Reboot loop. Yes, I read a thousand other answers already.
iPhone SE came to me with a broken screen. The LCD and touch both OK but the glass broken; points of impact are in the white area to the right of the home button, and about two inches above that on the right edge. Simple job, done it a bunch of times before. Careful disassembly, swap the components from old to new screen, plug it in and go.
I read about a thousand pages, on iFixIt and off, about this issue going as far back as the iPhone 5. Things I've been able to filter out:
- It's not the battery. If I disconnect the battery and power it from the Lightning port, the issue persists.
- It's not the replacement screen. I tried another replacement screen, and tried the original broken screen, and the issue persists.
- It's not the front camera flex cable. I tried it with the cable disconnected, and the issue persists.
- It's not the digitizer cable (on any of the screens). I tried it with the digitizer cable disconnected, and the issue persists.
- It's not the home button. I tried it with the home button connected and disconnected, and the issue persists.
- It's not the rear-facing camera. I tried it with the camera connected and disconnected, and the issue persists.
- I can't get the phone into DFU mode. The reboot cycle is so quick (just a few seconds on the Apple logo) that it doesn't seem to "recognize" the DFU request. The computer doesn't see the phone being connected via USB for the same reason, and obviously iTunes doesn't see it either.
At one point I reconnected the original screen + digitizer and I got the phone to boot to the PIN screen. I entered the PIN but then the phone froze. I had to disconnect the battery, and when it rebooted it went back into the reboot loop.
All of the connectors on the motherboard and in perfect shape. There is no moisture damage to the device.