PCB lamination design

- Nov 17, 2018-

PCB lamination design is not a simple stack of layers, in which the arrangement of strata is the key, which is closely related to the arrangement and direction of signals.Compared with the common PCB, the multi-layer board is designed with the necessary signal routing layer added. The most important thing is to arrange the independent power source and stratum (copper layer).In the high-speed digital circuit system, the advantages of using power source and stratum to replace the previous power source and ground bus mainly lie in:

1) provide a stable reference voltage for digital signal transformation. 

2) uniformly add power to each logic device at the same time. 

3) effectively inhibits crosstalk between signals. 

The reason is that using large area of copper as power source and stratum greatly reduces the resistance of power source and ground, makes the voltage on the power source layer uniform and stable, and can ensure that each signal line has a close ground plane relative to each other, which also reduces the characteristic impedance of signal line and can effectively reduce crosstalk.Therefore, for some high-end high-speed circuit designs, it has been clearly stipulated that a layering scheme of 6 layers (or above) must be used, such as the requirement of Intel on PC133 memory module PCB.This is mainly due to the electrical characteristics of the multi-layer board, as well as the suppression of electromagnetic radiation, and even the ability to resist physical and mechanical damage is significantly better than the low-layer PCB. 

In general, laminated design is carried out according to the following principles:Satisfy the minimum principle of signal loop;To minimize signal interference in PCB;It satisfies the symmetry principle.Specifically, the following aspects should be paid attention to when designing multi-layer boards:

1) a signal layer should be adjacent to a copper layer, and the signal layer and copper layer should be placed at intervals. It is best that each signal layer is adjacent to at least one copper layer.The signal layer should be tightly coupled with the adjacent copper layer (i.e. the media thickness between the signal layer and the adjacent copper layer is very small). 

2) the power supply copper and ground copper should be tightly coupled and located in the middle of the lamination.Shortening the distance between power source and stratum is beneficial to the stability of power source and reducing EMI.Try to avoid clipping the signal layer between the power supply layer and the stratum.The close proximity of the power supply plane to the ground plane is like forming a plate capacitance. The closer the two planes are, the larger the capacitance is.The main function of the capacitor is to provide a low-impedance reflow path for high frequency noise (such as switching noise), so that the power input of the receiving device has smaller ripple and enhances the performance of the receiving device itself. 

3) in the case of high speed, redundant formations can be added to isolate the signal layer, and multiple copper layers can effectively reduce PCB impedance and common mode EMI.However, it is recommended not to add more power layer isolation, which may cause unnecessary noise interference. 

4) the high-speed signals in the system shall be embedded in the inner layer and between two copper plates, so that the two copper plates can provide shielding for these high-speed signals and limit the radiation of these signals to two copper plates. 

5) give priority to the transmission line model of high-speed signals and clock signals, and design a complete reference plane for these signals, avoiding cross-plane segmentation as much as possible, so as to control the characteristic impedance and ensure the integrity of the signal backflow path. 

6) the situation that the two signal layers are adjacent.For the boards with high-speed signals, the ideal lamination is to design a complete reference plane for each high-speed signal layer, but in practice we always need to make a tradeoff between the number of PCB layers and the cost of PCB.In this case, the phenomenon of two adjacent signal layers cannot be avoided.The current practice is to increase the spacing between the two signal layers and make the line of the two layers as vertical as possible to avoid the signal crosstalk between the layers. 

7) the copper layer should be set in pairs. For example, the two, five or three or four layers of the six layers should be copper layer together. This is to take into account the requirement of balancing structure on the process, because unbalanced copper layer may cause warping and deformation of PCB. 

8) the sub-surface (i.e. the layer close to the surface) is designed to be stratified, which is conducive to reducing the EMI. 

9) the required number of signal layers is estimated based on PCB device density and pin density, and the total number of layers is determined. 

The plate structure is a very important factor to determine the EMC performance of the system.A good laminate structure has a good effect on inhibiting radiation in PCB.In the current common high - speed circuit system mostly USES the multi-layer board instead of the single panel and double panel.The following is a simple explanation on the design of four, six, eight and ten layers.