Improvement on printed circuit board inspection using injected AC current and GMR magnetic sensor

This paper is a continuation of our former work [1-2]. In this paper, injected AC current and Giant Magneto-Resistance (GMR) sensor were used to detect small defects in the order of few hundred micrometers in thin layers using two connectors and detecting the perturbation of the magnetic field due to a defect using a GMR sensor. A sample of silicon wafer with copper metallization of few micrometers in thickness was used. On the conductive surface a 2.5 mm long scratch was created where the width of the scratch was 0.2 mm. Four measurements were conducted to detect this defect. The first one, was performed when the sensitive axis of GMR sensor was perpendicular to scratch. The second measurement was conducted when the sensitive axis of the sensor was parallel to scratch. In the third and forth measurements the silicon wafer was flipped over to the side of the none-conducting layer in order to determine the resolution of the GMR. AC current was injected on the bottom of the wafer; the 2.5 mm length scratch was easily detected where GMR was parallel, and perpendicular to scratch for the third and fourth measurement respectively. It was found that the sensor output amplitude was maximum when the sensor axis was parallel to the scratch (0.33 V and 0.25 V for measurements 2 and 3 respectively), while the sensor output amplitude was minimum in the case where the sensor axis was perpendicular to the scratch (0.13 V and 0.1 for measurements 1 and 4 respectively). It was found that the amplitude which was recorded in the case where the sensor was parallel to scratch on the surface (measurement 2) is about 25% larger comparing with the sensor which was parallel to scratch and GMR sensor was over the back side of the same wafer.