X-ray or gamma ray systems or devices – Specific application – Tomography
Reexamination Certificate
2001-12-21
2002-12-31
Dunn, Drew A. (Department: 2882)
X-ray or gamma ray systems or devices
Specific application
Tomography
C378S025000, C378S058000, C382S147000, C382S149000
Reexamination Certificate
active
06501822
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to the rapid, high resolution inspection of circuit boards using a computerized laminography system, and in particular, to systems which use electronic means to adjust the Z-axis location of the inspection site with respect to the circuit board.
BACKGROUND OF THE INVENTION
Rapid and precise quality control inspections of the soldering and assembly of electronic devices have become priority items in the electronics manufacturing industry. The reduced size of components and solder connections, the resulting increased density of components on circuit boards and the advent of surface mount technology (SMT), which places solder connections underneath device packages where they are hidden from view, have made rapid and precise inspections of electronic devices and the electrical connections between devices very difficult to perform in a manufacturing environment.
Many existing inspection systems for electronic devices and connections make use of penetrating radiation to form images which exhibit features representative of the internal structure of the devices and connections. These systems often utilize conventional radiographic techniques wherein the penetrating radiation comprises X-rays. Medical X-ray pictures of various parts of the human body, e.g., the chest, arms, legs, spine, etc., are perhaps the most familiar examples of conventional radiographic images. The images or pictures formed represent the X-ray shadow cast by an object being inspected when it is illuminated by a beam of X-rays. The X-ray shadow is detected and recorded by an X-ray sensitive material such as film or other suitable means.
The appearance of the X-ray shadow or radiograph is determined not only by the internal structural characteristics of the object, but also by the direction from which the incident X-rays strike the object. Therefore, a complete interpretation and analysis of X-ray shadow images, whether performed visually by a person or numerically by a computer, often requires that certain assumptions be made regarding the characteristics of the object and its orientation with respect to the X-ray beam. For example, it is often necessary to make specific assumptions regarding the shape, internal structure, etc. of the object and the direction of the incident X-rays upon the object. Based on these assumptions, features of the X-ray image may be analyzed to determine the location, size, shape, etc., of the corresponding structural characteristic of the object, e.g., a defect in a solder connection, which produced the image feature. These assumptions often create ambiguities which degrade the reliability of the interpretation of the images and the decisions based upon the analysis of the X-ray shadow images. One of the primary ambiguities resulting from the use of such assumptions in the analysis of conventional radiographs is that small variations of a structural characteristic within an object, such as the shape, density and size of a defect within a solder connection, are often masked by the overshadowing mass of the solder connection itself as well as by neighboring solder connections, electronic devices, circuit boards and other objects. Since the overshadowing mass and neighboring objects are usually different for each solder joint, it is extremely cumbersome and often nearly impossible to make enough assumptions to precisely determine shapes, sizes and locations of solder defects within individual solder joints.
In an attempt to compensate for these shortcomings, some systems incorporate the capability of viewing the object from a plurality of angles. One such system is described in U.S. Pat. No. 4,809,308 entitled “METHOD & APPARATUS FOR PERFORMING AUTOMATED CIRCUIT BOARD SOLDER QUALITY INSPECTIONS”, issued to Adams et al. The additional views enable these systems to partially resolve the ambiguities present in the X-ray shadow projection images. However, utilization of multiple viewing angles necessitates a complicated mechanical handling system, often requiring as many as five independent, non-orthogonal axes of motion. This degree of mechanical complication leads to increased expense, increased size and weight, longer inspection times, reduced throughput, impaired positioning precision due to the mechanical complications, and calibration and computer control complications due to the non-orthogonality of the axes of motion.
Many of the problems associated with the conventional radiography techniques discussed above may be alleviated by producing cross-sectional images of the object being inspected. Tomographic techniques such as laminography and computed tomography (CT) have been used in medical applications to produce cross-sectional or body section images. In medical applications, these techniques have met with widespread success, largely because relatively low resolution on the order of one or two millimeters (0.0425 to 0.08 inches) is satisfactory and because speed and throughput requirements are not as severe as the corresponding industrial requirements.
In the case of electronics inspection, and more particularly, for inspection of electrical connections such as solder joints, image resolution on the order of several micrometers, for example, 20 micrometers (0.0008 inches) is preferred. Furthermore, an industrial solder joint inspection system must generate multiple images per second in order to be practical for use on an industrial production line. Laminography systems which are capable of achieving the speed and accuracy requirements necessary for electronics inspection are described in the following patents: 1) U.S. Pat. No. 4,926,452 entitled “AUTOMATED LAMINOGRAPHY SYSTEM FOR INSPECTION OF ELECTRONICS”, issued t
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Baker et al.; 2) U.S. Pat. No. 5,097,492 entitled “AUTOMATED LAMINOGRAPHY SYSTEM FOR INSPECTION OF ELECTRONICS”, issued to Baker et al.; 3) U.S. Pat. No. 5,081,656 entitled “AUTOMATED LAMINOGRAPHY SYSTEM FOR INSPECTION OF ELECTRONICS”, issued to Baker et al.; 4) U.S. Pat. No. 5,291,535 entitled “METHOD AND APPARATUS FOR DETECTING EXCESS/INSUFFICIENT SOLDER DEFECTS”, issued to Baker et al.; 5) U.S. Pat. No. 5,621,811 entitled “LEARNING METHOD AND APPARATUS FOR DETECTING AND CONTROLLING SOLDER DEFECTS”, issued to Roder et al.; 6) U.S. Pat. No. 5,561,696 “METHOD & APPARATUS FOR INSPECTING ELECTRICAL CONNECTIONS”, issued to Adams et al.; 7) U.S. Pat. No. 55,199,054 entitled “METHOD AND APPARATUS FOR HIGH RESOLUTION INSPECTION OF ELECTRONIC ITEMS”, issued to Adams et al.; 8) U.S. Pat. No. 5,259,012 entitled “LAMINOGRAPHY SYSTEM AND METHOD WITH ELECTROMAGNETICALLY DIRECTED MULTIPATH RADIATION SOURCE”, issued to Baker et al.; 9) U.S. Pat. No. 5,583,904 entitled “CONTINUOUS LINEAR SCAN LAMINOGRAPHY SYSTEM AND METHOD”, issued to Adams; and 10) U.S. Pat. No. 5,687,209 entitled “AUTOMATIC WARP COMPENSATION FOR LAMINOGRAPHIC CIRCUIT BOARD INSPECTION”, issued to Adams. The entirety of each of the above referenced patents is hereby incorporated herein by reference.
Several of the above referenced patents disclose devices and methods for the generation of cross-sectional images of test objects at a fixed or selectable cross-sectional image focal plane. In these systems, an X-ray source system and an X-Ray detector system are separated in the Z-axis direction by a fixed distance and the cross-sectional image focal plane is located at a predetermined specific position on the Z-axis which is intermediate the Z-axis locations of the X-ray source system and the X-ray detector system. The X-ray detector system collects data from which a cross-sectional image of features in the test object, located at the cross-sectional image focal plane, can be formed. In systems having a fixed cross-sectional image focal plane, it is necessary to postulate that the features desired to be imaged are located in the fixed cross-sectional image focal plane at the predetermined specific position along the Z-axis. Thus, in these systems, it is essential that the positions of the fixed cross-sectional image focal plane and the plane with respect to the obj
Agilent Technologie,s Inc.
Dunn Drew A.
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