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What Is PCB Overlay, Mechanical Layer, Solder Layer, Signal Layer, Drilling Data Layer?

When drawing a PCB board, everyone knows that there are many layers on the circuit board. Therefore, first of all, we need to know which layers are on the PCB board. By providing detailed answers to the various layers of a PCB, I hope it will be helpful for everyone to further understand the composition and design of a PCB.


pcb-layers

Layers Commonly Seen in PCB Design:

  • Mechanical

  • Keepout layer

  • Signal layer

  • Internal plane layer

  • Top overlay

  • Bottom overlay

  • Top paste

  • Bottom paste

  • Top solder

  • Bottom solder

  • Drill guide

  • Drill drawing

  • Multilayer


Top Layer: Also known as component layer, it is mainly used to place components, and can be used for wiring on double-layer and multi-layer boards.


Mid Layer: Up to 30 layers, used for laying signal lines in multi-layer boards.


Bottom Layer: Also known as soldering layer, it is mainly used for wiring and soldering, and sometimes can also be used to place components.


Top Overlayer: Used to annotate the projection contour of components, the label, nominal value or model of components, and various annotation characters.


Bottom Overlayer: The function is the same as that of the top silk screen layer. If various annotations are included in the top silk screen layer, then they are not needed in the bottom silk screen layer.


Internal Plane: Commonly referred to as the internal electrical layer, it includes the power supply layer, reference power supply layer, and ground level signal layer. The internal power layer outputs in negative form.


Mechanical Layer: The mechanical layer defines the appearance of the entire PCB board and is generally used to set the external dimensions, data markers, alignment markers, assembly instructions, and other mechanical information of the circuit board.


It can provide 16 mechanical layers, which are generally used to set the external dimensions, data markers, alignment markers, assembly instructions, and other mechanical information of the circuit board. These information may vary depending on the requirements of the design company or PCB manufacturer. In addition, the mechanical layer can be attached to other layers to output the display together.


multi-layers-pcba

Solder Mask: It provides 2 solder mask layers: Top Solder and Bottom Solder.

There are two layers, the Top Solder Mask and the Bottom Solder Mask, which are automatically generated by the PCB corresponding to the solder pads and through-hole data in the circuit board file. They are mainly used for laying solder mask. This board layer adopts negative output, so the solder pads and through-holes displayed on the board layer represent the areas on the circuit board that are not covered with solder mask, which are the parts that can be soldered.


Because it is a negative output, the actual effect of the parts with Solder Mask is not green oil, but tin plated, appearing silver white! 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 used to match solder pads during the design process and is automatically generated.


Solder mask refers to the part of the board that needs to be coated with green oil. In fact, this solder mask layer uses negative film output, so after the shape of the solder mask layer is mapped onto the board, it is not green oil solder mask, but rather exposed copper skin. Usually, in order to increase the thickness of the copper sheet, a line is drawn on the solder mask layer to remove green oil, and then tin is added to achieve the effect of increasing the thickness of the copper wire.


Apply a layer of paint to various areas outside the solder pad, usually green oil, blue oil, etc., to prevent tin from being applied to these areas. The solder mask layer is used to match solder pads during the design process and is automatically generated. The solder mask layer is a negative film output, and the solder mask layer is not covered with oil, while other areas are covered with oil.


pcba-smt

The function of the Paste mask layer is similar to that of the solder mask layer, with the difference being the corresponding solder pads for surface mounted components during machine welding. Altium Designer provides two soldering layers: Top Paste and Bottom Paste. Mainly targeting SMD components on PCB boards. Before attaching SMD components to the PCB board, solder paste must be applied to each SMD pad. The steel mesh used for tin coating must have this Paste Mask file in order for the film to be processed. The most important point to be clear about the Gerber output of the Paste Mask layer is that this layer is mainly aimed at SMD components. At the same time, compare this layer with the Solder Mask introduced above to understand their different roles, because from the film image, these two film images are very similar.


Distinguishing between solder mask layer and flux layer:


Solder mask refers to the part of the board that needs to be coated with green oil; Because it is a negative film output, areas without solder mask need to be coated with green oil, so the actual effect of the parts with solder mask is not coated with green oil.


Soldering aid layer: pass mask is used during machine SMT, which corresponds to the solder pads of all SMT components. It is the same size as the top layer/bottom layer layer layer and is used to open the steel mesh for tin leakage.


Signal layer: The signal layer is mainly used to arrange the wires on the circuit board. It provides 32 signal layers, including the Top layer, Bottom layer, and 32 internal layers.

Past Mask: There are two layers of solder paste, the Top Paste Mask and the Bottom Paste Mask, which refer to the exposed copper and platinum that we can see on the outside. For example, if we draw a wire in the top wiring layer, what we see on the PCB is only one wire, which is covered by the entire green oil. However, if we draw a square or a point on the Top Paste layer at the position of this wire, the square and point on the board will not have green oil, but copper and platinum. Its function is similar to that of the solder mask layer, except for the solder pads corresponding to surface mounted components during machine welding.


Keep Out Layer: Used to define the area on a circuit board where components and wiring can be effectively placed. Draw a closed area on this layer as the effective area for routing, which cannot be automatically laid out and routed outside of that area.


It is Used to draw the outer boundaries and positioning holes of printed boards, which means that after defining the prohibited wiring layer, the electrical wires we lay in the future wiring process cannot exceed the boundaries of the prohibited wiring layer. Used to define the area on a circuit board where components and wiring can be effectively placed. The function is to draw the prohibited wiring area. If there is no mechanical layer drawn in the printed circuit board, the printed circuit board manufacturer will use this layer as the PCB shape for processing. If both the KEEPOUT LAYER layer and the mechanical layer are present, the default is to use the mechanical layer as the PCB shape. However, the technical personnel of the printed circuit board manufacturer will distinguish it themselves. If they cannot distinguish it, they will default to using the mechanical layer as the outer layer.

Internal plane layer: It provides 32 internal power/ground layers. This type of layer is only used for multi-layer boards, mainly for arranging power layers and grounding layers. We refer to double-layer board, four layer board, and six layer board, generally referring to the number of signal layers and internal power/grounding layers.



Multi Layer: Pads and through-hole pads on a circuit board need to penetrate the entire circuit board and establish electrical connections with different conductive graphic layers. They are usually combined with through-hole or through-hole pad designs to describe the layer characteristics of voids. 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 up an abstract layer - multi-layer. Generally, pads and vias need to be set on multiple layers. If this layer is turned off, pads and vias cannot be displayed.


Drill: The drilling layer provides drilling information during the manufacturing process of circuit boards (such as pads, through holes require drilling). Altium Designer provides two drilling layers: Drill guide and Drill drawing.


Silkscreen layer: The silk screen layer is mainly used to place printed information, such as component contours and annotations, various annotation characters, etc. Altium Designer provides two silk screen layers: Top Overlay and Bottom Overlay.

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