Brazing is a thermal process for the metallurgical bonding of metallic materials inwhich a third metal component (brazing alloy or solder) is added. In addition aflux or processing in a protective atmosphere is applied to eliminate oxidation ofthe non-precious carrier. The melting range of the brazing alloy starts at thebeginning of the melting (solidus temperature) all the way to complete liquidphase (liquidus temperature). This range always is below the melting points ofthe two materials to be joined. During the brazing process with solubility of thematerials in each other diffusion processes are thermally activated by whichelements of the base material diffuse into the brazing alloy and elements of thebraze alloy diffuse into brazing alloy. This increases the bond strength andtherefore the mechanical stability of the brazed joint.

For attachment of contact parts to carrier base materials only brazing alloys (asopposed to solders) are used. The reason is the higher softening temperatureand melting point as well as higher mechanical strength and electricalconductivity of these alloys. The brazing alloys and fluxes used for electricalcontact attachment are listed in Chapter 4 in more detail. Following the mostfrequently used brazing methods are described.References to the bond quality are given according to the test methodsdescribed in Chapter 3.4.

The simplest way to produce braze joints is the use of a gas torch fueled by aburning gas and air or oxygen containing gas mixes. For higher productionvolumes partial automation is applied. The parts to be assembled aretransported after adding the suitable amounts of brazing alloy and flux through aseries of fixed gas burners on a turntable or belt driven brazing machine.To limit the amount of flux or gas inclusions it is recommended to slightly movethe contact tips forth and back (also known as puddeling) as soon as thebrazing alloy is liquefied. The bonded area achieved in torch brazing is typically65 – 90% of the contact foot print depending on the size and geometry of thecontact tip.

Furnace brazing is usually defined as brazing in a protective atmosphere or invacuum. Both processes do not require the use of fluxes.

The protective atmosphere brazing is conducted in batch operation in eithermuffle or pot furnaces or as a continuous process in belt furnaces using areducing atmosphere of pure hydrogen (H₂) or dissociated ammonia (H₂,N₂).

A vacuum is a very efficient protective environment for brazing but using vacuumfurnaces is more complicated and rather inefficient. Therefore this process isonly used for materials and assemblies that are sensitive to oxygen, nitrogen, orhydrogen impurities. Not suitable for vacuum brazing are materials whichcontain components with a high vapor pressure.

Parts with oxygen containing copper supports should not be brazed in reducingatmosphere because of their susceptibility to hydrogen embrittlement. Similarlycontact tips containing silver–metal oxide should not be exposed to protectiveatmospheres because a reduction of the metal oxide even in a thin contactsurface layer changes the contact properties of these materials.

During induction brazing the heat energy is produced by an induction coil fed bya medium or high frequency generator. This creates an electromagnetic alternatingfield in the braze joint components which in turn generated eddy currentsin the work piece. Because of the skin–effect these currents and their resultingheat are created mainly on the surface of the assembly components. Thedistance of the inductor must be chosen in a way that the working temperatureis generated almost simultaneously in the full joint area. For different contactshapes the geometry of the induction coil can be optimized to obtain shortworking cycles. One of the advantages of this method is the short heating timewhich limits the softening of the material components to be joined.Typical bond qualities of > 80% can be reached with this method also for largercontact assemblies. The widely varying working times needed for the differentbrazing methods are given in Table 3.1.