The role of each layer in the PCB board and design considerations

Many PCB design enthusiasts, especially beginners, do not fully understand the various layers in PCB design. They do not know its function and usage. Here is a systematic explanation for everyone:

1. The mechanical layer, as the name implies, is the appearance of the entire PCB board for mechanical shaping. In fact, when we talk about the mechanical layer, we mean the overall appearance of the PCB board. It can also be used to set the dimensions of the circuit board, data marks, alignment marks, assembly instructions and other mechanical information. This information varies depending on the requirements of the design company or PCB manufacturer. In addition, the mechanical layer can be added to other layers to output and display together.


2. Keep out layer (forbidden wiring layer), used to define the area where components and wiring can be effectively placed on the circuit board. Draw a closed area on this layer as the effective area for routing. Automatic layout and routing is not possible outside this area. The forbidden wiring layer defines the boundary when we lay out the electrical characteristics of copper. That is to say, after we first define the forbidden wiring layer, in the future wiring process, the wiring with electrical characteristics cannot exceed the forbidden wiring. At the boundary of the layer, there is often a habit of using the Keepout layer as a mechanical layer. This method is actually incorrect, so it is recommended that you make a distinction, otherwise the board factory will have to change the attributes for you every time you produce.

3. Signal layer: The signal layer is mainly used to arrange the wires on the circuit board. Including Top layer (top layer), Bottom layer (bottom layer) and 30 MidLayer (middle layer). Top and Bottom layers place the devices, and the inner layers are routed.

4. Top paste and Bottom paste are the top and bottom pad stencil layers, which are the same size as the pads. This is mainly because we can use these two layers to make the stencil when we do SMT. Just dug a hole the size of a pad on the net, and then we cover the stencil on the PCB board, and apply the solder paste evenly with a brush with solder paste, as shown in Figure 2-1.

5. Top Solder and Bottom Solder This is the solder mask to prevent the green oil from being covered. We often say “open the window”. The conventional copper or wiring are covered with green oil by default. If we apply the solder mask accordingly If it is handled, it will prevent the green oil from covering it and expose the copper. The difference between the two can be seen in the following figure:

6. Internal plane layer (internal power/ground layer): This type of layer is only used for multilayer boards, mainly used to arrange power lines and ground lines. We call double-layer boards, four-layer boards, and six-layer boards. The number of signal layers and internal power/ground layers.

7. Silkscreen layer: The silkscreen layer is mainly used to place printed information, such as component outlines and labels, various annotation characters, etc. Altium provides two silk screen layers, Top Overlay and Bottom Overlay, to place the top silk screen files and the bottom silk screen files respectively.

8. Multi layer (multi-layer): The pads and penetrating vias on the circuit board must penetrate the entire circuit board and establish electrical connections with different conductive pattern layers. Therefore, the system has set up an abstract layer-multi-layer . Generally, the pads and vias must be arranged on multiple layers. If this layer is turned off, the pads and vias cannot be displayed.

9. Drill Drawing (drilling layer): The drilling layer provides drilling information during the circuit board manufacturing process (such as pads, vias need to be drilled). Altium provides two drilling layers: Drill gride and Drill drawing.