I am designing an 4-port USB 3.0 hub and using fpc 0.5mm connector to connect it to other usb devices. Will using fpc 0.5mm cable have interference because of high data rate? I am talking about the interference inside the cable itself from D+,D- to SSRX or SSTX wire. I have been reading and only found out that it cause interference to wireless devices but nothing about interference inside the wire.
Wow, this is an ambitious approach I think! I am already scared to do such an appraoch on USB 2.0 high speed. I think using a 2-layer flex cable (bottom side as a ground layer) instead of using a standard FFC cable will allow you to match the 90 ohm impedance and to get much better results. But I have to admit that I'm not a high-speed layout specialist as well.
Flexible cables are used everywhere in modern laptops and chromebooks. As the screen size gets bigger and mainboard size gets smaller due to increasing level of integration, the USB ports within laptop body appears to be well apart from processor, so flex cable is cheaper than PCB. The connectors, however, are pretty wide, 30-40 pins. The signal distribution is usually in groups GND-S-S-GND, GND are not shared with adjacent link. So it is G-S-S-G-G-S-S-G....
The cable MUST maintain 80-90 Ohm differential impedance, otherwise the signal will be killed by reflections. In many cases the cable has a solid ground reference plane. The 90-Ohm is a must. To find such a cable will be a challenge.