PCB Board Layers

Dry | Detailed explanation of multi-layer PCB laminated structure

Before designing a multilayer PCB circuit board, the designer needs to first determine the board structure to be used based on the size of the circuit, the size of the board, and the electromagnetic compatibility (EMC) requirements, that is, to decide whether to use a 4-layer, a 6-layer, or a higher number of layers. After determining the number of layers, the placement of the internal electrical layers and how to distribute the different signals on these layers is determined. This is the problem of selecting the layer structure of a multilayer PCB. Layer structure is an important factor affecting the EMC performance of a PCB board and an important means of suppressing electromagnetic interference. This section will introduce the multilayer PCB board layer good structure of the relevant content

1.1 Layer selection and stacking principles

Determine the layer structure of the multilayer PCB board needs to consider more factors. From the wiring side, the more layers the more conducive to wiring, but the cost and difficulty of the board will also increase. For manufacturers, the symmetry of the layer structure is the focus of attention when manufacturing PCB boards, so the choice of the number of layers needs to be considered in all aspects of the needs to achieve the best balance.

For experienced designers, after completing the pre-layout of components, they will focus on analysing the wiring bottlenecks of the PCB. Combined with other EDA tools to analyse the board's wiring density; and then a combination of special wiring requirements of the signal lines such as differential lines, sensitive signal lines, etc., the number and type of signal layer to determine the number of layers; and then according to the type of power supply, isolation and anti-jamming requirements to determine the number of layers of the internal electrical. In this way, the number of layers of the entire circuit board is basically determined.

Determine the number of layers of the circuit board, the next step is to reasonably arrange the placement of the layers of the circuit order. In this step, the factors to be considered are mainly the following two points.

(1) The distribution of special signal layers.

(2) Distribution of power and ground layers.

If the circuit board, the more layers, the special signal layer, ground layer and power layer of the arrangement of the combination of the more varieties, how to determine which combination is optimal is also more difficult, but the general principles of the following.

(1) The signal layer should be adjacent to an internal electrical layer (internal power/ground layer), the use of internal electrical layer of the large copper film to provide shielding for the signal layer.

(2) The internal power and ground layers should be tightly coupled, i.e., the dielectric thickness between the internal power and ground layers should be taken as a smaller value to increase the capacitance between the power and ground layers and to increase the resonant frequency. The dielectric thickness between the internal power and ground layers can be set in Protel's Layer Stack Manager. Select [Design] / [Layer Stack Manager...] Command, the system pops up the Layer Stack Manager dialogue box, double-click the mouse Prepreg text, pop-up dialogue box shown in Figure 11-1, you can change the thickness of the insulating layer in the Thickness option of the dialogue box.

If the potential difference between power and ground is not large, a smaller insulation thickness, such as 5 mil (0.127 mm), may be used.

(3) The high-speed signal transmission layer in the circuit should be the signal intermediate layer, and sandwiched between the two internal electrical layers. So that the copper film of the two inner layers can provide electromagnetic shielding for high-speed signal transmission, but also can effectively limit the radiation of high-speed signals between the two inner layers, do not cause interference to the outside.

(4) Avoid two signal layers directly adjacent to each other.