Looking for some advice. Is it possible, or even recommended to route ethernet signals or USB signals through a FFC or FPC flat cable? The ethernet signals would be pre-transformer, right out of the SitCore. I have a compact design requirement that could be solved by stacking two boards with the USB and Ethernet on a separate board.
The thing that’s important with routing ethernet and USB signals is that their impedenace needs to be correct. USB requires 90 ohms of differential impedance and ethernet 100 ohms of differential impedance.
I am not sure if you can get controlled impedance with a FFC or FPC flat cable, this could also depend on your boardhouse. The other problem would be your connector, I don’t think you’ll find a FPC connector with the right spec (some pins 90 ohms, some pins 100 ohms).
On the other hand, I have routed plenty of boards without keeping the impedance in mind and I never had any problems with it whatsoever. I think your best option would be to try it out and see for yourself.
I expect you won’t have much problems using FFC. Use a ground trace at both ends of the signal pair.
Like “GND signal signal GND”
Molex does have impedance controlled connectors and shielded FFC cables,