Hot Air Soldering Tips

Below you'll find some semi-organized thoughts I (crumblenaut) shared with a member on The Rossmann Group's Discord who proceeded to suggest that I upload it to this wiki for the reference of others in the board repair community. As of March 2nd, 2021 these are exclusively my thoughts, techniques, and take-aways and do not represent a consensus amongst everyone in the community!
===Introduction and Encouragement===
*Air soldering can be super intimidating when you're getting started because you'll almost always be working around a ton of tiny components.
*Don't worry, though - it's not rocket science and you'll pick it up quickly with a little practice.
*Everyone has different styles, and the technique you develop will have more to do with your results than what air soldering equipment you're using.
*All you need is one dead, donated logic board to practice on where you can make all of the mistakes you want without consequence and in doing so you'll become a pro in no time!
===Basic Settings for Heat Transfer - Temperature, Blower Speed, and Distance===
*I use my hot air gun at 425°C with the widest nozzle that came with my station, which has an interior diameter of ~11mm (~7/16").
*I always keep my gun's fan down to a value of 2-3 out of 10 in order to ensure that the airflow doesn't accidentally move any components that I don't want moving. The goal is to have enough fan speed to transfer the heat to the board, but not much more than that or else you'll risk blowing components around on (or straight-up OFF!) the board. 
**Please note that setting your air gun's fan speed to its minimum can lead to it overheating dangerously! I had it pegged at its minimum once by accident and found that the entire exposed metal portion of the gun - the nozzle AND the chamber that houses the heating element - heated up to the point that the whole thing was glowing red! Scary!
*The distance of the air gun's outlet from the surface on which you're soldering will control the temperature more than anything else.
*Having a fume extractor doing its thing nearby (which you should have!) may also impact how heat is transferred to the surface. I keep my fume extractor's inlet at least ~4" (~100mm) from where I'm air soldering, and elevated from the plane of the board so that it catches flux fumes as they rise off the board without redirecting the air gun's output appreciably.
===Planning Ahead===
*Before beginning your work, be sure to have the right tools and supplies ready and that your space is free of clutter and arranged so that everything you need is immediately accessible! You'll generally want to decide on, prepare, and arrange:
**The tweezers / dental tools / scalpels you're going to want to use on the areas / components you'll be working with
**The soldering irons you may want heated up and ready to go ahead of time (with tips appropriate for the task at hand)
***I use the soldering iron attached to my station for heavier desoldering and working with larger components/pads and a have a separate smaller soldering iron with an even finer tip that I use for almost all of my actual microsoldering work. Both use bent tips, because they rule.
**Your spare solder, desoldering braid, cutters for used braid, isoproyl alcohol, and flux
**Your tip cleaner (I use a brass soldering iron tip cleaner in an enclosure with a sponge with a hole cut out of it surrounding the opening)
**Any components you're going to be replacing set in a place where they won't get knocked away, lost, or confused for the old parts
*You'll also want to look at the board and work out the spatial reasoning of your approach and exit strategy ahead of time.
**Remember, Louis says that MacBook logic boards aren't special and you don't always have to keep them in one orientation!
**I have found myself - far more often than what I should admit to - reaching around at a horrible angle to try to solder a component awkwardly, only to realize partway through the process that rotating the logic board itself 90° in one direction or the other turned what was a real physical challenge into something quite trivial.
**Be sure that the approach you're going to be taking isn't going to lead to you melting any plastic bits like the external ports or any fan/keyboard/battery connectors, both on the approach and the withdrawal.
***If exposing plastics to heat is unavoidable (like working near a battery connector on a MacBook Air board, for example), you can wrap those plastic components with a couple layers of aluminum foil with an excess "tail" of foil coming off of the area (if it's good enough approach for a MacBook Air's M1 heatsink, it's good enough for our purposes).
*Finally, ALWAYS take photos of your board before you begin your work. You never know when it'll be helpful to tell a components orientation, or where damage was before cleaning, or which of the two SPI ROM chips you have in front of you was the original and which was from the donor board.
===Preheating and Thermodynamics===
*You'll always have better luck air soldering any individual components if the whole area around where you're air soldering has been pretty well heated. Since PCBs are coated in copper, the whole thing will act as a sink for any heat you apply. If this is unmitigated, it can lead to fragile solder joints and difficulty keeping the target components' solder in a reworkable liquid phase.
*If you don't have a board heater (or a dedicated toaster oven) to preheat boards, you can get the surrounding areas of the board to be warm by circling the air gun around the broader area from a distance of a few inches for a bit before focusing it on the target component itself.
**Proper industrial PCB preheating is generally given a range of ~95°C to ~150°C (~200°F to ~300°F), but even heating the PCB to the point that it's pretty warm to the touch but reasonable to handle with your bare hands (substantially UNDER the boiling point of 100°C / 212°F) will make a massive difference in the ease with which you can air solder and rework components as well as the impact of the thermal stress on the wellbeing of the board itself.
*Maintaining a gradual, spatially-distributed ΔT (change in temperature) is a good thing for chips, components, and their means of being attached to the board.
===Finally... AIR SOLDERING!===
*Either preheat your board separately or by circling around the area you'll be working with as described above.
*Next, apply AN APPROPRIATE AMOUNT OF FLUX to the components you're going to be working with. If you have a good fume extractor in operation, a little too much is generally better than having too little in place. 
**Other than removal of components that you absolutely don't care about whatsoever and in areas that aren't especially populated, ALL AIR SOLDERING SHOULD BE DONE WITH FLUX IN PLACE. This will ensure that the solder doesn't misbehave, and that the components in question - both your target components and any components surrounding your target - form as-strong-or-stronger bonds with the board than what they had originally.
*Now, focus the heat from your air gun on the target components or areas using tighter circles. 
**Holding the iron fully motionless CAN be okay and is sometimes necessary, but I've found that you'll often heat one side of the components or a smaller-than-useful area of your board, even with a wide nozzle.
**Moving the air gun in tight circles will add some chaos to the pattern of airflow, increasing the number of angles of approach the heat will take when striking the components in question and can help it get to places where it otherwise wouldn't get, like under chips. 
***This does a lot to help avoid situations where one half of a chip or one lead of an inductor has phase changed to liquid while the other half/lead has stayed solid.
*Now while you're applying heat, watch the exposed leads of your component and eventually you'll see the solder phase change from solid to liquid. Give it a couple seconds past that point before you begin working with it.
**If you're targeting a component such as a BGA (ball grid array) chip that doesn't have solder that's visible from where you're at, then what I'll do is watch the solder of the components in the surrounding area - once they're uniformly in a shiny, liquid phase, then the BGA component may be ready for removal / rework.
*At this point, simple components - resistors, capacitors, inductors, diodes, etc. can be worked with directly.
*If you have enough flux in place that the component is basically "swimming" in it - which is oftentimes AN APPROPRIATE AMOUNT OF FLUX - then you can very gently and ever so slightly poke the BGA chip to be sure it's fully loose and floating on liquid solder balls before attempting to lift it off. 
**DO NOT APPLY ANY REAL FORCE HERE! If it is ready to be removed, the only thing holding it in place will be the surface tension of the liquid solder. If it does not move with the slightest touch from your tweezers, some or all of its solder has not yet liquefied!
**Applying force to chips on a logic board is a GREAT way to damage the chip or lift pads off of the board, some of which may be literally impossible to replace.
**At this point the chip has been "reflowed" - any corrosion under the chip should (ideally) have been dissolved into the flux and any inappropriate under-chip bridges in the ball grid array should (again, ideally) have been eliminated. 
***In the case of, say, a liquid exposed CD3215 USB-C controller chip, you may want to let the board cool and test it again before deciding to simply replace the chip. In my experience, the great majority of these chips are fine but have junk underneath them, and reflowing the chip more often than not restores its functionality.
*Finally, remember that if you ARE replacing components, it's always best practice to fully remove any solder left over on the board from old components using desoldering braid. 
**After doing this, clean any flux left over on the board and apply fresh flux before retinning the pads with leaded solder.
**If you're going to be replacing a BGA chip with a replacement that comes pre-balled, it's best to air solder that to desoldered and cleaned pads rather than attempting to get a little bit of solder adhered to each pad ahead of time. The balls it comes with are enough for it to attach securely to effectively bare (only flat, minimally pre-tinned) pads.
*Once your components are all in place, you may want to be sure that they're fully flowed into place, meaning that the solder has wetted / wicked properly onto the pads of the board and the component. You can do this by simply giving it a few more seconds under direct, tight circles with the air gun.
*Finally, remove heat from the board semigradually by expanding the circles with your air gun for 5-10 seconds to be sure that the board's ΔT throughout its cooldown phase is also gradual and spatially-distributed.
*YOU DID IT!!! Now just let your board cool down, clean the flux off of it, and test it out!
===Additional Resources===
*NASA on Soldering Techniques, Ideal Outcomes, and Flaws (archive)
*Zephyrtronics on Board Preheating