I was able to successfully replace BGA ram chips. It's much easier to do with old hardware, which is soldered with leaded solder, because leaded solder is easier to melt than unleaded one.
You would need these things:
- hot air station (858D is the cheapest one, it's not good at all for modern lead-free hardware, but sufficient for leaded solder; it's around 40-50USD, while something more professional, like Quik 861DW is ~280USD)
MAYBE you could use hot air gun, but this is not recommended because it has uncontrolled stream of hot air.
- soldering iron with wide T12 tip for cleaning remaining solder (there are cheap T12 soldering irons you can power with a laptop power brick, like this one https://aliexpress.com/item/4001075834034.html)
HEAVILY recommended: preheater (you can use electric oven, pancake-maker hot plate, metal sheet or 5,25" CD drive metal case warmed up with tealights etc.; if you use a cheap, underpowered hot air station, a preheater is necessary)
- flux, eg. Amtech NC 559 TF
- solder wick
+ replacement memory chips, maybe you can buy brand new ones on ebay or aliexpress. They should come with pre-applied solder balls. Make sure that they are not lead-free. If they don't have solder balls, you need to apply the balls. Then you would also need:
- leaded BGA solder balls (probably 0.45mm for memory chips, but you should check the datasheet)
- BGA stencil
It's not rocket science, but requires an initial investment and some practice. If you have lots of hardware you need to fix, it might be worth investing. Cheap hardware may cost less than a single-time professional service, but you need to keep in mind that cheap hardware is imperfect, it doesn't heat up nearly as fast as professional machines, temperature control is not perfectly calibrated, requires more patience and you shouldn't try to use it to repair modern, lead-free hardware.
I found a video that shows reballing and replacing memory chips: https://www.youtube.com/watch?v=B5g1AcDIjZw
Sometimes it is enough to reball them, but not often. A preheater was not used in this video, but in your case you should heat the entire board up to 80-90*C and heat up the memory chips afterwards.
This is more professional workflow with a preheater: https://www.youtube.com/watch?v=jLViTC6XkpA