Compositions and methods for providing anisotropic conductive pa

Details Compositions and methods for providing anisotropic conductive pa Compositions and methods for providing anisotropic conductive pa

1995-06-29

1998-06-23

Mayes, Curtis

Adhesive bonding and miscellaneous chemical manufacture

Methods

Surface bonding and/or assembly therefor

1562739, 1562757, 156305, 252 6251R, 252 6255, 252500, B32B 3120, H01F 144, H01R 404

Patent

active

057699968

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

This invention relates to compositions and methods for providing anisotropic conductive pathways between two sets of conductors, and to compositions and to methods for making anisotropically-conductive bonds between two conductors. The invention is particularly for use in the electronics industry.


BACKGROUND ART

Electronic components such as semiconductor chips are often very small and have minimal gaps between connectors such as pins. Conventional solder may give rise to difficulties because the solder may bridge the gap between two pins. Therefore anisotropically-conductive adhesives have been proposed for electrical interconnection. An anisotropically conductive adhesive (ACA) conducts electricity in one direction only (usually denoted as the Z direction) and should eliminate conduction in the plane perpendicular thereto (the X and Y directions).
Various proposals for ACA's are reviewed by Ogunjimi et al. in Journal of Electronics Manufacturing (1992) 2, 109-118. They usually consist of an adhesive matrix in which conductive particles are dispersed. The particles may be metal particles, or non-conductive particles (e.g. plastic or glass) with a thin metal coat. After the adhesive has been applied between two conductors, bond line thickness may then be reduced by pressure applied during cure so that the particles in the adhesive contact the two conductors but do not contact one another laterally (see U.S. Pat. No. 4,740,657 Tsukagoshi et al.). Alternatively, conductive particles which are also magnetic may be aligned by use of a magnetic field so that they form a chain and provide an anisotropically conductive path along the direction of the field. The adhesive is then cured while the field is applied (see U.S. Pat. Nos. 3,359,145 Salyer et al; 4,548,862 Hartman; 4,644,101 Jin et al; and 4,170,677 Hutcheson). U.S. Pat. No. 4,737,112 Jin et al. uses single-particle bridging with essentially uniform distribution resulting from application of a magnetic field. Particles are magnetized N-S by the magnetic field, resulting in lateral repulsion between particles. The text at column 4 lines 6-8 suggests that the particles may have a non-magnetic, non-conductive core portion which is coated with a magnetic conductive coating. However no working Examples of the use of such particles are described. The Examples in the Jin et al. patent use gold coated nickel spheres which would have a solid core of magnetic material.
In an unrelated area of technology, it is known to make a magnetic liquid or "ferrofluid" consisting of a colloidal suspension of minute ferromagnetic particles in an non-magnetic carrier liquid. A typical ferrofluid may consist of magnetite particles (Fe.sub.3 O.sub.4) having a particle size in the range 2 nanometres to 0.1 micrometres (and a mean size of about 0.01 micrometres) in kerosene as carrier liquid with a surfactant to prevent agglomeration of the particles (see Skjeltorp "One- and Two-Dimensional Crystallization of Magnetic Holes" in Physical Review Letters, Volume 51, Number 25, 19 Dec. 1983, 2306-2309, the contents of which are incorporated by reference). Skjeltorp describes the production of "magnetic holes" inside a thin layer of magnetic fluid containing a monolayer of polydisperse polystyrene spheres with diameters in the micrometre range. U.S. Pat. No. 4,846,988 (Skjeltorp) describes a method for bringing bodies immersed in liquid to form regular structural patterns by dispersing non-magnetic, essentially monodisperse, particles having uniform sizes and shapes in a ferrofluid so that the particles create non-magnetic "holes" in the ferrofluid, and applying a substantially homogeneous magnetic field to the ferrofluid. Each of the dispersed non-magnetic particle bodies then assumes a magnetic moment corresponding to the volume of liquid displaced by the body, but inversely directed. Magnetic interaction forces then prevail between the particle bodies, which may thus be collectively controlled by the external magnetic field to assume structural patterns. When the pa

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