Chosen Solution
I was replacing a screen assembly on iphone 6. before putting the screws back in I turned the device on and tested it. it was working fine. The digitizer shifted a tiny bit (maybe it pulled a connector off?) but the phone went off entirely. I have not been able to get it back on since. I did try a second brand new replacement screen assembly (known good) and it does the same thing. I can power on the phone and iTunes DOES see and communicate with it. I installed the screen again (no home button or any connectors except the screen and digitizer) - powered on, used a flashlight and could NOT see anything on the screen in the dark. All pins on connectors look good. I dont thinks the backlight filter since I cant see anything with a flashlight. Any ideas or suggestions. Thanks
First thing would be to take a close look at the connectors and surrounding areas for any physical damage. If you cannot see anything at all even with a clean screen and a flashlight, you may be looking at a display IC. If just the backlight goes out you can still see a faint image, but if just the display driver (u1501) or display boost (u5302_RF) goes out you can usually still see a backlight on with the screen outside of the frame. Hope this helps, but always start with the easy stuff, like looking over the board for physical marks.
