Adhesives, electrically conductive adhesive matrix

Electrically conductive adhesives owe their conductivity as well as their high cost to the incorporation of high loadings of metal powders or other special fillers of the types shown in Table 9.8. If enough metal particles are added to form a network within the polymer matrix, electrons can flow across the particle contact points, making the mixture electrically conductive. Virtually all high-performance conductive products today are based on flake or powdered silver. Silver offers an advantage in conductivity stability that cannot... 

Electrically conductive adhesives perform two primary functions. Like other types of adhesives, these materials provide a physical bond between two surfaces. In addition, an electrical interconnection between the two bonded surfaces is formed. This dual functionality is usually achieved by composite materials composed of metallic particles dispersed in an adhesive matrix. The electrical resistivity of conductive adhesives is compared to values of pure metals and polymers in Table 1. 

The heat induced into the workpiece will conduct instantly into the adhesive, providing the catalyst for cure. If one of the two substrates is not electrically conductive, then an adhesive can be used that includes a small percentage of metal oxide. The metal oxide particles within the adhesive become heated in the induction field and provide the source of heat to cure the matrix material in the adhesive. (See Fig. 14.5.)... 

We have outlined factors which affect particle distribution in a matrix. This distribution depends partly on filler properties but predominantly on the combination of properties of the pair filler-matrix. Filler distribution in a matrix depends on intended application. Some, such as applications which use fillers for reinforcement, require a homogeneous distribution of particles. In others, such as mentioned above electrical conductive materials, adhesives), a uniform distribution of filler particles may decrease their effectiveness. 

In contrast to soldering, conductive adhesives are used for special electronic applications. Conductive adhesives simultaneously establish mechanical and electrical joints between PCBs and components by means of a particle-filled resin matrix. Whereas the polymer matrix is responsible for the mechanical interconnection, the fiUing particles (silver, palladium, or gold particles) provide the electrical contact between PCB and component. Therefore, in contrast to solder joints, conductive adhesive joints have a heterogeneous structure. 

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. 

Increasing the concentration of metal particles in an insulating adhesive matrix changes the electrical properties of the composite in a discontinuous way. Assuming a random dispersion of the metal filler, as the concentration increases no significant change occurs until a critical concentration, pc, is reached. This point, where the electrical resistivity decreases dramatically, called the percolation threshold, has been attributed to the formation of a network of chains of conductive particles than span the composite. A two-dimensional cartoon of a conductive adhesive below p and just above pc is shown in Fig. 3. A typical plot showing the relationship between particle concentration and electrical resistivity is shown in Fig. 4. 

Ho and Chung [114,115] used unidirectional and continuous carbon fiber Sn matrix composites for the packaging of the high temperature superconductor YBa2Cu307 delta by diffusion bonding at 170°C and 4 MPa. The Sn acted as the adhesive and increased ductility, normal state electrical conductivity and thermal conductivity. Carbon fibers served... 

A second, alternative, approach employed a readily available N2 laser and standard DHB as matrix [169]. In this case, a dedicated adapter target was used to introduce the TLC plate into the mass spectrometer, and the plate (which had an aluminum back to ensure electric conductivity) was then simply mounted onto the target using conductive adhesive tape (dedicated TLC adapter targets are now also commerciaUy available [170]). A selected TLC lane of a separated hen egg yolk extract, and some selected positive-ion MALDI mass spectra recorded directly from the developed TLC plate, are shown in Figure 7.10. 

After application of the matrix, the TLC plate is loaded directly into the MALDI device. The most obvious and simple approach is to fix the TLC plate with conductive adhesive tape onto a standard MALDI target (although a dedicated TLC adapter is now also available from Bruker Daltonics). To avoid charging effects in the MALDI source by residual charged particles on the surface and to enable successful ion desorption, an electrically conductive surface is needed. Therefore, the use of TLC glass plates is absolutely discouraged, whereas alumina TLC plates are perfect for this application. Such plates are commercially available with different stationary phases. 

As a result of their high aspect ratio and electrical conductivity it has been established that carbon nanotubes can form electrically conductive networks in epoxy adhesives and polymer matrix materials and ultimately make them electrically conductive which triggers the opportunity to develop in-situ SUM method. Another important factor is that the addition of carbon nanotubes served to increase the bonding strength and durability of epoxy joints [18]. 

Conductive Columns. Nitto Denko Corp. developed an ACF for fine pitch flip chip applications (27). The features of this ACF were (1) connectability between bumpless chips and fine pitch PCB (2) high electrical conductivity (3) repairabil-ity (easy to peal off chips from a printed circuit board at elevated temperatures) (4) high reliability and (5) potential storage at room temperature. There are other notable features too (7) ACF is usable at pitches down to 25 fim, (2) the conductive elements are micrometallic columns as opposed to random-shaped particles, and (3) this adhesive matrix consists of a thermoplastic polymer resin, conductive columns coated with an insulator, and a high Tg polymer, which completely separates the columns from the adhesive (Fig. 6). 

The interest in using CNTs as nanoflllers in a PMMA matrix is increasing [188-193]. For instance, Jin et al. reported on the electrical conductivity and elec-trorheological (ER) properties of MWCNT-adsorbed PS and PMMA microspheres prepared by a simple and potentially scalable process [191]. First, homogeneous aqueous dispersions of MWCNTs were obtained with various surfactants and then dispersions of PS and PMMA microspheres were dropped into a beaker containing the nanombe dispersions. The PS and PMMA microspheres with adsorbed nanotubes underwent slow sedimentation. Adhesion of the nanotubes to the PS and PMMA microsphere surfaces was believed to be related to the hydrophobic interaction. The electrical conductivity of these nanombe-adsorbed microspheres was investigated the DC conductivity of samples was in the range of 1.9 x 10 to 6.2 X 10 Scm at room temperature, based on the cross-sectional area, whereas... 

Conductive Adhesives. Electrically conductive adhesives are used today for specialized applications such as connections to LCD displays and attachment of small resistors and capacitors. These materials consist of conductive particles, usually silver flakes or carbon, suspended in a polymer matrix, most commonly epoxy.The electrical resistance of the contact to the PCB tends to be unstable over time, so these materials are not suitable for applications requiring a constant, low-resistance contact. The primary failure mechanism is moisture migration through the epoxy to the interface, resulting in oxidation of the contact metal. Adhesion strength is also a reliability concern. New materials suitable for a broader range of applications are under development. Further information can be found in Ref 39. 

Electromagnetic/radio-frequency interference shielding materials have to meet much lower demands in terms of overall electrical conductivity (typically 4-5 orders of magnitude lower than a silver-flake-filled adhesive). This means that cheaper conductive fillers can be employed, for example, silver-coated copper flake, nickel flake, and carbon black. Typically the adhesive has to form a compliant joint between two mating surfaces, and hence room temperature vulcanizing or heat-cure silicone is often a convenient choice of matrix material. 

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