Epoxy-based conductive adhesives

Henkel offers its Hysol KOI052 rapid cure, silver-filled epoxy-based conductive adhesive to bond the cathodes in the construction of surface mount tantalum capacitors. This adhesive is suitable for in-line curing in as little as 20 seconds at elevated temperatures (200 °C). Features of this adhesive include a low volatile content, reduced voiding, low electrical resistance and high peel strength. 

Conductive adhesives are generally formulated from base polymers that are low-viscosity, thermosetting resins such as epoxies. Where elastomeric properties are required, silver-filled flexible epoxy and silver-filled silicone rubber systems are commercially available. 

To reduce the thermal stress for components as well as for thermoplastic base substrates, conductive adhesives and selective soldering can be used. In working with adhesives, good interconnections with a maximum temperature of only about lOCfC can be established. This temperature is necessary to harden the epoxy material. In the selective soldering technique, the heat is transferred only to places where interconnections have to be made between the component s termination and the pad on the substrate. This reduces the overall thermal stress situation for the whole 3D MID. 

To function as electrical conductors, epoxies and other polymer resins, because they are inherently insulators, must be filled with electrically conductive particles such as metals. The selection of electrically conductive or insulative adhesives is based largely on their conductivities or, conversely, on their volume resistivities. Electrically conductive adhesives should have low resistivities initially and retain those values on aging, moisture exposure, thermal cycling, and other operating and test conditions. The resistivities of metal-filled epoxy adhesives can range from... 

Lu D, Wong CP. Properties of conductive adhesives based on anhydride-cured epoxy systems. J Electron Mfg. Sep. 1999 9(3) 223-32. 

A special study on aerospace structural adhesives was conducted in 1976 by the National Materials Advisory Board of the National Research Council. The final report was published as one of the series in Treatise on Adhesion and Adhesives (108). That report discussed primarily phenolic and epoxy-based mate rials, e.g., vinyl-phenolics, n itr ile-phenolics, epoxy-phenolics, nylon-epoxies, and elastomer-epoxies. Polyimide and polybenzamidazole adhesives were barely mentioned. Most of the high-temperature adhesives discussed in one of the preceding sections were either not invented or not developed at that time. 

Epoxy adhesives are also easy to modify for special purposes. For example, they can be filled with carbon, silver, or gold to provide electrical conductivity. Other additives can enhance thermal conductivity, while maintaining electrical insulation. Additional performance properties of epoxy-based adhesives that can be modified include impact resistance, shrinkage, glass transition temperature, high-temperature strength, surface specific adhesion characteristics, and chemical or moisture resistance. 

Figure 6.5 Incorporation of conductive wires into textiles for wearable biosensors (http //dx.doi.org/10.3929/ethz-a-005135763). (a) PETEX hybrid fabric, (b) Electrical circuits based on PETEX fabric, (c) The establishment of connection between crossing wires involves three steps, coating removal and wire cut, applying of conductive adhesive, and epoxy resin...

Die attachment adhesives are normally epoxy- or polyimide-based and contain 70 - 80 wt % silver flake. Silver flake is the common choice for conductive adhesives as the oxide layer formed on the flake is also conductive. 

Abstract In this chapter, classification of adhesive and sealant materials is presented. For this purpose, various categories are considered depending on the polymer base (i.e., natural or synthetic), functionality in the polymer backbone (i.e., thermoplastic or thermoset), physical forms (i.e., one or multiple components, films), chemical families (i.e., epoxy, silicon), functional types (i.e., structural, hot melt, pressure sensitive, water-base, ultraviolet/ electron beam cured, conductive, etc.), and methods of application. The classification covers high-temperature adhesives, sealants, conductive adhesives, nanocomposite adhesives, primers, solvent-activated adhesives, water-activated adhesives, and hybrid adhesives. 

One of the most effective techniques used to improve the electrical conductivity of polymers is the incorporation of conductive fillers in the polymer matrix. The most popular electrically conductive filler is silver due to its moderate cost and superior conductivity. Silver-coated inorganic particles and fibers are superior compared to carbon particles and fibers as components in epoxy-based adhesives regarding the electrical conductivity of the composite because the electrical conductivity of silver is much higher than that of carbon. In addition to the high electrical conductivity of silver, the addition/application of silver-plated... 

Illustration of "interphases" between an epoxy-based, approximately 80% wt silver flake-filled electrically conductive adhesive and stainless steel and piezoelectric substrates. The flake size is approximately 7 pm... 

High performance, epoxy-based systems are widely and increasingly used in structural aerospace and other applications as both adhesives and the matrix of fibre-reinforced composites. Many studies have been conducted on epoxy systems directed towards elucidating structure-property relationships. This has resulted in a greatly increased level of understanding of these complex systems but has also indicated the complexity of the interrelations between composition, thermal history and properties. 

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