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Thick film pastes for nitride ceramics for high power applications
Keywords: high power , nitride ceramic, thick film pastes
Since decades thick film technology offers a series of advantages in the design of power electronic circuits: Direct print of high performance resistors and conductors, embedding of single and power tracks, suitable for high voltage and high frequency applications, realization of multilayer circuits, double sided and any shape usage. For the permanent miniaturization needs advanced substrate properties are essential. Since several years AlN is used as ceramic substrate offering excellent electrical insulation and very high thermal conductivity of up to 200 W/(m•K). However, the flexural strength of AlN is low (220 MPa) [1], which results in a reduced number of thermal cycles to failure of the manufactured device. A suitable alternative is Si3N4. The material has a significant higher flexural strength (610 MPa), an excellent thermal shock resistance and high fracture toughness. This makes Si3N4 an excellent ceramic substrate material for the next generation of power electronics. Further, the thermal conductivity of up to 120 W/(m•K) is comparable to AlN [2]. On the other hand, the low thermal expansion coefficient of Si3N4 of about 3 ppm/K constitutes a major challenge in the development of suitable thick film pastes for Si3N4. A series of new resistor pastes on Si3N4 as substrate material was investigated. The pastes consist of AgPd as conductive phase, glasses of matched composition and little amounts of inorganic additives. The fired films give sheet resistances between 100 mOhm/sq and 10 Ohm/sp. The thermal coefficient of the resistivity (TCR) is obtained to be below 100 ppm/K. A series of high power performance tests by means of STOL (short term overload) investigations was examined. In addition, the influence of various encapsulating glasses and impacts of high temperature aging on the high power performance are in focus of the study. FESEM analysis of the film structures were carried out in order to formulate material-performance relations. Comparable investigations were performed on AlN in order to point at the pronounced influence of the substrate material. The results are discussed in terms of chemical and physical material interactions and their influence on the performance under high power loads in the application.
Marco Wenzel,
Fraunhofer IKTS
Dresden, Germany
Germany


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