Wondering what kind of solvent is actually used in leaded (or lead-free) solder paste.
I'm currently using the liquid from a flux pen if the paste gets too dry for a stencil.
But that's a pain to get out, and it would be easier to have it in a bottle, with dropper.
Can't find something like that on ebay.
Leo..
I have used IPA 99.9% with some success.
If the paste gets too dry for a stencil, I would say you're storing it improperly, or it's just too old.
Solder paste is cheap. Not worth futzing around with solder paste that doesn't work right, and enduring the reduced yield and time wasted fixing boards that didn't solder right when you reflowed it.
I used to scrape the solder paste left on the spatula back into the jar and use next time. Since I stopped trying to save it, the my failure rate (in terms of boards that require rework to fix soldering problems after reflow) has gone down to almost 0.
I have use(d) IPA 99.9% with some success.
Somebody at work asked me if I had any IPA. I said, "beer?"
I sometimes do panelized boards with ten 28-pin MCUs and a bunch of other chips and 0805 passives.
Can't afford any mistakes there. 'Fixing' bridges or bad reflowing is not an option.
If the solvent is IPA, and only that evaporates, then I could give it a try.
Want to make sure though before using that instead of the fluxpen.
Leo..
Not contaminating and keeping in a cooler (not the same one as food or beer 
keeps it fresh longer. I hate those containers they come in and I dump mine into little Tupperware where it’s easy to get to. IPA is a great thinner and cleaner. We have dunked boards into IPA to clean flux on boards.
DrAzzy:
If the paste gets too dry for a stencil, I would say you're storing it improperly, or it's just too old.
I find even a new jar sometimes just a bit too dry for fine work,
or it's a bit too cold in my workshop (winter here).
And yes, it's stored in the fridge, and used at room temp.
Leo..
Kester makes a full line of flux thinners. The safety data sheets show that most are isopropanol with other ingredients like methanol and rosin, depending upon the formula. Just google “kester flux thinner” for a list. Only catch is a gallon is their smallest size.
I have a bottle of liquid flux, and sometimes the paste needs few drops of flux mixed in to get to squeeze out of a syringe nice again.
I don't return solder paste to the jar after it's been dragged across stencils.
I disagree about it being cheap. A 150g tub is $43.50 plus shipping. I had a 500g before that, was really surprised when the 150 showed up and I was expecting 500, checked to see that 500g was no longer offered (cmlsupply.com).
Wawa:
Wondering what kind of solvent is actually used in leaded (or lead-free) solder paste.
I'm currently using the liquid from a flux pen if the paste gets too dry for a stencil.
But that's a pain to get out, and it would be easier to have it in a bottle, with dropper.
Can't find something like that on ebay.
Leo..
Have you tried a syringe ? Or an other way could be an empty bottle of some eye drop medicine ? The last have dropper.
I sometimes use an air compressor/syringe/dispenser setup, which needs the right consistency paste to work.
But the real question was what type of solvent to use when the paste gets too dry.
This might only happen once for a jar/syringe of paste (will not likely be repeated).
I gather that IPA instead of the pen liquid could also work.
Leo..