Soft soldering is the process of joining metal parts by heating and running a low-melting point alloy between the two surfaces being joined. As cooling takes place, the alloy solidifies and results in a secure joint.  For most purposes, the low- melting point solder alloy is a composition of lead and tin.  Solder melts at a temperature less than 300C, which is far below the melting temperature of the metal being joined and produces a low strength joint.

Soft soldering is used extensively in sheet metal work for joining parts which are not exposed to the action of high temperatures and are not subjected to excessive loads and forces. It is used not only to make a mechanical bond between surfaces but also to provide a leak proof seal when liquids have to be contained. It can also be used to provide a permanent electrical connection. To provide a secure joint, it is essential that the joint surfaces are perfectly clean and free from rust, grease or any other substance likely to prevent good metal to metal union. The most likely cause of a bad joint, assuming the surface is clean, is the thin oxide film, which is present on all metals. Oxide films can be removed using emery cloth or a flux.

Glossary of Terms

Fillet.  An external build up of joining material upon the surface of a joint to form a radius and provide additional strength.

Wetting. Adhesion of solder or brazing alloy to the metal surface during the process of soldering or brazing.

Capillary Attraction. Phenomenon associated with the attraction of liquid within confined areas without the application of external forces or pressure. In brazing and soldering it refers to the unassisted passage of molten joining material into the heated joint gaps.

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Wipe.  The process of spreading molten solder into semi-molten or plastic condition over a joint using a wiping cloth.

Dry Joint. The portion or surface of brazed or soldered joint, which has not been wetted by the joining material.

Creep Joint or Hard Joint. The ability of brazed or soldered joint to withstand long continued stresses.

Dipping. It is the process in which joints of assembly to be brazed or soldered are dipped into the bath of molten brazing alloy or solder which enters the joints by capillary action.

Dross. Layer of impurities, which form on the surface of the molten metal in a dipping bath.

Parent Metal. The material of the parts to be brazed or soldered.

Joint Surface. The surface of the parent metal to which jointing metal will adhere to make the joint.

Joint Clearance. The space or distance between the joint surfaces.

Soldering Bit. The copper portion of the soldering iron, which when heated provides the means of heating the work and melting the solder during soldering operation.

Liquidus. The temperature above which solders, brazing alloys, etc. are completely molten.

Solidus. The temperature below which solders, brazing alloys, etc. are completely solid.

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