Adhesives, electrically conductive materials development

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. 

Properties of nanofillers recently developed nano materials are reported to display greater mechanical strength, greater thermal conductivity and improved electrical performance when compared to materials of normal particle sizes. Nano dimensional materials are being studied as fillers in polymer matrices in a variety of formulations for electrically conductive adhesives, thermally conductive adhesives, encapsulants, printed circuit boards, coatings, catalysts, underfills for flip-chip-attached devices and wafer-level connections. ... 

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]. 

By nature, polymers are insulating materials and some of them are employed without any added fillers. However, most applications require either electrical or thermal conductivity or both. According to the final use, thermal stabiUty and resistance to aggressive environment can be important criteria to select a particular type of polymer. In fact, few adhesives have been specifically developed for the electronic industry. Instead, most materials were preexisting compositions that have been tested and incorporated in production processes. Specific requirements such as ionic contamination, water absorption, and better thermal stability appeared later. Major chemical companies and semiconductor manufacturers, in particular in the USA and Japan, launched variants of these polymers fulfilling the demand for organic materials with enhanced properties. 

Materials for use as anisotropically conductive adhesives must satisfy requirements even more stringent than those defined previously for isotropically conductive adhesives. No specifications, however, have been defined specifically for these materials. When used for flip-chip applications, the adhesive not only serves as a physical and electrical interconnection between the device and the substrate, but also serves as the environmental protection and passivation layer. This fact, combined with high adhesive concentrations, makes the ionic contamination levels of these materials more critical than for isotropic conductive adhesives. In addition, the processing of these materials has a greater influence on joint reliability as the anisotropic electrical properties develop only after heat and pressure are applied to the joint. 

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