Elements of Soldering

Essential elements required for soft soldering are

(a) Soldering iron

(b) Solder

(c) Flux

(d) Source of heat

Soldering Irons

A soldering iron consists of a copper bit, secured to a iron holder, which is fitted with a wooden handle. The soldering irons are made in various sizes. The type of soldering iron selected must be large enough to heat the job adequately. The following soldering irons are used in service:

(a) Common Soldering Iron. It is used for general purpose. These are available in various sizes and shapes. The bit is heated by blow lamp or   brazing lamp.

(b) Electric Soldering Iron. This is a very convenient tool for repetition work. It is used in a workshop where other forms of heating are not available or desirable. Its great advantage lies in the absence of any open flame for heat. So, it is always clean and very seldom requires re-tinning. This comes in various sizes, shapes and varying voltage ratings. An iron must not be connected to a voltage other than that stated. A switch is fitted to the lead for temperature control, and must be switched off when iron is not in use.

(c) Alumino Thermic Soldering Iron. This type of iron is used where no other heating source is available. The copper bit incorporates a circular cavity, in which a ‘MOX’ tablet (Magnesium and Aluminium oxide) is placed. The tablet is ignited by a special match, termed as “Fusee” which heats the bit. These irons must not be used near aircraft, MT vehicle or other inflammable stores while the tablet is burning.

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Soft solders are mainly alloys of tin and lead. A small percentage of antimony is added to give a harder and stronger joint.  These are supplied in various forms such as bars, strips and wires. Some of the standard grades of soft solders and their uses are as follows:

Type of Solders Composition Percent Working temp.in oC. Uses & Properties
Lead Tin Antimony Silver Solid Liquid
Tinman’s 58 40 2 185 227 Fine and general work (termed as grade ‘C’)
Electrical 40 60 185 230 Resin-cored for instrument and electrical work
Lead Silver 97.5 2.5 183 304 High melting point solder. It retains the strength at higher temperature than Lead –Tin solder


These are necessary in both soldering and brazing. Flux is used for the reasons given below:

(a) To protect the work and the filler alloy from oxidation during heating.

(b) To dissolve oxides, which are already present or produced un-intentionally.

(c) It helps in easy flow of molten solder.


Fluxes can be divided into two main categories:

(a) Active or Chloride fluxes. These fluxes are acidic by nature and are available in various forms such as salt, fluid and paste.  It removes considerable amount of oxide, rust and grease. As it is acidic in nature, job must be cleaned after soldering operation.

(b) Safety or Protective fluxes. These type of fluxes are non- acidic by nature. These are used where acidic or corrosive action is prohibited. It is available in paste form or squeezed into the core of the solder wire.

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Method of Soldering

Whenever soft soldering is required to be carried out, the following process is to be adopted:

(a) Clean the parts thoroughly.

(b)  Tin the parts to be joined, if necessary.

(c)  Assemble the job in proper position.

(d)  Apply flux slightly.

(e) Melt the solder between the joints with a hot, cleaned and tinned soldering iron.

(f)   Allow the solder to get completely cooled.

(g) When it is cooled, wash well with clean water (normally warm soapy water).

(h) Remove all the traces of the flux, which will make the metal corrosive if left in contact with metal.

Causes of Faulty Soldered Joints – Causes of faulty soldering are as follows:

(a) Surface is not cleaned thoroughly.

(b) Surface is not assembled properly.

(c)  Wrong type of solder is used.

(d) Wrong type of flux is used.

(e) Soldering iron is not heated enough.

(f) Soldering iron is over heated.

(g) Soldering iron is not tinned properly.

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