An overview and evaluation of anisotropically conductive adhesive films for fine pitch electronic assembly

Conductive adhesives have been used in the electronics industry for several years to attach chips to package lead frames in the semiconductor industry and for general interconnection of components to flexible circuits for various consumer products. Generally, these materials conduct equally in all directions. To obtain pad isolation, the adhesives are screen printed to the pattern of the circuit pads. In the last few years, a new class of adhesives that are conductive in a single direction have been developed. These are referred to as Anisotropic Conductive Adhesive Films (ACAF). These anisotropically conductive adhesives provide electrical as well as mechanical interconnections for fine pitch applications. The conductivity of ACAF materials is only in the Z-direction (perpendicular to the plane of the board) while electrical isolation is maintained in the X-Y plane. Currently, at least 15 ACAF materials are commercially available. We have developed a methodology for evaluating these materials for their mechanical and electrical properties and interconnection use in the 8 to 15 mil pitch range. In addition, we characterized the materials as to their physical properties and cure characteristics. This paper details our findings with a comparison of physical form to assembly/cure and final electrical properties. We include in this study data from scanning electron microscopy, thermal analysis of the ACAFs, and cure and assembly studies on mixed substrate test vehicles. Information on initial electrical testing and long term reliability testing is also given