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64 The PCB Magazine • July 2016 IPC-TM-650 2.4.50: Thermal Conduc- tivity, Polymer Films "[…] This test method defines the procedure for determining the Thermal Conductivity of polymer coatings on inorganic substrates, such as polyimide on silicon wafer […]". First Conclusion Base material suppliers use many different methods, making difficult to compare the rela- tive performances of their products or to check the actual properties. Some methods are clearly not adapted to anisotropic materials. The flash method, described in both IPC-TM-650 2.4.50 and in ASTM E1461, is claimed to be adapted to measuring thermal diffusivity of only isotropic material (which could be discussed in reality). In any case, it does not allow to determine the XY plane thermal diffusivity of a thin anisotro- pic sample. Proposed Method to Measure XYZ Thermal Conductivity: Transient Fin Method The proposed method is called transient fin method. It aims at measuring the thermal diffusivity in the XY plane, with a simple set- up and a sample easy to produce. Associated with differential scanning calorimetry and the flash method, it allows to have thermal conductivity values along the Z-axis and in the XY plane. The method is based on heat generation by a copper conductor patterned on the dielectric material to analyze, and temperature measure- ment by optical sensor (infrared). The copper pattern is easily produced by a PCB manufac- turer. It does not request particular capabilities. Typically, starting from a laminate with copper on both sides, the pattern is produced by direct photolithography. When it is preferred to analyze the tempera- ture variation on the opposite side, in order to ease the centering of the optical sensor on this opposite side, some fiducials can be added also by photolithography of a copper pattern. How- ever, this copper pattern should be far enough of the analyzed zone, not to disturb the mea- surement. The principle is to inject a step of current between the two extremities of a copper track to generate heat, and to measure the tempera- ture variation in two distinct points at a defined distance from the heating track, by placing an optical sensor (infrared camera) either on the same or on the opposite side of the test sample. Some Physics… At first, the mathematical model of has to be determined. Once this done, the transient fin method consists of recording thermograms (output of the IR sensor), and find the best pa- rameters values to make the model stick to the experimental curves (thermograms). This model developed is based on the fol- lowing assumptions: • The sample is considered as initially at a uniform temperature, equal to ambient air tem- perature Ta. A step of current is supplied to the copper track, and generates heat flux F(t). Very interesting for the transient fin method: it is not necessary to know intensity or time variations of the heat flux. Figure 3: IPC-TM-650-2-4-50: Laser is flashed and the heat rise is measured on the back Al by the detector. Figure 4: Transient fin method principle. A THERMAL CONDUCTIVITY MEASUREMENT METHOD, ADAPTED TO COMPOSITE MATERIALS