Integration of Ni-Cu-Zn Ferrite in Low Temperature Co-fired Ceramics (LTCC) Modules

Integration of magnetic functional components in LTCC circuit boards calls for co-firing of dielectric and ferrite tapes. Ni-Cu-Zn ferrites with permeability of mu = 900 were fully densified at the standard LTCC sintering temperature of 900 degrees C. Co-firing of this ferrite with dielectric tapes requires the adaptation of the shrinkage behavior of the materials as well as the thermal contraction during the cooling period especially in the temperature range below the transformation point of the glass 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 laminates composed of Ni-Cu-Zn ferrite and tailored dielectric tapes are free of cracks and possess no open porosity. No significant interdiffusion between the ferrite and dielectric tapes was found using microprobe analysis. Pure ferrite laminates show homogeneous microstructure with large grains up to 50 mu m in diameter. However, growth of ferrite grains does not take place near the interface between ferrite and release or dielectric tapes. There is a strong correlation between permeability and volume fraction of large ferrite grains. Compared to pure ferrite laminates the permeability of co-sintered Ni-Cu-Zn ferrite layers is drastically reduced to 400, that is a decrease of more than 50%.