3W principle in PCB Design
In the PCB design, in order to reduce the crosstalk between lines, 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, such as clock lines, differential lines, video and audio signal lines, reset signal lines and other system key circuits. Not all wiring on the board is mandatory to comply with the 3W principle.
Meeting the 3W principle can reduce the crosstalk between signals by 70%, while meeting the 10W principle can reduce the crosstalk between signals by nearly 98%
Although the 3W principle is easy to remember, it should be emphasized that there are preconditions for the establishment of this principle. Considering the physical meaning of the cause of crosstalk, to effectively prevent crosstalk, the spacing is related to the stack height and 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 traces. The 3W principle is generally established under the condition of a 50 ohm characteristic impedance transmission line.
Generally, in the design process, if all 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 in PCB Design
It refers to the distance that the power layer is retracted by 20H relative to the ground layer, of course, it is also to suppress the edge radiation effect. Electromagnetic interference is radiated outward at the edge of the board. Retract the power plane so that the electric field is only conducted within the confines of the ground plane. Effectively improve EMC. 70% of the electric field can be confined within the ground edge if it is retracted by 20H; 98% of the electric field can be confined by the retraction of 100H.
The adoption of the “20H rule” refers to ensuring that the edge of the power plane is at least 20 times smaller than the edge of the 0V plane equivalent to the layer distance between the two planes.
This rule is often required as a technique for reducing side-fire emissions from 0V/power plane structures (suppressing edge radiation effects). However, the 20H rule will only provide a noticeable effect under certain specific conditions. These specific conditions include:
- The rise/fall time of current fluctuation in the power bus should be less than 1ns.
- 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 should be at least 20 times the distance between them and the power plane.
- The power bus structure does not resonate at any frequency of interest.
- The total derivative of the PCB is at least 8 layers or more.
Five Five Principles
The selection rule for 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 use a multi-layer board. This is a general rule. Sometimes, due to cost and other factors, a double-layer board is used. In this case, it is best to use one side of the printed board as a complete ground plane layer.