QFN is doable at home, though more difficult than TQFP leads or larger SOIC-chips. Still, far too many components are QFN today so its a good skill to pickup.
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The techninque is:
1. Reflow soldering brings the _whole_ board to soldering temperatures, and then relies upon surface-tension to pull all the devices into proper place.
2. Solder paste is applies ahead of time. You can cleanup solder paste with a toothpick before you bring the board up to temperature. Solder paste tends to go bad pretty quickly however. When doing prototypes, opt for the more expensive low-melt solder paste to minimize potential heat damage.
3. Prefer to use a stencil to apply solder paste. But its more than doable to be sloppy with a syringe and then rely upon the solder mask + surface tension to magically cleanup things during reflow temperatures.
4. Use a hot-air gun to fix any issues. The #1 issue you'll have is tombstoning, QFNs or TQFP chips are usually pretty good about settling into place. For TQFP issues (ex: bridging), you'll need solder-wick + soldering iron. EDIT: And flux: lots of flux helps. Also, flux goes bad, so throw away Flux all the time and keep buying new batches.
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BGA is also doable at home btw, thanks to the above principles. But its even harder than QFN. The real issue of BGAs is that you're looking at 4-layer minimum, maybe 6, 8, or 10-layer designs. There's also substantial grounding and other advanced PCB concepts you need before you can layout a BGA-capable PCB.
Its not so much the physical activity of soldering that's hard or difficult for BGAs. Its all the theory you need to study to breakout BGAs + minimize inductance + deal with impedance matching and trace-length matching.