Braze Process Optimization Involving Conventional Metal/Ceramic Brazing with 50Au-50Cu Alloy

Numerous process variables can influence the robustness of conventional metal/ceramic brazing processes. Experience with brazing of hermetic vacuum components has identified the following parameters as influencing the outcome of hydrogen furnace brazed Kovar{trademark} to metallized alumina braze joints: (a) Mo-Mn metallization thickness, sinter fire temperature and porosity (b) Nil plate purity, thickness, and sinter firing conditions (c) peak process temperature, time above liquidus and (d) braze alloy washer thickness. ASTM F19 tensile buttons are being used to investigate the above parameters. The F19 geometry permits determination of both joint hermeticity and tensile strength. This presentation will focus on important lessons learned from the tensile button study: (A) the position of the Kovar{trademark} interlayer can influence the joint tensile strength achieved--namely, off-center interlayers can lead to residual stress development in the ceramic and degrade tensile strength values. Finite element analysis has been used to demonstrate the expected magnitude in strength degradation as a function of misalignment. (B) Time above liquidus (TAL) and peak temperature can influence the strength and alloying level of the resulting braze joint. Excessive TAL or peak temperatures can lead to overbraze conditions where all of the Ni plate is dissolved. (C) Metallize sinter fire processes can influence the morphology and strength obtained from the braze joints.