High-speed signals trace impedance needs to designed to minimize the reflections in traces. The high speed protocol that is being designed for determines what the single and differential trace Impedance the traces need to meet as well as the tolerance for the impedance (50 Ω ±15%). To have designs be robust from PCB manufacturing errors and defects design the traces impedance be as close to the recommended value. The geometry of the traces, the permittivity of the PCB material and the layers surrounding the trace all impact the impedance of the signal trace.

In general, closely coupled differential signal traces are not an advantage on PCBs. When differential signals are closely coupled, tight spacing and width control is necessary. Very small width and spacing variations affect impedance dramatically, so tight impedance control can be more problematic to maintain in production. For PCBs with very tight space limitations (which are usually small) this can work, but for most PCBs, the loosely coupled option is probably best.

Loosely coupled PCB differential signals make impedance control much easier. Wider traces and spacing make obstacle avoidance easier (because each trace is not so fixed in position relative to the other), and trace width variations do not affect impedance as much, therefore, it is easier to maintain an accurate impedance over the length of the signal. For longer routes, the wider traces also show reduced skin effect and often result in better signal integrity with a larger eye diagram opening.