Determining Mechanical Stress Testing Parameters for FHE Designs with Low Computational Overhead

The goal of this article is to optimize the mechanical stress patterns required to adequately test all the potential fault locations on an FHE device. We reduce the number of cantilevers that need to be tested mechanically by utilizing two key insights. First, we observe that each fault resides in the path of multiple stress patterns. Therefore, we eliminate the patterns that stress redundant faults. Second, faults need to be stressed to a minimal level to emulate real world conditions. The minimum stress requirement can be obtained using the radius of curvature (ROC) specifications dictated by the application. This minimal level of stress is not exerted by all cantilever beams; hence, they can be eliminated from the test process. Using these two insights, we find the minimum number of stress patterns that cover all the fault locations. We validate our approach on an in-house FHE prototype. We use the COMOSOL multiphysics environment to obtain stress conditions for each fault location. Then, we develop a highlevel model to estimate the stress for unsimulated cantilever beams such that the testing time can be further reduced. Finally, we formulate a heuristic solution to optimize the stress patterns with low computational overhead.