Error 9 and 14 + Bluescreen how to fix?

Hello together, I bought a really cheap but broken iPhone 5s. In the past, I repaired my iPhone 4 and thought, the 5s will be no problem, because I need a new phone.

The problem: if I start the phone, I see the iTunes logo. If I try to restore the phone, the process bar goes to 25% and then I got the Bluescreen or the phone reboot. I cant enter the DFU mode. If I see the iTunes logo, I can't shutdown the phone.

So at first I replaced the battery and the Lightning-connector but without success. My PC is ok. No entries in the host file and I have no antivirus software. I know that the preowner opened the phone before.

I read that the error only comes, if the wrong screws were used. They are on the right position but I don't know what happend in the past.

So, what can I do? Is it really only the damaged board in case of the wrong screw? I read about micro soldering the board but what? I can solder but don't know what.