Restoring dried solder flux paste

[SilverSolder]

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

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. 

[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]

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

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.

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

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.

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.

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.

[jackthomson43]

50/50 always does the job .....  https://www.theengineeringprojects.com/2018/03/introduction-to-microcontrollers.html

[SilverSolder]

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.

I believe the original paste used diethylene glycol dibutyl ether as its solvent, but the sec-butanol in the thin flux works very well as an "additional solvent" even in very small quantities.

[SilverSolder]

50/50 always does the job ..... 

Yes, especially for a gin & tonic!

[ryan780]

As a fellow cheapskate, I just wanted to say thank you.  I don't know why it never occurred to me to use liquid flux to rejuvenated this tub of paste.  My paste wasn't quite as far gone as yours it sounds like so all I had to do was squeeze in the liquid flux and then stir with a stir stick.  Done!  I found this via a simple Google search and this was the first link that popped up.  I so rarely find searching for anything provides real answers anymore.  Instead the first 5 results are usually just links to buy things.  But when I found this on the first try I just had to register so I could say THANK YOU!  You've saved me having to spend money on a new tub of flux and you've made my Saturday.   

[T3sl4co1l]

Ethyl acetate (can be found in nail polish remover? not sure about pure sources though) also comes to mind, having a higher boiling point.

Turpentine is the OG solvent: it's the volatile fraction in natural pine resin (colophony) which gives it its rich piney smell.  It has a high enough boiling point that the sap stays just a little gooey and runs over the wound, but still volatile enough that, soon enough a crust forms, then eventually the deposit hardens.  100% organic!

Note, colloquial "turpentine" is more often mineral spirits (petroleum distillate, plain old kerosene).  Probably not a great idea.  Though I suppose any combination is about as flammable?  Guess it's worth a try.  Probably something with a slightly higher boiling point would be nice, so it doesn't bubble and fume so much when the soldering iron hits.

Tim

[SilverSolder]

As a fellow cheapskate, I just wanted to say thank you.  I don't know why it never occurred to me to use liquid flux to rejuvenated this tub of paste.  My paste wasn't quite as far gone as yours it sounds like so all I had to do was squeeze in the liquid flux and then stir with a stir stick.  Done!  I found this via a simple Google search and this was the first link that popped up.  I so rarely find searching for anything provides real answers anymore.  Instead the first 5 results are usually just links to buy things.  But when I found this on the first try I just had to register so I could say THANK YOU!  You've saved me having to spend money on a new tub of flux and you've made my Saturday.   

Welcome to the EEVblog, where we don't just use our solder flux - we take it apart!   

I actually used the "restored" flux (discussed in the original post) just a couple of days ago, and it still works like new - this is a durable fix!

[uli12us]

The tacky flux in syringes contain petroleum jelly as well. Unfortunately the fraction of it is a secret. I think a third of the weight can not be bad. So you have 1 part rosin, 1part IPA or other alcohol and 1 part petroleum jelly.

[Alex Eisenhut]

That's not cheap. Cheap is getting the rosin from a tree... Just need to pick some bark out of it and you're good. I actually cleaned some tips with it. My Dad has several leaky pine trees in his yard...

[T3sl4co1l]

Heh, way ahead of you, I actually picked some like a month ago on a walk...  Dissolved in acetone, filtered (ooh it's so sticky, be careful!), let to dry in a warm place.  Such a lovely amber-brown brew... not very tasty, even if you're into those really resinous-piney IPAs.

The turpentine fraction dries out very slowly indeed, and diffuses slowly, hence it skins over... right now I've got the container sitting above the stove pilot light, it's pretty toasty, but not enough to full melt.  It crusts over, leaving a liquid center.  Tempted to just put it in the oven and get it nice and cooked.  If I do, it'll smell like a saw mill in here...

As it is, yeah, it definitely works, but it's not as good as a proper blend.  The turpentine fraction evaporates rapidly on soldering heat, bubbling and melting everywhere, releasing nostril-burning fumes of turpentine and assorted light rosin compounds.

In a pinch?  Definitely recommended over nothing at all.

Tim

[Ian.M]

Let someone else boil out the turpentine!  In many less developed countries you can find good quality pale lump rosin at a street market spice stall.  In developed countries, you can usually get it affordably online - avoid 'violin rosin' as it may have additives undesirable in flux.  If the country isn't 'dry' you can probably also get 95% pure ethanol, though you may have to settle for denatured alcohol.   Provided you let the mixed flux settle out and decant it to remove undissolved solids, detritus that was embedded in the rosin, and any aqueous layer, the resulting liquid flux is comparable to commercial ROL0 (unactivated rosin 'R') flux.

[Alex Eisenhut]

And if you get enough of it it's time to boil some potatoes in it.

[SilverSolder]

I didn't have very good results thinning flux paste with either isopropyl alcohol or "regular" alcohol -  the product became grainy and opaque, and the alcohol evaporates out of the mix relatively quickly, leaving you back where you started (paste too thick) in short order.

The liquid fluxes made by the 'Big Flux' companies contain different/additional chemicals that provides desirable properties in addition to thinning the flux, such as longevity and (hopefully) safety!   And it works so well for thinning paste:  a few ml to do a whole tin.

[David Hess]

I didn't have very good results thinning flux paste with either isopropyl alcohol or "regular" alcohol -  the product became grainy and opaque, and the alcohol evaporates out of the mix relatively quickly, leaving you back where you started (paste too thick) in short order.

The liquid fluxes made by the 'Big Flux' companies contain different/additional chemicals that provides desirable properties in addition to thinning the flux, such as longevity and (hopefully) safety!   And it works so well for thinning paste:  a few ml to do a whole tin.

Ethylene glycol comes to mind as an additive but I do not remember if that is what they use.  The flux thinner I have commonly seen for liquid fluxes is 50/50 isopropyl alcohol and toluene.

Since Kester no longer provides flux or other chemicals in smaller packages, I would not mind learning how to process raw rosin into RA and RMA flux.

[SilverSolder]

I didn't have very good results thinning flux paste with either isopropyl alcohol or "regular" alcohol -  the product became grainy and opaque, and the alcohol evaporates out of the mix relatively quickly, leaving you back where you started (paste too thick) in short order.

The liquid fluxes made by the 'Big Flux' companies contain different/additional chemicals that provides desirable properties in addition to thinning the flux, such as longevity and (hopefully) safety!   And it works so well for thinning paste:  a few ml to do a whole tin.

Ethylene glycol comes to mind as an additive but I do not remember if that is what they use.  The flux thinner I have commonly seen for liquid fluxes is 50/50 isopropyl alcohol and toluene.

Since Kester no longer provides flux or other chemicals in smaller packages, I would not mind learning how to process raw rosin into RA and RMA flux.

The MG Chemicals product uses 2-Butanol and Ethanol, it seems to me the 2-Butanol is the "secret sauce" to getting a professional result.  I wonder if you could just dissolve raw rosin in the MG Chemicals' liquid flux, to thicken it up?

[jpanhalt]

I have used a lot of butanol in my days and can't stand its smell.

There are many alternative solvents with little or no smell:
1) Glycol ethers (https://en.wikipedia.org/wiki/Glycol_ethers)
2) Low molecular weight Cellosolves (e.g., ethyl or methyl cellosolve)
3) Diacetone alcohol, which is odorless to most people.

Of those three, diacetone alcohol is probably the weakest solvent, but it may still work.  It is used in "whiteboard" markers because it's a decent solvent and doesn't have a strong "chemical" smell that some people object to.

[tooki]

The tacky flux in syringes contain petroleum jelly as well. Unfortunately the fraction of it is a secret. I think a third of the weight can not be bad. So you have 1 part rosin, 1part IPA or other alcohol and 1 part petroleum jelly.

There's more to it. (Though I don't know what it is.) I tried some experiments with making flux paste from rosin, and nothing I did ever resulted in it actually mixing with the petroleum jelly. With alcohol, it just formed a separate liquid layer. Without alcohol, the rosin could be melted, but always separated back out, so just rosin particles suspended in grease. Awful to solder with.

Various commercial fluxes I've looked at use processed rosin (polymerized or various other processes I've forgotten). So I don't think they're actually just mixing in raw rosin.

[jpanhalt]

Kester (MSDS, SDS) shows that its rosin is 40% to 55% rosin ( CAS:8050-09-07) with an empirical formula of C19H29COOH and isopropy alcohol 55% to 70% (IPA).  The rosin is soluble in "alcohol," benzene, and ether.  "Alcohol" probably refers to ethanol (absolute).

If one searches on that CAS number you may get two different structural formulas (see here: https://www.sigmaaldrich.com/catalog/search?term=8050-09-7&interface=CAS%20No.&N=0&mode=partialmax&lang=en&region=US&focus=product). Of the two, only abietic acid has the empirical formula of C19H29COOH.  Moreover, it is commonly said that rosin flux is mostly abietic acid.

Wikipedia lists the solubility of abietic acid as very soluble in ethanol, acetone, diethyl ether, and petroleum ether.  The latter is a distillation product from petroleum and is mostly aliphatic hydrocarbons, but can contain other hydrocarbons as impurities.

Whether it is soluble in petroleum jelly is not clear.  Rate of dissolution is temperature dependent.  If heated above abietic acid's melting point, it may well dissolve or form a cloudy mixture. I know you can dissolve it in paraffin wax that way. (I used it as a vacuum sealer in high school.)

I suspect any relatively short chain alcohol or any of the substitutes* mentioned earlier will dissolve it.  Kester does list the principle component of its flux thinners as isopropyl alcohol, e.g,. 85% to 100%.

Since abietic acid is insoluble in water, I'd expect its solubility in "alcohol" to be highly dependent on whether the alcohol was absolute or had water in it.  For example, absolute ethanol is an excellent solvent for fats, but 95% ethanol is not.  Same goes for IPA.

*Even with a 10% to 30% water, cellosolves will dissolve rosin flux.

[SilverSolder]

[...] For example, absolute ethanol is an excellent solvent for fats, but 95% ethanol is not.  Same goes for IPA. [...]

That is very interesting, wonder why that is?  Do the water molecules somehow block the alcohol molecules by being more attracted to them than fat, or something like that?

[tooki]

Kester (MSDS, SDS) shows that its rosin is 40% to 55% rosin ( CAS:8050-09-07) with an empirical formula of C19H29COOH and isopropy alcohol 55% to 70% (IPA).  The rosin is soluble in "alcohol," benzene, and ether.  "Alcohol" probably refers to ethanol (absolute).

If one searches on that CAS number you may get two different structural formulas (see here: https://www.sigmaaldrich.com/catalog/search?term=8050-09-7&interface=CAS%20No.&N=0&mode=partialmax&lang=en&region=US&focus=product). Of the two, only abietic acid has the empirical formula of C19H29COOH.  Moreover, it is commonly said that rosin flux is mostly abietic acid.

Wikipedia lists the solubility of abietic acid as very soluble in ethanol, acetone, diethyl ether, and petroleum ether.  The latter is a distillation product from petroleum and is mostly aliphatic hydrocarbons, but can contain other hydrocarbons as impurities.

Whether it is soluble in petroleum jelly is not clear.  Rate of dissolution is temperature dependent.  If heated above abietic acid's melting point, it may well dissolve or form a cloudy mixture. I know you can dissolve it in paraffin wax that way. (I used it as a vacuum sealer in high school.)

I suspect any relatively short chain alcohol or any of the substitutes* mentioned earlier will dissolve it.  Kester does list the principle component of its flux thinners as isopropyl alcohol, e.g,. 85% to 100%.

Since abietic acid is insoluble in water, I'd expect its solubility in "alcohol" to be highly dependent on whether the alcohol was absolute or had water in it.  For example, absolute ethanol is an excellent solvent for fats, but 95% ethanol is not.  Same goes for IPA.

*Even with a 10% to 30% water, cellosolves will dissolve rosin flux.

We all know it dissolves in most solvents.

It definitely does not readily dissolve in petroleum jelly. Again, I've tried. Both with and without the help of alcohol (>99% IPA, specifically). I melted it to the point the rosin was fully liquid (and the petroleum jelly, too, of course, since its melting point is far lower). Oddly, the wiki article on abietic acid lists two wildly different melting points: "as low as 85 °C" in the body text, "172–175 °C" in the infobox, each with a citation. I'm pretty sure I exceeded 85 °C, but no way did I reach 172 °C.

When melted, they'd appear to mix, but as it cooled, it separated into two distinct phases.

 It might be interesting to try dissolving it with a hydrocarbon solvent and petroleum jelly, since I would expect each of those to be miscible with each other.

[jpanhalt]

Even pure substances can crystalize or form glasses from "solutions" of themselves.  We "all know that."  For example, you can purify hexadecane (cetane, a major component of diesel fuel) simply by crystallization from liquid hexadecane.  I would not necessarily assume petroleum jelly and rosin are miscible at all temperatures.  If the rosin precipitates, increase the amount of petroleum jelly, or reverse the ratios (i.e., more rosin, less petroleum jelly).  Maybe that is the reason the composition is a secret.