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70 The PCB Magazine • July 2016 line, as a function of time. Using the appropri- ate mathematical equations described above, it allows to determine the thermal diffusivity. Re- sults are in Table 5: Using the previously measured specific heat capacities and densities, it is now possible to compute the thermal conductivities in the XY plane and along the Z-axis: Results In this particular case, the base material manufacturer's datasheet differentiates the thermal conductivity along Z or in the plane but does not indicate specific values for 106 or 1080 prepregs, nor onto which laminate (thickness and composition in terms of glass- content) measurements were made. As indicat - ed in the datasheet: "Results listed above are typical properties, provided without warranty, expressed or implied, and without liability. Properties may vary, depending on design and application." Transient fin measurements gave lower val- ues than the typical values indicated in this datasheet. A quick look at the resin content of the 1080 and 106 indicates respectively 85% and 90% resin content. As seen before, glass is a better heat conductor than "normal" epoxy res- in, but this improved material does not contain "normal" epoxy resin. It has been filled with ce- ramic particles, precisely to improve its thermal conductivity, bringing it significantly above the ~1 W.m -1 .K -1 of the glass fibers. Therefore, it is normal to obtain a lower XY thermal conduc- tivity for 1080, containing more glass than 106. Conclusion In the PCB ecosystem, there is currently no unique and common standard method to mea- sure thermal conductivity. However, modern applications require an accurate knowledge of base material properties for thermal optimiza- tion of electronic systems. Thermally improved dielectrics become more and more popular in various printed circuit board applications. These copper-clad laminate products and corre- sponding prepregs are usually glass-reinforced, and therefore quite strongly anisotropic. Sig- nificantly expensive to buy and to process, the benefit brought by their use should be ac- curately known or at least easy to assess when designing a new product. 3D thermal models are nowadays common tools for system design- ers, but to provide consistent simulation results, these tools need accurate thermal properties to provide good simulations. Transient fin method presented in this pa- per is at its beginning. At this stage, it seems suitable and efficient enough to help suppliers and manufacturers to complete thermal con- ductivity information on most of dielectric base materials used in the printed circuit board in- dustry. It requires to manufacture a dedicated but simple test sample, to use a measurement chain with a current step generator, an IR sen- sor, and appropriate engineering skills. It can be improved by building a dedicated device, com- prising the current source, the sample holder, Table 5: XY thermal diffusivity measurements. Table 6: XYZ thermal conductivities calculated. A THERMAL CONDUCTIVITY MEASUREMENT METHOD, ADAPTED TO COMPOSITE MATERIALS