Hard Soldering Process and Flux

Soldering Process

A large number of joining processes are used in industry to make the finished products like aeroplanes, ships, cars, vehicles, radios, televisions, computers, machine tools and domestic appliances. Different methods used for joining components are riveting, brazing, welding, screw fastening devices, etc. In the engineering industry, for instance, soldering finds applications in making turbine blades and discs, pipelines, radiators, fins, in air-cooled engines, bicycle frames, industrial receptacles, gas equipment, etc.

Soldering is the process of joining two or more similar or dissimilar pieces of metal by the application of low melting alloy (solder) and heat.

Before describing soldering process in detail, in order to produce successful soldered joints following important points need to be adhered:

(a)          Metals to be soldered together must be chemically and mechanically cleaned. All the oxides, grease and dirt must be removed.

(b)          Metals to be soldered together must be heated to required temperature.

(c)          Metals to be soldered together must be firmly supported during the soldering operation and until they cool.

(d)          Proper flux must be used. Flux must be fresh and chemically pure.

(e)          Soldering operation should be done as quickly as possible.

(f)           An excess solder is useless and looks ugly.

(g)          All the flux traces must be thoroughly cleaned after the completion of soldering operation.

Stages of Soft Soldered Joint

In general, there are two stages of making a soft soldered joint. These stages are given below:

(a)          Tinning the metal surfaces

(b)          Filling the spaces between the tinned surfaces with solder

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Soldering Operation

The three most common soldering operations are:

(a)        Tacking

(b)        Sweating

(c)        Floating

Tacking.        Local joining of metal parts by welding, brazing or soldering to assist location prior to completing the assembly process.

1          Hot copper bit

2          Stick of soft solder

3          Melting surfaces pressed together with the aid of a wooden stick

4          Small ‘blobs’ of solder

Sweating.     It produces a continuous joint as shown. A thin coating of a suitable flux is applied to the mating surfaces, which have been previously cleaned. A uniform coating of solder is applied to the surfaces to be joined. This process is called tinning. Solder flows more rapidly between surfaces that have been pre-tinned. The joint edges are then placed together with their tinned surfaces in contact and are held by pressing them with a stick or an old file. A heated soldering iron is placed on one end of the seam ensuring maximum surface contact between a face of the bit and the joint metal.


As the solder between the two surfaces begins to melt and flow out from under the edges, the bit is slowly drawn along the seam followed by the stick pressing down on the joint to keep the thickness to a minimum. The success of sweating depends upon having an adequate and constant supply of heat energy.

Floating.        This is used to seal self-secure joints. One method is to use a specially shaped bottoming soldering iron. The joint is fluxed inside and outside and held at 45 on a wooden block. A small quantity of soft solder in the form of ‘buttons’ or ‘blobs’ are dropped into the container and melted with the heated and tinned bottoming iron. This molten solder is then carefully flooded along the inside of the joint as the container is slowly rotated. The quantity of solder is uniformly controlled by steady manipulation of the soldering iron, adding blobs of solder as and when required.

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If the bottoming soldering iron is not available, an alternative method is to use a flame as a heat source as shown. An asbestos glove should be worn as protection from the flame and the heat radiation from it. The flame is carefully applied to the outside of the joint, and the same procedure as shown is to be followed. A section through the shield joint is shown.

Scratch Awl

Scratch awl is a sharp point hand tool and is used during soldering for clearing dirt and greases on the metal parts where the joint is being provided.

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