... "incomplete cleaning" pretty much describes most attempts I've made to clean a hand soldered board. I never want to use as much IPA as it apparently necessary. Usually end up with white residue over more of the board than originally had flux from the soldering.
The amount and technique are one thing, but another aspect, and I cannot emphasize this enough, is this:
IPA is not a good cleaner for many fluxes. This has been argued on the forums ad infinitum, and some people refuse to accept it and dismiss it as nonsense, but there is a reason commercial flux removers exist, and why they contain ingredients beyond IPA.
No amount of IPA can flush away compounds that aren’t soluble in alcohol. And some compounds benefit from synergistic action of solvent combinations, dissolving in the combination better than they can in any of the constituent solvents alone.
Yes,
some fluxes do clean up perfectly with IPA, but many do not. And warm IPA in an ultrasonic cleaner performs way better than when used for hand cleaning. But my point is that it is not universally successful for hand cleaning as many people wrongly insist. Commercial cleaners do better, and even among them, I’m sure some fluxes respond better to some cleaners and vice versa.