The brazing flux is made of a mixture of solid potassium salts at room temperature. It is the essential element and it is imperative it possesses the following properties:
- Melt at a temperature at least 100 °C below the solidus (Correlation between solder and assembled parent metals).
- Remain stable until the required maximum temperature (melting range).
- Remain active for the duration of the soldering cycle (Lifecycle).
- Dissolve the metal oxides of the different metals involved (Flux type choice).
- It must be sufficiently fluid, even though it is loaded with dissolved oxides to be pushed away by the solder alloy in the capillary joint interstices.
- Must form a continuous thin layer on the joint surface to prevent oxidation.
- Leave little or no residue which will be easy to remove after soldering.
The surfaces to be brazed must be cleaned beforehand, in order to get rid of the grease and oxides present on the surface before and during the brazing process in the air and whose presence hinders the wetting of the solder.
Some precautions must be taken when using them. The fluxes contain irritating and / or toxic products and must not remain in contact with the skin. Moreover, at certain temperatures, the fluxes are unstable and release toxic vapors which could damage the eyes and the mucous membranes. It is necessary to ventilate the workshops and the brazing area.