​What type of Flux is used in Electronics Soldering? Try these!

by Joost Nusselder | Updated on:  June 20, 2021
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Soldering is the process of joining two metals with each other with another metallic substance by flowing a filler metal, for a stronger and robust joint. This technique of connecting metals with one another is widely used in electronics. Plumbing and metalworks also have extensive use of this technique.

Different types of fluxes are used in different cases. Electronics soldering is a rather sensitive field where the flux used should have certain attributes like non-conductivity. We will tell you about the types of flux that are used in electronics soldering, and what you should consider before using one of them yourself.

What-Is-Flux

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How to strip wire fast
Why Is Flux Required in Electronics Soldering

While you try to fill the joining point of two metals with another metal, which is essentially soldering, dirt and debris on those metal surfaces will create hindrance in creating a good joint. You can remove and clean the non-oxidizing dirt from those surfaces easily, but you have to use flux when you’re trying to remove oxidation.

Why-Is-Flux-Required-in-Electronics-Soldering

Oxidation | Is It a Bad Thing?

Oxidation is a natural thing. But that doesn’t mean it’s all good. All metals react with the oxygen in the air and from complex chemical compounds on the metal surface that is hard to remove and makes it very difficult to solder. Oxidation is commonly called rust on iron.

Use of Flux to Remove Oxidation

Flux is another chemical compound which it reacts with the oxidation, under high temperature, dissolving and removing the oxidation. You need to frequently use flux to clean the oxidation from your soldering iron tip because oxidation gets accelerated at higher temperatures. Keep this in mind if you’re intending to make your own soldering iron.

Use-of-Flux-to-Remove-Oxidation

Different Types of Flux in Electronic Soldering

The flux that is used on electric circuit boards is not of the same type as those used on wires. Because a circuit board and wire require different properties from flux. We will tell you everything about all the types of flux that are available on the market for electronics soldering.

Different-Types-of-Flux-in-Electronic-Soldering

Rosin Flux

Beating all other fluxes in terms of age is the rosin flux. During its early days of production, rosin fluxes were created from pine sap. After collecting the sap, it is refined and purified into the rosin flux. However, different other chemicals and fluxes are mixed with refined pine sap to produce rosin flux nowadays.

Rosin flux turns into liquid acid and flows easily when it’s heated. But upon cooling, it becomes solid and inert. It is very effective in removing oxidation from metals. After using it on circuits, you can leave them in their solid, inert state. They won’t react with anything else unless they are heater enough to turn into acid.

If you want to remove the residue after using rosin flux, you need to use alcohol. They are not water-soluble. That’s why you have to use alcohol instead of plain water. But there’s no harm in leaving those residues as they are unless you wish to do a wise job of keeping your circuit board clean.

Using Rosin-Flux

Organic Acid Flux

Organic acids like citric acid, lactic acid, and stearic acids are used to create this type of flux. The weak nature of these acids, combined with isopropyl alcohol and water forms organic acid fluxes. The biggest advantage of organic acid fluxes is that they are completely water-soluble, unlike the rosin flux.

In addition to that, as the acidic property of organic acid flux is higher than rosin fluxes, they are stronger than rosin fluxes. As a result, they can clean oxides off of metal surfaces more quickly. Couple this oxidation removing power with the soluble nature of the flux, and you have an easy to clean flux residue. No alcohol required.

Nevertheless, this cleaning benefit comes at a cost. You lose the non-conductivity property of the rosin flux residue. Because the residue from organic acids are electrically conductive and can affect the overall performance and operation of a circuit. So, make sure that you remove the flux residue after soldering.

Organic-Acid-Flux pour

No-clean Flux

Just like the name suggests, you don’t need to clean the residue obtained from this type of flux. It creates a significantly small amount of residue compared to the other two fluxes. It is based on organic acids and some other chemicals. These often come in syringes for the convenience of usage.

For circuits that use surface-mount technology, it is better to use this type of flux. Also, the ball grid array is a type of surface-mounted board that heavily benefits from no-clean fluxes. The small amount of residue it produces is not conductive or corrosive. You can use them on boards that are harder to access after installation.

However, some users find a surprisingly large amount of residue that is hard to remove, apart from being conductive. Be careful when using these flux on analog boards with high impedance. We recommend doing further inquiry before using the no-clean flux you are planning to use.

No-clean-Flux

Type of Flux to Avoid in Electronics Soldering | Inorganic Acid Flux

Inorganic acid fluxes are produced from a mixture of strong acids including but not limited to hydrochloric acid. You must avoid inorganic flux on circuits or any other electronic parts as both the flux and its residue can be corrosive. They are meant for stronger metals, not electronic parts.

Type-of-Flux-to-Avoid-in-Electronics-Soldering

​Type of Flux for Electrical Soldering

It’s imperative that you have flux before you start soldering. The flux is what prevents oxidation during the process. Otherwise, it wouldn’t be all that possible, Well, there’s a way around flux but that is rarely feasible. So, let’s take a look at the different types.

Type-of-Flux-for-Electrical-Soldering-FI

Can you Use Solder Without Flux?

Yes, you can use the solder without flux. But it would be a challenge as soldering with fluxes is quite unsafe and troublesome. The most common solder wire is rosin core, so it has fluxes inside it. You can go for those soldiers too. If you want to solder without flux, you have to apply heat, a lot of rubbing alcohol, lint-free cloth, and scouring pads.

But still, the solder may not flow well. So you’ll also need a lot of patience. The temperature should not be that high as low heat is suitable for better joints. IP alcohols are best for cleaning.

Can-you-Use-Solder-Without-Flux

Solder Paste vs Flux

Solder paste is a composition of small solder spheres with flux and it’s grey in color. Fluxes are some chemicals that help to wash away the contaminants on the metal surface. Both solder paste and fluxes are used for the same purpose, to remove the oxides, damageable particles, and dust to help in better soldering.

Fluxes are quite aggressive on metal surfaces. As they are a combination of compositions, it’s better not to leave them on board. It can harm the PCB and the circuits. But for solder pastes, there’s no risk to leave them on the surface after completing the soldering. It is designed to be adaptable to the materials on the PCB.

Solder-Paste-vs-Flux

Type of Flux for Electrical Soldering

The included flux within the core of a solder wire can be good enough. But there are particular situations where the use of an extra flux is useful and recommended. There are different types of flux you can go for such as rosin flux, organic acid flux, inorganic acid flux, and special fluxes.

Type-of-Flux-for-Electrical-Soldering

Organic Acid Flux

Organic acid flux is a water-soluble flux. It’s an extremely popular flux and very easy to use. Usually, these fluxes use common weak acids like citric, lactic, or stearic acids. The weak acid is added with solvents like isopropyl alcohol-water. Though they use weak acids, they are stronger than rosin fluxes.

You can wash oxides from metal surfaces much more speedily. As these fluxes are water-soluble, the PCB can be easily cleaned with just regular water. But don’t let water touch the components that shouldn’t get wet.

As organic acid products are conductive, it’s needed to be removed after soldering. Otherwise, the operation of a circuit may get affected.

Organic-Acid-Flux

Inorganic Acid Flux

An inorganic acid flux uses a blend including a strong acid like hydrochloric acid, zinc chloride, or ammonium chloride. They work better with stronger metals like copper, brass, and stainless steel.

Because of using strong acids, it’s very important to completely remove all the corrosive residues from the surface. Otherwise, it can damage your solder joint.

Inorganic-Acid-Flux

Special Fluxes

Most of the materials are very hard and rough to solder. That’s why in some particular cases different special fluxes are needed to be used for better performance. Due to the passivation layer formation of the aluminum oxide, aluminum, and its alloys are hard to solder.

In this case, it needs to be perfect to breach this layer expedite wetting by the solder. The metal ions that are nobler than aluminum undergo a redox reaction that dissolves aluminum on the surface layer, and makes a deposit. That intermediate layer can be wetted with the help of a solder.

The example of this type of flux is a perfect composition of different chemicals like fluoroboric acid, triethanolamine, and cadmium fluoroborate. Another type of special flux that can help in these cases is an unbelievable flux composed of ammonium chloride and fluoride and zinc chloride or you can also say tin(II) chloride.

Special-Fluxes

What Is Rosin Flux and What Is It Used For?

Rosin is an urbane and aerified pine sap. Though rosin fluxes are one of the oldest types of flux, it’s still used today. However modern rosin fluxes used a blend of various fluxes to enhance its performance.

Rosin fluxes are potent and acidic only when it’s turned into liquid. This hot state helps in its easy flow, clear away oxides quickly, and helps to abolish foreign particles from the surface of the metal you’re soldering. However, the great advantage is when the rosin flux cools down, it becomes inert and solid.

That means you can leave it on the surface after using unless it becomes liquid and starts eating away the connections. Therefore, it’s suggested to remove the flux after soldering. Alcohols are used to remove these fluxes easily.

Rosin-Flux

Soldering Techniques Using Flux

If you are worried about how to use flux for soldering, then here is the solution. You can use flux in both manual hand soldering and automated processes used by PCB contract manufacturers. For hand soldering, you can apply flux by using a flux pen or generally in the core of the solder wire. For the automated process, there are a couple of different ways to apply the flux such as wave soldering, solder reflow, and selective soldering.

Soldering-Techniques-Using-Flux

Wave Soldering

The flux used for wave soldering is generally made up of more solvents. For wave soldering flux will be sprayed on the surface. Once it goes through the solder wave the components will be cleaned of any oxide layer. If the board is using a less corrosive flux, it’s needed to go through a pre-cleaning before the flux is applied.

Wave-Soldering

Solder Reflow

A paste made of a sticky flux and small spheres of metal solder is used for the solder reflow process. This paste holds the material parts in place until the heat of the oven triggers the flow of the solder particles. This flux paste not only cleans the surface but also seals out the fair to prevent further oxidation.

Solder-Reflow

Selective Soldering

For selective soldering, you can apply the flux by spraying it or by applying a more exact drop jet process.

Selective-Soldering

FAQ

Q: Is it safe to use Vaseline instead of flux?

Ans: Yes, you can use vaseline as petroleum jelly is one of the best substitutes for soldering fluxes.

Q: How long do solder wires last?

Ans: Solid wires have an infinite lifetime if it’s stored in a dry, non-corrosive environment between 10-40 degrees celsius. But other types of solder wires usually have a lifetime of 3 years from the date of manufacturer.

Q: Does solder flux expire?

Ans: Yes, it has a limited lifetime determined by the solder wire. For the wires that contain 70% of lead, the flux life is 2 years from the date of manufacturer. But for other alloys, the lifetime is about 3 years from the date of manufacturer.

Conclusion

The key to getting a perfect solder joint is in using a flux before and during the process. Fluxes have made soldering much more easy and safe. Without any hassle, they clean the total surface. But it’s important to know which type of flux you need.

Hopefully, the article will introduce you to the necessary information you wanted to know before going for soldering with fluxes. Once you know about the types and techniques, the work will be easier and you’ll get the best result.

Conclusion

As you could see, all types of flux have their advantages and disadvantages, and it is nothing to worry about how to use flux for soldering. You now have a range to choose from while doing your soldering work on electronics. And just like us, nobody can declare any one of those fluxes as the best one out there.

If you work on circuits that use the surface-mount technology, your best bet would be on the no-clean flux. But be careful about the extra residue thing. And for other circuits, you can choose anyone between organic acid flux and rosin flux. Both of them do an excellent job.

Joost Nusselder, the founder of Tools Doctor is a content marketer, dad and loves trying out new equipment, and together with his team he's been creating in-depth blog articles since 2016 to help loyal readers with tools & crafting tips.
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