3W rule: In order to reduce the crosstalk between lines, the line spacing should be large enough. When the line center spacing is not less than 3 times the line width, 70% of the electric field can be kept without interfering with each other, which is called the 3W rule. For 98% of the electric fields not to interfere with each other, a 10W spacing can be used.
Five rules: the selection rule of the number of layers of the printed board, that is, when the clock frequency reaches 5MHz or the pulse rise time is less than 5ns, the PCB board must use a multi-layer board. This is a general rule. In the case of a double-layer board structure, in this case, it is best to use one side of the printed board as a complete ground plane layer.