What is Solder Flux? A Complete Guide

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Mar 3, 2021|Product Innovations and Design

 

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. 

Rosin Flux

Rosin, or “Type R” flux,  is the most basic soldering flux that has actually been used for thousands of years. Rosin is naturally derived from pine tar rosin, which can be farmed from trees. When it’s used as a soldering flux, rosin is then dissolved as a solvent in isopropyl alcohol. Type R flux is used without additional acid activators, and is clear white rosin that has many different functions.

 

When activators are added to any soldering flux, they increase the natural ability of the flux to dissolve heavier oxide films produced at higher soldering temperatures. Any activated flux can be mildly activated (RMA) or rosin-activated (RA). There are many different types of activators used, with common ones being organic acids, monobasic and dibasic organic salts, compounds containing chlorine or bromine, amides, and more. Activators are helpful but corrosive, and should always be removed from the circuit board to improve the reliability of the PCB.

 

While rosin fluxes are the most reliable, activated and mildly activated fluxes can actually leave traces of chloride ion residues or other corrosive residues. These must be removed from all PCBs after soldering, as to prevent any long-term failures—you don’t want the cause of corrosion on your boards to be because of the flux you used. Flux residues can also be tacky, which attracts dust and can cause shorts or electrical failures. Lead-free solder alloys are becoming more and more prevalent, and highly activated fluxes help overcome oxidation at higher soldering temperatures. Rosin flux retains its popularity because it leaves no residue on its own, and because it reduces the amount of corrosion when making a mechanical and electrical connection.

Rosin flux is important to the entire PCB soldering process. It removes oxides, grit, and grime from metal surfaces. It also improves the quality of intermetallic bonds and primes a metal surface, preventing any oxides from interfering with the soldering process.

No-Clean Flux

This type of soldering flux can be constructed from natural rosin or other synthetic rosins. While no-clean rosin-based soldering solutions are the same as Type R fluxes, they usually contain much lower concentrations of natural gum rosin—less than what is used in R, RMA, and RA flux solutions. No-clean fluxes that are truly synthetic have all the desirable properties of the natural flux rosin, but can also contain additional activators. Any soldering rosin with excess activators can leave behind residues that create corrosion.

 

No-clean fluxes were designed as a quality of life improvement, to help PCB assemblers by reducing the time and expense required to clean a board after soldering. Since no-clean fluxes leave far less residue behind, they can be a boon in many PCB manufacturing situations. They leave behind less residue than conventional Type R fluxes, and do not interfere with board operation or cause long-term corrosion.

 

While the residue left behind by no-clean fluxes may attract dust or feel sticky/tacky, and some cleaning may still be required depending on the job.  For instance, if your PCB is to be conformal coated (in order to protect the circuitry during operation), the board surface has to be completely free of all flux residues. No-clean flux can leave behind minimal residues, preventing the good adhesion of conformal coating. While no-clean fluxes have many benefits and fewer drawbacks, they still have their place in assembly. 

Water Soluble Flux

These fluxes are made with water-soluble rosins, and they have a residue that can be flushed away with a water rinse. Many water soluble fluxes are made with water-based solutions, eliminating the need to use more alcohol-based solder fluxes. VOC emissions can be reduced with water soluble fluxes, especially for those PCB manufacturers who operate under the most stringent regulations. There are also many acid activators used in water soluble fluxes, such as chlorine and bromine halogenated compounds, organic acids, monobasic and dibasic organic salts, and amides. These activators are corrosive and should be removed from any PCB to ensure longevity and reliability, and decrease the chance for corrosion to occur. Water soluble fluxes provide some of the best soldering conditions possible, and almost never burn off during the soldering process. They’re also much safer on metals than other solder fluxes.

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. 

When to Use Flux When Soldering?

What is flux, and why do you need to use it? Well, consider flux to be the paint primer of your PCB manufacturing process. When conquering any soldering project, flux should be used to build a strong connection and improve your soldering joints. Flux is a chemical agent that comes in many different varieties, and helps reduce oxidation, improve solder flow, and improve electrical contact. When flux is applied properly to an electrical lead, it fully helps to remove oxidation. It prevents airflow to the lead, protecting the electronic and mechanical components of a PCB. Solder flux also attaches more easily to any non-oxidized metal, which improves flow over electrical lead. Overall it increases conductivity and reduces the chance for shorts or corrosion to occur.

How to Use Flux When Soldering Electronics?

Flux is both a chemical cleaning agent and an inhibitor of corrosion. There are automated processes for applying flux used by many PCB manufacturers, but soldering flux can also be applied by hand. When soldering, you prepare the metal surface of the PCB by cleaning away impurities. When metal is exposed to air, oxides are formed which can increase the chances of corrosion and prevent the formation of good joints. Flux fully protects the surface of any board, and also helps with the soldering process. It is recommended to use a guide to PCB soldering.

Soldering flux is a paste or liquid substance that helps with the soldering and welding processes. Flux is necessary for soldering, and any workpieces that are connected mechanically or electronically are referred to as “soldered.” Solder flux can connect wires or repair a circuit board, as well as fill in any gaps between mechanical, electrical, or thermal workpieces. When soldering, you should only use a small amount of flux. Thicker paste melts in the heat, creating large puddles on your board. Liquid tack fluxes are often the way to go as they do not leave behind messes on your PCB. In some cases, acidic fluxes must be used which leave behind residues that lead to corrosion. It’s important to always clean residues away after a soldering job.

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.

Alternatives to Soldering Flux

Are there alternatives to soldering flux? You can solder without using flux, but you will still need something to break down the oxides on the metal surface. If you solder without doing this, you will damage the surface or leave it improperly cleaned. One alternative to soldering flux is petroleum jelly, which is naturally anti-corrosive so as to not damage your material or PCB. 

It’s a good idea to use soldering flux to ensure a strong bond that will not corrode or short out. It doesn’t matter how much you clean your metal surface, you’ll need a flux to make the solder stick. Without this, a solder joint simply will not be strong enough or conductive enough, which is why soldering flux is recommended. Also, solder will chase a heat source, and it can be difficult to get any molten solder off your iron without a flux. 

Depending on the work you’re doing, it might be possible to get by without soldering flux. Remember that without a flux the joint will not be as strong, which may be in your benefit if you are doing temporary work.

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.  

Conclusion

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. 

FAQs

Is Soldering Paste the Same as Flux?

Soldering paste and flux, together, are mixtures that glue your metals together. When working in PCB assembly, metallurgy, plumbing, circuit board repair and more, the difference between solder paste and flux can make or break your job.

Solder paste or solder cream helps to bind metals. Once heated, solder paste conjoins metal surfaces as an impenetrable glue. It creates solder joints by holding two metal, mechanical, or electronic surfaces together, and can even help the transmission of electric signals. Solder paste allows wiring and other objects to transmit their electricity without interruption. 

Solder paste is a specialized product that glues together different parts of metal alloys. Flux is the ingredient that you add to the metal surface either before or after using the solder paste because it acts as a cleaning agent during the soldering process. Soldering flux makes solder pastes more effective.

What is the most common type of flux when soldering?

Rosin flux or Type R flux is the most common type of flux used when soldering. It is commonly used to solder electrical components and is only active when heated. Rosin flux does not react with electric circuits while in its unheated state, making it a popular neutral soldering flux.

What Is No Clean Flux?

No-clean flux is designed to not require cleaning, though some no-clean fluxes still leave residue behind. No-clean residues can actually be more difficult to remove than other fluxes, because their trace residues aren’t designed to easily wash off. No-clean flux can be better described as low-residue solder paste because it leaves behind non-corrosive resinous residue at the solder joints. While no-clean flux may be the choice for many PCB jobs, it can be a hassle to deal with if you need other materials to adhere to a clean board.

Can Plumbing Flux Be Used on Electronics?

Solder itself doesn’t naturally adhere to metal as it melts, and requires solder to flow smoothly onto a connection. Plumbing solder uses an acid flux that differs from traditional electrical solder, which commonly uses rosin flux. Plumbing solder is very corrosive because it is used to strip away layers of oxidation, such as on pipes. As plumbing solder melts, it allows adherence to waterproof joints. Since this corrosive acidic flux