Opportunities and limits of wafer level robustness tests for life time prediction and forecast of field failure rate and the correlation to package level tests

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Dr. Takács Gábor
Department of Electron Devices

Shocks are fundamental loading condition to beam failure analysis. In this report, a shock response spectrum has been defined as a response to certain accelerations induced from several heights for a lumped mass. The mathematical modeling has been obtained based on linear vibration theory to describe the dynamic behavior of the beam testing system known as "Drop Tester". Finally, a complete dynamic description of the Drop Tester has been presented to assess the influence on beam robustness for wafer level drop. A classical theory of cantilever beam has been discussed to apprehend the first mode of frequency, shapes, expected region of stress and the influence of angular acceleration to induce a degree of torsion for several drop heights while subjected to drop. Matlab simulation has been performed to determine the first mode shape and frequency based on Euler–Bernoulli theory with thickness variation of the beam. Due to the combination of loading and complex geometric structure of the beam, FEA analysis has been performed by Ansys Workbench further to measure the stress and effect of torsion for the robustness. A case study has been discussed to evaluate a possible reason to obtain failed beams if not subjected to angular acceleration but to initial crack for repetitive loading for single crystal silicon. Finally, a reliability model has been developed based on Weibull Distribution to determine the survival and failure probability. Polynomial regression has been performed based on this reliability theory to estimate the ultimate fail of beams while undergoing repetitive stress in terms of drop numbers for wafer level drop test. Therefore, an overall understanding of cantilever beam with respect to testing condition has been analyzed extensively in the report.


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