What is flux for soldering?

Have you ever heard of soldering? Most people might have at one time or the other come across a soldering iron but didn’t recognize it. Soldering is a crucial component in the electronics industry as well as other industries.

Soldering is the process by which metal components are fixed onto a circuit or some other metallic surface with the help of an alloy of tin and lead known as solder. Soldering creates a strong bond between metals and is integral to having them joined together to form a solder joint.

Soldering can be of three types

  • Lead-Free Soldering
  • Lead-Based Soldering
  • Flux Soldering

Lead-free soldering doesn’t use lead, while lead-based soldering uses lead to join metallic components. On the other hand, Flux soldering is where a cleaning agent (Flux) is used to increase the effectiveness of the soldering process.

What is soldering flux used for?

Flux is a cleaning agent used to clean soldering surfaces before the soldering process can begin. Flux can be in three forms: liquid flux, solid flux, or even paste flux. That will, however, depend on the way you use it in the soldering process.

Flux works by eliminating any impurities, chemicals, or solid on the surfaces that are about to be soldered together. Oxides are the major impurity that flux removes from such surfaces before soldering. It also prevents re-oxidation of the surfaces.

Types of Solder Flux

The types of flux vary as much as the soldering process. With each flux, the type comes advantages as well as some drawbacks. That being said, there are three main types of flux soldering.

1. Rosin flux

This type of flux has rosin as the main ingredient alongside other ingredients such as abietic acid and other different acids. You can therefore call it an organic acid flux because it is naturally occurring in pine trees. Rosin fluxing itself is also of three varieties. The varieties all have varying levels of activation, which informs their activity when used in soldering. Rosin flux, rosin mildly activated, and Rosin activated.

Rosin flux works best where the surface to be soldered isn’t too dirty or has too many impurities. It gets rid of any remaining impurities such as metal oxides and leaves no flux residue after cleaning.

Rosin mildly activated works best for the filthy surfaces and isn’t that good at eliminating all the residues. You will need a flux cleaner to eliminate the residues that remain behind after using mildly activated rosin. On the other hand, Rosin is an excellent cleaner, although the residues it leaves behind are of significant levels. It, therefore, wouldn’t be a good option for soldering joints.


  • It has several varieties
  • Some of its types leave behind no residues
  • It is naturally occurring in Pine trees
  • They protect the soldered surface


  • Rosin activated leaves behind a significant amount of residues when used to clean.
  • Rosin cleaning is quite costly
  • You need a solvent to utilize it fully

2. No-clean flux

No-clean flux is a type of flux with negligible chemical composition. You can, therefore, clean metal surfaces without worrying about cleaning up later. Thanks to its cleaning process, it doesn’t need too much chemistry or activation to clean surfaces. It, however, offers pretty commendable soldering.

However, the no-clean flux contains Rosin as one of the components and other inorganic fluxes that allow it to clean effectively. Although initially meant to clean without chemistry, it brought about issues of ineffective flux amount, getting rid of all the oxide layers that might be on the base metal.

Depending on the flux components, you will find that at times the no-clean flux leaves behind residues. However, the residues aren’t harmful in any way and might not affect the base metal that much.


  • You don’t have to clean up residues after fluxing
  • Readily available and effective


  • It might not be as effective as other flux types

3. Water Soluble Flux

This flux type is typically made from organic glycol bases, making it one of the best organic fluxes. It is a pretty effective flux with the only issue being the formation of bonds with the circuit board or other metallic components. It, therefore, will require you to thoroughly clean after using.

It also is quite strong for an oxidant which makes it more powerful than rosin flux. However, its aggressive oxidation might be undesirable at times since it interferes with the integrity of the cleaning surface.


  • It is strong and effective
  • It leaves behind no residues


  • It forms bonds with metal surfaces or circuit boards
  • At times its oxidation properties might be more aggressive than required

What type of flux is used for electrical soldering?

The different types of soldering each require a specific type of flux. Not only does that increase the effectiveness of the soldering, but it also eliminates the chances of the soldering decomposing later on.

No clean fluxes will be the best option for electronics because they don’t require you to clean up afterward. On the other hand, water-soluble flux is highly corrosive and might need you to clean up after use.

Rosin-based flux can also be a good option for electronics as long as you avoid using variants that will give you a hard time cleaning up the residues.

The below flux versions will be a good start if you are soldering electronics.

What is flux for soldering

1. SRA Soldering (Rosin Plate Flux)

This is a rosin-based flux that helps bond electrical components to circuit boards. It is highly preferred thanks to its high-temperature range. The high-temperature range makes the best flux for electronics circuit boards which might at times get too hot.

It has a lower soldering temperature meaning it will melt over a lower range of temperature.

2. LA-CO 22101 Regular Flux Paste

This is a water-soluble flux effective in the soldering of copper. You, therefore, can use it on copper circuit boards. It works best with non-acidic cleaning agents.

It, however, wouldn’t work that well for electronics with surface mount components made of lead. Leaded solder is a better option for such circuit boards.  

You, however, should clean it thoroughly after using it since it could erode the circuit board.

What is the best flux for silver soldering?

Almost 90% of all silver soldering applications utilize a white paste flux. Various types of flux would work best with silver soldering.

  1. Stay-Silv White Brazing Flux
  2. Dynaflow Auto Dispensing Flux
  3. Stay-Silv Black Brazing Flux

The above options will enable you to solder the different types of silvering.

Do you need flux with rosin core solder?

Rosin core solder is a metal alloy containing pitch-like organic chemical compounds which melt rapidly and attach well to most metals, especially copper, forming a strong electrically conducting bond.

It comes with embedded flux and shouldn’t need you to add any extra flux to work well.

How to choose flux for soldering?

There are several things to consider when choosing the best flux for soldering.

1. Activity

As much as you will want a flux with high activity, you will need to consider the activity level and whether it could have any consequences for the surface. Therefore, the activity level is one of the main things you need to look for in flux for soldering.

2. Type of Flux

As we have already mentioned, different flux types work in different ways. Some leave behind a considerable amount of residue, while some don’t need any cleaning at all. You will have to analyze which one is best for you and pick a type.

3. Residues

It is imperative to look at the residues a flux will leave behind when using it. Some residues could be detrimental to the circuit and cause decomposing or eroding of the circuit’s components. Therefore when choosing a flux type, always ensure you pick one that has less corrosive residues as possible.

That is if it will be impossible to get one that doesn’t leave any residues.

4. Temperature

A suitable flux for soldering shouldn’t melt at a low temperature. That will be especially important if you will be working with electronics which usually generate lots of heat inside. The flux should have a high-temperature range to ensure it doesn’t react and damage the circuit board.

5. Safety

This might not seem like an important thing to consider, but it is. You need to ensure you get safe flux for yourself and the surfaces you will be working on. That means it shouldn’t have an overly high reaction rate.

Frequently asked question (FAQs)

Q. What type of flux should I use?

Ans: The type of flux you use will depend a lot on what circuit you will use. There are particular flux types for each material, from copper, silver to other metals. The type of flux you use should be safe for use with any surface you will be working with.

Q. Can you use Vaseline as flux?

Ans: It is complicated to use Vaseline as Flux. That’s because it doesn’t possess the necessary properties to get rid of oxides on the surfaces. That doesn’t mean it is entirely impossible to avoid using flux. It is possible, but it would make your soldering process needlessly hard.


We hope you are now conversant with the main types of soldering flux. Remember to choose the type of flux you use wisely since it could backfire if things go out of hand.


  1. https://www.tempoautomation.com/blog/understanding-soldering-part-4-how-to-use-flux-when-soldering-electronics/
  2. https://www.electrical4u.com/what-is-flux-types-of-flux/
  3. https://www.circuitspecialists.com/blog/choosing-soldering-flux/
  4. https://www.electronicshub.org/soldering-fluxes-types/
  5. https://www.electronicshub.org/best-soldering-flux-for-electronics/