In the surface mount technology, Ball Grid Array (BGA) has been used in a production of PC boards, because of their excellent characters such as high density of the lead pin pitch, better lead rigidity and self-alignment during re-flow processing. This paper deals with the development of image analysis for the detection of defects at BGA solder joints in PC boards by using X-ray images. In the conventional IC boards, it is possible to detect defects of solder joints by visual inspection, because the lead of IC package is set on its outside. However, we can't detect visually defects at BGA solder joints, because they are hidden under the IC package. In a production line, the inspection of BGA in PC boards depends on the function test of electric circuits in the final process. To improve a cost performance and the reliability of PC boards, an inspection of BGA is required in the surface mount process. Types of defects at BGA solder joints are solder bridge, missing connection, solder voids, open connection and miss-registration of parts. As we can find mostly solder bridge in these defects, we pick up this to detect solder bridge in a production line. The problems of image analysis for the detection of defects at BGA solder joints are the detection accuracy and image processing time according to a line speed of production. To get design data for the development of the inspection system, which can be used easily in the surface mount process, it is important to develop image analysis techniques based on X-ray image data. At the first step of our study, we attempt to detect the characteristic of the solder bridges based on an image analysis. </p

Azuma, Yoshiharu

Furukawa, Noboru

Goto, Sachiko

Maruyama, Toshinori

Mondou, Munehiro

Okada, Saburo

Sumimoto, Tetsuhiro

Okayama University Scientific Achievement Repository

PhysicsWeights & Measures fieldsOkayama University Year 2003Development of image analysis fordetection of defects of BGA by usingX-ray imagesTetsuhiro Sumimoto∗ Toshinori Maruyama† Yoshiharu Azuma‡Sachiko Goto∗∗ Munehiro Mondou††Noboru Furukawa‡‡ Saburo Okada§∗Okayama University†Okayama University‡Okayama University∗∗Okayama University††Eastern Hiroshima Prefecture Industrial Research Institute‡‡Eastern Hiroshima Prefecture Industrial Research Institute§National Institute of Advanced Industrial Science and TechnologyThis paper is posted at eScholarship@OUDIR : Okayama University Digital InformationRepository.http://escholarship.lib.okayama-u.ac.jp/weights and measures/12IMTC 2003 ~ Instmmentation and Measurement Technology Conference Vail, CO, USA, 20-22 May 2003 Development of Image Analysis for Detection of Defects of BGA by Using X-Ray Images Tetsuhiro Sumimoto’), Toshinori Maruyamal), Yoshiham Amma’), Sachiko Goto’), Munehiro Mondou’), Noboru Furukawa’) and Saburo Okada3) 1) Okayama University, Medical School 2-5-1, Shikata-cho, Okayama 700-8558, JAPAN Phone: +8 1-86-235-6892, Fax: t 8  1-86-222-37 17, Email: sumimoto@md.okayama-u.ac.jp 2) Eastern Hiroshima Prefecture Industrial Research Institute 3-2-39 Higashi Fukatsu-cho, Fukuyama, 721-0974, JAPAN Phone: +8 1-84-93 1-2404, Fax: t81-84-931 -0409, E-mail: mondou@toubu-kg.pref.hiroshima.jp National Institute of Advanced Industrial Science and Technology Phone: +81-823-72-1952. Fax: +8 1-823-72-1999, E-mail: s-okada@aist.go.jp 3) 2-2-2 Suehiro Hiro, Kure Hiroshima, 737-0197, JAPAN Ablracf-  I n  the surface mount fechnology, Ball Grid Array PGA) has been used in a production of PC boards, because of their excellenf churocters such (IS high densify of the leud pin pitch, bener lead rigidify and self-alignment during re-flow processing. This paper deals with fhe development of image analysis for the defection of defects a1 BGA solder joints in PC boards by using X-ray images. I n  fhe convenfional IC boards, it ispossible to defect defects of solder joints by visual inspection, because the lead of IC package is set on ifs oufside. However, we can’t defecf visually defects nf BGA solderjoinfs, because they are hidden under the IC package. I n  a production line, fhe inspection of BGA in PC boards depends on fhe function Lest of electric circuits in thefinalprocess. To improve a costperformance and the reliabilig of PC boards, an inspection of BGA is required in the surface mounf process. Types of defects at BGA solder joinfs are solder bridge, missing connection, solder voids, open connecfion and miss-regislrafion of parrs. As we con find mostly solder bridge in fhese defects, wepick up this to detect solder bridge in a production line. The problems of image analysis for fhe detection of defecfs af BGA solderjoinfs are the detection accuracy and image processing rime according lo a line speed ofproduelion. To get design dafa for the developmenf of fhe inspection sysfem, which can be used easily in the surface mounfprocess, it is important lo develop image analysis fechniques based on X-ray image data. At  thefirsf sfep of our sfudy, we attempt to detect the characteristics of the solder bridges based on an image analysis. I. INTRODUCTION In a conventional IC package, the lead pin of IC is set on the outside of IC package and the defects of the solder joints of lead pin to the PC hoard has been done by the visual inspection [ I ] .  Recently according to the high density surface mount, Ball Grid Arrays (BGA) and Chip Scale Packages (CSP) are used in PC boards, because they are easily mounted to the surface of PC boards [2,3]. However, we can’t inspect directly the solder joints of BGA, because these are hidden under IC package. In a production line, many companies that product the PC hoard with BGA have done the inspection of BGA in the function test ofelectric circuits in the final process. To improve the cost of performance in manufacturing IC packages, it is required to detect defects at BGA solder joints in the process of surface mount. Therefore, i t  is important to develop image analysis techniques for the inspection system in a production line. Types of defects at BGA solder joints are solder bridges(short of two balls), missing connection, solder voids, open connection and miss-registration ofparts. In actual production line, we can find mostly the solder bridges. In order to prevent a had package is sent to the next process, i t  is required to detect solder bridges in the surface mount process. We pay attention to detect solder bridges in a production line. In this paper, we propose to develop image analysis techniques for the detection of defects at BGA solder joints by X-ray imaging. The problems of image analysis for the detection of defects of BGA are summarized in the following, one is the detection accuracy, that is, BGA is very small and we must inspect many BGA according to a production line speed. The solder ball diameter is 0.76 mm and one IC package has three hundred solder balls. The other is the processing speed, that is, huge image data must be analyzed in the real time manner. At the first step of our study, it is important to develop image analysis techniques for the detection of defects at BGA solder joints. We attempt to detect BGA bridges based on X-ray images. 0-7803-7705-2/03/$17.00 02003 IEEE 1131 11. FUNDAMENTAL EXPERIMENT In order to get design information of defects at BGA solder joints for the development of inspection system, which can be used easily in a production line, we need the fundamental experiment with the test of IC package. A .  Test IC Package BGA is an important technology for utilizing higher pin counts, without the attendant handling and processing problems of the peripheral leaded packages. They are used in manufacturing PC boards, because of their higher ball pitch(1.27 mm pitch), better lead rigidity, and self-alignment characteristics during re-flow processing. Fig 1 shows the flow of a surface mount process with BGA. PC board comes into this process. At the first step, solder paste is printed in the circuit and at the next step BGA with fine pitch are mounted and solder joints between IC package and the surface ofprinted circuit are made by re-flow process. BGA solder joints could not been inspected and reworked using the conventional methods. In Chip Size Packages (CSP), Mondou et al. have proposed to measure precisely the surface stmcture by using the CO focal optics before re-flowing[4,5]. In BGA, the ability to inspect visually the solder joints is desired in a production line to provide confidence in solder joint reliability. Loader I Print of solder pastcl IMount ofBGA with fine pitch I Shipment Fig.1 Flowchart ofsurface mount. In the most case of defects at BGA solder joint, the solder bridges between two balls are founded in a production line. This results from excess solder or misplaced solder, because dirty solder paste stencils are often founded in a production line. In manufacturing PC boards, IC package used with BGA is CPU for main function in an electronics circuit. In the actual production line, we can find the test IC packages based on the final electrical circuit test. Fig.2 shows a photog-aph of one example of a test IC board. The thickness of PC board is 2 mm and it has six layers. IC package is mounted with BGA to the surface of the PC board. The solder ball diameter is 0.76 mm and the ball pitch is 1.27 mm and the number of 13GA is two hundred and fifty six. The size of IC package is 27x27 mm. This test package does not pass the electrical hnction test. We consider this package has defects at BGA solderjoints. Fig2 Photograph of one example of test board. B. Capture ofX-ray Image Data of BGA We try to capture X-ray image data by using an X-ray computed tomography (CT) [ 6 ] .  To detect the inner defects, this apparatus was made to get computed toniography of mechanical parts such as a ball bearing, a cylinder and a battery. In these parts, the object for measuring is one unit. In this apparatus X-ray focus is 4 pm and resolution is 68 lines pairlcm. The X-ray source and the detector of image are fixed. Besides, a test sample is set on the stage. Then, we can get images data by rotating the stage. We can adjust an image size of the test sample by changing the distance between the X-ray focus and the test sample. X-ray radiated from the focus transmits the test sample on the stage and comes to the detector. X-ray detection system consists of an image intensifier of 23 cm diameter and a CCD camera of four hundred thousand pixels. X-ray image is converted to the visible light by the image intensifier and image data is captured by 213 inch CCD camera as 8 bit gray levels. It is difficult to capture the X-ray image at one scene, because a solder ball is small. Thus the number of it amounts over two or three hundred. We tried to change the image size of a solder 1 I32 ball to analyze the characteristics of an abnormal solder ball. It isiimpossible to get computed tomography data of each solder ball, because there are many solder balls in one IC package. Therefore, we have captured a projection X-ray image of a IC package. We set vertically the test package on the stage and rotated the test table from 0 degree to *SO degrees on every I O  degrees as shown in Fig.3. By rotating the test package, we can take X-ray image with inclined penetration and attempt to detect BGA bridges from different direction. When the angle of inclination is over 50 degrees, we can not distinguish each solder halls, because of overlapping of images. The condition for capturing image data is as follows. Fig.3 Apparatus for CapNring X ray-image, X-ray tube voltage: I85 KV, X-ray tube current: 160mA, exposure times: 30 seconds. We did three hundred times to smooth out image data and to improve image quality. We got a series ofX-ray.image data and captured data is stored in the Magnetic Optical Disk as bit map data. 111. ANALYSIS OF X-RAY IMAGE DATA In the actual X-ray image data in PC boards, the image data of each solder ball is very small and we must process huge data. It is very difficult to process directly the image data of BGA. Therefore, at the first step, we need image analysis of BGA to get fundamental data for the development of an inspection system used easily in a production line. We propose the following image process fechniques. Fig.4 'shows a flowchart of an image processing for X-ray image data obtained by the above apparatus. Image data is an input data to the personal computer for analysis. It is converted to the binary data to detect accurately the counter of a solder ball. Threshold level is determined based on signal profile on horizontal line. We selected 54 count gray levels as the threshold level and converted to black and white image data to measure accurately the following factors of BGA. After labeling, first we measure the area of each solder ball and center of X axis and Y axis. Next we measure the perimeter and the radius ratio of each solder ball. A normal pattem of a solder ball is a circle. If a solder hall has defects such as bridge, the shape of the object separate from a true circle. In the case of solder bridges, two solder balls are shorted with the narrow path and we can observe the different pattem such as connected with the bridge. Convert to binary data Measurement of Characteristics of BGA + I Center o fX  and Y axes Perimeter +io Judgment R=l & Bad BGA Good BGA Yes Fig.4 Flowchart of image analysis. In order to judge whether the solder joints are connected normally to the base pad in a surface mount process or not, we pay attention to the radius ratio and the roundness of a solder ball. Roundness R is calculated by the following equation. R=L2/4nS ( I )  1133 Where L (m) is the perimeter of a solder ball, S (m’) is the area of a solder ball. If the object is a true circle, the radius ratio and the roundness equal to 1. According as the shape of the object separate from a true circle, the radius ratio and the roundness become lager than I .  The judgment whether BGA is good or not is determined by the radius ratio and the roundness. If the value of these terms equals to 1, we judge BGA is normal. Then, if the value of these terms overrun equal to 1, we judge BGA is abnormal. IV. RESULT AND DISCUSSION Fig.5 shows an example of the original image data series captured by the above apparatus with inclined penetration of X-ray (inclination angle :-37 degrees). In this picture, we can observe one abnormal BGA. We analyzed this image data based on the above method. Fig.6 shows the binary image data after labeling. Fig.7 shows the radius ratio of each solder ball. F i g 3  shows the roundness. The roundness is one for the true circle by equation ( I ) .  The actual radius ratio and the roundness of a solder ball is a little over one as shown in Table I .  This table shows an example of the result of the image analysis. Fig.5 Original image data (minus 31 degrees) When the radius ratio is below 1.5 and the roundness is below 1.1, we judge BGA is normal and if two terms overrun these values, we judge BGA is abnormal. In this table, we can find one abnormal solder ball as shown in the data number 58. The radius ratio is over 1.5 and the roundness is over 1.2, namely, 1.97657 and 1.22408 respectively. Therefore, we can wam this solder ball is abnormal. This abnormal image data is shown as number 58 in Fig.6. This test package is inspected in the function test of the electrical circuit and determined as an abnormal board. Except only one or two solder balls, we can’t find another abnormal point in this test board. In the X-ray image, we could not find obviously two balls short but can find a ball having tail. The radius ratio of number 33 civenuns 1.5 but the roundness is below 1.2. Then we judge this BGA is normal in this table. Fig.6 Binary image data (minus 37 degrees) Fig.7 Radius ratio. 18 58 8: Obpd number Fig.8 Roundness. 1 I34 Fig.9 Original image data (minus 43 degrees). Fig.10 Binary image data (minus 43 degrees). Table I Example ofresult of image analysis. Y 105 11117115 265904'11 5'19.811 11,190 106360 $5 111 2.088179 26603.1, 6024.15 116511 196411 56 116 161.9841 266.6111 61083.6 ll.961 l O 7 l S l  57 111 196,1167 ~ 6 7 , 0 6 1 0  60111836 11,730 1116191 b, 'j8d:~ i , ?. . * b h & $ & . & & & & & U  59 140 151?12648 261,Pllll 6,77168 121110 111121 60 3.6 1851,410 16.16810 6,31266 1 1 0 1 9 3  1D76PO 61 3711 *1710'1219 269,8171 66"l 1 1 1 4 6 1  106314 By rotating this test hoard, we can find another solder bridge with the inclination angle of minus 43 degrees, as shown in Fig.9. Fig.10 shows the binary image data after labeling. We can observe that the bridge connects two balls. This bridge is observed in the inclination angle between minus 40 degrees and minus 47 degrees. In this case, twn halls are labeled as one pattem. The radius ratio and the roundness are 20.71083 and 1.98948 respectively. If this bridge connects two halls completely, we can observe this bridge in any inclination angle. In this case, it is considered that two balls have excess solder under the solder hall and we can observe the bridge in the penetration angle between minus 43 degrees and minus 50 degrees. It is reasonable that the radius ratio and the roundness of a solder hall are effective to detect the solder hall bridge based on X-ray image data. Besides, we can detect defects under the solder hall by changing the inclination angle of X-ray. In the actual production line, we founded some abnormal PC hoards based on the functional test of electrical circuit. Each board has only one or two solder bridges. We wonder if every joint on every hoard needs inspection. We hope to inspect everything to provide higher confidence of reliability of PC hoards: But members of a company that products PC hoards say that they need to inspect every BGA, when the condition of a production is changed. Once a process runs well, a manufacturer could inspect only a test sample of PC hoards. V. CONCLUSION For a practical purpose of developing the inspection system to detect defects at BGA solder joints in a production line, we have proposed the image analysis techniques, in order to carry out the inspection of the IC package having BGA. At the first step of our study, we deal with an image analysis of the test package, and significant results are obtained as follow. I )  To find BGA bridge, the radius ratio and the roundness of a solder hall is effective. For a normal solder hall, we can get these values equal to nearly I .  On the other hand, for an abnormal solder hall. it is cleared that the radius ratio and the roundness o v e r "  1. 2) To analyze accurately the radius ratio and the roundness of a solder ball, it is enough to get image data having 20 pixels diameter in each solder ball. 3) To detect defects under the solder hall, it is effective to change the penetration angle of X-ray. It is concluded that the image analysis based on X-ray image data proposed in this study is an effective method for the detection of defects of BGA bridge. To realize the inspection system of BGA in a production line, further studies are desired such as the construction of control system of X-ray focus for covering all BGA in one IC package and image analysis algorithm according to a line speed of production. ACKNOWLEDGMENT The authors wish to thank Interface Corporation for providing IC test package and Western Hiroshima Prefecture Industrial 1135 Research Institute for technical support to capture X-ray image data by using the X-ray computed tomography. REFERENCES [I]  Toshimitsu HAMADA, Kozo NAKAHATA, Satoru FUSHIMI, Yoshifumi MORIOKA and Takehiko NISHIDA, “Aoutomatic Solder Joint lnspcction System by X-ray Imaging”, Joumal of the Society of Precise Engineering, Va1.59, No.1, 1993, pp. 65-71. X-rays Expose Hidden Connections”, Test and Measurement EUROPE August-September Vol. 8, No.4,2001, pp.8-13. Yasuhika HARA, “Non Destructive lnspcction Technologies Used for Electronic Components and Packages”, Joumal of Japan Institute of Electronics Packaging, Vol. 4, No.6,2001, pp.470-474. Munehiro Mandou et al., “3-D Visual Inspection far Solder Side of Printed Circuit Board (IV)-Development of Inspection System for Printed Circuit-”, Technical repon of Eastem Hiroshima Prefecture Industrial Research Institute, No. 9, 1996. pp.29-32. Munehiro MONDOU, Tomomitw KUSINO, Katsuhisa HIROKAWA and Naboru FURUKAW, “Three-Dimensional Measurement for LSI Package Surface III”, Technical report of Eastem Hiroshima Prefecture Industrial Research Institute, No. 14,2001, pp.13-16. hnp:lluww.seisan-ac.kure.hiroshimajpi 121 131 [4] [SI [6] X-ray CT inspection apparahs, I I 3 6  

Development of image analysis for detection of defects of BGA by using X-ray images

Okayama University Digital Information Repository

Development of image analysis for detection of defects of BGA by using X-ray images

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