Multilayer PCB is a multilayer routing layer, with a dielectric layer between each two layers, which can be made very thin. Multilayer PCB has at least three conductive layers, two of which are on the outer surface and the remaining one is synthesized into the insulation plate. The electrical connection between them is usually through the plated through hole on the cross section of the circuit board. The PCB determines the process difficulty and processing price according to the number of wiring surfaces. Ordinary PCB can be divided into single-sided and double-sided wiring, commonly known as single-panel and double-panel wiring. However, due to the product space design factors, high-end electronic products can overlay multi-layer wiring in addition to surface wiring. During the production process, after each layer of wiring is made, it can be positioned by optical devices. Cohesion allows multiple layers of lines to be overlaid in one circuit board. Multilayer PCB can be referred to as any circuit board with a layer greater than or equal to 2.

Multilayer PCB can be used for high frequency applications, are not susceptible to environmental changes and have stable dielectric properties. Multilayer printed circuit boards suitable for high frequency ranges include at least two printed circuit boards with interlayer sticky structures in their middle layers. At least one of these two printed circuit boards includes: an insulating film, An adhesive layer containing thermoplastic polyimide is arranged on at least one surface of the insulating film. And a metal line layer laid on the adhesive layer. The interlayer adhesive component contains thermoplastic polyimide.

The increased packaging density of integrated circuits results in a high concentration of interconnects, which makes the use of multiple substrates necessary. In the layout of the printed circuit, unforeseen design problems such as noise, stray capacitance, crosstalk, etc. have occurred. Therefore, the design of printed circuit boards must focus on minimizing the length of signal lines and avoiding parallel routes. Obviously, in a single panel, or even in a double panel, these requirements cannot be answered satisfactorily due to the limited number of cross-overs that can be achieved. In order to achieve a satisfactory performance in a large number of interconnection and crossover requirements, the printed circuit board has to expand the board to more than two layers, resulting in the emergence of Multilayer PCB, so the original intention of making Multilayer PCB is to provide more freedom in choosing an appropriate routing path for complex and noise-sensitive electronic circuits.