iPad will not boot at all. Logic board damage, but where?

I replaced the digitizer on an iPad 3 Wifi (A1416) and since, have not been able to get it to boot.

Hard reset has done nothing. After doing everything else I could think of, I built a new iPad 3 out of shop parts (all of which are confirmed working) essentially creating a completely new device around the logic board. Still nothing.

I put a good logic board into the newly built iPad and it works without a hitch.

So we can confirm that it's the board that's damaged.

The question is: Where?

I did a visual inspection under the microscope, and the thing is immaculate, with no obvious damaged traces or components, and no water damage.

our charging bay uses color coded lights to display charge status of plugged in devices, and normally when an iDevice is hard reset while plugged into that charging bay, the light will shut off and then come back on at the moment the device is reset.

when this iPad is plugged in, there is no "light response" to an attempted hard reset. The light is just a solid charging indicator throughout. I'm not sure what this means, exactly, but it might be helpful to someone who knows their way around an iPad 3 logic board.

Also, it's worth mentioning that my customer let me know that this iPad had been badly bent previously and built into a new enclosure. Obviously that helped nothing as far as the board is concerned.

Any suggestions for where to start troubleshooting this board?