2-layer board is the most basic laminated structure of PCB, and it is also the introductory layout of most electronic engineers. 2-layer board PCB, the distance between the two layers of circuits is related to the thickness of the board. The thicker the board, the greater the distance. But for four-layer board, some people think that the spacing of its four layers is equally distributed. In fact, it is not, but it is often distributed as shown in Figure 2. Do you know why it is distributed like this?