Pcb3w design principle
In 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 electric fields can be kept from interfering with each other, which is the 3W rule.
The 3W principle means that when multiple high-speed signal lines are routed over a long distance, their spacing should follow the 3W principle. For example, clock lines, differential lines, video and audio signal lines, reset signal lines and other key circuits of the system need to follow the 3W principle, but not all wiring on the board must 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 prior conditions for the establishment of this principle. Considering the physical significance of the causes of crosstalk, to effectively prevent crosstalk, the spacing is related to the stack height and wire width. For the four-layer board, the height distance between the wiring and the reference plane (5~10mils), 3W is enough; However, the height distance between the two-layer board and the reference layer (45~55mils) and 3W may not be enough for high-speed signal wiring. The 3W principle is generally established under the condition of 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 processing for sensitive signals, such as clock signals and reset signals.
PCB design 20H principle
It refers to the distance that the power supply layer shrinks 20H relative to the formation, and of course it is also used to suppress the edge radiation effect. Electromagnetic interference will radiate outward at the edge of the board. The power layer is retracted so that the electric field is only conducted in the range of the ground layer. Effectively improve EMC. 70% of the electric field can be confined to the grounding edge if it is retracted for 20 hours; 98% of the electric field can be confined by retracting for 100H h
The adoption of the “20H rule” means to ensure that the edge of the power plane is at least 20 times the layer distance between two planes when compared with the edge of the 0V plane.
This rule is often required to be used as a technique to reduce the side shot emission from the 0V/ power plane structure (suppress the edge radiation effect). However, the 20-hour rule will only provide obvious effect under certain conditions.
These specific conditions include:
1. The rising/falling time of current fluctuation in the power bus should be less than 1ns.
2. The power supply plane should be on the internal level of PCB, and the upper and lower levels adjacent to it are all 0V planes. The distance between these two 0V planes should be at least 20 times the layer distance between them and the power plane.
3. The power bus structure will not resonate at any frequency of interest.
4. The total derivative of PCB is at least 8 layers or more.
Five-five principles of PCB design
The selection rule of PCB layers, that is, if the clock frequency reaches 5MHz or the pulse rise time is less than 5ns, the PCB must adopt multi-layer boards, which is a general rule. Sometimes, due to cost and other factors, when a double-layer board structure is adopted, in this case, it is best to make one side of the PCB as a complete ground plane layer.
