1. Field of the Invention
The present invention generally relates to the art of automated inspection of electrical components, and more particularly to a method and apparatus for optical inspection of solder joints which determines the shape of the joint from the shading or reflected light intensity of its surface.
2. Description of the Related Art
Commercial mass production of electrical circuitry relies heavily on automated soldering of components to circuit boards. Although manufacturing technology has advanced to the point where as many as 97% of mechanically soldered joints may be defect free, quality control requires that the solder joints be inspected in an attempt to reject defectively soldered assemblies before they are shipped to end users. Government specifications require that solder joints be inspected visually, and detail the visual appearance of good and bad solder joints.
Solder joint inspection has conventionally been performed by human inspectors viewing the joints through microscopes. However, there may be great variation among the decisions made by different inspectors, and even the performance of a single inspector can vary markedly over time. This is understandable since the inspectors may be required to view as many as three thousand solder joints in a single thirty minute period.
Several systems have been proposed for replacing human inspectors with automated solder joint inspection apparatus, including the use of X-ray imaging and computerized processing of the X-ray image. An exemplary system of this type is manufactured by the IRT Corporation of San Diego, Calif., and designated as model no. CXI-5210. The apparatus passes X-rays from a collimated, narrow-beam source through a solder joint on a circuit board to form a visible light image on a fluorescent screen. A video camera produces a digitized or pixelized image of the visible image on the screen, which is input into a computer for making inspection decisions for each image based on software programmed with the quality standards for the particular circuit board. Although X-ray systems are capable of providing detailed inspection results, they are incompatible with visual inspection standards.
Another system which utilizes an infrared detector to sense the thermal signature of a solder joint subjected to extreme temperature changes is presented in a paper entitled "Combining Soldering with Inspection", by R. Vanzetti et al, IEEE Control Systems Magazine, Oct. 1988, pp. 29-32. The main disadvantage of this system when applied to actual practice is a low level of reliability in distinguishing good solder joints from bad joints, and does not measure visual criteria.
Systems based on visible light and machine vision detection have also been developed. One such system is described in a paper entitled "A Three-Dimensional Approach to Automated Solder Joint Inspection", by S. Chen, in Electronic Manufacturing, Nov. 1988. The system is based on structured light, or light of a known shape or structure. It is projected onto an object, and the reflection of the light is viewed by a camera located at a fixed angle from the projector. The distortion of the reflected light from the known projected shape is used to compute the distance of every point on the object from the projector and camera. The reconstructed three dimensional shape is processed by software programmed with predetermined inspection criteria.
Another three dimensional optical system is described in an article entitled "A Tiered-Color Illumination Approach for Machine Inspection of Solder Joints", by D. Capson et al, IEEE Transactions on Pattern Analysis and Machine Intelligence, vol. 10, no. 3, May 1988, pp. 387-393. In this system, a tiered lighting arrangement is used to generate color contours on the solder joint for the detection and classification of defects. Each type of defect gives rise to a characteristic pattern of color contours which are processed using binary image techniques on each color plane of the image. Geometric descriptors measuring the shape of the color contours and the color level intensities of the solder joint images are used to identify each defect class. The resolution of the system, however, is very coarse, and may be insufficient to provide reliable solder joint inspection in many applications.
Another three dimensional optical system is manufactured by Robotic Vision Systems, Inc., (RVSI) of Hauppauge, N.Y., under the product designation HR-2000. The apparatus uses an optical triangulation three dimensional vision method to make thousands of measurements on every solder joint. The resulting data set forms a high resolution geometric map of the solder joint and the surrounding board and component. All data are directly measured, and no data have to be inferred to make critical decisions.
The main disadvantage of three dimensional optical image processing systems is that large and expensive computers and specialized hardware including moving parts are required to obtain acceptable speed and throughput.