Novel Tests for the As-Manufactured Strength of Mn-Zn Ferrite Inductor Cores

Abstract

∗† ‡ § Ferrites are magnetic ceramic (oxide) materials used as inductors and power transformers. Two new tests were developed to assess the reliability of ferrite cores in a high-reliability application. The tests intended to assess the as-manufactured strength of the cores in two different locations that would be subject to relatively high tensile stress during operation. A cantilever bend test was used to evaluate a corner in the as-sintered condition, while a biaxial test was used on a machined surface. Parts from two manufacturers were tested, with two batches tested from one of the manufacturers. The results of the test showed differences in strength that could be correlated to processing-related features such as density, pore size and distribution, and surface preparation. Differences in cantilever bend test failure loads between two batches of cores from the same vendor illustrate the potential use of the cores as a check on manufacturing quality control. I. Introduction Ferrites are magnetic ceramic (oxide) materials used as inductors and power transformers. They are often used in pairs with matching machined surfaces to maintain air gaps in the path of magnetic flux. They are especially useful in high-frequency applications because their low electrical conductivity relative to other magnetic materials means they are less prone to core losses arising from eddy currents. The use of ferrites in high-reliability applications may require mechanical property data to establish safety margins and determine probability of failure under mechanical loads. There is limited information of the mechanical strength of ferrite material, however, and published reports indicate that the strength of Mn-Zn ferrites can be highly dependent on composition and processing conditions 1 . The strength of manufactured cores may be more dependent on surface flaws introduced by machining than on the intrinsic properties of the material. The finished core may include both as-sintered and machined surfaces, so the reliability of the core under stress is a function of both the applied load and the surface condition. The effect of machining and residual stress on magnetic properties has been addressed 2-4 , but the strength of manufactured cores has been reported only to a limited extent 5, 6 . Two new mechanical tests have been developed to address particular concerns about the strength of ferrite cores in an application in which they were to be epoxy bonded to a printed circuit board for a high-reliability space-based application. The ferrites selected for the design were commercial products manufactured without source control and their mechanical properties were not part of the initial design considerations. The cores featured both as-sintered and machined surfaces that would be subjected to stresses, but the relative strengths of these two types of surface condition were not known. The prevalence of shrinkage cracks at a potentially high-stress location in the cores raised concerns about the strength of the cores during testing and operation, especially due to thermal-mechanical stresses arising from mismatches in coefficient of thermal expansion (CTE). The tests were used to characterize the strength of ferrite cores acquired from two vendors. The tests demonstrated differences in strength between vendors and between batches of cores from one of the vendors. These differences had implications for margins of safety and reliability of the design.