PCB Tech - Three principles in PCB design

3W principle

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.

The 3W principle means that when multiple high-speed signal lines are routed over a long distance, the spacing should follow the 3W principle. For example, clock lines, differential lines, video and audio signal lines, reset signal lines and other system key circuits need to follow the 3W principle. Not all wiring on the board is mandatory to comply with the 3W principle.

Satisfying the 3W principle can reduce the crosstalk between signals by 70%, and meeting 10W can reduce the crosstalk between signals by nearly 98%

Although the 3W principle is easy to remember, it should be emphasized that there are previous conditions for the establishment of this principle. Considering the physical meaning of the causes of crosstalk, to effectively prevent crosstalk, the spacing is related to the height of the stack and the wire width. For a four-layer board, the height distance between the trace and the reference plane (5-10mils), 3W is enough; but for a two-layer board, the height distance between the trace 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 50 ohm characteristic impedance transmission line.

Generally, in the design process, if all the signal lines cannot meet 3W due to too dense wiring, we can only use 3W for sensitive signals, such as clock signals and reset signals.

20H principle

The distance between the power supply layer and the ground layer is reduced by 20H. Of course, it is also to suppress the edge radiation effect. Electromagnetic interference is radiated outward at the edge of the board. Shrink the power plane so that the electric field is only conducted within the ground plane. Effectively improve EMC. If you shrink by 20H, you can limit 70% of the electric field to the ground edge; if you shrink by 100H, you can limit 98% of the electric field.

The adoption of the "20H rule" means to ensure that the edge of the power plane is at least 20 times smaller than the edge of the 0V plane, which is equivalent to the layer distance between the two planes.

This rule is often required as a technology to reduce the emission from the side of the 0V/power plane structure (suppressing the edge radiation effect). However, the 20H rule will only provide significant results under certain specific conditions. These specific conditions include:

1. The rise/fall time of current fluctuations in the power bus should be less than 1ns.

2. The power plane should be on the inner layer of the PCB, and the upper and lower layers adjacent to it are both 0V planes. The distance between the two 0V planes extending outwards must be at least 20 times the distance between each of them and the power plane.

3. At any frequency of interest, the power bus structure will not resonate.

4. The total derivative of the PCB is at least 8 layers or more.

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Five Five Principles

The rules for selecting the number of layers of the printed board, that is, if the clock frequency reaches 5MHz or the pulse rise time is less than 5ns, the PCB Board must be a multilayer board. This is a general rule. Sometimes due to cost and other factors, a double-layer PCB board is used. In the structure, in this case, it is best to use one side of the printed board as a complete ground plane.