iPhone 5 boot loop after parts replaced

Hello,

Today I tried to replace some parts of a broken iPhone 5. Before I opened the phone the power button was broken and the screen was cracked but still working.

I replaced the flex cable, and the screen and while I was at it I also replaced the battery.

I closed the phone and tried to boot. Screen still black but the phone did vibrate few times. I put the old cracked screen back in and there was the apple logo. But it disappeared and came back again and it is repeating this proces over and over again. I tried to use the old battery, the old lcd but nothing helps. I just can't get it to boot in iOS!

Does anyone have an idea on this problem?

Any help is more than welcome!

Thank you in advance.