Before designing multilayer PCB board, the designer needs to first determine the circuit board structure used according to the circuit scale, circuit board size and electromagnetic compatibility (EMC) requirements, that is, to decide whether to use 4 layers, 6 layers, or More layers of circuit boards. After determining the number of layers, determine where to place the internal electrical layers and how to distribute different signals on these layers. This is the choice of multilayer PCB stack structure.
Laminated structure is an important factor that affects the EMC performance of PCB boards, and it is also an important means to suppress electromagnetic interference. This article introduces the relevant content of the multilayer PCB board stack structure.
After determining the number of power, ground and signal layers, the relative arrangement of them is a topic that every PCB engineer cannot avoid;
The general principle of layer arrangement:
1. To determine the laminated structure of a multilayer PCB board, more factors need to be considered. From the perspective of wiring, the more layers, the better the wiring, but the cost and difficulty of board manufacturing will also increase. For manufacturers, whether the laminated structure is symmetrical or not is the focus that needs to be paid attention to when PCB boards are manufactured, so the choice of the number of layers needs to consider the needs of all 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 wiring bottleneck. Combine with other EDA tools to analyze the wiring density of the circuit board; then synthesize the number and types of signal lines with special wiring requirements, such as differential lines, sensitive signal lines, etc., to determine the number of signal layers; then according to the type of power supply, isolation and anti-interference The requirements to determine the number of internal electrical layers. In this way, the number of layers of the entire circuit board is basically determined.
2. The bottom of the component surface (the second layer) is the ground plane, which provides the device shielding layer and the reference plane for the top wiring; the sensitive signal layer should be adjacent to an internal electrical layer (internal power/ground layer), using the large internal electrical layer Copper film to provide shielding for the signal layer. 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 film of the two inner electric layers can provide electromagnetic shielding for high-speed signal transmission, and at the same time, it can effectively limit the radiation of the high-speed signal between the two inner electric layers without causing external interference.
3. All signal layers are as close as possible to the ground plane;
4. Try to avoid two signal layers directly adjacent to each other; it is easy to introduce crosstalk between adjacent signal layers, resulting in circuit function failure. Adding a ground plane between the two signal layers can effectively avoid crosstalk.
5. The main power source is as close as possible to it correspondingly;
6. Take into account the symmetry of the laminated structure.
7. For the layer layout of the motherboard, it is difficult for the existing motherboards to control parallel long-distance wiring. For the board-level operating frequency above 50MHZ (refer to the situation below 50MHZ, please relax appropriately), it is recommended to arrange the principle:
The component surface and the welding surface are a complete ground plane (shield);No adjacent parallel wiring layers;All signal layers are as close as possible to the ground plane;
The key signal is adjacent to the ground and does not cross the partition.
Note: When setting up the specific PCB layers, the above principles should be flexibly mastered. Based on the understanding of the above principles, according to the actual requirements of the single board, such as: whether a key wiring layer, power supply, ground plane division is required, etc. , Determine the arrangement of the layers, and don’t just copy it bluntly, or hold on to it.
8. Multiple grounded internal electrical layers can effectively reduce ground impedance. For example, the A signal layer and the B signal layer use separate ground planes, which can effectively reduce common mode interference.
The commonly used layered structure:4-layer board
The following uses an example of a 4-layer board to illustrate how to optimize the arrangement and combination of various laminated 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 layer and the ground layer and should not be adopted.
Then how should options 1 and 2 be selected?
Under normal circumstances, designers will choose option 1 as the structure of the 4-layer board. The reason for the choice is not that Option 2 cannot be adopted, but that the general PCB board only places components on the top layer, so it is more appropriate to adopt Option 1.
But when components need to be placed on both the top and bottom layers, and the dielectric thickness between the internal power layer and the ground layer is large and the coupling is poor, it is necessary to consider which layer has fewer signal lines. For Option 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, Option 2 should be used to make the board.
If a laminated structure is adopted, the power layer and the ground layer are already coupled. Considering the requirements of symmetry, scheme 1 is generally adopted.
After completing the analysis of the laminated structure of the 4-layer board, the following uses an example of the 6-layer board combination to illustrate the arrangement and combination 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 (Inner_4), Siganl_4 (Bottom).
Solution 1 uses 4 signal layers and 2 internal power/ground layers, with more signal layers, which is conducive to the wiring work between components, but the defects of this solution are also more obvious, which are manifested in the following two aspects:
① The power plane and the ground plane are far apart, and they are not sufficiently coupled.
② The signal layer Siganl_2 (Inner_2) and Siganl_3 (Inner_3) are directly adjacent, so the signal isolation is not good and crosstalk is easy to occur.
(2) Siganl_1 (Top), Siganl_2 (Inner_1), POWER (Inner_2), GND (Inner_3), Siganl_3 (Inner_4), Siganl_4 (Bottom).
Scheme 2 Compared with scheme 1, the power layer and ground plane are fully coupled, 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 are directly adjacent to each other. The signal isolation is not good, and the problem of crosstalk is not solved.
(3) Siganl_1 (Top), GND (Inner_1), Siganl_2 (Inner_2), POWER (Inner_3), GND (Inner_4), Siganl_3 (Bottom).
Compared to Scheme 1 and Scheme 2, Scheme 3 has one less signal layer and one more 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 plane and ground plane are tightly coupled.
② Each signal layer is directly adjacent to the inner electric layer, and is effectively isolated from other signal layers, and 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 interference from the outside world to the Siganl_2 (Inner_2) layer and the interference from Siganl_2 (Inner_2) to the outside world.
In all aspects, scheme 3 is obviously the most optimized one. At the same time, scheme 3 is also a commonly used laminated structure for 6-layer boards. Through the analysis of the above two examples, I believe that the reader has 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, due to the fact that the circuit board layer design 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 have no determined priority for reference. But what is certain is that design principle 2 (the internal power layer and the ground layer should be tightly coupled) needs to be met first in the design, and if high-speed signals need to be transmitted in the circuit, then design principle 3 (high-speed signal transmission layer in the circuit) It should be the signal intermediate layer and sandwiched between two inner electrical layers) must be satisfied.
PCB typical 10-layer board design
The general wiring sequence is TOP–GND—signal layer—power layer—GND—signal layer—power layer—signal layer—GND—BOTTOM
The wiring sequence itself is not necessarily fixed, but there are some standards and principles to restrict it: For example, the adjacent layers of the top layer and bottom layer use GND to ensure the EMC characteristics of the single board; for example, each signal layer preferably uses the GND layer as a reference Plane; the power supply used in the entire single board is preferentially laid on a whole piece of copper; the susceptible, high-speed, and preferred to go along the inner layer of the jump, etc.