Mechanical properties considerations for selecting braze filler metals

This paper describes recent efforts at Sandia National Laboratories in the area of metal/ceramic brazing, primarily for electronic applications. The necessary elevated temperature mechanical properties of braze alloys used in metal/ceramic brazing are inherently different than those intended for metal/metal joining. Braze alloys used for metal/ceramic brazing must readily deform during cooldown from the brazing temperature so as to help to accommodate thermal expansion mismatch between the dissimilar base materials. Another requirement for braze alloys used in metal/ceramic brazing is that ordering reactions, if present, do not result in extensive alloy strengthening. By contrast, braze alloys used for metal/metal structural applications are expected to have excellent high temperature mechanical properties - including creep and tensile strength - but without strict limitations on ductility. These diverging design requirements will be discussed in view of the high temperature mechanical properties of the AWS BAu-4 filler metal (82wt.%Au-18Ni), its active metal alloy (82Au-15.5Ni-0.75Mo-1.75V) counterpart, and other braze alloys appropriate for metal/ceramic joining.