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18 SMT Magazine • June 2014 The temperature distributions obtained by IR thermography in the steady state are compared for all four sample types in Figure 4. During ex- periments with the nominal forward current a voltage drop of 4Uf = 13.5 V was measured (four single LED elements in one module, con- nected in series). Assuming an average pho- tonic efficiency of h = 20% we considered a continuous power loss of 7.56 W per sample. Since establishing of the junction temperature was related to high uncertainties we defined the pad-to-cooler thermal resistance Rth,p- c as the relevant parameter characterizing the thermal performance of the PCBs in the follow- ing way: (4) where Tpad is the temperature of the cop- per pad in the vicinity of the LED submount body and Tcooler is the temperature of the copper block. Using the respective temperature values from the IR thermography images and the values mentioned above equation (4) the re- sults as listed in Table 1 were obtained. Thermal Measurements and Results From the IR thermography picture of the DK6 sample a well-developed heat spreading effect can be recognized (Figure 5). The tem- perature gradients in the vicinity of the LED chip are the lowest ones compared to the oth- er three types, thus the heat flux is conducted into the cooler through an enlarged area of the dielectric. This high heat spreading effect is due to the excellent thermal coupling of the two copper layers by the microvia arrays. However, because of the thick dielectric—a 90 µm thick prepreg with moderate thermal conductivity of only 0.3 W/(mK), the temperature distribu - tion and therewith Rth,p-c reaches the highest values. From view point of the copper layout the DK2 and DK6 samples are very similar (not that the thin copper layer at the "plugged" via front faces does not contribute much to the heat dissipation). Though the thickness of the ADvAnCeD THeRMAL MAnAgeMenT SOLuTIOnS continues feaTure figure 5: Comparison of results obtained from Ir thermography measurements.