Electrical silver filled

Supreme 10 HT/S Master Bond 60 min at 125°C 35-45 min at 150°C Silver-filled version of Supreme 10HT for electrical conductivity... 

Licari, et al. (6) showed that by placing a drop of deionized water across a 20 mil gap between a conductor and a silver-filled adhesive, and then applying a one volt DC potential, silver dendrites began to grow across the gap within 30 seconds. Bridging, causing an electrical short, occurred within 3-4 minutes. Within eight minutes, silver particles had completely filled the gap. 

Thermally conductive adhesives may be filled with metal, ceramic, or inorganic particles. Silver-filled epoxies have high thermal conductivities, but may not be used where there is a risk of electrical shorting. In such cases, epoxies or other polymers filled with electrically resistive, but thermally conductive materials such as aluminum nitride, boron nitride, alumina, or beryllia must be used. Some applications for thermally conductive adhesives include attachment of power devices, heat sinks, large components such as capacitors and transformers, large ceramic substrates, and edge connectors. 

Optimum curing conditions are also important in attaining the highest conductivity. The improvements in electrical conductivity resulting from increased time and temperature cure conditions for a commercial silver-filled epoxy adhesive are found in Table 2.3. This low temperature-curing adhesive is used in assembhes having temperature-sensitive components. 

Electrical conductivities of fully cured, silver-filled epoxies are quite stable, decreasing only slightly when measured at elevated temperatures (Fig. 2.12, Table 2.4). Extended cures and aging at 150 °C for 1,000 hours have even improved their conductivities. 

For electrically conductive metal-filled adhesives, dielectric constants and dissipation factors are not meaningful parameters, but volume resistivities and contact resistances are significant. The volume resistivities for the best silver-filled epoxies range from 1 x 10 to 8 x 10 " ohm-cm. 

Thermally conductive films are used to bond heat dissipating components to heat sinks. Electrically conductive silver-filled epoxy films when used to attach substrates also serve as ground planes and provide RF/EMI shielding. They are also used in the high production automated attachment of leadframes to chips and to bond chips vertically in chip stacks. 

Adhesives used for screen or stencil printing in surface-mount applications are generally electrically insulative types whose functions are mechanical attachment and thermal dissipation. However, electrically conductive, silver-filled epoxies have been used for many years as ohmic contact adhesives to interconnect bare-chip devices in hybrid microcircuits and are used as solder replacements for surface mounting of components on printed-circuit boards. Regardless of their... 

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 instability of silver-filled epoxies on non-noble metal surfaces when used as replacements for solder. 

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... 

Partici Shape and Size. The most common morphology of conductive fillers used for ICAs is flake because flakes tend to have a large surface area, and more contact spots and thus more electrical paths than spherical fillers. The particle size of ICA fillers generally ranges from 1 to 20 /rm. Larger particles tend to provide the material with a higher electrical conductivity and lower viscosity (45). A new class of silver particles, porous nano-sized silver particles, has been introduced in ICA formulations (46,47). ICAs made with this type of particles exhibited improved mechanical properties, but the electrical conductivity is less than ICAs filled with silver flakes. In addition, short carbon fibers have been used as conductive fillers in conductive adhesive formulations (36,48). However, carbon-based conductive adhesives show much lower electrical conductivity than silver-filled ones. 

Fig. 17. Schematic depicting the effect of galvanic corrosion of a nonnoble metal pad on electrical conduction of a silver-filled ICA. (a) Good electrical conduction before corrosion, (b) Poor electrical conduction due to the formation of a metal hydroxide or oxide formation as a result of galvanic corrosion.

Reliability. Silver-filled ICAs have a potential for silver migration, which cause electrical shorts especially in fine pitch applications. A palladium-treated silver filler exhibited a much improved antimigration characteristics compared to standard silver-filled ICAs. However, the preparation of the Pd-coated silver particles is expensive. Some low cost approaches must be developed. More comprehensive understanding of the fatigue resistance of ICA joints is required. Activities in this area have been limited and nonconclusive. 

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