Fault isolation is the most important step for Failure Analysis (FA), and it is closely linked with the success rate of failure mechanism finding. In this paper, we will introduce a case that hard to debug with traditional FA skills. In order to find out its root cause, several advanced techniques such as layout tracing, circuit edit and Infrared Ray–Optical Beam Induced Resistance Change (IR-OBIRCH) analysis had been applied. The circuit edit was performed following layout tracing for depositing probing pads by Focused Ion Beam (FIB). Then, IR-OBIRCH analysis with biasing on the two FIB deposited probing pads and a failure location was detected. Finally, the root cause of inter- metal layer bridge was found in subsequent physical failure analysis.