Anisotropically conductive adhesives adhesive matrix

From the viewpoint of their conduction and mechanical joining, ACAs are similar to ICAs, except that they have lower concentrations of conductive particles. This lower concentration provides unidirectional conductivity in the vertical or z-direction (perpendicular to the plane of the substrate), which is why they are called anisotropic conductive adhesives. In the same way, ACA materials are prepared by dispersing electrically conductive particles in an adhesive matrix at a concentration far below the percolation threshold. The concentration of particles is controlled, so that sufficient particles are present to ensure reliable electrical conductivity between the assembled parts in the z-direction, while insufficient particles are present to achieve percolation conduction in the x-y plane (Kim et al. 2008b). O Figure 50.6 shows a schematic description of an ACA interconnect, showing the electrical conductivity in the... 

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. 

Thermosetting matrices, such as epoxies and thermosetting hot-melt adhesives, are used where increased reliability is required. Repair of anisotropically conductive interconnections assembled with thermoset adhesives is problematic, however, as the adhesive matrix must be removed completely from the substrate and device prior to reassembly. 

Nitto Denko Corporation developed an anisotropic conductive film for fine-pitch flip-chip applications [33]. This ACF (1) is connectable between bumpless chips and a fine-pitch printed circuit board (PCB) (2) has high electrical conductivity (3) is repairable (easy to peal off chips from a printed circuit board at elevated temperatures) (4) has high reliability and (5) can be stored at room temperature. The other notable features are it is usable at pitches down to 25 pm the conductive elements are micrometallic columns as opposed to random-shaped particles this adhesive matrix consists of a thermoplastic polymer resin and the conductive columns are coated with an insulator and a high- Tg polymer that completely separates the columns from the adhesive (Fig. 8). 

Polymer matrices of isotropic conductive adhesives are similar to anisotropically conductive adhesives. An ideal matrix for ICAs should exhibit a long shelf hfe (good room temperature latency), fast cure, relatively high glass transition temperature (Tg), low moisture pickup, and... 

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