Need help in learning how to successfully reball iPhone NAND

Hi, I’m having a hard time overall just trying to get NAND reballing to work, whether it be iPhone 4, 4S, 5, 5S, or 5C. I’d really appreciate it if I can get it to work at least once. I’ve been practicing for a few months, but I still feel like I haven’t gone anywhere. I’ve varied my temperature and air flow, and I can only cleanly remove the NAND at 350°C, air setting 2, after 3 minutes, with a 13x13mm rectangular nozzle on a Hakko FR-810B.

How can I check if the CPU or motherboard is broken without successfully soldering the NAND on top?

Using a reball stencil does not work very well for me. Is it possible to add the paste by hand using a very thin syringe? Or use solder balls? The low temperature solder paste is very liquidy and does not work very well for me.

I’m sad that I’ve been trying to learn and practice so much, but haven’t progressed. After I try to heat the NAND and see it go down, I try to start the motherboard, but the NAND starts to get really hot, or the motherboard does not turn on.

So far I have been able to test with an iPhone 4S that I can heat the NAND for 4 minutes straight each time at 310°C, 330°C, 350°C, and 370°C. At 390°C and above, the motherboard finally stops working, so I don’t think it is my temperature or amount of time. I just cannot seem to resolder the NAND well. How do I know when to stop applying heat when resoldering?

I have a machine to read the contents of the NAND, and sometimes, it can read everything. But most of the time, after desoldering, it does not read the information.

Please, does anyone have good advice particularly for the NAND? I really want it to work! Youtube video tutorials make it look so easy.