In this work we present a new defect localization capability on Wafer Level Chip Scale Packages (WLCSP) with small-scale Cu pillars using advanced 3D X-ray microscopy (XRM). In comparison to conventional microcomputed tomography (Micro-CT or microCT) flat-panel technology, the synchrotron-based optically enhanced 3D X-ray microscopy can detect very small defects with submicron resolutions. Two case studies on actual failures (one from the assembly process and one from reliability testing) will be discussed to demonstrate this powerful defect localization technique. Using the tool has helped speed up the failure analysis (FA) process by locating the defects non-destructively in a matter of hours instead of days or weeks as needed with destructive physical failure analysis.