I am routing a PCB that uses a USB connection. The differential pair traces are 10 mil distant from each other, and they are about 1mm different in length. Is it going to be a problem? What is the recommended maximum difference in length and the minimum distance between them?
Answer
While the length and impedance are both important, 1mm of length differential will not affect your system's performance in any way, even for usb-2.0 high-speed.
From the USB spec:
7.1.3 Cable Skew
The maximum skew introduced by the cable between the differential signaling pair (i.e., D+ and D- (TSKEW)) must be less than 100 ps and is measured as described in Section 6.7.
Assuming a perfect propagation velocity (i.e. C, the speed of light), a differential length of ~2.99 cm would produce a skew of 100 ps. As such, your 1 mm of trace length differential will not be a problem.
Added: On a real PCB, your signals travel slower than speed of light. For a stripline (inner layer) you divide the speed of light in vacuum by the square root of the relative dielectric constant (e_r). So about half speed. This means the 100ps is more like 15mm. For the outer layers, the speed is slightly higher (about 10%).
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