Powers off when disconnected from power source

I have read as many articles as i can find but they are few and limited...

I replaced the screen and when i tried to turn the phone on nothing happened. I connected it to a power source and it turned on but was stuck in a boot loop. When it is disconnected from a power source it turns off. I have replaced the battery, and the charging port cable assembly, with no change. Cleaned all cables and terminals, checked for logic board damage and cracked or broken terminals. Nothing. I also cleaned all grounds with a soft brush till they shine. I am at a loss any one with any ideas?