Touch ID Fingerprint Sensor not working after screen replacement on an iPhone 6.
When does this happen?
It is a very rare occurrence -- about 1 out of 1000 screen replacements on average.
Are any iPhone models more susceptible to this failure?
The iPhone 6 Plus (as acknowledged by Apple), the iPhone 6 and 6S are more likely to exhibit this Touch ID issue than are other models.
Why does it happen?
There are a number of possible reasons why this particular issue may arise:
1. Damaged Home Button Cable:
An easily diagnosed problem - damage to this cable can be seen with the naked eye.
This damage is not repairable and can only be addressed by Apple (phone replacement for a fee).
2. Damaged Home Button Extension Cable:
This cable runs from the bottom of the iPhone to the top between a heat-shield and the iPhone LCD.
The extension cable is not usually replaced with a screen (on iPhone 6) as damage to this cable infrequent.
The screen assembly frame must be disassembled to remove this cable and replacement usually solves the problem.
This problem is repairable.
3. Damaged Main-board Circuits:
After impact the behavior and reliability of electronics will ALWAYS be questionable.
Apple has acknowledged main-board failure on some models under less severe circumstances than screen breakage.
Touch ID failure can happen after a screen replacement (even if it was working after the breakage) for the following reasons:
During screen replacement - the home button must be disconnected from the main-board.
After reconnecting - underlying main-board failure begins to exhibit the symptom of Touch ID not working.
Even after reconnecting the original screen the Touch ID failure can persist.
Disconnecting the battery and disconnecting the components necessary for the screen replacement can result in capacitors on the main-board which were previously constantly conducting electricity and maintaining a charge to drain their electricity and exhibit failure when the power is re-connected.
The only option is replace the phone.