Calculating Differential Impedance for USB Differential Pair

[Apocx]

Hi all,

I'm trying to route some 90? differential pairs for some USB 2.0 High-speed data lines. To calculate the trace width/spacing, I used SaturnPCB. However, when using EEWeb's impedance calculator, I get much different values. I'm not sure which calculator to trust. Should I just go with SaturnPCB's values and pray it works? The difference seems pretty severe (10%) but I guess would fall within the 25% tolerance, so it might be OK either way--I don't know.



I thought about calculating it myself manually, but when I looked at the equations I was a bit lost...

The board stack (OSHPark) is as follows:

• 1 mil solder resist
• 1 oz copper (1.4 mil)
• 6.7 mil prepreg
• 0.5 oz copper (0.7 mil)
• 47 mil core
• 0.5 oz copper (0.7 mil)
• 6.7 mil prepreg
• 1 oz copper (1.4 mil)
• 1 mil solder resist

Any help is greatly appreciated!

[Neganur]

Ask the board manufacturer for the values. You can use all the calculators you want if you don't know the exact board make up they're using; they should be able to help you.

Edit: also, think ahead if you are going to use ground pours around the traces.

[Scrts]

I usually send the gerbers to the PCB manufacturer and tell which differential lines needs specific impedance. They modify the gerbers themselves and send me to approve.

[Apocx]

Ask the board manufacturer for the values. You can use all the calculators you want if you don't know the exact board make up they're using; they should be able to help you.

The board make up is listed at the bottom of my post, unless I'm misunderstanding you. Or are you saying the manufacturer would be able to supply values for track width/etc.? If so then I will ask them.

This page shows the stackup and states the dielectric constant is 3.66 at 1GHz, so I used that value in the impedance calculators. As far as I know all the input values I provided the calculators are accurate. They also include the full datasheet at the bottom.
http://support.oshpark.com/support/solutions/articles/122263-4-layer-board-stackup-and-specifications

Edit: also, think ahead if you are going to use ground pours around the traces.

Can you elaborate please? This is what my unfinished design looks like so far, using 11 mil traces with 8 mil gap. As I understood the guidelines, the diff. pairs (red) should not cross any plane splits, and should have a contiguous ground plane beneath it (yellow). Am I doing it wrong?




Thanks for your help!

[Apocx]

I usually send the gerbers to the PCB manufacturer and tell which differential lines needs specific impedance. They modify the gerbers themselves and send me to approve.

Oh wow, I didn't know that was something they'd do for you. I'm not sure OSHPark does that as it's basically a community batch of PCBs that is then sent to the actual manufacturer, but I can ask I suppose.

Thanks!

[Neganur]

Exactly that, I prefer asking them for the correct values instead of letting them edit the gerber.

They should also be able to give you the gap distance to ground planes in the same layer. But maybe your design does not have any ground plane in that area and so you don't need it.

Edit: added a pic, not sure how well the mobile app handles the size.

[tggzzz]

Edit: added a pic, not sure how well the mobile app handles the size.

What produced that picture?

[jahonen]

Looks like Polar Instruments Si8000 : http://www.polarinstruments.com/products/cits/Si8000.html

And regarding to the USB impedance requirements, there is also a common mode impedance specification of 30 ohms which should be considered in addition to the differential one.

Regards,
Janne

[Scrts]

Exactly that, I prefer asking them for the correct values instead of letting them edit the gerber.

Tried that.. It's still not perfect in the end. I let them do the edit. Never had any issues to be honest.