[MOVED] Power plane in a 4 layer PCB

[sworks30]

G'Day my dear friends,
I've just started routing my first 4 layer PCB with a pretty much standard layout; signal - GND plane - 3V3 plane - signal.
The circuit is powered from a 24V DC source and I use a step down converter to get 3.3V, which powers more or less all of my circuitry. My question is, should I place the 3V3 plane under the 24V region of the PCB, or should I remove it there?

Thanks mates!

 

[tmd_63]

you can leave the 3v3 plane if your components are on the signal side closer to the GND plane. Or you can place a copper area in the 3v3 plane and connect it to GND to give you a double thickness GND under the 24V components. The main area to watch for is the switching circuits and currents. Is does not matter around the 24V DC supplies (IE, you can have the 3v3 plane under the input traces but try to avoid any inductors).

 

[sworks30]

Thanks mate! Good idea on putting an extra ground beneath the 24V region. I'll doo that for sure.
One more thing. Do you make it a habit, to pour GND over the top and bottom layer as well? Can that affect signal integrity? (BTW, the PCB is not impedance controlled, but I still have a few 8MHz digital signals on it which I'm worried about).

Thanks again!!!

 

[tmd_63]

It depends on a number of factors. Always make sure that there are no islands of unconnected copper (a classic mistake of engineers trying to be PCB designers). How much copper is on the outer layers for the traces used? Make sure you have traces large enough for the currents involved. Try searching for UltraCad or SaturnPCB for programs that can help with the trace sizes for current ratings. I use a maximum of 10°C rise above ambient. If there is a lot of space without copper, then yes. Pour more copper. It will help with heat removal. Make sure your high frequency traces are as short as possible and keep the copper underneath the traces free from other traces (for the return path current).