Thermal modulation system and method for locating a circuit defect

1. Field of the Invention A system and method for locating a circuit defect, such as a short or an incipient open, in an electric circuit in a workpiece, such a Printed Circuit Board (PCB) or MultiChip Module (MCM). The circuit is connected to a device for sensitively measuring any resistance change. A thermal stimulus is applied to various subsets of the surface of the workpiece, the thermal stimulus being temporally modulated, and the resistance change measurement correlated with this modulation. By applying well-designed thermal stimulus subsets, resistance measurements may be logically combined which correspond to the plural thermal stimulus subsets. Further, the search region where the defect may be located may be iteratively refined. By measuring the time delay between the thermal stimulus and corresponding resistance change, the depth of a defect below the surface of the workpiece is further determined. Thus the system and method efficiently and accurately locates a defect in three dimensions, even if the workpiece is large, and the defect is small and underneath the surface.