so im in the process of trying to make my first PCB with USB 2.0 on it.

Right now im trying to work out what the trace width and clerance for the diffrential pair should be to get the correct impedance.

According to this: http://www.usb.org/developers/docs/hs_usb_pdg_r1_0.pdf

they normally use 7.5 mil(0.19mm) traces with 7.5mil clerance on a 63 mil board (1.6mm)

That should give the traces around 90 Ohms diffrential impedance acording to the PDF, but when i try to input it to my impedance calculator (Saturn PCB toolkit V7.03) it gives me:enter image description here almost 160Ohm - and even if use 53mil "Conductor height" or anything i cannot get the 90 Ohm diffrential impendace - What are im doing wrong here?

Best regards Martin


Those 60mil are NOT the board thickness but the distance to the next contiguous plane (GND or POWER).

I don't know your layer stack but if this is a 2 layer board and you want to keep that, you will need 1mm thick tracks which is probably a little .. well ...

You better want to stick to a 4-layer stack.

  • \$\begingroup\$ Well, at a 2 layer PCB 1.6mm with 2x35um copper, it the distance between bottom and top layer is 60mil - but i assume that the PDF is talking about a 4 layer PCB with the traces way closer then? \$\endgroup\$ – Martin.E Jun 12 '17 at 7:18
  • \$\begingroup\$ Whenever you require a specific impedance, the distance between the signal lines and the next plane is what you're looking for. If you cannot achieve the track thickness / spacing with your stackup, it's time to look at a different stackup. Some PCB fabricators specify default stackups. If yours doesn't, you can always request your desired stackup and specify the required impedances. The manufacturer will then calculate the track thickness and spacing for you (because the material in use is also relevant) and tell you those values. \$\endgroup\$ – Tom L. Jun 12 '17 at 7:48

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