OK. Careful with these suggestions. There are two sentences and two potential problems. Let me explain (I am an electronic engineer. Trust me
)
1)
When using a no-clean flux ALWAYS WASH THE BOARD AFTERWARDS. Use flux remover or metho and a toothbrush. Why? No-clean flux is designed to leave no ionic residues on the PCA once it has been activated. If you put lots of flux on the board (you do because it is no clean, right?) the only part of it that gets activated is where the soldering iron gets the temperature above a certain value for a certain time. The unactivated flux is hygroscopic and when it absorbs water it creates conductive paths between the tracks. Then dendrites of metal grow along these paths and periodically short between traces. As you can imagine it isn't great for reliability. It is also still corrosive. PCAs in production typically go through an oven the entire assembly is heated and all the flux activated. If you use a flux-cored solder that has no-clean flux
IN IT there is no need to clean.
2) Reworking joints that are already acceptable increases the thickness of the intermetallic bond and makes them more brittle. If a joint is acceptable, don't attempt to make it look pretty as you will just be compromising reliability. If it is sub-standard then of course you need to fix it.
I can see why people want to do both of these things but when you dig beneath the surface into the materials, properties and processes you can adversely affect performance of the assembly.
Sorry for the bad news
. A lot of things relating to soldering are counter-intuitive.