The advantage is that the N900 flash, while being a manufactured "problem zone", does not seem to be failing on its own. In all cases that I heard of, it took being dropped. So, I have hopes that if reflowed once, it has some chances to last. The graphics chips are under much higher thermal stress.

Actually, I got a used backup N900 off ebay beforehand. Now my girl-friend is a happy user of the reflowed N900.

I didn't dare to point my fat heat gun at a delicate board like this. There is a lot of plastic (for the elevated SD card holder) which would melt away like ice. So, I went and bought a hot-air station ZD-939L and after finding out that it doesn't make any sense without a preheater, I also bought a second-hand preheater off ebay which was a real bargain because the previous owner had modded it with a great mechanism for holding PCBs perfectly fitting the N900 board.

I also bought an IR thermometer but the one I got is crap and shows funny values and doesn't focus well.

About temperature: I can't really tell. Preheater was at about 120 °C. The hot air, I cannot exactly remember and I took about four attempts until I dared to go high enough. It takes quite some heat to melt the balls below the chip. I guess something like 350°C which is quite high, but I used a small nozzle and constantly moved around in order to spread the heat. I guess that's why I needed more then generally recommended. Of course, it's much better not to take four attempts because that all stresses the PCB layers and eventually will lead to delamination. Really glad, it didn't happen to me.

I also added some flux paste around the chip which melts well due to the preheater and flows under the chip. I removed that with isopropanol afterwards.

Also: I tried on the tiny static discharge chips to reball them: It's terribly difficult with mere solder and a solder iron. Solder just prefers to stick to the iron due to surface tension. So, even though it's recommended to reball: If the balls on your flash still look OK, I would maybe resolder the surface of your PCB and try to keep the balls on the chip.

Also: The interface to the flash chip is serial, so only very few of those like 70 balls actually carry a signal. It's enough for them to have good contact. The rest is mechanical support.

Where in Germany are you? PM me.