How To Clean Flux From Pcb

rosin based flux is generally not conductive unless you baked it at such high temperatures it turned caramel color or black ...

no-clean flux is just that. no-clean. you can leave it on. no problem

-low solids flux is a flux that under a correctly controlled process totally evaporates. if there is a lot of residue your process is wring and you need to remove it as it can be conductive

- water soluble flux needs to be removed simply because it is hygroscopic and leaves a soap-scum like residue on the board that is conductive.

isopropylalcohol does not attack plastics ( apart from one kind of transparent material that just falls apart like a smashed car window. very weird) ipa is perfectly safe for electronics.

There are some very agressive fluxes like RA that must be removed as they contain acidic substances. RMA is the most prevalent rosin flux ( RMA ; Rosin Mildly activated. RA = rosin activated )
it is actually not the rosin that is the flux , but another substance in the compound. the rosin is just a carrying vessel for the real active ingredient.

flux works on two fronts :

When activated ( temperature controls this, most fluxes kick in around 100 degree C ) flux is a very reactive element that want to bind with oxygen molecules. It is so reactive it can strip oxygen molecules that have already bonded with copper. So it converts copper oxide back into copper.

The second thing it does is lower the surface tension of the solder. this lets the solder 'wlow' over the to be soldered surface.

under a correct process the flux is given some time to activate and do its work before soder is applied. wave solder machines apply warm flux to the board first, let it soak for a few seconds as the conveyer moves over a warming plate and the flux is now activated. then it goes through the solder wave where the flux totally evaporates leaving a clean board.

solder paste works in a similar way. when doing reflow you have a rampup to flux activation, a hold phase to ledt the flux do its work and evaporate for almost 75% and then you ramp up to liquid phase where the solder melts and flow on the remaining flux. if reflow is done correctly there is little or no flux residue and board wash is not required.

boards that have been reworked ( manually touching 1 component ) will have residue and those need washing with iehter solvent based flux removers or water.

the flux in solder wire is a different composition than the flux found in pens or liquid form ( for starters the flux in solder wire is dry powder .. ) it is also more agressive as it has a shorter time to do its work.

and then there are specially formulated fluxes like the gel fluxes or tacky fluxes. those are formulated for rework and the idea is to use a lot of it. so much that there is residue and you need to remove it.
The same goes for pen fluxes. Those are for rework. fluxes for wavesoldering come in 50 gallon drums ... not something to have on your workbench ...

in general :
when doing handwork you will need to clean the board unless no-clean is used
when doing reflow or wave correctly ( note the usage of the word correctly ) there will be no need to clean the board afterwards unless in special circumstances like medical or military stuff where they are extremely anal ( with perfectly fine reasons) .

if you care about the esthetical aspect : clean it up

every kind of flux has its own specific flux remover. some flux removers are broad spectrum. like chemtronics flux-off.
and flux remover need a LOT of remover to wash a board. simply squirting a tiny bit on does not work. it just leaves goopall over the board like a sticky film. you need ot squirt some on , let it soak for 10 seconds, scrub with brush then spray more on to wash the 'spent' stuff off. let the board drip out , spray once more.
apply warm air to dry. NEVER EVER blow air using your mouth . as the flux evaporates it cools down the board which pulls moisture out of the air. human breath contains saliva that is acidic .... you don't wan that on the board.
So : warm air is the way to go. use your reflow hot air gun.

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