Abstract:
A method and apparatus allows adapting a standard flying prober system to probe test point targets on Printed Circuit Assemblies (PCAs) having irregularities in their planarity. The method and apparatus involves positioning a camera utilized by the prober system to predetermined offset positions relative to previously established test and/or other points for sets of images. Each set of images is processed by determining offsets in coordinates needed to align the images in a predetermined manner. The offsets for each measured point are translated into actual height values to be used during subsequent testing of a PCA.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This is a non-provisional patent application claiming priority of provisional application for Pat. No. 60/507,680 filed Oct. 1, 2003. 

   BACKGROUND OF THE INVENTION 
   1. Field of Use 
   The present invention relates to flying prober systems and more particularly to systems for enabling reliable testing of printed circuit assemblies (PCAs). 
   2. Prior Art 
   Modern Flying Probers are a class of In-Circuit Test (ICT) equipment which uses a plurality of moving probes in lieu of a standard bed of nails fixture to provide all or most connections between the PCA undergoing test and the test unit. Generally, flying prober tests are a subset of the tests that might have been performed by an ICT unit, because of the limited number of simultaneous connections possible between the PCA and tester. However, in present day test operations, such contact requirements have diminished compared with the recent past. 
   The principal benefit of flying probers is cost avoidance, in eliminating standard ICT bed of nails fixtures costing tens of thousands of dollars and having a short useful lifespan and little residual value. Another benefit is avoiding the delay associated with constructing such a fixture. Most flying probers have four moving probes, mechanically positionable to any board location by a relatively high speed mechanism. The list of board locations to be used in testing a given PCA type is generally derived from computer aided design (CAD) files provided as part of nearly all modern PCA designs. The same files are similarly used to define nail location points in a bed of nails fixture in non-flying prober testing. Alignment of the PCA on the flying prober is accomplished using electro-optical methods, whereby registration holes of the PCA that would be engaged when mounted on a bed of nails are, instead, found by image recognition methods and their precise locations recorded. Then, instead of mechanically aligning the PCA to the tester, the list of probing locations is recalculated to take into account the actual positions of the registration holes, essentially aligning the tester to the PCA instead of aligning the PCA to the fixture mounted on the tester. 
   The electro-optical system usually is or is the equivalent of a miniature television camera connected through a digitizer to the computer used to control the tester in its execution of a test program. In addition to its usefulness in PCA to tester alignment, the electro-optical system is used for other purposes related to testing. For example, the probing points may be sighted one by one for the benefit of the test programmer in verifying that the CAD data, upon which the test program is based, indeed matches the PCA for which a test program is being developed. Usually, the television camera used for this purpose is mounted on the carrier that also holds one of the probes. It is mounted in a position that is a predetermined offset from the probe itself. Thus, while the television camera system may not be able to display the probe as it touches the PCA, it can be placed directly over the point where the probe would touch the PCA had not the offset been applied. The camera&#39;s optics are aligned perpendicularly to the ideal plane of the PCA, allowing cross hairs or similar positional markings to be added to the image, creating a bombsight effect and allowing confirmation of theoretical probe positioning to a very high degree of precision. This display may also be used to verify the lack of probing obstacles in the vicinity of probing points or that the point is otherwise suitable for probing. One or more additional cameras are sometimes employed to further aide in test programming and/or execution, showing, for example a larger area of the PCA, and at an angle that allows watching some probes as they are extended to touch test points. The electro-optical system employed is sometimes sufficiently complex to allow Automated Optical Inspection (AOI) testing to be performed in conjunction with electrical flying prober tests. 
   While the nails of a standard ICT bed of nails fixture are mounted perpendicularly to the plane of the PCA while in its test position, flying fixture probing is performed at angles somewhat off perpendicular (Z axis). Angles of between five and sixteen degrees to the Z axis have been noted in some modern flying prober specifications. In some systems, the probe angle may be altered by test program commands. The angles are necessary to allow probing a series of closely spaced points by probes which are, by necessity, attached to relatively large drive mechanisms to allow speedy extension and withdrawal. Those mechanisms are in turn mounted to the carrier driven by an X-Y positioning mechanism. Two types of X-Y mechanisms used are linear motor and lead screw. Furthermore, the probes may be at angles to the X or Y axis as well as the Z axis. A single X-Y table of probe points suffices, regardless of the number of probes, variety of angles, or thickness of the particular PCA type being tested, by applying appropriate offsets as compensation for these effects in determining the precise point at which the PCA will be contacted. 
   However, the compensation discussed above is based upon the assumption that the probing points of the PCA exist in a perfect plane, or at predetermined differences from a perfect plane. Warpage of the PCA is both non-planar and unpredictable. Hence, planarity variations result in probing variations. In some cases, the intended test probing target may be probed slightly askew from the intended point of contact, usually the center. In other cases, the probe may miss the target altogether. For example, consider the case of a probe which is fifteen degrees from perpendicular attempting to probe a target point which is 35 mils in diameter. The required probing accuracy would be +/−17 mils, assuming the probe will not slide once one physical contact is made (not a safe assumption). At an angle of fifteen degrees, a 17 mil error occurs when the height of the intended target is approximately 1/16″ (0.017″/0.268, the tangent of 15 degrees). The actual safe region for contacting a 35 mil target with a 15 degree probe is a matter of opinion. If, however, half the error were considered safe (about 8 mils), the height would have to be predictable to within approximately 1/32″. Maintaining a planarity tolerance of +/− 1/16″ is generally not possible in a manufacturing environment with PCAs measuring 16″ by 16″ or more. Even with larger test targets, the planarity requirements are often impractical to maintain. 
   In the art of PCA testing using flying probers, planarity variations that cause a misprobe are a known problem, but there is a paucity of detailed data as to its significance. One result of a misprobe is a false error (e.g., when testing for resistance and getting an open indication) or a missed error (e.g., when a short was present but not detected). But, there are so many possible reasons for such errors, such a large quantity of such errors and so little engineering time to devote to making exact determination of error causes (real vs. false), that the effectiveness of available attempted solutions to the planarity problem has never been fully tested. Such attempted solutions are, e.g., standoff posts used to support a concave PCA from the underside during probing (but which may have no effect on convex PCAs) and standoff posts which attempt to apply either an upward or downward force, as needed, by means of vacuum applied over so small an area as to effect only relatively flexible PCAs. In all such cases, the required planarity cannot be guaranteed in a production environment. 
   Misprobes may also occur when the probing target is large enough that it will not be missed because of height differences. In such cases, the force applied by the probe may be inadequate, causing a lack of contact between the probe and PCA or too great, causing marking of the contact area. The marking may be in the form of a pit or a scored line, the latter resulting when the probe is pushed by excessive force. The marking occurs because probe contact depends upon spring force. In normal operation, contact force is achieved by attempting to drive the probe perhaps 50 to 100 mils further than would be required for the tip to make contact with the PCA probing point. At contact, the probe will stop moving and its internal spring will compress to take up the distance, providing contact pressure. In a case where the PCA contact point was significantly closer to the probing mechanism, early contact would be made and the probe might be driven 100 mils before the 100 mils previously referenced, for a total of 200 mils. In some cases, the spring might even fully compress, causing the probe to be driven against the PCA with the maximum force the probe extension motor can produce. In certain applications, the significant marking of PCA targets is not tolerated and the PCA has to be reworked or scrapped. 
   Accordingly, it is a primary objective of the present invention to provide a method and apparatus for overcoming variations in PCA board planarity of PCAs mounted for testing in a flying prober system. 
   It is a further objective of the present invention to utilize extensively hardware typically found on existing flying prober systems, making the retrofitting of such systems to incorporate the present invention practical. 
   SUMMARY OF THE INVENTION 
   The above and other objects of the present invention are achieved in a preferred embodiment of a method and apparatus which can be easily incorporated into an existing or standard flying prober system or included in the design of a new flying prober system. The method of the present invention includes additional processing and analysis of the data obtained from the electro-optical system, or television camera, referenced previously as used to provide a bombsight display to check the match of CAD data to the PCA being programmed and for other programming and test purposes. It involves extending the X-Y probe list to include height data (i.e. an X-Y-Z list). Once the height data has been added to the list, the appropriate offsets may be calculated and applied during the test program on a probe by probe basis in the same manner as the application of angular offsets previously described as being part of the standard flying prober system. 
   The apparatus utilized by the present invention, which may be considered optional in many cases, provides additional illuminators to enhance the television imaging process. Such apparatus many also include a new television imaging device able to operate closer to the PCA and/or having a wider field of view and/or greater resolution. 
   According to the present invention, the imaging system used to perform a bombsight check of alignment is positioned with points to be checked near the diagonal edges of the image instead of at the center. The points to be checked may be derived from the X-Y probing point list or otherwise selected, the former being convenient to use simply because it exists as part of the PCA CAD data and need not be separately obtained. Two images are obtained of each point selected for use. After the PCA and tester are in alignment, by virtue of the registration points having been found by the imaging system and the X-Y list recalculated as described previously, the imaging system is moved, by automatic program control, to a first point of which a height measurement is to be made. However, an offset is applied to the X-Y drive mechanism such that the point is positioned near one corner of the image. The image is then stored for subsequent processing. Next, the imaging system is moved to a point corresponding to the original X-Y location used before applying the previous offset and an equal but opposite offset is applied. Thus, the point selected for processing will appear in the opposite diagonal corner of the image from the image previously stored. The new image is then stored. 
   Also, according to the present invention, the area immediately surrounding the places where the points of interest in each of the two images is selected for comparison and one image is offset until it matches the other image within limits of reasonable certainty. The amount of offset required is proportional to the height of the probing point, as caused by the magnification of the imaging system lens. That is, if the PCA point being checked is higher (closer to the lens) than that of a PCA with perfect planarity, the offset required to match the two images would be greater than the offset required to match the two images found at normal height. Conversely, the offset required to match the images when the PCA was further from the lens would be less than the offset required to match two images found at normal height. Prior to testing of production PCAs and at regular intervals, a calibration method is used to determine the offset requirements for matching images resulting from points positioned at various distances from the lens, such that offsets may be related to specific distances. 
   In the preferred embodiment, to provide for consistent images of the area immediately surrounding the points being checked, an illumination system may be used. The system has two units, one unit which is as closely positioned as practical to being directly above the point being checked when the imaging system is ready to store the first image discussed above. The second unit will be similarly positioned as close as practical to directly overhead the same point when the imaging system is ready to store the second image discussed above. The two units are used one at a time as part of the imaging processing of the points directly under them. 
   A like series of steps are applied at a sufficient number of target points on the surface of the PCA board. The height of all X-Y target points of the PCA may be interpolated and later applied in determining individual offsets to be used in probing all target locations by any of the probes. 
   The above objects and advantages of the present invention will be better understood from the following description when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an area of a sample PCA in detail, with the principal probing point to be used in explaining the operation found in the center of the drawing. 
       FIG. 1   c.  is a first section of  FIG. 1 , also centered on the same point, but showing a view that might be seen using the bomb sighting imaging system according to the present invention. 
       FIG. 1   a.  is a second section of  FIG. 1  showing the view that would occur when a first offset is applied to the X-Y drive system that would otherwise be used to locate the imaging system directly above the point of interest shown in  FIG. 1   c.    
       FIG. 1   b  is similar to  FIG. 1   a,  except that it shows the result of applying to the imaging system locator, an equal but opposite offset to the offset applied in  FIG. 1   a.    
       FIG. 1   d  is the same as  FIG. 1   a,  except that a box has been included to point out the area of that image that will be used for comparison. 
       FIG. 1   e  is the same as  FIG. 1   b,  except that a box has been included to point out the area of that image that will be used for comparison. 
       FIG. 1   f  is an illustration of how the offsets required to match the images from each of the two diagonal corners relate to the apparent distance between the two points caused by the magnification of the imaging system. 
       FIG. 2  shows the details of a flying prober probe carrier and the mounting of the probe mechanism and imaging system thereon. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   With reference to  FIGS. 1 and 2 , the principles of the present invention will now be described relative to its application to a particular system. The system is a standard or existing flying prober that uses an imaging system camera  220  with a diagonal field of view of about 20 degrees and a horizontal optical center positioned approximately four inches above the PCA being tested,  100 . A probe carrier  200  is attached by means not shown to a conventional mechanism not shown used for moving carrier  200  about the flying prober in X-axis and Y-axis directions on a plane parallel to the ideal plane of PCA  100 . Camera  220  and a bracket  201  are attached to carrier  200 . A pair of illuminators  230  and  231  are attached to bracket  201  and are positioned along one horizontal axis formed by a diagonal of the rectangular image formed by the imaging system. Illuminators  230  and  231  are further positioned to be both equidistant from the vertical optical center of the imaging camera lens and as close as possible to 1.46 inches apart, measured from the vertical axis center of one illuminator to the equivalent point on the other. Each of the illuminators  230  and  231  may be designed to provide point sources of light except where they must be mounted significantly further apart than the desired distance, in which case only diffused light may be found to have the greatest effect. The illuminators  230  and  231  are selected to provide sufficient light to allow the imaging system lens to be operated at a small aperture, providing maximum depth of field without loss of resolution. A pair of cables  232  and  233  individually connect the two illuminators to a control system, allowing them to be selectively and separately turned on or off. In some cases, ambient lighting from other parts of the flying prober may be found sufficient, obviating illuminators  230  and  231 . A cable  221  connects the imaging system camera  220  to other parts of the imaging system not shown. 
   A probe  214  is shown in a retracted position, nominally more than two inches above PCA  100 . This distance allows X-axis and Y-axis movements of carrier  200  without striking most large components that might be mounted on a PCA. Probe  214  and a barrel  213  form an integral assembly within which a spring exists, causing a controlled force to be exerted between the tip of probe  214  and a point which it first contacts and then is further pushed against by moving barrel  213  closer to PCA  100 . A probe holder designed to accommodate the probe assembly including barrel  213  and probe  214  forms part of a linear motor arm  212 . Arm  212  is extended from or retracted into a linear motor module  210  through its connection to a control system via cable  211 . The extension of arm  212  may be selectively controlled in increments of 10 mils from zero to 2.1 inches on an axis 15 degrees from perpendicular to the ideal plane of PCA  100 . The 15 degrees is the angle from perpendicular at which motor module  210  is mounted to carrier  200 . It should be noted that motor module  200  may or may not be aligned to the Y-axis of carrier  200  movement as a matter of flying prober system design choice. 
   In  FIG. 1 , a PCA area of nominally 2 by 3 inches is shown. A registration hole  120  is used in automatically aligning PCA  100  to the test system. Two other similar holes, not shown, would normally be used as part of the registration process. Integrated Circuit device(s)  160  is attached to PCA  100  by the surface mount process of soldering legs  161  to pad(s)  151 . Similarly, discrete components  162  are surface mount process soldered to pad(s)  150 . Etch(es)  140  are connected to device pad(s)  150  and/or  151  and to plated through hole(s)  130 . Plated through hole  131  is similar to other holes  130 , but is the target point of interest in determining a single item of PCA height data. Holes  130  and  131  are shown as solder filled, but could equally well be empty. 
   It is important to note that comparison at each point is made between two images of the same point from different camera positions and not to images of the same point on another PCA, such as might be termed a standard PCA of the type being tested That is, the present invention does not rely on consistency between one PCA and the next. For example, a particular target location on one of a series of PCAs of the type may be a solder filled hole, whereas another PCA of the same series may have an unfilled hole in the same location. Generally speaking, such variations would not negatively affect proper operation of the present invention. 
     FIGS. 1   a,    1   b,    1   c,    1   d  and  1   e  show the 1 inch by 1.33 inch area of the image produced by camera  220  in three camera positions.  FIG. 1   c  shows the bombsight position directly above the X-Y location of probing location  131 . In  FIG. 1   a,  carrier  200  has been moved 0.728 inches from the position of  FIG. 1   c  in a first direction along the diagonal axis of the image of  FIG. 1   c.  In  FIG. 1   b,  carrier  200  has been moved 0.728 inches in the opposite direction from the position of  FIG. 1   c  along the same axis. In  FIG. 1   d,  a 0.15 by 0.15 inch area has been highlighted (boxed) in the corner of the image occupied by hole  131  of  FIG. 1   a.  Similarly, in  FIG. 1   e,  a 0.15 inch by 0.15 inch area has been highlighted in the corner of the image occupied by hole  131  in  FIG. 1   e.  In  FIG. 1   f.,    FIGS. 1   d  and  1   e  are merged into a single image to aid in developing a better understanding of how the individual corner images relate to an effective separation between the same point viewed from two distinct positions. The images of  FIGS. 1   a  through  1   f  represent those of a section of a PCA and, in particular, of an individual test point  131 , which is located upon the ideal test plane (i.e., neither higher nor lower). Height variation of the PCA would cause the images to be magnified or reduced, thereby causing test point  131  to be further from or closer to the center of each image and, in particular, towards the inner or outer corners of image capture areas  170  and  171 . (The inner corners being those capturing the portion of the image closest to the center of entire field captured by the imaging system, the outer corners being those diagonally opposite.) Hence, for example, a test point  130  which is closer to the imaging system lens by virtue of a PCA height variation will produce images where that test point  130  is proportionally closer to the outer corners of image capture areas  170  and  171  than would be the case of the equivalent test point  130  on a PCA having no height variation. 
   Description of Operation 
   With reference to  FIGS. 1 and 2 , operation of the preferred embodiment will now be described. The operation will be described relative to a specific example but the invention is not in any way limited to such use. 
   It is assumed that the system has been previously calibrated such that the image qualities of a single pair of offset images on a plane of a 3/16 inch thick calibration PCA and a single pair of offset images on a plane of a ¼ inch thick calibration PCA are both stored within the system. It is further assumed that a series of points have been predetermined that are more or less evenly spaced across the production PCA and are sufficiently close enough to each other that the height of any point on the PCA may, with sufficient accuracy, be estimated by interpolating from the list of nearby points for which the height has been actually determined using the present invention. As stated previously, these points may be directly extracted from the system list of X-Y probable points or a list separately developed or from a combination thereof. In addition to the stated list of distributed points, a list of alternative nearby points also may be included to provide for cases where the height of a particular location cannot be determined for one reason or another. 
   The operation according to the present invention proceeds as follows using the list of points. 
   Step 1. Select a first point from the list. 
   Step 2. Turn illuminator  230  on and illuminator  231  off (allow for lamp delay if necessary). 
   Step 3. Move camera  220  to X, Y of selected point then apply an offset of X−0.591, Y−0.425. (illuminator  230  is thereby directly above X, Y position.) 
   Step 4. Capture a first 150 mil by 150 mil corner portion  170  of the image under illuminator  230 . 
   Step 5. Turn illuminator  230  off,  231  on (allow for lamp delay if necessary). 
   Step 6. Move camera to X, Y of selected point then apply an offset of X+0.591, Y+0.425. (Illuminator  231  is thereby directly above X, Y position.) 
   Step 7. Capture a second 150 mil by 150 mil corner portion  171  of the image under illuminator  231 . 
   Step 8. Process the first and second portions of the images to determine offset needed to achieve favorable match between any 100 mil by 100 mil portion of the image captured in Step 7 with any same sized portion of the image captured in Step 4. If no match made, select a nearby alternate point, if available, or the next point of any further points on the list, if not. 
   Step 9. Translate the X, Y offsets at each point measured into actual height values. An offset of zero would be found where the PCA top surface (the surface closest to the probes) was 3/16″ above the surface of an ideal PCA of zero thickness. An X, Y offset of the imaged equivalent of adding 23 mils to the distance between the same points on the first and second images (in steps 4 and 7) is the value previously stored in the calibration phase when comparing the relative distances of the 3/16 inch thick and ¼ inch thick portions of the calibration PCA. (The image equivalent of 1.457 inches becomes 1.481 inches equivalent when 1/16 inch closer to the lens.) 
   It is seen from the above that the apparatus and method of the present invention may readily be incorporated by those skilled in the art into standard flying prober systems with few modifications. Such modifications may be exclusively software modifications, if they are, in and of themselves, found to provide sufficient height measurement accuracy for the PCAs desired to be tested. Additionally, electro-optical systems with greater resolution and accuracy may be incorporated, including those using wider or non-linear fields of view or zoom lenses and various forms of illumination to augment those systems 
   While in accordance with the provisions and statutes there has been illustrated and described the best form of the invention, certain changes may be made without departing from the spirit of the invention as set forth in the appended claims and that in some cases, certain features of the invention may be used to advantage without a corresponding use of other features.