I have an analog circuit with few opamps (max freq. 500Hz) and a DC-DC converter with 500 kHz frequency for power supply. It seems I need to use both sides of PCB for components and routing if I use 2 layers. Or, I can go for 4 layers with dedicated power and ground plane. I see in most cases dedicated planes are used, or at least one side is used for ground. Do I need a good ground plane in my case? Or would it be sufficient if I use thick trace for power and ground?
If you really know what you're doing, and:
- you really need to cut down the cost (4 layer PCB is not that much more expensive)
- you can afford the extra work needed to route the PCB in two layers
- you can afford the risk of a few respins that might be needed to achieve satisfying EMC performance
the ground and power planes are not necessarily needed. I mean, it is possible to make a good design without them, but it is much much more difficult.
The nice thing about 4 layers is you can have shielding/ground planes that create capacitance to the smaller planes. Since the smaller planes located around a DC to DC converter are usually being switched in some manner, they make excellent antennas.
Another thing that a ground plane does is act as a shield for magnetic fields and provides a path for return currents.
If the design needs to go through FCC regulatory, I would suggest a 4 layer design would be best in almost all cases.
There are some good guides on how to do routing for DC to DC converters, AN139 being one of them.