Mar 3, 2021|General


From Bluetooth controls to the computers we work with every day, circuit boards appear in all aspects of our day to day life. Of course, sometimes you need to make a minor repair so that your machine keeps moving or creating a necessary upgrade. That’s where a soldering iron, flux, and a little know-how come in handy. 

The solder and know-how make sense, but why do you need flux to solder? Read on to learn what it is, why you need it, and if you can use any alternatives for your project.

What is Solder Flux?

Solder flux is a substance used when soldering electronics. More specifically, flux clears away oxidized metal from the surfaces and seals out the air so that you don’t worry about more oxidation. Additionally, using flux improves the wetting aspects of the liquid solder. 

The Application of Different Types of Flux

You have three categories of flux to work with depending on the type of electrical components you need to solder. Each class uses unique chemical compositions and application methods that make them more compatible with specific electronics. 

  • Wave soldering requires flux with more solvents that clean the components and remove any existing oxide layers. It’s usually sprayed on the board before you start soldering. Boards with less corrosive products would require a pre-cleaning before spraying on the flux.
  • The solder reflow approach involves paste made from the sticky flux and small balls of metal solder. This paste keeps parts stable until you can apply heat to make the solder bits reflow and bond appropriately. The flux cleans, seals out air, and encourages flow as the solder melts.
  • Selective soldering involves spray or drop jet processes to coat the circuit board.

Soldering electronics is a precise process that requires careful management. You need to carefully apply the correct amount of flux so it can do its job without interfering with the soldering process. Too much flux or heating the components too fast could create gaps or splatter across areas of the board that shouldn’t have solder. 

As you can see, it’s essential to choose the right category for your circuit board. If you’re not sure which flux to use, contact your PCB manufacturing source for their recommendations. 

Cleaning Flux from Electronics

Once you finish the job, you probably need to clean the flux off the electronic components, especially if you use a corrosive type. You need to know what kind of flux you use so that you can take appropriate steps to clean your product. 

  • Rosin based flux requires chemical solvents with fluorocarbons.
  • Remove water-soluble flux with detergents and deionized water.
  • As suggested by their name, no-clean flux leaves behind a limited amount of residue. However, you may want to wipe away anything unseemly to preserve the bond and improve the cosmetic finish.

Of note, even the no-clean flux products can leave behind a film that interferes with some testing and inspections. Though it’s more important to clean off corrosive flux products, it’s in your best interest to do a general cleaning of every piece you use solder.

What is the Difference Between Solder Flux and Solder Paste?

There is a fine line between liquid flux and solder paste flux, but each substance has unique properties. For most purposes, the primary differences between the paste and liquid versions are the physical properties and compositions.

Liquid flux is fluid and mostly consists of solvents with some activators, rosins, and other additives. It’s usually diluted with water and sprayed onto the components, so many liquid flux products fall under the no-clean category. 

Solder paste flux is a jelly-like substance that you can combine with solder powder to create a gray paste that looks much like putty. As you can imagine, solder paste flux generally requires more clean-up.

The other difference between liquid solder flux and solder paste flux is the way they work.  Liquid solder is up to 25% active ingredients, while the jelly-like solder paste has between 60% and 80% active ingredients. 

Chemical composition remains one of solder flux paste’s most intriguing aspects. Once mixed, a solder paste can be up to 90% metal, which sounds quite heavy to use. However, the non-metallic parts have lower density and create about half of the product’s volume.

What Can Substitute for Solder Flux?

Flux is necessary for soldering, but what happens if you run out? It’s not always convenient to stop in the middle of a project, but you may have a suitable substitute sitting around your house. 

Can You Use Vaseline as Flux? 

Yes, Vaseline works just as well as commercial flux, and you can even buy a generic brand. Petroleum jelly is primarily made from waxes and mineral oils so that it won’t corrode your components. Plus, it cleans away the gunk and eliminates metal oxides that could compromise your bond.

Are There Other Alternatives to Commercial Flux?

Of course, there are other alternatives if you don’t happen to have Vaseline on hand. There are a few less effective options that could work in a pinch. Be careful working with these substitutes because you need to pre-clean the surfaces before using them.

Lemon juice is another common household item that can fill in for commercial flux. Citric acid is effective against metal oxides, and one lemon could suffice for a small job. However, you may not want to make it a habit because lemons could get expensive.

You can also make pine tar flux from pinecones or rosin flux from crystallized Colophony. It takes a little effort to make both substances, and you need to have the supplies on hand. Unless you live near pine trees, play the violin, or spend a lot of time rock climbing, you may not have the necessary ingredients.  


Flux is a critical part of the soldering process because it keeps components clean and strengthens the bond. However, that doesn’t mean you need to choose commercial flux for your project because petroleum jelly can be equally effective. 

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