Crane Aerospace & Electronics in Redmond, WA, develops, qualifies, and manufactures a variety of implantable devices for leading medical companies. An unusual phenomenon was discovered during a routine visual inspection step on a recent development project. On the flexible portion of a rigid-flex PCB assembly, visible radial cracks were found on the annular rings of ENIG-plated thru-holes. The thru-holes remain unpopulated during Crane’s assembly process, and the defect was primarily seen on the top side of the PCB. Additionally, the occurrences of these cracks vary in frequency and severity. This paper describes the testing and analysis Crane performed in order to determine the root cause of this defect.