Thermal conductivity silver-filled adhesives

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. 

In practice, several techniques can be used to maximize the dissipation of heat. Of course, solder and other metallurgical attachments provide the lowest thermal resistance but, where adhesives must be used, those having the highest thermal conductivities should be selected. Some silver-filled adhesives, formulated with unique suspensions of silver, resin, and carrier fluid are reported to have thermal conductivities of 20 W/m-Kf l approaching those of ceramics and some metals. An adhesive having a high silver loading is produced once the carrier fluid has been removed, and the adhesive is fully cured. 

Two silver-filled adhesives, Johnson Matfliey JM 7000 and JM 7800, have respective elastic modulus of 10 and 5.8 GPa, 7g 250 and 210°C, thermal conductivity 1.1 and 1.6Wm K , volume resistivity 2x10 and 5 X 10 n cm. According to the manufacturer, the thermal stresses, determined by measuring the radius of curvature of silicon chips, are far from the level of stress where the risk of cracking and delamination increases considerably. In 1994 Johnson Matthey proposed a low-stress cyanate adhesive (JM 2500) exhibiting a modulus of elasticity of only 0.4 GPa. The low stress properties of this material were demonstrated by the large radius of curvature (Im)ofa 15x 15 mm die bonded to a 0.15 mm thick leadframe. This value remained constant after KKX) thermal cycles from — 65 to 150°C. 

Curves of Figure 19 compare the data published for (a) boron nitride [37,40] (b) aluminium (c) diamond-[37-39] (d) aluminium nitride [37-42] (e) crystalline silica. It can be seen that, at 45 vol.%, the maximum thermal conductivity achieved with diamond powder is 1.5 W m K, while crystalline boron nitride at 35 vol.% affords 2.0Wm K. The thermal conductivity of silver-filled adhesives was studied by using silicon test chips attached to copper and molybdenum substrates [43]. The authors outline the importance of the shape factor A, related to the aspect ratio of the particles, to achieve the highest level of thermal conductivity. Another study reports the variation of the effective thermal resistance, between a test chip and the chip carrier, in relation to the volume fraction of silver and the thickness of the bond layer [44]. The ultimate value of bulk thermal conductivity is 2 W m at 25 vol.% silver. However, the effective thermal conductivity, calculated from the thermal resistance measurements, is only one-fifth of the bulk value when the silicon chip is bonded to a copper substrate. 

The electrical conductivity of gold- and silver-filled adhesives is addressed in Section 6.3.2 and the qualification standards in Section 6.4.2.9. Although this latter directive limits the thermal ageing conditions to 150°C, different studies have been published on the behaviour of organic adhesives at higher temperatures [133,134]. The volume resistivity of commercial adhesives is of the order of... 

Adhesives are poor conductors of heat and electricity, but both can be increased by filling with powdered metals, especially silver. To increase thermal conductivity alone, metal oxide fillers can be used. The most effective of these is beryllium oxide, which is both toxic and expensive aluminum oxide is a practical alternative. Some values of thermal conductivity are collected in 0 Table 18.6. 

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. 

Electrically conductive film adhesives, if filled with silver particles, are also highly thermally conductive. Thermal conductivities range from 0.9 to 7 W/m K, depending on the formulation. Aluminum-to-aluminum lap shear strengths range from 2200 psi to 5100 psi. Films are commercially available in thicknesses from 2 to 6 mils. 

The main advantages of silver-glass adhesives are their very high thermal conductivities and thermal stabilities compared with filled polymer resin formulations. Thermal conductivities of 65 W/mK to over 80 W/m K are reported. ... 

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 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. 

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... 

Epoxy adhesives are also easy to modify for special purposes. For example, they can be filled with carbon, silver, or gold to provide electrical conductivity. Other additives can enhance thermal conductivity, while maintaining electrical insulation. Additional performance properties of epoxy-based adhesives that can be modified include impact resistance, shrinkage, glass transition temperature, high-temperature strength, surface specific adhesion characteristics, and chemical or moisture resistance. 

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