Meaning of PCB Layers

The meaning of each layer of PCB (eagle and protel)
1. Signal layer
The signal layer is mainly used to arrange the wires on the circuit board Protel 99 SE provides 32 signal layers, including Top layer, Bottom layer, and 30 MidLayer.
2 .Internal plane layers
The Protel 99 SE provides 16 internal power layers/grounding layers. This type of layer is only used for multi-layer boards and is mainly used for arranging power and grounding wires. We refer to double-layer boards, four layer boards, and six layer boards, which generally refer to the number of signal layers and internal power/grounding layers.
3 .Mechanical layers
The Protel 99 SE provides 16 mechanical layers, which are generally used to set the external dimensions, data markers, alignment markers, assembly instructions, and other mechanical information of a circuit board. These information may vary depending on the requirements of the design company or PCB manufacturer. Executing the menu command Design | Mechanical Layer can set more mechanical layers for the circuit board. In addition, mechanical layers can be attached to other layers to output and display together.
4 .Solder mask layers
Apply a layer of paint, such as solder mask, to various areas outside the solder pad to prevent tin from being applied to these areas. The solder mask layer is automatically generated to match the solder pad during the design process Protel 99 SE provides two solder mask layers: Top Solder and Bottom Solder.
5. Paste mask layer
It has a similar function to the solder mask layer, except for the solder pads corresponding to surface mounted components during machine welding Protel 99 SE provides two solder paste protection layers, Top Paste and Bottom Paste.
6 .Keep out layers
Used to define the area where components and wiring can be effectively placed on a circuit board. Draw a closed area on this layer as the wiring effective area, which cannot be automatically laid out and wired outside of that area.

7. Silk screen layer
The silk screen layer is mainly used to place printed information, such as component contours and annotations, various annotation characters, etc Protel 99 SE provides two silk screen layers, Top Overlay and Bottom Overlay. Generally, various annotation characters are placed on the top silk screen layer, and the bottom silk screen layer can be turned off.

8 .Multi layers
The solder pads and through-holes on the circuit board need to penetrate the entire circuit board and establish electrical connections with different conductive graphic layers. Therefore, the system specifically sets an abstract layer, multi-layer. Generally, solder pads and through-holes need to be set on multi-layer. If this layer is closed, the solder pads and through-holes cannot be displayed.

9 .Drill layer
The drilling layer provides drilling information during the manufacturing process of circuit boards (such as pads, through holes require drilling) Protel 99 SE provides two drilling layers: Drill grid and Drill drawing.
Correspondingly, there are also many layers in Eagle (commonly marked in green).