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Soldering, Brazing, and Solid-State Diffusion Bonding

The three joining processes that will be described here are soldering, brazing, and solid-state diffusion bonding. Soldering and brazing both involve using a filler metal that is heated above its melting point, made to wet the mating surfaces of a joint, with or without the aid of a chemical fluxing agent,leading to the formation of metallurgical bonds between the filler and the respective components.

By convention, the joining process is defined as soldering if the filler metal melts below 450 C and as brazing if it melts above this temperature. In both soldering and brazing, it is uncommon for the original surfaces of the components to be eroded by reaction with the filler beyond the microscopic level (<100m). Solid-state diffusion bonding involves placing surfaces of two components in contact under a loading that at the least is provided by the weight of the upper component and heating the assembly until the voids at the interface have been removed by diffusion (see next section).

There is another important type of joining process—welding—that involves the fusion of the touching joint surfaces by controlled melting by heat being specifically directed toward the joint. Welding, using a laser or a microplasma torch, is being successfully applied in chain making (see under Karat Gold Brazing Alloys later).

Certain properties of gold are used to advantage in metal joining. In particular,

• Gold does not oxidize when heated in air, neither does it tarnish. This metal possesses
excellent corrosion resistance, and in alloys it confers resistance to oxidation.
• Gold forms eutectic alloys with other metals covering a wide range of temperatures,
encompassing soldering and brazing temperature regimes.
• Gold is relatively easy to apply as a surface coating by both physical vapor deposition and
chemical plating.
• Many gold alloys possess enhanced mechanical properties, especially at elevated
temperatures.
• Gold’s low elastic modulus and rapid self-diffusion are beneficial for solid-state diffusion
bonding.

Gold is unusual in that it is the only element on which both brazes and solders are based. However, because gold is the most expensive constituent of solders and brazes, the use of gold filler alloys tends to be limited to high value applications, such as electronics, photonics, and jewelry manufacture.