Our thanks to ACL Staticide for allowing us to reprint the following.
There are a lot of myths and perhaps just misunderstandings about cleaning solder flux from printed circuit boards.
This article from ACL will hopefully address some of those primary myths and concerns regarding cleaning and PCB preparation for a variety of rework and repair operations.
Firstly, we clean and remove most flux residues in order to prevent metal oxidation and to prevent generally poor electronic circuit connections.
Often, technologists and technicians alike are advised to clean boards post soldering processes with isopropyl alcohol (i.e. IPA). There are proven chemical reasons for not using a hygroscopic solvent like IPA (hygroscopic: it draws moisture from the air to itself as an equilibrium mechanism) for basic board cleaning and further process preparation.
In most instances, IPA is too aggressive for most plastics and can intrude into parts and spaces creating undesirable effects. IPA is known to dissolve polystyrene and other types of plastic capacitors. By dissolving various materials, a potential low-ohm and conductive film is spread over the surface of the PCB while the flux itself (typically a very high solids material) is left untouched in a host of board surface areas, and in particular, at the solder joints. And, left over flux materials can be detrimentally conductive in many instances.
Now, briefly looking at flux materials:
Rosin based flux is generally not conductive unless it is baked on at extremely high temperatures (caramel or black in color). Generally, no-clean flux is just that. It does not always require cleaning and you may leave it on. Many low solids flux materials when applied under a correctly controlled process will completely evaporate with time. Most water soluble fluxes need to be removed simply because they leave behind a soap-scum like residue which is generally conductive. Additionally, there are some very aggressive fluxes like RA that must be removed as they contain very acidic substances. RMA is the most prevalent rosin flux (RMA rosin mildly activated and RA rosin (less) activated). It is actually not the rosin that performs the flux activity, but another substance in the flux compound which promotes spreading and adhesion. The
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