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July 2016 • The PCB Magazine 59 materials are preferred for heat sink applica- tions, while materials of low thermal conduc- tivity are used for thermal insulation. Using dielectric materials having a high thermal conductivity to build a PCB will make the whole structure more thermally conductive, and will help to transfer heat generated by the components to a colder region. This is generally the strategy to maintain both the PCB and the components to an acceptable operating tem- perature. Thermal conductivity is probably the most interesting characteristic, when it is about to compare different base materials in order to optimize a printed circuit board thermal man- agement, or simply to assess the operating tem- perature of a system. This is why suppliers indi- cate usually thermal conductivity properties in their dielectric base materials' datasheets. There are a number of possible ways to mea- sure thermal conductivity, each of them suit- able for a limited range of materials, depend- ing on the thermal properties and the medium temperature. Two classes of methods exist to measure the thermal conductivity of a sample: steady state and non-steady-state (or transient) methods. In general, steady-state techniques perform a measurement when the temperature of the material measured does not change with time. The transient techniques perform a mea- surement during the process of heating up. Thermal Diffusivity In heat transfer analysis, thermal diffusivity is the thermal conductivity divided by density and specific heat capacity at constant pressure. It measures the ability of a material to conduct thermal energy relative to its ability to store thermal energy. In a material with high thermal diffusiv- ity, heat moves rapidly through it, because the material conducts heat quickly relative to its volumetric heat capacity or 'thermal bulk.' It characterizes the heat propagation velocity in a material, without information on its intensity. Thermal diffusivity is often measured with the flash method (this is the case for ASTM E1461 and IPC-TM-650-2-4-50). It involves heating a strip or cylindrical sample with a short energy pulse at one end and analyzing the temperature change (reduction in amplitude and phase shift of the pulse) a short distance away. Thermal diffusivity is usually denoted . The following formula explains the relationship between thermal conductivity l, the density ρ, and the specific heat C p . Where a: thermal diffusivity (m 2 .s -1 ) l: thermal conductivity (W.m -1 .K -1 ) ρ: density (kg.m - ³) Cp: specific heat capacity (J.kg -1 .K -1 ) NB: ρCp is called volumetric heat capacity (J.m -3 .K -1 ). When ρCp is known, measuring thermal diffusivity is very interesting to assess thermal conductivity. Current PCB Base Material Thermal Conductivity Measurement Situation There are a number of possible ways to measure thermal conductivity, each of them suitable for a limited range of materials, de- pending on the thermal properties and the medium temperature. Every field of applica- tion, from building to semiconductor industry have its own nature of materials, performance expectations, and range of temperatures. Most of printed circuit board materials are made of woven glass-fiber impregnated with resin. Glass and resin do not have the same prop- erties. Therefore, thermal conductivity in the XY plan (along the glass fibers) cannot be the same as thermal conductivity along the Z-axis (thickness of the PCB). Table 1: Thermal conductivities of normal PCB base materials main components (orders of magnitude, W.m -1 .K -1 ). A THERMAL CONDUCTIVITY MEASUREMENT METHOD, ADAPTED TO COMPOSITE MATERIALS