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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 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 ip. This comparison may not be accurate to some, but you get the gist.  

Nevertheless, the following guide 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 bonds 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.

Why is Solder Flux Used in Soldering?

Since solder flux acts mainly as a purifying and deoxidizing agent, PCB manufacturers and fabricators use it to remove impurities from circuit boards.

This ensures the reliability of the soldering process, as oxides tend to result in improper or poor solder joints. Poor soldering can lead to connection issues and, ultimately, inferior PCBs. 

Solder flux can also make the solder-wetting process more effective. The solder flux can remove oxides from the circuit board’s copper tracks and electronic component’s lead/pins.

This, in turn, ensures a more reliable bond between the two. Likewise, solder flux can ensure that molten solder sticks more effectively to the surface mount lands.

Soldering Iron Tip Solder Flux

Soldering Iron Tip Solder Flux

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 not 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 aremanyw 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-free Easy application Minimum cleaning requiredConvenient packaging90 to 315°CCrystalizes after a year of use Thick and dry Dark residue
Delcast – Rosin Soldering Flux PasteFast-actingStrongCan work as a wetting agentQuick-dryingGreat for accuracy in solderingStops metal oxidation and corrosionThe packaging could be better Has a strong smell
LA-CO 22101  – Regular Flux PasteCompletely water soluble No residue for water pipesGreat for copper and brass solderingGreat for all types of weatherComplies with ASTM B813 national standards Zinc-Chloride free Non-acidic, non-toxic, and lead-freeInadequate for lead-containing electronics
Novacan – Old Masters FluxGreat for copper wire and leadVersatileAffordableEasy to cleanThe packaging could be better. Has a strong smell
Harris SSWF1 –  Stay Silv FluxGood quality packagingAffordableContain 56% silver content Great smoothness and thickness Easy to work withHigh melting pointIncompatible with silver Not 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 work with a PCB by hand, you can apply flux to it using a flux pen. Alternatively, you can use a multicore solder containing flux. 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. 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 spraying it on or using 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. 

Considerations for Effective Soldering with Solder Flux

There are different types of solder flux. Some may not be compatible with the components you are attempting to solder. Using incompatible solder flux can result in damage from corrosion. Another factor to consider is the solder flux’s activity level. 

Solder flux comes in three activity levels:

  • Low (L):  <0.5% halide
  • Medium (M): 0 to 2% halide
  • High/H: 0 to >2% halide

The activity level indicates how aggressive the solder flux is. The higher the level, the faster it removes impurities,s but also the more likely it is to cause corrosion or damage. As such, it’s best to aim for medium activation.

Skilled Man Hands Soldering Electronics

Repair Restore Pcb Uninterruptible Power Supply

In What Cases Can You Solder Without Utilizing Flux?

Solder flux may not be necessary in situations where you are soldering smaller components. Generally, pre-tinned pads and solder paste contain minute amounts of flux.

This flux may be sufficient to ensure effective flow and bonding during the soldering process. 

With that being said, there areseveralf factors to consider before you attempt to solder without flux.

For instance, the condition of the metal surfaces, the type and quality of the materials, and the type of solder.

This decision will ultimately impact the reliability and strength of your solder joints.       

What Precautions Must be Taken When Handling Flux?

Flux can be toxic to people, especially when inhaled or ingested. It may be irritating or even damaging to the skin – especially if it has a higher activation level.

As such, human technicians and operators must take care when handling it.

Flux must only be used in clean, well-ventilated areas. This decreases the risks of direct inhalation.

We recommend that technicians and handlers use protective gloves and eyewear when repeatedly working with flux. The gloves should use a neoprene material (or better). 

Skilled Man Hands Soldering Electronics

Skilled Man Hands Soldering Electronics


Do you need flux to solder?

Flux isn’t necessary for soldering – especially for smaller components. However, soldering without flux can increase the risk of weak connections and improper solder joints. Before you decide to forgo flux, you must consider the materials in use.  

What is a good flux for electronics?

A lot of people, who are mostly experts, believe that rosin-based solder flux is the best flux type for electronics – for good reason, too.

Rosin-based flux is non-corrosive, features a low melting point, and is residue-free.

Ultimately, if you consider all these things, you’ll discover that It’s a highly effective deoxidant and cleaning agent.  

Is it okay to leave flux on circuit boards?

It’s never okay to leave flux on circuit boards, as most flux residue is corrosive. This is due to the fact that most flux is acidic. Consequently, it’s important that you remove it from your circuit board as soon as it’s feasible. If not, flux residue can cause irrecoverable damage to your circuit board.   

Can solder flux go bad?

Solder flux can indeed go bad. In fact, you’ll notice that most solder fluxes come labeled with an expiration date.

However, in most cases, you can refrigerate your flux and still use it even past this date.

Leaving flux out at room temperature increases the risk of it going back.

Nevertheless, we would not recommend using flux that has reached its expiration date, regardless of whether you’ve refrigerated it or not.

It can be less effective or, worse, damage your circuit boards. 

What is soldering flux made of?

You can find fluxes in various chemical make-ups. For instance, most organic fluxes use natural compounds taken from pine extracts, rosin, and resin.

You can also find inorganic fluxes on the market which mainly consist of chemicals such as fluorides, borates, and chlorides. On the other hand, acidic organic fluxes use stearic, lactic, and citric acid in their compositions.    

Zinc Chloride White Flux

Zinc Chloride White Flux

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.  

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Emma Lu
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