System-level test (SLT) has recently gained visibility when integrated circuits become harder and harder to be fully tested due to increasing transistor density and circuit design complexity. Albeit SLT is effective for reducing test escapes, little diagnostic information can be obtained for product improvement. In this paper, we propose an unsupervised learning (UL) method to resolve the aforementioned issue by discovering correlative, potentially systematic defects during the SLT phase. Toward this end, HDBSCAN  is used for clustering SLT failed devices in a low-dimensional space created by UMAP . Decision trees are subsequently applied to explain the HDBSCAN results based on generating explainable quantitative rules, e.g., inequality constraints, providing domain experts additional information for advanced diagnosis. Experiments on industrial data demonstrate that the proposed methodology can effectively cluster SLT failed devices and then explain the clustering results with a promising accuracy of above 90%. Our methodology is also scalable and fast, requiring two to five orders of magnitude lower runtime than the method presented in .