How to choose a soldering flux

most important element in the repair of any equipment is soldering.Solder, flux, solder paste - all this must be considered when carrying out repair work.If everything is relative to the solder is clear - this is usually applied to tin-lead solders with different melting points (depending on the alloy composition), then what about the flux?Why is it needed?

main purpose of flux - removal of surface oxides, as well as reducing the surface tension in order to improve the spreading of the solder.In addition, the flux for soldering junction is protected from the external environment.

What are the fluxes

On the action exerted on the metal surfaces, fluxes are of the following types.

  • active (also known as acid).In their composition contains hydrochloric acid, fluorides and chlorides metals.Active soldering flux actively dissolves the oxide film formed on the metal surface, thereby providing a high mechanical strength of the future connection.It should be remembered that the flux activity is not suitable for the repair of electrical equipment, as its remnants eventually corrode the brazed.
  • Acid-free .These include rosin and rosin cooked with added fluxes turpentine, alcohol or glycerin.Acid-free soldering flux does not only cleans the metal surface of oxides, but also protects it from further oxidation.Furthermore, the use of rosin causes corrosion of the surface.Typically used for soldering copper, bronze and brass products.
  • Activated .Are prepared from rosin, which is added in small amount of phosphoric or aniline hydrochloride, salicylic acid and diethylamine hydrochloride.Activated soldering flux may be applied to a wide variety of metals and alloys (iron, copper, nickel, bronze, nichrome, silver, steel).It can also be used for brazing parts of oxidized copper alloys without pre-stripping.
  • Anti .These fluxes are made of phosphoric acid with addition of various organic solvents and compounds.Furthermore, the composition of some of these fluxes may contain organic acids.Corrosion flux is used for brazing copper and its alloys, as well as silver, platinum and constantan.
  • Protective .These include olive oil, wax, petrolatum, powdered sugar.Protective fluxes do not have a chemical effect on the metal, as well as protect the cleaned surface from oxidation.

Furthermore, soldering of cast iron, carbon steels, and copper and its alloys are used borax (sodium tetraborate), which is a white crystalline powder having a melting point 741oS.

Also borax (more precisely, its mixture with boric acid in a ratio of 1: 1) is used for brazing stainless steel and superalloys solid.

as a flux for brass products use a mixture of equal parts salt and calcium chloride.

for brazing aluminum products required flux having a low melting point.Typically, the flux for aluminum brazing comprising 30 to 50% potassium chloride.

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

What is important to know when the soldering

When selecting a flux for soldering is taken into account not only the material from which made the parts to be soldered, and a solder is used.The melting point of the flux should not exceed the melting temperature of the solder.

Regardless of the type of place soldering flux after use should be wiped with a cloth soaked in acetone or alcohol rectified.Then clean the place the brush or brush moistened with any solvent to remove flux residue.This is especially true of the active flux, as its decomposition products are not only pollute the soldering, but also are a hotbed of corrosion.