Early improvements in inspection methods were concerned with better work-piece illumination, more accurate location, object image acquisition, flaw identification, recognition and finally rejection against given criterion. The focus of evolutionary inventive steps in these directions may be seen by overview of the following patents:
The need for diffused lighting to reduce reflections and shadows in close-up photography has been long recognized. Shank, in U.S. Pat. No. 3,737,226 discloses apparatus in which an indirect light source illuminates a small object through a series of pyramidal reflectors. Light is thus diffused around four sides of the object before reflection to a camera. This invention was for close-up photography and would have limited value for high speed inspection of microelectronic elements.
Kanade et al., in U.S. Pat. No. 4,427,880, provide an array of discrete light-emitting sources which are used to sequentially illuminate a symmetrical work piece object. Reflections are focused on a light responsive position sensor so as to provide continuous indications of distance, surface orientation and curvature of the object. Details of surface geometry are not provided. This approach is most effective for measuring distance if the reflective surface is flat. Based on the position of the reflected light spots on the surface, the distance between the surface and the optical sensor can be calculated and determined. The system does not use continuous illumination for visual identification of the object and its orientation. U.S. Pat. No. 4,508,452 to DiMatteo et al provides for determining the surface profile of an object by projecting a pre-coded pattern onto the surface. By matching the newly acquired image pattern to a pre-determined image pattern, the profile of the newly acquired image can be extracted. An object surface is scanned by a moving projector and subdivided into the large number of coded sections. Comparisons are made of progressive photographs of the work-piece with those of a standard reference surface. The entire surface of an object may therefore be mapped. The system is not applicable to improving contrast between very small three dimensional objects, such as wires, and the reflective background.
Imamura et al. in U.S. Pat. No. 4,568,835, detects foreign matter such as dust particles on a plane substrate by means of scattering of the reflections from a laser beam. As a specimen work-piece such as a photomask is scanned by an oblique incidence laser illumination beam, reflections from foreign materials are less directly scattered than are those from the edges of the circuit pattern. The illumination incidence angle is 80 to 60 degrees off normal, with a portion of the beam being reflected from the substrate surface while the remainder is refracted into the substrate medium from which it is internally reflected then externally scattered outward. This approach does not consider circular illumination used with a highly reflective, low refractive background medium.
In a different surface measurement application, Schachar, in U.S. Pat. No. 4,695,163, determines the contour of a cornea by scanning the surface with coherent light from different positions along a rectilinear path. Reflections received by detectors along the track are maximally polarized when the incidence angle equals Brewsters's angle. From a knowledge of the index of refraction of the medium and of Brewster's angle, the relative spacial locations of points over the surface may be determined. The system should provide slow but precise information when a refracting medium is under inspection, but will have limited utility with highly reflective objects.
An object locating system for use with robotic systems is described in U.S. Pat. No. 4,791,482 to Barry et al. The system projects a known geometrical image from a light source onto the surface of an object. The plane of the image on the object is determined by finding a normal to the surface from known geometrical relationships. Comparison of normals at different surface points are used to calculate distances and angles between the points. Gaussian images are generated for comparison between referenced objects and the unit under test.
In the field of solder joint inspection systems, Sanderson in U.S. Pat. No. 4,876,455 discloses a fiber optic solder joint inspection approach, in which light from multiple sources is reflected from a specular object to a fixed array of transducers. The individual light sources are derived from a single source which is scanned and piped to a plurality of optical fibers which lead to individual openings spaced around a semicircular illumination frame. For a given surface attitude, reflections to the fixed transducers will result from only one illumination source, assuming essentially specular reflection from the surface. Given known surface features of the object, an approximate reconstruction of the shape is made. The point source is usable with solder joint fillet inspection, but not with the variably curved and positioned wiring connections of microelectronic assemblies.
A related invention, U.S. Pat. No. 4,988,202 to Nayar et al, extends the above approach to include generation of an Extended Gaussian Image representation of a solder joint which is then evaluated as to acceptability.
A system for inspection of the uniformity of the surface of a flat circuit board component such as a dual inline package, employing computer vision is taught by Chemaly in U.S. Pat. No. 4,972,493. Illumination is provided by low angular light at the surface edge. Anomalies on the flat surface of dual in-line packages are inspected for pits, holes, blisters, grease, marks, chips and cracks Marks on the surface are distinguished from planned surface irregularities by comparison of grey scale brightness. The two directional lighting is not developed for specular surfaces such as wires, bonds and wedges.
Inspection of the circuit board components when soldered in place is taught by Ikegaya et al. in U.S. Pat. No. 5,027,418. Component lighting is provided by a standard ring illuminator positioned normal to the board. Board masking is provided to make an assessment of soldering condition independent of component lead placement on the circuit board lands.
It may be noted that none of the above inspection systems treat identification and inspection of variably curved and placed circuit elements such as microelectronic wires and bonds. Further, none teach the use or advantages of dual annular illumination sources, each disposed at different angular relationships with respect to normal, each of which provides optimal viewing contrasts for different classes of microelectronic wires and bonds relative their similar background.
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