Research On Machine Vision System For High-precision Positioning Of Ic Packaging

With the requirements of high accuracy and high speed in wire bonding, machine vision positioning system has become one of important factors which influence the quality of IC packaging. In this case, it can be used as a feedback of control system to achieve accurate positioning of chips. A positioning system using machine vision technologies has been designed and implemented in this dissertation to obtain accurate positioning. The calibration method and visual localization algorithm are researched, and the integration of vision and motion is studied. The main works and contributions in this dissertation are as follow:1) In terms of hardware of vision system, the structure of the optical system has been designed based on analyzing packaging equipment's requirements. The hardwares of vision system are selected base on two different optical lenses. In order to verify the imaging effect, experiments are done, which indicate that clear images can be obtained using vision systems2) In terms of camera calibration method, this paper presents a novel approach of camera calibration under the near parallel condition based on geometric moments. The camera calibration method is carried out by using a calibration board with arranged rectangular features. Before calibration, feature regions on the image of calibration board are extracted, and centroid and rotation of each feature are detected using geometric moments. Then, the closed-form solutions to the parameters are provided by combining Gauss lens model with camera model, and all camera parameters are refined by a nonlinear minimization procedure according to the distortion coefficients. Simulations and experiments are performed to verify the proposed camera calibration algorithm. The experimental results indicate that the algorithm is feasible with a printed calibration board and simple operating steps, and the the precision of camera calibration is high.3) A calibration method for micro-vision system is present using a reticule with grid as calibration pattern. Experiments are performed which show that the calibration of micro-vision system is reliable. Beacause of the calibration pattern is convenient to obtain, this method has practicability.4) In terms of visual localization algorithm, the method of precision positioning for chip and its leadframe is present. The normalized Fast Fourier Transform is applied for locating weld spots on chip, and a positioning method with contour extraction based on invariant moments is proposed for locating leadframe. According to experimental results, the precision of these methods can satisfy packaging requirements. The localization is implemented by Labview program.5) In terms of system integration, the experiment system is established by combining vision and motion system. Coordinate transformation between vision and motion coordinate system, error compensation model and block diagram of visual feedback are proposed. On the basis, the position error, calculated by visual localization algorithm, is converted from vision coordinate to motion, and high precision positioning is achieved.