Initially, three manufacturing methods for multilayer boards were disclosed: the Clearance Hole Method, the Build Up Method, and the Plating Through Method (PTH). The Clearance Hole method is not practical because it is time-consuming to manufacture and is limited in densification. Layer increase method is quite complicated because of the manufacturing method, plus the advantages of high density, but because the demand for high density is not as urgent as now, has been unknown; recently, due to the increasing demand for high-density circuit boards, once again become the focus of research and development of various manufacturers. As for the PTH method, which is the same process as double-sided boards, it is still the mainstream manufacturing method for multilayer boards.