Abstract:
A probe test assembly for testing electronic devices maintained at lowered temperatures includes heaters and flows of dried air to prevent condensation from forming on the devices. The devices have pins received in an electrically non-conductive system board, and the probe test assembly includes a heat conductive and electrically conductive apertured probe plate to transfer heat from the heaters to the system board. Grounding pins extend from the probe plate to grounding pads on the system board, and a non-conductive apertured pattern plate spaced above the probe plate protects exposed ends of the grounding pins. In an alternate embodiment, the pattern plate is omitted in favor of a conductive intermediate plate and a superposed non-conductive contact plate. Signal pins extend from the system board to the contact plate in order to transfer contact points for electronic device from the system board to the contact plate. A multipin connector can be connected to the contact plate to enable the transmission of signals from the devices under test to a computer.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for making high frequency measurements of super cooled system boards for the purposes of testing and/or implementation of engineering changes. More specifically, the present invention relates to a probing system for super cooled system boards by which moisture condensation and corrosion formation on the test site are avoided. 
     In the testing of large systems during the initial bring up and including debugging of system hardware, special modifications are typically made to the product. A metal stiffener used to support the large system boards is machined so that an open access is provided to e.g., pins of a Device Under Test (DUT), such as a Multi Chip Module (MCM), as well as to other points of interest. A method of measuring system operations utilizing holes drilled through a probe template made of an insulating material offers a full range of interconnections at all signal locations and selected ground or voltage reference locations of the DUT. 
     With the ever increasing operational speeds of computer systems, including mainframes, it is becoming more and more difficult to provide accurate measurements of operational parameters such as switching noise and signal integrity, jitter measurements, measurements of differential signals, and differential measurements of voltage to ground disturbances. To achieve higher operational speeds, future systems require that the temperature of the electronics&#39; operational point be reduced to near zero degrees C and below. Any testing with one side of a system at extremely low temperatures and the other side in an exposed room environment will cause condensation to form. This condensation will, over a period of time, cause corrosion of exposed metal interface connections. This is permissible in a dedicated system that will be scrapped, but not for machines that are to be used over long periods of time. 
     TRADEMARKS 
     S/390 and IBM are registered trademarks of International Business Machines Corporation, Armonk, N.Y., U.S.A. and Lotus is a registered trademark of its subsidiary Lotus Development Corporation, an independent subsidiary of International Business Machines Corporation, Armonk, N.Y. Other names may be registered trademarks or product names of International Business Machines Corporation or other companies. 
     SUMMARY OF THE INVENTION 
     A probe test assembly according to the present invention which prevents condensation from forming on a Device Under Test (DUT) cooled to a temperature approaching 0âC. or even less than 0âC. includes a heat conducting probe plate mounted on one side of a large system board on the opposite side of which the DUT is mounted, with pins of the DUT extending into openings in the probe plate. Electrical resistance heaters are secured in heat-conducting relationship on the probe plate, which conducts the heat to the large system board and to the DUT, thereby raising the temperature of the pin side of the board above a level at which condensation forms. A large recess formed in the underside of the probe plate defines with the large system board a chamber which is pressurized by desiccated air is fed under pressure through passages in heat transfer relationship with the probe plate. Desiccated air under pressure is also fed directly to a space between the large system board and the DUT. Ground pins extend from openings in the probe plate to grounding pad locations at openings in the large system board. A non-conductive pattern plate having openings in alignment with the openings in the probe plate is space above the probe plate, and electric test probes are inserted through the aligned openings in the pattern plate and the probe plate to the openings in the large system board receiving the pins of the DUT. In view of the foregoing, the probe test assembly of the present invention provides for testing of DUT&#39;s which is non-permanent, non-destructive and free from condensation. 
     In an alternate embodiment, the pattern plate is omitted in favor of a conductive intermediate plate space above the probe plate and a non-conductive contact plate spaced above the intermediate plate. Both the intermediate plate and the contact plate have openings aligned with the openings in the probe plate. Ground pin connections between the contact plate and grounding pad locations at openings in the large system board are made either by ground pins each extending all of the way from the contact board to the large system board or by a first set of grounding pins extending from the contact plate to the intermediate plate and a second set of grounding pins extending from the probe plate to the large system board. Signal pins extend from openings in the large system board, through openings in the probe plate and the intermediate plate, all the way to the contact plate, thereby transferring contact points for the DUT from a confined area of the large system board to the contact plate, which is unconfined. A multipin connector connected to the signal pin receiving openings of the contact plate can be mounted on the contact plate, so that the probe test assembly can be connected to a computer. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, wherein: 
     FIG. 1 is a perspective view of a stiffener with a probe test assembly in accordance with the present invention; 
     FIG. 2 is an enlarged cross section of the probe test assembly in accordance with the present invention taken along the line  2 — 2  in FIG.  1  and positioned above a device under test; 
     FIG. 3 is an enlarged cross section of an alternate embodiment of the probe test assembly in accordance with the present invention; and 
     FIG. 4 is a further enlarged perspective view of a portion of the embodiment of FIG. 3, showing a different arrangement of signal pins and ground pins and an additional forward portion of the pattern board. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, a metal stiffener  10  used to support a large system board  11  has an opening  12 , sometimes called a “manhole”, defined (e.g., machined) therein. The use of a metal stiffener (or other supporting structure) to support a large system board is well known. The opening  12  in the stiffener  10  is located to provide access to an area of interest on the large system board  11 , such as the pin side of a Device Under Test (DUT)  13 , e.g., a Multi Chip Module (MCM), whose pins are received in openings in the large system board. It will be appreciated that the scope of the present invention encompasses providing access for testing (or other purposes) of any component that is normally covered by a stiffener and is not limited to an MCM. A probe test assembly  14  is positioned at the opening  12  when testing (e.g., a system test, such as when error injection and recovery, is required to understand and circumvent system failure mechanisms) is desired, thereby providing access to the pins of the DUT  13 , as is described hereinafter. The probe test assembly  14  is preferably shaped similar to the opening  12  in the stiffener  10 , although any shape may be employed. In the present example, the probe test assembly  14  is generally rectangular (as is the opening  12 ). 
     The probe test assembly  14  comprises a frame  22 , a pattern board or plate  24 , and a probe plate  26 , wherein the frame supports the pattern plate spaced above the probe plate. The frame  22  has opposing surfaces, with one of the surfaces facing and bearing on the stiffener  10 . A plurality of alignment pins (not shown) are received in alignment holes in the frame  22 , extending above and in corresponding alignment holes in the stiffener  10  to correctly position the frame  22 , and therefore the probe test assembly  14 , relative to the pins of the DUT  13 . The frame  22  has four mounting holes  28  therethrough (FIG. 2) which align with corresponding mounting holes  30  in the stiffener  10 . The probe test assembly  14  is secured onto the stiffener  10  by screws  32  in the mounting holes  28  and  30  or by other suitable fastening arrangement. The frame  22  is preferably comprised of an electrical insulation material, such as FR4, thereby insulating the probe plate  26  from the stiffener  10 . The probe test assembly  14  of this exemplary embodiment is particularly well suited for high frequency measurement applications, as described more fully hereinafter. Further, the probe test assembly  14  provides for nondestructive probing of the pins of the DUT  13 . 
     The pattern plate  24  has a lower surface facing the probe plate  26 . A pattern or array of holes  34  corresponding to the patter of pins on the DUT  13  is provided through the pattern plate  24  to provide an insulated guide path for a probe  36 . The pattern plate  24  has a plurality of mounting holes  38  therethrough which align with a plurality of mounting holes (not shown) in the probe plate  26 . The pattern plate  24  is secured onto the probe plate  26  by screws  41  through the mounting holes  38  and the mounting holes in the probe plate or by other suitable fastening means. The pattern plate  24  is preferably made of an electrical insulation material, such as FR4. Preferably, nomenclature (not shown) indicative of the I/O pins of the DUT  13  and their positions relative to the holes  34  of the pattern plate  24  is provided on the outer surface of the pattern plate. 
     The probe plate  26  has a surface facing the large system board  11 . A pattern or array of holes  42  corresponding to the input/output locations on the large system board  11  is machined through the probe plate  26 , which is made of a nonferrous metal, such as gold-plated brass. For clarity of illustration, only a few of the holes  42  have been shown, but it is understood that holes  42  are present all across the probe plate  26  and that there are actually thousands of such holes. The pattern of holes  34  in the pattern plate  24  may comprise a full compliment of the I/O locations in the probe plate  26 , thus providing access to all locations. Alternatively, the pattern of holes  34  in the pattern plate  24  may comprise a limited number of holes suitable for testing applications that require multiple testing of a limited number of signal locations. Such limited testing access limits the incidence of probing errors and the possibilities of causing a DUT to cease functioning, especially in an environment where the DUT is mission critical and can not be stopped. A plurality of alignment pins (not shown) are received in alignment holes in the probe plate  26  and in corresponding alignment holes in the frame  22  to position the pattern plate  24  and the probe plates  26  on the frame  22  and, ultimately, to position the pattern and the probe plates relative to the pins of the DUT  13 . The probe plate  26  has four mounting holes therethrough which align with a plurality of mounting holes in the frame  22 . The probe plate  26  is secured onto the frame  22  by screws (or other suitable fastening means) through these mounting holes. 
     In high frequency applications, the probe plate  26  is metal and is part of the measurement system. Resilient ground pins, or terminals,  50  are pressed into selected holes  42  in the metal probe plate  26  to provide a low impedance ground return path for test measurements. The ground pins  50  provide a permanent return path that is uniform and consistent every time the probe test assembly  14  is used. An exemplary ground path is from the probe  36 , through the metal probe plate  26  to a ground pin  50 , and through the ground pin  50  to a ground pad on the system board  11  which is contacted by the ground pin. The probe  36  is a high frequency probe which is used to access signal points (i.e., pins of the DUT  13 ) through the appropriate holes  34  in the pattern plate  24  and the corresponding holes  42  in the probe plate  26 , with the signal return path being provided by the close proximity of the ground pins  50 . The pattern plate  24  provides a non-conductive mechanical cover for the exposed ends of the grounding pins  50  in the metal probe plate  26 . As described hereinbefore, probe plate  26  is insulated from the stiffener  10  by the insulating material of the frame  22  to enhance measurement integrity, thereby insuring that the noise generated by other package components are not coupled in the measurements. 
     The DUT  13  is maintained at a temperature lower than ambient temperature, typically at a temperature nearly as low as 0âC. or even below 0âC. In contrast, the ambient temperature, that is, the temperature on the side of the large system board  11  opposite the side on which the DUT  13  is positioned, is typically room temperature. As a result, condensation tends to form on the side of DUT  13  facing the large system board  11 , including the pins of the DUT. In addition, the ends of the DUT pins are exposed to the relatively warm, moist ambient air, and the same air moves through unoccupied openings in the large system board  11  and into contact with other portions of the DUT  13 . 
     Electrical resistance heater coils  52  are mounted on the ambient in side of the probe plate  26  to heat the probe plate, particularly by conduction. In turn, the heated probe plate  26 , which covers the area of the large system board  11  in which the testing is being done and contacts a portion of the large system board surrounding that area, transfers heat to the DUT pins and the large system board  11 , and through the DUT pins and the large system board to other portions of the DUT  13 . A thermal pad  53 , for example, in the form of a thin copper strip, can be interposed between the perimeter of the probe plate  26  and to the large system board  11  to enhance heat conduction. The thickness of the thermal pad  53  has been exaggerated in FIGS. 2 and 3 for clarity of illustration. The thermal pad  53  can be an integral part of the large system board. A recess  54  is defined in the underside of the probe plate  26 , the recess extending over the entire area of the large system board  11  in which testing is being done. The heat of the probe plate  26  heats the air in the recess  54 . The electrical heater coils  52  are contained in housings  56  which are secured by, for example, screws  58  to the outer surface of the probe plate  26 , outside but adjacent to the area containing openings for the probes  36 . The housings  56  can be secured alternatively by welding or physical compression in a channel, or can be formed from one piece with the probe plate  26 , such as by casting. Leads  59  extend from the electrical heater coils  56  to a source of electrical power, and conventional equipment can be used to control the heat generated by the electrical heater coils. In this regard, a feedback system for temperature control can be provided by one or more thermocouples (not shown) imbedded in or attached to the probe plate  26 . 
     In addition, a plurality of dry air supply conduits  60  provide dry air under pressure to the recess  54  on the underside of the probe plate  26 . Since the probe plate  26  contacts the large system board  11  all along a line surrounding the recess  54 , the recess and the upper surface of the large system board define a chamber  61  pressurized with dry air under pressure from a source of dry air (not shown). The source of dry air removes moisture from air in a conventional way, such as by using desiccant crystals. As can be appreciated from FIG. 1, the conduits  60  are located in spaced positions on the pattern plate  24 , such as one of the conduits being located near each of the corners of the pattern plate. As can be seen from FIG. 2, each conduit  60  extends through the pattern plate  24  and the probe plate  26  into communication with the chamber  61 . A specific structure for a conduit  60  can be a passage  62  in the probe plate  26 , one end of a relatively rigid tube  64  received in one end of the passage and held there by, for example, a frictional fit, and an end of a flexible tube  66  connected to the relatively rigid tube  64 , the opposite end of the flexible tube being connected to the source of dry air. A flange  67  extends downward from the probe plate  26  between the passage  62  and the recess  54  to define with the large system board  11  a nozzle N. Dry air issues from the nozzle N and sweeps across the large system board  11  toward the center of the probe plate  26 , removing any moisture from around the ground pins  50 , the ends of the probes  36 , and exposed ends of pins of the DUT. 
     The dry air pressurizes the chamber  61  defined by the recess  54  and the large system board. As a result, dry air flows from the chamber  61  out through the openings  42  in the probe plate  26 , thereby preventing relatively moist air from entering the chamber. The dry air also flows from the chamber  61  through the openings in the large system board  11  and into a space  68  between the cold side of the large system board and the DUT  13 , thereby keeping dry the space  68  and the pin side of the DUT  13 . Furthermore, the movement of the dry air through the passage  62  in the probe plate  26  and into the recess  54  on the underside of the probe plate heats the dry air, thereby giving the air greater drying ability when it reaches the pins and other portions of the DUT. The thermal pad  53  coacts with a thermoelastic seal  69  on the underside of the probe plate  26  to help prevent the loss of dry, heated air from the chamber  61  through the sides of the chamber and the ingress of moist air into the chamber through the sides. 
     A test head alignment aperture  70  is defined in the large system board  11  to receive a test head alignment pin  72  associated with the test probe assembly to precisely align the openings  42  in the probe plate  26  and the openings  34  in the pattern plate  24  with the DUT pin receiving openings in the large system board  11 . Typically, the test head alignment pin  72  passes through alignment openings in the pattern plate  24  and the probe plate  26 . In the probe test assembly  14  of the present invention, the test head alignment pin  72  is a relatively rigid tube  74  connected by a flexible tube  76  to a source of dry air. As a result, dry air passes directly from the source, through the test head alignment aperture  70  in the large system board  11 , to the space  68  between the large system board and the DUT  13 . Since the relatively rigid tube  74  passes through the heated probe plate  26 , and the recess  54 , the dry air flowing through the tube is heated so as to provide additional drying ability. As an option, lateral apertures  79  can be provided in the tube  74  within the recess  54  to supply additional dry air to the chamber  61 . 
     As can be seen from FIGS. 3 and 4, in an alternate embodiment of the present invention, provisions are in a probe test assembly  14 ′ made to permit probes to contact points spaced above the large system board. Other than as specifically described and/or illustrated in connection with this embodiment, the embodiment has the same features as the previously described embodiment. Instead of the pattern plate  24 , an electrically conductive apertured intermediate plate  80  is positioned just above the probe plate  26 . The intermediate plate  80  is spaced above the openings  42  in the probe plate  26  to accommodate and protect portions of the ground pins  50  which project above the probe plate. The spacing is provided by a depending spacer frame  22 , like that of FIG.  2 . An apertured non-conductive contact board  84  is supported at its perimeter on the intermediate plate  80 , with a large central portion of the contact board spaced above apertures  86  in the intermediate plate  80 . This spacing can be provided by a seal frame  86  extending along the underside of the perimeter of the contact board  84 . The contact board  84  is secured to the intermediate plate  80  by screws or the like (not shown) extending through the seal frame  86 . The positions of the apertures in the contact board  84  correspond to the positions of the pins of the DUT  13  for both measuring and grounding purposes. 
     Other apertures can be provided in the contact board  84  for grounding purposes. The test head alignment pin  72  also passes through precisely positioned alignment openings provided in the intermediate plate  80  and the contact board  84 . Probe pins or signal pins  88  extend from pins of the DUT  13  exposed in the openings of the large system board  11 , through aligned openings  42  in the probe plate  26  and the intermediate plate  80  to apertures in the contact board  84 . As a result, contact points for the DUT are transferred up from the large system board  11 , in an area confined by portions of the probe test assembly  14 ′, to an unconfined area on an opposite side of the probe test assembly and on an opposite side of the heating arrangement from the large system board. The unconfined area is especially significant because large numbers of DUT inputs and outputs are often used f simultaneously, and the confined area within the probe test assembly  14 ′ is typically  6  inches by  6  inches. As can be seen from the left side of FIG. 3, grounding can be provided by a first level of grounding pins  50  extending from grounding pads on the large system board  11  to the probe plate  26  and a second level of grounding pins  50  extending from the intermediate plate  80  to grounding pads at apertures in the contact board  84 . As an alternative to the two levels of grounding pins  50 , double sided grounding pins  90  can be used which extend from the large system board  11 , through openings in the probe plate  26  and the intermediate plate  80 , to the contact board  84 . 
     A multi-pin interface connector  92  or other surface mount connection apparatus is provided on the contact board  84  so that a computer can be connected quickly and easily to the probe test assembly to record and analyze measurements obtained therefrom, especially when a large number of signals are transmitted at one time and/or when repeated measurements are to be made. Conductive paths are provided on the contact board  84  from apertures receiving the signal pins  88  to apertures receiving pins  94  of the multi-pin interface connector  92 . The grounding pins  50  or the double-sided grounding pins  90  in the arrangements described above provide return current paths for the multi-pin interface connector  92  to the large system board  11 . 
     While the preferred embodiment to the invention has been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.