In high-speed PCB design, signal integrity, electromagnetic compatibility (EMI), and routing efficiency are crucial. To ensure high-quality designs, engineers need to follow a series of routing rules, but sometimes the reasons behind these rules are unclear! Below are some common rules explained, which we hope will be helpful.
1. High-Speed Signal Routing Shielding Rule
Reason: Unshielded or incompletely shielded critical high-speed signal lines, such as clock signals, can lead to EMI leakage.
Specific Measures: It is recommended to use vias to ground every 1000mil of shielding to reduce EMI interference.
2. High-Speed Signal Routing Closed-Loop Rule
Reason: Closed loops created in multi-layer PCB routing can form loop antennas, increasing EMI radiation intensity.
Specific Measures: Avoid creating closed loops in multi-layer PCBs for high-speed signal networks, such as clock signals.
3. High-Speed Signal Routing Open-Loop Rule
Reason: Open loops created in multi-layer PCB routing can form linear antennas, also increasing EMI radiation intensity.
Specific Measures: Avoid creating open loops in multi-layer PCBs for high-speed signal networks.
4. High-Speed Signal Characteristic Impedance Continuity Rule
Reason: Discontinuities in characteristic impedance during inter-layer switching increase EMI radiation.
Specific Measures: Ensure continuous width of traces within the same layer and continuous impedance of traces across different layers.
5. Routing Direction Rules in High-Speed PCB Design
Reason: Non-perpendicular traces between adjacent layers cause crosstalk, increasing EMI radiation.
Specific Measures: Adjacent routing layers should follow a horizontal-to-vertical routing direction to suppress crosstalk.
6. Topology Rules in High-Speed PCB Design
Reason: A suitable topology directly affects the characteristic impedance control and performance under multiple loads.
Specific Measures: In high-speed PCB design, it is recommended to use a rear-end star-shaped symmetrical structure instead of the daisy-chain topology commonly used in low-frequency applications.
7. Trace Length Resonance Rule
Reason: When the trace length and signal frequency resonate, electromagnetic waves are radiated, generating interference.
Specific Measures: Check signal trace lengths and avoid making them integer multiples of 1/4 of the signal wavelength to prevent resonance.
8. Return Path Rules
Reason: High-speed signals without a good return path will experience significantly increased radiation.
Specific Measures: Ensure the return path for high-speed signals such as clock signals is minimized. Radiation is directly proportional to the area enclosed by the signal path and the return path.
9. Decoupling Capacitor Placement Rules
Reason: Improper placement of decoupling capacitors will fail to achieve the desired decoupling effect.
Specific Measures: Decoupling capacitors should be placed close to power supply pins, and the area enclosed by the power and ground traces of the capacitors should be minimized.