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

Abstract

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 pattem 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 Lipplications. 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 elecmcal properties. We include in this w d y 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.

Failure mechanism study of anisotropic conductive film (ACF) packages

“…Manuscript Failures in ACF parts have been reported after temperature cycling, moisture preconditioning and autoclave. Available literature has suggested a variety of driving factors for interconnect failures, including oxidation of conductive particles, initial misalignment due to coplanarity issues, thermal stresses due to coefficient of thermal expansion (CTE) mismatch, post-assembly residual compressive stresses, polymer expansion due to moisture absorption, and degradation of the polymer at hightemperature [1]- [5]. However, no conclusive explanations or direct evidence can be found to pinpoint the major cause of ACF electrical failures.…”

Reliability aspects of electronics packaging technology using anisotropic conductive adhesives

“…The unidirectional conduction is achieved by using a relatively low volume fraction of conductive fillers. This low filler loading is insufficient for inter-particle contact and prevents conduction in the X − Y plane of the adhesive, but enough particles are present to assure reliable conduction between bonding electrodes in the Z direction [2] . Because of the anisotropy, ACAs can be deposited over the entire contact region, greatly expanding the bonding area.…”

“…Both thermoplastic and thermosetting resins have been used as adhesive matrices. The principal advantage of thermoplastic ACAs is the relative ease to disassemble the interconnections for repair operation, while thermosetting adhesives possess higher strength at elevated temperature and form more robust bonds [2] . The commonly used conductive fillers include silver and nickel particles and polymer spheres coated with metal (Ni/Au).…”

Electrical Characterization of NCP- and NCF-Bonded Fine-Pitch Flip-Chip-on-Flexible Packages

“…Largescale integrated chips with high input-outputs (I/Os) often provide a solution for the shrinkage of product size and weight. The adhesive flip-chip assembly has gained popularity in chip packaging as it offers a low-cost, lead (Pb)-free, fine-pitch, and a simple and low-temperature processing solution [1]- [3]. The launch of anisotropic conductive adhesive in the market enhanced the development of display products by offering very fine-pitch development.…”