In order to reduce line crosstalk in the PCB design, it is necessary to ensure that the line spacing is large enough. When the line center spacing is not less than 3 times the line width, the majority of the electric field can not be disturbed. This is the principle of 3W.

As shown in the following figure: Satisfying the 3W principle can reduce the crosstalk between signals by 70%, while satisfying 10W can reduce the crosstalk between signals by nearly 98%.

3W Although the principle of easy to remember, but to emphasize that this principle is established to have the previous conditions. From the physical meaning of crosstalk causes, to effectively prevent crosstalk, the spacing and stack height, wire width related. For a four-layer board, the traces are highly spaced from the reference plane (5 to 10 mils), and the 3W is sufficient; however, the two-layer board, the alignment distance from the reference layer (45 to 55 mils), and the 3W for high-speed signal traces may not be sufficient. The 3W principle is generally established at 50 ohm characteristic impedance transmission lines.

3W principle refers to a number of high-speed signal line long distance when the spacing should follow the 3W principle, such as clock line, differential line, video, audio signal line, reset signal line and other system key circuit need to follow the 3W principle, and Not all the wiring on the board must be mandatory to meet the 3W principle.