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July 2016 • The PCB Magazine 63 This table gives an overview of the range of thermal conductivities of typical existing dielectric base materials, and thermally im- proved glass-epoxy. Because of this specific- ity, only supplier D considers anisotropy, and specifies thermal conductivity according to the direction of measurement. All others provide a single value. Thermally improved dielectrics are more ex- pensive than standard products, and more dif- ficult to process. Therefore, a good knowledge of thermal conductivities in all directions may be important for the designer to optimize their products, from a pure technical prospective, but also from a cost efficiency prospective. Indeed, heat can be transferred to colder regions later- ally, on component side, on opposite side, or combination of these three options. Thermal Conductivity Measurement Methods Used by Material Suppliers ASTM E1461: "Standard Test Method for Thermal Diffusivity by the Flash Method" "[…] this test method covers the determina- tion of the thermal diffusivity of primarily ho- mogeneous isotropic solid materials. Ther- mal diffusivity values ranging from 0.1 to 1000 mm 2 s -1 are measurable by this test method from about 75 to 2800 K. […]This test method is applicable to the measurements performed on essentially fully dense (preferably, but low porosity would be acceptable), homogeneous, and isotropic solid materials that are opaque to the applied energy pulse. […]" ASTM F433: "Standard Practice for Evaluat- ing Thermal Conductivity of Gasket Materials" […] This practice covers a means of measur- ing the amount of heat transfer quantitatively through a material or system. This practice is similar to the Heat Flow Me- ter System of Method C 518, but modified to accommodate small test samples of higher ther- mal conductance. ASTM D5930: "Standard Test Method for Thermal Conductivity of Plastics by Means of a Transient Line-Source Technique" […] This test method covers the determina- tion of the thermal conductivity of plastics over a temperature range from -40 to 400°C. The thermal conductivity of materials in the range from 0.08 to 2.0 W/m.K can be measured cov- ering thermoplastics, thermosets, and rubbers, filled and reinforced. […] There is no known ISO equivalent to this test method." ASTM C518: "Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus" "[…] This test method covers the measure- ment of steady state thermal transmission through flat slab specimens using a heat flow meter apparatus […] Applicable to the measure- ment of thermal transmission through a wide range of specimen properties and environmen- tal conditions. The method has been used at ambient conditions of 10 to 40°C with thick- nesses up to approximately 250 mm, and with plate temperatures from -195°C to 540°C at 25 mm thickness […]. To meet the requirements of this test method the thermal resistance of the sample must be greater than 0.10 K.m.W -1 in all directions […]" Laser Flash: "Method to measure thermal diffusivity" The laser flash method is used to measure thermal diffusivity of a thin disc in the thick- ness direction. This method is based upon the measurement of the temperature rise at the rear face of the thin-disc specimen produced by a short energy pulse on the front face. With a reference sample, specific heat can be achieved, and with known density the thermal conduc- tivity is calculated. Figure 2: Printed circuit board thermal management possibilities. A THERMAL CONDUCTIVITY MEASUREMENT METHOD, ADAPTED TO COMPOSITE MATERIALS