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June 2014 • SMT Magazine 17 ADvAnCeD THeRMAL MAnAgeMenT SOLuTIOnS continues feaTure experimental Setup Test objects were selected with following specifications: • DK2, (FR-4, thickness = 1 mm, pads with plugged thermal vias) • DK6, (FR-4, thickness = 0.2 mm, open thermal vias with 0.3 mm diameter, laminated onto 1.5 mm thick Al substrate with a 90 µm thick prepreg) • IMS1, (70 µm thick copper clad, 110 µm thick dielectric, thermal conductivity = 0.5 W/(mK), Al substrate thickness = 1.5 mm) • IMS3, (70 µm thick copper clad, 125 µm thick dielectric, thermal conductivity = 4 W/(mK), Al substrate thickness = .5 mm) All PCB samples were prepared with a size of 30 mm x 40 mm. The layout allowed test- ing of two power LEDs at the same time (Figure 4a) whereby the comparatively large via arrays should help to maximize heat spreading. Most important qualities of an experimen- tal setup are to simulate boundary conditions as close as possible to the following extreme con- ditions: a) The PCB itself acts as a heat sink by conducting the heat flux in the plane direction and dissipating (e.g., by natural convection and radiation) b) The PCB acts as mean to interconnect the power component with a cooler and to help spreading the heat flux uniformly over the entire attachment surface of the cooler In this section, we compare the thermal performance of FR-4-based PCBs with thermal via arrays (two different modifications: with open vias and with plugged vias) and IMS in an experimental setup according to condition b). For this purpose a water-cooled copper block was used as a heat sink with constant tempera- ture. The samples were attached to the copper block using an equally distributed thin layer of thermal grace with a thermal conductivity of 0.6 W/(mK). The samples were pressed against the copper block with pointed plastic screws in order to keep the thermal resistance of this interface as stable as possible without heat re- moval from the top side (Figure 4b). Temperature distributions of the top side of the samples during operation with the nominal forward current (If = 700 mA) were recorded using a high-end thermography sys- tem. An emissivity correction was made by a calibration measurement with a miniaturized thermocouple at the LED pad. From evaluation of transient thermal re- sponse on power-on-steps with the nominal power value we see that the final temperature distribution on the entire sample surface is ob- tained within only a few seconds in all cases. figure 4: LeD module mounted on water-cooled heat sink: a) top view of test sample; b) sample under test.