Powers on, screen does not work, is the logic board damaged?

I have an iPhone 6 which had a working lcd display but the digitizer was failing on it so I performed a screen replacement (install new lcd + digitizer assembly) which I have done many many times on this model iphone. I took the screen off and transferred everything to the new screen and reconnected the battery then before snapping the phone back together (I did not reinstall the shield plate over the ribbon cables yet) I plugged the phone into a charger. The device powered on because I could hear it vibrate to indicate charging but the display never came on. I cannot figure what is wrong.

Things I have tried already:

-two different brand new screens

-with and without the camera flex

-two different lcd Shields

-used magnifying glass to inspect all connections on the logic board, everything looks good

-this is not long screw damage because I never reinstalled the screws on the board

-power cycled the device

-with and without the home button

-there's no water damage

Does anyone know what is wrong??