Differential Pairs

PCIe and USB require 90Ω impedance matching for P/N differential pairs. Important reference: Digi-Key Tutorial (opens in a new tab)

This is pretty important to keep in mind:

H_t (distance between traces and top ground plane): 10000 mm (we want this to be large to approximate infinity, as we are using an external layer for our differential pair--set this to the FR4 thickness if you are routing your USB lines on an internal layer sandwiched between two ground planes)