I have run a small repair business for 7 years now. I pride myself on quality work, so when I began to have about a 20% return rate for phantom touch issues I knew I had to dig my heals in to try and figure this out.
First thing first - I do not use OEM displays. All my displays however are high quality copy with TIANMA LCD’s and I have used the same supplier and same quality level for years without issues.
I’ve come up with the following procedures during iphone 7/8 repairs and have not had a single return since. I’ve also done these procedures on returned units with the same display and fixed the issue.
First - the Metal rectangular cover on the flex cable of the new screen has to be positioned perfectly. The best way to determine the position is to take a pair of calipers and measure from the frame of the phone to the edge of the loudspeaker. For example, the Iphone 7 measures 30.50mm and the iphone 7+ measures 44.50mm. I use a piece of kapton tape to hold the flex cables in place, measure from the edge of the screen to the edge of the cover, adjust, measure etc. Once you have it where you want it, install the metal backplate. If you make the actual position of the edge of the rectangular cover match this distance within +0.2mm/-0mm you’ve completed step 1.
Second - I’ve noticed that on the original displays, the rectangular chip cover is one solid piece covering all the critical components on the flex. The chips installed on the aftermarket units are wider then original, and they have made these fit under the cover by cutting the edge of the cover leaving the chip exposed. If yours are the same as described, cover this exposed chip with a piece of Yellow Kapton tape.
Third - When installing the screen back onto the phone, install all 4 screws in the flex cable cover loosely. Side the metal cover upwards as far as you can with all 4 screws loose. When you have it as far upwards towards the top of the phone as you can, tighten the 4 screws.
I’ve got photos of all the critical steps if anyone would like a copy, send me a message,