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Flux Boards: What is Flux & Why is it so Important?

“Flux” in the flux board is one of the most widely used words in the English dictionary. It can refer to the flow rate of particles, the amount of discharge of a given substance, uncertainty or confusion, etc. In electronics and PCB manufacturing, you can consider flux as one of the most important components and steps in the PCB soldering process.

You could compare using flux to cleaning and sterilizing a wound before stitching and bandaging it up. This comparison may not be accurate to some, but you get the gist.  

Nevertheless, in the following guide, we will discuss what flux is and which flux types are most common in soldering. We will also explore how to use flux on printed circuit boards and how to remove residual flux from electronics. 

Flux Boards–What is Flux on a Circuit Board?

Flux on a Circuit Board

Earlier, we compared using flux to sterilizing a wound. However, cleaning a surface before applying glue would be a more apt simile. If a surface is not clean enough before you apply a bonding agent, it will not bear effective adherence.

One of the key challenges in successful soldering is an unclean surface or site. This could be due to obvious aberrations such as dirt, dust, or oil. Alternatively, it could be due to more subtle impurities such as oxidation.

While you can remove the obvious aberrations through mechanical or chemical means, you may find it harder to tackle the oxidation. This is because the soldering process innately encourages oxidation and re-oxidation.

This is where flux comes in. Not only do most modern fluxes reduce and prevent oxidation during the soldering process, but they may also stop corrosion.

Some fluxes help solder bond more effectively by helping it discharge and flow (flux) more easily. Charcoal was one of the earliest forms of flux as it works as a deoxidizing agent. Since then, there have been cleaner and more efficient fluxes. We will review them in the next section.

Flux Boards–Types of Flux for Circuit Boards

Types of Flux

Besides charcoal, some of the most common early fluxes included lime, phosphorus-containing minerals, baking soda, borax, and potash. 

Today, fluxes are either rosin-based, acid-based, water-soluble, or no clean. Traditional rosin fluxes can be either non-activated, mildly activated, or fully activated. We usually mix activated rosin-based fluxes with activating agents that increase the wettability of metal.

There are quite a few flux products out there, and most of them are rosin-based. Which ones are best for your project? In the following table, we highlight a few of them as well as expand on their pros and cons.

Best Fluxes For Soldering and Working With Electronics

Soldering FluxProsCons
SRA Soldering Products – Rosin Paste FluxDesigned especially for electronics Zinc-freeEasy applicationMinimum cleaning requiredConvenient packaging90 to 315°CCrystalizes after a year of useThick and dryDark residue
Delcast – Rosin Soldering Flux PasteFast-actingStrongCan work as a wetting agentQuick-dryingGreat for accuracy in solderingStops metal oxidation and corrosionThe dispenser is tricky to work withNot good for thick or dirty wiring
LA-CO 22101  – Regular Flux PasteCompletely water solubleNo residue for water pipesGreat for copper and brass solderingGreat for all types of weatherComplies with ASTM B813 national standardsZinc-Chloride freeNon-acidic, non-toxic, and lead-freeInadequate for lead-containing electronics
Novacan – Old Masters FluxGreat for copper wire and leadVersatileAffordableEasy to cleanPackaging could be betterHas a strong smell
Harris SSWF1 –  Stay Silv FluxGood quality packagingAffordableContain 56% silver contentGreat smoothness and thicknessEasy to work withHigh melting pointIncompatible with silverNot easy to clean

Flux Boards–How to Use Flux with Electronics

How you apply flux will depend on its container and the tools you have at your disposal. It will also depend on the consistency of the Flux. Flux can be pasty, fluid, thick, or solid.

If you are working with a PCB by hand, you can apply flux to it using a flux pen. Alternatively, you can use a multicore solder containing flux. And this allows you to apply flux while you solder joints or components.

The flux that PCB manufacturers apply during the automatic manufacturing process is either through wave soldering, solder reflow, or selective soldering. 

Wave soldering involves spraying the flux onto the board before the automatic soldering process. Solder reflow involves a mixture of thick flux and metal solder that holds the PCB’s components in place until the oven heats the board. This initiates a reflow.   

The automatic process applies the flux during selective soldering by either spraying it on or a precise drop jet process.

Flux Boards–Can I Use Tinning Flux on Electronics

Flux on Electronics

People commonly use Tinning flux for plumbing and pipes. Nevertheless, we would not advise using tinning flux for electronics. It may be too corrosive for PCBs. This is especially true for plumbing and pipe fluxes. 

How to Remove and Clean Residual Flux

Flux often leaves residue behind. This is particularly true for flux with higher traces of solids. Furthermore, the difficulty in the cleaning of flux depends on the type of flux it is. While some fluxes are water-soluble, others will require you to use a solvent.

Flux residue may form corrosive spots on solder joints. However, you do get no-clean flux that does not leave volatile or corrosive residue and thus does not need the cleaning process. Technicians typically use isopropyl alcohol to clean residue off the PCB. This acts as a solvent. 

Alternatively, there is a purchasable specialized flux removing chemicals. Some of them come in the form of sprays and drops for precision. 

In Summary

If you have reached this point in the guide, you should understand what flux is and why it is so important in the PCB soldering process. You should also identify a few fluxes and how to clean residue off a PCB. Nevertheless, we hope you have found the above content to be helpful. Thank you for reading.  

Hi, I am Hommer, the founder of WellPCB. So far we have more than 4,000 customers worldwide. Any questions you can feel free to contact me. Thanks in advance.