Micross

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Thick film pastes for aluminium nitride and silicon nitride ceramics
Keywords: Thick film technology, aluminium nitride , silicon nitride
Since decades thick film technology offers a series of advantages in the design of power electronic circuit: 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. In order to improve the performance of the printed circuit and enable additional miniaturization advance substrate material are essential. Since several years AlN is used as ceramic substrate offering excellent electrical insulation and very high thermal conductivity of up the 200 W/mK. However, interactions of common thick film paste additives such as bismuth or copper oxide with the substrate restrains of the variety of available thick film pastes for AlN. In addition, legal provision such as the ‘Restriction of Hazardous Substances Directive’ (RoHS) and ‘Registration, Evaluation, Authorisation and Restriction of Chemicals’ (REACH) prevent the utilisation toxic materials such as lead and cadmium, which are used in many thick film pastes. At Fraunhofer IKTS, we developed a complete REACH and RoHS conform paste system, consisting of contacts, a full range of different resistors and overglaze, suitable for thick film technology on AlN. New additives, adopted organic vehicle, and modified manufacture technologies are used to tune properties such as the temperature coefficient of the resistor (TCR) and adhesion strengths of solder joints. The influence of these parameters on the performance of the developed environmental friendly paste system will be discussed. In addition we will present our reason developments on thick film paste systems for Si3N4. This material has an excellent thermal shock resistance and high fracture toughness that outranges the performance of AlN. Where, thermal conductivity and material density are comparable. This makes Si3N4 an excellent ceramic substrate material for the next generation of power electronics. However, the low thermal expansion of about 3 ppm/K compared to approximately 4.3 ppm/K for AlN and 6.5 ppm/K for Al2O3 is the major challenge in the processing of thick film pastes on Si3N4. Therefore, new glasses and glass-ceramic composites were development. We will present the performance of the new paste system for Si3N4 and discus material-property relations.
Marco Wenzel,
Fraunhofer IKTS
Dresden, Saxon
Germany


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