Anisotropically conductive materials

The following list includes several commercial manufacturers of conductive adhesives commonly used for die attach as well as other interconnection applications. Many manufacturers sell both isotropically as well as anisotropically conductive materials. The list is not intended to be inclusive but merely to provide initial guidance. 

ULTRAHIGH ANISOTROPIC CONDUCTING MATERIALS BY CONSTRUCTION OF CONDUCTING POLYMER LB MULTILAYERS [2, 13,16-18]... 

Since side-chain-type LC polyacetylene derivatives were first synthesized [10-18], various types of LC conjugated polymers have been synthesized and evaluated from the standpoint of their electrical and optical properties [19-46]. Among them, LC aromatic conjugated polymers, including LC polythiophene derivatives, have been of recent interest because they are available for linearly polarized luminescent materials, and also anisotropically conducting materials [28-30, 40-53]. 

The connection of flexible circuits with anisotropic conductive materials is not new idea. An organic adhesive material that has conductive particles becomes conductive only in the z direction when sandwiched between a flexible circuit and the other device under a pressure... 

The connections have relatively high resistance with a high noise level.The low-heat resistance resulting from the organic matrix of the anisotropic conductive materials is another barrier to apply as a reliable termination under sever operating temperatures. 

Anisotropic conductive materials 60 -5 High dimensional accuracy (-0.01%)... 

Hampel, B. (1994). Experiences with heat seal connections and anisotropic conducting materials for the car industry. Adhesives in Electronics 94, International Conference on Adhesives Joining Technology Electronics Manufacture, VDI/VDE-IT. 

The aligned-DNA, transparent, self-standing, and flexible film is of interest as a new naturally-occurring functional material, as well as an anisotropic conductive film. For example, the DNA-lipid film is effective as an adsorption filter of carcinogens such as acridine orange and ethidium bromide. The aligned-DNA film also shows polarization of light. 

The essential idea of EC inspection of CFRP is to use the anisotropic conductivity of the material [1,2]. Along the carbon fibres much higher conductivity is found than in the opposite direction (9... 15 kS/m and 0.07... 

Abstract Liquid-crystalline (LC) polymers that exhibit ionic or electronic conduction are described. Anisotropic and efficient transportation of electrons and ions is expected for these materials. The ordered LC nanostructures of LC polymers having ion- or electron-active moieties are important for efficient anisotropic transport. For electron-conductive materials, we focus on side-chain LC polymers. 

Shante s theory of hopping conduction in highly anisotropic disordered materials ... 

A new material may or may not be nonstoichiometric. Most complexes are stoichiometric. If an inorganic complex exhibits high anisotropic conductivity, the complex may be nonstoichiometric. 

Electrically conductive materials can be made to be either isotropically conductive (IC) or anisotropically conductive (AIC). IC materials are the most common and conduct electricity in all directions. AIC materials conduct materials in one axis when compressed between two contact points. 

There are two types of conductive adhesives conventional materials that conduct electricity equally in all directions (isotropic conductors) and those materials that conduct in only one direction (anisotropic conductors). Isotropically conductive materials are typically formulated by adding silver particles to an adhesive matrix such that the percolation threshold is exceeded. Electrical currents are conducted throughout the composite via an extensive network of particle-particle contacts. Anisotropically conductive adhesives are prepared by randomly dispersing electrically conductive particles in an adhesive matrix at a concentration far below the percolation threshold. A schematic illustration of an anisotropically conductive adhesive interconnection is shown in Fig. 1. The concentration of particles is controlled such that enough particles are present to assure reliable electrical contacts between the substrate and the device (Z direction), while too few particles are present to achieve conduction in the X-Y plane. The materials become conductive in one direction only after they have been processed under pressure they do not inherently conduct in a preferred direction. Applications, electrical conduction mechanisms, and formulation of both isotropic and anisotropic conductive adhesives are discussed in detail in this chapter. 

Conductive adhesives are one of the feasible alternatives to lead for electronics assembly. Isotropically conductive adhesives are suitable for standard pitch (50- to 100-mil) surface-mounted components and numerous commercial materials are available (see commercial suppher Ksting, Section VI.E). Anisotropically conductive adhesives are more suited to flex to rigid connections, fine pitch components (15- to 20-mil pitch), and flip-chip assembly (4- to 12-mil pitch) [22]. Adhesives are not ready to replace solder throughout the electronics industry, however, due to questions that remain concerning the reliability of electrical interconnections. Their implementation is currently limited to low-cost applications using polyester substrates and specialty appHcations where solder cannot be used. Additionally, the lack of equipment for large-volume assembly with anisotropically conductive adhesives, which require the simultaneous appUcation of heat and pressure, impedes the acceptance of these promising materials. 

Several processing techniques have been developed to achieve anisotropic conductivity. One method aligns the conductive metal particles in a magnetic field [41-43]. Alternatively, anisotropy can be achieved in materials with random dispersions of metal particles. Using an adhesive composite where the concentration of metal particles is below but close to pc results in a material with a short-range percolation length If the separation distance between substrate and device is less than whereas the pitch is much greater than anisotropic conduction is achieved. This approach is difficult to implement, as shorts between adjacent conductors, as well as opens between the substrate and device, are statistically possible. 

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