Soldering is a delicate operation of joining two metal parts. The presence of any dirt, grease or oxide on the surfaces being soldered will adversely affect the quality of soldered joint. Seme chemicals called soldering fluxes are used in soldering, depending on the metals to be soldered. The soldering fluxes are available in the form of powder, paste, liquid or solid. Flux also improves joint strength. It also protects the metal from oxidation.

After the flux is applied on the joint surface (before soldering), it is heated by a soft flame or soldering iron. The flux loses its liquid, then melts and decomposes to form hydrochloric acid which dissolves oxides film from the surfaces of the workpiece. The fused flux provides a protective cover over the soldered joint.

Soldering Fluxes


a) Inorganic or acid corrosive fluxes consist of zinc chloride and ammonium chloride. These are acidic in nature. A flux with zinc chloride (75%) and ammonium chloride (25%) with melting point 177°C gives a very good cleaning effect and covering properties. This flux is used in liquid form because during soldering, the water or solvent evaporates and the flux melts. Zinc chloride type fluxes are used for soldering low carbon steels, low alloy steels and copper alloys (brass).

b) Mild fluxes are composed of organic acids which are less corrosive than the inorganic acids. Although as applied, the organic fluxes are also corrosive, on heating, these become mildly corrosive. These fluxes comprise lactic acid, stearic acid, etc. Organic fluxes are useful in situations where sufficient heat can be used to fully decompose the flux so that corrosive constituents may be volatilized.

c) Non-corrosive fluxes are the rosin fluxes and these are non-corrosive, and are used for soldering of electrical and electronic joints. This flux is obtained from tars found in pine trees. Since this flux acts slowly (its spreading is slow), some hydrochloride compound is added to rosin to work as an activator. Rosin fluxes are used on copper and copper alloys.

Surface Cleaning Agents Used in Soldering Although flux also serves the purpose of cleaning the surface, before starting the soldering operation, the workpiece surface to be soldered is first cleaned off dirt, grease and other foreign matter. Cleaning can be done by wire brushing, scrapping, grinding or sanding Some chemical agents are also used for cleaning. These are (a) alkaline degreasing solvents so clean oil or grease, and (b) pickling (acid cleaning) to remove rust, scale or oxides


1.     The surfaces to be joined by soldering must be thoroughly cleaned off any den grease or oxide by acid cleaning (pickling). Wipe out the joint clean

2.     The joint surfaces are heated with a soft oxy-acetylene flame

3.     Soldering flux is later applied on to the heated surfaces of the joint. It is beated further sometimes.

4.     The solder is then applied with the help of soldering iron and it is allowed to melt only by the heat present in the heated surfaces of the workpiece 

5.     Soon after the soldering operation is over, the soldering flux should be cleaned from the joint with a wire brush or by filing.


Soldering methods are classified according to the method of applying heat to melt the solder and also for pre-heating the surfaces to be soldered. These include soldering iron method, torch method, dip bath method, resistance method, ultrasonic method, induction method. wave soldering, etc. Only the most common methods of soldering have been elaborated in the following

Soldering Iron Method

The soldering iron method (or soldering copper method) is one of the oldest methods used for lead-tin alloys soldering operations. The method is shown in Fig. 8.2. The soldering iron consists of a square or octagonal solid copper rod having at its front end a four-sided tapered copper point and the tool is available in different sizes. The copper point helps in quick transmission of heat from the tool to the workpiece joint. Solders are available in the form of wire or strip. The heated end of soldering copper touched with the solder to melt a part of it, which is subsequently tinned over to the copper point transferred to the joint.

The soldering copper may be heated by gas flame, blow lamp, or an electrical resistance heating system. Electric soldering irons are available which maintain uniform heat. A 200-watt iron is commonly used for most sheet metal soldering. The tip of the soldering iron must be kept clean and tinned. Soldering iron gives concentrated heat which is most desirable for soldering. It is easier with soldering iron to spread or smoothen the solder on the joint surface.

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