A printed circuit is a conductive pattern attached to the surface of an insulating substrate to connect electronic components. The finished board of a printed circuit becomes a printed circuit board, or PCB. Printed circuit boards (PCBs) use conductive tracks, pads, and other features to mechanically support and electrically interconnect electronic or electrical components. These features are etched onto and/or between layers of a non-conductive substrate, forming one or more copper layers. Components are typically soldered to the PCB for electrical connection and mechanical fixation. Printed circuit boards are used in all but the simplest electronic products. They are also used in some electrical products, such as passive switch boxes. Alternatives to PCBs include wire-wrapping and point-to-point construction, both of which were once popular but are now rarely used. PCBs require additional design work to lay out the circuit, but manufacturing and assembly can be automated. Specialized CAD software can be used to accomplish most of the layout work. Using PCBs to mass-produce circuits is cheaper and faster than other wiring methods because component mounting and wiring are done in a single operation. Large quantities of PCBs can be manufactured simultaneously, while the layout only needs to be done once. PCBs can also be hand-crafted in small quantities, but the efficiency will be reduced. PCBs can be single-sided (one copper layer), double-sided (two copper layers on both sides of a substrate layer), or multi-layered (outer and inner copper layers alternating with substrate layers). Multi-layer PCBs allow for higher component density, as otherwise the circuit routing in the inner layers would occupy surface space between components. With the adoption of surface mount technology, multi-layer PCBs with more than two (especially more than four) copper layers have become popular. However, multi-layer PCBs make circuit repair, analysis, and field modification more difficult and often impractical.

　　 Classification of PCBs

　　1. Rigid PCBs: Rigid PCBs have a certain mechanical strength, and the components mounted on them have a certain bending resistance. They are usually in a flat state during use. Generally, rigid PCBs are favored by electronic devices for their good flatness and excellent bending resistance.

　　2. Flexible PCBs: Flexible PCBs are made of flexible laminated plastics or other flexible insulating materials. Circuit boards made of flexible PCBs can be easily bent and stretched, which means that people can bend them arbitrarily according to installation requirements when using flexible PCBs. Flexible printed boards are used in special occasions, for example, the display screen of some digital multimeters can be rotated, which is due to the popularity and application of flexible PCBs.

　　3. Single-sided PCBs: Single-sided PCBs are basic and simple PCBs, with components arranged on one side and circuits on the other. Because there is only one layer of conductor, it is called a single-sided PCB (Single-sided PCB or one layer PCB). Single-sided PCBs are usually made of laminates or fiberglass cloth, and their conductive patterns are relatively simple, so most of them are made using screen printing or wet film processes. In addition, a small number of various products usually use photoresist to pattern the circuits.

　　4. Double-sided PCBs: Double-sided PCBs (also known as double-sided plated through-holes or DSPT, usually made of epoxy boards or fiberglass cloth) are a pathway to more advanced applications. They allow for tighter (and even more) routing by using through-holes to alternate between the top and bottom layers.

　　5. Multilayer boards: Multilayer boards are printed circuit boards with three or more layers of conductive patterns. The conductive patterns of the inner layers of multilayer boards are laminated with insulating adhesive sheets, and the outer layers are covered with metal foil, pressed into a whole multilayer board. The holes of the multilayer board must be metallized in order to lead out and connect the printed circuits sandwiched between the insulating substrates, which is PTH (Plate Through Hole).

　 　Features of PCBs:

　　I. General characteristics:

　　1) Higher density: Over the past century, with the improvement of integrated circuits and the advancement of mounting technology, the high density of PCBs has been developed.

　　2) Higher reliability: After a series of inspections, tests, and aging tests, it ensures that the PCB works reliably for a longer period (generally 20 years).

　　3) Good designability: Through the standardization and standardization of the design, it can meet the requirements of various PCB performances (electrical, physical, chemical, mechanical, etc.), with short time and high efficiency.

　　4) Good productivity: Using modern management, standardized, large-scale, and automated production can be carried out to ensure the consistency of product quality.

　　5) Easy testability analysis: A relatively complete set of testing methods, testing standards, various testing equipment and instruments have been established for testing and evaluating the qualification and service life of PCB products.

　　6) Assembly capability: PCB products are not only easy for the standardized assembly of various components, but also can achieve automated and mass production, and can assemble PCBs and various components into larger components, systems, and even complete machines.

　　7) Good maintainability: Because PCB products and components are designed, manufactured, and assembled according to standards, they are also standardized. When the system malfunctions, the system can be quickly, conveniently, and flexibly replaced, and the system can be quickly restored.

　　 II. Features of multilayer PCBs:

　　1) The use of integrated circuits can miniaturize the entire machine and reduce the weight of the entire machine.

　　2) Improve wiring density, reduce the distance between components, and shorten the signal transmission path.

　　3) Reduce the number of component solder joints and reduce the failure rate.

　　4) Due to the use of shielding layers, the signal distortion of the circuit is reduced.

　　5) Introduce a grounding heat-conducting surface to reduce local overheating and improve the reliability of the entire machine.