Isotropic conductive adhesives adhesive matrix

The electrical conductivity of isotropic conductive adhesives is established in all directions by metal particles, generally silver flakes. These silver flakes are embedded in a matrix consisting of a base polymer, generally epoxy resin, and hardeners such as amines or carboxylic acids. It is the hardeners that ensure the mechanical strength of the joint. The choice of these two components can influence reaction speed and curing temperature. Broadly speaking, any chemical reaction will be fast at a high temperature and slow at a low temperature. 

Figure 5.3 shows that each isotropic conductive-adhesive system has its optimum range for the proportion of fillers. Below what is commonly referred to as the percolation threshold only short conductive chains form, surrounded by an insulating polymer matrix, because at best only a few filler particles are in contact with each other. Increasing the proportion of filler above the percolation threshold enables the first conductive paths to form. Specific conductivity increases dramatically up to a plateau. The critical concentration depends on different factors such as... 

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... 

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. 

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