Routing Practices

Signal Routing
Microstrip or stripline routing are ways to route signals on a PCB.
Microstrip routing refers to a trace routed on an outside layer of the PCB
separated by a dielectric from the reference plane (GND or VCC). Stripline
routing refers to a trace routed on an inside layer with two reference
planes.


Clock Signal Routing

Considering routing techniques can help to maximize the quality of clock
transmission lines. Use the following routing techniques for clock signals:
■ Avoid using serpentine routing; clock traces should be as straight as
possible.
■ Avoid using multiple signal layers for clock signals.
■ Avoid using vias in the clock transmission line, since vias can
contribute impedance change and reflection.
■ Route the clock trace on the microstrip (preferably top layer) to
minimize the use of vias and delays, since air is the dielectric material.
Air has the lowest dielectric constant (Er = 1).
■ Place a ground plane next to the outer layer to minimize noise. If
using the inner layer for routing the clock trace, sandwich the layer
by ground planes to reduce delay.
■ Terminate clock signals properly.


Differential Signal Routing

For quality signal transmission, consider routing techniques of
differential signals in PCB designs. Use the following techniques for
differential signal routing:
■ Make D > 2S to minimize crosstalk. See Figure 5.
■ Route the two traces of a differential pair as close to each other as
possible after they leave the device to ensure minimal reflection.
■ Maintain a constant distance between the two traces of a differential
pair over their entire length.
■ Keep the electrical length between the two traces of a differential pair
the same. This minimizes the skew and phase difference.
■ To minimize impedance mismatch and inductance, avoid using vias.