QFN case I usually solder with SMD hot air rework station, final result is like factory solder. Anyway I will suggest usage of soldering flux with this case. First add soldering flux on soldering place/contacts, then add thin layer of soldering material on all pins and in middle area. Then again add soldering flux, add QFN IC over that, and use hot air station.
Also there is way to solder QFN with PCB hole in centre of QFN place, but I personally dont like this method.
Additional there is special soldering paste, which is used with hot air soldering station, oven or IR spot soldering.. This is easiest way. But that special soldering paste is expensive and have lifetime around 6 month after production. I use this a lots, and this is really easiest way.
How to Solder QFN MLF chips
About fluxes...
Fluxes for soft solder are currently available in three basic formulations:
Water-soluble fluxes - higher activity fluxes designed to be removed with water after soldering (no VOCs required for removal).
No-clean fluxes - mild enough to not "require" removal due to their non-conductive and non-corrosive residue.[5] These fluxes are called "no-clean" because the residue left after the solder operation is non-conductive and won't cause electrical shorts; nevertheless they leave a plainly visible white residue that resembles diluted bird-droppings. No-clean flux residue is acceptable on all 3 classes of PCBs as defined by IPC-610 provided it does not inhibit visual inspection, access to test points, or have a wet, tacky or excessive residue that may spread onto other areas. Connector mating surfaces must also be free of flux residue. Finger prints in no clean.
Traditional rosin fluxes - available in non-activated (R), mildly activated (RMA) and activated (RA) formulations. RA and RMA fluxes contain rosin combined with an activating agent, typically an acid, which increases the wettability of metals to which it is applied by removing existing oxides. The residue resulting from the use of RA flux is corrosive and must be cleaned. RMA flux is formulated to result in a residue which is not significantly corrosive, with cleaning being preferred but optional.
Flux performance needs to be carefully evaluated, a very mild 'no-clean' flux might be perfectly acceptable for production equipment, but not give adequate performance for a poorly controlled hand-soldering operation.
https://en.wikipedia.org/wiki/Soldering
I currently use MultiCore MF220. :roll:
Best regards,
Peter