PCB routing is a critical step in the PCB design process. Improper routing will impair circuit performance, and even lead to circuit instability or complete failure. Therefore, correct PCB routing is of paramount importance.
PCB routing is a critical step in the PCB design process. Improper routing will impair circuit performance, and even lead to circuit instability or complete failure. Therefore, correct PCB routing is of paramount importance.
First, PCB routing shall follow the principles listed below:
- Shortest Trace Length: Traces should be as short as possible to reduce signal transmission time and the impact of noise. Shortening the trace length also helps minimize signal attenuation, and reduces the cost and size of the PCB.
- Impedance Matching: For high-speed clock and data signals, ensuring signal quality is critical. Therefore, the impedance of the traces shall be matched with the impedance of the circuits on the board to guarantee signal integrity.
- Signal Layering: To avoid signal interference, the layers of the PCB shall be arranged with interleaved VCC, GND and signal layers. The power layer and ground plane shall be placed as close as possible to each other for shielding.
- Avoidance of Signal Loops: To prevent signal return and mutual interference between differential signals, signal traces shall be arranged with other signal lines, ground or VCC between the two lines of the differential pair.
- High-Frequency Signal Isolation: High-frequency signals cause severe interference to the circuit. Therefore, for high-frequency signal traces, adequate isolation measures must be implemented to ensure signal quality.
With these principles in place, we can discuss the specific routing strategies:
- Low-Level Signals: For low-level signals, noise interference can be avoided by shortening the trace path. Priority can be given to arranging low-level signal lines in a more compact layout area, such as the central area of the board.
During routing, attention shall also be paid to reducing trace bends and ensuring an appropriate line width. An excessively narrow line width will cause signal attenuation, while an excessively wide line width will lead to signal reflection. Meanwhile, sufficient space shall be reserved during layout to leave room for subsequent revisions.
- Complex Medium-Speed Signals: For complex medium-speed signals (1 MHz to 100 MHz), a layered layout can be adopted. The existing production process can be subdivided into four layers: two layers for ground and power supply, and the other two as signal layers, which will not introduce RF noise interference at the same time.
During routing, horizontal, straight and point-to-point traces shall be prioritized, and routing solutions that do not require adapters and extension cables shall be given preference. If necessary, curved traces can also be used, but the bends shall be smooth and not excessive.
- High-Speed Signals (>100 MHz): During layout, high-speed signals shall be isolated, arranged as differential pairs, and a layered layout shall be applied. For high impedance matching requirements, PCB transmission lines using only copper etching technology require germanium or P-type semiconductors for chip-level matching.
During routing, traces with the minimum available line width shall be preferentially selected, and differential routing shall be the first choice for straight traces without intermediate branches from pin outputs. For cross-line connections, signal traces shall not cross each other; instead, signal transmission shall be realized through vias, pin rearrangement, loop routing or lead-out methods.
In summary, PCB routing is a key link in PCB design, which requires compliance with specific principles and strategies. PCB routing can be optimized and improved through online analysis, combined with simulation and actual measurement. The correctness of PCB routing will directly affect the performance of the circuit. Therefore, the time invested in this link will definitely provide the strongest guarantee for the engineering project with a much shorter R&D cycle.
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