Active metal brazing

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 

These brazes have experienced major metallurgical and fabrication developments in the last few years. Thus the concentration of Ti has been reduced from about 10% or 5% to 2 or 1% by slightly varying the Ag/Cu ratio or by introducing elements such as Sn and In which seem to increase the thermodynamic activity of Ti. The purpose of the decreases of Ti concentration is mainly to improve the ductility of the brazed joints (Ti intermetallics are usually brittle), 

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. 

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Table 10.5. Some commercially available active metal brazes.

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... 

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. 

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,. 

Active metal brazing (AMB) is a process to braze metals, such as Kovar , titanium, copper, and molybdenum, to ceramics. The process does not require a metallization on the ceramic. For this chapter, only copper brazing will be addressed. The AMB process is used by some manufacturers as an alternative to DBC as it provides higher adhesive strength while using a less critical furnace profile. It also does not rely on oxide formation as in the DBC process. 

Weinshanker, S., Private conversation with A. Krum, Etching with Active Metal Brazing, A. Krum, August 20, 2003. 

There are three fundamental methods of metallizing ceramic suhstrates thick film, thin film, and copper, which includes direct bond copper (DEC), plated copper, and active metal braze (AMB). Not all of these proeesses are compatible with all suhstrates. The selection of a metallization system depends on both the application and the eompatihility with the substrate material. 

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