Question: I've carried out a couple of repairs on my iPhone 5 over the last fortnight, including replacing the screen. I detached the home button from its mounting in the original screen, and placed it in the new screen. It works fine, but I need to glue it to the contact that it sits on top of. What glue does Apple use to do this? Where can I buy this, or a suitable alternative?

Answer: 
I realize this is an older question, but a common problem for several iphone models. Might help someone.
 The whole assembly is larger than the home button hole. Problem is the round plastic button does come loose from the whole assembly. It is glued in place and can break free. 
As long as everything is functioning correctly, it wouldn't hurt to put a dab of glue on and stick it back in. Be careful not to use too much, as it could cause more problems!
Good luck
I use 0,5mm thick vhb tape that I cut around the edge of the button on each side.
This settles the button in its lodgement, and prevent casual wobbling.