Adhesives, electrically conductive

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. 

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... 

Most unmodified polymeric resins have a very low thermal conductivity. There are certain applications where high levels of thermal conductivity are required. For example, power electronic devices and other heat-generating components are bonded to heat sinks and other metal sources. Metal powder filled adhesives, such as those described above for electrically conductive adhesives, can conduct both heat and electricity. 

The other concepts associated with nanotechnology are those described above, positional assembly and self-replication. Nanotechnology is currently being used for light-emitting polymer films, in computer applications, electrically conductive adhesives, and other areas. 

Licari,J.J. Perkins,K.L. Caruso,S.V. Guidelines For The Selection of Electrically Conductive Adhesives for Hybrid Microcircuits, NASA CR-161978, 1981. 

Filler Adhesive component in a solid, finely dispersed form that specifically modifies the processing properties of the adhesive and the properties of the adhesive layer (e.g., metal particles in electrically conductive adhesives, chalkstone, carbon black to increase viscosity). Fillers are not reactive partners in adhesive curing. 

Xu, S. Y. and Dillard, D. A., "Determining the Impact Resistance of Electrically Conductive Adhesives Using a Falling Wedge Test," Jeee Transactions on Components and Packaging Technologies, Vol. 26, No. 3, 2003, pp. 554-562. 

In electronics production, two main principles of interconnection are used soldering using metal-based alloys and adhesive bonding with electrically conductive adhesives (Rahn 1993). 

After mechanical attachment to a substrate, a leadframe, or to the inside of a package, bare die or chip devices are electrically connected by one of five methods wire bonding, flip-chip bonding, TAB, solder attachment, or attachment with electrically conductive adhesives. Fig. 1.6 shows some of these methods. 

Epoxies became popular for the first generation of electrically conductive adhesives introduced in the mid-to-late 1960s. The early adhesive formulations contained organic solvents, which often became trapped in the bondhne during cure and resulted in voids, especially under large chips and substrates. These early adhesives also contained large amounts of ionic impurities, notably chloride ions, which, in the case of nonconductive adhesives, resulted in reduced electrical resistivity, ion migration, and corrosion, especially in humid environments. Early epoxies were also... 

A fourth generation of electrically conductive adhesives based on cyanate esters and modified cyclo-olefin thermoset (MCOT) resins was introduced in the 1990s. These resins were specially formulated for low moisture absorption, high thermal stability, and low stress to be compatible with solder-reflow conditions. They were developed to prevent or reduce the so-called popcorn effect, a failure mode attributed... 

High thermal conductivity LED attach adhesives improved moisture resistance, hermetic sealing, extremely low moisture and air permeation/penetration for OLEDs Stress-free electrically conductive adhesives for filling small vias... 

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. 

A second key function for adhesives is to form ohmic contacts or electrical connections between active and passive devices, connectors, leadffames, I/O leads, or other electronic parts to form a circuit. Other applications for electrically conductive adhesives include electrostatic dissipation and electrical grounding connections. 

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... 

Electrically conductive adhesives may be isotropic (conduction equally in all directions) or anisotropic (conduction in the z-direction only). Both types are widely used in the assembly and packaging of electronics. 

Li L, Lizzul C, Kim H, SacoUck L, Morris JE. Electrical, structural and processing properties of electrically conductive adhesives. IEEE Trans Components Hybrids Mfg Technol. Dec. 1993 16(8) 843-51. 

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. ... 

Of particular interest to adhesives formulators are nanofillers such as carbon nanotubes (CNT), silica, alumina, magnesium oxide, titanium dioxide, zirconium oxide (Zn02), silver, copper, and nickel). Of these, carbon nanotubes are the most widely studied for electrically conductive adhesives to attach microdevices, to interconnect microcircuits and to increase I/O densities at the device level. ... 

Ryszard K, Mosicicki S. Electrically Conductive Adhesives in SMT - The Influence of Adhesive Composition on Mechanical and Electrical Properties. Proc. IMAPS Poland Conf 2000 199-206. 

Lyons AM. Electrically Conductive Adhesives Effect of Particle Composition and Size Distribution. Society of Plastics Engineers Annual Tech. Conference. 1990 843-845. 

Several approaches using electrically conductive adhesives instead of solder have been explored and are proving successful. Anisotropic adhesives (See Chapter 1), for example, have been used to connect flex circuits and cables from the separate driver circuits to the panel, avoiding the use of solder connections. More importantly, integrated-circuit chips can be bonded directly to the ITO conductor traces on the panel, a technology called chip-on-glass (COG). IC chips can be flip-chip bonded, then underfilled with a stress-free underfill adhesive if necessary. For protection, the chips may then be encapsulated with epoxy (glob-topped). 

Smart cards are among the fastest growing applications for electrically conductive adhesives. Smart cards are pocket-size plastic cards with thin embedded ICs that store, process and transfer data much in the manner of a mini-computer. Smart labels are also being developed where the substrate is paper instead of plastic. Flip-chip transponder chips and flexible antenna are incorporated producing, for example, low-cost radiofrequency identification (RFID) units. 

Electrically conductive adhesives are being used to interconnect flip-chip devices in smart cards resulting in thinner and smaller structures. Flip-chip silicon devices that have been thinned to several mils may be connected to a substrate with silver-filled paste epoxy or with anisotropic film adhesive instead of solder, then embedded and laminated to form a card that is less than 40-mils thick. Requirements for adhesives used in smart cards, in many respects, are more severe than those for other commercial applications. Besides having to withstand high humidity and temperature extremes, smart cards must take the continued abuse of human handling, repeated bending, exposure to human sweat and salt residues, and exposure to ultraviolet radiation from sunlight. 

Although important for structural adhesive bonds, fracture mechanics is not as critical for non-structural low load-bearing adhesives as used in most electronic modules. For the most part, passing minimum specification requirements for peel and tensile strengths both at ambient conditions and accelerated test conditions are sufficient. However, computer-simulated modeling and reliability analysis have been used for evaluating electrically conductive adhesives as used in electronics assembly. ... 

Thermal conductivity requirements are specified in both military and industry specifications generally as 1.5 W/m K or greater for electrically conductive adhesives and 0.15 W/mK or greater for electrically insulative adhesives. 

Silver-filled epoxies and other electrically conductive adhesives are widely used to electrically connect chip devices or packaged components to interconnect substrates or printed-circuit boards. Chip capacitors, resistors, transistors, diodes, and magnetic components may be attached with silver-filled epoxies whose volume resistivities range from 1 x 10 " to 3 x 10 " ohm-cm or with gold-filled epoxies whose volume resistivities are approximately 8 x 10 ohm-cm. Conductive adhesives are also finding use as replacements for solder balls in flip-chip devices. In all cases, to achieve reliable connections, initially low-contact resistances or volume resistivities must remain low on aging and on exposure to operational stress conditions, such as humidity, temperature, vibration, shock, and power. 

Su B. Electrical, Thermomechanical, and Reliability Modeling of Electrically Conductive Adhesives. PhD Dissertation. Georgia Institute of Technology May 2006. 

Kim HK, Shi FG. Electrical reliability of electrically conductive adhesive joints dependence on curing condition and current density. Microelectron J. 2001 32. 

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. 

Zwohnski M, Hickman J, Rubon H, Zaks Y. Electrically conductive adhesives for surface mount solder replacement. In Proc. Second Int. Conf. Adhesive Joining Coating Tech, in Electronics Mfg. Stockholm Sweden 1996 333-340. 

Kraus HS. Pecuhar Behavior of Electrically Conductive Adhesives on Aluminum Surfaces, Ahlestik Laboratories (undated). 

The method and type of equipment used also vary with the type of adhesive. As an example, the viscosity of electrically conductive adhesives is measured according to ASTM D1824, Apparent Viscosity of Plastisols and Organosols at Low-Shear Rates by Brookfield Viscosity The viscosity of typical electrically conductive die-attach adhesives is measured with a Brookfield HBT viscometer with Spindle TB and Speed 5. For higher-viscosity conductive adhesives and for underfill adhesives, a Brookfield RVT or RVF viscometer is used with Spindles 6 or 7 at speeds of 4—10.4 rpm. Another Brookfield viscometer, the Cone-and-plate viscometer with a CP-51 spindle is used for low-to-intermediate viscosity adhesives. Finally, the Brookfield HAT and HBT instruments are used for the high-viscosity (1-2 million cP at 1 rpm) adhesives typically used in SMT applications. ... 

Electrically conductive adhesives that are used as solder replacements are evaluated for material characteristics and contact resistance on solder surfaces. In one study, the requirement for volume resistivity is given as 0.001 Q-cm or less and for contact-resistance change as 20% or less after 500 hours of 85/85-exposure. ... 

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