Soldering is a method of joining similar or dissimilar metals by the application of heat and using a filler metal or alloy called solder, whose liquidus temperature is below 4500 C. The molten filler metal is made to flow between the two closely placed adjacent surfaces by the capillary action.

Though soldering obtains a good joint between the two plates, the strength of the joint is limited by the strength of the filler metal used. Soldering is used for obtaining a leak proof joint or a low resistance electrical joint. The soldered joints are not suitable for high temperature applications because of the low melting temperatures of the filler metals used.

The purpose of using the flux is to prevent the formation of oxides on the metal surface when the same is heated. The fluxes are available in the form of powder, paste, liquid or in the form of core in the solder metal. It is necessary that the flux should remain in the liquid form at the soldering temperature and be reactive to be of proper use. The filler metals used are essentially alloys of lead and tin. The composition of solder used for different purposes are as given below

Soft solder - lead 37% tin 63%

Medium solder - lead 50% tin 50%

Plumber’s solder - lead 70% tin 30%

Electrician’s solder - lead 58% tin 42%

Soldering is classified into soft soldering and hard soldering.

Soft soldering is used extensively in sheet metal work for joining parts that are not exposed to the action of high temperatures and are not subjected to excessive loads and forces or vibrations. Soft soldering is also employed for joining wires and small parts. The solder is mostly composed of lead and tin. In soft soldering, Zinc chloride and ammonium chloride are the most common soldering fluxes used which are quick acting and produce efficient joints. But because of their corrosive nature the joint should thoroughly cleaned of all the flux residue from the joint. These are to be used only for non-electrical soldering work. Rosin and rosin plus alcohol based fluxes are least active type and are generally used for electrical soldering work.

Hard soldering employs solder which melts at higher temperatures ( 6000 C to 9000 C) is stronger than used in soft soldering. Hard solder is an alloy of copper and zinc to which silver is added some times. German silver, used as a hard solder for steel is an alloy of copper, zinc and nickel.

Sequence of operations:

The following operations are required to be performed sequentially for making soldered joints.

Shaping and fitting of metal parts together: The two parts to be joined are shaped to fit closely so that the space between them is extremely small and filled completely with
  1. solder by capillary action. If a large gap is present, capillary action will not take place and the joint will not be strong.
  2. Cleaning of surfaces: In order to obtain a sound joint, the surfaces to be soldered are cleaned to remove dirt grease or any other foreign material.
  3. Application of flux: The flux is applied when the parts are ready for joining.
  4. Application of heat and solder: The parts are held in a vice or with special work holding devices so that parts do not move while soldering.

The soldering iron or bit may either be heated electrically or by a gas flame. The soldering bit is heated sufficiently so that the heat acquired by it is sufficient enough to melt the solder immediately when the latter is applied to it. A useful form of soldering iron is the universal type which has a copper bit tapered to form an edge at its end. This facilitates the bit to be inclined at any desired angle to suit the odd shaped jobs.

After the soldering iron has been heated to the desired heat, its surface is cleaned and then dipped in a mixture of flux and solder. The solder is then melted into the joint is smoothed over and finished by the use of the soldering iron. Another practice is to first dip it in a mass of flux followed by the application of solder. This enables the solder to melt and spread over the hot surface of the bit to form a coating over it. This operation is known as tinning. After this, the bit is again dipped in the flux to remove the oxides from its surface, if any, and then in the solder again to pick up its required quantity. It is then ready for application to the work. The solders which have low percentage of tin have a higher melting point. Sometimes it becomes necessary to heat up the job instead of the bit to get good results. In soldering big jobs, the solder is used in form of wire, sometimes having a core containing flux.