Abstract:
A technique for monitoring the alignment of groups of spring-loaded pins extending from a test head of an automatic component tester with conductive pads for receiving the spring-loaded pins includes a printed circuit board having an array of resistive pads. The resistive pads have substantially uniform sheet resistivity across their surfaces and have at least one electrode extending therefrom. The printed circuit board is placed against the spring-loaded pins in place of the conductive pads, so that the pins make contact with the resistive pads. When a pin makes contact with a pad, it forms an impedance with each electrode of the pad. By measuring each impedance, the location of the pin relative to the pad can be determined. Impedance measurements can be conducted at high speed under computer control. In addition, measurement results can be stored and analyzed for diagnosing and predicting faults due to misalignments of pins to pads.

Description:
[0001]    This invention relates generally to automatic test equipment for electronics. More particularly, this invention relates to monitoring the alignment of pins used to make contact with conductive pads to form connections between different portions of an automatic test system.  
         BACKGROUND OF THE INVENTION  
         [0002]    Automatic test equipment (ATE) plays a significant role in the manufacture of semiconductor devices. Manufacturers generally use automatic test equipment—or “testers”—to verify the operation of semiconductor devices at the wafer and packaged device stages of semiconductor manufacturing processes.  
           [0003]    To accommodate the testing of different types of devices, testers generally employ specialized device interface boards or “DIBs.” A DIB maps interface nodes of a tester to interface nodes of a device under test, or “DUT.” Different DIBs are generally provided for testing different types of DUTs.  
           [0004]    The input and output nodes of a tester generally connect to a DIB via conductive, spring-loaded pins. The spring-loaded pins extend from the tester, and make contact with conductive pads on the DIB. Conductive traces within the DIB convey signals between the tester and the DUT, and allow the tester to exercise the DUT. Once a DIB is placed against the tester, the spring-loaded pins are compressed against the pads of the DIB, and connections are established between the tester and the DUT.  
           [0005]    As devices become more complex, more spring-loaded pins are needed to thoroughly test the devices. As devices become smaller, the pins must be spaced more closely together. Consequently, testers typically employ extremely small spring-loaded pins. For example, the Catalyst™ tester manufactured by Teradyne, Inc., of Boston, Mass. uses spring-loaded pins with diameters of only 0.635 mm (25 mils). The conductive pads on the DIBs with which these pins make contact have widths of only 1.524 mm (60 mils). Plans are already in place for shrinking pad widths to only 1.143 mm (45 mils).  
           [0006]    Maintaining alignment between pins and pads having such small diameters presents a significant challenge. Misalignments arise from a number of sources, including coarse misalignments between the DIB and the tester, misalignment of the pins within the tester, bent pins, and dimensional instabilities of the materials that constitute the DIB and the tester.  
           [0007]    Successful testing of components relies on pins making good contact with pads. Accordingly, procedures have been developed to verify pin-to-pad alignment. One common method involves replacing the DIB with a transparent board or sheet. Circular patterns are inscribed on one surface of the board in the areas where the conductive pads are normally found on the DIB. Using a hand-held magnifier, a human operator manually inspects positions of the pins with respect to the circular patterns. Properly aligned pins fall within the circumference of the corresponding circular patterns on the transparent board.  
           [0008]    This technique suffers from a number of drawbacks. Because it is manual, this technique takes a great deal of time to perform, particularly when a tester has a large number of pins. Results can be inconsistent and suffer from human error. Perhaps more importantly, the manual process does not provide quantitative information about pin-to-pad alignment. Unless great effort is expended, no data is collected about the alignment of pins to pads, or about changes in pin-to-pad alignment between different uses or over time. Because the misalignment of pins and pads will likely present a significant obstacle to future testing, we believe that a quantitative method of monitoring pin-to-pad alignment will be crucial to the success of ATE systems.  
         SUMMARY OF THE INVENTION  
         [0009]    With the foregoing background in mind, it is an object of the invention to quantitatively monitor the alignment of pins with respect to pads in automatic test systems.  
           [0010]    It is another object of the invention to monitor the alignment of pins and pads at much higher speeds than current methods permit.  
           [0011]    To achieve the foregoing objects and other objectives and advantages, a system for monitoring the alignment of pins with respect to conductive pads in an automatic test system includes an array of resistive pads. The array of resistive pads is physically positioned in place of the conductive pads, so that the resistive pads occupy the space normally occupied by the conductive pads. The array of resistive pads has a layout that substantially matches the layout of the conductive pads that the resistive pads replace. Each resistive pad has an electrode extending from a portion of the resistive pad. When a pin makes contact with a resistive pad, the pin forms an impedance with the electrode. The location of the pin relative to the electrode, and thus relative to the pad, is then determined from the impedance. Impedance measurements can be repeated at high speed for each resistive pad, thus allowing high-speed monitoring of pin-to-pad alignment.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    Additional objects, advantages, and novel features of the invention will become apparent from a consideration of the ensuing description and drawings, in which  
         [0013]    [0013]FIG. 1 is a top, plan view of one resistive pad as provided in accordance with the invention;  
         [0014]    [0014]FIG. 2 is a simplified, equivalent schematic of the resistive pad of FIG. 1;  
         [0015]    [0015]FIG. 3 is simplified schematic of an array of resistive pads for monitoring pin-to-pad alignment according to the invention;  
         [0016]    [0016]FIG. 4 is a flowchart showing a process for manufacturing a diagnostic circuit board including an array of resistive pads; and  
         [0017]    [0017]FIGS. 5 a  and  5   b  are top, plan views of the diagnostic circuit board, as viewed during different steps of the manufacturing process shown in FIG. 4.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]    Structure and Equivalent Circuit of Resistive Pad  
         [0019]    [0019]FIG. 1 illustrates a top view of a single resistive pad  100  constructed in accordance with the invention. The resistive pad  100  is preferably fabricated on an outer surface of a printed circuit board using a thin-film resistive material, such as Ohmega-Ply® material from Ohmega Technologies, Inc., of Culver City, Calif.  
         [0020]    As shown in FIG. 1, the resistive pad  100  includes a resistive body  110 . The resistive body  110  has a substantially uniform sheet resistivity across its surface and can be regarded as including four immediately adjacent, substantially even quadrants, numbered I-IV. Electrodes  112 ,  114 ,  116 , and  118  respectively form electrical connections with, and extend from, the first through fourth quadrants of the resistive body  110 . Tabs  122 ,  124 ,  126 , and  128  extend outwardly from the resistive body  110 , to form connections with the electrodes  112 ,  114 ,  116 , and  118 , respectively.  
         [0021]    When a conductive pin makes contact with any point along the surface of the resistive body  110 , it forms an impedance with each of the electrodes  112 ,  114 ,  116 , and  118 . These impedances can be measured to ascertain the location of the pin along the surface of the resistive body  110 , where the pin makes contact with the resistive body  110 .  
         [0022]    [0022]FIG. 2 is a simplified, equivalent schematic  200  of the resistive pad  100 . Terminals  212 ,  214 ,  216 , and  218  respectively represent the electrodes  112 ,  114 ,  116 , and  118  of FIG. 1. As indicated, a conductive pin  210  makes contact with a resistive pad  100  through a corresponding contact resistance  220 .  
         [0023]    The impedances formed between the pin  210  and the electrodes  112 ,  114 ,  116 , and  118  are complicated functions. They include terms related to the geometry of the pad  100 , the geometry of the pin where the pin makes contact with the pad, and the point along the surface of the pad where contact is made. To a first-order approximation, however, these impedances can be modeled as simple potentiometers or “slide-wires.” In particular, a horizontal slide-wire is formed among the pin  210  and the electrodes  214  and  218  of quadrants II and IV, and a vertical slide-wire is formed among the pin  210  and the electrodes  212  and  216  of quadrants I and III.  
         [0024]    As shown in FIG. 2, the horizontal slide-wire formed between the pin  210  and the electrodes  214  and  218  includes five components:  
         [0025]    1. A contact resistance  220  (R contact ).  
         [0026]    2. An impedance  222  (R T-Left ) corresponding to the tab  128 ;  
         [0027]    3. An impedance  224   a  (R Left ) corresponding to the path between the pin  210  and the tab  128 ;  
         [0028]    4. An impedance  224   b  (R Right ) corresponding to the path between the pin  210  and the tab  124 ; and  
         [0029]    5. An impedance  226  (R T-Right ) corresponding to the tab  124 .  
         [0030]    Using standard Ohmega-Ply® material with a surface resistivity of 25-ohms/square, the contact resistance  220  between a 25-mil diameter, gold-plated pin and a pad has been empirically measured to be approximately 52-ohms. We have found that contact resistance is substantially constant over time and over multiple contacts between a pin and a pad. Assuming a round pad with a diameter of 1.524 mm (60 mils), the resistance across the pad can be calculated to be approximately 30-ohms. The impedances of the tabs  222  and  226  have been empirically measured to be approximately 5-ohms each.  
         [0031]    Given these values, the impedance R Left  can then be determined as follows:  
                       Z   1     =                  R   Contact     +     R   Left     +     R   TLeft         ,              or                   R   Left     =                  Z   1     -     57                 ohms         ,                 (     EQ   .              1     )                               
 
         [0032]    where Z 1  is a measured impedance between the pin  210  and the terminal  218 . Similarly, the impedance R Right  can be expressed as 
           R   Right   =Z   2 −57ohms,  (EQ.2) 
         [0033]    where Z 2  is a measured impedance between the pin  210  and the terminal  214 .  
         [0034]    With these resistances known, the horizontal distance of the pin from a left edge  132  of the resistive body  110  can be expressed in terms of R Left :  
                 D   H1     =     W   *       R   Left       R   TotH           ,           (     EQ   .              3     )                               
 
         [0035]    where W is the width of the pad (1.524 mm, or 60 mils) and R TotH  is the horizontal resistance across the resistive body  110  (i.e., 30-ohms). This same distance can also be expressed in terms of R Right :  
               D   H2     =       W        (     1   -       R   Right       R   TotH         )       .             (     EQ   .              4     )                               
 
         [0036]    Either of the expressions D H1  and D H2  can separately determine the horizontal position of the pin relative to the pad. We have recognized, however, that there is an advantage to combining the two expressions. We have recognized that the precise dimensions of the tabs  124  and  128  can vary slightly, and that these variations can introduce errors in determining the position of the pin relative to the pad. We have found, however, that the average value of D H1  and D H2  is insensitive to variations in tab dimensions, as long as the variations are symmetrical with respect to the center of pad. Therefore, the distance of the pin from the left edge  132  of the pad is preferably expressed as follows:  
               D   H     =           D   H1     +     D   H2       2     .             (     EQ   .              5     )                               
 
         [0037]    The relationships developed above apply analogously to the vertical slide-wire formed among the pin  210  and the terminals  212  and  216  of quadrants I and III. Like the horizontal slide wire, the vertical slide-wire includes five components:  
         [0038]    1. A contact resistance  220  (R Contact ).  
         [0039]    2. An impedance  228  (R T-Top ) corresponding to the tab  122 ;  
         [0040]    3. An impedance  230   a  (R Top ) corresponding to the path between the pin  210  and the tab  122 ;  
         [0041]    4. An impedance  230   b  (R Bottom ) corresponding to the path between the pin  210  and the tab  126 ; and  
         [0042]    5. An impedance  232  (R T-Bottom ) corresponding to the tab  126 .  
         [0043]    The impedances RT Top  and R Bottom  are determined as follows:  
               D   V1     =     H   *       R   Bottom       R   TotV                 (     EQ   .              8     )                   D   V2     =     H        (     1   -       R   Top       R   TotV         )         ,           (     EQ   .              9     )                               
 
         [0044]    where Z 3  is an impedance measured between the pin  210  and the terminal  212 , and Z 4  is an impedance measured between the pin  210  and the terminal  216 .  
         [0045]    With these resistances known, the vertical distance between the pin and a bottom edge  134  of the resistive body  110  can be expressed using either of the following equations:  
                 R   Top     =       Z   3     -     57                 ohms         ,   and           (     EQ   .              6     )                 R   Bottom     =       Z   4     -     57                 ohms               (     EQ   .              7     )                               
 
         [0046]    where H is the height of the pad (1.524 mm, or 60 mils) and R TotV  is the vertical resistance across the pad (i.e., 30-ohms). The distance of the pin from the bottom edge  134  of the resistive pad is preferably expressed as the average of these values:  
               D   V     =           D   V1     +     D   V2       2     .             (     EQ   .              10     )                               
 
         [0047]    With the horizontal and vertical distances D H  and D V  known, the location of the pin relative to the pad is simply the point that is both a distance D H  to the right of the left edge  132  and a distance D v  up from the bottom edge  134  of the resistive body  110 .  
         [0048]    Diagnostic Circuit Board  
         [0049]    To monitor the alignment of pins to pads in an automatic test system, an array of resistive pads  100  is fabricated on a diagnostic circuit board. The array of resistive pads forms a geometrical pattern that substantially matches the geometrical pattern of conductive pads normally found on the DIB. To monitor pin-to-pad alignment, the diagnostic circuit board is installed in place of the DIB so that the resistive pads occupy the locations normally occupied by the conductive pads of the DIB. The resistive pads then make contact with the pins extending from the tester, just as the conductive pads on the DIB make contact with the pins of the tester during normal operation.  
         [0050]    [0050]FIG. 3 schematically illustrates an exemplary array  300  of resistive pads  310 . Each resistive pad  310  is preferably identical to the resistive pad  110  of FIG. 1. As shown in FIG. 3, the top electrode  1   12 , bottom electrode  1   16 , left electrode  118 , and right electrode  1   14  of each resistive pad  310  respectively connects the top, bottom, left, and right electrode of each of the other resistive pads  310  in the array  300 .  
         [0051]    A selector  312  is coupled to the resistive pads  310  and selects from among the top, bottom, left, and right electrodes. The selector  312  has four inputs: a first input connected to all of the top electrodes  112 ; a second input connected to all of the bottom electrodes  116 ; a third input connected to all of the left electrodes  118 , and a fourth input connected to all of the right electrodes  114 . The selector  312  selects one of these four inputs to conduct the input to an output  314  (“LO”).  
         [0052]    Conventional testers generally include independently controllable relays connected in series with their pins. When these relays are opened, the pins effectively “float,” i.e., they form no electrical connections within the tester. Conventional testers also include a switching matrix connected to the pins and analog instruments that can be connected to each pin through the switching matrix. By properly configuring the switching matrix and the series-connected relays, a terminal of an analog instrument can be connected to one and only one pin. Another terminal of the analog instrument can be connected to the output  314  of the selector  312 . With the analog instrument configured for measuring impedance, separate impedance measurements can then be made for each pin. By varying the position of the selector  312 , all four impedances Z 1 -Z 4  described above can be measured for each pin-to-pad connection.  
         [0053]    The exact method employed for measuring impedance can be varied substantially within the scope of the invention. In the preferred embodiment, an accurate fixed current, for example 1 mA, is sourced into one pin at a time, while the other pins float. A voltmeter is connected between the pin sourcing the current and the output  314  of the selector  312 . The output of the selector is coupled to ground. For each position of the selector  312 , the voltmeter measures the voltage induced between the pin and the output  314  of the selector  312 . Each impedance is then computed as the respective measured voltage divided by the sourced current. From the measured impedances, the alignment of pins to pads for each pad  310  in the array  300  is then determined, using the equations derived above.  
         [0054]    Manufacturing Process  
         [0055]    [0055]FIG. 4 is a flowchart that illustrates a manufacturing process for fabricating a diagnostic circuit board that includes an array of resistive pads. FIGS. 5 a  and  5   b  depict a diagnostic circuit board  500  during different stages of the manufacturing process.  
         [0056]    At step  410 , a laminate is provided that includes at least three layers: a first layer  510  of insulating material; a second layer  512  of resistive material bonded to the first layer; and a third layer  514  of conductive material bonded to the second layer and forming an outer surface of the circuit board  500 .  
         [0057]    At step  412 , a first mask is applied to the outer surface of the circuit board  500 . The first mask defines regions on the circuit board  500  that are resistant to chemical etching. These regions correspond to areas where neither resistive material  512  nor conductive material  514  are to remain on the surface of the circuit board  500  after etching.  
         [0058]    At step  414 , a first chemical bath is applied to the surface of the circuit board  500 . The first chemical bath etches away conductive material  514  not protected by the first mask. A second chemical bath is then applied at step  416 . The second chemical bath etches away resistive material  512  not protected by the first mask. At the conclusion of step  416 , the surface of the circuit board  500  appears substantially as shown in FIG. 5 a . The first mask is then removed.  
         [0059]    At step  418 , a second mask is applied to the circuit board  500 . The second mask defines areas on the surface of the circuit board  500  where conductive material is to be etched away.  
         [0060]    At step  420 , a chemical bath is applied to the circuit board, and conductive material not protected by the second mask is removed. At the conclusion of step  420 , the circuit board  500  appears substantially as shown in FIG. 5 b.    
         [0061]    The resultant features on the circuit board  500  include an array of resistive pads  110 , as well as electrodes and conductive connections between them. The circuit board  500  preferably includes at least one additional layer of conductive material, for example on the back surface of the board  500 , for establishing electrical connections among the different resistive pads  110 .  
         [0062]    In the preferred embodiment, the laminate includes standard Ohmega-Ply® material with a sheet resistivity of 25-ohms/square. Ohmega-Ply® is a thin-film nickel alloy (a resistive layer  512 ) electrodeposited onto copper foil (a conductive layer  514 ). Ohmega-Ply® is conventionally used to manufacture buried, thin-film resistors between layers of circuit boards. By bonding Ohmega-Ply® to an insulating layer  510  of FR 4 , for example, the three layers described above can be realized. We have not previously observed Ohmega-Ply® being used at an outer surface of a circuit board; however, we have found it to be durable material that is highly resistant to scratches and oxidation.  
         [0063]    All circuit boards are known to suffer from dimensional errors. To minimize dimensional errors in manufacturing, a direct write tool is used to apply masks to the circuit board  500 . Direct write tools are highly accurate lithography instruments that operate by moving a circuit board around a scanned point of light. Direct write tools are manufactured by Carl Zeiss, Inc., having U.S. headquarters in New York, N.Y., and are currently in use for circuit board fabrication by Sanmina Technology Center, of Haverhill, Mass.  
         [0064]    Depending upon the accuracy of the manufacturing process, the tabs  122 ,  124 ,  126 , and  128  can be either enlarged or reduced. In general, the tabs ensure that no conductive material (i.e., copper) comes into direct contact with the body  110  of the resistive pad  100 . If copper were to make direct contact with the resistive body  110 , the geometry of the body  110  would change, and the ability to predict the location of a pin based on measured impedances would be impaired. Therefore, smaller tabs can be provided when highly accurate manufacturing processes are used. Larger tabs should be used with less accurate processes.  
         [0065]    The techniques described herein allow high-speed monitoring of pin-to-pad alignment in automatic test systems. Conventional digital multi-meters normally found within testers can measure hundreds of impedances per second, and conventional relays can switch in under 1 ms. Accordingly, using the techniques described, an array of 100 pins can be tested in under a second. An ATE system can store results for later reference. Different test results can be compared over time, and faults can be diagnosed and predicted.  
         [0066]    Alternatives  
         [0067]    Having described one embodiment, numerous alternative embodiments or variations can be made. For example, the preferred embodiment provides that four different impedances Z 1 -Z 4  be measured to determine the position of a pin relative to a pad. Only two impedances are strictly required, however. By measuring just one of Z 1  and Z 2  and one of Z 3  and Z 4 , the distances D H  and D V  can be determined (See EQ. 3 and 8, for example). The two additional impedances merely compensate for dimensional inaccuracies of the tabs  122 ,  124 ,  126 , and  128 . If these inaccuracies are controlled, two measurements can be eliminated for each pad. Accordingly, the resistive pads  100  would include only two electrodes, one horizontal electrode and one vertical electrode. Providing two electrodes instead of four allows higher packaging density of pads, and simpler circuit boards.  
         [0068]    Sometimes it may be desirable to monitor pin-to-pad alignment along one dimension only, for example, in the horizontal dimension. This may arise when pins are physically constrained from moving vertically, but not from moving horizontally. In these instances, two electrodes may be used, spaced apart by approximately 180 degrees, for estimating the position of a pin. Only one electrode is strictly required, however, provided that dimensional inaccuracies of the tab can be controlled.  
         [0069]    The preferred embodiment provides that a rigid printed circuit board be used to hold an array of resistive pads. Flexible printed circuit media, such as flex boards, can also be used.  
         [0070]    Other types of compressible pins can be used besides spring-loaded pins. For example, the invention can be applied to connections made with small, S-shaped wires called “MicroSprings,” made by FormFactor, Inc., of Livermore, Calif. Other compressible pins not employing conventional springs can also be used.  
         [0071]    The embodiment described above applies to compressible pins that make contact with fixed pads. The invention also applies, however, to rigid pins making contact with compressible pads. According to this variation, a compressible circuit board, for example, one made of an elastomeric material, can be used instead of a rigid circuit board, for holding the array of compressible, resistive pads.  
         [0072]    In addition, pins have been described as extending from the tester, and pads have been provided on the DIB. This arrangement can obviously be reversed, with pins extending from the DIB and pads on the tester.  
         [0073]    The techniques described above can also be used for checking the alignment of pins on subassemblies of a tester. For example, the techniques can be used to check pin alignment on probe towers, before the probe towers are installed within a tester. In addition, the invention is not limited to testers. It can also be used to measure pin alignment on a wide variety of electronic assemblies.  
         [0074]    As described above, the resistive pads  100  are substantially round and can be divided into four equal quadrants. Nothing requires this specific shape, however. Pads found in real electronic assemblies assume a wide variety of shapes, and the resistive pads  100  used to check the alignment of pins with these pads should be tailored to match these shapes. As described above, impedances Z 1 -Z 4  are measured by forcing an accurate current into each pin and measuring the resultant voltage imposed across the pad. As ready known to the skilled practitioner, however, there is a wide variety of methods for measuring impedance. For example, a simple ohmmeter can be used. Alternatively, a fixed voltage can be imposed between the pin and each electrode of a pad, and the resultant current measured to deduce the impedance.  
         [0075]    In addition, impedances need not be measured using a centralized analog instrument within the tester. An external instrument, for example an IEEE-488 bench-top instrument, or a VXI instrument, can be used. Alternatively, each pin of the tester can include a separate instrument for measuring impedance at that pin.  
         [0076]    The conventions “left,” “right,” “top,” and “bottom” have been used above to indicate relative orientations of the electrodes and different quadrants of the pads. These are provided as arbitrary designations for reference only.  
         [0077]    Each of these alternatives and variations, as well as others, has been contemplated by the inventor and is intended to fall within the scope of the instant invention. It should be understood, therefore, that the foregoing description is by way of example, and the invention should be limited only by the spirit and scope of the appended claims.