A multi-layer printed circuit board is a printed circuit board that distributes electrical connections on multiple levels. Its wiring is more complex than that of a single-sided or double-sided PCB and can significantly reduce the size of the circuit board. This type of circuit board is usually composed of 3 to 16 layers, or even more copper foil layers, and contains holes for completing the connection. In order to avoid blind hole misalignment, the inner wall of the hole is often plated with conductive metal, which is also more challenging in terms of process. 

Multi-layer printed circuit boards are widely used, especially in electrical products. It is achieved by using more single-sided or double-sided wiring boards, for example, through a double-sided inner layer and two single-sided outer layers of printed circuit boards, or two double-sided inner layers and two single-sided outer layers of printed circuit boards, which are alternately connected through a positioning system and insulating adhesive materials, and the conductive patterns are interconnected according to the design requirements, thereby forming a four-layer or six-layer printed circuit board, also known as a multi-layer printed circuit board. With the continuous development of SMT (surface mount technology) and the continuous introduction of a new generation of SMD (surface mount devices), such as QFP, QFN, CSP, BGA (especially MBGA), electronic products have become more intelligent and miniaturized, which has promoted major reforms and progress in PCB (printed circuit board) industrial technology, making the design of multi-layer printed circuit boards more flexible, stable and reliable, with superior economic performance, and now widely used in the production and manufacturing of electronic products.

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