差分对布线指南 — 长度匹配与阻抗

USB 2.0: 90Ω ±10% differential. USB 3.x / USB-C SuperSpeed: 90Ω ±10% differential. Ethernet (100BASE-TX, 1000BASE-T, 10GBASE-T): 100Ω ±10% differential. PCIe Gen1–Gen5: 85Ω ±10% (note: not 100Ω). HDMI/DisplayPort: 100Ω ±10%. MIPI CSI/DSI: 100Ω. LVDS: 100Ω. SATA: 100Ω. The PCIe 85Ω is the common surprise — it's lower than other interfaces because of historical chipset driver design. Always check the specific interface spec, not generic 'high-speed = 100Ω' assumptions.

Intra-pair skew is the length difference between the + and − traces of one differential pair. Recommended limits: USB 2.0 — 150 mil. USB 3 — 5 mil. PCIe Gen3 — 5 mil. PCIe Gen4/5 — 2 mil. DDR4 strobe pairs — 2 mil. The cause: any length mismatch converts differential signal into common-mode signal, which radiates EMI and reduces eye height. Measure intra-pair skew at the same reference plane (vias don't count toward length until both ends use them). KiCad's length-tuning meander tool, Altium's xSignals, and EasyEDA's pair-routing assistant all report this number directly.

Inter-pair skew is the length difference between separate pairs in the same group (e.g. four PCIe lanes, eight DDR data byte lanes). Limits are looser than intra-pair: PCIe lanes — 200 mil, DDR4 byte-to-byte — 100 mil, DDR4 within byte — 25 mil. The reason: inter-pair skew is corrected by the receiver's per-lane equalizer/training (PCIe LTSSM, DDR write leveling), so longer mismatch is tolerated. Intra-pair skew has no equivalent correction — the signal is already corrupted by the time it arrives. Match intra tightly, inter loosely.

Strongly preferred but not strict. Both traces of a pair on the same layer share the same reference plane and the same dielectric, which keeps Zdiff stable and intra-pair skew predictable. When pairs must change layers (BGA escape, connector breakouts), match the via count for both traces (one via on + and one via on − at the same point) and place ground stitching vias adjacent to maintain the return path. Avoid splitting one trace through inner layers and one through outer layers — the propagation velocity differs by 5–10% and you'll never match the skew.

Tightly coupled pairs run with edge-to-edge spacing equal to or less than the trace width (S ≤ W). This makes the pair's coupling field dominate over external noise. Loosely coupled pairs (S > 2W) rely more on the ground plane reference and tolerate small spacing variations better, but pick up more external noise. Most modern interfaces (USB 3, PCIe, HDMI) use tightly coupled pairs because the routing density and EMI requirements favor it. The fab's stackup spec defines the trace width to hit your target Zdiff at your chosen S/W ratio — use what they recommend, don't deviate.