iPhone 5S Press/Hold Powr - Logo appears, then disappears. No boot.

Hello all,

We have an iPhone 5S in for repair. Originally customer told us it was a battery issue and cracked screen. We installed new batt + new screen and the issue still occurs.

Issue: Apple logo appears when pressing down power, but logo disappears after approx. 4 seconds.

Testing we've performed and discoveries:

- Hold down POWER and logo appears. Disappears after 4 seconds give/take. Can press POWER again, same issue occurs.

- When I press down POWER for a second or so until logo appears and release, logo disappears and I can no longer hold or press power to get logo to reappear. When this occurs I can hook it up to an Apple charge cable, unplug from cable and can duplicate the same issues again.

- Hold POWER + HOME for 10 seconds does nothing.

At this time we've tried:

- Another battery

- Another test screen

- Issues not resolved

- Hooking up to PC and power/home to reset/put in restore mode (PC does not recognize device)

We are thinking:

- Possible charge port issue?

- Possible power button issue?

If anyone has thoughts/suggestions please let me know.