What Types of Flux is Used in Electronics Soldering?

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.

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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.

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.

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.
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.

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.

Summarizing Everything

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.