Translate the interface specification into constraints
Identify the exact interface, transmitter, receiver and required operating mode. Define differential impedance, permissible skew, channel length or loss limits and relevant topology. Use the device documentation to decide which constraints apply to each group of signals.
Two equal CAD lengths do not guarantee equal propagation delay. Different layers, transitions or dielectric environments can produce different behavior. Pair routing must preserve the intended electrical symmetry across the full channel, including packages and connectors where relevant.
Preserve a deliberate reference path
Differential signaling does not make reference conductors irrelevant. Common-mode behavior and asymmetries still depend on the surrounding structure. Review the pair together with nearby planes, voids and changes of reference.
On a two-layer USB board, determine whether the actual dielectric and routing arrangement can meet the required geometry and return-path needs. Do not assume that using two layers is automatically acceptable or impossible. Check the USB mode and device-specific requirements, then validate the implemented channel.
Review breakouts, transitions and tuning
Keep the geometry controlled where practical and inspect unavoidable departures at pads or connectors. Evaluate via arrangement, stubs and reference connections together. A fixed rule about bend angle or separation cannot replace the analysis of the actual structure.
Add tuning only to satisfy a defined timing or skew requirement. Closely packed serpentine segments can interact, so physical added length may not provide the simple delay expected. Review tuning locations and extracted behavior rather than maximizing visual symmetry alone.
Check manufacturing and measurement assumptions
Use the fabricator-approved stackup to set width and spacing. State which adjustments require approval and retain the final values. If material weave or other variation matters to the timing budget, assess it for the chosen construction rather than relying on a generic allowance.
Verify the routed pair against the constraints and the intended test conditions. Include representative connector and fixture effects in measurements. Keep the resulting acceptance evidence linked to the board and firmware revision so later changes can be reviewed consistently.