How to Choose Flux for your Soldering or Brazing Application

Let’s flux about it…
 
You’ve decided to make a repair; you have your soldering gun or brazing torch ready to go, but it hits you! “I don’t have any flux!”
Not to worry, we will help you choose what flux is right for you, but first you will know the difference between the applications.
 
 “What’s the difference between soldering and brazing?”
The American Welding Society (AWS) defines brazing as a process which involves a filler metal that turns to liquid above 842 degrees Fahrenheit (450 degrees Celsius). Brazing applications are a mechanically strong joint. You would typically see this application being used in the automotive industry, for jet engines and in HVAC. You may not know, but your cookware and utensils are also put together using this application. Did you ever wonder how your cookware was so durable? Probably not, but it’s still good to know!

Soldering also involves a filler metal but turns to liquid below 842 degrees Fahrenheit (450 degrees Celsius). This makes the joint electrically strong and can withstand any electric currents running through. If you are soldering any type of electronics, you will need to keep in mind that the temperature needs to stay below 842 degrees Fahrenheit so that your electronics don’t burn up. Yikes! 
Since soldering has a lower processing temperature, the joint is not as strong, compared to the brazing application. This is sometimes the most desirable application, because it has a lower liquidus state compared to brazing.

Of course, always follow your procedure or spec sheet for the proper application for every job!
 
Now we know the difference between the applications, but we need something else to complete the job. “We need flux!”

“What is flux?”
Flux is a chemical cleaning agent, flowing agent, or purifying agent. Flux is used in extracting and joining metals and is crucial to promote melting of the filler metal.
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“Do I even need flux?”
Yes. It is imperative to use flux for the following reasons:

1. Removes oxides from metals to be joined (you will still need to clean your metals prior to application).
2. Prevents oxidation after completion.
3. Improves “wetting out” characteristics and heat transfer, which allows the filler metal to flow and not ball up.

“What type of flux do I need?”
There are many options available, but they all have their advantages and disadvantages.  You will have to take many variables into consideration, before you grab the nearest flux on the shelf. Not all flux is created equal!
Here are the things you need to know, in order to find the right flux for the job:

• Application
• Process
• Type of metals to be joined
• Filler metal
• Thickness
• Diameter of filler metal
• Desired appearance/cleanliness
• Any additional information that is relevant for the job

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​You will have choices when it comes to the type of flux you will need. Here are some types that are out there:

• Flux coated filler metals
• Rosin core solders
• Powder
• Liquid
• Paste
• Gel
• Flux that is “injected” into the flame from the fuel supply

 
 Solder Fluxes:
No Clean Flux- Residue, if any, is minimal and will “vanish” from the heat produced. If residue needs to be removed, it will require solvent cleaning. It’s very hard to remove and could become an issue because it can interfere with electronics. The residue left behind from the flux is conductive.                  Typical primary ingredient: Isopropanol

Rosin Flux- The reaction between the flux and the metallic oxides on the joint metals make the flux flow easily and provides the cleaning required to “wet” the joint. Rosin flux becomes inert when cooled and solid. If electronics heat up enough to make the residue liquid, the acid can start deteriorating connections. Residue can be removed with alcohol. Typical primary ingredient: Rosin (tree sap)

Water Soluble Flux/Organic- Highly acidic and corrosive. Residue should always be removed when complete. Clean with water, steam, or water-based cleaning product. Typical primary ingredient: Isopropanol
 
Brazing Fluxes: 
General Purpose Powder Flux- Active range 1400-2200 degrees Fahrenheit. Use with low fuming Bronze, Nickel Silver, Copper base alloys, Steel and Cast Iron. This is the same flux that is coated on flux Bronze. Typical primary ingredient: Boric acid, Borax

White Flux- Low temperature (1050-1600 degrees Fahrenheit) silver brazing. This flux is used with most ferrous and non-ferrous metals and not recommended on Aluminum, Magnesium, or Titanium. Typical primary ingredient: Boric acid, potassium fluorohydroborate

Black Flux- High temperature (1050-1800 degrees Fahrenheit) silver brazing. Used on heavy parts where overheating or prolonged heating may occur. This flux is recommended when brazing stainless steels. Residue can be more difficult to remove.
​Typical primary ingredients: Boric acid, potassium fluorohydroborate.

Aluminum Flux- A powdered flux designed for use with aluminum alloys (915-1115 degrees Fahrenheit). You can apply this by sprinkling on dry, mixed with water or alcohol to form a paste. Typical ingredients: potassium chloride, sodium chloride.

Flux Injected in Flame- This flux is introduced into the fuel gas and passed through flame. This increases strength, reduced filler metal consumption, minimal post braze cleaning, quicker braze times, increased penetration, and can cut costs by 40%.
Typical primary ingredient: Trimethyl borate.

Hybrid Fluxes- Some manufacturers have hybrid fluxes available for difficult applications. Brown fluxes have been tested to work like black flux, but easier to remove residue.
 
Although there is no “silver bullet” when choosing the appropriate flux, you may find that multiple types of flux are needed but testing and evaluating may be required for best results. Please contact an Earlbeck Gases & Technologies Representative to help determine the best flux for your job!

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Brian Dressel

Account Manager at Earlbeck Gases & Technologies