The meaning of layers in PCB

The base material of FR4 PCB is generally fiber glass. In most cases, the fiber glass substrate of PCB generally refers to the material "FR4". The solid material "FR4" gives the PCB hardness and thickness. In addition to the FR4 substrate, there are flexible circuit boards produced on flexible high-temperature plastics (polyimide or similar) and so on.

In the production of copper clad laminate, heat and adhesive are used to press copper foil onto the FR4 substrate. On double-sided PCB board, the copper foil will be pressed to the front and back sides of the FR4 substrate. In some low-cost occasions, copper foil may only be pressed on one side of the substrate. When we refer to "double-sided board" or "two-layer board", we mean that there are two layers of copper foil on our board. Of course, in different PCB designs, the number of copper foil layers may be as few as 1 layer, or more than 16 layers.

1. PCB signal layer
Signal Layers, the signal layer is used to place copper traces that connect digital or analog signals. The Altium Designer circuit board can have 32 signal layers, where Top is the top layer, Mid1 to Mid 30 is the middle layer, and Bottom is the bottom layer. Traditionally, the Top layer is also called the component layer, and the Botton layer is also called the soldering layer.

2. Soldermask Layer
Above the copper layer is the soldermask. Because generally green soldermask is used, this layer normally makes the PCB look green. The soldermask layer covers the traces on the copper layer to prevent the traces on the PCB from contacting other metals, solder or other conductive objects to cause short circuits. The existence of the soldermask allows everyone to solder components in the right place and prevents solder bridging. We can see that the soldermask covers most of the PCB (including the traces), but the component hole ring and the SMD pad are exposed for soldering.

3. Pastemask layer (solder paste layer, SMD layer)
Its function is similar to that of the soldermask, but the difference is solder paste layer is the pad of the corresponding surface-mounted component during machine soldering. Protel99 SE provides two layers of solder paste, Top Paste (top layer) and Bottom Paste (bottom layer), mainly for SMD components on PCB boards. If all components on the board are Dip (through hole) components, there is no need to export Gerber files of this layer.
Before placing SMD components to the PCB board, solder paste must be applied to each SMD pad. The stencil producing step needs this Paste Mask file. The most important point of the Gerber output of the Paste Mask layer is for SMD components. At the same time, compare this layer with the Soldermask layer described above to find out the different of these two layers.

4. Silkscreen Layer
The white characters are the silk screen layer. Letters, numbers and symbols are printed on the silk screen of the PCB, which can facilitate assembly and guide everyone to better understand the design of the board. We often use symbols on the silkscreen layer to mark certain pins or LED functions etc..

5. Mechanical Layer, Keep-Out Layer (outer shape layer and forbidden wiring layer)
The mechanical layer defines the appearance of the entire PCB board. In fact, when we talk about the mechanical layer, we mean the overall appearance of the PCB board. The forbidden wiring layer defines the boundary when we layout the electrical characteristics of copper. That is to say, after we first define the forbidden wiring layer, in the future layout process, the electrical characteristics cannot exceed the boundary of this layer.

6. Drill layer
The drilling layer provides drilling information during the circuit board manufacturing process (such as pads and vias need to be drilled). Generally PCB Gerber file will provide two drilling layers, Drillgride (drilling instruction map) and Drill drawing (drilling drawing).