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Packaging-induced stresses can cause mechanical failures of various forms in the Cu/low-k interconnects. Here we report a time-dependent failure mode of the interconnects underneath the copper pillar bump. Delayed catastrophic fracture is observed in the interconnect dielectrics when a sustained shear load is applied on the bump using a single bump shear setup. The time to failure is found to be highly sensitive to the load level and temperature, but not to the environmental humidity. However, moisture diffusion through intentionally broken moisture seal can accelerate the failure process. Quantitative analysis suggests the delayed failure can be well captured over a wide range of testing conditions by a model based on subcritical crack growth in the interconnect dielectrics.