Bridge Rectifier Problem or something else?

Question

I am designing a 30V 5A power supply and after simulations working fine I decided to make it on actual hardware. But during the first run, something shorted and the power supply didn't even turn on.

These are before and after photos of PCB:

Photos (facing some issue on forum, so shared google drive link)

The black area on PCB is around the bridge rectifier. The bridge rectifier is shorted and some tracks have been removed due to high current(maybe). Other area on PCB is completely fine.

This is the whole schematic of power supply if required: link

The diodes used for bridge rectifier is RL207 (Datasheet) which has 2A 1000V rating and 70A surge current rating.

The max surge current according to the simulation is supposed to be around 20A which the RL207 should be able to handle even after derating.

Can anyone help me to find the actual problem?

The reasons that I could think of but not sure:

1. The residue flux after soldering shorted some tracks when 230VAC was connected. I had already verified all the connections using a multimeter before powering the circuit and orientation of diodes are also correct.

2. The inrush current was too high for diodes to handle during charging of 470uF capacitor due to which rectifier shorted and therefore AC mains got shorted.

Some extra information if required:

1. Clearance for copper pour on PCB is 1mm.
2. RL207 diodes were taken from an old computer power supply in working condition.
3. NTC 10D-9 thermistor was also taken from the same old computer power supply.
4. The yellow and black wire taped together was used in place of fuse as I didn't had any at that time.

Answer 1

Diode D1 (1N4148) cannot handle the high reverse voltage and is of no use here anyway. It's just a short circuit after some µs.

Answer 2

The diode rating appears to be ok. But I cannot tell from the google drive schematic how these are even used, so cannot tell if the surge current would exceed 70A.

The issue is most likely contamination between the high-voltage traces. Any one or more of the following areas could be to blame:

Keep in mind that 240VAC = 340VDC peak-to-peak. Anything causing the tiny gap between traces to diminish could have initiated an arc with up to 340V across it. Solder blobs, flux bridges, trace being too close, mote of dust, etc.

Such areas must be exceptionally clean and free of any contaminants, including flux; flux dissolves metal slightly, so can become slightly conductive itself. Usually not a problem at 5V or 12V, but can be critical at 340V. Even those brands of flux which claim to be "no-clean" - clean it anyways. Top-side also. Lacquer thinner or Acetone cleans it well. Isopropyl alcohol also works, but is slow.

It would be better to use larger creepage and clearance distances on such high-voltage traces to lessen the chance of arc-flash.

Three additional things:

1. I don't see a fuse. A thermistor (used as a current-limiting device) is not a fuse. Mains-powered anything must have a fuse, to prevent fire.
2. Consider using a Variac (adjustable auto-transformer) to vary the AC voltage being supplied to this unknown supply. This allows you to test at different input voltages and maybe would have alerted to to an issue (and the Variac fuse blown) before the voltage got high enough to severely damage things.
3. Consider powering an unknown supply from a series incandescent lamp. If the lamp bulb lights then goes out, then the supply has charged and should be working. If the bulb stays on, then the supply is shorted and damage is reduced because the full input current is dissipated by the lamp. This doesn't necessarily work for all supplies (some may actually oscillate), but it shouldn't cause damage for a quick three-second test.