Basic PCB Knowledge

Basic PCB Knowledge

PCB (PrintedCircuitBoard), Chinese name is PCB, also known as printed circuit board, printed circuit board, is an important electronic component, is the support of electronic components, is the provider of electrical connections of electronic components. Because it is made by electronic printing, it is called a "printing" circuit board.

Classified according to the number of circuit layers: divided into single-panel, double-panel and multi-layer boards. Common multi-layer boards are generally 4-layer boards or 6-layer boards, and complex multi-layer boards can reach dozens of layers.

PCB boards have the following three main types of division:

Single sided PCB

Single-Sided Boards On the most basic PCB, the parts are concentrated on one side and the wires are concentrated on the other side. Since the wire only appears on one side, this PCB is called a single-sided. Because single-panel has many strict restrictions on the design line (because there is only one side, the wiring can not cross and must be around the path), so only the early circuit used this type of board.

Double sided PCB

Double-Sided Boards These boards have wiring on both sides, but to use the two-sided wires, you must have proper circuit connections between the two sides. The "bridge" between such circuits is called a via. The via hole is a small hole filled or coated with metal on the PCB, which can be connected to the wires on both sides. Because the area of the double panel is twice as large as that of the single panel, the double panel solves the difficulty of wiring interleaving in a single panel (which can be conducted through the hole to the other side), which is more suitable for circuits that are more complicated than single panels.

Multilayer PCB boards

Multi-Layer Boards To increase the area that can be routed, multi-layer boards use more single or double-sided wiring boards. Using a double-sided inner layer, two single-sided outer layers or two double-sided inner layers, two single-sided outer printed circuit boards, alternating between the positioning system and the insulating bonding material and conductive patterns printed circuit boards that are interconnected according to design requirements become four-layer, six-layer printed circuit boards, also known as multilayer printed wiring boards. The number of layers of the board does not mean that there are several layers of 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 the outermost two layers are included. Most of the motherboards are 4 to 8 layers of structure, but technically they can achieve nearly 100 layers of PCB. Large supercomputers mostly use quite a few multi-layer motherboards, but because such computers can already be replaced by clusters of many common computers, super-multilayer boards have gradually disappeared. Because the layers in the PCB are tightly coupled, it is generally not easy to see the actual number, but if you look closely at the motherboard, you can still see it.

Classified according to soft and hard

Divided into rigid circuit board and flexible circuit board, soft and hard board. The PCB shown in the first figure below is generally referred to as a rigid (Rigid) PCB, and the yellow connection line in the second figure is referred to as a flexible (or permeable) PCB. The intuitive difference between a rigid PCB and a flexible PCB is that the flexible PCB can be bent. Common thicknesses of rigid PCBs are 0.2 mm, 0.4 mm, 0.6 mm, 0.8 mm, 1.0 mm, 1.2 mm, 1.6 mm, 2.0 mm, and the like. The common thickness of a flexible PCB is 0.2 mm. The place where the part is to be welded will be thickened with a thick layer of 0.2 mm and 0.4 mm. The purpose of understanding these is to provide them with a spatial reference when designing the structural engineer. Common materials for rigid PCBs include: phenolic paper laminates, epoxy paper laminates, polyester glass mat laminates, and epoxy glass cloth laminates; flexible PCB materials commonly include: polyester film, polyamide Amine film, fluorinated ethylene propylene film.

PCB raw materials

A copper clad laminate is a substrate material for making a printed circuit board. It is used to support various components and to achieve electrical or electrical insulation between them.

PCB aluminum substrate

PCB aluminum substrate (metal-based heat sink including aluminum substrate, copper substrate, iron substrate) is a low-alloyed Al-Mg-Si high-plastic alloy plate (see the figure below), which has good thermal conductivity and electrical insulation properties. And machining performance, currently the mainstream aluminum substrate Fosslet.

PCB polychlorinated biphenyl

PCB polychlorinated biphenyl is a synthetic organic compound used commercially in North America from 1929 until the late 1970s and has been widely used in electrical equipment insulation, heat exchangers, hydraulic systems, and other special applications. Although this chemical has not been processed in Canada, it has been widely used.

After decades, people realized the pollution of PCBs to the global environment. It is a mixture of various chlorinated biphenyls, which is extremely harmful to the human body. The Canadian government has taken steps to eliminate PCBs, but illegally imported, processed and sold PCBs in Canada in 1977, and illegally released PCBs into the natural environment in 1985, while the Canadian Constitution allowed PCB equipment owners to continue to use them. PCB until the life of the device. Since 1988, provincial governments in Canada have begun to regulate the storage, transportation and destruction of PCBs.

PCBs are not easily decomposed in the natural environment, and they travel very far. PCBs enter the air, soil and rivers and oceans during production, processing, use, transportation and waste disposal. Small marine organisms and fish inhale PCBs into the body. They become food for large marine life, so that the PCB enters the body of all marine life, including mammalian marine life. The accumulation of PCB in marine organisms far exceeds its content in water, almost a few thousand times the number.

Contact processing

The solder mask green paint covers most of the copper surface of the circuit and only exposes the terminal contacts for soldering, electrical testing and board insertion. This endpoint requires an additional protective layer to avoid oxides at the terminals of the anode (+) that are connected during long-term use, affecting circuit stability and creating safety concerns.

[Electroplating hard gold] On the plug-in end of the board (commonly known as gold finger) is coated with a layer of nickel and a highly chemically blunt gold layer to protect the end points and provide good connection performance, which contains an appropriate amount of cobalt, which is excellent. Wear characteristics.

[Spray tin] Covers a layer of tin-lead alloy on the soldering end of the board with a hot air leveling to protect the board ends and provide good soldering performance.

[Pre-welding] Overlay the upper layer of anti-oxidation pre-weld film on the soldering end of the board, temporarily protect the soldering end point and provide a flat soldering surface before soldering, so as to have good soldering performance.

[Carbon ink] A layer of carbon ink is printed on the contact end of the board to protect the end points and provide good connection performance.

Mold cutting

Cut the board into a CNC molding machine (or die press) to the customer's desired size. When cutting, the plug is fixed on the bed (or mold) through the previously drilled positioning hole. After the cutting, the gold finger portion is further processed by the bevel angle to facilitate the plugging of the circuit board. For the multi-chip forming circuit board, it is necessary to add an X-shaped broken line (known in the industry as V-Cut) to facilitate the customer to split and disassemble after the plug-in. Finally, the dust on the board and the surface ionic contaminants are washed.

Final inspection package

The final electrical conduction, impedance test, and solderability and thermal shock resistance tests were performed on the board prior to packaging. And with appropriate baking to eliminate the moisture and accumulated thermal stress of the board during the process, and finally packaged in a vacuum bag.

Edit this paragraph features

PCB  can be used more and more widely because it has many unique advantages, as outlined below.

Can be high density. For more than 100 years, the high density of printed boards has grown with the integration of integrated circuits and advances in mounting technology.

High reliability. Through a series of inspections, tests and aging tests, the PCB can be reliably operated for a long period of time (usually 20 years).

Designability. For the various performances of PCB (electrical, physical, chemical, mechanical, etc.), the design of printed boards can be realized through design standardization and standardization, with short time and high efficiency.

Productivity. With modern management, it can carry out standardization, scale (quantity), automation and other production to ensure product quality consistency.

Testability. Established relatively complete test methods, test standards, various test equipment and instruments to detect and identify PCB product qualification and service life.

Assemblyability. PCB products not only facilitate the standardized assembly of various components, but also can be automated, large-scale mass production. At the same time, the PCB and various component assembly components can be assembled to form larger components and systems up to the complete machine.

Maintainability. Since PCB products and various component assembly components are produced in a standardized design and scale, these components are also standardized. Therefore, once the system fails, it can be replaced quickly, conveniently and flexibly, and the system can be quickly restored. Of course, you can say more about it. Such as miniaturization and weight reduction of the system, high-speed signal transmission, and the like.