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Towards Highly Conductive Silver Pastes for LTCC Power Electronics
Keywords: conductor pastes, LTCC, power electronics
Silver shows excellent conductivity and a comparable low price. Moreover, its sintering temperature is lower than 900°C. Thus, it is the preferred conducting material in the highly integrative LTCC multilayer technology. From another point of view, silver has a high mobility at elevated temperatures which can be an issue during co-firing with Low Temperature Co-fired Ceramics (LTCC) because these materials consist of considerable amounts of glass phases. Often silver enters the LTCC glass phase changing its viscosity and sintering behavior, which can have disadvantageous effects of the geometry of an assembly like cambering [1]. The development of modern LTCC circuits for power electronics for e.g. automotive technology or LED packaging is driven by a strong need to handle higher current and heat amounts in order to realize buried inductors or heat dissipation vias. On one hand, the silver amount and density in those conductors has to be maximized, on the other hand, the interaction of those highly silver containing structures with the LTCC surrounding must be controlled carefully. Based on theoretic calculations, we maximized the tap density of silver powder mixtures by blending powders of different particle sizes to values higher than 7 g/cm-3. Then, pastes were made of the respective highly packed silver powders and detailed investigations regarding paste sintering kinetics and interactions between paste ingredients among one another as well as between paste ingredients and LTCC substrates were made. The results of this work are presented and discussed in terms of performance of the resulting conductors, critical parameters in the paste composition and LTCC chemistry. [1] M. Totokawa et al., Study of the Strain Mechanism of Multilayer Ceramic Substrate, IMC 1994 Proceedings, Omiya, April 20-April22 (1994)
Markus Eberstein, Group Manager Thick Film Technology, Photovoltaics
Fraunhofer IKTS
Dresden, Saxony 01277,
Germany


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