Microstructural studies of Cu brazing on AlN
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The microstructures and phase compositions of Cu–Ag–Ti active-metal brazing alloys have been studied by scanning electron microscopy and energy dispersive x-ray spectroscopy to evaluate alloy wetting on AlN and Cu brazing on AlN. Titanium is segregated from the original alloy, and a Ti-rich layer is formed between the brazing alloy and AlN substrate. The alloy components are able to penetrate into the grain boundary of AlN during wetting or brazing, and the interfacial reaction takes place along the grain and outer boundary of AlN. The bonding of brazing alloys to AlN substrate often induces cracks in the AlN side.
I. INTRODUCTION
Electronic packaging encompasses ceramics, polymers, and metals used to support, interconnect, and protect circuits ranging from discrete components to multichip modules.1 Specific requirements are imposed on packaging materials used for high-power circuits, and aluminum nitride (AlN) meets these criteria.2–4 The high thermal conductivity and low thermal expansion coefficient of AlN are well suited to dissipate heat from high current conductor lines and high frequency devices. However, prior to building circuit components on AlN substrate, surface metallization of AlN is required. Compared to ceramic materials, such as alumina (Al2O3) and beryllia (BeO), aluminum nitride suffers from poor adhesion to metals because it does not form metal oxide bonding layers and has very little grain boundary phase for metal films to penetrate.2 Specific techniques are required to promote adhesion of metals on AlN, and activemetal brazing is a simple and effective method to join metal foils, such as copper (Cu) and silver (Ag), to AlN.5 Active-metal brazing typically requires moderate to high temperatures to melt the brazing alloy and initiate reactions between the alloy and the substrate to form the bond.6 Despite the availability of commercial brazing alloys for brazing metals to AlN, the bonding mechanisms between metals and AlN are still not well understood. Furthermore, the quality of brazing is affected by many variables, for example, surface roughness of brazing materials, atmosphere, brazing time, and temperature. Thus, understanding the brazing mechanism and the effects of brazing variables on brazing quality is crucial in optimizing the entire brazing process and achieving a high-quality metal braze on AlN. In the last decade, some progress has been made in attempting to understand the brazing mechanisms and structure–property relationships of the interfaces be670
J. Mater. Res., Vol. 16, No. 3, Mar 2001
tween brazing alloys and AlN.7–11 Research has concentrated on the wetting behavior of various alloys on AlN and other nitrides. The interface interaction between brazing alloys and some oxides or nitrides has been studied when the intended joining metals, such as Cu and Al, were absent. For instance, Brow et al. studied the wetting behaviors of several commercial brazes on AlN at 900 °C.8 Commercial brazes containing varied titanium (Ti) concentration were selected, and they found that the
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