When designing a multi-layer PCB circuit board, it is first necessary to determine the circuit board structure used according to the size of the circuit, the size of the circuit board and the EMC requirements, that is, to choose a four-layer, six-layer or multi-layer circuit board. After determining the number of layers, determine the placement of the internal electrical layers and how to distribute various signals to each layer. This issue involves the choice of multilayer PCB layer structure. The laminated structure is an important factor affecting the EMC performance of the circuit board, and it is also an important means to suppress EMI. This article introduces the stacking structure of multilayer PCB circuit boards.
After the number of power supply, ground layers and signal layers is determined, the relative arrangement between them is a topic that every PCB engineer cannot avoid;
General principles of PCB layout of printed circuit boards:
1. There are many factors that need to be considered to determine the laminated structure of the multi-layer PCB board. In terms of wiring, the more layers, the better for wiring, but the cost and difficulty of making boards will also increase. For manufacturers, whether the laminate structure is symmetrical or not is the focus of attention when manufacturing PCB circuit boards, so the selection of the number of layers needs to consider the needs of various aspects to achieve the best balance. For experienced designers, after completing the pre-layout of the components, they will focus on the analysis of the PCB routing bottlenecks. Combine other EDA tools to analyze the wiring density of the circuit board; then combine the number and types of signal lines with special wiring requirements such as differential lines, sensitive signal lines, etc. to determine the number of layers of the signal layer; then according to the type of power supply, isolation and anti-interference requirements to determine the number of inner electrical layers. In this way, the number of layers of the entire circuit board is basically determined.
2. Below the component surface (the second layer) is the ground plane, which provides the device shielding layer and the reference plane for the top layer wiring; the sensitive signal layer should be adjacent to an internal electrical layer (internal power supply/ground layer), using the large size of the internal electrical layer. Copper film to provide shielding for signal layers. 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.
3. All signal layers should be adjacent to the ground plane as much as possible;
4. Try to 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 the two signal layers can effectively avoid crosstalk.
5. The main power supply should be adjacent to it as much as possible;
6. Take into account the symmetry of the laminated structure.
7. For the layer layout of the motherboard, the existing motherboard is difficult to control the parallel long-distance wiring. For the board-level operating frequency above 50MHZ (the case below 50MHZ can be referred to, and appropriate relaxation), the recommended layout principle: component surface , The welding surface is a complete ground plane (shielding); there is no adjacent parallel wiring layer; all signal layers are adjacent to the ground plane as much as possible; key signals are adjacent to the ground layer and do not cross the partition area.
Note: When setting the layers of a specific PCB, the above principles should be flexibly grasped. On the basis of understanding the above principles, according to the needs of the actual single board, such as: whether a key wiring layer, power supply, and ground plane division are required, etc. , to determine the arrangement of the layers, do not rigor or hold on to it.
8. Multiple grounded inner electrical layers can effectively reduce the grounding impedance. For example, the A signal layer and the B signal layer use separate ground planes, which can effectively reduce common mode interference.
Commonly used laminated structures of PCB printed circuit boards:
four-layer circuit board
The following uses the example of a four-layer board to illustrate how to optimize the arrangement and combination of various laminated structures. For commonly used four-layer boards, there are several stacking methods (from top to bottom).
a. Siganl_1 (Top), GND (Inner_1), POWER (Inner_2), Siganl_2 (Bottom).
b. Siganl_1 (Top), POWER (Inner_1), GND (Inner_2), Siganl_2 (Bottom).
c. 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?
Under normal circumstances, designers will choose scheme 1 as the structure of the four-layer board. The reason for the choice is not that scheme 2 cannot be used, but that general PCB boards only place components on the top layer, so scheme 1 is more appropriate.
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.
If a stacked structure is used, the power layer and the ground layer are already coupled. Considering the requirement of symmetry, scheme 1 is generally adopted.
Six-layer circuit board
After completing the analysis of the laminated structure of the four-layer circuit board, the following is an example of the combination method of the six-layer circuit board to illustrate the arrangement and combination of the laminated structure of the six-layer circuit board and the preferred method.
a. Siganl_1 (Top), GND (Inner_1), Siganl_2 (Inner_2), Siganl_3 (Inner_3), POWER (Inner_4), Siganl_4 (Bottom).
Scheme 1 adopts 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:
①. The power layer and the ground layer are far apart and not fully coupled.
②. The signal layer Siganl_2 (Inner_2) and Siganl_3 (Inner_3) are directly adjacent to each other, and the signal isolation is not good, and crosstalk is prone to occur.
b. Siganl_1 (Top), Siganl_2 (Inner_1), POWER (Inner_2), GND (Inner_3), Siganl_3 (Inner_4), Siganl_4 (Bottom).
Compared with scheme 1, scheme 2 has sufficient coupling between the power supply layer and the ground layer, which has certain advantages over scheme 1, but the Siganl_1 (Top) and Siganl_2 (Inner_1) and Siganl_3 (Inner_4) and Siganl_4 (Bottom) signal layers are directly Adjacent, the signal isolation is not good, and the problem of easy crosstalk has not been solved.
c. Siganl_1 (Top), GND (Inner_1), Siganl_2 (Inner_2), POWER (Inner_3), GND (Inner_4), Siganl_3 (Bottom).
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.
①. The power layer and the ground layer are tightly coupled.
②. Each signal layer is directly adjacent to the inner electrical layer, and is effectively isolated from other signal layers, so crosstalk is not easy to occur.
③. Siganl_2 (Inner_2) is adjacent to the two inner electrical layers GND (Inner_1) and POWER (Inner_3), which can be used to transmit high-speed signals. The two inner electrical layers can effectively shield the external interference to the Siganl_2 (Inner_2) layer and the interference of Siganl_2 (Inner_2) to the outside world.
Considering all aspects, scheme 3 is obviously the best, and at the same time, scheme 3 is also a commonly used laminated structure in 6-layer circuit boards. Taking the above two examples as an example, I believe that the reader has a certain understanding of the laminated structure, but in some cases, a certain laminated structure cannot fully meet the requirements, which requires consideration of the priority of various design principles. rights issue. Since the layer design of the circuit board is closely related to the characteristics of the actual circuit, and the anti-interference performance and design focus of different circuits are also different, these principles cannot be considered as a priority in practical work. However, it can be determined that the design principle 2 (the internal power supply layer and the ground layer should be tightly coupled) should first meet this requirement in the design. In addition, if high-speed signal transmission is required in the circuit, then the design principle 3 (on the signal transmission line) The signal intermediate layer, and sandwiched between two inner circuit layers) should meet this requirement first.
Ten layer circuit board
Typical ten-layer PCB design The general wiring sequence is TOP--GND---signal layer---power layer---GND---signal layer---power layer---signal layer---GND- --BOTTOM
The wiring order itself is not necessarily fixed, but there are some standards and principles to constrain it: for example, the adjacent layers of the top layer and the bottom use GND to ensure the EMC characteristics of the board; for example, each signal layer is preferably used as a reference GND layer Plane; the power supply used by the entire single board is given priority to laying a whole piece of copper; the one that is susceptible to interference, high-speed, and the inner layer along the transition is preferred, and so on.
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