Patent Publication Number: US-6990227-B2

Title: Method for printed circuit board inspection

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to methods for printed circuit board inspection and particularly to an improved inspection system for PCBs that can quickly and efficiently inspect any arrangement, orderly or not, of PCBs, identical or not. 
   BACKGROUND OF THE INVENTION 
   Printed circuit boards (PCBs) are generally arranged on flat panels for inspection by automated inspection systems. Reference information regarding spatial coordinates and geometry of features found on the PCB, such as pads, holes and the like, is typically input to the inspection system from a data base of computerized design software used to lay out the PCB. The inspection system senses the coordinates and geometry of the features, such as by optical sensors, and compares the sensed information to that of the data base. In this manner, the system detects which PCBs are acceptable and which have flaws. 
   In general, a multiplicity of the PCBs to be inspected rest on flat panels with typical dimensions of 18×24 inches. In prior art systems, reference information is generated for an entire panel. Alternatively, reference information may be generated for a single PCB. However, the PCBs must be arranged in orderly columns and rows so that the inspection system can find a reference point for each PCB from which coordinate measurements are made. For example, the lower left corner may be designated (0,0) and all measurements in (x,y) coordinates are based on that origin. Once the reference points for the PCBs have been found, the inspection system can simply inspect all of the PCBs, since they are all identical to each other without generating a reference for the entire panel. 
   However, prior art inspection systems cannot readily inspect the PCBs if they are not arranged in an orderly fashion or are not identical to each other. 
   SUMMARY OF THE INVENTION 
   The present invention seeks to provide an improved inspection system for PCBs that can quickly and efficiently inspect any arrangement, orderly or not, of PCBs, identical or not. 
   In the present invention, PCBs on a panel to be inspected are grouped into clusters. Each cluster is defined in terms of the number of PCBs in that cluster, the location of the PCBs and their orientation, the concept of orientation including mirroring. An image is created and defined by the location and orientation of each of the clusters on the panel. An image map is generated for the panel to be inspected, for use by a computerized automated inspection system. The image map includes the location and orientation of each of the clusters on the panel and the information about the geometry and features of the PCBs in each cluster. Using the image map, the inspection system commences with a first defined PCB and proceeds until all of the PCBs on the panel have been inspected. 
   There is thus provided in accordance with a preferred embodiment of the present invention a method for printed circuit board (PCB) inspection, including providing a multiplicity of PCBs placed on an inspection panel, defining each non-identical PCB in terms of geometry and features which are to be inspected, grouping the PCBs into at least one cluster, the at least one cluster being defined in terms of an amount, location and orientation of the PCBs in the at least one cluster, creating a reference image for the panel defined by a location and orientation of the at least one cluster on the panel, and inspecting the panel by comparing sensed information with the reference image. 
   In accordance with a preferred embodiment of the present invention pattern recognition methods are used to group the PCBs into the at least one cluster. Alternatively, PCBs are grouped, and their orientations defined, manually. 
   Further in accordance with a preferred embodiment of the present invention an image map is generated for the panel, the image map specifying the location and orientation of the at least one cluster on the panel, inputting information about the geometry and features of the PCBs into the image map, and using the image map by a computerized automated inspection system to inspect the PCBs. 
   Still further in accordance with a preferred embodiment of the present invention the inspection system learns attributes of, and creates and stores a reference for, each non-identical PCB. 
   In accordance with a preferred embodiment of the present invention the inspection system uses the image map to duplicate the stored references for duplicate locations of each individual PCB. 
   Further in accordance with a preferred embodiment of the present invention the reference image is used to create a mirror image of a PCB. 
   Still further in accordance with a preferred embodiment of the present invention the PCBs are grouped into a cluster that includes at least two sub-clusters which are not contiguous with each other. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which: 
       FIG. 1  is a simplified flow chart of a method for PCB inspection in accordance with a preferred embodiment of the present invention; and 
       FIGS. 2 ,  3  and  4  are simplified illustrations of layouts of PCBs that can be inspected with the methods of the invention. 
   

   DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
   Reference is now made to  FIG. 1  which illustrates a flow chart of a method for PCB inspection, in accordance with a preferred embodiment of the present invention, and to  FIG. 2  which illustrates a layout of PCBs to be inspected with the method of the invention. The PCBs may rest upon a flat inspection panel  5 , as in conventional inspection systems. The PCBs may include any number of non-identical PCBs, arranged in any manner. 
   In the method of the invention, a multiplicity of PCBs, such as PCBs A, B, C, D, E and F, are each defined in terms of geometry (shape and size) and features which must be inspected, such as holes, pads and the like. The PCBs are then grouped into one or more clusters, such as clusters  10 ,  12  and  14 . Each cluster is defined in terms of the number of PCBs in that cluster, the location of the PCBs and their orientation. For example, cluster  10  contains 6 PCBs of type A, reference numerals  21 ,  22 ,  23 ,  24 ,  25  and  26 . The dimensions of each PCB type A are, for example, 19 by 15, with reference to coordinate axes (x,y). The reference points of the PCBs are (1,1), (21,1), (41,1), (61,1), (81,1) and (100,1). The orientation of PCBs  21 ,  22 ,  23 ,  24  and  25  are such that the PCBs are generally aligned with coordinate axes (x,y). The orientation of PCB  26  is such that it is rotated at an angle α with respect to the x-axis. Additionally the orientation of PCBs may be mirror imaged in the same cluster, for example PCBs  27  and  28  in cluster  14 . Typically a cluster includes a single type of PCB, although multiple types of PCBs may also be provided in the same cluster. 
   Pattern recognition methods and algorithms may be used to group the PCBs into the clusters and to determine their orientation. Such methods can be used to optimize and speed up the process of grouping the PCBs into clusters. Of course, the grouping and orientation determination can be done manually instead. 
   The arrangement of clusters  10 ,  12  and  14  on panel  5  defines an image  30 . In other words, image  30  is defined by the location and orientation of each of the clusters on panel  5 .  FIG. 2  is just one example of a cluster arrangement, and other examples are now described with reference to  FIGS. 3 and 4 . 
   In  FIG. 3 , a cluster  40  of 6 identical and identically oriented PCBs A is located at the bottom of the panel, and a cluster  42  of 6 other identical and identically oriented PCBs A is on the left side of the panel, the PCBs of cluster  40  being oriented differently than those of cluster  42 . A cluster  44  of 4 identically oriented PCBs B is on the middle right side of the panel. Two identical clusters  45  of 4 identical and identically oriented PCBs C are on the right side of the panel. Clusters  45  can be combined into one cluster  46  of 8 PCBs C, even though cluster  44  intervenes between clusters  45 . By using cluster  46  instead of clusters  45 , processing time is saved. Thus,  FIG. 3  illustrates that the clusters can “overlap” one another and that clusters need not be contiguous. 
   In  FIG. 4 , a cluster  50  of 4 identically oriented PCBs G is on the left side of the panel. Partially overlapped with cluster  50  is a cluster  52  of 4 identically oriented PCBs H. Another cluster  54  of 6 identically oriented PCBs G′ is on the right side of the panel. It is noted that each PCB G′ of cluster  54  is the mirror image of the PCB G in cluster  50 . Thus the system can conveniently define PCBs G′ by simply defining them as the mirror image of G without having to waste processing time to define them all over again. 
   It is appreciated that any arbitrary arrangement of clusters and PCBs may be handled by the methods of the present invention. 
   A reference image map is generated for the panel to be inspected, for use by a computerized automated inspection system. The image map includes the location and orientation of each of the clusters on the panel and the information about the geometry and features of the PCBs therein. In accordance with a preferred embodiment of the present invention, the inspection system learns the attributes of, and creates and stores a reference for, each non-identical PCB. Using the image map, the inspection system duplicates the stored references for duplicate locations of each individual PCB and inputs the image of each non-identical PCB into its appropriate location in the image map. This step may be performed prior to inspection or “on the fly” during inspection. This increases the efficiency and operative speed required to prepare a reference image and that of the inspection system in general, and also reduces memory and disk space required. 
   In summary, by grouping the PCBs of the panel into clusters, the inspection system can readily inspect any regular or irregular arrangement of PCBs, by sequentially scanning the panel area. 
   It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of the features described hereinabove as well as modifications and variations thereof which would occur to a person of skill in the art upon reading the foregoing description and which are not in the prior art.