Flux Used in Soft Soldering

The operation of brazing, soldering and welding has brought out the use of fluxes in service. Flux is a chemical substance, which is made for cleaning the surface of the job and preventing oxidation in operations, such as welding, soldering and brazing. It is rather difficult to have satisfactorily brazed or soldered joints without the use of fluxes. Therefore it is very essential to use flux in soldering, brazing and welding. Soldering flux may be defined as a substance (solid or liquid) applied to a metal to make solder flow readily and gives a permanent joint. The success of a solder depends to a large extent upon the proper choice of flux.


The soldering fluxes are needed due to the following reasons:

(a)          To clean the surface of the base metal during heating.

(b)          To eliminate impurities appearing on metal surface.

(c)          To breakdown the surface tension of molten solder which helps the easy flow.

(d)          To prevent the formation of fresh oxides by forming a protective layer on the solder.

Qualities of a Good Soldering Flux

The requirements of a good flux are:

(a)          It must remain liquid at the soldering temperature.

(b)          In its liquid state it must act as a cover over the joint and exclude the air.

(c)          It must dissolve any oxide film present on the surface being joined.

(d)          It should be readily displaced from the joint surfaces by the molten solder.

Action of Flux

A  –  Flux solution lying above oxidized metal surface

B  –  Boiling flux solution removing the film of oxide (as Chloride)

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C  –  Bare metal in contact with fused flux

D  –  Liquid solder displacing fused flux

E  –  Tin reacting with the base metal to form compound

F  –  Solder solidifying

Types of Fluxes

Soft soldering fluxes can be divided into two categories:

(a)          Active or Chloride Fluxes

(b)          Safety or Protective Fluxes

Active or Chloride Fluxes

Chloride fluxes are used in most of the soldering works. These fluxes quickly dissolve the oxide, rust and grease from the base metal and at the same time act as a barrier to prevent the further oxidation. Unfortunately, all active fluxes have a corrosive residue, so the joint must be carefully washed with hot water after completion of soldering. Otherwise residue of such flux may cause corrosion. The complex type of soldered joint should be immersed in a solution of 1 to 2% (by volume) of hydrochloric acid (HCl) in water then washed carefully in water.

Because of difficulty of avoiding residual corrosion, the use of active fluxes is banned in electrical and other works which cannot be washed effectively. These fluxes are made in various forms as paste, salt or fluid.

Safety or Protective Fluxes

These fluxes are less corrosive in action as compared to the chloride fluxes and less active in removing surface oxides. These prevent oxidation during soldering but will not remove oxide which is already present on the joint surfaces. So surface must be cleaned properly before soldering. These types of flux are especially used on electrical, radio and computer assemblies or other works where complete freedom from acid or corrosion is necessary. These are supplied in four forms as follows:

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(a)          Paste / powder flux

(b)          Soldering fluid

(c)          A soldering cream

(d)          Cored solder wire

Specific Fluxes for Different Metals

Metals Fluxes
Steel Ammonium chloride
Lead Tallow
Zinc and galvanised work Dilute hydrochloric acid
Brass Zinc chloride (Killed spirits) or resin
Tin Zinc chloride

Different Types of Fluxes and Uses

Nomenclature of Flux Type Uses
Compound tinning Active Tinning big-end bearings
Flux, soldering


Safety Soldering of stainless steel conductors of ignition cables
Paste, soldering Safety All metals, except stainless steel: where a non-corrosive joint is required
Solution, soldering Active For general purpose
Hydrochloric acid(HCl) Active For general purpose, zinc and galvanised iron
Resin Safety Electrical work
Tallow Safety Soldering of lead and zinc metals
Olive oil Safety Pewter (Brittania metal)
Flux, non-corrosive Safety For use in affixing identification labels to steel tubing
Ammonium chloride or Sal-ammoniac Active For tinning large surfaces, cooking tins, tinning soldering iron
Phosphoric acid Active Stainless steel pipes
Zinc chloride or D.T.D. 81 or Killed Spirit Active General engineering work (i.e. sheet metal work)