Home page » Useful information » Basics of Choosing Fluxes for Soldering
Basics of Choosing Fluxes for Soldering
Basics of Choosing Fluxes for Soldering/Brazing
Flux is a substance consisting of a mixture of various components, used to remove the upper layer of oxidized particles from a metal surface to ensure high-quality soldering/brazing.
Fluxes can be classified as follows:
- By type of solvent: aqueous or non-aqueous.
- By activity temperature (relative to 450°C) and type of activator.
- By the state of the flux — liquid, solid, and gaseous fluxes are distinguished.
- By type of action — chemical and electrochemical action, protective and reactive.
For high-quality soldering/brazing, the flux must:
- react with the metal’s oxide particles before the solder itself melts
- provide sufficient surface wettability
- retain its composition within the required temperature range
- be such that, after the process is complete, it can be easily removed and does not corrode the surface
Materials for compounding a flux are selected based on the characteristics of the oxides of the material being processed. For example, for acidic oxides a basic flux is used, and for a basic oxide film, acidic fluxes are used.
Metal fluorides may be introduced to increase activity.
When soldering/brazing materials with poorly soluble oxide films, alloying additives of certain metals are added to the fluxes for better dissolution of these films.
In industrial soldering/brazing, fluxes are usually prepared from pure materials. The most common component is Zinc chloride of “pure” grade or its foreign-made equivalent. Ammonium chloride is also often used.
Differences in Components for Different Metals:
For soldering/brazing Aluminum, its alloys and materials containing it, as well as heat-resistant and hard-alloy materials, fluorides are used — Hydrofluoric acid, Sodium fluoride, Ammonium fluoride.
Cryolite is used when soldering/brazing Aluminum and its compounds. Both sodium cryolite and potassium cryolite are used. Such compounds are also called sodium fluoroaluminate, or the complex mineral potassium fluoroaluminate (“KM-FAK”).
Hydrochloric acid is often used for soldering/brazing non-ferrous metals and alloys.
For Zinc and copper alloys, Paraffins, waxes, Stearin, and others are used.
Borax and boric acid are used for steels and cast irons, hard and heat-resistant alloys.
Organic acids and Orthophosphoric acid are used for bronze and certain steels.
After the operation is completed, flux components remain partly on the metal surface, so an additional surface-cleaning operation is required. This is carried out with solvents, weak alkaline solutions, for example Caustic soda; treatment with concentrated Nitric acid or its weak solutions is also possible. In some cases, rinsing with plain water is possible.
Fluxes based on Ethylene glycol or rosin do not react with the metal surface, so their residues do not need to be removed, or alcohol and other solvents can be used.
The rinsing method depends on the flux components.
