Resistance welding.

One common factor in all the above processes is that each of them is dependent upon a suitable source of heat energy. The application or choice of a particular thermal joining process may vary according to the temperatures used to produce an efficient joint and, of course, according to the materials being joined.

Soft soldering

Soft soldering is a low temperature thermal process in which the actual metals being joined are not melted. The process involves the use of a suitable low-melting-temperature alloy of tin and lead which is “bonded”, by the application of heat, to an unmelted parent material.

Soft solders, therefore, must have a lower melting point than the metals they join.

It is an essential feature of a soft soldered joint that each of the joined surfaces is “tinned” by a film of solder and that these two films of solder are made to “fuse” with the solder filling the space between them.

Basically the action of a soft solder when applied to a prepared metal joint which has been heated to the required temperature is to:

a) flow between the parent metal surfaces which remain unmelted;

b) completely fill the space between the surfaces;

c) adhere thoroughly to the surfaces;

d) solidify.

In a correctly soldered joint, examination under a microscope has shown that in the action of tinning metals, such as brass, copper and steel, a definite chemical reaction takes place. The metal surface and the tin in the solder react together to form an “intermetallic compound” which, to some extent, becomes part of the parent metal.

In general there are two stages in making a soft-soldered joint:

1. Tinning the metal surface.

2. Filling the space between the tinned surfaces with solder.

Preparing the joint

The “tinning action” of solder cannot take place unless the two surfaces to be joined are chemically clean. Not only must the mating surfaces be free from dirt and grease, but also of any oxide film.

Cleaning for soldering

Prior to any soft soldering, brazing or welding operations it is necessary to expose bare metal at the mating or joining surfaces. Dirt, grease, oil or oxides are themselves unsolderable and would act as barriers between the molten solder and the metal surfaces to be joined.

Oxygen combines with substances much more rapidly at elevated tempera­tures. Therefore, it is very important that metal surfaces must be prevented from oxidizing whilst they are being heated to the soldering temperature. A suitable soldering flux is used for this purpose.

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