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Evaluation of Fracture Toughness of Ceramic Substrates for Power Modules
Keywords: Ceramic substrate, Fracture toughness, Power modules
The rapidly growing market of electric vehicle and hybrid electric vehicle demands higher power density of the circuit in power modules. The heat dissipating board for the power modules usually consists of a thin ceramic substrate such as aluminum nitrides or silicon nitrides sandwiched by cupper plates. Mechanical sustainability of the substrate after sever heat cycles is a critical issues since thermal stress due to thermal expansion mismatch between Cu and ceramic plates generates cracks in the substrate and damages the components. The fracture toughness of the thin ceramic plate is a main factor which governs the thermal fatigue of the heat dissipating board. However, fracture toughness evaluation of such a thin ceramic plate with a thickness of ~0.32 mm has not been studied systematically. In this study, the fracture toughness of both thin aluminum nitrides and silicon nitrides was measured using a single-edge precracked beam. Several test specimens with the size of both 0.32 x 4.0 x 40 mm3 and 0.62 x 4.0 x40 mm3 were machined from a sintered bulk of AlN or Si3N4, together with the standard-sized specimens with the size of 3 x 4 x 40 mm3. A small, thin single-edge notched beam was bonded on one side of a brass beam and the assembly was deformed in three-point bending to introduce the precrack. The single edge-precracked specimen was then removed from the beam and the fracture toughness was measured using standard single edge-precracked beam (SEPB) method with a modified jig. The fracture toughness measured using this method was almost identical to those obtained for the standard specimens machined from the same bulk, which demonstrated the validity of our new evaluation method. Part of the research work was supported by NEDO, Japan.
Hiroyuki Miyazaki, Senior researcher
National Institute of Advanced Industrial Science and Technology
Nagoya, Aichi Prefecuture

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