Sinter-active soft ferrites with adequate permeability profiles are required for the fabrication of multilayer ferrite inductors (MLFI). For MLFI fabrication, a Low Temperature Ceramic Co-firing (LTCC) process is used. Substituted hexagonal ferrites of Y-, and M-type represent an important family of soft ferrites which might operate at high-frequency conditions up to 2 GHz. However, for Ag-based multilayer inductor applications a sinter process at 900°C is required. Low-temperature sinter-ability is provided by the use of sub-micron powders and/or sintering additives.
Substituted Y-type hexagonal ferrites Ba2Co2-x-yZnxCuyFe12O22 were obtained after sintering at 1000°C. Substitution of Cu for Co improved the low-temperature sintering behavior. The addition of 5wt.% Bi2O3 guarantees almost complete densification at 900°C. The saturation magnetization and permeability are significantly affected by the Zn-concentration. A maximum permeability of μ′ = 10 and cut-off frequency fg~2GHz was observed for a ferrite with y = 0.4.
Co/Ti-substituted M-type BaFe12-2yCoyTiyO19 ferrites can also be used for multilayer inductors. The magneto-crystalline anisotropy changes from uniaxial to planar upon Co/Ti-substitution, and ferrites with y≥1.1 exhibit soft magnetic behavior. Ferrite powders were prepared at 1000°C. The addition of a sintering aid shifts the temperature of maximum shrinkage down to below 900°C and dense samples were obtained after firing at 900°C. A permeability of μ′ = 16 and a resonance frequency of 1 GHz was observed. Substituted M-type ferrites are stable during co-firing at 900°C and show no sign of decomposition, i.e. these materials are LTCC-compatible.
Ferrite tapes were prepared by tape casting and multilayer structures were fabricated by screen printing, stacking, lamination and final co-firing. Firing was performed at LTCC conditions i.e. 900°C. We report on the co-firing behavior, microstructure and permeability of monolithic laminates. It is shown, that hexagonal Co2/Zn2Y- and Co/Ti-M-type ferrites are excellent magnetic materials for multilayer inductors.
Bondability of Copper Joints Formed Using a Mixed Paste of Ag2O and CuO for Low-Temperature Sinter Bonding
