PCB (printed circuit board) is a printed circuit board, which is one of the important components of the electronics industry. Almost every kind of electronic equipment, from electronic watches, calculators, to computers, communication electronic equipment, military weapon systems, as long as there are electronic components such as integrated circuits, in order to make the electrical interconnection between each component, printed board. The printed circuit board is composed of an insulating substrate, connecting wires, and pads for assembling and soldering electronic components, and has a dual role of a conductive circuit and an insulating substrate. It can replace complicated wiring and realize the electrical connection between various components in the circuit, which not only simplifies the assembly and welding of electronic products, reduces the wiring workload in the traditional way, and greatly reduces the labor intensity of workers; Volume, reduce product costs, and improve the quality and reliability of electronic equipment. The printed circuit board has good product consistency. It can adopt standardized design, which is conducive to mechanization and automation in the production process. At the same time, the entire printed circuit board after assembly and debugging can be used as an independent spare part, which is convenient for the interchange and maintenance of the whole product. At present, printed wiring boards have been widely used in the production of electronic products.
The earliest printed circuit boards used were paper-based copper-clad printed boards. Since the appearance of semiconductor transistors in the 1950s, the demand for printed boards has risen sharply. In particular, the rapid development and wide application of integrated circuits have made electronic devices smaller and smaller, and the density and difficulty of circuit wiring have become more and more difficult. This requires printed boards to be constantly updated. At present, the variety of printed boards has developed from single-sided boards to double-sided boards, multilayer boards and flexible boards; the structure and quality have also developed to the level of ultra-high density, miniaturization and high reliability; new design methods, design supplies and Board-making materials and board-making processes continue to emerge. In recent years, various computer-aided design (CAD) printed circuit board application software has been popularized and promoted in the industry. Among specialized printed circuit board manufacturers, mechanized and automated production has completely replaced manual operations.
The creator of the PCB was the Austrian Paul Eisler. In 1936, he first used printed circuit boards in radios. In 1943, Americans mostly applied this technology to military radios. In 1948, the United States officially recognized that the invention could be used for commercial purposes. Printed circuit boards have only been widely used since the mid-1950s. Printed circuit boards are found in almost every electronic device. If there are electronic parts in a certain kind of equipment, they are also mounted on PCBs of different sizes. The main function of the PCB is to make various electronic components form a predetermined circuit connection, and to play the role of relay transmission. It is the key electronic interconnect of electronic products, and is known as the “mother of electronic products”.
The reason why PCB can be used more and more widely is because it has many unique advantages, as follows:
For many years, the high density of printed boards has been able to develop correspondingly with the increase in integrated circuit integration and advances in mounting technology.
Through a series of inspection, testing and aging tests, it can ensure that the PCB works reliably for a long time (usually 20 years).
The requirements for various performances (electrical, physical, chemical, mechanical, etc.) of the PCB can be achieved through design standardization and standardization. The design time is short and the efficiency is high.
PCB adopts modern management, which can realize standardization, scale (quantity), and automated production, thereby ensuring the consistency of product quality.
Established relatively complete test methods and test standards, which can be used to test and identify the qualification and service life of PCB products through various test equipment and instruments.
PCB products are not only convenient for standardized assembly of various components, but also for automated, large-scale mass production. In addition, the PCB and other various components are integrated to form larger components and systems, up to the entire machine.
Since the components of the PCB product and various components are assembled as a whole with standardized design and large-scale production, these components are also standardized. Therefore, once the system fails, it can be replaced quickly, easily and flexibly, and the system work can be quickly restored.
PCB also has some other advantages, such as miniaturizing and lightening the system, and speeding up signal transmission.
The PCB has the following functions in electronic equipment.
(1) Provide mechanical support for fixing and assembling various electronic components such as integrated circuits, realize wiring and electrical connection or electrical insulation between various electronic components such as integrated circuits, and provide required electrical characteristics.
(2) Provide solder mask patterns for automatic welding, and identify characters and graphics for component insertion, inspection, and maintenance.
(3) After the printed board is used for electronic equipment, due to the consistency of similar printed boards, errors in manual wiring are avoided, and automatic insertion or placement of electronic components, automatic soldering, and automatic detection can be achieved to ensure electronic products Quality, improve labor productivity, reduce costs, and facilitate maintenance.
(4) Provide the required electrical characteristics, characteristic impedance and electromagnetic compatibility characteristics for the circuit in high-speed or high-frequency circuits.
(5) The printed board embedded with passive components inside provides certain electrical functions, simplifies the electronic installation procedure, and improves the reliability of the product.
(6) In large-scale and ultra-large-scale electronic packaging components, an effective chip carrier is provided for chip packaging of miniaturized electronic components.
Edit by level
Classification according to the number of circuit layers: Divided into single panel, double panel and multilayer board. Common multi-layer boards are generally 4-layer boards or 6-layer boards, and complex multi-layer boards can reach dozens of layers. There are three main types of PCB boards:
Single-Sided Boards On the most basic PCB, the parts are concentrated on one side, and the wires are concentrated on the other side (when there is a patch component, it is the same side as the wire, and the plug-in device is on the other side). Because the wires only appear on one side, this type of PCB is called Single-sided. Because the single panel has many strict restrictions on the design circuit (because there is only one side, the wiring cannot cross and must go around its own path), only early circuits used this type of board.
Double-Sided Boards This type of circuit board has wiring on both sides.
To use two-sided wires, you must have a proper circuit connection between the two sides. This “bridge” between circuits is called a via. A via is a small hole filled or coated with metal on the PCB. It can be connected to the wires on both sides. Because the area of the double-sided board is twice as large as the single-sided board, the double-sided board solves the difficulty of wiring interleaving in the single-sided board (which can be conducted to the other side through the hole), and it is more suitable for more complicated circuits than the single-sided board.
Multi-Layer Boards In order to increase the wiring area, multiple layers
The board uses more single or double-sided wiring boards. A printed circuit board with one double side as the inner layer, two single sides as the outer layer, or two double sides as the inner layer, and two single sides as the outer layer. The conductive pattern is alternately connected together through a positioning system and an insulating bonding material. The 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, an empty layer is added to control the thickness of the board. Usually, the number of layers is even, and it includes the two outermost layers. Most motherboards have a structure of 4 to 8 layers, but technically it can achieve nearly 100 layers of PCB boards. Large supercomputers mostly use fairly multilayer motherboards. However, because such computers can already be replaced by clusters of many ordinary computers, ultra-multilayer boards have gradually become unused. 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.
Edit by soft and hard
Divided into rigid circuit boards, flexible circuit boards, and rigid-flex boards. Generally, the PCB shown in the first picture below is called a Rigid PCB, and the second one is
The yellow connection lines in the figure are called flexible (or disturbing) Flexible PCBs. The intuitive difference between rigid PCB and flexible PCB is that flexible PCB can be bent. Common thicknesses of rigid PCBs are 0.2mm, 0.4mm, 0.6mm, 0.8mm, 1.0mm, 1.2mm, 1.6mm, 2.0mm, etc. The common thickness of a flexible PCB is 0.2mm. Where parts are to be soldered, a thick layer is added behind it. The thickness of the thick layer ranges from 0.2mm to 0.4mm. The purpose of understanding these is to provide them with a spatial reference when designing. Common materials for rigid PCBs include: phenolic paper laminates, epoxy paper laminates, polyester glass mat laminates, epoxy glass cloth laminates. Flexible PCB materials commonly include: polyester film, polyimide Amine film, fluorinated ethylene propylene film. 
PCB design principles
To get the best performance from electronic circuits, the layout of components and the layout of wires are important. In order to design good quality, low cost PCB. The following general principles should be followed:
First, consider the PCB size. The PCB size is too large, the printed lines are long, the impedance is increased, the anti-noise capability is reduced, and the cost is also increased; if it is too small, the heat dissipation is not good, and the adjacent lines are susceptible to interference. After the PCB size is determined, the location of the special components is determined. Finally, according to the functional units of the circuit, all components of the circuit are laid out.
Observe the following principles when locating special components:
① Shorten the connection between high-frequency components as much as possible, and try to reduce their distribution parameters and mutual electromagnetic interference. Vulnerable components should not be placed too close to each other, and input and output components should be kept as far away as possible.
② There may be a high potential difference between some components or wires, and the distance between them should be increased to avoid accidental short circuit caused by discharge. Components with high voltage should be placed as hard as possible during the debugging.
③ Components weighing more than 15 g should be fixed with brackets and then soldered. Those large, heavy, and heat-generating components should not be installed on printed boards. Instead, they should be installed on the chassis base of the entire machine, and heat dissipation should be considered. Keep the thermal element away from the heating element.
④ For the layout of adjustable components such as potentiometers, adjustable inductors, variable capacitors, and micro-switches, the structural requirements of the whole machine should be considered. If it is adjusted inside the machine, it should be placed on a printed board where it is easy to adjust. If it is adjusted outside the machine, its position should be compatible with the position of the adjustment knob on the chassis panel.
According to the functional units of the circuit, the layout of all components of the circuit must meet the following principles:
① Arrange the positions of each functional circuit unit according to the flow of the circuit, make the layout convenient for signal circulation, and keep the signals in the same direction as possible.
② Take the core components of each functional circuit as the center and make a layout around it. The components should be pulled evenly, neatly and compactly on the PCB to minimize and shorten the leads and connections between the components.
③ For circuits operating at high frequencies, the distribution parameters between components must be considered. Generally, the components should be arranged in parallel as much as possible. In this way, it is not only beautiful, but also easy to mount and weld, and easy to mass produce.
④ The components located on the edge of the circuit board are generally not less than 2 mm away from the edge of the circuit board. The optimal shape of the circuit board is rectangular. The aspect ratio is 3: 2 or 4: 3. When the size of the circuit board is larger than 200 mm✖150 mm, the mechanical strength of the circuit board should be considered.
The principles are as follows:
① The wires used for input and output should be avoided as much as possible. It is best to add a ground wire between the wires to avoid feedback coupling.
② The minimum width of printed circuit wires is mainly determined by the adhesion strength between the wires and the insulating substrate and the value of the current flowing through them.
When the thickness of the copper foil is 0.05 mm and the width is 1 to 15 mm, the current will not exceed 3 ° C through a current of 2 A. Therefore, the width of the wire is 1.5 mm. For integrated circuits, especially digital circuits, a wire width of 0.02 to 0.3 mm is usually chosen. Of course, as long as you can, use wide wires, especially power and ground wires.
The minimum spacing of the wires is mainly determined by the worst-case insulation resistance and breakdown voltage. For integrated circuits, especially digital circuits, as long as the process allows, the pitch can be as small as 5 to 8 mm.
③ The bend of the printed conductor is generally circular, and the right angle or the included angle will affect the electrical performance in high-frequency circuits. In addition, try to avoid the use of large-area copper foil, otherwise, the copper foil will easily swell and fall off when heated for a long time. When a large area copper foil must be used, it is best to use a grid shape, which is conducive to excluding the volatile gas generated by the adhesive between the copper foil and the substrate when heated.
The pad center hole is slightly larger than the device lead diameter. Pads that are too large are prone to false soldering. The pad outer diameter D is generally not less than d + 1.2 mm, where d is the lead hole diameter. For high-density digital circuits, the minimum pad diameter can be d + 1.0 mm.
PCB board software editing
The providers of commonly used PCB design software are Altium, Cadence, Mentor, etc. Among them, Altium (formerly known as Protel International) has successively introduced Protel 99 SE, Pi’otel DXP, AltiumDesigner, etc., which are widely used in China. At present, Altium Designer has new versions released every year. For PCB design software, learn to use one, and the rest It’s easier to learn, because the functions of such software are very similar.
Industry chain editing
Classified according to the upstream and downstream of the industrial chain, it can be divided into raw materials, copper clad laminates, printed circuit boards, and electronic product applications. The relationship is as follows:
Fiberglass cloth: Fiberglass cloth is one of the raw materials of copper-clad laminates. It is woven from glass fiber yarn and accounts for about 40% (thick) or 25% (thin) of the cost of copper-clad laminates. Glass fiber yarns are calcined in a kiln into a liquid state by using raw materials such as silica sand. They are drawn into ultra-fine glass fibers through extremely small alloy nozzles, and then hundreds of glass fibers are entangled into glass fiber yarns. The investment in the construction of the kiln is huge, generally requiring hundreds of millions of dollars, and once it is ignited, it must be produced continuously for 24 hours, and the cost of entry and exit is huge.
Copper foil: Copper foil is the raw material that accounts for the largest proportion of the cost of copper clad laminates, accounting for about 30% (thick) or 50% of the cost of copper clad laminates.