Epoxy adhesives silver fillers

Figure 3.16 Effect of increasing amounts of silver filler and of temperature on the thermal conductivity of an epoxy adhesive. The concentrations are hy volume (23% hy volume corresponds to 80% hy weight). ...

Table 3.8 Silver fillers used in epoxy adhesives 1... 

Table 3.8. Silver Fillers Used in Epoxy Adhesives[ H59]... 

ICAs cannot accommodate this large strain, the silver filler particles must move relative to one another as the epoxy matrix is strained. The most common pattern of resistance change was only increased to a point corresponding to about a 70% loss in interface contact resistance before sudden failure. This was an indication that the interface crack slightly propagated into the adhesive (74). 

Because the silver fillers are encapsulated with an epoxy layer, however, Ag migration is not likely to occur in conductive adhesives under test conditions relevant in practice, e.g., 85 °C (185 °F) and 85% RH or 60 °C (140 °F) and 90% RH under 5 V bias (Ref 12). However, under more severe conditions, such as the presence of a liquid water film, higher bias, and smaller pitch spacing, Ag migration does occur. For example, short circuit between 0.20 mm (8 mil) spaced pads has been observed after 2000 h of 85 °C (185 T) and 85% RH test with 15 V bias (Fig. 6). In Ref 28, the migration of Ag particles was also observed in ICA joints subjected to the current-induced aging (10 30A) and the consequent electrical degradation was reported. 

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. ... 

Surface treatments for Cab-O-Sil also influence the viscosity and thixotropic properties of an adhesive as shown in Fig. 3.18 where MS-5 is untreated Cab-O-Sil, TS-720 is treated with polydimethylsiloxane, and TS-610 and TS-530 are treated with dimethyldichlorosilanes. The silica filler treated with polydimethylsiloxane (TS-720) produces greater van der Waals forces of attraction and thus higher viscosity compared to TS-610 and TS-530.1 The surface area of the filler has a pronounced effect on viscosity. As the surface area of silver-flake filler increases, the frictional forces between particles increase resulting in higher viscosity. Epoxy formulations filled with constant-size silver flakes of various surfaces areas show this effect when viscosity is plotted as a function of the filler surface areas (Fig. 

Many alternatives exist and the choice between thermoplastic and thermosetting resins depends on the expected lifetime and maximum operating temperature. Styrene-butadiene block copolymers and acrylic resins can be used to produce low-end adhesives with an acceptable stability up to 100°C, whereas epoxy and silicone thermosets are preferred for their robustness at 150-200°C. In the most severe environments, poly(imide-siloxanes) and polyimides can sustain medium-term exposures to 250 and 300°C, respectively. Various conductive fillers are cited in the literature, including noble metals such as gold or silver, and low-cost metals such as copper, nickel, chromium, and soft solders. 

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