GMR magnetic sensor for scratch detection on PCBs

The inspection of defects on printed circuit boards (PCBs) by GMR sensor and injecting current was proposed in this paper. This technique is based on injecting AC currents through the metallic layer using two probes and detecting the perturbation of the magnetic field. Two experiments were performed on a sample of rectangular piece of PCB to detect 3 mm length of scratch which was located on the sample. Two measurements were conducted to detect this defect. The first measurement was performed when the sensitive axis of GMR sensor was perpendicular to scratch. The second measurement was performed when the sensitive axis of the sensor was parallel to scratch. The PCB sample was made of aluminum of dimensions 50 mm times 15 mm. The metal layer in this sample is about 100 microns in thickness. Two wires were soldered at the two ends of the rectangular sample. An AC current was supplied through these wires to the metal layer of the sample. The optimal frequency for detecting the small scratch was 1 kHz. By comparing the voltage output amplitude for both measurements, it was found that the scratch was successfully detected. It was verified that the signal-to-noise ratio (SNR) is much more than 3 (minimum for detection) in both measurements. Applications of injecting current technique include detecting defects in thin metallic layers for IC manufacturing.