iPhone 4 stop working then overheating
I have an iPhone from a friend who, overnight has stopped working. It does not light. The phone has no warranty. He asked if I could help. It overheated when it is plugged to the charger.
After an initial disassembly (remove the motherboard and put in place), he made a sign of life. Except that the screen was the rainbow colored sky randomly. He started and recognized by iTunes. The external buttons work, the vibrator, the home button and sound. As against the screen and the touch screen does not work. The temperature rose even more.
I did a hard reset and and there, the screen is on but it starts black and it overheated even more.
The overheating is mainly on the side of the phone and through the motherboard. More I live time on the charger more it rises in temperature.
What could it be? A faulty motherboard? a faulty chip or a bad welding?
Thank you in advance for your replies