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The Importance of Flux in Soldering Sheet Metal

The Importance of Flux in Soldering Sheet Metal

If you are soldering any kind of sheet metal, it is not only important to have the right solder and equipment, but also to have the right flux.  Flux plays a vital role in soldering as it enhances the bonding of the joints as well as helping the performance of the soldering process. 

Below are 4 reasons why flux is needed in soldering sheet metal:

1. Flux helps with solder flow and wetting:

Flux plays a key role in solder flow and wetting. Wetting is the ability of the solder to spread and adhere to the metal surface to help ensure a solid bond. Flux, made of an activator, facilitates wetting by helping to reduce the surface tension of the solder allowing it to flow more uniformly and smoothly over the sheet metal.  Proper wetting means that the solder joint was correctly formed.

2. Flux prevents oxidation

All metals oxidize faster during the heating process of soldering.  Oxidation will form an outside layer on the surface of the sheet metal that can potentially inhibit the solder from adhering to the surface.  Flux protects the metal surface from oxidation as it reacts chemically with the oxides.  Therefore, when you apply flux before soldering, it allows proper solder wetting to occur.

3. Flux helps with heat transfer:

Another function of flux is to improve the heat transfer during the soldering process. Flux acts as a thermal conductor and it helps to distribute the heat evenly across the metal surface. This helps to create a consistent soldering temperature and prevents either under or overheating. By achieving the ideal soldering temperature, it helps to create optimal solder flow.

4. Flux acts as a cleaning agent:

Flux also acts as a cleaning agent as it removes dirt, grease, and contaminants from the surface of the sheet metal.  These impurities hinder the solder from bonding correctly and lead to weak solder joints.  Flux dissolves these impurities and allows the solder to adhere correctly to the clean metal surface.  Proper preparation of the surface with flux helps ensure the contact between the solder and the sheet metal and results in more reliable and stronger connections.

The Best Flux for Sheet Metal

Not only is flux required for soldering, but it is important to choose the right flux for your particular type of sheet metal.

The best flux for sheet metal should be a strong and active flux with the activators that work on the type of metal you are soldering.   Each type of metal has its own oxide which determines the chemical flux type required.   For instance, a flux that is effective on copper oxides will not work on steel oxides.

While flux is necessary for soldering, it is very important to clean off any flux residues post soldering that are left behind.  The flux can be easily removed by flushing the surface with water or wiping it down with a damp or wet cloth. By cleaning off the flux residues, you help both the longevity of the solder joints and avoid corrosion after assembly.  


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