Thermal expansion behavior of silver matrix composites
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I. INTRODUCTION
THE two most important purposes of electronic packages are to protect integrated circuit (IC) chips from electrical, thermal, chemical, and mechanical damage, and to act as heat sinks to spread the heat generated during IC operations.[1] Hence, a high mechanical strength, high thermal conductivity, and good chemical stability are the required properties for effective packaging materials. Low thermal conductivity and high permeability of humidity of polymer materials limit their application to the high-power IC packaging industry.[2] Ceramic materials have been considered as upgraded packaging materials, but high dielectric constants and low thermal conductivity cause major problems in their development.[3] Kovar alloy is the most used metallic packaging material because of its good weldability to glass. However, a molybdenum adhesion layer is necessary to enhance the thermal conductivity of Kovar alloy.[4] Copper has been applied mostly as the packaging material for high electrical- and thermalconduction requirements, such as high-power hybrid circuits.[4] Pure copper has a relatively low mechanical strength and is usually alloyed by aluminum or silver to enhance its strength. However, the electrical and thermal conductivity of copper decreases drastically because of a large lattice strain caused by the addition of a small amount of alloying element.[5] Aluminum alloys are used as packaging materials for microwave hybrid circuits and aerospace electronics, but their extremely high CTE of 24 3 1026/ 8C results in large thermal stresses, leading to the formation of cracks between them and the chips.[6] Metal matrix composites (MMCs), combining the high strength and low thermal expansion of reinforcements with
SHOU-YI CHANG, Postdoctor, and SU-JIEN LIN, Professor, are with the Department of Materials Science and Engineering, National Tsing Hua University, Taiwan, Republic of China. MERTON C. FLEMINGS, Professor, is with the Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted April 27, 1999.
METALLURGICAL AND MATERIALS TRANSACTIONS A
the high ductility and high electrical and thermal conductivity of metallic matrices, have been rapidly developed since the 1960s. Among them, silver matrix composites have been well developed and used as upgraded electrical contact materials in the electronics industry because of their good electrical and thermal conductivity.[7,8] In such applications, materials are subjected to severe damage during arc erosion operations.[9,10,11] Because silver has the best thermal conductivity of 419 W/m ? k,[12] these silver matrix composites also satisfy the requirement of fast heat spread and are worthwhile candidates for electronic packaging materials. However, because of the complicated and expensive fabrication processes for these composites,[7] their application to the electronic packaging industry has been limited. Recently, the application of electroless silver plating, followed by hot pressing,
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