Electrically conductive adhesives for electronic packaging and assembly applications

Some recent advances in materials, structures and properties that affect the mechanical and electrical characteristics and reliability performance of electrically conductive adhesives (ECAs) are discussed. This is done in the context of electronic packaging and assembly applications. Micro-silver-filled epoxy ECAs (with an average silver particle size of 5 pin) modified by addition of a conducting polymer, low melting point (LMP) alloys, or nanoparticles (average particle size of 80 nm) are described and compared with respect to volume resistivity, tensile strength and adhesion to copper. To understand the conduction and sintering behaviors of the various ECA formulations, SEM, optical microscopy and X-ray photoelectron spectroscopy (XPS) are used to investigate the micro-structure and chemical nature of the cured ECAs. Volume resistivity of the specially formulated adhesives is in the range of 10(-4) to 10(-6) Ohm cm. Adhesives formulated with a conducting polymer exhibit the greatest tensile strength with copper when compared to the other formulations investigated. The conducting polymer-modified ECA exhibited electrical conductivity on the order of that achieved with conventional ECAs. Hence, good electrical performance is achieved concurrent with superior mechanical properties. It was found that with increasing curing temperature, the volume resistivity of all ECAs decreased. This is attributed to sintering of metal particles at higher temperatures. Incorporation of nanoparticles reduces sintering temperature. Sintering of ECAs with micrometer-scale silver particles was further evaluated using high temperature/pressure lamination. A continuous metallic network resulted when the lamination temperature was greater than 300 degrees C. Electrical stability of ECA joints with aluminum was evaluated by stress testing in an environment of elevated temperature and humidity. A thin layer of silane-based coupling agent on the aluminum surfaces retards the degradation of the electrical properties of the joint. In general, proper preparation of the metal surface to which an ECA is mated is critical for maintaining both electrical and mechanical performances. (C) Koninklijke Brill NV, Leiden, 2008.