Image analysis – Applications – Manufacturing or product inspection
Reexamination Certificate
1998-12-11
2002-11-05
Johns, Andrew W. (Department: 2621)
Image analysis
Applications
Manufacturing or product inspection
C382S144000
Reexamination Certificate
active
06477266
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the production of printed circuit assemblies, and more particularly, to the vision inspection of mounted printed circuit components for defect identification and resolution.
2. Description of the Prior Art
A printed circuit assembly may have a large number and variety of components mounted on one or both sides of a printed circuit board. After component installation, but before final soldering, it is desirable to inspect each component-bearing side of the board for correct component assembly. This entails a check for wrong components, reversed components, broken components, mis-wired components or wire jumpers, and other defects.
Component-side inspection of circuit boards is often performed manually, either with or without the aid of optical magnification. Manual inspection is based on the inspector's familiarity with the product or by comparing the printed circuit assembly against a known reference such as a document, photograph, or an actual known good assembly. Such comparisons tend to be tedious and error prone because the inspector must repeatedly alternate attention between the reference and the board being tested (“Test Board”). As a result, manual component-side inspection of circuit boards is usually performed on a sample basis or is performed as a first board check before running many new boards.
As an alternative to manual inspection, automatic vision systems using roving cameras or line scan imagers have been utilized in some printed circuit assembly inspection applications. In an automatic system, many similar printed circuit assemblies are presented to the system such that it can learn the acceptance and rejection criteria. Optical character and shape recognition may also be used in conjunction with board data such as CAD generated information. Such systems can be very complex and prohibitively expensive. They are most effective in low product mix/high product volume environments.
A third approach to printed circuit assembly inspection, representing a compromise between manual and automatic inspection systems, is Blink Overlay Comparison. According to this method, alternating images of a known good board (“Golden Board”) and a Test Board are repeatedly presented to a human operator. The two images appear in alternating fashion at the same visual position in the field of view of the observer. If the two boards are exactly alike and their images are presented to an observer in alternating fashion at some rate (such as five times per second), then the observer will not notice any movement of the images. On the other hand, if there are any differences between the two boards, then only the parts of the images with the differences will flash at the animation rate. These differences are generally due to the assembly defects mentioned above. By displaying the two images in the same visual position, the movement of the observer's attention and focus is eliminated. This increases the speed and accuracy of the inspection.
Several approaches to Blink Overlay Comparison have been used in the past. One such implementation is based on a pure optical comparison. In this method, two optical imaging systems are used in conjunction with a shutter system. The Golden Board is fixtured in a frame at a precision location under the lens of one of the imaging systems. The Test Board is similarly fixtured under the lens of the other imaging system. The two fixtures that hold the boards are coupled mechanically so that they can be moved simultaneously in X-Y directions under their respective lenses. The images from the two lenses are alternately displayed through a common optical eyepiece as in a microscope. Alternation between the two images is achieved with a motorized shutter system. Because the two boards are fixtured using a common reference, such as their respective tooling holes, their images are presented in the same relative position in the eye piece as they appear during alternate intervals.
In another implementation of Blink Overlay Comparison, the optical imaging systems described above are replaced with standard broadcast-grade color video cameras. The optical viewing system is replaced with a color monitor. The fixturing for the Golden Board and the Test Board is similar to that used in the optical system. The images of the two boards are electronically switched and alternatingly displayed on the video monitor.
Both of the foregoing Blink Overlay Comparison systems suffer from several basic problems. First, these systems require that the Golden Board be physically present. This means that an inventory of Golden Boards needs to be maintained. As an example, if a facility produces 100 different types of boards at an average cost of $500.00, then a $50,000 inventory of Golden boards is tied up at all times. Second, both the Golden Board and the Test Board must be placed very accurately in their respective fixtures so that the two images line up properly. Third, due to the limited resolution of the optical and video systems, only a very small portion of the Golden Board and the Test Board can be viewed at one time. This requires many movements of the two boards under the lenses.
One prior art system overcomes certain of these problems. It includes a downward looking video camera that is mounted on an X-Y positionable mounting assembly and a precision board-holding fixture. First, a Golden Board is placed on the precision board holder using the Golden Board's tooling holes. This provides a repeatable position for the Golden Board and the subsequently imaged Test Board. The video camera has a limited resolution, for example, 500×500 pixels. In order to read the value of components on a board for comparison purposes, the camera has to be zoomed in so close that on a typical 16″×16″ board, only a very small portion (such as 4″×4″) of the board is captured at one time. In that case, the camera moves and grabs images at 16 different locations on the Golden Board. In this way a library of images for each Golden Board is created and stored.
When running the production of any one board type, the Golden Board file for that board is loaded. In order to compare a newly manufactured Test Board, it is placed in the precision board holder in the same manner as the Golden Board and the imaging sequence is started. The camera moves through the 16 slices, according to the example above, stops at each position, and animates between the Golden Board and Test Board images on a color monitor for comparison. The system then moves the camera to the next slice upon a signal from the operator.
By digitizing and storing the Golden Board images, this prior art system eliminates the need for an inventory of actual Golden Boards. However, there are several remaining problems with this design. First, the board holder is a precision fixture that locates the Golden Board and the Test Board by engaging fixture pins into the tooling holes in each board. Therefore, various fixtures need to be built for different types of boards. Second, the fixturing of a board requires that it be taken off-line and placed in the fixture. This slows production. Moreover, fixturing can be difficult unless the board is already soldered and there are no loose components that can be disturbed or dropped. However, the ideal place to inspect for assembly defects is before soldering when the defects can be easily corrected. Third, due to the low resolution of the camera, an elaborate zooming and x-y camera positioning system is required. This adds complexity and can slow down system operation.
Accordingly, there is a need in the art for a vision comparison inspection system for printed circuit assemblies that overcomes the foregoing disadvantages of the prior art. What is required is a system that allows rapid and efficient inspection of circuit board assemblies in a production line environment without the need for time-consuming fixture set ups and complicated camera zooming and x-y positioning manipu
Azarian Seyed
Duft Walter W.
Johns Andrew W.
Lucent Technologies - Inc.
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