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Insulated Metal Substrate for High-Temperature Power Electronics Packaging
Keywords: high-temperature packaging, power electronics, insulated metal substrate
Direct bond copper substrates (DBC) are commonly used in power electronics packages and modules. The thick copper layer bonded to the ceramic substrate provides high current handling capacity while the ceramic has a higher thermal conductivity (e.g., AlN) than a glass or glass-ceramic. While it has exhibited good stability under elevated temperature soaking, early failure of the DBC substrate was observed during relatively aggressive temperature cycling tests (-55C to 250C at ~2 cycles/hr), raising concerns regarding its reliability and suitability for high-temperature power packages. Aluminum nitride substrates with the copper etched to specific dimensions failed after only 20 cycles or less during preliminary cycling tests with the copper layer peeling off and the crack staying in the ceramic just below the metal. An insulated metal substrate consisting of a cordierite-base glass-ceramic sintered on a molybdenum substrate that was developed for other applications is being investigated as a potential alternative to DBC as well. The glass-ceramic has a slightly lower coefficient of thermal expansion (CTE) of 4.1 ppm/C than the molybdenum such that it is under compression below the processing temperature making the coating stable. The glass crystallizes during the firing process and obviates the need to add reinforcing particles to strengthen it. A potential advantage seen with this substrate is that the molybdenum can simultaneously serve as the baseplate for the module and heat spreader/heat sink, thus eliminating several interfaces from the heat dissipation path. The adhesion of the coating is excellent and the interfacial fracture toughness was determined from indentation tests to range from 10 to 19 J/mm2. The substrate has outlasted the DBC substrates under identical temperature cycling conditions with no visual evidence of incipient failure after 45 cycles indicating that it can survive much longer.
Jesus N. Calata, Postdoctoral Associate
Virginia Polytechnic Institute and State University
Blacksburg, VA

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