What to pay attention to when designing PCB stackups
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 required to achieve the functions required by the circuit. The determination of the number of plane layers is related to the requirements of circuit functions, signal integrity, EMI, EMC, and manufacturing costs. For most designs, there are many conflicting requirements on the PCB's performance requirements, target costs, manufacturing technology, and system complexity. The PCB stackup design is usually determined by taking into consideration various factors. High-speed digital circuits and radio-whisker circuits are usually designed with multilayer boards.
Here are 8 principles to pay attention to in cascading design:
In a multilayer PCB, the signal layer (S), power (P) plane, and ground (GND) plane are usually included. The power plane and ground plane are usually solid planes without divisions. They 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 strip line or an asymmetric strip line. 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 from the traces.
2. Determine single power reference plane (power plane)
Using decoupling capacitors is an important measure to address power integrity. Decoupling capacitors can only be placed on the top and bottom layers of the PCB. The decoupling capacitor's traces, pads, and vias will seriously affect the effect of the decoupling capacitor. This requires that the design of the traces connecting the decoupling capacitors should be as short and wide as possible, and the wires connected to the vias should also be Keep it short. For example, in a high-speed digital circuit, a decoupling capacitor can be placed on the top layer of the PCB, and the second layer is assigned to a 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 for high-speed digital devices.
3. Determine the multiple power reference plane
The multi-power reference plane will be divided into several solid regions with different voltages. If the signal layer is next to the multiple power supply layer, the signal current on the nearby signal layer will encounter an undesired return path, making a gap in the return path. For high-speed digital signals, this unreasonable return path design may bring serious problems, so high-speed digital signal wiring is required to be kept away from the multiple power reference plane.
4. Identify multiple ground reference planes (ground planes)
Multiple ground reference planes (ground planes) provide a good low-impedance current return path and reduce common-mode EMl. The ground and power planes should be tightly coupled, and the signal plane should be tightly coupled with the adjacent reference plane. This can be achieved by reducing the layer-to-layer dielectric thickness.
5. Reasonably design the wiring combination
The two layers that a signal path spans are called a "routing combination." The best wiring combination design is to avoid the return current from flowing from one reference plane to another, but from one point (area) of one reference plane to another point (area). In order to complete the complicated wiring, the interlayer conversion of the wiring 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 routing 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 smooth for return current. Although it is possible to reduce the ground bounce by placing a decoupling capacitor near the via or reducing the dielectric thickness between the reference planes, it is not a good design.
6. Set the wiring direction
On the same signal layer, the direction of most of the 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. Use an even-numbered structure
It can be found from the designed PCB stack-up that almost all of the classic stack-up designs are even-numbered layers, rather than odd-numbered layers. This emergency is caused by many factors, as shown below.
It can be understood from the manufacturing process of the printed circuit board that all conductive layers in the circuit board are saved on the core layer. The material of the core layer is generally a double-sided overlay. When the core layer is fully used, the conductive layer of the printed circuit board The number is even.
Even-layer printed circuit boards have cost advantages. Due to the lack of a 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-numbered 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-numbered printed circuit board is significantly higher than the even-numbered 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. Before laminating and bonding, the outer core layer needs additional processing, which increases the risk of the outer layer being scratched and etched incorrectly. The increased outer layer treatment will significantly increase manufacturing costs.
After the printed circuit board is bonded to the multilayer circuit, when the inner layer and the outer layer are cooled, different lamination tensions will cause the printed circuit board to bend to different degrees. And as the thickness of the circuit board increases, the risk of bending of a composite printed circuit board with two different structures increases. Odd-layer circuit boards are easy to bend, and even-layer printed circuit boards can avoid circuit board bending.
During the design, if there is a stack of odd layers, the following methods can be used to increase the number of layers.
If the power supply layer of the printed circuit board is designed to be an even number and the signal layer is an odd number, a method of adding a signal layer may be adopted. The added signal layer does not cause an increase in cost, but can shorten the processing time and improve the quality of the printed circuit board.
If the power supply layer of the printed circuit board is designed to be an odd number and the signal layer is an even number, the method of adding a power supply 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, to print the circuit board according to the odd number of layers, and then copy a ground layer in the middle.
In microwave circuits and mixed media (dielectrics with different dielectric constants) circuits, a blank signal layer can be added near the center of the printed circuit board stackup, which can minimize stacking imbalance.
8. Cost considerations
In terms of manufacturing costs, with the same PCB area, the cost of a multi-layer circuit board is definitely higher than that of a single-layer and double-layer circuit board, and the more the number of layers, the higher the cost. However, when considering the realization of circuit functions and circuit board miniaturization, ensuring signal integrity, performance indicators such as EMC, EMC and other factors, multi-layer circuit boards should be used as much as possible. Comprehensive evaluation, the cost difference between the multilayer circuit board and the single-layer circuit board will not be much higher than expected.