active/passive component embedded substrates is crucial not only to achieve Resistor Capacitor improved performance and power density Al2O43 T = 1,000 nm Vgs2+ but also to ensure the reliability of the sysMet. Block Met. Block SiC Die tem. The various thermal interface materiT = 320 nm Si N als, such as heat spreaders, and active and 3 4 Vgs1+ passive cooling systems must be able to SUS410 Heat Sink transfer high heat fluxes up to several hun(a) (b) dred W/cm2 to the heat sink. Even more challenging is the task to ensure that the entire structure will handle the thermome- FIG 14 (a) A schematic diagram and (b) cross section of a 600-V hightemperature SiC module. Met: metal. (Figure courtesy of NEDO.) chanical stress induced by thermal cycling-an often life-time-limiting potential failure mode. Maintaining coefficient of thermal expansion (CTE) match and mechanical symHeat metry within the system under dynamic thermal load conSources ditions is essential. Figure 14 shows the cross section of a carefully optimized SiC power module. Special care was taken to minimize warpage by maintaining symmetry and deploying Vapor CTE-Matched compatible materials. A group of Japanese companies in Cavity Thermal Casing the New Energy and Industrial Technology Development Nanostructured Interface Heat Sink Organization developed this sandwich-structured power Wick Material module [12]. An interesting feature of the design is the use of a stainless steel (SUS410) heat sink for its matching CTE FIG 15 The TGP. of 10 ppm/oC even though the thermal conductivity of the material is only 26 W/mK. Thermal ground planes (TGPs) are planar heat conductor devices constructed either from high-thermalconductivity solid materials (thermal inserts and heat spreaders) or low-profile vapor chambers [13]-[15]. Depending on their particular design, the thermal conductivity of vapor chambers can reach values up to 4,000 W/mK. Their thickness varies from submillimeters to the range of tens of millimeters. Figure 15 is a cross-sectional illustration of a TGP formed by vapor chamber. The heat removal takes place by means of a continuous cycle of evaporation and condensation of the cooling fluid encapsuleated within the chamber. (a) The condensed liquid (often water) returns to the heat source along the path of the porous wick structure of the inner surface of the chamber. Pumped two-phase cooling systems are the primary choice for high-power and high-power-density applications where high heat fluxes in the range of tens to hundreds of watts per square centimeter are encountered. These systems can produce very uniform temperature gradients within 1- 2 °C over a large surface area while achieving high heat-transfer coefficients [16]. Advanced heat-exchanger designs require very small amounts of coolant circulation that can be maintained by a small and reliable pump with (b) very low power consumption. Internal surface preparation and coating is critical. FIG 16 The implementation of an advanced pumped twoThe two best-performing coatings are the thermally conphase cooling-loop heat exchanger: (a) uncoated channel and ducting microporous coating (TCMC) and the high-tem(b) coated channel. (Photos courtesy of Advanced Cooling Technologies Inc. [16].) perature TCMC (HTCMC). Figure 16(a) illustrates a 1-in # December 2015 z IEEE PowEr ElEctronIcs MagazInE 37