Chosen Solution
I replaced the screen and battery off my iPhone 6s and it won’t turn on. It was working perfectly fine before but now all it does is the back flashes three times (flashlight) and vibrate twice. It stays in a completely dark screen.
Unplug the battery, then plug the battery back in then turn on the phone. Shine a flashlight onto the screen and see if you see an apple logo on the dim screen, if you do you have a blown backlight circuit which requires micro-soldering to fix it. You may be able to find someone locally in your area that offers board-level / micro soldering repairs for iPhones. Blown backlight is very common on the 6s models due to it’s LCD screen connector design as lots of people unplug or plug in the screen with the battery still connected which most of the time ends up in a blown backlight circuit.
