Epoxies electrically conductive adhesives

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... 

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... 

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). 

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. 

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. 

Electrical-andjor thermal-conducting adhesives [1,9]. The epoxies and acrylates described above are filled with metal powders to get electrical-conducting adhesives. For special applications polyimide and silicone adhesives are used also. Since the metallic particles must touch each other inside the resins to reach a sufficient level of conductivity, a metal content of 70 to 80 wt % is necessary. Silver is the metal generally used, since specific resistances of the filled adhesives down to about 10 " S2cm can be achieved (metallic silver has a specific resistance of 1.6 x 10 S2cm). Using other metals, such as copper or nickel, the accessible electrical conductivity is too small. On the other side, copper-filled resins show good thermal conductivity and are therefore used for such... 

Epoxies became popular for the first generation of electrically conductive adhesives introduced in the mid-to-late 1960s. The early adhesive... 

Dispensible electrically conductive adhesive Epoxy Resin (Bisphenol-F) 15-25% Diepoxide diluent 1-15% Aromatic amines 5-15% Silver flake (Ferro 26 LV, Silflake 282, etc.) 50-80% Ablebond 84-1 series, Epo-Tek 3001, Ablebond 967-1, Epo-Tek H20 series... 

Electrically conductive adhesives, primarily silver-filled epoxies, are finding uses as replacements for solder in surface-mounting components on printed-circuit boards and in flip-chip attachments. There are several driving forces for this application, a major one being the trend to eliminate lead and tin-lead solders because they may be health hazards. Also associated with the use of solder, is the need to eliminate ozone-depleting solvents presently used to clean and remove flux residues. Electrically conductive polymer... 

Lin and Chiu [55] studied the effects of silver or copper particle composition (silver coated or uncoated copper), on particle shape (flake or spherical), particle size and oxidation temperature on the electrical properties of copper-filled epoxy resin electrically conductive adhesives. They also studied pressure dependent conduction behaviour of compressed copper particles. The silver-coated copper particles showed significantly greater oxidation resistance than un-coated copper particles because the... 

Some applications call for an electrically conductive adhesive. Masterbond offers EP21TDC/S Medical which is a two-component, electrically conductive, silver-filled tough epoxy adhesive with a high peel strength as well as low volume resistivity. 

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. 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. 

Electrically conductive adhesives are used for such applications as bonding lead wire to electrodes, solder paste replacement in the assembly of SMD, die attachment, and repair of conducting paths. Adhesives such as epoxies or acrylics are highly filled (typically 80%) with conductive metal particles. Metals such as nickel and copper cannot be used because of their susceptibility to oxidation. Precious... 

Figure 4 Chemical formulae of di- and poly functional epoxies conunonly used to manufacture electrically conductive adhesives Dainippon Epiclon 830 18, Ciba-Geigy ERE 1359 19, Ciba-Geigy MY 0510 21, Ciba-Geigy MY 720 23, Dow Chemical DEN 438...

To reduce the cost of flip chip interconnection, an organic process was developed at Epoxy Technology Inc. [81]. The polymer flip chip technology (PFc ) a polymer dielectric paste and an electrically conductive adhesive. 

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