Thermal conductivity epoxy adhesives

Titanium or beryllium oxide also provides a degree of improvement in thermal conductivity to epoxy systems. Magnesium oxide and aluminum oxide have also been commonly used for this purpose, although the degree of improvement is not as great as with the fillers discussed above. The effect of various fillers on the thermal conductivity of cured adhesive is shown in Fig. 9.6. The incorporation of metal fibers with metal powders has been shown to provide synergistic improvement to the thermal conductivity of adhesive systems,... 

Ecrotherm . (Emerson ft Cuming] Silicone or epoxy compds. thermally conductive greases, adhesives, coatings, enctpsulants. 

A typical thermally conductive epoxy system used as an adhesive, as well as for other purposes, has a thermal conductivity of 0.0026 cal/cm/sec/°C and a volume resistivity of 1.5 x 10 ohm.cm (1.5 x 10 ohm.m). Fillers include alumina (aluminum oxide), beryllia (beryllium oxide), other unspecified inorganic oxides, boron nitride, and silica. Boron nitride is an excellent choice as a thermally conductive filler except that its content reaches a maximum at about 40% by weight in epoxy resins. The resultant products are always thixotropic pastes. BerylUa powder has excellent thermal conductivity by itself, but when mixed with a resin binder its conductivity drops drastically. It is also highly toxic and high in cost. Alumina is a commonly used filler to impart thermal conductivity in resins. ... 

Epoxies are the most commonly used adhesives (qv). Silver and gold are sometimes added to an epoxy to improve its thermal conductivity. Polyimide, also used as an adhesive, has low shrinkage as well as low viscosity and can be cured at 180°C its primary drawback is a tendency to absorb water, as much as 6% by weight. 

Pot lives of DETA and TETA adhesives are on the order of 20 to 30 min at room temperature. When mixed with DGEBA epoxy resins in large batches, the exotherm can be significant due to the reactivity. This generally limits the amount of mixed adhesive that can be prepared at one time, and it also limits the amount (mass) of adhesive that can be applied to a joint [although often thin bond line and the thermal conductivity of the substrate (e.g., metals) will diminish exotherm effects]. 

Aluminum powder, in particular, is frequently employed at relatively high concentrations in high-temperature epoxy adhesive formulations. The filler provides improvement in both tensile strength and heat resistance, and it increases the thermal conductivity of the adhesive. Aluminum powder fillers also reduce undercut corrosion and, hence, improve adhesion and durability of epoxy adhesive between bare steel substrates. It is believed that this is accomplished by the aluminum filler providing a sacrificial electrochemical mechanism.27... 

These processes have an advantage in that the heat penetrates deeply into the joint and into the epoxy material itself. With conventional thermal energy processes, the heat must be conducted into the mass of the epoxy adhesive from outside the joint. This is hindered by the presence of the substrates, the substrate geometry, and the relatively low thermal conductivity of the epoxy itself. 

When the epoxy adhesive cannot be made flexible enough, the thermal conductivity and thermal expansion coefficient are controlled by appropriate fillers. General-purpose room temperature cured epoxy-polyamide adhesive systems can be made serviceable at low temperatures by the addition of appropriate fillers to control thermal expansion. 

Relatively low thermal expansion coefficient and high thermal conductivity compared to most epoxy adhesives... 

A third difficulty in bonding metal surfaces is that they have a higher thermal coefficient of expansion and thermal conductivity than most epoxy adhesive systems. As explained in other chapters of this book, the difference in rates of thermal expansion results in internal stresses in the adhesive joint, especially when the adhesive bond is cured at elevated temperatures or when it is exposed to low temperatures or repeated thermal cycling. 

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. 

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... 

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). ...

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

CSP Ultralow MOE, stress and thermoset (MCOT), silicone materials, highly filled thermally conductive adhesives Epoxy blends, cyanate-ester... 

Once the epoxy bumps have been formed, the wafer is thinned from the backside, the devices are singulated, and their edges beveled at 45° to expose gold pads at the periphery of die. The chips are then stacked and bonded with thermally conductive, electrically insulative epoxy (Figure 5.16). Paste adhesives are cost effective and can be dispensed with existing epoxy dispensing equipment. However, with... 

Substrates such as diamond-copper combine excellent thermal conductivity and closely matched thermal-expansions but at a significantly higher cost. On the other hand, copper that has high conductivity and is low in cost, presents an expansion-coefficient mismatch with the LED materials. These problems may be resolved by formulating adhesives with modified epoxies, silicones, or flexibilizers to relieve stresses (see Chapter 2). 

In a typical case where a silicon IC is attached to an alumina ceramic substrate that, in turn, is attached to the inside of a metal or ceramic package, the two epoxy interfaces can easily contribute 2.5 °C/watt to the total resistance. However, some silver-filled epoxies are reported to have high thermal conductivities, thus contributing 0.6 °C to 1 °C/watt. Actual measurements may differ considerably from calculated values because of reported thermal conductivities that differ from the actual, differences in the thicknesses of bond lines, voids in the adhesive, and incomplete mating of surfaces. Further in the analysis, the effects of lateral flow of heat and interactions of heat flow among adjacent components are often neglected. 

DieMat DM 6030 Series, High thermal conductivity silver epoxy adhesive, DieMat Product Data Sheet. March 19, 2008. 

Thermal-transfer adhesives that are electrically insulating also exhibit wide ranges of thermal conductivities, depending on the filler type and amount. The thermal conductivities of epoxies filled with boron nitride or diamond are approximately 4 W/m K and 12 W/m K, respectively, while those of the more common aluminum-oxide-filled adhesives range from 1 to 2 W/m K. 

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... 

Thermal dissipation. Thermally conductive adhesives may be filled with metal, ceramie, or inorganie partieles. Silver-filled epoxies have high... 

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