Blindly PCB Layout will be in vain!
#1 Requirements for limited order of wiring
1. Priority for key signal lines
Priority is given to key signals such as power supply, analog small signals, high-speed signals, clock signals and synchronization signals.
2. Principle of priority for wiring density
Start wiring from the device with the most complex connection relationship on the single board. Start wiring from the area with the densest connection on the single board.
3. Precautions for key signal processing
Try to provide dedicated wiring layers for key signals such as clock signals, high-frequency signals, and sensitive signals, and ensure their minimum loop area. If necessary, shielding and increased safety spacing should be adopted to ensure signal quality.
4. Impedance control
Networks with impedance control requirements should be arranged on the impedance control layer, and their signals must be avoided from being split across.
#2. Wiring crosstalk control
1. Explanation of the 3W principle
The distance between lines is kept 3 times the line width. In order to reduce crosstalk between lines, the line spacing should be large enough. If the line center distance is not less than 3 times the line width, 70% of the line electric fields can be kept from interfering with each other, which is called the 3W rule.
2. Crosstalk control
Crosstalk refers to the mutual interference between different networks on the PCB caused by long parallel wiring, mainly due to the distributed capacitance and distributed inductance between parallel lines.
The main measures to overcome crosstalk are:
Increase the spacing between parallel wiring and follow the 3W rule;
Insert grounded isolation wires between parallel lines;
Reduce the distance between the wiring layer and the ground plane.
#3. General rules for wiring
1. Orthogonal structure of adjacent plane routing directions
Avoid running different signal lines in the same direction on adjacent layers to reduce unnecessary inter-layer crosstalk; when this situation is difficult to avoid due to board structure limitations (such as some backplanes), especially when the signal rate is high, consider isolating each wiring layer with a ground plane and isolating each signal line with a ground signal line.
2. small discrete device routing must be symmetrical
SMT pad leads with relatively close spacing should be connected from the outside of the pad, and direct connection in the middle of the pad is not allowed.
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3. Minimum loop rule
The loop area formed by the signal line and its loop should be as small as possible. The smaller the loop area, the less external radiation and the less interference from the outside world.
4. No STUB
No STUB is allowed in the routing.
5. The same network wiring width should be consistent
The wiring width of the same network should be consistent. The change of line width will cause uneven characteristic impedance of the line, and reflection will occur when the transmission speed is high.
Under certain conditions, such as the similar structure of the connector lead wire and the lead wire of the BGA package, the change of line width may not be avoided due to the small spacing, and the effective length of the inconsistent part in the middle should be minimized.
6. Prevent signal lines from forming self-loops between different layers
This kind of problem is easy to occur in multi-layer board design, and self-loops will cause radiation interference.
7. Avoid sharp angles and right angles
Sharp angles and right angles should be avoided in PCB design, which will generate unnecessary radiation and poor PCB production process performance.
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