3W principle in PCB design

In order to reduce the crosstalk between the lines in the PCB design, the line spacing should be large enough. When the line center spacing is not less than 3 times the line width, most of the electric fields can be kept from interfering with each other. This is the 3W principle.

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

Although the 3W principle is easy to remember, it should be emphasized that this principle is established with previous conditions. From the physical meaning of the cause of crosstalk, it is necessary to effectively prevent crosstalk, which is related to the stack height and the wire width. For a four-layer board, the distance between the trace and the reference plane is (5~10mils), 3W is enough; but for the two-layer board, the distance between the trace and the reference layer is high (45~55mils), and 3W may not be enough for high-speed signal routing. The 3W principle is generally established under the condition of a 50 ohm characteristic impedance transmission line.

The 3W principle refers to the long-distance routing of multiple high-speed signal lines, the spacing should follow the 3W principle. For example, clock lines, differential lines, video and audio signal lines, reset signal lines, and other system critical circuits need to follow the 3W principle. Not all wiring on the board is mandatory to comply with the 3W principle.