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3W principle of PCB design
Nov 17, 2018

In order to reduce the crosstalk between lines in PCB design, the spacing between lines should be large enough. When the spacing between the center of lines is no less than 3 times the width of lines, most 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 the crosstalk between signals by 70%, while satisfying 10W can reduce the crosstalk between signals by nearly 98%.

3W principle of PCB design

Although the 3W principle is easy to remember, it should be emphasized that the principle is established under prior conditions.Considering the physical meaning of the cause of crosstalk, it is necessary to prevent crosstalk effectively.For the four-layer plate, the distance between the line and the reference plane height (5~10mils), 3W is enough.However, with two layers, the distance between the line and the reference layer (45~55mils), 3W May not be enough for high-speed signal.The 3W principle is usually established under the condition of the 50-ohm characteristic impedance transmission line.

The 3W principle refers to that when multiple high-speed signal lines travel long distances, their spacing should follow the 3W principle. For example, the clock line, difference line, video, audio signal line, reset signal line and other key system circuits should follow the 3W principle, but not all the wiring on the board should be forced to conform to the 3W principle.