Electronic – USB differential pair length

Assuming you're only using USB-low-speed or full-speed, you should be fine.

Generally, layout considerations only really have to be taken if you're going long distances (many inches), or using USB-2.0. Even then, USB is surprisingly tolerant.

• USB 1.1 or USB2.0 low/full speed

  • You really don't need to worry. There are (possibly apocraphyal) stories of people running USB2.0 low-speed of 50' of CAT-5 wire. As long as you keep your wire-runs a few inches or less, I wouldn't worry.
  • The fastest edge you will need to worry about in low/full speed applications is 12 Mhz. As such, you're not really approaching the point where making sure your traces/wiring is properly transmission-lined/impedance-controlled is that important, at least as long as your overall uncontrolled-impedance sections are less then, say ~6".
  • As I said, most USB controllers are impressively tolerant of USB devices that are widely out-of-spec. If this is something for production, I would spend the effort to do it properly (there is one guy out there who has a motherboard that throws a hissy-fit if anything connected to it deviates from the spec by a tiny amount), but if it's just a test-board, I'd say just lay it out neatly, and don't worry about it.

• USB2.0 High-speed.

  • Here layout becomes more important. USB2.0 High-Speed has a maximum edge rate of 480 Mhz. As such, even short traces start to approach the wavelength of the data, and as such proper impedance-control becomes important.
  • Assuming you EDA package has proper impedance-controlled routing options, just set your differential-pair impedance to be ~90Ω, and you should be fine. Be careful to make sure you have a contiguous ground-plane, though

• USB3.0

  • So you hate yourself?

Summarizing comment trail as an answer:

The requirement is for a PCB layout for a pass-through between USB2.0 A and B connectors on a PCB. The rest of the circuit on the PCB does not interact with the USB signal path.

Suggested solution:

By changing the physical arrangement of the two sockets to be close together rather than at opposite sides of the board as originally envisaged, the signal trace length and transmission effect concerns are alleviated.

Further, by setting the two connectors at right angles to each other, at one corner of the board area, the need to leave space between them to allow cables to be plugged in, is addressed: The cables would be connected along different edges of the board and would not touch each other.

This allows simplification of routing as well:

• The recommendation for equal length signal paths is inherently addressed
• The arrangement does not interfere with rest of PCB layout, as it is off in a corner
• With the indicated small trace length, transmission line and antenna effects are negligible for USB 2.0 High-Speed transmissions

(as posted by OP).

Concerns that may need addressing:

• Physical robustness of PCB to cope with stresses of repeated cable insertions - A mounting bolt at the corner between the connectors should address this.
• Effective total length of USB cable, adding up the A-side and B-side cables, may exceed USB maximum cable length. The very short PCB section would act merely as an extension of the cable.
• Creative solutions needed for suitably boxing the board with connectors at the corner.