LVDS pairs need 100 ohms differential impedance. However, I am having trouble achieving that in my design.
My design requires that the pairs travel over very thin flexible PCB, about 50mm total transmission length. The substrate is 50um, with a 50um coverlay. Using Saturn's PCB Toolkit, it seems to be extremely difficult to achieve 100R impedance without making the tracks too thin to manufacture, or the S/H ratio going out of spec.
However, I can achieve 80R impedance without too much difficulty.
My question is: Can I just use 80R tracks, and terminate them with a 80R resistor?
As far as I understand it, as long a every part of my transmission line has the same impedance, I shouldn't get any reflections. The only down side will be a reduced voltage at the receiver, but since I should have very few other losses in the system, that should still be plenty of voltage swing for the receiver.
Best Answer
You might see if your flex vendor can provide 0.075 mm trace width. This capability is fairly common for rigid board vendors now, and my understanding is that flex has historically been able to support smaller features than rigid (but I don't design flexes regularly).
With 0.075 mm traces it's pretty easy to get 100 ohm differential:
That said, if your driver IC can handle an 80 or 85 ohm load, I don't see any reason not to use that instead. You'll get lower loss and probably more accurate characteristic impedance with the wider traces.