Three types of circuit board
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Circuit board systems are classified into the following three types
Single panel
We just mentioned it, so we call this PCB single-sided. 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.
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.
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.
inner circuit
The copper foil substrate is first cut into a size suitable for processing and production. Before laminating the substrate, it is usually necessary to properly roughen the copper foil on the board surface by brushing, micro-etching, etc., and then adhere the dry film photoresist tightly to it at the appropriate temperature and pressure. Send the substrate with the dry film photoresist attached to it into an ultraviolet exposure machine for exposure. The photoresist will undergo a polymerization reaction after being exposed to ultraviolet rays in the light-transmitting area of the film (the dry film in this area will be processed later in the development and copper etching steps. Retained as an etching resist), and transfer the circuit image on the negative film to the dry film photoresist on the board surface. After peeling off the protective film on the film surface, first use sodium carbonate aqueous solution to develop and remove the areas that are not exposed to light, and then use a mixed solution of hydrochloric acid and hydrogen peroxide to corrode and remove the exposed copper foil to form a circuit. Finally, the dried dry film photoresist is washed away with sodium hydroxide aqueous solution. For inner circuit boards with more than six layers (inclusive), use an automatic positioning punching machine to punch out the riveting reference holes for inter-layer circuit alignment. Multi-Layer Boards
In order to increase the area that can be wired, multi-layer boards use more single or double-sided wiring boards. Multilayer boards use several double-sided panels, and an insulating layer is placed between each layer and then glued (pressed).
The number of layers on the board represents how many independent wiring layers there are. Usually the number of layers is an even number and includes the two outermost layers. Most motherboards have a 4 to 8-layer structure, but technically a PCB board with nearly 100 layers is possible. Most large supercomputers use multi-layer motherboards. However, because such computers can be replaced by clusters of many ordinary computers, ultra-multilayer boards have gradually fallen out of use. Because the layers in the PCB are tightly integrated, it is generally not easy to see the actual number, but if you look closely at the motherboard, you may be able to see it.
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The automatic detection technology of circuit boards has been applied with the introduction of surface mounting technology, and the packaging density of circuit boards has increased rapidly. Therefore, automatic detection of circuit boards is not only basic, but also economical, even for low-density, average-number circuit boards. In complex circuit board inspection, two common methods are bed-of-needle testing and dual-probe or flying-probe testing.
Working level
The circuit board includes many types of working layers, such as signal layer, protective layer, silk screen layer, internal layer, etc. The functions of various layers are briefly introduced as follows:
⑴Signal layer: Mainly used to place components or wiring. Protel DXP usually contains 30 middle layers, namely Mid Layer1~Mid Layer30. The middle layer is used to lay out signal lines, and the top and bottom layers are used to place components or lay copper.
⑵Protective layer: Mainly used to ensure that areas on the circuit board that do not need tin plating are not tinned, thereby ensuring the reliability of the circuit board operation. Among them, Top Paste and Bottom Paste are the top solder resist layer and the bottom solder resist layer respectively; Top Solder and Bottom Solder are the solder paste protective layer and the bottom solder paste protective layer respectively.
⑶ Silkscreen layer: Mainly used to print the serial number, production number, company name, etc. of components on the circuit board.
⑷Inner layer: Mainly used as a signal wiring layer. Protel DXP contains a total of 16 internal layers.
⑸Other layers: Mainly include 4 types of layers.
Drill Guide (drilling orientation layer): mainly used for the location of drilling holes on printed circuit boards.
Keep-Out Layer: Mainly used to draw the electrical border of the circuit board.
Drill Drawing (drilling drawing layer): mainly used to set the drilling shape.
Multi-Layer: Mainly used to set multi-layer.