What causes flux to suck-in solder?

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In summary, flux is used to remove the oxide layer on the parts, to prevent re-oxidation, and to create clean wettable conditions. This allows capillary action to draw the molten solder into the joint.
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Stephen Tashi
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The flux applied to copper pipes and fittings before they are soldered together is said to suck the solder into the space between the fitting and the pipe. Empirically, the solder does flow into the narrow space. What physical phenomenon causes this? - an actual vacuum?
 
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Same reason water rises up a capillary tube I would think.

The flux is used to destroy the oxide layer on the parts, and to prevent re-oxidation.
It acts upon the surface tension, and all those other inter-molecular forces, so that you just don't end up with a ball of solder.
Note that it is easier to solder copper and silver versus a steel.

A comprehensive discussion.
https://opentextbc.ca/physicstestbook2/chapter/cohesion-and-adhesion-in-liquids-surface-tension-and-capillary-action/
 
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  • #3
When soldering copper pipe you must remove the oxide layer first with sand paper or what ever. Needed is a bright shiney oxide free surface. Perhaps it is a reduction in surface tension that draws the solder in.
 
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"In soldering of metals, flux serves a threefold purpose: it removes any oxidized metal from the surfaces to be soldered, seals out air thus preventing further oxidation, and by facilitating amalgamation improves wetting characteristics of the liquid solder. "

https://en.wikipedia.org/wiki/Flux_(metallurgy)#Soldering
 
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  • #5
Flux doesn’t suck solder in, it creates clean wettable conditions as mentioned above so that capillary action can draw the molten solder in.
 
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darth boozer said:
"In soldering of metals, flux serves a threefold purpose: it removes any oxidized metal from the surfaces to be soldered, seals out air thus preventing further oxidation, and by facilitating amalgamation improves wetting characteristics of the liquid solder. "

https://en.wikipedia.org/wiki/Flux_(metallurgy)#Soldering
If the oxide layer on copper pipe is not removed mechanically before soldering the joint will most likely leak.
 
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As Marcus above states, it is the capillary action between the close-fitting pipe and fitting joint. When molten, the solder flows very nicely TOWARDS the heat, so that is why one heats the joint to be soldered, and then apply the solder from the outside. You know you are at the right temp when the solder melts smoothly from the stick and flows cleanly along the flux edge. The flux is to to keep the surface oxide clear while heated, since metals oxidize easiest when heated. But it is a combination of exactly where the heat in the joint is and the capillary effect. When molten you can pull the solder along by heating where you want it to go, it will flow towards the heat.

As a US Navy Welder, this is something I had to be GOOD at, and not just tin/lead solders, but high grade .30 and.50 grade silver solders for high pressure steam and air lines. Is actually closer to brazing than soldering, but it is a matter of the use and different temps. Tin/lead is such a low melt point compared to even a soft silver solder, but they still work the same way in how they 'wet' and flow along the surface and into the joint being soldered.

I could bring a clean meniscus to both the outside and inside surfaces of the solder joint. This was one of the tests, along with pulling a 1/4 oz of grade 3 silver solder through 6 feet of 1/2 inch thin walled copper pipe. Lots of prep, knowing what you are doing with the equipment and a knowledge of physics made a big difference in my training time. I was top of my class with Welding/Hull Tech School.
 
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1. What is flux and what is its role in soldering?

Flux is a chemical compound that is used in soldering to remove oxides from the metal surfaces being joined. It also helps to prevent new oxides from forming, ensuring a strong and clean bond between the solder and the metal.

2. How does flux cause solder to be "sucked-in"?

Flux creates a chemical reaction with the metal surfaces, causing them to heat up and become more reactive. This increased reactivity pulls the molten solder into the joint, creating a strong and secure bond.

3. What are the common causes of flux not being able to "suck-in" solder?

Some common causes of flux not being able to "suck-in" solder include using the wrong type of flux for the metal being soldered, using too little flux, or using old or expired flux. Contaminants on the metal surfaces can also prevent the flux from working effectively.

4. Can flux cause any negative effects on the solder or the metal being joined?

Flux can cause negative effects if it is not used properly. Using too much flux can lead to excessive residue, which can be difficult to clean and may affect the electrical conductivity of the joint. Additionally, using flux with corrosive properties on certain metals can cause damage to the metal surface.

5. Are there any alternatives to flux for soldering?

Yes, there are alternative methods for soldering that do not involve the use of flux. These include using a soldering iron with a built-in flux core, using solder paste, or using a fluxless soldering technique. However, flux is still widely used and recommended for its effectiveness in creating strong and clean solder joints.

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