Ni‐Cu‐Zn ferrites with high Curie temperature for multilayer inductors with increased operating temperatures

Abstract We studied the sintering behavior and magnetic properties of Ni _0.60‐y Cu _y Zn _0.42 Fe _1.98 O _3.99 ferrites. The shrinkage is shifted toward lower temperature with increasing Cu content y. The addition of Bi ₂ O ₃ sintering aid induces enhanced shrinkage at T < 900°C and dense ceramics are obtained after sintering at 900°C. Such ferrites exhibit a permeability of µ = 135‐250 depending on the composition, sintering temperature and concentration of sintering additive. Ferrites with y = 0.20 show a high Curie temperature of T _c = 307°C. Multilayer inductors were fabricated and cofired at 900°C using ferrite tapes without and with 0.75 wt% Bi ₂ O ₃ . The compatibility of ferrite tapes with different metal pastes (Ag, AgPd, and Au) was evaluated. Ferrite tapes were also integrated between layers of low‐ k dielectric CT708 tapes and successfully cofired at 900°C. Preliminary tests indicate that the multilayer inductors can be operated up to temperatures of 250°C. This demonstrates that high‐T _c Ni‐Cu‐Zn ferrites are promising magnetic materials for inductive components for high operating temperatures.