Conductive adhesives alternative

Silver filler is more preferable due to excellent corrosion resistance and conductivity, but its high cost is serious disadvantage. Hence alternative conductive filler, notably nickel-carbon nanocomposite was chosen to further computational simulation. In developed adhesive/paste formulations metal containing phase distribution is determined by competitive coordination and cross-linking reactions. 

The adhesion of conductive polymers can be improved by mixing heterogeneous phases. The mixing of a non-polar polymeric adhesive phase with a polar conductive aqueous phase comprised of a wato receptive polymer humectant and an electrolyte has been disclosed (22,23). The two phase composite consists of a continuous phase of ionically conductive, hydrophilic pressure-sensitive adhesive (PSA) composition and a discontinuous phase of domains of hydrophobic PSA and provides for enhanced adhesion to manunalian skin, while retaining the critical alternating current impedance. 

FIG. 4 Illustration depicting an isotropic conductive adhesive alternative. 

FIG. 5 Illustration depicting an anisotropic conductive adhesive alternative. 

There is a seemingly infinite variety of tests for specific applications, and some even have alternate adaptations for particular industries or customers. This section contains a few examples of common application tests for urethane foams, coatings, adhesives, and elastomers that may be conducted in addition to those described above. Some of these are just specialized versions of tests discussed in die previous section but are presented here because they have been developed for specific applications. Others are more broadly useful and very common. 

Electrical Conductivity. Unmodified polymeric resins are natural insulators and do not exhibit electrical conductivity. There are certain applications, however, where electrically conductive adhesives provide a significant value. One such application is the use of conductive adhesives as an alternative for wire or circuit board soldering. Another application, with less of a requirement for conductivity, is the assembly of components that are shields or protection from electromagnetic interference. 

Besides this prime role of attachment, electrically conductive adhesives are widely used to form electrical contacts between components and the printed-wiring board or other interconnect substrate, such as thin-film or thick-film ceramic substrates or flexible cable. For this function, adhesives serve as low-cost alternates to wire, solder, and other metallurgical connections. 

Jagt JC, Beric PJM, Lijten GF. Electrically conductive adhesives a prospective alternative for SMD soldering. IEEE Trans Comp, Packaging, Mfg Tech 1995 18(2) 292-298. Part B. 

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. 

Up to now the electrical connection of devices and conducting tracks is made by solder processes, and therefore the adhesives have to withstand temperatures of about 260°C for a short time without remarkable losses in their performance. Up to now, conducting adhesives for electrical connections as an alternative have been nsed in special applications only. 

Both ends of each wire are stripped and one end is soldered onto the outer edge of the uncoated conductive part of the substrate. Indium solder is often a good choice for soldering the lead wires to the substrate due to its high conductivity and good adhesion to surfaces. For better adhesion, a thin layer of indium can be deposited first onto the substrate (Fig. 3.1b). Next, more indium is used to attach the wires onto the substrate (Fig. 3.1c). Alternatively, silver paint can be used, in which case the sample should dry in air for a few hours or annealed at 80 °C for 20-30 min. 

Bioimpedance is usually measured with a tetrapolar measurement setup with two electrodes injecting a harmless alternating current into the body, and two other electrodes measuring the voltage drop over the body part of interest [6]. In practice, this is conducted by standard adhesive Ag/AgCl electrodes Common electrode positions are whole-body positions (wrist-to-ankle) and hand-to-hand positions (Figure 3). 

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