How to choose a flux for soldering

The most important element in the repair of any equipment is soldering. Solder, fluxes, solder paste - all this must be taken into account when performing repair work. If with solder all is relatively clear - usually for this use tin-lead solders with different melting points (depending on the composition of the alloy), then what about the flux? What is it for?

The main purpose of the flux is removal from the surface of the oxides, as well as a decrease in the surface tension in order to improve the spreading of the solder. In addition, the flux for soldering serves to protect the junction from the effects of the external environment.

What are the fluxes?

According to its effect on metal surfaces, the fluxes are of the following types.

• Active (also called acid). They contain hydrochloric acid, fluoride and chloride metals. The active flux for soldering intensively dissolves the oxide film formed on the surface of the metal, thus ensuring a high mechanical strength of the future joint. It should be remembered that the active flux is not suitable for repairing electrical equipment, since its residues corrode the soldering site over time.
• Acid-free . These include rosin, as well as rosin-based fluxes with the addition of turpentine, alcohol or glycerin. The acid-free flux for soldering not only cleans the surface of the metal from oxides, but also protects it from further oxidation. In addition, the use of rosin does not cause corrosion of the surface. It is usually used for soldering copper, bronze and brass products.
• Activated . Prepared from rosin, which is added in small amounts of phosphoric or hydrochloric acid aniline, salicylic acid and hydrochloric acid diethylamine. Activated flux for soldering can be used for a wide variety of metals and alloys (iron, copper, nickel, bronze, nichrome, silver, steel). It can also be used for soldering oxidized parts from copper alloys without their preliminary stripping.
• Anticorrosive . These fluxes are made from phosphoric acid with the addition of various solvents and organic compounds. In addition, some of these fluxes may include organic acids. Anticorrosive flux is used for soldering copper and its alloys, as well as silver, platinum and constantan.
• Protective . These include olive oil, wax, petroleum jelly, powdered sugar. Protective fluxes do not have a chemical effect on the metal, and also protect the cleaned surface from oxidation.

In addition, for soldering products made of cast iron, carbon steels, and also copper and its alloys, a drill (sodium tetraborate) is used which is a white crystalline powder having a melting point of 741 ° C.

Also, the borax (more precisely, its mixture with boric acid in a ratio of 1: 1) is used for soldering stainless steel and hard refractory alloys.

As a flux for brass products use a mixture consisting of equal parts of table salt and calcium chloride.

Soldering of aluminum products requires a flux having a low melting point. Usually, the flux for soldering aluminum contains from 30 to 50% of potassium chloride.

Flux for soldering can be produced in the form of powder, liquid or paste. In addition, there are special solder paste, in which the solder particles are already contained together with the flux.

What is important to know when performing soldering

When choosing a flux for soldering, not only what material is made of the parts to be soldered, but also what solder is used, is taken into account. The melting point of the flux should not exceed the melting point of the solder.

Regardless of the type of flux used, it is necessary to wipe the place of soldering after work with a cloth dampened in acetone or in alcohol-rectificate. Then clean this place with a brush or brush moistened with any solvent to remove the flux residues. This is especially true for the active flux, because the products of its decomposition not only contaminate the soldering site, but also are a source of corrosion.