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Power device packaging targeting high temperature operation with maximum Tcase = 250ºC
Keywords: Thermostable packaging, Ag sinter die-attach, nanocomposite molding
Post-silicon wide band-gap semiconductors like SiC and GaN are emerging into the market though the current packaging technology is basically similar to those for Si devices. The inherent high operation temperatures of these wide band-gap semiconductors are widely discussed, but yet not achieved in a production level reliability and quality. Here we demonstrate a set of packaging technologies with excellent thermal stability for high temperature SiC device operations over 200ºC in a mass-product quality. Our thermostable packaging method is realized by a well-tuned combination of thermostable molding and metal-paste sintering die-attach [1]. The former is an imide-based polymers of silsesquioxane nanocomposite [2], and the latter a Ag micro-flake paste involving SiC nanoparticles [3]. The die-attach can be processed in 30 min sintering at 250ºC, in air, without loading. The molding by thermal injection is conducted at 180ºC for 5 min. followed by 5 hrs of post-mold curing at 270ºC. The resulting Tg of the mold material reaches 400ºC, where both the CTE and the Young's modulus are carefully tuned for those of other parts in the device package. At the die-attach interface layer, sintered Ag and nanocomposite polymer finally forms an ideal microscale network structure to relax the thermal stress, and mitigates atomic migrations and grain growth of Ag interconnection. The tests samples of SiC SBD devices in TO-247 package have survived various stress tests: high temperature storage (HTS) at 250ºC for 1000 hrs, 500 cycles of heat-shock tests (HST) of from -50ºC to 250ºC, and power cycle tests (PCT) with maximum Tcase = 250ºC up to many thousands of cycles. Characterizations of the tested devices are made in electronic and thermal properties, as well as the detailed analysis of the microstructures in the die-attach layer. On the basis of these experimental observations, we discuss our packaging design in terms of the thermal stability, the device reliability, and the expected lifetime of the products. We would finally outlook the future of high operating-temperature power devices.
Shijo Nagao, Associate Professor
Osaka University
Ibaraki, Osaka

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