PCI Express Width and Spacing

Coupling of the intra-pair differential signals and increased spacing to neighboring signals help to minimize harmful crosstalk impacts and Electro-Magnetic Interference (EMI) effects. In the microstrip case, a differential trace should be 5 mils wide, with a 7 mil wide air gap spacing between the two traces of a pair. Any uncoupled sections whose intra-pair space exceeds 7 mils can be routed as a 7 mil wide trace, if the section is 100 mils or longer. In the stripline case, a differential trace should be 5 mils wide with a 5 mil wide gap between the two traces of a pair.

The spacing between pairs and to all non-PCI Express signals should be 20 mils or four times the dielectric height, whichever is greater. If the non-PCI Express signals have significantly higher voltage levels or edge rate than the PCI Express signal, the space should increase to 30 mils in order to avoid coupling.