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|Silver in LTCC - Interfacial Reactions, Transport Processes and Influence on Properties of Ceramics|
|Keywords: LTCC, silver conductors, sintering process|
|Silver pastes have been preferentially used for screen printed conductors and vias in the production of LTCC modules. Reasons are high electrical conductivity, the possibility to sinter in air and the moderate price of silver compared to other noble metals as gold and palladium. But there is a well-known disadvantage: During sintering the glassy phase around conductors and vias can assimilate silver. A successful development of LTCC components requires a control of such silver migration to prevent a local degradation of materials properties. The local silver concentration in LTCC multilayers was investigated in dependence on sintering temperature, sintering atmosphere and type of LTCC material. Electron microprobe investigations on tempered samples of LTCC containing alumina and glass prove that silver assimilation of the glass starts above 600 °C. The silver transition into the glassy phase stops when the silver conductors are hermetically sealed in LTCC. Further investigations show: Silver amount around inner conductors is much lower than around surface vias and after sintering in nitrogen atmosphere no silver was found in the glassy phase. Besides interactions between silver and standard low-dielectric LTCC materials containing glass also the behaviour of silver screen printed between almost glass-free ferrite or capacitor tapes was investigated. No silver was found after sintering at 900 °C in both materials, but silver moved through these materials and was detected in glassy phases of other co-fired LTCC materials. All results approve the assumption that surface oxidation of silver, evaporation and transport via gas phase are the mechanisms for the transfer of silver from conductors in the glassy phase. Silver in the glassy phase of LTCC can change the local sintering and crystallization behaviour and the material properties, as permittivity and colour. Examples of property change are given and the significance for the functionality of LTCC modules will be assessed.|
|Torsten Rabe, Research Manager
Federal Institute for Materials Research and Testing (BAM)