The big problem here getting the heat right. Doing it with hot air is tricky, because the only way to heat that center pad enough is via the body of the chip. I am concerned about frying the chip just getting it back on the board. It is quite a lot of heat to get the old one off. Plus, you don't actually know if you have gotten the solder on the central pad to melt or not.
If you didn't think about it, think about shielding the components (even small ones really next to the IC) around with some kapton tape, you will feel way more comfortable heating to re-solder.
As far as I know it's quite difficult to fry a chip like that (except for very sensitive ones) just by soldering it with hot air ;-) . If you need to be more comfortable to take an old computer board or something like that and tune your Hot Air station and your gesture by training on it.
A way to be sure the under pad is soldered is to, really clean very well the other pads and solder only the under pad by heating on top (and letting cool down, it's bigger it can take a will). If the component doesn't move (and nothing else is soldered) it should be OK. But I'm not sure it's a good idea or useful. Other way is to put melted soldering tin (not sure in english) on the central pad to have a little bump + a lot of flux after, and either put very small amount of it (then clean with soldering wick + add flux) or solder past for the outside pads (not too much), this will be nervously annoying to place the component, but that way when it'll be hot enough the under will melt and outside also, you will be sure because otherwise the component will not be perfectly horizontal and down... However I hink it's doable in one shot with solder past and flux.
I would just solder it using hot air and, solder past around, because normally when the outer pads will be soldered all around (try to heat everywhere especially center of the IC) everything will be OK. I've done it on two boards, an AVR Dragon where I changed the ic of the useless and poor designed boost regulator (to comply with USB 4.25-5.25V when the integrated uC need at least 4.5V) and with a custom board using an incredibly thin and small H-Bridge, because there wasn't any other low-voltage H-Bridge on the market for my use. -- The kapton tape really really made my life easier on the Dragon, suddenly the capacitors were shielded and I could heat more to get the under pad OK.
I totally understand your fears, I find that frighting when I first began this... and I'm still "ignorant" of this technic despite being able to do a lot of difficult things with it.
You can take a look at my first really tiny nasty component try (not repair, soldering) :
https://twitter.com/SikulaRobotik/status/726065207919955968And I really need the thermal pad soldered otherwise it's dissipation would not be correct. However it's a custom board, it was harder because I had no idea if anything was wrong or not... but the outer pad plan is somehow exposed through the left. On the other (AVR Dragon) it was exactly like you, repair / rework, nasty faulty components, with an insane amount of component around. ;-)
I hope my little experience can help you, if you have any question do not hesitate.
I hope one day to have time to go to the Shenzen maker fair, because, their makers are rebelling and soldering BGA without proper tools, something just basic uncontrolled hot air, or basic hot air station, I really want to master such skills.I am wondering about actually sending this off to someone with a decent reflow oven. There's a fair chance it would be fine with the gun, but the trouble is there is no way to know until you plug it back up. And of course, if it works but the solder didn't flow underneath, there is a possibility of failure due to overheating in the future.
I am going to think about it a bit.
In fact it's not such a good idea, rework stations exist for a reason, this one ;-) . You can't really put the board in a reflow oven without removing all the connectors, plastic LEDs, (seen on the picture you attached), or a least protecting them, which can prove trickier than reworking with hot air.
Heating under the all board at around 100-150°C, is what the quartz pre-heater for rework allows. If you find a way to properly do that, It's a nice way to eliminate mechanical constraint and ease you top heating also. (however you can "pre heat" the the board, by using the hot air flow around all the board with a correct distance and not too hot temp, when you have nothing it's OK, and it's work pretty well).
Just an idea, I have never done this, but - you may want to check out "chip quik" - Using chip quik you likely could prevent the components on the back from coming off by lowering the temperature of the top components quickly using hot air, once it mixes in then the difference (in melting temperatures, if you're careful) will be large.
GET OTHER PEOPLES OPINION ON THIS!
Another idea blow a cooling fan across the bottom while heating the top with hot air. But that could also cause problems, with mechanical stresses.
As far as I'm concerned, I would not do that : rework usually imply "preheating" all the board to a consistent temperature (like with a super pricy quartz heater), this is to prevent any mechanical constraint... This is even why it's such a good idea, when you have limited tools, to get the board temperature rising by heating around up, down everywhere (if you are not too close to the board, it will slowly rise without desoldering anything and melting any connector).
The hot air temperature is actually falling when the distance from the output of hot air is rising, try with an apple or something like that, I had fun with my hot air station, and it's incredibly difficult to even burn the surface if the distance rise, even at 500°C max airflow... and it's a decent one trust me.I just discovered their is an amazing (and $$$) tool, which is a quartz under pre-heating with a top directed & controlled IR hot beam, to rework small BGA, seek Aoyue it's a new tool from them.... well cost too much for me but it's amazing.EDIT : I was writing when you had kapton like me
Kapton (polyamide) tape is a plastic tape that doesn't melt except at very high temperature which can be used to shield parts you don't want detaching.
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