The starting point for most PCB stackup design is determining what impedance or
impedances to use in each signal layer. A number of impedances have been used for
controlled impedance PCBs. Among these are 62 or 65 ohms for PCI buses, 72 or 75
ohms for video signals, 50 ohms for ECL and high speed CMOS, 28 ohms for Rambus
and an assortment of differential impedances for various differential signaling protocols.
The most common impedance found in multilayer PCBs is 50 ohms.