Chosen Solution
I had to replace the battery in my iPhone 6s. I removed my screen to make sure I didn’t damage it during the process. When I put it back together, the screen was blank. Every once in awhile it will flash on at full brightness. The phone still works and you can see the faint images on the screen. It smelt like something may have been smoked. I have tried a few different screens but no luck. I am thinking it may be the backlight filter?? I have someone looking at it more today, but want to see what everyone thought before I committed to starting a repair. Would the issue be something that could be tracked down? Thanks!
As @soros says, failure to disconnect the battery before removing the screen can short out the backlight circuit. It is very sensitive on the 6S. It may be as simple as just the backlight filter (FL4211) or it may have impacted more components of the circuit, such as the diode(s), coil(s) or even the driver IC. This requires a micro-soldering repair and is not DIY. You should look for a repair shop that does micro-soldering repairs in your area or one that does mail-in repairs.
If you removed the screen before the battery or reconnected the screen after the battery its a good chance you burned something on the motherboard. :(
