Detection of Metallic Bead by EddyCurrent Probe with GMR Sensor

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Detection of Metallic Bead by Eddy-Current Probe
with GMR Sensor

• T.Hagino, S.Yamada, M.Iwahara
• Institute of Nature and Environmental Technology
• Kanazawa University

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Outline of Presentation

• Background
• Eddy-Current Testing technique (ECT)
• Micro solder ball on the Ball Grid Array
  (BGA)
• Structure of ECT probe
• Meander coil/GMR sensor type probe
• Characteristics of GMR sensor
• Experimental results
• Numerical analysis
• Characteristics of detection of metallic
  bead
• Conclusion

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Eddy-Current Testing Techniques
ECT technique is one of non-destructive inspection

• Features of ECT
• Nondestructive testing
• Simple structure
• Faster inspection
• Inspection of crack on both
• surface and bottom side

ECT technique is applied to inspect defects as
nonconductive fragment on the uniform metallic
parts and product.
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New application of ECT

• The progress of the ECT probe with micro
  magnetic sensor becomes possible to apply to
  various applications.

Detection of metallic bead
ECT techniques is applied to inspect the defect
on the uniform metallic parts and product
This attempt can be considered as a new
application of ECT techniques
One of the purpose of this attempt is, the
application of the solder ball on Ball Grid Array
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Ball Grid Array (BGA)
The BGA technology enable us to package LSI
elements on high-density printed circuit board

• Package substrate
• Efficient soldering
• Miniaturization is possible

The BGA surface mount needs the small solder
ball to connect LSI elements
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Micro solder ball

• The BGA surface mount needs the small solder
  ball to connect LSI elements.
• The size of the solder ball decreases in
• order to increase the density of packaging.
• The mount system needs the recognition
• of both size and position of solder ball.

Optical system is used at this inspection
apparatus generally
Influence of noise of optics or dust
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Configuration of ECT probe

• The meander coil as exciter
• The GMR senor as receiver

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Configuration of ECT probe (Cross-section)
The distance between the solder ball and the
probe is 100 mm. The distance between GMR sensor
and the solder ball is 185 mm.
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Giant Magnetic Resistance (GMR)
GMR is magnetic sensor as head element on the
hard disk device
Bz
The ac characteristics of resistance
change of GMR sensor
Magnetic field is 200mT.
The DC constant current of 5mA is fed to GMR
sensor
High sensitivity only to magnetic flux density
in z-direction
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Comparisons between Magnetic sensors
GMR is about 100 times higher than hall element
in sensitivity and the frequency characteristics
is much better.
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Principle of detection for metallic bead
The exiting current flow
The eddy-currents flow on the surface of the
solder ball
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Analysis of eddy-current distribution
The computed distribution of eddy
currents flow inside solder ball
The sinusoidal current is 0.1A and frequency is
5MHz
Eddy currents inside solder ball are flowing in
the counter clockwise
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Characteristic of detection
Condition Diameter of ball 0.76mm Scanning step
0.1mm Frequency 5MHz
Figure (b) is the numerical gradient values to
remove offset and to enhance the output signal
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Definition of signal and noise
We discuss the characteristics of detection
based upon this strip chart
The signals of solder ball,Vs, and the noise,Vn,
are defined by the peak to peak amplitude.
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Relationship between size and signal
Solder ball with dimension of 0.25 to 0.76 mm are
tested
These solder balls with dimension of 0.25 to 0.76
mm can be detected clearly
The signal of solder ball will decrease when the
diameter of the ball decrease
It is possible to evaluate the size of the ball
from the size of signal
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Relationship the lift-off height and signal
The lift-off height is the distance between
meander coil and solder ball
The signal should be higher than noise signal
more than 3 times to keep the realization of
defection
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Detection of position of solder ball
Condition Frequency 5 MHz Scanning step 0.1
mm Diameter of ball 0.3 mm
The point at that the maximum values are taken
seems to be the middle of solder ball
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Conclusions

• We proposed the detection of metallic bead by
  ECT techniques as a new application.
• The existence, size and position of metallic bead
  can be detected from the signal by using ECT
  technique.
• The experimental verification of the model proves
  the prospective possibilities for the development
  of biosensor, physical measurement systems, and
  other.