Electronics > Repair
Restoring dried solder flux paste
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jpanhalt:
I am in the same shape as both you guys are with a tub of #44.  It is at least 25 years old.  I just use it and avoid the crystals.  As I recall, I once put it in the microwave or more likely used a heat gun for a short time to remelt the surface -- long enough ago I don't remember .  It still works.  I would not add additional solvents to it unless as a last resort.

Maybe by the weekend, if we are not snowed in, I will have something to report.
T3sl4co1l:
FYI, I tried melting some rosin into (very old) paste flux of that sort of composition (sublimates acicular crystals); it seems it sunk to the bottom, and remains brittle and gummy.  I didn't try much mixing, just stirring.  Hard to see if it mixed much, everything is pretty brown and opaque...

Perhaps the commercial product is an emulsion, but it doesn't seem to be a solution anyway, if the rosin isn't chemically altered.

I wonder which alterations would do.  Oxidation of unsaturated bonds?  Cleavage of esters?  And what is "activation" anyway?  (I was poking around a little earlier, and found one 1960s patent using light alkyl polybromides -- yikes!)

Tim
helius:
I think hydrogenation of abietic acid esters is more common than oxidation. Those fluxes would then be categorized as "REsin".

As you found, activators are frequently halides, but not always. Some recent formulas are "halide and halogen free" so their activators are in another class, such as citrates, imides or (pyro)phosphates. They are there to react with oxides and slightly etch the metal surface so solder can wet it better.

The alkyl bromides might be scary from a toxicity standpoint, but the main concern is to avoid corrosion or metal migration in the assembled product. Having the halide ion bound up in an organic molecule makes it less likely to migrate and react with copper.
T3sl4co1l:
Indeed. :) In the same patent, they detailed the tests used to qualify formulations -- some MIL and ASTM standards I think they were, where a copper sheet is thoroughly cleaned and buffed (with steel wool), floated on a lead bath (not tinned, at least not thoroughly!) and left to oxidize in air for an hour at 400C, or a minute at 500.  They say the coating is fractional microns of cuprous oxide, giving it a delightful flat brown appearance.  Seemingly even more impressive is using the solder on it, getting significant wetting (measured by how far a blob of solder and rosin, I seem to have missed how much of each -- spreads out).  I know the fluxes I have here, and my patience level, well enough to get out the sandpaper or steel wool if I saw a surface like that. ;D

They also describe a corrosion test, which is vacuum deposited copper on glass: very thin, so when the rosin is cooked on it (how hot and for how long, was garbled, I was just reading the OCR..), if the copper oxidizes much due to air or rosin, it will disappear pretty much instantly; and the affected area is measured optically.

Hmm, hydrogenation should make the "resin" more lipophilic than alcohol-philic (heh, I don't think there's really such a thing as that, it would just be plain hydrophilicity, wouldn't it), that may be why it doesn't mix with petroleum jelly.

Citrates would be like partial esters, I suppose?  Alkylated to get them in solution, but not fully so there's free acids to be active (and the oxygen-studded citrate will be more active than the chunky abietic acid is); and pyrophosphates would be the same idea?  And imides like the uh, whatever the OSP stuff is, probably?  Yeah, I can see that.  Some of those might be pretty easily accessible to the home chemist; hydrogenation a bit less so.  (I imagine Pd:C or such should do the job, and H2 gas is easy enough to generate on demand -- it's one of the easier organic reactions to perform -- but it's a reaction (and subsequent workup, augh, filtering catalyst from sticky sticky resin solution? :P ) nonetheless, not just mixing raw materials together.)

Tim
SilverSolder:

Who knew flux was so complicated!  :D

Presumably the recipes for some old school fluxes are more realistic to execute at home?
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