Phone heated up, smell burnt and wont turn on
I am repairing an iPhone 6 with cracked screen for my customer. Before doing anything, I unplugged the battery and put a new screen in to test it. I was testing the new screen fine then the phone suddenly heated up and I smell burnt; it then won't turn on. The battery is not original, I tried another battery and the phone still won't turn on.
Anyone knows what the cause of this problem? Is it because of the battery or sth else?
Is this a good question?