Abstract:
A PC board test fixture translates various stimulation signals produced from a base grid of an automatic test system to randomly positioned test points of a printed circuit board to be tested. The test fixture includes a translator board having conductive pads aligned with probes of the base grid. Conductive pads on the top surface of the translator board are aligned with spring-loaded probes carried by a probe support fixture and aligned with the random test points of the printed circuit board. Conductive feedthroughs connect selected conductive pads on the bottom of the translator board to the randomly located pads on top of the translator board. A computer-executed algorithm compares the locations of the pads on the top and bottom of the translator board to determine the locations of the feedthroughs.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to apparatus for probing printed circuit (PC) boards to apply stimulus signals to selected points of the PC boards and to measure responses at other points of the PC boards to enable the responses to be compared with the expected response and thereby determine the operability of the PC board. The invention relates more particularly to fixtures that translate or conduct signals between conductive probes of a base grid having fixed center-to-center spacings and randomly oriented conductive test points of the PC board to be tested. 
     Typical state of the art printed circuit board testers include large numbers of programmable signal drivers for producing various stimulation signals to be applied to a printed circuit board under test, and also include a large number of programmable signal receivers for receiving signals produced by the printed circuit board under test in response to the stimulus signals, and further include means for comparing the response signals to expected responses. Typically, such a printed circuit board tester includes an array of conductors or pointed probe tips arranged in a rectangular array having 100 mil centers. When a particular printed circuit board is to be tested, it ordinarily will have a large number of probe points or test points which are randomly positioned on a surface of the printed circuit board. For unloaded printed circuit boards, such probe points often will be the locations of conductive feedthroughs. A fundamental problem encountered in the industry is how to reliably and economically provide connections between the various ones of the base grid conductors or probe points (which have been programmed to effectuate testing of a particular printed circuit board) and the randomly positioned probe or test points of the PC board to be tested. Use of translator boards that are &#34;hand designed&#34; to align conductive points of the base grid with the randomly positioned probe points of the PC board to be tested is excessively expensive, as there may be several thousand probes points on a particular PC board. 
     Automated techniques for translating or conducting signals between the base grid of the automatic tester and the PC board test points have been needed. However, what has been provided in the past are translating devices that utilize long, or solid spring-loaded probes, referred to herein as &#34;probe pins&#34;, the lower ends of which contact signal pads or probes of the base grid. The upper ends of the probe pins contact a probe point of the printed circuit board to be tested. An upper guide plate having clearance holes therein aligned with the probe points of the printed circuit board to be tested is aligned with the printed board to be tested. A lower guide plate has clearance holes aligned with the conductors of the base grid. The spring-loaded probe pins then are dropped through holes in the upper guide plate and pass through holes in the bottom guide plate to make contact with various base grid conductors. The clearance holes are large enough to allow the spring-loaded probe pins to tilt sufficiently to ensure that they can pass through guide holes in both the upper and lower guide plates (since the guide holes in the upper and lower plate normally are not aligned due to the random placement of the guide holes in the upper plate). 
     A basic problem with some implementations of the foregoing prior art approach is that there is no precise way of determining exactly which conductors of the base grid make electrical contact 2 which probe points of the printed circuit board under test. Therefore, test algorithms which exercise the board under test (BUT) are inherently &#34;non-deterministic&#34;. What this means is that circuit design data cannot be used to produce the expected response with which the actual response produced by the board under test in response to the stimulation is to be compared. Instead, various patterns of test stimuli must be applied to a particular printed circuit board that is known to be &#34;good&#34;. The resulting response is stored and used as the basis of comparison with subsequently tested boards. Furthermore, the spring-loaded probe pins need to be quite long so that their angles of tilt or inclination, caused by misalignment of the randomly positioned PC board test points with the 100 mil centered base grid conductors, are not too great. As test point spacing decreases, the tilted probe pin approach becomes increasingly impractical. 
     Another problem with the foregoing approach is that a relatively large volume of air must exist between the base grid and the bottom of the printed circuit board to be tested. This large volume of air is rather incompatible with vacuum fixturing techniques wherein a vacuum is produced in the volume between a vacuum sealed pressure plate over the BUT and the base grid of the tester in order to force the BUT against the spring-loaded probe pin points. 
     It is clear that there remains an unmet need for an improved apparatus for translation of automatic tester base grid conductor signals to randomly positioned PC board test points. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an inexpensive deterministic apparatus for translation of automatic PC board tester base grid signals to and from randomly positioned test point or conductors of a PC board under test. 
     It is another object of the invention to provide a deterministic base grid to test point translation device which is highly compatible with present vacuum fixturing technology for printed circuit board testers. 
     It is another object of the invention to provide an automated technique for manufacturing an apparatus for translating test signals between the base grid of an automatic tester and probe points of a PC board to be tested. 
     It is another object of the invention to provide an automated testing apparatus for translating electrical signals between a base grid conductor array having one density of signal concuctors to a printed circuit board having a higher density of conductive test areas. 
     It is another object of the invention to provide an improved probe pin assembly for use in automatic PC board test apparatus. 
     Briefly described, and in accordance with one embodiment thereof, the invention provides an apparatus and method for translating test signals from base grid conductors of an automatic test system to randomly positioned test points of a printed circuit board by providing vertical feedthroughs in a translator board having conductive pads on a bottom surface thereof precisely aligned with and adapted for contact to test signal conductors of the base grid, the translator board also having conductive pads on its upper surface precisely aligned with the randomly oriented test points of the PC board to be tested. The translator board is fastened to the bottom of a probe pin support jig having holes precisely aligned with the conductive pads on the top surface of the translator board. Spring-loaded dual plunger probe pins are retained in the holes in the pin support fixture by a lip on the lower end of a sleeve within which the spring-loaded plungers move. The probe pin support jig includes a recess or other alignment means that receive a printed circuit board to be tested and hold it in precise alignment with the upper plungers of the spring-loaded probe pins. Each of the upper and lower plungers of the spring-loaded probe pins has an outer point, the upper point contacting a probe point of the printed circuit board and the lower point contacting an upper pad of the translator board. To manufacture the translator board, digitized data representing the locations of the pads on the bottom of the translator board is compared with digitized data representing the random locations of the probe points of the printed circuit board to be tested to determine the points of overlapping of the upper and lower pads of the translator board. The intersection information is used to determine the locations of the conductive feedthroughs that connect each upper pad of the translator board to a predetermined lower pad of the translator board. The digitized data representing the random locations of the PC board probe points also is utilized to produce artwork which then is used to photoetch or otherwise produce the conductive pads on the upper surface of the translator board; that digitized data also is used to drill the holes in the probe pin support fixture into which the probe pins are inserted. 
     In another embodiment of the invention, half of the base grid probes are omitted to leave a checkerboard pattern, since usually the number of test points of a printed circuit board is much less than the number of available base grid probes, and it is highly desireable to save the expense of unused base grid probes and associated electronics. Selective shorting is provided between conductive pads on the lower surface of the translator board which do not contact probes of the base grid and ones that do, and appropriate feedthroughs are provided from such shorted lower conductive pads to overlapping upper pads of the translator board. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial perspective view of a test fixture of the present invention. 
     FIG. 2 is an enlarged section diagram of a probe pin used in the test fixture of FIG. 1. 
     FIG. 3 is a partial top view of a translator board for the test fixture of FIG. 1. 
     FIG. 4 is a section view taken along section line 4--4 of FIG. 3. 
     FIG. 5 is a partial bottom view of the translator board 4 of FIGS. 1 and 3. 
     FIG. 6 is a partial top view of an alternate translator board. 
     FIG. 7 is a partial bottom view of an alternate translator board for use on a underlying sparse checkerboard base grid. 
    
    
     DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, particularly FIGS. 1-5, printed circuit board (PCB) fixture 1 includes a &#34;base grid&#34; 2 that contains a large number (typically 4,000 to 100,000) of conductive pointed test signal probes 3. Each of the test signal probes 3, which is spring-loaded, is programmably connected to a separate signal source circuit or signal receiver circuit. Various programmable test systems are commercially available. For example, a Model TM5/BBT, manufactured by Testsystems, Inc. provides a base grid having 49,000 conductive probes thereon. Typically, the conductive probes 3 of base grid 2 are arranged in a rectangular array, with 100 mil centers between the individual conductive probes. 
     Test assembly 1 also includes a translator board 4 having an upper surface 4A and a lower surface 4B (see also FIG. 5). The upper surface 4A has a number of round conductive pads, collectively referred to by reference numeral 5, disposed thereon. The conductive pads 5 may, for example, be 80 mils in diameter. Their respective locations are precisely aligned with the locations of the probe points of the printed circuit board to be tested, as subsequently explained. 
     On the bottom surface 4B, best seen in FIG. 5, a plurality of square conductive pads 28 are disposed. The conductive pads 28 have 100 mil centers. The midpoint of each of the bottom square pads 28 is precisely aligned with the pointed tip of a respective one of the base grid probes 3 when translator board 4 is properly aligned with base grid 2. 
     As best seen in FIGS. 1, 3, and 4, a plurality of conductive feedthroughs 33 electrically connect edge portions of various upper conductive pads 5 to particular &#34;overlapping&#34; lower pads 28, respectively. 
     Referring again to FIG. 1, test fixture 1 includes a probe pin support fixture or jig 7 having a number of vertical holes 8 therein. Each of the vertical holes 8 is precisely aligned with a corresponding one of the upper translator board pads 5. A dual plunger spring-loaded probe pin 9 is disposed in each of the vertical holes 8 of probe pin support jig 7. 
     The structure of a typical probe pin 9 is best seen with reference to FIG. 2. Each probe pin 9 includes an outer cylindrical sleeve 20 having a lower lip or flange 21 that rests against the bottom surface of probe pin support plate 7 and retains probe pin 9 in hole 8 when translator board 4 is fastened (typically by screws such as 37) in aligned relationship against the bottom surface of probe pin support jig 7. 
     Probe pin 9 includes an upper plunger 17A having a pointed upper tip 17 that contacts a test point on the bottom of the printed circuit board 13, which is the BUT. Plunger 17 includes a lower nipple 23 about which an upper end of a compression coil spring 19 tightly fits to provide a reliable electrical and mechanical connection between plunger 17A and spring 19. Thus, plunger 17A can move upward and downward in the directions of arrows 26 within sleeve 20. Probe pin 9 also includes a lower plunger 18A having a pointed bottom tip 18 which electrically contacts a particular translator board pad 5 with which that probe pin 9 is aligned. A nipple 24 extends upward from the upper surface of plunger 18A and is tightly inserted into the bottom end of conductive coil 19 to provide a good mechanical and electrical connection thereto. Compression of coil spring 19 is resisted by shoulder 22 which surrounds nipple 23 and shoulder 25 which surrounds nipple 24. Dimples 35 retain the plungers 17A and 18A and the coil spring 19 in sleeve 20, which in turn is retained in the assembly including translator board 4 and probe pin support plate 7 by means of flange 21. 
     Probe pin support jig 7 includes a recess 10 in its upper surface for receiving printed circuit board 13. Recess 10 is bounded by a vertical wall 11. The dimensions of recess 11 can be precisely the same as the dimensions of printed circuit board 13, with a suitable tolerance to allow insertion of PC board 13 into recess 11, to thereby allow precise alignment of printed circuit board 13 with the test assembly 1. In most cases, the recess 11 will be larger than the printed circuit board, and tooling pins or the like will be used to obtain precise alignment of the printed circuit board and the probe pin support jig. 
     Typically, printed circuit board 13 includes a number of flat conductors 14 and a large number of feedthroughs 15. Especially for unloaded printed circuit boards, a main objective of testing is to check the continuity of the feedthroughs. Typically, the center of each feedthrough is aligned with the pointed upper end 17 of a probe pin 9. 
     Suitable apertures (not shown) to allow a vacuum to be produced between printed circuit board 13 and base grid 2 can be provided. The volume of air to be expelled is very small, making vacuum fixturing techniques conveniently implementable in conjunction with the test fixture 1. 
     Typically, the thickness of translator board 4 is 62 mils. It can be composed of ordinary PC board material, the upper conductors 5, and the lower conductors 28 being produced by photoetching of the initially copper clad surface of the original &#34;blank&#34; board. Typically, probe pin support jig 7 may be 0.5 inches thick, and can be composed of suitable insulating structural material, such as phenolic, fiberglass resin, etc. 
     In accordance with the present invention and the method of manufacturing the translator board 4, it should be appreciated that typically a customer which wishes to have printed circuit boards such as 13 tested, will supply electronically readable media containing data indicating the location of each of the feedthroughs 15 that need to be probed. If such media is not supplied, the locations of a sample board can be digitized to obtain such media with such data. As previously mentioned, ordinarily, the locations of the feedthroughs will be random relative to the locations of the conductive spring-loaded probes 3 of base grid 2. 
     A supply of &#34;stock&#34; translator boards such as 4 will be kept available. The bottom pads 28 of the stock translator boards will already have been defined on a phototool for photoetching, as the bottom pads are always the same. Digital data corresponding to the locations of each of the lower translator board pads 28 will, of course, be known since the bottom pads 28 are all of the same size and center-to-center spacing. Such digital data then is fed into a computer program which determines the location of an imaginary &#34;subsquare&#34; 30 (FIGS. 3 and 5) in each of the bottom pads 28. Each subsquare or dotted line 30 typically is located 10 mils from the outer edges of that pad 28. Within each of the round top pads 5 is a &#34;subcircle&#34; 31, or dotted line circle, typically 10 mils from the edge of that pad. 
     The computer program which locates the feedthroughs 33 selects an intersection between a subsquare 30 and a subcircle 31 and thereby determines the location of one feedthrough to connect the corresponding upper pad 5 and lower pad 28. This technique provides the needed lateral &#34;translation&#34; between a particular base grid probe 3 and one PC board test point 15. The computer program which performs the function of determining the intersections of subcircles 31 and subsquares 30 is quite straighforward, and can be easily implemented by one skilled in the art. However, Appendix A attached hereto includes a printout of a program, written for a HARTRONIX model 4S32A computer, available from Hartronix, Inc. of Tempe, Arizona, to define the feedthrough locations. 
     The distance between each of the subcircle dotted lines 32 and the outer edge of the upper pad 5 is typically 10 mils. 
     Once the above-mentioned computer program has determined the desired locations of the feedthroughs 33, this information, in the form of digital data stored in an electronically readable medium used to control a numerically controlled drilling machine is used to drill the feedthrough holes for the feedthroughs 33. A standard feedthrough plating procedure then is utilized to produce the conductive feedthroughs of translator board 4. 
     The location of the upper translator board pads 5 is obtained by using the above-mentioned data corresponding to the locations of the probe points or feedthroughs 15 (of printed circuit board 13) to drill 80 mil diameter holes in a piece of opaque film. Such holes are, of course, precisely aligned with the feedthrough holes or probe points 15. This piece of film then is used to expose photoresist on the upper surface of the translator board 4. Using standard photoetching technology, the copper clad upper surface of the stock translator board is etched away, leaving only the upper pads 5 at the desired locations (corresponding to test points of the BUT). 
     The probe pin support plate 7 is inverted, and a large number of probe pins 9 are placed thereon in a vibratory apparatus that causes the probe pins to be automatically loaded into the holes 8 in the probe pin support jig 7. In accordance with the present invention, the probe pins 9, which typically are only 0.5 inches long, can be reliably and automatically loaded in this fashion. Then, the translator board 4 is aligned with and fastened to the bottom surface of the probe pin support jig 7. The structure then is ready to be conveniently stored until needed. When needed, the assembly 4, 7 with probe pins loaded thereon, is easily aligned over and lowered onto the base grid 2, and individual PC boards 13 can be rapidly tested. 
     Since the computer program that determines the translator board feedthrough locations provides data that matches every probe point of PC board 13 to exactly one predetermined base grid probe 3, the non-deterministic relationship between the base grid conductors of the prior art and the probe points of the printed circuit boards to be tested is avoided. In many cases, this allows the expected response of the printed circuit board to be determined in advance, and simplifies the task of obtaining a suitable expected response. 
     The above described technique works well as long as the probe points of the printed circuit board to be tested are not substantially smaller and closer to each other than the bottom pads 28 of translator board 4. Howevever, with the state of the art for printed circuit boards rapidly advancing, some printed circuit boards may have substantial numbers of probe points which are located on 40 mil centers or 50 mil centers, rather than the much greater 100 mil centers of the bottom pads 28 of translator board 4. For such a high density of probe points, the situation can arise wherein two of the probe points would be aligned over a single pad. FIG. 6 illustrates this situation, in which reference numerals 5-1 and 5-2 correspond to upper pads on a translator board 4. The upper pads 5-1 and 5-2 are considerably smaller in diameter than the upper pads disclosed in FIGS. 3 and 4, and their center-to-center spacing is considerably less. Direct feedthroughs such as 33 in FIGS. 3-5 from upper pads 5-1 and 5-2 in FIG. 6 would both contact the same bottom pad, namely pad 28-1. 
     To avoid this problem, only feedthrough 33-2 connecting upper pad 5-2 to lower pad 28-1 has been provided. An extension conductor 41 from upper pad 5-1 has been provided, electrically connecting it to a point over adjacent bottom pad 28-2. A feedthrough 33-1 then is provided between the end of conductor 41 and bottom pad 28-2. This technique will be satisfactory as long as there are not too many more upper pads such as 5-1, 5-1, etc., than lower pads, and as long as it is not necessary for the extensions such as 41 from different upper pads to cross over each other in order to reach a point above an available bottom pad 28. Use of the technique described with reference to FIG. 6 is not as simple as use of the technique described with reference to FIGS. 1-5, because the computer algorithm which determines the routes of the extensions 41 and the locations of the feedthroughs such as 33-1 connecting a bottom pad such as 28-2 with the end of an extension 41 is more complex. Furthermore, the pattern on the upper surface of a translator board 4 is photoetched cannot be accomplished simply by drilling. Other techniques must be provided for exposing or cutting the film regions corresponding to the extensions 41 in order to achieve the desired photoetching of the top surface of the translator board. Nevertheless, the technique described with reference to FIG. 6 would be useful in certain instances. 
     Another desireable feature of this invention allows one to take advantage of the fact that on printed circuit boards 13 to be tested, the density of test points 15 is almost always much less than the underlying base grid 2. With current commercial electronic packaging, individual devices have their leads on 100 mil centers on rows that are usually spaced 300 mils or greater apart, and are never mounted on a printed circuit closer than 100 mils from each other. This means that with the maximum packing density, only 50 percent of the underlying base grid points will be assigned to the test points on the printed circuit board 13. 
     FIG. 7 illustrates a translator board 4 that will work in this case with an underlying base grid 2 where only every other test point is installed in the base grid in a checkerboard pattern. The contacts in the base grid 2 would only contact the bottom pads such as 28A, which are crosshatched in the drawing while the pads such as 28B would have no contact to the test system connected to the base grid 2. 
     The computer program would operate as previously described to place the drilled feedthrough contacts 33 to connect the upper conductor pads 5 to the bottom conductor pads 28. Those feedthrough contacts 33 that end up connected to cross-hatched pads 28A will have a conductive path established. The computer program would then look at those pads 28B-1, 28B-2, etc, that have feedthrough contacts 33 connected to them and then find an adjacent pad such as 28A-1, 28A-2, etc, that does not have a feedthrough 33 connected to it. It would then cause the photo artwork that will be used to etch the pads 28 on the translator board 4 to be modified so as to connect those two pads together. 
     This may be done by drilling a hole such as 32 in the negative photo image of the array of pads 28. When the translator board is then fabricated, a conductive path will be established from a base grid point 3 contacting pad 28A-1 which is connected to pad 28B-1 through a conductive path established by 32-1 and then through feedthrough 33-1 making connection to pad 5-1 which then contacts probe assembly 8 contacting test point 15. 
     The considerable advantage to this method is that the machine establishing the underlying test grid need only have half of the test points required by prior art tester systems. Typical test machines typically cost $5.00 to $10.00 per test point. A reduction from 40,000 test points to 20,000 test points could save $100,000 to $200,000 per machine. Several other advantages accrue by allowing simple mechanical structures to support the force required to depress the spring-loaded test points 3. Typical spring pressure is 4 ounces, thereby requiring 10,000 pounds of force on a 40,000 point underlying grid. 
     While the invention has been described with reference to a particular thereof, those skilled in the art will be able to make various modifications to the described embodiments without departing from the true spirit and scope of the invention. It is intended that all test assemblies and methods that are equivalent to those described herein, in that their respective elements and/or steps perform substantially the same function in substantially the same way to accomplish substantially the same result, be considered equivalent to the embodiments described herein. ##SPC1##