Short conductive fillers

Carbon Black (CB), Short Carbon Fiber (SCF), or Ni as Conductive Fillers. 79... 

Partici Shape and Size. The most common morphology of conductive fillers used for ICAs is flake because flakes tend to have a large surface area, and more contact spots and thus more electrical paths than spherical fillers. The particle size of ICA fillers generally ranges from 1 to 20 /rm. Larger particles tend to provide the material with a higher electrical conductivity and lower viscosity (45). A new class of silver particles, porous nano-sized silver particles, has been introduced in ICA formulations (46,47). ICAs made with this type of particles exhibited improved mechanical properties, but the electrical conductivity is less than ICAs filled with silver flakes. In addition, short carbon fibers have been used as conductive fillers in conductive adhesive formulations (36,48). However, carbon-based conductive adhesives show much lower electrical conductivity than silver-filled ones. 

On the other end of the scale are the conducting and semiconducting polymers. Several types exist. For example, conducting fillers may be added, such as short metallic fibers that touch each other or carbon black. Alternately, an ionic polymer may be employed, or a salt may be added to the polymer. Conductivity in the latter systems depends on the moisture content of the polymer. 

Chen, P.W., Chung, D.D.L., 1995. Improving the electrical conductivity of composites comprised of short conducting fibers in a nonconducting matrix the addition of a nonconducting particulate filler. Journal of Electronic Materials 24 (1), 47—51. 

Another common antistatic agent is carbon black, a form of amorphous carbon with intrinsic conductivity. The conductivity of carbon black, therefore, does not change with humidity. Carbon black is used in coating formulations and as a conductive filler in polymeric materials, especially in the electronics industry. The main drawback of carbon black is its intrinsic black color and the tendency to generate particles that dissipate in the clean room air and may cause undesired shorts. 

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... 

Classification by particle size is helpful in classification since particle size will affect performance but, by itself, falls short as a criterion when selecting fillers for applications which require certain levels of conductivity (thermal or electric) or of chemical interaction, etc. In one publication, materials were divided into particulates, fibers, and colorants. These distinctions are not helpful for a material designer. For a classification to be useful in filler applications, it must include the most important properties of fillers which affect the resultant material. The eight most important are as follows ... 

To lower the probability of conduction in the X Y plane (i.e., reduce the short-range percolation coherence length ), particles are used with an aspect ratio as close to 1 as possible. In contrast, isotropically conductive systems use flakes with high aspect ratios as fillers. Particle size distributions are minimized so that each particle can potentially serve as an electrical bridge between substrate and device. 

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