PCB Blog - PCB board stackup design

1. General principles of layer arrangement:
1.1 There are many factors that need to be considered to determine the laminated structure of the multilayer 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 that needs to be paid attention to when manufacturing PCB boards, so the selection of the number of layers needs to consider the needs of various aspects to achieve a balance. For experienced designers, after completing the pre-layout of components, they will focus on analyzing the routing bottlenecks of the PCB board. 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.
1.2 The ground plane below the component surface (the second layer) provides the shielding layer of the device 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 copper of the internal electrical layer. 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 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.
1.3 All signal layers should be adjacent to the ground plane as much as possible;
1.4 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.
1.5 The main power supply should be adjacent to it as far as possible;
1.6 Take into account the symmetry of the laminated structure.
1.7 For the layer layout of the motherboard, the existing motherboard is difficult to control 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 principles:
The component surface and the welding surface are complete ground planes (shielding); there are no adjacent parallel wiring layers; all signal layers are adjacent to the ground plane as much as possible; key signals are adjacent to the ground plane and do not cross the partition area. Note: When setting the layers of a specific PCB board, it is necessary to flexibly grasp the above principles. 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 are required. Wait, determine the arrangement of the layers, and do not rub it hard, or hold on to it.
1.8 Multiple grounded inner 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.

2. Commonly used stacked structures:
2.1 4-layer board
The following is an example of a 4-layer board to illustrate how to optimize the arrangement and combination of various stacked structures.
For common 4-layer boards, there are several stacking methods (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? Under normal circumstances, designers will choose scheme 1 as the structure of the 4-layer board. The reason for the choice is not that Option 2 cannot be used, but that ordinary PCB boards only place 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.

2.2 6-layer 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 6-layer board laminated structure 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). Scheme 1 adopts 4 layers of signal layers and 2 layers of internal power/ground layers, which 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, and 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 (Inner_4), Siganl_4 (Bottom). Scheme 2 Compared with scheme 1, the power supply layer and the ground layer are fully coupled, 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. (3) 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 internal 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 outside interference to the Siganl_2 (Inner_2) layer and the Siganl_2 (Inner_2) interference to the outside world. Taking all aspects into account, scheme 3 is obviously a kind of chemistry. 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 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 ground layer should be tightly coupled) needs to be satisfied first in the design, and if high-speed signals need to be transmitted in the circuit, then design principle 3 (the high-speed signal transmission layer in the circuit) should be the signal intermediate layer, and sandwiched between the two inner electrical layers) must be satisfied.

2.3 10-layer board
Typical 10-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 PCB board, and the inner layer along the transition is preferred, and so on.