There can be several causes to boot looping. Remove the screen assembly and connect your device to iTunes. Does iTunes recognize your phone? You could also try a tool like 3uTool to put it or remove it from recovery mode. If that doesn't work, you can also disconnect the back camera to see if that helps.
If the phone works without the screen assembly, then gradually add the LCD and digitizer, then the front camera and then the sensor flex. This may help you in troubleshooting the device at a modular level.
You could have a logic board level issue as well but try these steps first and then come back and comment on my answer so that I get notified of your updates.