB. The traces, pads, and vias of the decoupling capacitor will seriously affect the effect of the decoupling capacitor. This requires that the trace connecting the decoupling capacitor should be as short and wide as possible, and the wire connected to the via should also be considered. As short as possible. For example, in a high-speed digital circuit, you can place the decoupling capacitor on the top layer of the PCB, assign the second layer to the high-speed digital circuit (such as a processor) as the power layer, the third layer as the signal layer, and the fourth layer Set to high-speed digital circuit ground.
In addition, it is necessary to ensure that the signal traces driven by the same high-speed digital device use 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 multi-power reference plane
The multi-power reference plane will be divided into several physical areas with different voltages. If the signal layer is next to the multi-power layer, the signal current on the signal layer near it will encounter an undesirable return path, causing a gap in the return path. For high-speed digital signals, this unreasonable return path design may cause serious problems, so the high-speed digital signal wiring should be far away from the multi-power reference plane.
4. Determine multiple ground reference planes (ground plane)
Multiple ground reference planes (ground planes) can provide a good low impedance current return path, which can reduce common mode EMI. The ground plane and power plane should be tightly coupled, and the signal plane should also be tightly coupled to the adjacent reference plane. This can be achieved by reducing the thickness of the medium between layers.
5. Reasonable design and wiring combination
The two layers spanned by a signal path are called a "wiring combination". The best wiring combination design is to avoid the return current from flowing from one reference plane to another reference plane, but from one point (plane) of one reference plane to another point (plane). In order to complete complex wiring, the layer-to-layer conversion 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 use adjacent layers as a wiring combination. If a signal path needs to cross multiple layers, it is usually not a reasonable design to use it as a wiring combination, because a path through multiple layers is not smooth for the return current. Although it is possible to reduce ground bounce by placing decoupling capacitors near the vias or reducing the thickness of the dielectric between reference planes, it is not a good design.
6. Set the wiring direction
On the same signal layer, it should be ensured that the direction of most of the wiring is the same, and it should be orthogonal to the wiring direction of the adjacent signal layer. For example, the routing direction of one signal layer can be set to the "Y axis" direction, and the routing direction of another adjacent signal layer can be set to the "X axis" direction.
7. Use even layer structure
It can be found from the designed PCB stackup that the classic stackup design is almost all even-numbered layers, not odd-numbered layers. This phenomenon is caused by various factors, as shown below.
① It can be understood from the manufacturing process of the printed circuit board that all conductive layers in the circuit board are coated on the core layer. The material of the core layer is generally a double-sided copper clad laminate. When the core layer is fully utilized, the conductivity of the printed circuit board The number of layers is even.
② Even-layer printed circuit boards have cost advantages. Due to the lack of one layer of dielectric and copper, the cost of raw materials for odd-numbered printed circuit boards is slightly lower than the cost of even-numbered printed circuit boards. However, because the odd layer printed circuit board needs to add a non-standard laminated core layer bonding process on the basis of the core structure process, the processing cost of the odd layer printed circuit board is significantly higher than that of the even layer printed circuit board. Compared with the ordinary core structure, the addition of copper clad outside the core structure will lead to a decrease in production efficiency and a longer production cycle. Before lamination bonding, the outer core layer requires additional processing, which increases the risk of scratches and incorrect etching of the outer layer. The added outer layer treatment will greatly increase the manufacturing cost.
③ When the printed circuit board is in the multi-layer circuit bonding process, when the inner and outer layers are cooled, different lamination tension will cause the printed circuit board to bend to different degrees. And as the thickness of the circuit board increases, the greater the risk of bending the composite printed circuit board with two different structures. Odd layer circuit boards are easy to bend, and even layer printed circuit boards can avoid circuit board bending.
In the design, if there is a stack of odd layers, you can use the following method to increase the number of layers.
① If the power layer of the printed circuit board is designed to be an even number and the signal layer is an odd number, the method of increasing the signal layer can be used. The increased signal layer does not cause an increase in cost, but can shorten the processing time and improve the quality of the printed circuit board.
② If the power layer of the printed circuit board is designed to be an odd number and the signal layer is an even number, the method of increasing the power layer can be adopted. And another simple method is to add a ground layer in the middle of the stack without changing other settings, that is, first print the circuit board wiring according to the odd layer, and then copy a ground layer in the middle.
③ In microwave circuits and mixed dielectric (mediums with different dielectric constants) circuits, a blank signal layer can be added near the center of the printed circuit board stack, which can minimize stack unbalance.
8. Cost consideration
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. But when considering the realization of circuit functions and circuit board miniaturization, to ensure signal integrity, EMI, EMC and other performance indicators, multi-layer circuit boards should be used as much as possible. Comprehensive evaluation shows that the cost difference between the multilayer circuit board and the single-layer circuit board is not much higher than expected.
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