What Is Flux? | Soldering

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Flux is a very important part of soldering. Flux is necessary to reduce the oxides that tend to form whenever you have hot metals in contact with the air.

In electronics, we use a rosin-core solder or a mild solder or water-soluble solder. Make sure that you choose something that's not plumbing flux or acid-core flux, because that will tend to corrode the parts over time. It's okay for pipes but not for electronics.

So in this case, we're looking at rosin solder flux. It comes in a paste form. If the temperature's a little bit colder, it will tend to crystallize and solidify, but it can be melted with a little bit of heat from the soldering iron.

Now, when you heat flux it tends to produce smoke, so it's good to have some ventilation or some kind of vapor extractor to be sure that you're not breathing in a lot of the caustic vapors from the flux. What the flux does, when you heat a workpiece, is it reacts with these metal oxides that are forming with the temperature and exposure to the air, that tend to interfere with the metal-forming process of soldering. So this chemically reacts with these oxides, and makes for a nice, clean metal that can then form a nice, perfect alloy.

I should mention that many solders come with flux in the core. For example, this is flux-core solder. It's actually hollow, and inside there's a central core of flux. Then as you feed the solder into your workpiece, the flux then distributes itself over the surface of the work, and does its action in reducing those metal oxides, preventing them from occurring as you're doing the soldering. It happens to be at the right moment in time and place, so having a flux-core solder is often very, very helpful.

And finally, we should talk about water-soluble flux, which is a recent improvement in flux. With water-soluble flux, cleaning becomes much, much easier. With rosin-core flux, the flux is not water-soluble. It takes some kind of solvent in order to dissolve it and to remove it from the work piece. Solvents such as trichloroethylene and other toxic solvents must be used.

Water-soluble flux is definitely easier. It may not be appropriate in all circumstances. It can leave a residual layer that may interfere with high-impedance circuits, where you have resistances of ten megaohms or greater. The flux may actually lower that resistance and cause the circuit to malfunction. And on those cases you'd want to use a rosin-core solder flux. But in most cases, the water-soluble flux is the better choice. It's environmentally friendly and it's easier to clean when you're finished.

[drewgormley]

If you're soldering WITHOUT flux, you'll find it hard to get a good clean quick bond. 

IF you use flux, you'll get a nearly instant clean joint, right away because it allows the solder to bond rapidly because there is no interfering oxidation. 

Use flux to pre tin your work areas. Joints are nearly instant once temperature is reached with flux and pre tinned work. 

The only downside is you have to clean your work after, which should be standard practice anyway. 

[lurkerrekrul]

I always used to say that flux was what made solder stick to whatever you were trying to solder. And someone would always correct me and say that it "cleans" the parts. Regardless of the actual mechanism at work, with flux, solder flows onto and sticks to the parts. Without it, the solder would rather roll up into a ball and stick to the iron. So from the layman's perspective, flux is the magic paste that makes solder stick to things.

[tannerslice]

I've never soldered before in my life. All I wanted to know was how to use flux. I still don't.

[haydo]

Okay, I just finished eating the flux, is that all that is required?

[NilsAlbertsson]

I watched this video, and now actually know less about flux than before!

[hyqhyp]

As little information as possible with as many words as possible.

[MrMatt]

If you have a base idea of what Flux is, this video just further explains. Flux is used in welding also, so I had an idea of what it does. It is used as a "barrier" between the work piece and the air, so the air won't negatively react with the work piece, keeping it free of defects and oxidation.

[MaryStewart]

lols at the "sodder" vs "solder" argument.

[ghffrsfygdhfjkjiysdz]

I LOLed at 0:20 "... In electronics we use water soluble solder..." I sure hope not.

[ZeroChannelZero]

This video is sort of like asking your dad what's an 8-track tape? and having to sit through his entire collection of Arlo Guthrie.

[Baerchenization]

Jesus Christ. He didn't answer the damn question...

[nicholasbrown]

'What is flux?' ...Something everyone should have when soldering! 😁

[johnathancrumm]

Good description on it. Now on to how to use it.

[mpcoley]

Great, thanks for showing us how use it. Cuz it's called Howcast.

[maaly]

thanks for these tips. I had an easier time soldering.

[mlight]

Excellent for understanding why flux is needed.

[yojellquiom]

thank you for this.. i've learn so much..

[JoshuaAugustusBacigalupi]

'What is Flux' != 'How to Solder'. There are plenty of videos for how to solder. I don't know why ppl are confused; the video answered all I wanted to know re: 'What is Flux?'

[simonriddick]

Don't worry I googled it myself.

The flux paste will draw in the solder after the pipe is heated using a blow torch. The acid in the flux attracts the solder into the seam between the two copper pieces and fills the gap and creates a semi-permanent bond between the pieces.

[insylem]

Thanks for mention that mega ohm bit. THere are times when I need to solder with an insulation resistance of at minimum 3.6Giga Ohms.