Abstract:
An optical inspection system with an improved illumination system. The improved illumination system used to illustrate the invention has a base formed from a printed circuit board. Substrates for mounting lighting elements, which are exemplified by light emitting diodes, are formed also on printed circuit boards. These circuit boards have serrated edges and the diodes are mounted to the serrations. This configuration allows the light emitting elements to be focused on the focal point. Also in the exemplary illumination system, the light emitting elements have different beam widths so that variations in the illumination intensity as a function of elevation angle are reduced.

Description:
This invention relates generally to automated optical inspection systems and more specifically to the lighting arrangement for such inspection systems. 
   Automated optical inspection is used during the manufacture of printed circuit boards and in other fields. A computer controlled system passes a camera over the printed circuit board or other object being inspected and takes images of various regions on the board. The images are then scanned by the computer to verify that features expected to be present in a properly manufactured board are in that region and have an expected shape. Alternatively, the system might verify that there are no features in that region that should not be present. If the features are verified, the board passes inspection. If they are not verified, the board is either labeled as defective or passed on for human inspection or additional testing. 
   When the object being inspected in a printed circuit board, the features of interest will be things such as edges of electronic components, metal traces or solder. Many of these features will be captured in an image, though, only if the light strikes the object at a certain angle relative to the camera that is forming the image. For example, if the object has a shiny surface that is oriented to reflect light away from the camera, the camera will capture no light from the object. If the camera captures no light reflecting from the object, the area where the object is will appear black in the image formed by the camera. 
   Thus, to create an image that is useful for automated inspection, the lighting of the inspection area must be controlled relative to the cameras. Some automated inspection system use annular light sources that provide light from all around the area being inspected. However, these types of light sources had several drawbacks. First, annular light sources generally allowed a camera to be focused on the inspection area only from the top looking through the annulus from a direction perpendicular to the printed circuit board. Often, features on a board being inspected will not be readily apparent in an image taken from this direction. 
   Often, various “lighting modes” are desired to take multiple images of the same area under inspection. In each lighting mode, the area under inspection is illuminated from a different angle or a different combination of angles. 
   Annular light sources are known. Some such sources have been made with arrays of light emitting diodes. When the light emitting diodes are operated in sectors, the inspection area can be illuminated from different directions. When the light emitting diodes are operated in groups based on their distance from the center of the annulus, the inspection area can be illuminated from different elevation angles. Lighting from different elevation angles is useful in highlighting different kinds of features on the printed circuit boards. The drawback of annular lighting systems, though, is that the light emitting elements that are used to illuminate the inspection area from a low elevation angle are further from the inspection area than those light emitting elements used to illuminate the inspection area at a high elevation angle. Additionally, the illumination area is further from the center of the beam for light emitting elements at the periphery of the annulus. Consequently, the inspection area is illuminated at a lower intensity when lighting from a low elevation angle is required. Such variations in lighting intensity are not desirable for an inspection system. 
   U.S. Pat. No. 5,245,421 shows a lighting apparatus for inspecting printed circuit boards with surface mounted components that is better suited for providing multiple lighting modes. FIG. 1 of that patent is reproduced as  FIG. 1  herein.  FIG. 1  shows a portion of the inspection apparatus  10 . Apparatus  10  contains an inspection head  12 . 
   Inspection head  12  is mounted on an X-Y table  13  so that it may be moved over the surface of a printed circuit board to be inspected. The remainder of the apparatus is not shown, but it includes mechanical structures to move printed circuit boards into and out of an inspection area under X-Y table  13  and other conventional components of an inspection, such as a computer controller. 
   Inspection head  12  includes cameras that acquire images of the board. In the illustrated embodiment, four cameras  14 ,  15 ,  16  and  17  are shown. The cameras  14 ,  15 ,  16  and  17  are mounted in a hollow cylinder  30  with an open bottom  31 . Inside cylinder  31  are mounted light groups, of which  23  and  24  are visible in FIG.  1 . However, the patent describes that there are four light groups inside cylinder  30 . The light groups can be operated singly or in combination to provide various lighting modes at the open bottom  31 . In use, open bottom  31  is placed near the printed circuit board under inspection and centered over the area being inspected. That area is lighted from various angles by selective activation of the light groups and images are formed of components on the board using the cameras. 
   U.S. Pat. No. 5,060,065 describes the lights inside cylinder  30  in greater detail. That patent describes that the light groups are formed from arrays of light emitting diodes distributed around the surface of a dome inside cylinder  30 . In a commercial embodiment of the lighting apparatus pictured in U.S. Pat. No. 5,060,065, identical diodes are mounted to curved printed circuit boards that are soldered to a ring. In this way, each light emitting diode is essentially the same distance from the inspection area near open bottom  31 . Further, the beam center of each light emitting diode is focused on the inspection area. Such an arrangement provides much greater uniformity in illumination. In addition, space between printed circuit boards carrying the light emitting diodes allowed the multiple cameras to be focused through the dome onto the inspection area. 
   However, to adequately illuminate components on the printed circuit board, the open bottom  31  has to be very close to the surface of the printed circuit board being inspected—roughly 1.75 mm (0.7 inches) or less. This restriction prevents the system from being used to manufacture printed circuit boards with tall components such as transformers or large value capacitors. We have invented a way to allow a light source to operate with taller components on the surface of the printed circuit board. As will be described in greater detail below, we have also invented ways to simplify the manufacture of the lighting structure. 
   SUMMARY OF THE INVENTION 
   With the foregoing background in mind, it is an object of the invention to provide an improved lighting structure for an automated optical inspection system. 
   The foregoing and other objects are achieved in a lighting structure having a substantially planar base and a plurality of light emitting elements supported by the base. In the preferred embodiment, the light emitting elements are diodes attached in groups to substrates projecting from the base. In the presently preferred embodiments, the base and substrates are both printed circuit boards and are interconnected with plug type connectors. 
   In the preferred embodiments, the substrates are mounted along lines that radiate from a common point and the light emitting elements are mounted to focus their beams at an inspection area below this common point. Another feature of the preferred embodiments is that light emitting elements mounted further from the common have a narrower beam angle than the light emitting elements mounted closer to the common point. Using light emitting elements with different beam angles reduces variations in the size of the area illuminated by each light emitting source and also reduces variations in beam intensity in the inspection area from light emitting elements at different distances from the inspection area. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood by reference to the following more detailed description and accompanying drawings in which 
       FIG. 1  is a sketch of a prior art illumination system; 
       FIG. 2  is a sketch of a surface of an illumination system according to the invention; 
       FIG. 3  is a sketch of the opposite surface of the illumination system of  FIG. 2 ; 
       FIG. 4  is a cross sectional view of through the illumination system taken through the line  4 — 4  in  FIG. 2 ; and 
       FIG. 5  is a sketch of printed circuit boards used to form the illumination system of FIG.  2 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 2  shows an illumination system  210  according to the invention. In use, illumination system  210  would be used in place of cylinder  30  in FIG.  1 . In a preferred embodiment, illumination system  210  is used in an automated optical inspection system for printed circuit boards. Illumination system  210  would provide lighting on the board at various stages of its manufacture. It can be used to provide lighting for inspection of printed circuit boards with tall components such that the clearance between the illumination system  210  and the surface of the board being inspected must be 1.7 mm or more. 
   Such a system would be used in the manufacture of printed circuit boards. Circuit boards are often inspected at several steps during their manufacturing process. The inspection is sometimes used to alter the work flow for the individual boards. For example, if an inspection reveals a missing component on the board, the board might be re-routed for rework. The precise manner in which the system of the invention is used to alter the work flow in the manufacturing is not important to the invention. 
   Illumination system  210  has a base, which in the illustrated embodiment is printed circuit board  212 . Board  212  can be a conventional printed circuit board, with conductive traces in it. These traces carry the signals that control individual lighting elements  310  (FIG.  3 ). Electrical connectors  216 , which can be traditional pin and socket type connectors, are mounted in convenient locations on board  212 . In the illustrated embodiment, board  212  is generally square and connectors  216  are mounted in the corners of the board. 
   Board  212  also has an opening  214  in it. Opening  214  provides locations for cameras (not shown) to be focused through illumination system  210  to an inspection area on a printed circuit board under test In the illustrated embodiment, there are five such locations. There are four camera locations  218 A,  218 B,  218 C and  218 D ( FIGS. 2-4 ) located at different angles around opening  214 . There is a fifth camera location  218 E ( FIG. 4 ) in the center of opening  214 . These camera locations allow cameras to form images of an inspection area from multiple angles. 
   Attached to board  212  are a plurality of lighting substrates. In the illustrated embodiment, lighting substrates are provided by a plurality of printed circuit boards  220   a  and  220   b . Manufacture of boards  220   a  and  220   b  are described in greater detail below in conjunction with FIG.  5 . 
   Both boards  220   a  and  220   b  extend into opening  214 . Neither boards  220   a  nor  220   b  extend to the center of opening  214 . By not extending the boards to the center of opening  214 , camera location  218   d  is formed. In addition, boards  220   a  are shorter than boards  220   b , extending only part way into opening  214 . In this way, there are unobstructed passages through opening  214  that form camera locations  218   a  . . .  218   d.    
   Boards  220   a  and  220   b  are better seen in  FIG. 3 , which shows that side of illumination system that would face a printed circuit board under inspection during operation of the system. Each board  220   a  and  22   b  have a plurality of lighting elements attached to them. In the preferred embodiment, the lighting elements are photodiodes. Because boards  220   b  are longer than boards  220   a , they are shown to have more lighting elements attached to them. 
   In the illustrated embodiment, each of the boards  220   a  and  220   b  is attached to board  212  along a line that emanates from the center of opening  214 . Lighting elements  310  are attached to boards  220   a  and  220   b  in a pattern that also generally follows the lines emanating from the center of opening  214 . 
   Such an arrangement is useful when the center of opening  214  is aligned with the area to be inspected of a printed circuit board. In this way, lighting elements  310  are disposed in circles of varying radii around the area to be inspected. As in the prior art, the lighting elements  310  can be simultaneously activated in sectors. Activating lighting elements in one sector causes the inspection area to be illuminated from one side. Instead or in addition, only lighting elements that are a specified radius from the inspection area results in illuminating the inspection area from different elevation angles. 
   As shown in  FIG. 3 , each of the boards  220   a  and  220   b  is mounted to project from board  212 . In the preferred embodiment, each of the boards  220   a  and  220   b  projects at a substantially right angle from board  212 . This configuration places one edge of each board  220   a  and  220   b  against board  212  and a second edge of each board  220   a  and  220   b  generally facing the printed circuit board being inspected. These edges of boards  220   a  and  220   b  provide convenient points of attachment for electrical connectors ( 412   a  and  412   b  in  FIG. 4 ) to connect each board  220   a  and  220   b  to board  212 . These edges also provide convenient points of attachment for light emitting elements  310 . 
   Traces within boards  220   a  and  220   b  run between the connectors and the lighting elements. In this way, electrical signals can be routed to the individual lighting elements. In particular, electrical signals can be routed from a system controller (not shown) through connectors  216 , through traces on board  212 , through connectors  412   a  and  412   b , through traces on boards  220   a  and  220   b  to lighting elements  310 . Many separate paths for separate lighting elements  310  are provided in a way that is simple to manufacture. In particular, components and connectors are soldered to printed circuit boards to create subassemblies, which are then plugged together into a finished assembly. 
   Turning to  FIG. 4 , a cross section of illumination system  210  is shown taken along the line  4 — 4  in FIG.  2 . In this view, two boards  220   b  are visible. The connectors  412   a  and  412   b  are shown as are lighting elements  310 . As seen in  FIG. 4 , each of the lighting elements  310  is mounted to direct its beam at an inspection area, which in this embodiment falls below the center of opening  214 . With the light emitting diodes used in the preferred embodiment, to direct the beams in this fashion, it is necessary that each of the light emitting diodes be mounted at a different angle. 
   In the preferred embodiment, the light emitting diodes are mounted at different angles by first forming platforms (of which some are numbered  510   a  . . .  510   c  for illustration in  FIG. 5 ) along the edge of boards  220   a  and  220   b . As shown in  FIG. 5 , conducting pads are formed on the platforms (such as those numbered  510   a  . . .  510   c ) to which leads of the light emitting diodes may be soldered or attached using any convenient mechanism. The platforms give the edges of printed circuit boards  220   a  and  220   b  a serrated appearance. The orientation of the platform is selected to point the lighting element at the illumination area. This pattern could be termed a Fresnel pattern. 
   In the preferred embodiment, the light emitting elements on each board  220   b  are not the same. Those light emitting diodes  416  mounted closer to the inspection area  414  have a wider beam angle that those light emitting diodes  418  that are mounted furthest from the inspection area  414 . In this way, the intensity at the inspection area  414  is equalized. While theoretically each lighting element on each board  220   b  could have a different beam angle, there are practical limits to the beam angles that are readily available. In one embodiment, only two different types of light emitting diodes are used, with the one having the closest beam angle to what is desired for a particular location being selected. 
   Turning now to  FIG. 5 , a sketch illustrating the manufacture of boards  220   b  is shown. Boards  220   a  would be formed in a similar fashion. Several boards  220   b  are formed form a single sheet of fiberglass stock  530  or other suitable material for making printed circuit boards. As is conventional, traces in the desired pattern are formed on two surfaces of the stock  530 , including pads  512  for attachment of lighting elements. Channels, such as  520   a  and  522   b  are then milled into the stock  530 , thereby forming the desired edges of board  220   b . As illustrated, channel  520   a  has a shape that forms platforms, such as  510   a  . . .  510   c . Score lines  524   a  and  524   b  are also cut, but are not cut completely through stock  530 . At the completion of all handling steps, the epoxy stock can be broken away along the score lines  524   a  and  524   b , leaving a plurality of printed circuit boards  220   b.    
   Having described one embodiment, numerous alternative embodiments or variations might be made. For example, it was described that lighting elements of different beam angles were used to reduce variations in intensity of illumination at different elevation angles. Other ways might be used to equalize intensity. Because groups of lighting elements can be separately controlled, the intensity from some groups might be increased by giving higher input power to that group. Or, where lights are flashed, some groups might be controlled to stay on longer. 
   Also, it was described that light emitting diodes are used for the preferred embodiment of the lighting elements. Other light sources might be used. For example, optical fibers might be used to distribute light from the desired locations and in the desired directions. 
   Further, it was described that the base and substrate elements are manufactured as subassemblies and then connected together to form the illumination system. While such a construction is desirable because it simplifies the manufacturing process, it is not required. It would be possible, for example, to have the lighting elements attached to other kinds of support structures. For example, the support structure might be a unitary member. Alternatively, lighting elements disposed along several rays might be attached to the same member to create subassemblies with more lighting elements than pictured herein. 
   Additionally, it is described in the preferred embodiment, that printed circuit boards are used. Traditional epoxy fiberglass boards with traces might be used, though such are not the only suitable elements. Other substrates are available to form printed circuit boards, such as glass or ceramic. Additionally, ways other than photoetching can be used to create conductive traces. Deposition techniques can be used. Individual wires might also be attached to a substrate to create an equivalent structure. Further, it is not necessary to achieve the advantages of the invention for the traces to be integral with the substrate of the “printed circuit board.” 
   Also, it was pictured that the diode lighting elements rest on the edge of printed circuit boards  220 . It should be appreciated that any mounting arrangement that allows the lighting light to project beyond the edge of the circuit boards  220  could work. Thus, alternative ways of mounting the lighting elements along the edge could be used. 
   Further, it was described that diodes are soldered to the edges of printed circuit boards. Other methods of mounting the diodes might be used. For example, the diodes might be mounted in sockets. 
   Therefore, the invention should be limited only by the spirit and scope of the appended claims.