Epoxy, silver-filled

Hydrophobic, silver filled epoxy Silver-filled epoxy paste Silver-filled cyanate ester paste... 

Silver-filled epoxy paste Aluminum nitride filled thermoplastic epoxy Silver-filled thermoplastic epoxy... 

MMM-A-189 Synthetic Rubber for Hot and Cold Bonding. MMM-A-193 Adhesive, Vinyl Acetate Resin Emulsion. MMM-A-1931 Adhesive, Epoxy, Silver Filled, Conductive. 

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. 

Figure 5.32. Conductivity of epoxy resin filled with silver coated glass beads vs. volume concentration. [Adapted, by permission, from Lekatou A, Faidi S E, Lyon S B, Newman R C, J. Mat. Res., 11, No.5, 1996, 1293-304.1...

Thermal conductivity and expansion are important properties of adhesives used in electronics. Both properties influence the performance of computer chips. Generally, the chip has a protective cover which is attached by an adhesive. The adhesive bond must be maintained during thermally induced movement in the chip. The chip is bonded to its base with an adhesive which must also take thermal movement and, in addition, transfer heat from the chip. Two epoxy adhesives were used in the study silica filled epoxy (65 and 75 wt% SiO2 epoxy) and epoxy containing 70 wt% Ag. Figure 15.6 shows their thermal conductivities. The behavior of both adhesives is completely different. The silver filled adhesive had a maximum conductivity at about 6()"C whereas the maximum for SiOz filled adhesive was 120"C. The Tg of both adhesives was 50 and 160 C, respectively. Below its Tg, the thermal conductivity of the adhesive increases at the expense of increased segmental motions in the chain molecules. Above the Tg the velocity of photons rapidly decreases with increasing temperature and the thermal conductivity also decreases rapidly. 

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. 

Lastly, and more importantly, suppliers have reformulated their standard adhesives to reduce or prevent bleedout. For example, the widely used Henkel s Ablebond 84-1 LMl (Low Mobile Ions), a silver-filled epoxy, was reformulated as a no-bleed version Ablebond 84-1 LMl NB (No Bleed). This NB version, however, was found to have reduced wettability to capacitor terminations and, as a result, yet another version (NBl) was formulated as a compromise. Adhesives that are 100% solids are more prone to bleedout than solvent-based types since they contain low-molecular-weight reactive diluents that have a greater tendency to migrate prior to complete curing. Some surface-related and material-related observations concerning bleedout mechanisms may be summarized as follows ... 

Thixotropy occurs when clusters of filler particles break up and fluidity increases. At rest, fillers exist in clusters and their strength, size, and shape determine the static viscosity of the adhesive. The total surface area of the filler particles also contributes to static viscosity. Shear thinning occurs when the clusters, such as silver flake in silver-filled epoxies, break apart and viscosity decreases then, as stress is removed, the clusters form again and viscosity increases. ... 

The resistivities of the best silver-filled epoxies used for die and chip attachment are in the 10 " to 10 ohm-cm range. Although these values are still several orders of magnitude worse than pure silver metal, they are adequate for most circuit applications. 

Table 2.3 Volume resistivity versus cure conditions for a silver-filled epoxy adhesive ...

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. 

Figure 2.12 Volume resistivity versus temperature for silver-filled epoxy paste adhesive.

Table 2.4 Volume resistivities for silver-filled epoxy adhesives (ohm-cm)...

Graphs of the actual and predicted thermal conductivities for a silver-filled epoxy composition are shown in Fig. (2.15). 

The thermal conductivities of unfilled epoxies, as with all other unfilled polymers, are quite low, typically 0.1-0.2 W/mK. When filled with metal or thermally conductive nonmetal fillers up to 80-85% by weight, the thermal conductivities increase a minimum of tenfold. Some silver-filled epoxies are reported to have thermal conductivities as high as 6.0 to approximately 8 W/m K (AI Technology ESP 8450 W and ESP 8456-00, respectively). 

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. 

Bjorneklett A, Halbo L, Kristiansen H. Thermal Conductivity of Epoxy Adhesives Filled with Silver Particles. Inti J Adhesion. Apr. 1992 12(2). 

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... 

In a similar process, known as polymer-film interconnect (PFI), an insulative thermoplastic film is laminated over the devices at the wafer stage, and vias are opened over the bonding pads using a laser. At that point, either the normal solder bumps can be formed or a silver-filled conductive adhesive can be stencil printed into the vias to form polymer bumps. After printing, the epoxy is B-staged and the flip-chip devices are diced. In assembly, the devices are heated to a temperature that completes the cure of the B-staged bumps and simultaneously reflows the thermoplastic underfill material. 

---