5441337089

WeC’5-3

2007 IEEE International Conference on Control and Automation Guangzhou, CHINA - May 30 to June 1, 2007

An Integrated Inspection Method based on Machinę Yision for Solder Pastę Depositing

Shenglin Lu

the College of Medianical Engineering South Chin a University of Technology Guangzhou, Chin a [e-mail]

Xianmin Z hang

the College of Mechanical Engineering South China University of Technology Guangzhou, China zhangxm@ scut.edu.cn

Yongcong Kuang

the College of Mechanical Engineering South China University of Technology Guangzhou, China

Abslrad—The solder pastę depositing inspection is very i nipo limit in the process of Surface Mnunting for print Circuit hoard (PCB). An integrated inspection approach based on the machinę rision was presented in the study. The method developed in this paper can identify the major deferts of the solder pastę depositing, such as displacement, deficiency, excess, bridgc and oeerflow. Firstly, position compensation is applied to improve the inspection accuracy. Secondly, an image enhancemenl algorithm based on the teature is detdoped, which can enlarge the ditterences of the grayscale values hetween the PCB images and the solder pastę images. Tliirdly, the images of the printed PCB are analyzed by using the particie analysis method and the two-dimensional (2D) inspection results are ohtained. A pseudo three-dimeusional (3D) inspection approach is furthcr proposed to identify the defects in the JD stale. Finally. experimeut results are presented, wliich illustrate the yalidity of the approach.

Keywords- machinę rision; inspection; solder pasie depositing I. INTR0DUCT10N

Tlie serem printer is one of the key eąuipments of the SMT industry [1], Its performances directly affect the subsequent handling and the product ąuality. Thus, it is necessaty to inspect the Printing Circuit Board (PCB) afler ser ecu printing. It was repotted that 50~70% PCB assembling defects were rrom the stage of solder pastę depositing [2]. The inspection of the solder pastę depositing is one of tire most important stages in the process control of SMT production linę [3], The defects of solder pastę depositing include displacement, deficiency, excess, bridge, overflow, and so on. The typical inspection methods for the solder pastę depositing include manuał inspection, the laser scan inspection [4] and the inspection based cn madiine vision [5], The manuał inspection is a popular method because of its simple operation; however, it has many serious disadvantages, such as Iow efficiency and high probability of miss judgment. The laser scan approach provides morę detailed defect information because of its 3D inspection function. However, the 3D model of the solder pastę is acąuired by point or linę seans of a laser [6], so the inspection very slowly. Furthermore, the laser scan device is expensive. The method based on machinę vision has the merits of high inspection accuracy and speed, thus it can be used in PCB solder pastę depositing inspection of the screen printer. Moreover, no extra hardware cost is needed, because Ihe approach can make fu.il use of the existing hardw’are resource of the vision system of Ihe screen printer.

The rnain procedures of solder pastę inspecticn based on machinę vision include image acquisition, image enhancement and defect analysis. In order to improve the performance of this inspection approach, the position compensation method is utilized to determine precise position before acąuiring PCB images. An image enhancement method based on texture is developed d. which can enlarge the differences of the grayscale yalues between PCB base images and solder pastę images. In addition. the printed PCB images are analyzed by using the 2D inspection algorithm based on Ihe particie analysis method. A pseudo 3D inspection approach is applied to inspect the defects of solder pastę depositing in volume. Finally, Ihe experiment results illustrate the yalidity of the proposed method.

U.    THESCREEN PRINTER WORKFLOW

As one of the key ecuipments. the main function of the solder pastę screen printer is to deposit Ihe solder pastę on PCB precisely. The screen printer is composed of conveyer, squeegee system, machinę yision system and planar 3 degrees of freedom (DOF) adjusting unit, etc. The machinę yision system is used to align the PCB and the stencil, and to inspect solder pastę depositing. A cycle of work flow of the screen is shown in Fig. 1. 'Ihe position and pose of the PCB is not fixed after being loaded by the ccnveyer. Thus, the machinę yision system need to measure the marks positioning by' paltem matching so tłiat the printer can Snd the differences of the position and pose of the PCB and the stencil. Then, these differences can be compensated by use of the planar 3-DOF adjusting unit.

Mamiscript received November 1,2006. This woric was supperteć by the Teacbing and Research Award Program for Outstanding Y oung Teacher in Ifigher Education Institulions of MOE, P.R.C., the Guangcong Hong Kong Technology Cooperation Funding (Dongguan Proiect 20061682) and the Research Project of Ministry of Education and Guangdong Prosince (2006D90304001). Shenglin Lu is with the Sou* China University of Technology, Guangzhou, GuangDong 510640 China

1 ^244-0818-0/07/S20.00 © 2007 IEEE

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