Various key factors have to be kept in mind while designing a PCB for a quality design. An important principle of PCB design, which becomes more critical in mixed signal design in order to avoid noisy signals and abnormal functioning is keeping analogue and digital grounds separated. 

Since in auto-routing the computer recognizes it as a single ground and routes accordingly, grounds are to be routed manually to avoid it. In addition to this:

1. For each digital IC, capacitors should be placed between its input power and ground
2. High frequency tracks should be kept short
3. Signals with high frequency should have a ground plane below them; they shouldn’t be run over gaps
4. Analogue circuits and digitals circuits should be kept separated
5. Analogue ground and digital grounds should be kept separated but connected to a common point
6. 90o bends in tracks should be avoided, instead 45o bends can be used to avoid noise and signal reflections.
7. Power and ground tracks should be wide so that they have a higher current limit, also they should be placed first
8. Auto-routing should be avoided when possible
9. There shouldn’t be any copper fills unconnected on a PCB. They can be grounded to act as a heat sink or removed.
10. Vias shouldn’t be placed under the components
11. Through hole components ca be used as vias since the hole is connected to layers underneath
12. Drill size for holes and vias should be kept minimum