Braze active metal

This special class of brazes reacts chemically with the surfaces of ceramic components to produce wettable products with metallic characteristics, such as TiO, TiC x or TiN x as described in Sections 6.3 and 7.2. Thus the wetting is due to an in situ metallization . By definition, the brazes must contain chemically reactive elements such as Ti that are often added to eutectic brazes similar to those developed for joining metal components. Many sessile drop experiments have shown that active metal brazes can wet a wide range of ceramics when a suitable inert environment is used. Particularly high standards of environmental inertness... 

Table 10.5. Some commercially available active metal brazes.

Ni brazes are attractive when components must endure high service temperatures and evaluation of the potential of Ni eutectic brazes containing additions of Cr, such as BNi-5 and BNi-7 (Ni-19Cr-1 OSi and Ni-14Cr-10P), as the active metal for joining Si3N4 and SiC ceramics has been vigorous, (McDermid et al. 1989,... 

Rapid fluid flow cannot be achieved with active metal brazes because of the need to form solid wettable reaction product layers for their liquid fronts to advance. Equations (10.1) to (10.2) relating liquid flow rates to the opposed effects of surface energy imbalances and of viscous drag are not relevant. Actual penetration rates are so slow, usually of the order of 1 pm.s, that the usual practice is to place the active metal braze alloy within the joints rather than expecting it to fill them, and, as explained already, gap width is not the dominant consideration when designing ceramic-metal joints. 

The compositions of useful braze alloys have been carefully devised to ensure satisfactory wetting of the proposed component materials, but other adjustments have been made to achieve desirable mechanical characteristics of the joints. This adjustment has been made for virtually all commercial braze families and has been particularly important in the development of active metal brazes for ceramic components. 

The prime requirement of an active metal braze is that it should be able to change the chemistry of the ceramic surface to make it wettable, usually by forming hypostoichiometric TiC, TiN or TiO. This necessitates using alloys with high Ti activities, but alloys with high Ti concentrations are seldom suitable as brazes. Thus, Cu alloyed with 5 or 10 wt.% of Ti wets many ceramics well but... 

Solid state bonding Metallizing + brazing Active brazing Ceramic frit process... 

Braze alloys for ALOs-based ceramics might be Ag-Cu, Au-Ni, or Ag-Cu-Zn, but these alloys generally do not wet ceramics. The oxidation potentials of Cu and Ag are less than that of Al, so they do not react with the ceramic. If a small percentage of an active metal, e.g., Ti, is added, then the high oxidation potential of the Ti causes it to undergo a redox reaction with the ceramic (AI2O3). 

In designing an air braze filler metal, it is critical that there is some degree of mutual solubility between the metal and oxide constituent(s) in the liquid state. Experimental evidence suggests that the likely indicators for this behavior are measurable oxygen solubility (i.e., oxygen activity) in the liquid metal constituent, compatible melting points between the metal and oxide to be alloyed, and multiple valence states in the cation species of the oxide constituent that allow it to serve as an efficient oxygen buffer in the noble metal. Phase formation and phase equilibria in the filler metal system must be appropriate for the application of interest. For... 

The method of deposition is what differentiates the hybrid circuit from other packaging technologies and may be one of two types thick film or thin film. Other methods of metallizing a ceramic substrate, such as direct bond copper, active metal brazing, and plated copper, may also be considered to be in the hybrid family, but do not have a means for directly fabricating resistors and are not considered here. Semiconductor technology provides the active components, such as integrated circuits, transistors, and diodes. The passive components, such as resistors, capacitors, and inductors, may also be fabricated by thick- or thin-film methods or may be added as separate components. 

Brazing of ceramics is not without difficulty. In contrast to glasses, many metals and alloys bead up on ceramic substrates, that is, the molten metal does not wet the ceramic. To promote wetting, the surface must either first be metallized, or a braze containing a chemically reactive or active metal, often titanium, must be used. Titanium reacts with the ceramic and generally facilitates wetting via the formation of a more metallically bonded reaction product at the interface. 

The process flow for active metal braze substrates usually involves coating the braze alloy on the ceramic substrate of interest in a paste form, or as a metal foil. The copper foil is then placed on top of the braze alloy, and the whole assembly is heated in an inert atmosphere. The braze alloy melts and forms a strong bond with the copper and substrate. In many cases, the braze alloy and copper are patterned before bonding to eliminate the need for... 

Thermal performance of SijN, and AIN active-metal brazed structures. With copper metallization above 0.25 mm in thickness, the AIN laminates fail as a result of the CTE mismatch and the lower flexural strength compared to Si N,. 

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