Inspection of the PCB defects by using ECT technique with GMR sensor

1
Inspection of the PCB defects by using ECT
technique with GMR sensor

• K. Chomsuwan, S. Yamada , Y. Fukuda, M. Iwahara
• Kanazawa University, 2-40-20 Kodatsuno, Kanazawa
  
• 920-8667, Ishikawa JAPAN
• H. Wakiwaka
• Shinshu University, 4-17-1 Wakasato, Nagano
  380-8553
• Nagano JAPAN
• S. Shoji
• TDK corporation, 543, Saku, Nagano 385-8555
• Nagano JAPAN

2
Outline of presentation

• Introduction of ECT technique and PCB inspection
  system.
• ECT probe for PCB inspection.
• Construction.
• Basic principles for PCB inspection.
• SV-GMR sensor characteristics.
• Inspection results.
• Inspection signal.
• Inspection of bare PCB model.
• Conclusions.

3
Inspection of bare PCBs

• Conductive tester (Pin probe)
• Mechanical scratch by contact actions.
• Imperfection defects (non-uniformity in width and
  thickness)
• High precision position mechanism.
• Optical inspection (CCD camera)
• Inspection over coating surface.
• Only visible defects is able to inspect.

4
Advantage of ECT to inspection of bare PCBs

• Non-contact inspection.
• No mechanical stress.
• No high precision position mechanism system.
• Measuring conductivity.
• Conductor disconnection and short circuits
• Imperfections on PCB conductor.
• Simple structure (Inexpensive).
• High sensitivity for surface properties.

5
Structure of the proposed ECT probe
Top view of the probe

• Meander coil (exciting coil)
• SV-GMR sensor (magnetic sensor)

Cross section of the probe (Distance from PCB
conductor to GMR sensor is around 135 mm.)
6
Principle of defect inspections on bare PCBs
7
Eddy-current paths for partial defects
8
SV-GMR sensor characteristics

• Normal resistance 627 W
• Constant current of 5 mA was fed to SV-GMR
  sensor
• MR ratio 10.37

The sensitivity is around 6 /mT in Z-axis.
Characteristics of GMR sensor in its sensing axis
Characteristics of GMR sensor at each of its axis
9
System configuration
Scan pitch 0.05 mm
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Inspection signal obtained from SV-GMR sensor
To eliminate offset and to enhance the signal at
defect points, the numerical gradient technique
is applied
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Signal and noise amplitude gradient
Noise signal has quit stable value with less than
10 mV/pitch
12
S/N ratio between solenoid coil and GMR sensor
vs. PCB conductor width

• The probe should be placed as close as possible
  to the bare PCBs to achieve the good inspection
  results.

Utilization of GMR sensor can improve the S/N
ratio up to 15 dB
13
Bare PCB model inspection
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Conclusions

• This research proposes the new application of GMR
  sensor for bare PCB inspection based on ECT
  technique.
• The proposed probe able to inspect the conductor
  disconnections and partial defects on bare PCBs
• Cause by SV-GMR sensor features, the defects on
  bare PCB is not difficult to identify.
• From experiment results, the ECT probe with GMR
  sensor is possible to apply for high-density PCB
  inspection (PCB width lt 100mm).

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Acknowledgement

• This work is supported in part by a Grand-in-Aid
  for Scientific Research from Japan Society for
  Promote of Science (Category B, 14350218).