won't power on, no DFU, but battery seems to charge?

Hello all,

I've been given a dead iPhone and I'm trying to bring it back to life.

I was told that this iPhone was dropped on the floor (about 2-3 feet), turned off and never turned on again.

I dismantled it and I can tell it has never been opened before, no water damage.

I replaced: LCD/digitalizer, dock, battery, volume/mute/jack/power with new ones: no luck.

I looked at the motherboard with a magnifier, nothing wrong.

The iPhone is not powering on, hard reset and DFU mode are not working, it looks totally dead.

The funny thing is that the phone is charging the battery when plugged to a PC or a wall charger! So I guess the power microchip is not dead.

What can I test? Any idea/suggestion?

Thanks in advance.