Blog  /  Soldering Flux – Ultimate Guide to Understanding Soldering

Soldering Flux – Ultimate Guide to Understanding Soldering

About understanding soldering, Like any other electrical project, you'll need a clean surface before working on it. And a significant problem with metallic surfaces is oxidation.

Metal oxides reduce a metal's wetting ability making the solder ball up instead of spreading uniformly. Hence the need for a soldering flux.

What is it? What is its use? At OurPCB, we've got answers to all that, but first, read on.


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What is Soldering Flux?



Rosin flux used in soldering

Fig 1: Rosin flux used in soldering

Source: Wikipedia


Soldering flux is a cleaning agent used during soldering and desoldering to remove oxide films from metal surfaces. It improves the wetting ability of the solder, allowing it to flow smoothly over the surfaces without dewetting.


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The Application of Different Types of Flux


Here we're looking at four types of flux that you can use in your soldering processes.


Rosin Flux


Rosin is an essential component of the soldering process and is found in its natural state in pine tree stumps. However, modern rosin is mixed with other chemicals to improve its performance and durability.

We can further divide the rosin flux into:

  • Non-activated flux (R)
  • Mildly activated flux (RMA)
  • Started flux (RA)


Non-activated Rosin Fluxes (R)


From the name, this flux type is non-activated and suits cleaning weakly oxidized surfaces. Therefore, they're more suited for soldering copper wires, PCBs, and semiconductor materials and surfaces.


Rosin Mildly Activated Fluxes (RMA)


The RMA fluxes are stronger cleaners than the R fluxes and are used to remove stubborn oxides. Additionally, they're better for cleaning higher-containment leads for electronic components, PCBs, and general use cables.


Rosin Activated Fluxes (RA)


The RA fluxes are intense and have the best cleaning capacity among the rosin fluxes. As a result, it's the best choice for soldering surfaces that are hard to clean.


Low Residue or No- clean Flux and Solder Paste


A close-up view of an engineer soldering 

Fig 2: A close-up view of an engineer soldering


The low residue flux rose in popularity with the widespread ban on CFCs.

Furthermore, it's common practice in Europe to solder without cleaning the rosin fluxes. Consequently, using low residue fluxes saves on capital expenditure and cleaning costs.


Organic Acid Fluxes


The organic or water-soluble fluxes are made of organic materials such as stearic, lactic, and citric acids. These weak acids are combined with solvents such as water and isopropyl alcohol to improve their performance.

More so, organic acid fluxes are quicker and stronger than rosin fluxes when soldering. Additionally, once you're done with your assembly, you can wipe off the extra flux material with water.


Inorganic Acid Fluxes


Inorganic acid fluxes are stronger and bond better than their organic counterparts. Consequently, they're better with stronger metals such as stainless steel, copper, and brass.

Also, they include sodium fluorides, zinc chlorides, stannous chlorides, hydrofluoric acids, and hydrochlorides.


Understanding Soldering-  How to Use Soldering Flux


The first step in soldering metals is knowing their material, the best soldering process, and the best fluxes. Secondly, hot soldering flux is very corrosive, and you'll have to find ways to work around it for better results.

Here's a look at the best soldering processes for a longer-lasting connection.


Understanding Soldering- Choose the Right Equipment


A modern gas blowtorch

Fig 3: A modern gas blowtorch

Source: Wikipedia


Different rosin fluxes work better on specific surfaces while not so well on others.


  • First, use rosin-based fluxes for electrical soldering. You don't need anything too corrosive on your PCB, as it'll melt the connections.
  • Second, use acid flux when working on bigger pipes as it's more corrosive and removes large areas of oxidation quickly.
  • Third, go with a leaded solder when soldering electronics as it has lower melting points. You don't want to disfigure your boards by applying excessive heat to melt the solder.
  • Fourth, opt for silvered solder for soldering pipes and bigger surfaces, as silver has a higher melting point than lead.
  • Fifth, clean the tip of the soldering iron with a damp sponge before working on the electronics.
  • Last, go with a propane blowtorch when soldering bigger pipes. It's bigger and hotter, reducing the time to heat the surfaces.


Understanding Soldering- How to Solder Wires With Soldering Flux


  • First, twist the exposed ends of the wires together while ensuring not to leave any pointed ends.
  • Second, apply the soldering flux on the wires using your fingers or a small paintbrush. Furthermore, soldering flux is non-corrosive in paste form; hence you're safe working on it.
  • Third, press the soldering gun gently on the wires to melt the soldering flux.
  • Fourth, when the wires are hot enough, place solder on the opposite side and see if it's melting. Also, ensure to keep the soldering iron pressed to the wires.
  • Fifth, retract the soldering iron to let the solder cool and connect strongly together.


Understanding Soldering-  How to Solder Pipes With Soldering Flux


Using the gas burner and the solder for copper pipes soldering

Fig 4: Using the gas burner and the solder for copper pipes soldering


Soldering pipes is slightly different from how you solder wires. Here's how you'll go about it.

  • First, clean the area you'll solder with an abrasive material such as steel wool or sandpaper.
  • Second, use a small paintbrush to apply flux to the ends and insides of your pipes. Additionally, ensure that the flux layer is smooth and devoid of bumps.
  • Third, hold the two sections you want to connect with a female connector or fitting before soldering.
  • Fourth, heat the female connector with a blowtorch or soldering iron and press the solder to the pipe's opposite end.
  • Fifth, inspect your connection once the pipes have cooled a little to check if they're evenly coated.


Understanding Soldering-  The Best Soldering Flux Cleaners


  • Rosin-Based Flux - There are specific chemical agents you can use to clean rosin-based fluxes.
  • Water-Soluble Flux - You can use simple detergents with deionized water to clean off water-soluble flux.
  • No-Clean Fluxes - The best cleaner for no-clean fluxes are a solution of water and a saponifier. No-clean fluxes are safe and any residue won't harm your board. However, I'd recommend cleaning as no-clean fluxes lower the adhesion effect of the subsequent coating.




Flux is essential to ensure good connections that become brittle and break under pressure.

However, soldering fluxes are surface-specific and may react badly on other surfaces. Therefore, be careful when purchasing one for your board or project.

Finally, contact us for more information on soldering flux or soldering in general.



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