Types of circuit boards and the purpose of each layer

Circuit board systems are classified into the following three types:

1. Single panel

We also call this PCB single-sided board. Because there are many strict restrictions on the design of single-board circuits (because there is only one side, the wiring cannot intersect and must take separate paths), so only early circuits use this type of board.

2. Double panel

This type of circuit board has wiring on both sides. However, to use wires on both sides, there must be appropriate circuit connections between the two sides. This "bridge" between circuits is called a via. A guide hole is a small hole on a PCB that is filled or coated with metal and can be connected to wires on both sides. Because the area of a double-sided board is twice that of a single-sided board, and because the wiring can be interleaved with each other (can be wrapped around to the other side), it is more suitable for use on more complex circuits than a single-sided board.

3. Multilayer board

For more complex application requirements, circuits can be arranged into a multi-layer structure and pressed together, and through-hole circuits are built between the layers to connect the circuits on each layer.

The purpose of each layer of the PCB:

PCB is composed of multiple layers, including electrical and non-electrical layers, understanding the purpose and meaning of each layer will help us to design a PCB better. Here is an example of a 4-layer PCB.

• Silkscreen Layers: The silkscreen layer is divided into the silkscreen top layer and the silkscreen bottom layer. General PCB on the white (if the board soldermask with white, the silkscreen for black) characters and wireframe is silkscreen layer, used to label the bit number, component box, and some note information. As shown in the figure below:

• SolderMask Layers: Soldermask is divided into the top layer of soldermask and bottom layer of soldermask. General PCB board surface of the green oil (of course, there are other colors, more commonly used in blue, black, white, red, yellow, just the default use of green) is the soldermask, play an insulating role. Note that the layer is negative, that is to say, there is a drawing of the place is not the green oil, not painted places have green oil. As shown in the figure below:

• PasteMask Layers: The flux layer is divided into a flux top layer and a flux bottom layer. Also known as the SMD layer or stencil layer. General PCB board pads on a layer of tin or gold-plated is the flux layer, the layer is to play an auxiliary welding so that welding is easier. In the design, the layer is a positive piece, that is, the drawing of the place of flux, no drawing of the place there is no flux. This is shown in the picture below:

• Electrical Layers: The electrical layer, which includes the top layer, the middle layer (multilayer board) and the bottom layer, is the one that has the electrical connection properties, and most of the design work is to design that layer. The electrical layer is used to make reasonable alignments and connect the pins of the component packages in a reasonable manner. Copper layup is also done on the electrical layer. For 4-layer boards, the inner layer will be set up as the ground and power layer using the negative form. As shown in the figure below:

• Mechanical Layers: The mechanical layer is used to design some physical and mechanical properties, such as borders, slots, holes, and so on. Generally there can be more than one mechanical layer, you can customize the role of the layer, you can also carry out dimensional labeling and so on. There will be differences between this software, for example, the PCB in the picture uses Allegro and does not have multiple mechanical layers (Altium Designer has multiple mechanical layers). The board is framed and hollowed out using Outline.
• Assembly Layers: The assembly layer is divided into assembly bottom layer and assembly bottom layer. Generally used in the PCBA processing, labeling component assembly location and direction, of course, the layer can also use silkscreen layer instead. The layer is no longer shown in the physical PCB, just can be printed out for staff to view. As shown in the figure below:

• Keep-Out Layer: Keep-Out layer includes top layer, middle layer, bottom layer, and all layers. Keep-Out layer is mainly used for constraints. For example, if there are some places where copper is not allowed to be laid or there can't be routes and cross-holes, etc., you can mark the area on the Keep-Out layer. The outside box is RouteKeepIn and the circle is RouteKeepOut.

• Drill Layer: The drill hole layer contains drill hole data such as over-holes and through-hole pads. It is shown in the figure below:

• Substrate: The substrate is the insulating medium between the electrical layers, commonly used FR-4 epoxy fiberglass, of course, there are aluminum substrates, flexible boards and so on. Choose the appropriate substrate for your specific application.

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