Evaluation and control of residual stresses in BaTiO₃-based Ni-MLCCs
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The residual stresses in BaTiO₃ based multilayer ceramic capacitors (MLCCs), showing X7R and Y5V characteristic, have been estimated using microindentation technique. The influence of the plane with laminated direction and the distance from electrode on the internal stresses, especially residual stresses, was investigated in order to predict the failure mechanisms of MLCCs. The internal stresses induced on the housing regions (margins) of MLCCs at the x and y planes parallel to the electrode represent relative tensile stress in the x and y directions (parallel direction to the electrode), and large compressive stress in the z direction (perpendicular direction to the electrode), respectively. Moreover, the other plane perpendicular to the electrode, z plane, showed a different tendency in the residual stresses. In the case of X7R, the residual stresses showed a reverse tendency, showing the tensile and the compressive stresses in the parallel and the compressive stresses in the parallel and the perpendicular directions to the electrode, respectively. And, in the case of Y5V, the residual stresses showed the compressive stresses in all the directions. However, the values or magnitudes of the residual stresses are different with character(X7R, Y5V) of MLCCs and the number of the active layer. These results indicate that the residual strsses created on MLCCs depend not only on the laminated direction, parallel and perpendicular directions, but on the distance from the electrode. The control of residual stresses in BaTiO₃ based MLCCs was performed with post process using pressure(30MPa) and heat treatment(900℃). The crack length was dramatically reduced at almost every planes of X7R and Y5V, so the difference in the crack length between each direction was reduced. The residual stesses induced on the cover layer of MLCCs before the post process were enhance and changed to compressive stress after the post process, respectively. Especially, the post process was more effective in the parallel direction to the electrode, showing relatively large change of the value of the residual stresses. Also, the residual strsses in the MLCCs , showing Y5V characteristic, showed uniform distribution after the post-process. At all the planes, there was not evidence for the tensile stress in MLCCs before the post process.