I am working on a backplane PCB in which we have five PSU modules (230 VAC input, 5V DC output) and five custom designed application boards. I will be using 70 um thick layers in backplane PCB.
In PCB designs, we sometime use two layers for DC power which we call power planes and two layers for Gnd but this is for DC power and Gnd.
Can we also do the same for AC main single phase lines on the PCB ? I mean to route each AC main line on two layers using wide tracks.
Yes you can route AC power across 2 layers. Whether that is a preferred idea is unknown since we have no idea what your circuit looks like or how much power need to pass from point to point.
Each PSU modules (230 VAC input, 5V DC output) is 100 Watt and there are five in total on the backplane. The backplane is 40 cm wide. If I route the AC main lines on 2 layers , would there be a risk of phase mis alignment or anything else that can be considerable while routing AC main lines on two layers.
I doubt there will any phase related issues with 230VAC. The expect that clearances will be the most interesting item to pay attention to.
As @timothy.phillips says, clearances are very important. The obvious one is layer to layer separation. While standard FR4 materials will generally withstand surprisingly high voltages, it is easy to overlook the clearances needed around any vias or holes for through-hole components, even if they do not connect to either of the planes. Don’t forget to keep a good distance from any mounting holes, even if they’re not plated, many PCB fixings are made of metal and being too close to the AC layers would not be good.
Another thing to consider is that these AC planes must be kept well in from the edges of the PCB.
One benefit of keeping the high voltage AC connections as traces rather than planes is that it is obvious where the high voltages are. This is better than spreading them across your board unnecessarily. If you have a total of 500W of power to deliver, and the AC voltage is 230V, then the current is only around 2.2A. This is not very much and traces carrying this level of current do not need to be all that wide. This is not hard to implement on most boards.
However you do things, don’t forget to partition your design carefully so that nothing from the high voltage AC side comes anywhere near the low voltage DC side … and vice-versa (it is just as bad when you route the AC as nice short, wide traces, but then forget to keep any 0V or DC power planes from getting very close to the AC through-hole connections.
While it is possible to put AC on planes, when the current is this small the question is whether you gain anything by doing so.
Answering your final question about phase alignment of standard mains AC, the answer is no. To get a 10% phase difference the board would have to be 375 miles long. On any realistic size of board, the phase differences between all the convertors is irrelevant.
Thank you very much for your comments and suggestions. You have highlighted important points that need to be considered while routing AC main lines on the PCB.