Before designing multi-layer PCB circuit board, the designer needs to first determine the circuit board structure according to the scale of the circuit, the size of the circuit board and the requirements of electromagnetic compatibility (EMC), that is, decide whether to use 4-layer, 6-layer or more layers of circuit board. After determining the number of layers, determine the placement position of the internal electrical layer and how to distribute different signals on these layers. This is the choice of Multilayer PCB laminated structure. Laminated structure is an important factor affecting the EMC performance of PCB, and it is also an important means to suppress electromagnetic interference. This section will introduce the related contents of Multilayer PCB laminated structure.
Selection and superposition principle of layers
Many factors need to be considered to determine the laminated structure of Multilayer PCB. In terms of wiring, the more layers, the better the wiring, but the cost and difficulty of board making will also increase. For manufacturers, whether the laminated structure is symmetrical or not is the focus of attention in PCB manufacturing, so the selection of layers needs to consider the needs of all aspects to achieve Zui good balance.
For experienced designers, after completing the pre layout of components, they will focus on the analysis of the wiring bottleneck of PCB. Analyze the wiring density of circuit board combined with other EDA tools; Then the number and type of signal lines with special wiring requirements, such as differential lines and sensitive signal lines, are integrated to determine the number of signal layers; Then the number of internal electric layers is determined according to the type of power supply, isolation and anti-interference requirements. In this way, the number of layers of the whole circuit board is basically determined.
After determining the number of layers of the circuit board, the next work is to reasonably arrange the placement order of each layer of the circuit. In this step, the following two main factors need to be considered.
(1) Distribution of special signal layer.
(2) Distribution of power layer and stratum.
If the number of layers of the circuit board is more, the types of arrangement and combination of special signal layer, stratum and power layer will be more. How to determine which combination method Zui is better will be more difficult, but the general principles are as follows.
(1) The signal layer shall be adjacent to an internal electric layer (internal power supply / stratum), and the large copper film of the internal electric layer shall be used to provide shielding for the signal layer.
(2) The internal power layer and the stratum should be closely coupled, that is, the dielectric thickness between the internal power layer and the stratum should be taken as a smaller value to improve the capacitance between the power layer and the stratum and increase the resonant frequency. The media thickness between the internal power layer and the stratum can be set in Protel's layerstackmanager. Select [design] / [layerstackmanager...] to open the layer stack Manager dialog box. Double click the prepreg text with the mouse to open the dialog box as shown in Figure 11-1. You can change the thickness of the insulating layer in the thickness option of the dialog box.
If the potential difference between the power supply and the ground wire is small, a smaller insulating layer thickness can be used, such as 5MIL (0.127mm).
(3) The high-speed signal transmission layer in the circuit should be the signal intermediate layer and sandwiched between two internal electrical layers. In this way, the copper film of the two inner electric layers can provide electromagnetic shielding for high-speed signal transmission, and can effectively limit the radiation of high-speed signal between the two inner electric layers without causing external interference.
(4) Avoid two signal layers directly adjacent. Crosstalk is easily introduced between adjacent signal layers, resulting in circuit failure. Adding a ground plane between the two signal layers can effectively avoid crosstalk.
(5) Multiple grounded internal electric layers can effectively reduce the grounding impedance. For example, a signal layer and B signal layer adopt separate ground planes, which can effectively reduce common mode interference.
(6) Give consideration to the symmetry of the floor structure.
