PCB Design Principles
During the process of Protel hardware development, 3W and 20H PCB design principles are very important.
3W refers to the distance between lines should keep 3 times the line width. To reduce crosstalk between lines, it is necessary to ensure that the line spacing is large enough. If the line center distance is more than 3 times the line width, 70% of the line electric fields can be kept from interfering with each other, which is called 3W principle. To make 98% of the electric field does not interfere with each other, 10W principle can be used.
20H principle means that the power layer should be indented 20H inward compare to the ground layer, which also can suppress the edge radiation effect. The effect refers to electromagnetic interference radiates outward from the edge of the board. Therefore, indent the power layer inwards make the electric field conducted in the ground plane only, effectively improve the EMC. Indent 20H inwards can limit 70% of the electric field to the ground edge; while, 100H inwards can limit 98% of the electric field to the ground edge.
The use of 20H principle means ensuing the edge of the power plane is at least 20 times indented than the 0V plane edge, which is equivalent to 20 times of the layer spacing between the two planes.
This principle is often required to suppress edge radiation effects. However, it is effective only under certain conditions, including:
1.The current fluctuation in the power bus rises/falls less than 1 ns.
2.The power plane is at the internal level of the PCB, and the two levels adjacent to it are 0V planes. The distance of these two 0V planes extends outward should be at least 20 times the distance between their respective planes and the power plane.
3.The power bus structure does not resonate at any frequency you concerned.
4.The PCB has a total derivative of at least 8 layers or more.