Conductive Adhesives for Electronic Assemblies
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Conductive Adhesives for Electronic Assemblies
C.T. Murray, R.L. Rudman, M.B. Sabade, and A.V. Pocius Abstract A number of different types of adhesives are used in the assembly of electronic components and devices. This article provides an overview of such adhesives that also have another job—they work at conducting electricity or heat. The resins or binders in these adhesives range from thermosetting to pressure-sensitive. Conductivity is obtained by the judicious choice of filler. For electrically conducting adhesives, the fillers range from silver flake to silver-coated fibers. For thermally conducting adhesives, the fillers range from aluminum oxide to boron nitride. We also discuss a specific type of electrically conducting adhesive—the z-axis film adhesive. In these adhesives, particles are oriented in such a fashion that allows conduction in the direction perpendicular to the adhesive, but not in the plane of the adhesive. Keywords: adhesives, anisotropic conductivity, electrical conductivity, electronic assemblies, fillers, isotropic conductivity, thermal conductivity.
Introduction Adhesives are widely known for their consumer applications. Many consumers are familiar with brand names such as Elmer’s Glue, Scotch tape, and Super Glue. Industrially, adhesives are known for their role in the assembly of such diverse items as furniture, automobiles, and aircraft. One lesser known but equally important industrial application is the use of adhesives in the assembly of electronic components. As in other industries, this segment has certain specific requirements for adhesives. Such requirements include ease of use, rapid cure for thermosets, and reliability in manufacturing and in the properties of the resulting component. In spite of the normally benign conditions in which most electronic components are used, the reliability tests in this industry are among the most stringent. This is due to the fact that the industry has established a good correlation between those tests and the life of an electronic component. Adhesives are used for a multitude of reasons in electronic applications. In this article, we examine some of the materials science of electronic-grade adhesives that also have the property of being thermally or electrically conductive. In the MRS BULLETIN/JUNE 2003
simplest view, one can divide the classes of adhesives that are used in the electronics industry into two types: isotropically conductive and anisotropically conductive. In the next section, we discuss two types of isotropically conductive adhesives: electrically conductive and thermally conductive. Anisotropically conductive adhesives are discussed afterwards.
Isotropically Conductive Adhesives Background Isotropically conductive adhesives (ICAs) conduct electricity or heat in all directions through the adhesive material. This feature separates their applications from those of z-axis film (ZAF) adhesives that conduct electricity in one direction only. Since directionality of conduction is not a characteristic of ICAs, there are a broad ra
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