Electrically Conductive Adhesives

Abstract

Electrically conductive adhesives are being used in electronic packaging for several decades. A brief review of the dynamic development of conductive adhesives under the influence of the miniaturization, the adaptation of environmental friendly manufacturing processes is presented. With respect to the importance of isotropically conductive adhesives (ICA), a new contact model to analyze the principle influences of e.g., particle size, particle geometry, and filler content on the percolation threshold is introduced. With this model the arrangement of the particles within a contact is calculated by considering different types of forces (elastic, friction, adhesion, and inertia). Taking into account the electrical properties of the filler particles, the electrical contact behavior including its changes due to aging is investigated. Finally, typical applications of isotropically conductive adhesives are presented. One example shows how the thermal requirements for attaching a GaAs heterojunction power transistor can be fulfilled using an adhesive with an extremely high filler content (thermal conductivity: >60 W/mK). In another case it is demonstrated how extreme thermomechanical requirements resulting from a thermal expansion mismatch of parts of a sealed IR sensor housing can be corresponded using an adhesive with a comparatively low glass transition temperature. A further example shows a packaging concept of a miniaturized, biocompatible multichip module. For mounting both, narrowly spaced SMDs and bare chips, an isotropically conductive adhesive has been applied. 21