​Type of Flux for Electrical Soldering

by Joost | Updated on:  June 20, 2021
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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.

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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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Selective Soldering

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


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.


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.
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.