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Dear fellow users of Electronics StackExchange,

I'm in the process of trying to upgrade my electronics lab to be able to comfortably drag-solder dense SMD packages (SO's and PLCC's) and I'm wondering what flux paste or liquid to use. My part of the world (Europe) is specific in that electronics repair gear and materials seem pretty expensive, and the domestic brands seem different from the "U.S. and global golden standard" :-)

I've noticed some reasonable-looking flux pens in the shop of a friendly nearby distributor. The trouble is: there are two models and mine is the difficult freedom of choice! :-) Please help :-D

The dispenser pen by Stannol is probably a pretty standard job, but speaking about the liquids inside, I'm painfully undecided between the X33S-07i and the X32-10i. Any German hackers around, with practical advice? :-)

I mean are there any practical differences for my purpose? Does one drag better than the other? Does one smell worse than the other? Both are no-clean non-corrosive halide-free liquids, supposed to leave an alcoholic smell, and both pens cost the same. The X33 is also specified to be resin-free (synthetic? inorganic?), while the X32 does not have that note in the datasheet = apparently resin-based. As for "intended application" and practical properties, the datasheet texts are almost the same (except that they're not copy+paste).

Hmm. They look the same, they're supposed to smell the same, they cost the same, the declared application is the same... so I guess either will probably work fine for me :-) Any comments welcome.

I do not expeect to use the flux pen every day. Should I be worried that the solvent in the liquid dries out over time? Should I rather just get some paste in a can (a thick resin-based oily emulsion) and just work with that? :-) Note that Stannol have a "contact soldering paste" in a can, for electronics, that is not acid-based / corrosive...

References to further reading:

Gosh this whole post looks like a Stannol ad. In that context I'd like to declare that I am not in any way affiliated to Stannol. I'm just googling around for some information on Flux pastes/liquids and I find their papers pretty educational and well explained. Even if they don't go into sweet details about the chemistry of the various ingredients and additives, it's difficult to find such public information elsewhere. Should the moderators find my text "over the top", I welcome their wise verdict and action.

Note that Stannol is a traditional German brand and the papers have their originals in German. Thus, some cultural references may not ring a bell to English speakers around the world ("the one traditional solder paste in a blue can" etc).

===== EDIT =====

Based on the answer by Spehro Pefhany, I googled some more and found further interesting reading. Hence this edit.

...or maybe I should just say Google "RMA flux" :-)

And I also found other, cheaper local brands of flux pastes and liquids, which I'm now able to understand better even without a detailed "application information". Next I actually have to try some :-)

==== EDIT ====

In case someone was interested in a more thorough reading on various Flux ingredients and the chemistry background, there's a book called the Handbook of Lead-Free Solder Technology for Microelectronic Assemblies - the chemistry is concentrated in maybe 20 pages around page 400.

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  • \$\begingroup\$ You might want to single out your questions a bit better, not everyone will read the whole post because of it's length and a bit of a chatty nature. So condensing the questions for TL;DR comments might be helpful. Can't answer your questions though. \$\endgroup\$
    – Arsenal
    Apr 5, 2018 at 9:52

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Neither German, nor a 'hacker', but I wouldn't use either one of them.

They contain "no clean" type flux, which is okay for digital circuits but can cause issues with sensitive analog circuits. Also very hard to clean the (non-visible) residue when you have to clean.

I suggest RMA liquid flux if you can't find it in pen form. There may be some refillable pens available that you can put whatever flux you like into them.

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  • \$\begingroup\$ Thanks for the recommendation :-) I can see RMA04 also from Stannol within my reach, in a syringe apparently. At least twice as expensive as the "no clean" pens for the same qty of the stuff, but you've probably just told me the reason. \$\endgroup\$
    – frr
    Apr 5, 2018 at 14:26
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Looking back, I see that my final purchases were yet very different from the products debated in my original question :-) so I guess I might as well formulate this as an answer...

Where I live, there's an important local distributor called TME (operating across the EU from Poland). From them, I know a Polish brand of chemistry products for Electronics - called Termopasty. And, a few days after asking my question here, I purchased a few samples of their products in TME. Since then, a small set of their fluxes has become a stable, indispensable part of our two soldering workplaces :-) I have no interest in selling their products, I'm just a happy customer - so if I'm not yet across the line here with inappropriate advertising, let me mention a few particular products:

The TK-83 is a basic brownish RMA flux (liquid). The label says "no clean", but realistically this one is probably not so bad, leaves the classic rosin residue that can be easily cleaned by IsoPropyl-Alcohol ("IPA"). I use this for general soldering work in electronics, where I don't need extreme tin dragging capability and I don't mind a bit of rosin residue. This is my flux of first choice. It works slightly better than a homebrew mix of rosin + IPA + Glycerine.

The RF800 is a slightly more refined RMA flux (liquid), marketed namely for SMD work. I have used this successfully for drag soldering. And although it says "no clean", it clearly does leave a bit of transparent residue that can be cleaned by IPA.

Then there is an agressive, inorganic flux called the TS-81 (liquid) - which makes tin stick to steel, nickel, and some other difficult to solder materials (the label says acidproof steel). By the smell, it contains or releases a small amount of hydrochloric acid, so it's not at all pleasant to inhale - but I only use this on special occasions. Doing "mechanical" soldering to steel nuts or bolts, or a really special application: this liquid is priceless to soak your dull oxidized soldering iron tips with tin! I have a coin-sized piece of sheet iron (I believe scavenged from a tin can), I put a small droplet of the TS-81 on the small metal sheet, prepare solder wire and touch the droplet with the dull tip. Roll the tip around a bit, in the drop of molten tin and flux, caressing the metal sheet - Lo and behold, the tip is again perfectly soaked with tin. Some people say that tips can inherently wear off in a single day of busy work, and are just no good afterwards... and perhaps this chemistry is not doing the tip surface the best possible service, eating away some constituent metal out of the steel alloy or something... but ahh well: in terms of prolonging the tip's service life, it works best of anything I've tried! It is especially useful for the hollow desoldering tips.

Each of these liquid fluxes is sold in several different packages - including a flux pen in the case of the TK-83. I tend to buy them all in the smaller bottle available, which comes with a neat brush on a stick, mounted in the bottle cap. The downside of these bottles is that they're easy to tip over, inadvertently. To prevent this, we've invented simple foolproof holders, made of heavy left-over pieces of aluminum profiles that we have lying around. Just something that rests flat on the table, while holding the tiny bottle upright.

Apart from the liquids, Termopasty also have a gel flux for SMD work. I've purchased one syringe, it does work as advertised, but in the end I mostly work with the liquids in the workshop, and the syringe is lodged in the "travel kit", forever waiting for its true opportunity :-) So unless this is your particular preference, you probably don't need the gel if you have the liquids...

Then there's another gel, sold in a can, with a chemistry optimized for coarse tinsmith work (rain gutters, buckets, maybe copper heating pipes - not sure). I have this in my "blacksmith toolbox", it works, but I hardly ever do that kind of work...

Termopasty also have two solder pastes, with and without lead, containing powdered solder - for SMD hot air soldering. So far I've never had a need for this = can't serve as a referee here...

Speaking of reviving a soldering tip, there's a product called Tippy by Stannol - neat, powedered solder mixed with solid flux, but realistically the aggressive TS-81 flux is more capable :-) if you don't mind the fumes...

So... these are some products relevant to me, in my eastern-leaning corner of Europe :-) Reasonably priced, and available within 2-3 days of placing an order, from a shop where we nowadays order stuff every other day...

P.S.: get some brass shavings to clean the tip of the coarse stuff. Some say that this is more sensitive to the tip than a wet sponge. The thermal shock of a wet sponge reportedly shortens the life of the tip. And after all, the brass shavings actually work at least as well as the sponge...

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