Electronics > Repair
Restoring dried solder flux paste
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SilverSolder:

Cheapskate project last night, that worked so well I thought it might appeal to other cheapskates out there!   :-DD

I have a pot of rosin flux paste that had dried out to the point of becoming a little difficult to use.  I don't use it often, but when I do, I want it to work...

Rosin flux paste is thinned at the flux factory with chemicals that are a little difficult or expensive to buy in small quantities, e.g. diethylene glycol dibutyl ether and other esoteric compounds.

In the past, I have tried to make flux paste thinner with isopropyl alcohol and other thinners, but the ones I tried were never really satisfying and in any case always ended up evaporating relatively quickly, leaving the flux just as hard as before.

Then it struck me:  why not use a liquid rosin flux as a solvent for the partially dried paste?  The idea being that the thin flux already has an appropriate type of rosin solvent in it...

So, while my wife was not looking, I went to work in the kitchen...  I added a small amount of the liquid flux to the pot (one or two ml),  then heated the mixture in the microwave oven for about 15 seconds, enough to melt everything - stirred thoroughly.  Let sit overnight to solidify.

The result is awesome -  soft, supple, beautiful flux!   A miracle cure - absolutely worth the effort.  8)



shakalnokturn:
Beware heating things in the microwave, the right stuff in the wrong conditions is a recipe for having the door blown open.

 What would be in the liquid rosin paste to keep it liquid?

I break small chips off the solid rosin, dissolve it in acetone and keep that in a small syringe for use. The most common use being running the needle over the four sides of TQFP's.
SilverSolder:

--- Quote from: shakalnokturn on May 01, 2020, 02:53:23 pm ---Beware heating things in the microwave, the right stuff in the wrong conditions is a recipe for having the door blown open.

--- End quote ---

The flux paste melts like wax, at a fairly low temperature.  It did not start to smell or anything.  Obviously we are talking seconds in the microwave - it would definitely not be a good idea to heat this up too much.


--- Quote --- What would be in the liquid rosin paste to keep it liquid?

--- End quote ---

2-Butanol and Ethanol, about 25% of the total product of each.  The "secret ingredient" is the 2-Butanol which will not evaporate as fast as alcohols.


--- Quote ---I break small chips off the solid rosin, dissolve it in acetone and keep that in a small syringe for use. The most common use being running the needle over the four sides of TQFP's.

--- End quote ---

That's basically how I use the liquid product in the picture.  It is really good, and now I have another use for it, every couple of years hopefully!
David Hess:
I have also restored paste flux using a microwave oven for heating.  For a solvent, I have had good results with acetone and toluene.  50/50 toluene and isopropyl alcohol is a common rosin flux thinner and cleaner.
rubidium:
Completely agree with the 50/50 2-butanol (AKA sec-butanol) / ethanol mixture. It's more than likely this was the original solvent mixture in the paste.
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