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Integration of Ni-Cu-Zn Ferrite in LTCC-Modules
Keywords: LTCC Modules, Ferrite, Co-firing
Integration of magnetic functional components in LTCC circuit boards calls for co-firing of dielectric tapes, silver conductors and ferrite tapes. Ni-Cu-Zn ferrites with high permeability of µ = 900 were developed which can be sintered to high density at standard LTCC temperature of 900 °C. Successful co-firing of this ferrite with dielectric tapes requires the adaptation of the shrinkage behavior of the materials as well as adaption of the thermal expansion during the cooling period - especially in the temperature range below the transformation point of the glassy phase of the dielectric tape. To match these preconditions, a new dielectric LTCC material with steep sintering curve and high thermal expansion coefficient was designed. Sintered multilayer composed of Ni-Cu-Zn ferrite and tailored dielectric tapes are free of cracks and possess no open porosity. Compared to pure ferrite laminates the permeability of co-sintered Ni-Cu-Zn ferrite layers is drastically reduced to µ = 400, i.e. a decrease of more than 50 %. To investigate the origin of this permeability decline, the effect of the multilayer laminate architecture and of the sintering conditions (pressure) were studied. Ni-Cu-Zn ferrite laminates were sintered separately, in combination with alumina sacrificial tapes, and with dielectric tapes, respectively. Ni-Cu-Zn ferrite layers show inhomogeneous microstructures after co-firing with dielectric tape. The ferrite grain size is strongly reduced near the interface. Furthermore, distinctive local changes of ferrite composition were found. SEM and EDX measurements confirm severe interdiffusion between the ferrite and dielectric tapes. Correlations between microstructure and composition on one hand and ferrite permeability on the other hand will be discussed. As an alternative to prevent silver diffusion into surrounding ferrite and dielectric layers, co-firing of multi-component LTCC laminates under reduced oxygen partial pressure was investigated. However, in the case of Ni-Cu-Zn ferrites this is confined by the limited stability of the ferrite at lower pO2. Phase stability and permeability of the ferrite were studied at different oxygen partial pressures of the sintering atmosphere.
Torsten Rabe,
Federal Institute for Materials Research and Testing (BAM)
Berlin 12200,
Germany


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