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How can we professionally design a multilayer PCB stackup?

About PCB Stack-up Design :
  1. The main steps that we will look for when a customer asks us to do layout design :
(1)Complete your schematic design and test the functionality
(2)Finalize your BOM, Netlist, and Part list
(3)Decide on the size that your PCB will need to inhabit within your device
(4)Decide on the number of layers you’ll need to take into account the size, number of components and their footprints as well as the clearances needed for the various elements of your PCB
(5)Many components will have very specific requirements for EMI or standoff distance to other components, so you’ll need to make sure you understand the basic characteristics of special components.
(6)You’ll need to decide on the trace widths, clearances, via, ring sizes and scoring needed on the board since these also take up space.
   2. General rules of PCB cascading design:
(1) The formation and the signal layer should be tightly coupled, which means that the distance between the formation and the power supply layer should be as small as possible, and the thickness of the medium should be as small as possible, so as to increase the capacitance between the power supply layer and the formation
(2) Try not to be directly adjacent between the two signal layers, which is easy to occur signal crosstalk, affecting the performance of the circuit.
(3) For multi-layer circuit boards, such as 4-layer boards and 6-layer boards, it is generally required that the signal layer should be adjacent to an inner electric layer (formation or power supply layer). In this way, a large area of the inner electric layer can be used to play the role of shielding signal layer, thus effectively avoiding crosstalk between the signal layers.
(4) For the high-speed signal layer, it is generally located between two inner electric layers. The purpose of this is to provide an effective shield layer for the high-speed signal on the one hand and limit the high-speed signal between two inner electric layers, on the other hand, to reduce the interference to other signal layers.
(5) Consider the symmetry of the layered structure.
(6) Multiple grounded internal electrical layers can effectively reduce the grounding impedance.
PCB laminate design requires attention to the following aspects when designing multilayer boards
The laminate design of the PCB isn’t a simple stack of layers, where the arrangement of the formation is critical, and it’s closely related to the arrangement and orientation of the signals. The design of the multilayer board is similar to that of a normal PCB. In addition to adding the necessary signal routing layers, the most important thing is to arrange independent power and ground (copper layer). In high-speed digital circuit systems, the advantages of using power and ground to replace the previous power and ground buses are:
     (1) Provide a stable reference voltage for the conversion of the digital signal.
     (2) Evenly apply power to each logic device at the same time.
     (3) Effectively suppress crosstalk between signals.

  The reason is that the use of large-area copper as the power source and the ground layer greatly reduces the resistance of the power supply and the ground, so that the voltage on the power supply layer is even and stable, and that each signal line has a close ground plane corresponding to each other. At the same time, the characteristic impedance of the signal line is also reduced, and crosstalk can be effectively reduced. Therefore, for some high-end high-speed circuit designs, it has been clearly stated that a 6-layer (or more) stacking scheme must be used, such as Intel's requirements for PC133 memory module PCB. This is mainly due to the electrical characteristics of the multilayer board, as well as the suppression of electromagnetic radiation, even in the ability to resist physical and mechanical damage is significantly better than the low layer PCB.
  Under normal circumstances, the stacking design is carried out according to the following principles: satisfying the characteristic impedance requirements of the signal; satisfying the principle of signal loop minimization; meeting the signal interference requirements in the PCB, and satisfying the principle of symmetry. Specifically, you need to pay attention to the following aspects when designing a multilayer board:
1) A signal layer should be adjacent to a copper layer, and the signal layer and the copper layer should be spaced apart, preferably each signal layer can be in close proximity to at least one copper layer. The signal layer should be tightly coupled to the adjacent copper layer (i.e., the thickness of the medium between the signal layer and the adjacent copper layer is small).
2) Power copper and ground copper should be tightly coupled and in the middle of the stack. Shorten the distance between the power supply and the ground, which is conducive to power supply stability and reduce EMI. Try to avoid sandwiching the signal layer between the power plane and the ground plane. The close proximity of the power plant to the ground plane is like forming a plate capacitor. The closer the two planes are, the larger the capacitance value. The main function of this capacitor is to provide a low-impedance return path for high-frequency noise (such as switching noise) so that the receiving device's power input has less ripple and enhances the performance of the receiving device itself.
3) In the case of high speed, the excess ground layer can be added to isolate the signal layer. Multiple layers of copper can effectively reduce the impedance of the PCB and reduce common mode EMI. However, it is recommended not to add more power layers to isolate, which may cause unnecessary noise interference.
4) The high-speed signal in the system should be in the inner layer and between the two copper layers so that two copper coatings can shield these high-speed signals and limit the radiation of these signals to the two copper-clad areas.
5) Give priority to the transmission line model of high-speed signals and clock signals, design a complete reference plane for these signals, avoid cross-plane partitions as much as possible to control the characteristic impedance and ensure the integrity of the signal return path.
6) The case where two signal layers are adjacent. For boards with high-speed signals, the ideal stack is to design a complete reference plane for each high-speed signal layer, but in practice, we always need to make a trade-off between PCB layer count and PCB cost. In this case, the phenomenon that two signal layers are adjacent cannot be avoided. The current practice is to increase the spacing between the two signal layers and make the two layers of traces as perpendicular as possible to avoid signal crosstalk between layers.
7) The copper layer is preferably set in pairs. For example, the 2, 5 or 3 and 4 layers of the six-layer board are to be copper-plated together, which is considered in consideration of the process-balanced structure, because the unbalanced copper layer may Will cause warpage of the PCB.
8) The subsurface ( the layer next to the surface layer) is designed as a formation to help reduce EMI.
9) Estimate the total number of layers based on PCB device density and pin density to determine the total number of layers.
The structure of the slab is an important factor in determining the EMC performance of the system. A good lamellar structure has a good effect on suppressing radiation in the PCB. Multilayer boards are often used in high-speed circuit systems that are common today, rather than single and double panels.


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