Abstract:
A test system including an apparatus for conveying signals between a first circuit board and a second circuit board includes a dielectric substrate having a first side forming a first surface and a second side forming a second surface. The apparatus also includes a plurality of contact pins each configured to convey electrical signals. Each of the contact pins may extend through the dielectric substrate and may protrude beyond the first surface and the second surface. In addition, one or more of the contact pins may be formed using a pliable resistive material.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to test systems and, more particularly, to boards having feed-through contacts for conveying signals of a device under test.  
         [0003]     2. Description of the Related Art  
         [0004]     Output signals from a device under test may be analyzed in a variety of ways. One way is to use a test circuit board or patch board. The output signals which are to be analyzed may be routed or coupled to the test board and then further routed to an analyzer port for connection to an analyzer. In some test systems, a test circuit board may be connected directly to the device under test using cables, connectors and sockets. In other test systems, the device under test may be mounted to a standard system board and the test circuit board may be coupled to the system board using cables, connectors or other means.  
         [0005]     Depending on the frequencies of the output signals, the loading placed on the output signals by the analyzer and by the wiring and traces of the test circuit board may be sufficient to distort the output signals. This distortion may cause incorrect measurements and may possibly even preclude normal system operation. Accordingly, when probing any signal it may be advantageous to keep the lead lengths of any probe wires as short as possible to reduce the amount of load that the probe adds to the output signal. In addition, it may be desirable to isolate the probe or test elements from the output drive of the device under test.  
       SUMMARY OF THE INVENTION  
       [0006]     Various embodiments of a test system including an apparatus for conveying signals between a first circuit board and a second circuit board. In one embodiment, the apparatus includes a dielectric substrate having a first side forming a first surface and a second side forming a second surface. The apparatus also includes a plurality of contact pins each configured to convey electrical signals. Each of the contact pins may extend through the dielectric substrate and may protrude beyond the first surface and the second surface. In addition, one or more of the contact pins may be formed using a pliable resistive material.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a block diagram of one embodiment of a test system.  
         [0008]      FIG. 2  is a perspective view drawing of one embodiment of the test system of  FIG. 1 .  
         [0009]      FIG. 3  is a cross-section of one embodiment of an interposer board of the test system of  FIG. 2 .  
         [0010]      FIG. 4  is a detailed view of a cross-section of one embodiment of an interposer board contact pin making a connection to a test board and a system board. 
     
    
       [0011]     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.  
       DETAILED DESCRIPTION  
       [0012]     Turning now to  FIG. 1 , a block diagram of one embodiment of a test system is shown. Test system  10  includes a device under test (DUT)  20 . DUT  20  includes an output signal contact  21  coupled to an input contact  31  of a receiver device  30  via a signal path  25 . In addition, output signal contact  21  is coupled to an input signal contact  51  of an analyzer unit  50  via signal path  45 . In addition, a series resistor R 1  is wired into signal path  45  and is shown as part of interposer board  160 .  
         [0013]     In one embodiment, DUT  20  may be a high performance processor, for example. During operation, DUT  20  may output signals on a variety of contacts. As described in greater detail below, in one embodiment, to capture and analyze the output signals on analyzer  50  the signals may be routed to analyzer  50  through an interposer  60  and test board (not shown in  FIG. 1 ). As will be described in greater detail below, interposer  160  may include a plurality of contact pins (not shown in  FIG. 1 ) which convey the signals.  
         [0014]     Depending on the frequency of the signal produced at output signal contact  21  of DUT  20 , the load created by signal path  45  and analyzer  40  may cause distortion of the output signal. Accordingly, lead lengths and associated test wiring should be minimized. In addition, series resistor R 1  may provide some signal isolation from analyzer  50 , thereby minimizing loading effects of the test circuit board and analyzer  50 . As will be described in greater detail below in conjunction with the description of  FIG. 2  through  FIG. 4 , series resistor R 1  may be implemented using a resistive material as the contact pin material of interposer  160 .  
         [0015]     Referring to  FIG. 2 , a perspective view drawing of one embodiment of the test system of  FIG. 1  is shown. Components corresponding to those shown in  FIG. 1  are numbered identically for clarity and simplicity. Test system  100  includes a device under test (DUT )  20  which may be mounted to a heat sink  120  and to a system board  150 . System board  150  is coupled to a test board  170  through an interposer  160 . Test system  100  also includes a backing plate  180  which provides a compressive mechanism to hold the various components together. Further, an analyzer connector  190  may be coupled to test board  170  via a corresponding connector on test board  170  for connection to an analyzer (not shown in  FIG. 2 ).  
         [0016]     Backing plate  180  may be used to provide a compressive force for “sandwiching” test board  170 , interposer  160 , system board  150 , DUT  20  and heat sink  120  together. In the illustrated embodiment, thumb-screws or other suitable fasteners may be used to fasten backing plate  180  to heat sink  120 . This arrangement may compress each contact on DUT  20 , interposer  160  and test chip  40  to their respective contact pads on their respective circuit boards.  
         [0017]     In the illustrated embodiment, DUT  20  uses a ball grid array (BGA) for its contact pinout. The BGA forms a given footprint pattern. The footprint pattern of DUT  20  is mated to a footprint pattern  155  on system board  150 . Footprint pattern  155  is provided on both the top and bottom surface of system board  150 . To keep lead lengths as short as possible, the footprint pattern on each board surface is symmetrically matching and also positioned opposite each other. Accordingly, a footprint pattern on the bottom surface of interposer  160  mates to footprint pattern  155  on the top surface of system board  150 . In addition, a footprint pattern on the top surface of interposer  160  mates to a footprint pattern on the bottom surface of test board  170 , and so forth. It is noted that although a BGA footprint pattern is used in the illustrated embodiment, other embodiments are contemplated in which other footprint patterns may be used.  
         [0018]     In one embodiment, system board  150  may be any circuit board which is used in the normal operation of DUT  20 . For example, if DUT  20  is a processor, system board  150  may be a processor motherboard. However, in other embodiments, system board  150  may be special circuit board designed to emulate a typical system environment as seen from DUT  20 .  
         [0019]     In the illustrated embodiment, test board  170  is a circuit board which provides signal paths for conveying output signals from DUT  20  to analyzer connector  190  for use by the analyzer (not shown in  FIG. 2 ).  
         [0020]     In the illustrated embodiment, interposer  160  may provide a means for conveying signals from system board  150  to test board  170  while allowing clearance of other components on system board  150 . In other words, interposer  160  may be a spacer which also conveys signals. In one embodiment, interposer  160  includes a plurality of contact pins  165  for conveying the signals. As described above, one or more of the contact pins may provide a series resistance, such as resistance R 1  of  FIG. 1 , to the signal that it conveys.  
         [0021]     It is noted that many conventional contact pin polymers used to convey signals are made using highly conductive materials (e.g., a silver-based polymer) having very low or even negligible resistance values.  
         [0022]     Turning to  FIG. 3 , a cross-section of one embodiment of the interposer of the test system of  FIG. 2  is shown. Components corresponding to those shown in  FIG. 1  and  FIG. 2  are numbered identically for clarity and simplicity. Interposer  160  includes a dielectric substrate  310  and a plurality of interposer contact pins  165 . The dielectric substrate also includes a plurality of through-holes  320  through which the contact pins  165  extend.  
         [0023]     In one embodiment, dielectric substrate  310  may be implemented using materials such as FR4, for example, which is commonly used to manufacture circuit board substrates. In addition, through-holes  320  may be bored completely through dielectric substrate  310 . Contact pins  165  may be positioned to extend through the through-holes and to protrude above the top and bottom surfaces of dielectric substrate  310 . As described above, contact pins  165  may be arranged across the surface of dielectric substrate  310  in a footprint pattern that matches a footprint pattern of another board such as footprint pattern  155  of  FIG. 2 , for example.  
         [0024]     In addition contact pins  165  may be implemented using pliable resistive material that may provide a compression connection when mated between system board  150  and test board  170 .  
         [0025]     To minimize the loading effects of analyzer  50  of  FIG. 1  and its associated wiring on the output signals of DUT  20 , in one embodiment, the pliable resistive material of contact pins  165  may be a polymer material that has a resistance value that is controllable and predetermined. The resistance may be increased or decreased during manufacture depending on how much resistance may be needed in a given application. As described above, the resistance value may provide a series resistance to the signals that are conveyed by interposer  160 . In one embodiment, the resistive polymer may be a carbon-based material although other embodiments may include other materials having suitable resistive properties may be used. It is note that in one embodiment, some of the contact pins  165  may use the conventional highly conductive materials while others may provide the series resistance using the pliable resistive material.  
         [0026]     In one embodiment, the resistance value of the resistive material may have the same order of magnitude as the characteristic impedance value of the signal traces and drives associated with the conveyance of the signal. Thus, in one embodiment, the resistive material may provide a resistance value greater than 5 ohms. For example, in one specific implementation, a resistance value of 20 ohms may be appropriate depending on the frequency of the signal, the signal trace characteristics and the impedance of the output driver of DUT  20 . This is in contrast to conventional conductive polymers which strive to keep the resistance value as small as practicable. It is noted that generally speaking, the resistive value should not be large enough to prevent propagation of the signals through interposer  160 .  
         [0027]     Referring to  FIG. 4 , is a detailed view of a cross-section of one embodiment of an interposer board contact pin making a connection to a test board and a system board is shown. Components corresponding to those shown in  FIG. 1  through  FIG. 3  are numbered identically for clarity and simplicity. Test board  170  includes a contact pad  176  that may be connected to a signal trace (not shown) for conveying signals to analyzer connector  190  (not shown in  FIG. 4 ). System board  150  includes a contact pad  156  that may be connected to a signal trace (not shown) for conveying signals to DUT  20  (not shown in  FIG. 4 ). Interposer  160  includes a contact pin  165 , which is in contact with contact pad  176  and contact pad  156 . As shown, a compressive force is being exerted such that the respective portions of contact pin  165  that protrude beyond each surface of interposer  160  have deformed to make electrical connections.  
         [0028]     As described above, contact pin  165  may be implemented using a resistive material such as a resistive polymer, for example, having a controllable and predetermined resistance which may provide a series resistance R 1  to a signal propagated through contact pin  165 .  
         [0029]     Although the embodiments above have been described in considerable detail, numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.