Ceramic soldering
The main problem with joining metal to ceramic using brazing is the different coefficients of thermal expansion (CTE) of the materials. The greater this difference and the higher the brazing temperature, the greater the stresses “frozen” in the ceramic-metal bond at room temperature. Therefore, attempts are made to use metal alloys such as Ni or Kovar, as these have a CTE similar to that of aluminum oxide at room temperature. In addition, attempts are made to adapt the shape and material thickness of the metal parts so that bond stresses are reduced through plastic deformation. Alumina Systems distinguishes between three types of brazing as joining techniques.
Passive Al2O3 Bracing of Ceramics and metal
The most common method for brazing ceramics and metals is passive Al2O3 brazing with an AgCu28 eutectium solder with a melting point of 779 °C. This always requires metallization of the ceramic. Various other solders are used depending on the requirements and material pairing. The solder is usually used in wire and foil form, and sometimes also as a paste. One advantage of passive brazing of ceramics is the good wetting and flow properties of the solder on the nickel-plated metallization layer. More than 90% of Alumina Systems’ vacuum-tight ceramic-metal systems are manufactured this way.
A solder fillet forms between the metallization and the metal part, providing optimal stress transfer. These joints therefore exhibit very high strengths (min. MPa). Brazing is usually performed under forming gas (90% nitrogen (N2) and 5% hydrogen (H2)) to protect against oxidation. This is not sufficient for stainless steel composite components, for example. That’s why soldering is done in a vacuum.
Active soldering in metal-ceramic solder joints
The main advantage of this process is the use of active solders, usually containing titanium, which allow a direct connection between the solder and the components without metallizing the ceramic. A disadvantage is the poor wetting properties of the ceramic. This can lead to problems when creating vacuum-tight connections.
Soldered technical ceramic connection using glass solder
Alumina Systems frequently uses glass solders for vacuum-tight ceramic-to-ceramic joints. This eliminates the need for metallization of the ceramic. Since the glass-based bond does not require metals such as nickel, such joints can also be used in areas exposed to magnetic fields.
Formation of a soldered ceramic-metal joint
The micrograph shows that a thin, soft, and flexible copper membrane (OF-Cu: oxygen-free copper) was soldered between the solid copper part and the ceramic. This membrane minimizes the bond stresses in the ceramic-metal system through deformation.