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What are the principles in PCB design?


3W principle: When multiple high-speed signal lines are long-distance, the spacing should follow the 3W principle. For example, clock lines, differential lines, video, audio signal lines, reset signal lines and other system critical circuits need to follow the

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3W principle. Not all wiring on the board is mandatory to comply with the 3W principle.
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.

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20H principle: refers to the distance between the power layer and the ground layer within 20H, and of course to suppress the edge radiation effect. Electromagnetic interference is radiated outward at the edge of the board. The power plane is shrunk such that the electric field conducts only within the range of the ground plane. Effectively improved EMC. If the shrinkage is 20H, 70% of the electric field can be confined to the grounding edge; if it is retracted by 100H, the 98% electric field can be limited.
The adoption of the 20H rule means that the edge of the power plane is at least 20 times smaller than the edge of the 0V plane to be equivalent to the layer spacing between the two planes.

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Five-five principle: the selection rule of printed board layer number, that is, the clock frequency to 5MHz or the pulse rise time is less than 5ns, the PCB board must use multi-layer board, which is a general rule, sometimes due to cost and other factors. In the case of a two-layer board structure, in this case, it is preferable to use one side of the printed board as a complete ground plane layer.

This rule is often required as a side-shooting emission technique (suppression of edge radiation effects) from the 0V/power plane structure.