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CAI et al.: NEW IC SOLDER JOINT INSPECTION METHOD FOR AN AOI SYSTEM 3
Fig. 1. AOI image acquisition system.
(red, green, and blue) hemispherical light-emitting diode array
illumination.
The red, green, and blue lights irradiate the solder joint
surfaces and are reflected onto the camera. Since the flats
and the slopes reflect different light components onto the
camera, the 3-D shape information of the solder joints may
be extracted from this acquired 2-D image. For the slopes of
the IC solder joints, a large amount of red lights and green
lights are reflected away, while the blue lights are reflected
toward the camera, which makes the solder joints appear to
be blue in the images. On the contrary, the flats of the IC pins
and the pads reflect the red lights toward the camera, while
green and blue lights are reflected elsewhere. Such flat regions
appear to be red in the images. An example of an obtained
IC solder joint image containing 16 qualified solder joints and
an unqualified one (the rightmost one) is shown in Fig. 2(a).
For a clear description, we zoom in on the third one on the
left in Fig. 2(a), which is marked by a white rectangle. It is
rotated counterclockwise by 90°, which is shown in Fig. 2(b).
In Fig. 2(b), Region A is located on the flat of the IC pin,
while Region B is located on the solder joint of this pin. That
is, Region B is located between the pin and the pad.
III. A
LGORITHM PRINCIPLE
In the ViBe algorithm for moving object detection, the pre-
vious frames of a video are input in sequence so as to produce
the templates, which approximately represent the model of real
background. The current frame is input and compared with the
trained templates. The parts of the frame failing to match the
templates are considered to be the foreground pixels.
A. Relationship Between Defect Inspection and ViBe
For moving object detection, each frame of a video contains
the background, the foreground, and some noise. To detect the
foreground, we should first build a background model. Then,
the foreground is detected by comparing the pixels of the input
frame with those of the model. It means that the input pixels
Fig. 2. IC solder joint image obtained by an AOI system. (a) Image with
17 IC solder joints. (b) Zoomed-in view of IC solder joint image.
mismatching the pixels in the model are considered as the
foreground pixels. This is similar to the problem of defect
inspection in an AOI system. For automatic inspection of
IC solder joints, an unqualified solder joint shows an abnormal
appearance at the specific location of the image compared with
a qualified one. That is, each unqualified solder joint image
contains the normal region, the abnormal region, and some
noise. Thus, detecting the abnormal region in a solder joint
image can be viewed as detecting the foreground in a frame of
a video. Therefore, we consider the unordered IC solder joint
images as the frames of an inspection sequence. Like moving
object detection, we can first use the inspection sequence to
establish a model representing a qualified solder joint. Then,
we detect the abnormal region by comparing the pixels of
the input solder joint image with the pixels of the established
model. In this paper, we use the ViBe background modeling
method [21] to train the qualified solder joint model. Since
our proposed inspection method statistically calculates the real
distribution of the hue channel values of a pixel using these
unordered images, we need not sort the frames of the sequence.
Since three channels of the hue-saturation-value model are
independent with each other, it is widely used in the fields of
image processing and computer vision. In addition, the hue
channel contains the major information for colors perception.
Thus, we use the hue channel of the solder joint image to
establish the qualified solder joint model. Here, the range of
the hue channel values is [0, 180).
For moving object detection, the background of the outdoor
scene often contains many nonstatic objects, such as tree
branches in the wind and ripples on the surface of the water.
The hue channel values of the pixels in these nonstatic
objects vary significantly. The ViBe algorithm models the
nonstatic objects and static scene excellently through several