Ni-Cu-Zn ferrite components are very sensitive to stress and this can cause unstable permeability. This work focuses on the source of stress, and the interaction between the residual stress after sintering and the further stresses contributed by subsequent manufacturing processes of multilayer chip inductors. The results show that the sources of stress include cofiring of ferrite and silver coils, and the operations involved in the manufacturing processes. The results also show that the stresses pass through the interfaces between materials to the body of component and that the stresses induced in the subsequent manufacturing processes would accumulate or counteract with the residual stress in the component, although the stress can also be released by changing the connection status of these interfaces. The results also show that the compressive and tensile stresses cause different levels of inductance shift.
Like other electrical components, the characteristics of magnetic components are significantly affected by not only magnetic fields, but also ambient temperature and inner/outer stress. In order to understand the nature of inductance shift of NiCuZn ferrite after thermal treatments, in this work we measured and analyzed the inductance shift of inductors which were treated separately by reflow and temperature cycling conditions. The results reveal that the inductance shift comes from not only residual stress release, which causes an increase in inductance during the cooling stage, but also the change in magnetic status by the peak temperature, which also causes an increase in inductance. Nevertheless, this magnetic status would be affected by the magnetic field to form a remanence that causes a fall in inductance later on, and this remanence level can be reduced under subzero temperature conditions. Comparing the construction of components for multilayer chips made of pure ferrite with those made of wound coil, this complex relation between heat, stress and the magnetic field of chip inductors are caused by the cofiring of ferrite and conductor material during the manufacturing process of chip inductors.
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