Before designing a multilayer PCB, the designer needs to first determine the structure of the circuit board according to the size of the circuit board, the size of the circuit board and electromagnetic compatibility (EMC) requirements, that is, to decide whether to use four, six or more layers of the circuit board. After the number of layers is determined, the placement of the inner electric layers and how to distribute different signals on these layers are determined. This is the choice of multilayer PCB. Cascade structure is an important factor affecting EMC performance of PCB board, and also an important means of restraining electromagnetic interference.