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Failure by fracture is a serious problem with multilayer ceramic capacitors (MLCCs), and the interior electrodes are known to strengthen MLCCs. Historically, it has been assumed that the dominant strengthening mechanism is crack tip shielding via direct crack tip-electrode interactions. However, we have found that residual stresses arising from differential thermal contraction after device sintering are actually responsible for the observed increase in strength. In addition, the fracture initiation sites in MLCCs are located outside of the electrode array, so the established idea that the electrical and mechanical failure controlling flaw populations are one and the same cannot be true. Weibull distributions were compared from the bending fracture of two populations of MLCCs with barium titanate (X7R) dielectric, nickel electrodes, and the same exterior geometries (but different electrode array configurations). MLCCs had characteristic strengths of 236 MPa versus a strength of 190 MPa for 19- and 3-electrode MLCCs, respectively. Fractography, a critical flaw size computation, an analytical residual stress approximation, and in situ electrical measurements taken during bending were also used to examine the fracture process and demonstrate that residual stress and not crack tip shielding is an important strengthening mechanism in MLCCs. |
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