Image Distortion Compensation by Using a Polynomial Model in an X-ray Digital Tomosynthesis System

X-ray technology has been widely used in a number of industrial applications for monitoring and inspecting inner defects which can hardly be found by normal vision systems as a ball grid array (BGA) or a flip chip array (FCA). Digital tomosynthesis (DT) is one of the most useful X-ray cross-sectional imaging methods for PCB inspection, and it usually uses an X-ray image intensifier. However, the image intensifier distorts X-ray images severely both of shape and intensity. This distortion breaks the correspondences between those images and prevents us from acquiring accurate cross-section images. Therefore, image distortion compensation is one of the most important issues in realizing a DT system. In this paper, an image distortion compensation method for an X-ray DT system is presented. It is to use a general distortion polynomial model on two dimensional plane that can cope with arbitrary, complex and various forms of distortion. Experimental results show a great improvement in compensation speed and accuracy. Introduction X-ray technology is a good solution for inspecting inner defects of industrial products. In case of conventional X-ray radiography images, however, it is very difficult to correctly recognize the internal shape of objects, because all objects on the projection line are overlapped in the images. On the other hand, X-ray cross-sectional imaging systems forming an arbitrary cross-section image of a 3D object make it easy to inspect the inner shape or structure of it. PCB solder joint inspection is one of the industrial fields which need such an X-ray inspection system. In particular, an X-ray crosssectional imaging system promises to be a good solution for new SMD (Surface Mount Device) technology such as BGA, FCA, and J-type lead [1]. An X-ray cross-sectional image can be obtained from two or more images projected from different directions by the method such as tomography, laminography or digital tomosynthesis. Tomography has been mainly used in medical area, but recently it is being applied in industrial fields such as precision inspection of casting products [2]. Laminography was originated by Bocage [3]. Its principle comes from the geometric focusing effect by a synchronized motion between an X-ray source and a detector, which is shown in Fig. 1. Digital tomosynthesis is a digital version of laminography, where a set of images of different views are stored and synthesized through computational operations in a computer [4]. It is one of the most useful X-ray cross-sectional imaging methods for PCB inspection because it can obtain a cross-section of a local inspection area very quickly. Thus it has been often applied to PCB solder joint inspection [1, 4-7]. For real time inspection of PCB by DT, an X-ray image intensifier usually is used. However, image intensifier distorts X-ray images severely both of shape and intensity. This distortion breaks the correspondences between those images and prevents us from acquiring accurate cross-section images. Therefore, image distortion compensation is one of the most important issues in realizing a DT system. In this paper, an image distortion compensation method for an X-ray DT system is presented. Key Engineering Materials Vols. 297-300 (2005) pp 2034-2039 online at http://www.scientific.net © (2005) Trans Tech Publications, Switzerland Online available since 2005/Nov/15 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 130.203.133.34-17/04/08,12:03:39) It is to use a general distortion polynomial model that can cope with arbitrary, more complex and various forms of distortion. And it is available for both of intensity distortion and shape distortion.