Differences between different types of flux in solder?

[GoodCrossing]

I've been looking at this list of Multicore 60/40 rolls and I see there are many different types of what I presume is the flux inside the solder wire. I've cheched out the datasheet for the different types but I don't really understand it  , so what's the difference, and do I need to choose which one to get or will they all work for general soldering?

[IanB]

The best kind of flux to get for general soldering is probably "rosin, mildly activated" or "RMA".

Another alternative would be "no-clean" flux.

A kind of flux to avoid is "water soluble" as this flux needs to be washed off the board after you have finished soldering.

[GoodCrossing]

Whoops, I thought I had attached the picture of the different types 

https://imgur.com/a/Lzk6p0F

[helius]

Crystal is an activated rosin flux, with the different grades having different levels of halides. Halides are additives used to make brighter joints on oxidized metal surfaces: if you are soldering to new bright tin parts and new boards, you can use halide-free fluxes, while some halides make rework easier on old assemblies with oxidation or corrosion. Crystal 511 has 1.1% halides, Crystal 502 has 0.2%, and Crystal 400 has 0%. These would all be considered RMA fluxes (rosin, mildly activated), which don't necessarily need cleaning unless your process requires it.

362, 366, 311, and x39 are also activated rosin fluxes, which would be considered RMA or no-clean. They have various levels of activators, as described in the datasheets, for different application requirements.

The above would all be suitable for normal hand soldering and rework.

Hydro-X is water soluble ("organic") flux and should not be used unless you have extensive capabilities for cleaning and testing. The data sheet says that soldered assemblies need to be destructively tested for ionic contamination and the wash water conductivity monitored It seems to be made of citric acid in a glycerol and PEG carrier.
Arax is an acid flux, made of ammonium chloride and ammonium fluoride in a urea buffer. Like all acid fluxes it is unsuitable for electronics.

These ones would not be recommended.

[schmitt trigger]

I also agree that for a hobbyist, it is best to use a no-clean flux.

A washable flux, if washed incorrectly, can cause long term reliability issues.

Some people have reported that they have washed boards using  warm water with mild soap, scrubbing them with a clean toothbrush, and rinsing them in de-ionized water.
The caveat I see here is the different tap water qualities around the world. Some locations have very hard water which could leave residues. Also some soaps, which are mostly intended for human use, have lotions and moisturizers. Again, it depends where in the world you get that soap, as formulations will vary.

Therefore, stick with a no-clean flux.

[GoodCrossing]

Crystal is an activated rosin flux, with the different grades having different levels of halides. Halides are additives used to make brighter joints on oxidized metal surfaces: if you are soldering to new bright tin parts and new boards, you can use halide-free fluxes, while some halides make rework easier on old assemblies with oxidation or corrosion. Crystal 511 has 1.1% halides, Crystal 502 has 0.2%, and Crystal 400 has 0%. These would all be considered RMA fluxes (rosin, mildly activated), which don't necessarily need cleaning unless your process requires it.

362, 366, 311, and x39 are also activated rosin fluxes, which would be considered RMA or no-clean. They have various levels of activators, as described in the datasheets, for different application requirements.

The above would all be suitable for normal hand soldering and rework.

Hydro-X is water soluble ("organic") flux and should not be used unless you have extensive capabilities for cleaning and testing. The data sheet says that soldered assemblies need to be destructively tested for ionic contamination and the wash water conductivity monitored It seems to be made of citric acid in a glycerol and PEG carrier.
Arax is an acid flux, made of ammonium chloride and ammonium fluoride in a urea buffer. Like all acid fluxes it is unsuitable for electronics.

These ones would not be recommended.

Thank you! They didn't have 362, so I'll see if they have 311 and I'll go with that or Crystal 511.

I also agree that for a hobbyist, it is best to use a no-clean flux.

A washable flux, if washed incorrectly, can cause long term reliability issues.

Some people have reported that they have washed boards using  warm water with mild soap, scrubbing them with a clean toothbrush, and rinsing them in de-ionized water.
The caveat I see here is the different tap water qualities around the world. Some locations have very hard water which could leave residues. Also some soaps, which are mostly intended for human use, have lotions and moisturizers. Again, it depends where in the world you get that soap, as formulations will vary.

Therefore, stick with a no-clean flux.

I see, it's nice to know that!

[tooki]

Also note that water soluble fluxes cannot be used with stranded wire, period, no exceptions. (Since it wicks up under the insulation where it cannot be washed - or the wash water wicks it up!!) So yeah, ignore those for home use.

I’d go for the RMA over no-clean, insofar as they tend to be a bit more aggressive. The crystal 511 should be awesome. (Note that RMA fluxes don’t normally need cleaning, either, for most applications. A good RMA flux like the Crystal will be just fine without cleaning. I clean just because I like to!)

[KL27x]

^
I recall a random video I watched. Guy ran a special effect studio. It's a well viewed video, like maybe in the millions. They use plumber's flux (zinc chloride, water soluble) on stranded copper wire, and they clean with water. This is their ironclad protocol, developed over many years and strictly adhered to, working with pyrotechnics and explosives and hundreds of people coordinated for 10 seconds of filming where the thing better work when it's supposed to.

For probably the first 2 years after I fell down this grand rabbit hole, I used plumber's flux from the hardware store on all kind of circuits. Along with my Radio Shack fire brand. The only issue I ever had was with steel. When the flux contacts any kind of steel, it doesn't matter if it is stainless or not. The steel will start to corrode, later, despite thorough washing... eventually causing joint failure to something like a battery terminal. Now there is a tiny bit of steel in most smd component leads, but I never had any issue with anything but actual steel like in battery terminals or or spring contacts or the like. I still have some of the PCB's and breadboarded stuff I made from back then, and they're completely fine.

From my own experience, I'm pretty sure is the chloride/halide part of the water soluble flux that causes the rusting of steel. The halide salts embed in the surface of the steel and can't be washed away with water, despite they're water soluble. This is why if you use HCl to derust a part, it will usually rerust within a few days, despite you slather the steel or cast iron with oil or wax, after washing. HCl is fast and cheap and is used for removing rust/scale, but often you want to follow HCl bath with another type of halide-free acid to remove the halides in the surface layer. For this reason, plumber's or acid flux is not kind to soldering iron tips, either.

That said, +1 on rosin flux.

I also agree that for a hobbyist, it is best to use a no-clean flux.

Not correcting you. Just clarifying for the OP. Most no cleans will be fine for hobbyist pcb work and the like. But to be clear, if rosin weren't made from trees (if rosin flux were synthetic but worked exactly the same way), it would be classified as a no clean, too. So while some no clean fluxes are exactly the same as rosin flux as far as anyone here would be concerned, some no cleans are not.

[exe]

No-clean doesn't mean cleaning is not required. It can only be left on the pcb if fully activated, i.e., you follow the process described in datasheet. Many fluxes remain active if not heated. Such fluxes are for reflow and not suitable for hand soldering. Flux can also be highly conductive if not fully activated.

I suggest use an RMA flux. Test it for corrosivness and conductivity. Don't buy from China, it's not worth the risk. And don't trust youtube videos, many people presenting their favorite fluxes really have no idea what they are suggesting.

[tooki]

No-clean doesn't mean cleaning is not required. It can only be left on the pcb if fully activated, i.e., you follow the process described in datasheet. Many fluxes remain active if not heated. Such fluxes are for reflow and not suitable for hand soldering. Flux can also be highly conductive if not fully activated.

Can you elaborate on the differences between no-clean and RMA? Because in the ordinary sales materials, both no-clean and RMA are described as being safe to leave in most situations. I’ve struggled to find a good analysis and description of their differences in actual use.

I suggest use an RMA flux. Test it for corrosivness and conductivity. Don't buy from China, it's not worth the risk. And don't trust youtube videos, many people presenting their favorite fluxes really have no idea what they are suggesting.

No kidding!!!! What is it about soldering that causes people whose skills barely take them past “novice” to be compelled to make tutorial videos?? It seems to me that for every good soldering tutorial, there are 20 others showing how not to do it (but without realizing that they’re showing awful technique).

[exe]

No-clean doesn't mean cleaning is not required. It can only be left on the pcb if fully activated, i.e., you follow the process described in datasheet. Many fluxes remain active if not heated. Such fluxes are for reflow and not suitable for hand soldering. Flux can also be highly conductive if not fully activated.

Can you elaborate on the differences between no-clean and RMA? Because in the ordinary sales materials, both no-clean and RMA are described as being safe to leave in most situations. I’ve struggled to find a good analysis and description of their differences in actual use.

They need to describe these "situations". Too bad most fluxes I have do not define this. So, I do my own evaluation before using any flux.

So, concerning your question about the difference between no-clean and RMA. I don't know and I don't care because these terms do not reflect the real performance.

If I don't forget, I'll post in the evening a photo of how different fluxes behave with and without activation over time.

[exe]

I attached two pictures: one is after one month, and one after like 9 months (I'm too lazy to label fluxes on the first picture, sorry). All of these fluxes except two or three are "no clean". Each flux I tested in two ways: 1) I just applied it 2) I heated it with a heatgun for a while. These are laying on my bench for more than 7 months.

I didn't try to reproduce reflow process, so by no means my experiment represents quality of fluxes in production environment. But it is close-enough to how I normally use them. So, some fluxes are corrosive only when cold, some corrosive only after heating. Besides that, I did conductivity tests, but that's another story.

My personal conclusion: do not waste time trying to find "the best" flux as these are just too active and require cleaning for my use cases. I stick with rosin-based fluxes that are truly no-clean. Their performance is not great, but most of my parts are new and shiny, so they solder well even without any added flux (my solder wire has flux).

When I was a beginner, I wanted to have best tools, best flux, etc. But it was a waste of time and money. Now I'm trying to get things done. And 99% of time my cheap tools more than enough. Expensive fluxes like fl-22 or amtech won't make me more productive. Although, there are situation when you might want a special flux. E.g., reworking BGA stuff, it needs non-boiling no-clean flux. Or, I have a high-temperature flux I used to solder enamel wires.

[Teledog]

I concur with Exe.
Plain rosin is best.
Made my own liquid rosin for years..with finely ground instrument bow rosin  & 99% IPA.
Coat boards with it  & no corrosion ..even years later.
It IS hygroscopic, so if you do use it on a board, bag it for archive/storage or wash it with IPA/flux cleaner after soldering.
 

[GigaJoe]

I don't know ...   I left a flux couple time, then a circuit started to oscillate in Mhz range,  20-50 mV  ...   now I use acetone to clean it, rather pulling hairs ..

[exe]

I don't know ...   I left a flux couple time, then a circuit started to oscillate in Mhz range,  20-50 mV  ...   now I use acetone to clean it, rather pulling hairs ..

Yep, a typical flux story . Cleaning the flux is always a safe bet (except when there are components sensitive to cleaning agent). Also, precise analog circuits may be sensitive to leakage currents (esp. high-impedance inputs).

There is another catch: rosin solution in spirit is conductive until it dries. So, the circuit may not work properly immediately after soldering (never had problems with this myself, but noticed this behavior while experimenting with fluxes).

I have to do a series of (unprofessional) videos on fluxes in a month or so.