First, Be certain you are willing to risk the loss of the phone if you are not successful and make any mistakes. I would recommend finding a "Donor" board and experimenting with it first, as there are several heat stations on the market and they are all a bit different with their heat settings and airflow.
(Personally...I try to get in and out quick, minimizing the time I am applying heat to the board. I set my airflow at 40% and heat no more than 380C but usually stick around 360C for this chip).
I apply Kapton tape around everything surrounding the IC and a couple coins to act as heat sync on either side, also move antenna cables out of the way. After putting a small amount of flux around the chip, apply heat using the smallest attachment directly on the center of the IC as it is so small (Not like the Baseband or NAND where you have to move around the chip in a circular motion).
Keep an eye on the solder on Capacitors, etc to see the solder begin to flow, gently nudge the ic as you are heating it until it begins to flow (Do not pull it up until you are certain that it is liquid or you will pull pads and end up having to run pain in the butt traces to repair the pads)...and keep heat pointed away from NAND and U_PMU_RF as you are heating.
Then, use "Low Melt" on the tip of your soldering iron on the pads after removing it to make it easier to clean the pads. Don't try wicking with an Iron...after using lowmelt, you can use a small piece of wick and tweezers to use it like a paint brush to gently brush the pads with your Air set at about 220C to 240C making sure you keep them moist with clean Flux.
When you reseat the new IC, (On cleaned pads and very smooth surface), add a small amount of flux first, place the IC keeping track of orientation and hold it still in place for a few so it doesn't cause bridges by sliding around. Once it gets set in place, you can let go and keep heating and it will find its way to its pads on its own if you cleaned well enough.
Watch several videos on Youtube first to get a good idea of what others are doing (Here is one from Chris Long) and again, if possible, practice a bit...even if you don't have a Iphone Logic Board, find an old piece of electronics you can scrap and play around with it.
Best of Luck!
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