PCB Technical - Multilayer printed circuit boards design method

Before designing a multilayer printed circuit boards, the designer needs to first determine the requirements of electromagnetic compatibility (EMC), according to the size of the circuit board, the size of the circuit board, the size of the circuit board, and the size of the circuit board. To determine the circuit board structure used, that is, to decide whether to use a 4-layer, 6-layer, or more layered circuit board. After determining the number of layers required, determine where to place the internal electrical layers and how to distribute the different signals on these layers. This is the choice of the multilayer PCB stack-up structure. Laminated structure is an important factor affecting the EMC performance of the PCB board. An important means of suppressing electromagnetic interference This section will introduce the related content of the multilayer PCB board stack-up structure.

1. The selection of layers and the principle of superposition

There are many factors to consider when determining the stack-up structure of a multilayer PCB. In terms of wiring, the more layers, the better for wiring, but the more layers, the better for wiring. For manufacturers, whether the laminated structure is symmetrical or not is the focus of attention when manufacturing PCB boards. Whether the laminated structure is symmetrical or not is the cost and difficulty of the board, which will also increase the symmetry of the laminated structure. Therefore, the selection of the number of layers needs to consider various aspects. needs to achieve a balance. For experienced designers, after completing the pre-layout of the components, the key analysis will be carried out on the routing bottleneck of the PCB. After completing the pre-layout of the components, the tool analyzes the wiring density of the circuit board; then integrates the signal lines with special wiring requirements such as differential lines, sensitive signal lines with special wiring requirements, such as differential lines and other EDA tools to analyze the circuit board. The number and type of signal lines with special wiring density requirements, such as differential lines, sensitive signal lines, etc., determine the number of layers of signal layers, and then determine the number of layers of signal layers according to the type of power supply; according to the type of power supply, isolation and resistance interference requirements to determine the number of inner electrical layers. To determine the number of layers of the signal layer, the number of internal electrical layers is determined according to the type of power supply, isolation and anti-interference requirements. In this way, the number of layers of the entire circuit board is basically determined. After determining the number of layers of the circuit board, the next job is to reasonably arrange the placement order of the circuits on each layer.
(1) The signal layer should be adjacent to an inner electric layer (internal power supply ground layer), and the large copper film of the inner electric layer should be used to provide shielding for the signal layer.
(2) The internal power supply layer and the ground layer should be tightly coupled, that is, the thickness of the medium between the internal power supply layer and the ground layer should be compared) The internal power supply layer and the ground layer should be tightly coupled, that is, a small value, to increase the capacitance between the power supply layer and the ground layer and increase the resonant frequency. A small value increases the capacitance between the power plane and the ground plane and increases the resonant frequency. (3) The high-speed signal transmission layer in the circuit should be a signal intermediate layer and sandwiched between two inner electrical layers. In this way, the copper films of the two inner electric layers can provide electromagnetic shielding for high-speed signal transmission, and at the same time, the radiation of the high-speed signal can be effectively limited between the two inner electric layers, so as not to cause external interference.
(4) Avoid two signal layers directly adjacent to each other. Crosstalk is easily introduced between adjacent signal layers, resulting in circuit failure; adding a ground plane between two signal layers can effectively avoid crosstalk.
(5) Multiple grounded inner electrical layers can effectively reduce the ground impedance; for example, the A signal layer and the B signal layer use separate ground planes, which can effectively reduce common mode interference.
(6) Taking into account the symmetry of the layer structure.

2. Commonly used stacked structure
The following is an example of a 4-layer board to illustrate how to optimize the arrangement and combination of various stacked structures: For commonly used 4-layer boards, there are the following stacking methods (from top to bottom):
(1) Siganl_1 (Top), GND (Inner_1), POWER (Inner_2), Siganl_2 (Bottom).
(2) Siganl_1 (Top), POWER (Inner_1), GND (Inner_2), Siganl_2 (Bottom).
(3) POWER (Top), Siganl_1 (Inner_1), GND (Inner_2), Siganl_2 (Bottom).

Obviously, option 3 lacks effective coupling between the power plane and the ground plane and should not be used. So how should option 1 and option 2 be chosen? In general, designers will choose option 1 as the structure of the 4-layer board. The reason is not that Option 2 cannot be used, but that the general PCB board only places components on the top layer, so it is more appropriate to use Option 1. However, when components need to be placed on both the top and bottom layers, and the dielectric thickness between the internal power supply layer and the ground layer is large and the coupling is poor, it is necessary to consider which layer has fewer signal lines. For scheme 1, there are fewer signal lines on the bottom layer, and a large-area copper film can be used to couple with the POWER layer; on the contrary, if the components are mainly arranged on the bottom layer, scheme 2 should be used to make the board. After completing the analysis of the laminated structure of the 4-layer board, the following is an example of the combination method of the 6-layer board to illustrate the arrangement and combination of the laminated structure of the 6-layer board and the preferred method:

(1) Siganl_1 (Top), GND (Inner_1), Siganl_2 (Inner_2), Siganl_3 (Inner_3), POWER (In). Scheme 1 uses 4 layers of signal layers and 2 layers of internal power/ground layers, and has more signal layers, which is conducive to the wiring work between components, but the defects of this scheme are also more obvious, which are manifested in the following two aspects:
a. The power and ground layers are far apart and not fully coupled
b. The signal layer Siganl_2 (Inner_2) and Siganl_3 (Inner_3) are directly adjacent to each other, the signal isolation is not good, and crosstalk is prone to occur

(2) Siganl_1 (Top), Siganl_2 (Inner_1), POWER (Inner_2), GND (Inner_3), Siganl_3 (In).
Compared with scheme 1, scheme 2 has sufficient coupling between power supply layer and ground layer, which has certain advantages over scheme 1, but Siganl_1 (Top) and Siganl_2 (Inner_1) and Siganl_3 (Inner_4) and Siganl_4 (Bottom) signal layers directly Adjacent, the signal isolation is not good, and the problem of easy crosstalk has not been solved. Compared with scheme 1 and scheme 2, scheme 3 reduces one signal layer and adds an internal electrical layer. Although the layers available for wiring are reduced, this scheme solves the common defects of scheme 1 and scheme 2:
a. The power and ground layers are tightly coupled.
b. Each signal layer is directly adjacent to the internal electrical layer, which is effectively isolated from other signal layers, and is not prone to crosstalk.
Through the analysis of the above two examples, I believe that readers have a certain understanding of the cascading structure, but in some cases, a certain scheme cannot meet all the requirements, which requires consideration of the priority of various design principles. Unfortunately, because the layer design of the circuit board is closely related to the characteristics of the actual circuit, the anti-interference performance and design focus of different circuits are different, so in fact, these principles do not have a definite priority for reference. But it is certain that design principle 2 (the internal power supply layer and the ground layer should be tightly coupled design principle (the internal power supply layer and the ground layer should be tightly coupled if high-speed signals need to be transmitted in the circuit, together) needs to be satisfied first in the design., In addition, if the circuit needs to transmit high-speed signals, then design principle 3 (the high in the circuit needs to be satisfied first in the design, if the printed circuit boards need to transmit high-speed signals (the high-speed signal transmission layer should be the signal middle layer, and sandwiched between two between the inner electric layers) must be satisfied. The signal transmission layer should be a signal intermediate layer and sandwiched between two inner electric layers) must be satisfied.