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68 The PCB Magazine • July 2016 Step 2 Flash method is then used to measure a Z , thermal diffusivity along Z axis (thickness of the base material). Combining a Z and volumet- ric heat capacity ρC p gives thermal conductiv- ity along the Z-axis. Here again, flash method is well documented and standards exist (cf. ASTM E1461, IPC-TM-650 2.4.50). Samples have to be prepared according to standard method, for in- stance ASTM1461. It will not be developed here. Step 3 In a third step, the proposed transient fin method is used. The specific transient fin sam- ples have to be made. With an electrical step stimulus, the copper track is heated-up while an optical thermal sensor (IR detector) records top surface temperature. The mathematical process described above is used to find the best a and h values to make the model stick to the experi- mental curve. It allows to get the thermal diffu- sivity in the XY plane, and later on, combined with the volumetric heat capacity (isotropic), it gives XY thermal conductivity value. Results In this case, glass-reinforced dielectrics have been measured as an example. Measurements were performed on two different prepregs of thermally improved base materials. Test sam- ples have been manufactured with these pre- pregs. For the transient fin method, test sam- ples were manufactured by hydraulic vacuum lamination, photolithography, and routing, in standard printed circuit board industry process conditions. • Sample 1080 = made of thermally optimized glass-reinforced epoxy 1080 type prepreg • Sample 106 = made of thermally optimized glass-reinforced epoxy 106 type prepreg Step 1: Volumetric heat capacity Measured by Differential Scanning Calorim- etry (model mdSc3 from SETARAM). Figure 8: Measurement methodology flow-chart. Table 3: Specific heat capacity Cp of 1080 and 106 prepregs measured by DSC. A THERMAL CONDUCTIVITY MEASUREMENT METHOD, ADAPTED TO COMPOSITE MATERIALS