Which flux should I buy?
So I found myself asking once I needed to desolder components from an overcomplicated PCB to verify my simpler design would work.
Little did I know what I was getting myself into...
Let's begin with a few terms you'll see when looking for flux on Digikey: Rosin (R), No Clean (NC), Rosin Mildly Activated (RMA), Rosin Activated (RA), and Water Soluble (WS).
And first, the...
Rosin-based fluxes (R, RMA, RA), comprise rosin, solvent, and activator (optionally).
Each of the three elements of Rosin Based flux has a function. We'll start with rosin.
Rosin is the substance that has the "wetting" property. When it makes contact with the mix of metals which compose solder, the metals flow like a liquid. Why? Don't ask me.
Rosin is a natural substance, you make it by just boiling pine sap (resin).
The only problem is, it's a solid, toffee-like thing on its own. String players apply it to their horsehair bows to make them stickier, gripping the strings better. Like in Devil went down to Georgia: Johnny, rosin up your bow and play your fiddle hard / 'Cause Hell's broke loose in Georgia and the Devil deals the cards.
So how do you apply the rosin to the solder to get it to flow? Well, you could just set the things against each other and heat. Rosin turns into a viscous, molasses-like form when it is heated. And in fact just sticking them together is exactly what's done with rosin-core solder.
The other option is to put the block of rosin in a liquid and have it dissolve. This special liquid is the solvent. It's just an alcohol.
At this point, you have a flux. Rosin (non-activated) (R) flux to be particular. You can buy it, the product category on Digikey is Flux - Rosin Non-Activated, Liquid. There's only one product in this category on Digikey at the moment, being FLS912-1G made by SRA.
However, if you tried to use it to apply some solder to a metal pad, you might be disappointed. The datasheet recommends it for use with copper, nickel, or even brass. But from looking at Nordson's solderability matrix (which I will discuss more later), I can see that you might have some with bronze, and definitely would with a steel.
What might happen is even though the solder flows (that is, becomes liquid, it would not stick to the metal pad. It might ball up on top or something. The problem is that there forms a thin layer of "rust" (oxidation) on the target metal which keeps the liquid solder from making contact with the target metal.
This is where the third ingredient comes in, the activator. It is an acid which dissolves the oxidation layer. Add it to the mixture and you have activated flux. Depending on how much you add, you might be left with Rosin Mildly Activated (RMA) or Rosin Activated (RA) flux.
The more activated the flux, the wider variety of finishes (the metal on the pad) you can solder on to (and the more damage to the board is done if the residue is not cleaned properly). There's a table in the explainer from Nordson linked in the sources which shows what metals require what level of activation. The more resilient the oxidation, the higher activity required. For example, stainless steel is resistant to all the rosin fluxes. It forms a small layer of oxidation during the "passivation" process. For stainless steel you need one of...
There are other ways to make fluxes, but the principle is the same. There is always some sort of activator which breaks down the oxidation, and there is something that has a wetting property. When you use a flux from one of the other categories: resin, organic acid, or inorganic, there is something doing each of these actions.
And they basically increase in activity level. Resin is similar activity to rosin generally. For stainless steel, maybe you could get away with an organic acid flux (as seen in that Nordson table), but probably an inorganic flux would be better, as you can get even higher activity with them. Organic acid flux can have a wide variety of activity levels. And the inorganic flux is usually quite high.
In fact there is a system for classifying fluxes based on their construction and activity. J-STD-004. This expert report from Almit explains the system better than I could, so I will just link to it here. That's an archived version in case the original ever disappears, the original is linked in sources.
You can see a typical line-up of products and their J-STD-004 classifications from SRA here.
The two terms from the beginning which are yet unexplained fit in that framework in this way:
A flux's classification in this system is always given in the datasheet, in my experience.
The chemical mixture, base, solvent, and activator, is put in a carrier, which determines the viscosity of the flux.
At a certain viscosity, the flux goes from liquid to tacky. Tacky flux goes by other names: gel, jelly, even "paste", not to be confused with solder paste.
Liquid flux comes in pens, bottles with droppers, and jugs, and it's used for soldering when the board can be flat so that the solder doesn't run off. You can also buy liquid flux for wave soldering, a method of automatically soldering THT components.
Tacky flux comes primarily syringes and tubes, but when it comes in a jar it's called "paste". It's used for soldering at odd angles, or even on a flat board if you just want the flux to stay still.
There are more physical properties that the Wikipedia page gives. Their whole list is: Activity, Corrosivity, Cleanability, Residue tack (nothing to do with "tacky flux", which refers to viscosity), Volatility, Viscosity, Flammability, Solids, Conductivity.
However we've already covered the main properties which are important for someone trying to buy a single syringe of tack flux off of Digikey for some simple rework. And so much more, goodness, how did we get here...
Which seems like a good place to wrap this up. I'll leave you with a nice YouTube video I found where a guy makes his own flux. It seems a little more powerful than anything I would need though...
Sources:
https://www.nordson.com/en/divisions/efd/resources/solder-selection-guide
https://en.wikipedia.org/wiki/Flux_(metallurgy)
https://en.wikipedia.org/wiki/Rosin
https://en.wiktionary.org/wiki/rosin
https://www.almit.de/export-reports/pdf-en/FLux_types_in_accordance_with_IPC-J-STD-004.pdf
https://www.tokimeku.com/blogs/news/different-types-of-flux
Please send comments to blogger-jack@pearson.onl.