Can You Use Too Much Flux When Soldering? Here’s the Truth

Flux is an essential component in ensuring the soldering process goes off without a hitch. Used to prevent oxidation before and during soldering, flux should be kept on hand whenever working on electronic circuit boards. But can you use too much of the material? 

There is no set amount of flux that should be used when soldering. However, any excess paste must be cleaned off the circuit board entirely. Otherwise, leftover flux can cause corrosion and therefore damage the very parts that are being fixed.

If you’re not sure whether you’re using too much flux when soldering, read on. This article will discuss the appropriate amount of flux needed to solder properly and help you determine whether you’re using too much or too little of the material. 

Can You Use Too Much Flux?

When using flux, it’s easy to go overboard and use more than necessary. However, this can be avoided by starting with a small amount and adding more as required. 

The biggest problem one can run into when using too much flux is not properly cleaning all of it off the circuit board once finished. When extra flux is left on the circuit board and not removed, it can cause damage to the pieces being soldered, such as tarnishing and corrosion. 

While there is no set amount of flux that should or shouldn’t be used, be sure that you can easily and thoroughly remove any extra paste that may be leftover before adding more.

How to Remove Extra Flux

Removing flux is an easy process, but extra care should be put into it, as small and fragile circuit board pieces can easily be damaged. Unless you are working with a water-soluble flux, you’ll need to have some type of chemical solvent on hand to remove the extra flux. According to Chemtronics, this can be as simple as isopropyl alcohol, which most people already have on hand.

To remove the flux itself, simply wet a soft cloth or cotton swab with water of a solvent of some type before gently massaging the area to remove the residue. Be sure to exercise extreme caution when doing to avoid damaging any pieces of the board. Additionally, you’ll want to pay great attention to this process, so you don’t leave any residue behind.

After removing excess flux, give the board a gentle rubdown with a dry cloth to remove any leftover moisture. If you still notice a bit of residue, repeat the process. The key when working with flux is to be as thorough as possible while still being careful not to upset any of your newly completed handiwork.

What Type of Flux Is Best for Avoiding Residue?

There are several different flux types available for soldering that are often broken down into three categories: rosin, water-soluble, and no-clean. The exact kind of flux you need to use will depend on the nature of the project being completed.

However, it’s worth noting that different types of soldering flux will produce different levels of excess material or residue that may need removal during the process: 

  • Rosin Flux: When working with large wires and an open surface, rosin flux is the best option. A combination of both rosin and solvent, this flux is known for being resistant to corrosion while leaving little to no residue on the project. However, if you find yourself with extra rosin flux, a specific chemical solvent will be needed to remove it.
  • Water-Soluble Flux: Generally, water-soluble flux is considered to be stronger and able to withstand more tarnishing than rosin flux. It is suggested to use water-soluble flux instead of rosin when attempting to solder something that will continue to be used for many years. As the name suggests, these fluxes are water-soluble and are therefore easier to clean, so long as the rest of the board can stand to interact with water.
  • No-Clean Flux: If you find yourself working with an intricate, easily disturbed board, you may find that no-clean flux is the best option for your project. Quite literally, this flux does not need to be removed and can be left on the board without causing any problems.

If you’re concerned about accidentally using too much flux during your soldering process, opt for no-clean flux, as it is least likely to cause corrosion or damage when left on the board. When working with the other two flux types, exercise more caution during the cleaning process to ensure all residue is properly removed.

Is Flux Necessary for Soldering?

While the application of flux is typically a crucial part of soldering, it isn’t always necessary. This is because many solder wires have the material inside the core already. 

Whether or not flux is needed depends on the soldering wire being used. According to DigiKey, solder wires with flux inside, also known as flux core solder, don’t require extra flux to be used.

Unless this specific type of wire is being used, keeping flux on hand is always a good rule of thumb. Though the balance between too much and not enough can become tricky, it’s always best to err on the side of caution instead of skipping out on flux altogether. As discussed in the next section, failing to flux at all can cause damage to the circuit board you’re working on.

What Happens If There’s Not Enough Flux?

Just as one can use too much flux when soldering, complications can arise when not enough flux is applied as well. Acting much like a paint primer, flux works to clean and prepare the surface for soldering and prevent oxidization during the process. Flux also aids in smoothly joining together joints.

When there isn’t enough flux being used during soldering, you’ll likely find it very difficult to join together the separate pieces. The quickly-forming oxidation makes it nearly impossible for the solder to stick, which is why using some amount of flux during the process is so critical.

While it is recommended to start small and add more if necessary, don’t be afraid to use enough flux to get the desired results. While the sweet spot between too much flux and not enough may come with practice, you can always resort to cleaning residue off using a towel and a chemical solvent. If you work diligently and carefully, you shouldn’t notice a problem with the amount of flux you choose to use.

How Much Flux Should I Use?

The exact amount of flux needed to complete a soldering project varies on two factors: the type of soldering being completed and the density of the soldering wire. 

According to Indium, flux percent is calculated by weight, and anywhere between 0.5 and 3% is an appropriate amount of flux for soldering. While higher percentages result in more residue, too low percentages and oxidation are more likely to occur.

A higher percentage of flux should be used when completing robotic soldering, as opposed to hand soldering. It is recommended to use just over 4% flux when working in a robotic setting.

Final Thoughts

Anyone familiar with the soldering process knows just how important flux is. The agent is used not only to clean the board’s surface but also to protect from oxidation during the soldering process. 

Often made out of rosin, flux can be purchased at virtually any hardware store. We hope that this article has offered helpful information and encouraged you to discover the perfect flux type for your projects!

If you liked this article, have a look at my other articles I wrote about the topic!

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Alexander Berk

A bit about myself: I am a certified international welding engineer (IWE) who worked in different welding projects for TIG, MIG, MAG, and Resistance Spot welding. Most recently as a Process Engineer for Laser and TIG welding processes. To address some of the questions I frequently got asked or was wondering myself during my job, I started this blog. It has become a bit of a pet project, as I want to learn more about the details about welding. I sincerely hope it will help you to improve your welding results as much as it did improve mine.

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