Basic Knowledge of PCB Circuit Board (2)

Fourth: Classification of PCB

According to the number of circuit layers, the PCB boards are divided into single-layer, double-layer and multi-layer boards. Common multi-sided boards are generally 4-sidedboards or 6-sidedboards, and complex multi-sided boards can reach dozens of layers.

Single-Sided Boards on the most basic PCB boards, the parts are concentrated on one side, and the wires are concentrated on the other side (when there are SMD components and the wires are on the same side, the plug-in devices are on the other side). Because the wires only appear on one side, this kind of PCB board is called single-sided. Because single-sided boards have many strict restrictions on the design of the circuit (because there is only one side, the wiring cannot cross and must be around a separate path), only old circuits use this type of board.

Double-Sided Boards: This kind of PCB board has wiring on both sides, but to use wires on both sides, there must be a proper circuit connection between the two sides. The “bridge” between such circuits is called a via. A via is a small hole filled or coated with metal on the PCB board, which can be connected with the wires on both sides. Because the area of ​​the double-sided board is twice as large as that of the single-sided board, the double-sided board solves the difficulty of interlacing wiring in the single-sided board (it can be connected to the other side through the hole), and it is more suitable for use in more complicated circuits than the single-sided board.

Multi-Layer Boards In order to increase the area that can be wired, multi-layer boards use more single or double-sided wiring boards. Use one double-sided as the inner layer, two single-sided as the outer layer, or two double-sided as the inner layer and two single-sided as the outer layer of the printed circuit board. The positioning system and the insulating bonding material alternately together and the conductive pattern Printed circuit boards that are interconnected according to design requirements become four-layer and six-layer printed circuit boards, also known as multilayer printed circuit boards. The number of layers of the board does not mean that there are several independent wiring layers. In special cases, empty layers are added to control the thickness of the board. Usually, the number of layers is even and includes the two outermost layers. Most of the motherboards have 4 to 8 layers of structure, but technically, it is possible to achieve nearly 100 layers of PCB boards. Most large supercomputers use fairly multi-layered motherboards, but because these types of computers can already be replaced by clusters of many ordinary computers, super-multilayered boards have gradually ceased to be used. Because the layers in the PCB board are tightly integrated, it is generally not easy to see the actual number, but if you look closely at the motherboard, you can still see it.

Fifth: Composition of PCB

Circuit and Pattern: The circuit is used as a tool for conduction between the originals. In the design, a large copper surface will be additionally designed as a grounding and power layer. The route and the drawing are made at the same time.

Dielectric Layer: Used to maintain the insulation between the circuit and each layer, commonly known as the substrate.

Through Hole / Via: The through hole can make the lines of more than two levels connect to each other. The larger through hole is used as a part plug-in. In addition, there are non-through holes (nPTH) that are usually used as surface mounts. It is used for fixing screws during assembly.

Solder Resistant /Solder Mask: Not all copper surfaces need to be tin-on parts, so the non-tin area will be printed with a layer of substance (usually epoxy resin) that insulates the copper surface from eating tin. Avoid short circuits between non-tinned circuits. According to different processes, it is divided into green oil, red oil and blue oil.

Legend /Marking/Silk Screen: This is a non-essential structure. The main function is to mark the name and position frame of each part on the circuit board, which is convenient for maintenance and identification after assembly.

Surface Finish: Because the copper surface is easily oxidized in the general environment, it can not be tinned (poor solderability), so it will be protected on the copper surface that needs to be tinned. The protection methods are HASL, ENIG, Immersion Silver, Immersion Tin, and Organic Solder Preservative (OSP). Each method has its advantages and disadvantages, which are collectively referred to as surface treatment.

Sixth: Appearance of PCB

The bare board (no parts on it) is also often called “Printed Wiring Board (PWB)”. The base plate of the board itself is made of materials that are insulated and heat-insulated, and not easy to bend. The small circuit material that can be seen on the surface is copper foil. The copper foil was originally covered on the entire board, but part of it was etched away during the manufacturing process, and the remaining part became a network of small lines. These lines are called conductor patterns or wiring, and are used to provide circuit connections for parts on the PCB.

Usually the color of the PCB board is green or brown, which is the color of the solder mask. It is an insulating protective layer that can protect the copper wire, prevent short circuits caused by wave soldering, and save the amount of solder. A silk screen is also printed on the solder mask. Usually words and symbols (mostly white) are printed on this to mark the position of each part on the board. The screen printing surface is also called a legend.

When the final product is made, integrated circuits, transistors, diodes, passive components (such as resistors, capacitors, connectors, etc.) and various other electronic components will be mounted on it. Through the wire connection, the electronic signal connection can be formed and the application function can be formed.