Coefficients of thermal expansion of metal-matrix composites for electronic packaging
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INTRODUCTION
PACKAGING materials used in electronic applications protect and support integrated circuits and other components while, at the same time, aiding in the removal of heat generated during the operation of the component. With rapid advances in microelectronics, packaging technologies play a key role in the design of active devices, which have seen increasing miniaturization in recent years. These devices, such as multichip modules wherein several chips are located in close proxity, are being designed for greater power densities, which demand improved cooling and thermal management. In addition, the need for lighter weight (as, for example, in avionics), protection from hostile environments (as, for example, in automotive applications), increasing packaging density, enhanced reliability, and lower costs has resulted in a search for newer materials for electronic packaging.tl~ Conventional metallic materials for packaging applications include Cu, A1, Invar and Kovar Ni-Fe alloys, Cu-Invar and Cu-Mo-Cu laminates, and Cu-W and Cu-Mo blends. These materials do not meet the stringent requirements in advanced electronic packaging applications for low coefficient of thermal expansion (which often needs to be tailored to the specific geometric design of the electronic device), high thermal conductivity, low specific gravity, and low cost. For example, the use of A1 or Cu promotes unacceptably large residual stresses as a result of a high coefficient of thermal expansion (CTE) in devices based on Si or gallium arsenide (GaAs). These thermal residual stresses are a common cause of brittle fracture of the integrated circuits and substrates. Molybdenum and W have high densities, while Kovar has a high cost and low thermal conductivity. These limitations of conventional materials have led to increasing focus on Al-matrix composites with SiC Y.-L. SHEN, Graduate Research Assistant, and A. NEEDLEMAN, Professor of Engineering, are with the Division of Engineering, Brown University, Providence, RI 02912. S. SURESH, Richard P. Simmons Professor, is with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted July 12, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
particle reinforcements as potential candidates for a variety of uses in advanced electronic packaging (Figure 1, and Table I). tq The SiC particles are available in hexagonal (a) and cubic (/3) crystal structures from inexpensive raw material sources. They have low density (p = 3.2 g/cm3), low CTE (a = 4.7 • 10 -6 K - l ) , high Young's modulus (E = 450 GPa), and a commercially available particle size range of 1 to 50 p~m; in addition, SiC is also available in single crystalline form. The thermal conductivity, K, of SiC is in the range 80 to 200 W/(m K), depending on purity and processing conditions. By contrast, A1 has the following physical properties: p = 2.7 to 2.8 g/cm 3, E = 70 to 80 GPa, a = 23 • 10-6 K -L, and K = 180 to 230 W/(m K). Aluminum alloys reinforced with SiC par
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