PCB (Printed Circuit Board) is an important electronic component that serves as a support structure for electronic components and a carrier for electrical connections. It is called a “printed” circuit board because it is produced using electronic printing techniques. PCB are one of the essential components in the electronics industry. Almost every electronic device, from small items like digital watches and calculators to large systems like computers, communication electronics, and military weapon systems, uses printed circuit boards to connect integrated circuits and other electronic components electrically.
A printed circuit board consists of an insulating substrate, connecting wires, and pads for assembling and soldering electronic components, serving both as conductive pathways and an insulating base. It can replace complex wiring to achieve electrical connections between various components, simplifying assembly and soldering processes, reducing the workload associated with traditional wiring methods, and significantly decreasing labor intensity. Additionally, PCB help reduce the overall size of devices, lower product costs, and improve the quality and reliability of electronic equipment. They offer good product consistency and can be standardized in design, facilitating mechanization and automation in production. Moreover, a fully assembled and tested printed circuit board can serve as an independent spare part, making it easier to replace and maintain complete products.
Printed circuit boards were originally made from paper-based copper-clad laminates. Since the introduction of semiconductor transistors in the 1950s, the demand for printed circuit boards has surged. The rapid development and widespread application of integrated circuits have led to smaller electronic devices with increasing circuit density and complexity, necessitating continuous innovation in printed circuit boards. Currently, PCB varieties have evolved from single-sided boards to double-sided boards, multilayer boards, and flexible boards. Their structure and quality have advanced to ultra-high density, miniaturization, and high reliability. New design methods, materials, and manufacturing processes are continually emerging. In recent years, various computer-aided design (CAD) software for PCB has become widely adopted in the industry, and mechanized and automated production has completely replaced manual processes in specialized PCB manufacturers.
Classification by Layer Count
According to the number of circuit layers, PCB can be classified into single-sided boards, double-sided boards, and multilayer boards. Common multilayer boards typically have four or six layers, while more complex boards can have dozens of layers. The three main types of PCB classifications are:
Single-Sided Boards
Single-sided boards (Single-Sided Boards) have components concentrated on one side and conductive traces on the opposite side (or both traces and surface-mounted components on one side, with through-hole components on the other). Since traces only appear on one side, this type of PCB is called single-sided. Due to the strict limitations on circuit design (traces cannot cross and must take separate paths), single-sided boards are typically used only in early circuit designs.
Double-Sided Boards
Double-sided boards (Double-Sided Boards) feature wiring on both sides, requiring proper electrical connections between the two sides. These connections, known as vias, are small holes filled or coated with metal that allow traces from both sides to connect. Double-sided boards, which have double the surface area of single-sided boards, solve the interleaving issues of single-sided designs (allowing connections to be made through vias). They are more suitable for circuits that are more complex than those typically handled by single-sided boards.
Flexible board, Rigid-flexible board
Multi-layer boards (Multi-Layer Boards) increase the available wiring area by utilizing multiple single-sided or double-sided boards. For example, a four-layer PCB may consist of a double-sided board as the inner layer, flanked by two single-sided boards as outer layers, or two double-sided boards as inner layers with two single-sided boards as outer layers. These printed circuit boards are interleaved with insulating adhesive materials and interconnected as per design requirements. The number of layers does not necessarily indicate the number of independent wiring layers; in special cases, empty layers may be added to control board thickness, with the number of layers typically being even and including the outermost two layers. Most motherboards consist of 4 to 8 layers, although technically, PCB can have nearly 100 layers. High-end supercomputers often utilize highly layered motherboards, but as clusters of standard computers can now replace such systems, ultra-multilayer boards are becoming less common. Each layer in a PCB is tightly integrated, making it difficult to discern the actual number of layers, though careful observation of a motherboard can reveal this information.
Flexible board, Rigid-flexible board
Flexible boards, rigid-flex boards are categorized into rigid circuit boards and flexible circuit boards. Generally, the PCB shown in the first image is referred to as rigid PCB, while the yellow connections in the second image are called flexible PCB. The intuitive difference is that flexible PCB can be bent. Common thicknesses for rigid PCB include 0.2mm, 0.4mm, 0.6mm, 0.8mm, 1.0mm, 1.2mm, 1.6mm, and 2.0mm. The common thickness for flexible PCB is 0.2mm, with thicker layers added on the back for soldering components, which can range from 0.2mm to 0.4mm. Understanding these details provides structural engineers with a spatial reference during design. Common materials for rigid PCB include phenolic paper laminates, epoxy paper laminates, polyester fiberglass laminates, and epoxy fiberglass laminates; common materials for flexible PCB include polyester film, polyimide film, and fluorinated ethylene propylene film.
