Hi,
I have time to time the same problem w. 4x4 & 5x5mm, 0.5mm pitch QFN 16 ICs and I use a more finer solder system from JBC control unit Type "BT",
www.jbctools.com, with 0.3mm tip & 0.3mm tinn & much flux...
It functions- such exemplars yet over months & in industrial environments.
What about your "back (thermo-)pad", that you can not solder/remelt if you are handsolder!!
Maybe is your problem not other, that "thermal altering"...
I mean, the bak/cooling pad is per handsolering not applicable, but for the most IC`s (Power or RF-technic) is a must. For such ICs it is to lowest potential (GND or -Vss) to contact/solder, but if you didnt an reflow process applyed: it is very randomly possible= is abs. not reliable!:-((
If thats your case= after them you fixed the IC on corners/diagonal with handsoldering -- I belief you must take a smaller hot air gun for reflowing the thinn >>better: solderpaste with minimal thickness between QFN IC and PCB!
On an experiment RF-Board I applyed i.e. a 2mm holes under my QFNs & I soldered "from back"-more or less
), but all these experiments are (very)problematics- not simple to practice & in all cases nothing for some serie production...
K.