Phone overheated charging now wont turn on

Hi, my trusty iphone 5S became very hot while charging this weekend and the screen went grey. After I shut it down I couldn't start it up again, and my computer won't recognize it.

So far I have tried a new screen and battery to see if there was some sort of short but neither happened. I was hoping that the new battery would give me a sign that the charging port shorted out because it would turn the phone on at least to show the battery sign because it had a slight charge., but got nothing. Leaving me to believe it is not the charging port, battery, or screen... meaning it is something in the motherboard?

How wrong am I?

Cheers!!!

Cole