Field Gradient
Electromagnetic inspection techniques evaluate subtle spatial variations in flux density by measuring the vector difference between two closely spaced magnetic sensing elements. Using differential magnetic scanning isolates localized perturbation fields generated by hidden current anomalies while suppressing uniform background magnetic interference. This measurement approach detects micro-cracks in inner-layer copper traces and microvia structural discontinuities during non-destructive circuit verification.
The scope of this scanning modality stops at non-conductive dielectric boundaries where no induced or applied electrical currents propagate.
Defect Resolution
Current distribution inside multilayer printed circuit boards creates low-intensity magnetic fields that decay rapidly with vertical distance from the active layer. Differential magnetic scanning utilizes pairs of magnetoresistive or Hall effect sensors configured in a bucking arrangement to cancel ambient electromagnetic field noise from nearby power supplies or industrial equipment. When an electrical current encounters a necked trace, incomplete copper plating, or a high-resistance solder joint, the local current density shifts, creating a localized magnetic gradient.
Dual-sensor probes record this gradient as a differential voltage output proportional to the spatial derivative of the magnetic flux density. High-speed motorized gantry tables move the sensor array across the board surface at fixed standoff distances, mapping minute current deviations across sub-millimeter conductor features without requiring direct physical contact with conductive pads.
Inspection Boundary
Signal interpretation relies upon uniform substrate planarity and precise gantry height control across the scan area. Physical separation variations introduce amplitude errors that resemble subsurface conductor flaws, requiring real-time distance compensation sensors on the probe assembly. Unenergized board networks cannot produce measurable signals under this technique unless an external low-frequency alternating magnetic field induces local eddy currents within the copper features.
