Abstract:
A probe card assembly that compensates for differing rates of thermal expansion is disclosed herein. The assembly is comprised of a multi-layered dielectric plate interposed between a probe head and a printed circuit board. The printed circuit board has arrayed upon its surface a first plurality of electrical contacts arranged in a pattern. The dielectric plate has a second plurality of electrical contacts arranged in a pattern matching the first plurality of contacts. A planarizing interposer is interposed between the plate and the printed circuit board and has a pattern of holes matching the pattern of electrical contacts on the printed circuit board and plate. The assembly further includes a plurality of electrical connectors disposed within each of the holes arrayed in a pattern upon the planarizing interposer a plurality conductive bumps or fuzz buttons making electrical contact with the first and second plurality of electrical contacts.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This patent application claims priority from U.S. Provisional Patent Application Serial No. 60/265,357 filed Jan. 31, 2001 and U.S. Provisional Patent Application Serial No. 60/265,708 filed Feb. 1, 2001, both of which are incorporated by reference herein in their entireties. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates to the manufacture of probe card assemblies used to test the electrical continuity of integrated circuits formed on semiconductor wafers, such as silicon or gallium arsenide.  
           [0003]    In the standard design of probe cards for performing such testing, a multi-layered ceramic (MLC) plate is inserted between a probe head that has a plurality of small diameter probes positioned to contact circuitry on an integrated circuit under test and a printed circuit board (PCB) that interfaces with an electric meter or other piece of test equipment. The plate is a space transformer having electrically conductive lines with conductive vias extending therethrough. The purpose of the MLC space transformer is to re-route electrical signals from the very finely pitched pattern of electrical contacts on the probe head to the more coarsely pitched pattern on the printed circuit board. By “pitched” it is meant that there exists spacing between adjacent lines or vias. In the process of testing an integrated circuit, electrical connection must be made between the MLC and the printed circuit board. Such a connection is typically achieved by soldering the MLC to the PCB using industry standard soldering techniques to form a rigid solder connection. When conducting such testing at elevated temperatures, for example between 75 degrees and 125 degrees Celsius, it is desirable to manufacture the probe head out of materials whose Coefficient of Thermal Expansion (CTE) closely matches that of silicon wafer being tested. The CTE of silicon is 3.2 ppm/degC. Accordingly, it is preferred that the CTE of the probe head be approximately  2  times that of silicon, such as less than 7 ppm/degC.  
           [0004]    Because the MLC makes electrical contact with the finely pitched contacts on the probe head, it too must have a CTE closely matching that of silicon. Preferably, the CTE of the MLC is also within 2 times of the CTE of silicon. This requirement presents a problem when mating the MLC to the PCB, as the CTE of PCB is typically 17 ppm, much higher than that of silicon. Because of the differing rates of expansion between the MLC and the PCB, there is a potential for the rigid solder connections to crack under the mechanical stress that results from heating and/or cooling the structure.  
           [0005]    Typically, all probes of the probe head must be planar within a tolerance of a few thousandths of an inch. Careful control of manufacturing and assembly tolerances, as well as additional grinding and polishing processes, are required to maintain such a degree of planarity.  
           [0006]    Existing circuitry testing methods and apparatuses do not account for this difference of CTEs. For example U.S. Pat. No. 5,623,213 entitled “Membrane Probing of Circuits” and U.S. Pat. No. 5,841,291 entitled “Exchangeable Membrane Probe Testing of Circuits” are directed to the use of electrically conductive bumps on a flexible substrate or membrane to test electrical circuits. There is no suggestion or discussion that the system compensates for the differences of CTE inherent in the overall system.  
           [0007]    Likewise, U.S. Pat. No. 5,973,504 entitled “Programmable High-Density Electronic Device Testing” is directed to a system for testing high-density electronic devices. More particularly, this patent discloses the use of a test system with a multi-chip module to route signals between pads of a device under test and a test circuit. This system employs a membrane probe card and conductive circuit connection bumps. The membrane probe card is screwed onto the housing using a frame ring and is positioned between a pressure mechanism and the device under test. The conductive bumps are grouped on the membrane to correspond with the connection pad arrays. In an alternative embodiment, electrical connection may be maintained using electrical button connectors. Again there is no suggestion that the system be designed to compensate for the differences of CTE inherent in the testing system.  
           [0008]    U.S. Pat. No. 5,623,213 entitled “Membrane Probing of Circuits,” U.S. Pat. No. 5,841,291 entitled “Exchangeable Membrane Probe Testing of Circuits,” and U.S. Pat. No. 5,973,504 entitled “Programmable High-Density Electronic Device Testing” are incorporated by reference herein in their entireties.  
           [0009]    Commonly owned U.S. Pat. Nos. 6,163,142 and 6,297,657, both entitled “Temperature Compensated Vertical Pin Probing Device,” are directed to improved probing devices useful in testing integrated circuits over large temperature ranges. Both of these patents disclose methods and apparatuses complementary to the method and apparatus disclosed herein. U.S. Pat. Nos. 6,163,142 and 6,297,657 are incorporated by reference herein in their entireties.  
           [0010]    Accordingly, there is a need for a method and apparatus useful to adjust the planarity of the probe head after assembly in order to correct for misalignment. Ideally, such a method and apparatus should minimize the need for critical machining and assembly processes.  
         BRIEF SUMMARY OF THE INVENTION  
         [0011]    It is an object of the invention to find a method of electrically and mechanically connecting the MLC to the PCB in a way that will allow for the differing rates of expansion while maintaining electrical contact.  
           [0012]    Additionally, it is an object of the invention to provide a process whereby one may adjust the planarization of the MLC relative to the PCB.  
           [0013]    One novel aspect of the present invention involves the attachment of the MLC to the PCB in a manner that allows for differing rates of thermal expansion as well as allowing for probe head planarization, while maintaining electrical contact between the MLC and the PCB.  
           [0014]    To compensate for the differing rates of thermal expansion, the present invention suitably employs a planarizing interposer that includes a plate positioned between the MLC and the PCB while maintaining a compliant electrical connection. For the purposes of this disclosure, “compliant electrical connection” shall refer to a connection that maintains electrical connection and the integrity of the components on either side of the interconnection regardless of differing rates of thermal expansion, and allows for planarization of the head assembly. Compliant electrical connection with the device under test (integrated circuit) may be maintained using fuzz buttons or conductive bumps. In embodiments where fuzz buttons are used, the plate has a plurality of holes (also called vias or via holes) drilled in a pattern that matches the pattern of electrical contacts on the PCB. Fuzz buttons are then positioned within the holes. In embodiments where conductive bumps are employed, the conductive bumps are screened or deposited onto land grid array pads of the MLC and/or the PCB.  
           [0015]    In a first embodiment, there is disclosed a probe card assembly for testing integrated circuits comprising: a multi-layered dielectric plate interposed between a probe head and a printed circuit board, the printed circuit board having arrayed upon its surface a first plurality of electrical contacts arranged in a pattern, the dielectric plate having arrayed upon its surface a second plurality of electrical contacts arranged in a pattern substantially matching the first plurality of electrical contacts; a planarizing interposer interposed between the ceramic plate and the printed circuit board, the planarizing interposer having a pattern of holes matching the pattern of electrical contacts on the printed circuit board and the dielectric plate; a mounting ring clamped to the plate and the mounting ring attached to the printed circuit board; and a third plurality of compliant electrical connectors disposed within a multiplicity of the holes arrayed in a pattern upon the planarizing interposer, the electrical connectors making electrical contact with the first plurality of electrical contacts and the second plurality of electrical contacts.  
           [0016]    In a second embodiment, there is disclosed a method of adjusting the planarization of a probe card assembly comprising the steps of: (a) screwing the adjustment screws sufficiently tightly so as render the mounting ring in close proximity with the printed circuit board; (b) determining the planarization of said probe head; and (c) adjusting the tightness of the screws to achieve a desired degree of planarization for the probe head.  
           [0017]    In a third embodiment, there is disclosed a method of assembling a probe card assembly comprising the steps of: (a) inserting each of a plurality of fuzz buttons into via holes using an electrically conductive tool; and (b) confirming electrical connectivity between the multi-layered dielectric plate and the printed circuit board.  
           [0018]    In a fourth embodiment, there is disclosed a method of assembling the probe card assembly of claim 1, comprising the steps of: (a) providing a temporary plate to verify electrical connection; and (b) replacing the temporary plate with a multi-layered dielectric prior to testing. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 is a schematic diagram of the probe card assembly according to the present invention.  
         [0020]    [0020]FIG. 2 is a cross-sectional representation of the probe card assembly.  
         [0021]    [0021]FIG. 3 is a cross-sectional representation of the probe card assembly depicting the use of fuzz buttons.  
         [0022]    [0022]FIG. 3 a  is a cross-section representation of the probe card assembly with through holes in the PCB for alignment purposes.  
         [0023]    [0023]FIG. 4 is a cross-sectional representation of the probe card assembly depicting the use of fuzz buttons with filled via holes.  
         [0024]    [0024]FIG. 5 is a cross-sectional representation of the probe card assembly depicting the use of conductive bumps between the PCB and the space transformer.  
         [0025]    [0025]FIG. 6 is a cross-sectional representation of the probe card assembly depicting the use of conductive bumps between the PCB and MLC. 
     
    
     DETAILED DESCRIPTION  
       [0026]    The present invention includes a planarizing interposer located between a MLC or other multi-layered dielectric and a PCB. The interposer uses a compliant interconnect (generally shown in FIG. 1), such as fuzz buttons (illustrated in FIGS. 2, 3, and  4 ), a Pin Grid Array (illustrated in FIG. 5), or conductive bumps (illustrated in FIG. 6), to maintain electrical IS connection between the PCB and the MLC. Throughout this specification reference to a MLC is intended to also include other suitable multi-layered dielectric plates.  
         [0027]    Turning to FIG. 1, an interposer  20  includes a plate with a plurality of through-holes formed in a pattern that matches the pattern of a first plurality of electrical contacts on the PCB  10 . Preferably, the plate is formed of a dielectric, such as rigid plastic. Most preferably, the plate is formed of Techtron PPS manufactured by Quadrant Engineering Plastic Products. The first plurality of electrical contacts are typically arranged in an array with pitch of 0.05 inches. While 0.05 inches is typical, one skilled in the art would recognize that other pitches may be suitably employed. Electrical connectors are deposited in the through-holes to provide an electrical pathway. A preferred electrical connector is that of a small cylindrically shaped element called a fuzz button. The fuzz button is a commercial product consisting of a thin electrically conductive metal wire, such as beryllium copper, that has been compressed into a die to form a cylindrical mass and having spring-like properties. Examples of preferred fuzz buttons include those manufactured by Tecknit of Cranford, N.J.  
         [0028]    The MLC  30  may be clamped to a mounting ring  40 , which is then attached to the PCB  10  in a manner, as described below, that allows the ring&#39;s planarization to be adjusted. The mounting ring  40  is mounted to the PCB  10  using a plurality of screws  45 . A preferred configuration employs three screws. On each screw, between the ring  40  and the PCB  10 , are placed stiff springs  25 , as shown in FIG. 2. Suitable stiff springs include spring washers, such as Belleville washers manufactured by HK Metalcraft Inc. of Lodi, N.J. Most preferably, the screws  45  pass through the mounting ring  40  and spring washer  25 , such that turning the screws allows the planarity of the mounting ring to be adjusted. The MLC  30  may optionally rest on a shelf  55  in the mounting ring  40 . Preferably, the MLC  30  is clamped onto the shelf in the ring.  
         [0029]    Preferably, the mounting ring  40  is attached so that the MLC  30  is in close proximity with the surface of the PCB  10 . The resulting compression causes the MLC  30  to be held in the ring against the fuzz buttons  90  (as shown in FIG. 2) in the interposer  20 . As a result, the fuzz buttons make electrical connection between the MLC  30  and the PCB  10 . Since the electrical connection is not rigid, the PCB  10  is free to expand at a different rate than the MLC  30  while continuing to make electrical contact. A probe head (also called a test head)  50  (shown in FIG. 2) can now be placed in a tester that determines the degree of planarization of the probe head  50 . If adjustment is necessary, the appropriate adjusting screw  45  may be unscrewed slightly so as to level the probe head  50 . The Belleville washers  25  are selected to provide sufficient pre-load between the ring  40  and the PCB  10  so as to ensure that the ring does not move under the normal loads applied during the probe test process. As shown in FIG. 2, optional pads  185  may be deposited or brazed onto the MLC  30  or PCB  10   
         [0030]    In an alternative embodiment, as shown in FIG. 3, the PCB  10  has hollow plated vias or via holes  100 . Fuzz buttons  90  are inserted in the via holes  100 . Preferably, the fuzz buttons  90  have diameters slightly larger than the vias  100  so that the fuzz buttons  90  are held in place by compression. For example, the via holes  100  may be approximately 0.03 inches in diameter and the fuzz buttons  90  slightly larger. In this embodiment, no interposer (shown in FIG. 2 as numeral  20 ) is required, saving a significant portion of the cost of the interposer  20 , and eliminating the need for alignment of the frame to the PCB  10 . Accordingly, because alignment is inherently ensured, this embodiment has improved reliability.  
         [0031]    Preferably, the fuzz buttons  90  are inserted from the non-test side  160  of the PCB  10  and are pushed through until an electrical contact is verified between the fuzz button  90  and the MLC  30 . The non-test side refers to the side of the board opposite the test head  50 . Such a configuration ensures that the final height of all the fuzz buttons  90  relative to the MLC  30  are nearly the same. To simplify the assembly process, it may be desirable to temporarily use an aluminum plate in place of the MLC. Use of such a plate simplifies the verification of an electrical connection. Preferably, a tool that is electrically conductive is used to push the fuzz buttons  90  through the vias  100 , so that contact may be verified when there is a closed electrical path from the tool, through the fuzz button  90  to a temporary plate and back to the tool. This temporary plate (not shown) temporarily replaces the MLC  30  for purposes of loading the fuzz buttons  90  and is removed and replaced with the MLC  30 . It may be desirable to make the temporary plate slightly thicker than the actual MLC  30 , such that the fuzz buttons  90  are compressed against the temporary plate. Removing the plate and replacing it with the MLC  30  then allows the fuzz buttons  90  to relax slightly while maintaining electrical contact, thereby minimizing the compressive force exerted on the MLC  30 .  
         [0032]    This configuration ensures the alignment of the fuzz buttons  90  relative to the PCB  10 . To further simplify the alignment of the MLC  30  to the fuzz buttons  90 , additional pads  185  may be optionally employed to the MLC&#39;s contact array (not shown) on the side of the MLC opposite the test head  50 . The pattern of the MLC contact array matches the LGA pattern on the PCB  10 . Two corresponding holes in the PCB  10  may be used to sight through the PCB  10  to the pads. The MLC  30  may then be adjusted so as to center the pads in the hole. A preferred arrangement includes two alignment pins, such as those used in pin grid array MLC&#39;s, that are brazed to the pads. These pads pass through two corresponding additional via holes in the PCB  10 , thereby aligning the MLC&#39;s LGA array to the PCB&#39;s via array.  
         [0033]    In an alternative embodiment (not shown), the stiff Belleville washers  25  may be eliminated or replaced with lower-stiffness washers, and the planarizing screws  45  may be counter-sunk into the mounting ring. In this alternative embodiment, the ring may be free to float relative to the PCB  10  and self-planarize during testing, while the counter-sunk screws suitably re-center the ring when the test load is removed. The degree of float between the ring and the PCB is compensated by the compliant electrical contacts (fuzz buttons or conductive bumps).  
         [0034]    [0034]FIG. 3 a  depicts the use of site holes  200  to assist in the alignment of the assembly.  
         [0035]    In another embodiment, as shown in FIG. 4, longer and softer fuzz buttons may be used with a plurality of blind vias  110 . In this embodiment, the non-test end  160  of the vias  110  are filled such that the depth of the vias allow a portion of the fuzz buttons to protrude beyond the PCB  10 . In this embodiment, the diameter of the vias  110  may be larger than that of the fuzz buttons  90  as friction is no longer preferred to maintain the fuzz buttons  90  position.  
         [0036]    In an alternate embodiment, as shown in FIG. 5, an interposer  20  is constructed using a Pin Grid Array (PGA)  180  brazed to the contact pads of the MLC  30 . The PGA mates with and is inserted into plated vias in the PCB. Spring washers  25  (such as Belleville washers) are placed under the mounting ring  40  at a plurality of locations, preferably three. Adjustment screws  45  (also shown in FIG. 1) pass through the washers  25  to allow the planarity of the ring to be adjusted. Once properly planarized, the tips of the PGA, which protrude through the back side (non-test side) of the PCB, are soldered to the back side of the PCB vias  100 . Optionally, to shorten the electrical path of the pins  180 , a conductive paste, such as a conductive epoxy or solder, may be injected or screened into the vias on the test head side of the board. Preferably, the inside diameters of the vias are larger than the pins so that when the pins are inserted into the vias, the paste fills the space between the pins and the vias to create an electrical connection. Such a configuration shortens the electrical path so that shorter pins may be used. The MLC  30  may then be aligned using the method set forth above. Because an electrical connection has been created between the MLC  30  and the PCB  10 , the test head may be mounted to the mounting ring and MLC  30 , and a final planarity check may be done with the completed assembly. If necessary, the planarity may be adjusted. Once properly aligned, the assembly is cured, such as by a low temperature cure cycle, to set the conductive paste. Once planarity is verified, an epoxy filler may be injected under the MLC  30  to fill the gap between the MLC  30  and the PCB  10 , increasing the bond strength between the MLC  30  and the PCB  10 .  
         [0037]    In an alternative embodiment as shown in FIG. 6, fuzz buttons  90  may be replaced with compliant conductive bumps  120 . These conductive bumps  120 , preferably made of conductive epoxy, such as a metal or graphite polymer with an electrical resistivity less than 0.005 ohm-cm, are screened or deposited onto land grid array pads of the MLC  30  and/or the PCB  10 . When the MLC  30  and PCB  10  are brought into contact, the bumps  120  form a compliant electrical path between the MLC  30  and the PCB  10 . The test head (not shown) is then mounted to the mounting ring  40  and aligned with the MLC  30 . The test head&#39;s planarity is then tested to ensure proper planarity and adjusted, if necessary, by turning the adjustment screw  45  (of FIG. 1) to bring the high side of the head closer to the PCB  10 . The PCB  10  and space transformer  130  are then heated until the epoxy is cured, so that the space transformer  130  is held to the PCB  10 . An epoxy under-filler is then injected under the MLC  30 .  
         [0038]    Optionally, the conductive bumps may be made of a solder paste preferably using a low melting point solder, such as 100 degC. Preferably, the solder is solid at room temperature. As the temperature rises during testing, the solder softens and re-flows, eliminating mechanical stress while maintaining electrical contact. When cooled, the solder re-solidifies.  
         [0039]    Thus, there has been shown and described an apparatus and method to planarize a probe card assembly prior to testing. One skilled in the art would recognize that other embodiments and modifications may be useful without detracting from the overall purpose of this invention. Any such modifications and applications recognized by one skilled in the art is intended to fall within the scope of this application.