計測自動制御学会産業論文集
Vol. 6, No. 9, 72/77 (2007)

Development of High Speed Oblique X-ray CT System
for Printed Circuit Board
Atsushi TERAMOTO*, Takayuki MURAKOSHI*, Masatoshi TSUZAKA**, and Hiroshi FUJITA***The high density LSI packages such as BGA (ball grid array) and CSP (chip scale package) are being utilized in the car electronics andcommunications infrastructure products. These products require a high-speed and reliable inspection technique for their solder joints. In thispaper, we propose the novel oblique CT system that can non-destructively obtain the 3D image representing solder shape of a printed
circuit board at a high-speed. This system obtains the projection images from various directions using the rotational flat panel detector andthe fixed mounted open-type X-ray generator that has a wide radiation angle, and it obtains the 3D CT image using the 3D FBP (Filtered
Back Projection) method. In addition, we developed the CT reconstruction acceleration unit for our CT system, and thus the 3D CT image
was obtained in real time. In the experiments, we analyzed the BGA mounted board and the solder crack that is subjected to a thermal
shock. These results demonstrated that the proposed system obtained the information that was able to judge the good or defective situation
in soldering.
Keywords: X-ray, CT, printed circuit board, BGA and inspection

achive the complete inspection, rapidity and mechanism assuming
the use by the production line are required.
In this paper, we describe an inspection system that can performaccurate and fast inspection of solder junctions in the BGA andCSP for the surface mount process.
X-ray CT technology is a precise technique used for inspecting
solder bumps 6) 7). Several industries use CT to obtain projection
data by rotating the sample about an axis that is orthogonal to theaxis that connects the X-ray generator to the detector. We call thismethod vertical CT in this paper. When thin and wide objects,
such as PCBs are inspected by the vertical CT, it is necessary to
cut out the region of interest of the PCB because it touches the
X-ray generator or detector during the scanning process. Since thistechnique is incapable of achieving non-destructive inspection, it
is not used for inspection before a product is shipped but is used
for defect analysis during research and development.
To overcome the disadvantages of vertical CT, oblique CT(OCT) was proposed 8) 9) 10), which obtains the projection image ofan object from an oblique direction by rotating the X-ray generator
and detector. And some systems employed rotation stage and
X-ray generator and detector are tilted for the rotational stage. In
the OCT, scanning objects move horizontally because X-ray
radiation is inclined; thin and wide objects such as PCBs do nottouch the X-ray generator and detector. Therefore, although the
PCB may be close to the X-ray generator, we can obtain a CT
image with high magnification without destroying the PCB.
However, when we introduce OCT to production process, there
are three issues. Firstly, it is not possible to achieve the high-speed
inspection required in the production process because moving
objects (X-ray generator and rotation stage) are very heavy.
Secondly, there is problem about the visual field size. The
projection images are collected by imaging tube as a 0.3M pixel inthe conventional system. The visual field size in OCT image isvery small for the size of the entire component at the necessary
magnification. There are large gap between inspection time andproduction cycle in the conventional system. Thirdly is anmechanical adaptaion to production process. Although standard
production process requires the PCB transfer system, it is very
difficult to introduce to the conventional system structural.
Based on these advantages and disadvantage of conventional

1. Introduction
Recently, BGA (ball grid array) and CSP (chip scale package)
have been widely used as an LSI package mounted on a PCB(printed circuit board) in electronic products. The electricaljunction of the BGA and CSP is formed by solder bumps that areplaced in a grid-like formation under the LSI package. The solder
shape cannot be observed by optical techniques. X-ray inspection
is suitable for the non-destructive inspection of the internal
conditions such as the solder shape of the BGA and CSP. Since theadvent of BGA and CSP, X-ray inspection has been widely used as
the major inspection method of PCBs in consumer electronic
products.
With regard to the inspection technique using X-ray fluoroscopy,
the authors have developed techniques for extracting the solderusing the energy subtraction method 3), quantitatively measuring
the small solder bump at high speed using the relation between
brightness and object thickness in the X-ray image 4) and detecting
microvoids (bubbles) included in a small solder bump by using
image data processing and statistical methods 5). The inspection
using X-ray fluoroscopy is suitable for the fast inspection of
consumer products because it employs simple mechanics and isable to obtain 2D information from the detector at a time. The
X-ray fluoroscopic image, however, is the projection image, andthe information in it is compressed along the z direction. Thus, it
is impossible to evaluate the 3D solder shape by this technique. In
particular, BGA/CSP is being applied to car electronics andcommunications infrastructure products. Obviously, highreliability is required from these products. In order to meet theinspection requirement for these products, the establishment of theinspection technique which can examine soldered joints more
directly is strongly required. In addition, the inspection of theentire shipped product is preferable to guarantee its reliability. To
1) 2)*Nagoya Electric Works Co., Ltd.
550 KATORI, TADO-CHO, KUWANA-CITY 511-0102**Department of Radiological Technology, Nagoya University
1-1-20 DAIKO-MINAMI, HIGASHI-KU, NAGOYA 461-8673***Department of Intelligent Image Information, Graduate
School of Medicine, Gifu University
1-1 YANAGIDO, GIFU-CITY 501-1194(Received January 30, 2007)
72

OCT, we developed a fast OCT system intended for the online
inspection in the production process of BGA and CSP mountedboard.
In this paper, the OCT system will be described as follows. In
Sec.2, a description of the OCT structure and processing
procedure is given. In Sec.3, we show some experimental results.
The conclusions are provided in Sec.4.

2. System Description
2.1. Structure and processing outline of OCT

Fig.2 Prototype of OCT

The structure of the OCT system developed by us and thephotograph of a prototype system are shown in Fig.1 and Fig.2,
respectively. The system consists of a rotational flat panel detector(FPD), an open-type X-ray generator that is set on the elevator and
the X-Y stage. The photograph of the FPD and the X-ray generator
is shown in Fig.3. By synchronizing the rotaion of FPD and the
X-Y movement of sample, the X-ray projection images fromvarious directions are obtained.
Unlike the conventional OCT, the X-ray generator needs not
rotate because the radiation angle of open-type X-ray generator is
130°. The light weight of the FPD results in a low mechanicalload; this further allows high-speed scanning. Regarding to thenumber of pixels in the projection image, FPD has 5.3M pixel, it
is able to obtain the 17 times area for the conventional imaging
tube (0.3M pixel).
As described in the previous section, the X-ray generator andthe detector do not interfere when the direction of the X-ray
irradiation is inclined. The projection image of a thin and widesample such as a PCB can be obtained under high geometricmagnification. The geometrical magnification increases as the
distance between a sample and the X-ray generator decreases. The
geometrical magnification of the OCT prototype is 200, and a
spatial resolution of 0.25 µm is attained at the maximum
magnification.
X-ray projection images are transferred to the host memory via
image grabber board, and the 3D OCT image is obtained byexecuting 3D reconstruction on the OCT reconstruction
accerelation unit. With regard to the PCB handling, it isautomatically inserted and exposured by the conveyor that
equipped on the X-Y stage, and the on-line inspection is feasible.

(a)

(b)

Fig.3 Key components of the OCT such as X-ray
generator(a) and flat panel detector(b).
2.2 Data processing
The data processing procedure in the OCT consists of (A)collection of the projection image, (B) image correction, and (C)3D image reconstruction. This processing is described as follows.
(A) Collection of the projection images
By synchronizing the rotaion of FPD and the X-Y movement ofsample, the X-ray projection images from various directions are
obtained. Here, the center of projection image must keep noticing
the identical position while correcting the projection images. The
X-Y stage is controlled so that identical position may rotate inradius of gyration RR synchronizing to the FPD as described in
Fig.4, and gyration RR is defined as follows.

RR = FOD ⋅ tan α(1)where the distance from the X-ray source to the sample is FOD,
and the radiation angle of the X-ray source and the detector is α.

FPD
CPU
FPD

Image grabber
RR
X-Y stage

CT reconstruction unit

FOD

X-Y stage
PCB
X-ray
generator

αInspection object

Stage controller
X-ray controller

X-ray focus spot
X-ray generator

Fig.4 Synchronization of X-Y stage and FPD

Fig.1 Structure of OCT73n(B) Image correctionz

The projection image have spatial non-uniformity in the
proposed system due to the following reasons:
1. The X-ray is irradiated from the focal point of the X-ray
generator; the brightest region of the projection image is at thecenter and the brightness decreases as the distance from the center
increases.
2. The focal condition of the X-ray changes with time bythermal change. This results in a change in the distribution of pixel
value in the projection image.
To correct the spatial non-uniformity of the image, anon-uniformity corrected projection image p(m,n) is calculated asfollows:

I (m , n)p ( m , n ) = − log e
I 0 (m , n)rcObjectr r
f (r )dr

2.3 Acceleration of 3D image reconstruction
In the OCT method, filter processing of the projection images
followed by back projection to the 3D space are performed, as
described in the previous section. The matrix size depends on thememory buffer size; the maximum size of our prototype system
was 2048 × 2048 × 32 at 32 bits per pixel or 2048 × 2048 × 64 at
16 bits. The number of calculations reaches four billion per
projection excluding the memory access. Moreover, a large
number of random accesses to the 3D memory buffer occur during
the back projection processing.
We implemented the reconstruction software on the Windows
platform. The processing time is 4.0 s per projection using a dual
Xeon 3.6 GHz processor with 4 GB RAM. However, the image
acquisition time, including a mechanical movement, is 1 s per
projection; the reconstruction is not calculated in real time. To
accelerate the reconstruction speed, we developed a CT
reconstruction accelerator unit for OCT. Fig.6 shows the block
diagram of the CT reconstruction acceleration unit. It consists ofthree FPGAs (field programmable gate arrays) with a fast memory
buffer connected to each of them, and the projection and 3D OCT
images are transferred over the PCI-X slot. The reconstruction
time is 0.3 s per projection using this unit, which is 13 times fasterthan CPU processing.
The acquisition of the projection image and the reconstruction
process can be executed in parallel, and the acquisition time islonger than the reconstruction time. Therefore, we can obtain the
3D OCT image in real time.

(3)rwhere r is the track from an X-ray generator to the detector, andrrrr
is defined as r = l c + m a + n b .
Provided that the X-ray projection is a parallel beam, the 2D
Fourier transform Pφ(u,v) of the projection image is equal to arr
vertical section of c in the 3D Fourier image F (ω ) based on
the projection slice theorem.

rr
F (ua + vb ) = Pφ (u , v)

(4)

The major methods of image reconstruction from projections
are the Fourier method and filtered back projection method (FBP).
In this study, we employed FBP to allow high speed reconstructionof 3D images. 3D image reconstruction using FPB is realized by
performing filtering and back projection. In the former, afrequency filter H(u,v) is applied to the projection image Pφ(u,v),
as shown in Eq.(5). In the latter, the filtered image gφ(m,n) is backprojected to the 3D space, as shown in Eq. (6).

g φ ( m, n) =1

Pφ (u , v) H (u , v )e
4π ∫ ∫2r
1
f (r ) =2π ∫0

2πr r r rgφ (r ⋅ a , r ⋅ b )dφ− j ( um + vn )

dudv

ｘ

Fig.5 Simplified scheme of OCT(2)

Fig.5 shows a simplified scheme of the OCT that is equivalentr
to the scheme in Fig.1. An object f (r ) is illuminated by a conerbeam X-ray at the rotation angle φ. We assume that a andrb are unit vectors along the horizontal and vertical directions ofrthe detector plane in the 3D space, and c is the vector thatindicates the direction of projection. The 2D projection pφ(m,n) ofrrr
the object f (r ) is denoted as the integral of f (r ) along the r :∞Detectory(C) 3D image reconstruction

−∞

m

r
a

X-ray source

where I(m,n) is the X-ray intensity of projection, and I0(m,n) is the
X-ray intensity without a sample.

pφ (m, n) = ∫rbφ

PCI-X Bus

Projection image(2048 × 2048)
Projection

3D CT image

image

FPGA1

(5)

Filtering

Image buffer

FPGA2
Addressing ofback projection

FPGA3
Accumulation ofback projection

Cache memory

Cache memory(6)

Cache memory

Fig.6 Block diagram of the CT reconstruction
acceleration unit74

3. Experimental Result and discussion
3.1 Evaluation using BGA mounted board
OCT was applied to the analysis of a BGA mounted board.
An evaluation board, on which a plastic BGA with a bump pitchof 1.0 mm and bump diameter of 0.6 mm was mounted were
employed. The PCB size was 150 × 150 mm2 and the thickness
was 2.5 mm. If we had used the conventional vertical CT, wewould have had to trim away sections of the PCB except for the
analysis region. On the other hand, we did not have to cut any
region of the PCB when using the OCT system. Regarding to theinspection speed, projection images were captured at thetheoretical limitative speed of FPD, and the reconstruction ended
under capturing. As a result, scanning was finished in 32 s. Whencycle time of production line is 150 s per board, this system allow
to inspect four BGAs without making production line waiting.
Table 1 shows the experimental conditions. Fig.7 shows theconventional X-ray fluoroscopic image of the BGA mountedboard. In this figure, the arrows indicate the soldering defect, andwe cannot determine which bump is defective. Fig.8 shows thehorizontal slice image, and Fig.9 shows the vertical slice image of
OCT. Fig.10 shows the 3D shape of the sample using thevolume-rendering technique.
In Fig.8 (b), the bump outline at the defective bump is unclear,which indicates that the solder is not connected correctly between
the bump and the PCB. Additionally, Figs 9 and 10 also indicatethat some bumps are incorrect at the PCB side.

BGA package
Slice 3
Slice 2
Slice 1Printed circuit board

500um

(a)

500um

100

X-ray current [uA]100

Matrix size of projection image (X × Y)

2048 × 2048

Matrix size of OCT image (X × Y × Z)

2048 × 2048 × 64

Geometric magnification

10

Spatial resolution (X × Y × Z) [µm]5.0 × 5.0 × 10.0

Number of projections

32

Inspection time [ sec / scan ]

32

500um

(c)
(d)
Fig.8 Evaluation result of OCT. (a) illustrates the crosssectional view of the BGA mounted board andthe slice position. (b) to (d) show the horizontal
slice image at slice 1 to 3, respectively.

Table 1 Experimental conditions
X-ray voltage [kv]

(b)

500um
(a)

Fig.9

(b)

Evaluation result of OCT. (a) is the horizontal
slice image which gives the slice line for verticalslice (white arrows). (b) shows the vertical slice
images generated from the 3D OCT image along
the white lines in (a).

500um

Fig.7 X-ray fluoroscopic image of BGA mounted board
Fig.10 Volume-rendering image

75

Although most of the solder bumps cannot be observed fromthe outside, only the outer bumps can be observed using a
microscope. To verify OCT, we compared the solder shape in the
OCT with that of the microscope. For the evaluation, we used thesame conditions as those listed in Table 1. We used VHX-500(Keyence) as the microscope, and captured the BGA photographfrom the outside of the 1.27 mm pitch BGA. The results are shownin Fig.11 (a) and (b). To the right of the bump in Fig.11 (a), solder
particles exist under the solder bump, indicating that the solder has
not melted. In Fig.11 (b), solder particles also exist on the righthand side. This result demonstrated that OCT cannon-destructively analyze the solder shape on the PCB.

Chip resistor
Solder

(a)

1.0mm

(b)

(a)

500um

(b)

Fig.11(c)Comparison between photo of micro-scope (a)and OCT vertical slice image (b).

3.2 Evaluation using chip resistor solder with crack
The generation of cracks due to thermal fatigue and mechanicalshock of the solder joint area is a major factor contributing to thedecrease in the reliability of electronic equipment, and various
researches have been conducted to study solder cracks 11) 12). We
evaluated the capabilities of crack analysis using a chip resistor
mounted board that was subjected to 3000 cycles of thermal
fatigue. The experimental conditions used for the evaluation are
shown in Table 2, and the construction of the evaluation board
and the OCT images are shown in Fig.12.
Fig.12 (b) is a volume-rendering image of the entire solder
shape, and (c)–(e) show the images that sequentially cut it fromthe solder top to the bottom in the volume-rendering image. The
width of the crack was 10~20 µm. This narrow crack can beclearly observed to progress from the surface of the soldering to
the inside.

Fig.12

4. Conclusion
In this paper, we proposed a oblique CT system that cannon-destructively obtain the 3D solder shape of a PCB at a highspeed. This system collects the projection images from various
directions using a rotational FPD and a fixed mounted open-type
X-ray generator that has a wide radiation angle, and obtains the3D CT image by the 3D FPB method. In addition, we developed aCT reconstruction acceleration unit for our CT system, and the 3D
CT image was obtained in real time. In the experiments, weanalyzed the BGA mounted board and solder crack of the chip
resistor. The results demonstrated that the proposed system wasable to obtain the effective information by non-destructive
inspection of the PCB.
Currently, more than 20 inspection systems are operating indomestic and foreign PCB manufacturing factories in order toinspect the BGA solder joint area of car electronics products.
In this paper, we developed the data acquisition system using
the OCT system; however, an automated inspection system isrequired for mass production lines. Presently, we are engaged in
the development of an automated algorithm for solder bumpinspection.

Table 2 Experimental conditions
X-ray voltage [kv]100

X-ray current [uA]100

Matrix size of projection image (X × Y)

2048 × 2048

Matrix size of OCT image (X × Y × Z)

2048 × 2048 × 64

Geometric magnification

20

Spatial resolution (X × Y × Z) [um]2.5 × 2.5 × 5.0

Number of projections

180

(d)
(e)
OCT images of solder joint with crack. (a)illustrates the inspection component, (b) is a
volume-rendering image of the entire solder
shape, and (c)–(e) show the images thatsequentially cut it from the solder top to thebottom in the volume-rendering image.

76

[ References ]Atsushi Teramoto（Member) He received B.S. and
M.E. degrees in electrical engineering from Meijo
University, Japan, in 1996 and 1998, respectively. In
1998, he joined Opto-electronics division, Nagoya
Electric works Co.,Ltd., where he is currently a
researcher. His research interests include image
processing, image analysis, and image recognition.

1) P.Haglund, P.Frisk, J.-O.Andersson : "BGA-MCM technology forharsh environmental applications", ISHM-Europe, 14/16 (1997)
2)

R.Ghaffarian : "BGAs for high reliability applications", Electronics
Packaging & Production, 38-3, 45/52 (1998)
3) A.Teramoto, I.Horiba: "Extraction of Solder Constituent from CSP
Boards by Means of X-ray Energy Subtraction Method", Journal ofJapan Institute of Electronics Packaging, 4-1, 68/71 (2001) (inJapanese)

Takayuki Murakoshi (Non Member) He received
B.S. degree in electrical engineering from Nagoya
University, Japan, in 1972. In 1972, he joined Nagoya
Electric Works Co.,Ltd., where he is currently asenior researcher. His research interests include data
sensing, image processing, and intelligent traffic
control system.

4)

A.Teramoto, T.Murakoshi, I.Horiba : "High Speed and High Precision
Solder Bump Inspection Method Using X-ray Fluoroscopy", IEEJ
Transactions on Industry Applications, 125-D-6, 652/658 ( 2005)(in Japanese)
5) A.Teramoto, T.Murakoshi, Masatoshi Tsuzaka, Hiroshi Fujita :"Automated Detection of Micro Void in Solder Bump", IEEJ
Transactions on Industry Applications, 126-11, 1514/1421, (2006) (inJapanese)
6) L.A.Shepp and J.B.Kruskal : "Computerized tomography: The new
medical X-ray technology", American Mathematical Monthly, 85,655/661 (1979)
7) Z.H.Cho : "General views on 3-D Image Reconstruction andComputed Transverse Axial Tomography", IEEE Transactions on
Nuclear Science, 21, 44/71 (1974)
8) H.E.Knutsson, P.Edholm, G. H.Granlund, and C.U.Petersson :"Ectomography-A New Radiographic Reconstruction Method-I.
Theory and Error Estimates", IEEE Transactions on Biomedical
Engineering, 27-11, 640/648 (1980)
9) C.U.Petersson, P.Edholm, G.H.Granlund, and H.E.Knutsson :"Ectomography-A New Radiographic Reconstruction Method-II.
Computer Simulated Experiments", IEEE Transactions on Biomedical
Engineering, 27-11, 649/655 (1980)
10) H.Matsuo, A.Iwata, I.Hoiba, N.Suzumura : "Three-Dimensional
Image Reconstruction by Digital Tomo-Synthesis Using Inverse
Filtering", IEEE Transactions on Medical Imaging, 12-2, 307/313(1993)
11) T.Terasaki, H.Tanie : "Fatigue Crack Propagation Analysis for Micro
Solder Joint with Void", Proceedings of 2005 International
Symposium on Electronics Materials and Packaging, 37/42 (2005)
12) T.Miyazaki, M.Oomiya, H.Inoue, K.Kishimoto, M.Amagai :"Evaluation of Fatigue Strength for Solder Joints after Thermal
Aging", Proceedings of 2005 International Symposium on Electronics
Materials and Packaging, 215/219 (2005)

Masatoshi Tsuzaka (Non Member) He received
the B.S. degree in electrical engineering from Meijo
University, Japan, in 1989, and Ph.D. degree from
Graduate School of Engineering, Gifu University in
1999. He is currently an Associate Professor in thedepartment of Radiological Technology, Nagoya
University School of Health Sciences, Japan. His
research interests include the measurement of
diagnostic X-ray spectra, study on radiation protection, image evaluation
in medicine, computer-aided diagnosis system, tele-Radiology, and highspeed networks in medicine.
Hiroshi Fujita (Non Member) He received the B.S.
and M.S. degrees in Electrical Engineering from Gifu
University, Japan, in 1976 and 1978, respectively, andPh.D. degree from Nagoya University in 1983. He
was a Research Associate at University of Chicago,
USA, from 1983 to 1986. He is currently a Chairman
and a Professor in the Department of Intelligent
Image Information, Graduate School of Medicine, Gifu University, Japan.
His research interests include computer-aided diagnosis system, image
analysis and processing, and image evaluation in medicine. Dr.Fujita has
published over 500 papers in Journals, Proceedings, Book chapters andScientific Magazines. He is currently a president of Japan Society of
Medical Imaging and Information Sciences.

77