At the beginning, multi-layer boards were made public using three manufacturing methods: Clearance Hole, Build Up, and PTH. Due to the labor-intensive nature of the gap hole method in manufacturing and the limitations of high density, it has not been practical. Due to the complexity of the manufacturing method and the advantages of high density, the increasing layer method has remained unknown due to its less urgent demand for high density; Due to the increasing demand for high-density circuit boards, Erjin has once again become a focus of research and development for various manufacturers. As for the PTH method, which is the same process as double-sided boards, it is still the mainstream manufacturing method for multi-layer boards