1. The odd number layer PCB needs to add non -standard layer of core layer bonding process on the basis of the core structure process. Compared with nuclear structures, the production efficiency of the PCB circuit board processing plant with foils outside the nuclear structure will be reduced. Before layer pressing, the outer core needs additional process processing, which will increase the risk of scratching and etching errors.

　　2. A balanced structure Avoid bending. The best reason for the design without a strange layer of PCB is that the strange layer of the circuit board is easy to bend. After the multi -layer circuit key combination process is completed, when the PCB is cooled, different layer of pressure tension will cause PCB to bend when the core structure and foil structure cool. As the thickness of the circuit board increases, the risk of curvage of two different structures of composite PCB is greater. The key to eliminating circuit boards is to use balanced layers. Although the curved PCB meets the specifications to a certain extent, subsequent processing efficiency will be reduced, resulting in increased costs. Because special equipment and processes are required during the assembly process, the placement accuracy of parts will be reduced, which will affect the quality.