Chosen Solution
I recently replaced my iPhone 5s battery, and after turning on my iPhone, the screen has begun to flicker at the corners. I have re-opened my iPhone twice and re-inserted all of the connectors. None of my connectors appear to be damaged. I have also done a hard reset on my phone and the flickering persists.
If bens method doesn’t work you may have damaged the screen during disassembly, where u careful when taking off the screen? The touch id cable is connected to the case and the screen components may be close to the cable.
Try check this aswell. If the adhesive battery tape snaps off whilst being pulled out, the iTech may use alcohol to release the tape. If the screen is still connected, the alcohol may seep under the screen connectors. Although most of the alcohol will evaporate, the alcohol under the screen connectors will take longer to disappear. This will cause the screen to flicker and may even damage the circuit board as the alcohol is causing mini shorts. Remove the screen connectors to ensure there is no alcohol underneath them. Additionally when using alcohol to dislodge the battery adhesive tape, dip the card / tool in to the alcohol as apposed to squirting alcohol in to the phone or under the battery. This would explain why switching the phone on and off several times may cause the issue to go away as the alcohol evaporates.
