Thermal conductivity unfilled epoxies

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). 

Encapsulation is often performed with resins containing fillers such as mica, aluminum oxide, milled glass fibers, and many others. Although epoxies are the resins most generally used, polyesters, filled and unfilled silicones, urethanes, and polysulfides are also used. By the proper choice of fillers it is possible to match expansion rates of the electronic part and the encapsulant, increase the thermal conductivity of the part, and considerably upgrade the electrical and mecharucal properties of the assembly. 

Table 2 lists thermal conductivity values for several metals as well as for beryllium oxide, aluminum oxide, and several filled and unfilled resins. Fig. 2 shows the thermal conductivity for an epoxy resin as a function of volume fraction of heat-conductive filler. 

The best current 100% solids epoxy adhesives contain about 70% aluminum oxide by weight and give thermal conductivities in the range of 0.8-1 in the English units shown in Table 2. For convenience, a conversion chart is included in Table 2 to permit conversion to any other set of units. The k values for the best alumina-filled epoxies are 10-12 times greater than for unfilled epoxy resins, but are still much lower than for pure metals or solders. Nevertheless, heat flow is adequate for bonding most components. For example, an adhesive with a thermal conductivity of 0.91 and a bond thickness of 3 mils would be able to transfer about 20 W/cm of surface area, with a AT only about 10 C above the heat sink temperatures ... 

The data of this table show that unfilled polymers and plastics have rather low X values and are very good thermal barriers or insulators. Furthermore, the thermal conductivity of most polymers falls in the tight range indicated for unfilled epoxy resins, about 0.2 W m which is more than three orders... 

Being inexpensive and providing an excellent shear strength, alumina is broadly used to formulate thermally conductive adhesives. The best solventless epoxy adhesives contain about 70% of aluminum oxide and give thermal conductivities in the range of 1.4—1.7 W m These values are 8—10 times more than for the unfilled epoxy resins but are still much lower than for pure metals or solders. Never-... 

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