Section 3 multilayer PCB cascade structure

Before designing a multilayer PCB, the designer must first determine the structure of the PCB based on the circuit size, circuit board size, and EMC requirements, that is, it is decided to adopt a 4-layer, 6-layer, or more layers of the circuit board. After determining the number of layers, determine the placement location of the inner electrical layer and how different signals are distributed on these layers. This is the choice of multilayer PCB stack structure. Stack structure is an important factor affecting pcb emc performance and an important means to suppress electromagnetic interference. This section describes the multilayer PCB stack structure.

11.1.1 how to select and stack Layers

Many factors need to be considered to determine the stack structure of multilayer PCB. In terms of wiring, the more layers, the more favorable the wiring, but the cost and difficulty of the Board will also increase. For the manufacturer, whether the stacked structure is symmetric or not is the focus of PCB Board manufacturing. Therefore, the choice of layers needs to consider the needs of various aspects to achieve the best balance.

For experienced designers, after completing the pre-layout of components, the PCB wiring bottleneck will be analyzed. Use other EDA tools to analyze the wiring density of the circuit board, and then combine the quantity and types of signal lines, such as differential lines and sensitive signal lines, to determine the layers of the signal layer; then, the number of inner electrical layers is determined based on the power supply type, isolation and anti-interference requirements. In this way, the number of Board layers of the entire circuit board is basically determined.

After the layers of the circuit board are determined, the next step is to arrange the placement order of each layer circuit reasonably. There are two main factors to consider in this step.

(1) distribution of special signal layers.

(2) distribution of power supply layers and strata.

The more layers the circuit board has, the more types of special signal layer, ground layer, and power layer are arranged and combined. the more difficult it is to determine which combination method is optimal, however, the general principles are as follows.

(1) The signal layer should be adjacent to an inner electrical layer (internal power supply/formation), and the large copper film of the inner electrical layer should be used to shield the signal layer.

(2) The internal power supply layer should be closely coupled with the formation. That is to say, the thickness of the medium between the internal power supply layer and the formation should be smaller to increase the capacitance between the power supply layer and the formation, increase the resonance frequency. The media thickness between the internal power supply layer and the formation can be set in the layer stack manager of PROTEL. Select Design or layer stack manager ...] Command. In the stack Manager dialog box that appears, double-click the prepreg text. The Dialog Box 11-1 is displayed. You can change the thickness of the insulation 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, for example, 5mil (0.127 ).

(3) The high-speed signal transmission layer in the circuit should be the signal intermediate layer and be clamped between two inner electrical layers. In this way, the copper film of the two inner electrical layers can provide Electromagnetic Shielding for high-speed signal transmission and effectively restrict the radiation of High-speed signals between the two inner electrical layers without external interference.

(4) avoid two signal layers directly adjacent. Crosstalk is easily introduced between adjacent signal layers, resulting in circuit function failure. Adding a ground plane between the two signal layers can effectively avoid crosstalk.

(5) Multiple grounding inner electrical layers can effectively reduce the Grounding Impedance. For example, a signal layer and B signal layer use their respective ground planes to effectively reduce common mode interference.

(6) consider the symmetry of the layer structure.

11.1.2 common cascade Structures

The following example shows how to optimize the arrangement and combination of various laminated structures.

For Commonly Used 4-layer panels, there are several cascade methods (from top layer to bottom layer ).
(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, the power supply layer and formation in solution 3 are not effectively coupled and should not be used.

So how should solution 1 and solution 2 be selected? Generally, the designer selects solution 1 as the 4-layer structure. The reason for selection is not that solution 2 cannot be used, but that General PCB boards only place components on the top layer, so solution 1 is more appropriate. However, when components need to be placed on the top and bottom layers, and the thickness of the medium between the internal power supply layer and the formation is large and the coupling is poor, the signal lines on which layer should be considered should be less. For solution 1, there are few signal lines at the bottom layer, and a large area of copper film can be used for coupling with the power layer. On the contrary, if the components are mainly arranged at the bottom layer, choose solution 2 to make the board.

If the cascade structure shown in 11-1 is used, the power supply layer and the ground layer are coupled. Generally, solution 1 is used to consider the requirements of symmetry.

After completing the analysis of the laminated structure of the 4-layered laminate, the following uses an example of a 6-layered laminate combination to illustrate the arrangement and combination of the 6-layered laminated structure and the optimization method.

(1) siganl_1 (top), Gnd (inner_1), siganl_2 (inner_2), siganl_3 (inner_3), power (inner_4), siganl_4 (bottom ).

Solution 1 adopts a layer-4 signal layer and a layer-2 internal power supply/access layer, and has many signal layers, which is conducive to wiring between components. However, this solution has obvious defects, the performance is as follows.

① The power supply layer and the ground layer are separated far from each other and are not fully coupled.
② The signal layer siganl_2 (inner_2) and siganl_3 (inner_3) are directly adjacent to each other. The signal isolation is poor and Crosstalk is prone.
(2) siganl_1 (top), siganl_2 (inner_1), power (inner_2), Gnd (inner_3), siganl_3 (inner_4), siganl_4 (bottom ).

Compared with solution 1, solution 2 has full coupling between the power supply layer and the ground layer, which has some advantages over solution 1. However, siganl_1 (top), siganl_2 (inner_1), and siganl_3 (inner_4) it is directly adjacent to the signal layer of siganl_4 (bottom). The signal isolation is poor and the problem of crosstalk is easily solved.

(3) siganl_1 (top), Gnd (inner_1), siganl_2 (inner_2), power (inner_3), Gnd (inner_4), siganl_3 (bottom ).
Compared with solution 1 and solution 2, solution 3 reduces a signal layer and an inner electrical layer, although the layers available for cabling are reduced, however, this solution solves the Common Defects in solution 1 and solution 2.

① The power supply layer is closely coupled with the ground layer.
② Each signal layer is directly adjacent to the inner electrical layer, which is effectively isolated from other signal layers and is not prone to crosstalk.
③ Siganl_2 (inner_2) is adjacent to two inner electrical layers Gnd (inner_1) and power (inner_3) and can be used to transmit high-speed signals. The two inner electrical layers can effectively shield external interference on the siganl_2 (inner_2) layer and siganl_2 (inner_2) external interference.

In all aspects, solution 3 is obviously the most optimal one. At the same time, solution 3 is also a commonly used stack structure of 6 layers.
Through the analysis of the above two examples, I believe that the reader has a certain understanding of the cascade structure, but sometimes, a certain solution cannot meet all the requirements, this requires consideration of the priority of the design principles. Unfortunately

The design of the Board layer 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. Therefore, in fact, these principles have no fixed priority for reference. But it can be determined that the design principle 2 (the internal power supply layer and formation should be closely coupled) needs to be met first during the design, and if the circuit needs to transmit high-speed signals, therefore, the design principle 3 (the High-speed signal transmission layer in the circuit should be the signal intermediate layer and be clamped between two inner electrical layers) must be met. Table 11-1 provides a reference scheme for the multilayer structure.