Homebrew PCBs

If you really want to use an iron, what I did was put a big via in the
middle of the pad - big enough to solder the pad from the back side
through the hole.  Don't rely on your smallest tip, though - use
something beefy enough to heat up all that copper.  You'll need flux
in there too.

But I find that using a hotplate makes QFNs much easier.

Also, don't skimp on the amount of pad that extends beyond the chip on
the PCB.  You'll need that for soldering the pins.  You'll need flux
there too.