3W Principles of PCB Design

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3W Principles of PCB Design [Copy link]

In order to reduce crosstalk between lines in 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 rule. As shown in the figure below.

Meeting the 3W principle can reduce the crosstalk between signals by 70%, while meeting 10W can reduce the crosstalk between signals by nearly 98%.

Although the 3W principle is easy to remember, it should be emphasized that this principle is established under certain conditions. From the physical meaning of the cause of crosstalk, to effectively prevent crosstalk, the spacing is related to the stacking height and the wire width. For a four-layer board, the height distance between the routing and the reference plane (5~10mils), 3W is enough; but for a two-layer board, the height distance between the routing and the reference layer (45~55mils), 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 means that when multiple high-speed signal lines are routed over long distances, their spacing should follow the 3W principle. For example, clock lines, differential lines, video and audio signal lines, reset signal lines and other key system circuits need to follow the 3W principle, but not all wiring on the board must be forced to comply with the 3W principle.