What flux is used in solder paste? Or how to salvage pasted boards that hardened

[Corporate666]

We stencil, place and reflow our own boards in-house.  Occasionally something happens and we'll have some boards that are pasted or partially placed that, for whatever reason, aren't reflowed.  Could be the P&P ran out of a needed part half way through a board, or reflow oven was down, or whatever.

Over time, the solder paste hardens to an something like concrete.  I've tried using off-the-shelf flux to apply it to the board and thin the hardened paste, but it doesn't seem to work too well.

Any ideas on what I could do to the boards to make them usable again?  I'm guessing it is just the solvent component of the flux that evaporates, so maybe soaking in IPA or something might help?  Or soaking in some sort of thin (or thinned with IPA) flux?  Or perhaps some kind of appropriate flux that could just be brushed or sprayed onto the boards before reflowing them?

[EvilGeniusSkis]

look up the MSDS for the solder paste, it will have the list of what is in the paste, from there you can figure out what solvent they are using, use that. If you had stated what solder paste you used I could be of more help

[ovnr]

Do you want to clean the boards completely to re-use them, only partially to place the missing components, or just reflow everything placed on them already?

I've found that most dried solder pastes re-liquify when heated to 50-150 degrees C. Then you can just wipe it off while hot. Alternatively, even if it is completely dried, you may very well be able to successfully reflow the components. There's still plenty of flux left, just no solvents.