Abstract:
A thermal head adapter for testing a device under test is provided that can accommodate a large device and will improve the airflow through the thermal head to the device under test and out into the shroud. The thermal head adapter comprises a first section with a first perimeter and a second section with a second perimeter. The shroud is sealed onto an upper surface of first section, and the base of the second section attaches to a printed board. The perimeter of the first section is greater than the perimeter of the second section. The upper surface of the first section may comprise ridges that effectively form a moat-like structure to capture fallen condensation from the shroud walls. A drain may take the liquid within the boundary of the ridges to a desired location outside of the thermal head adapter.

Description:
GOVERNMENT RIGHTS 
       [0001]    This invention was made with Government support under Prime Contract Number N00030-05-C-0007 awarded by the United States Navy. The Government may have certain rights in this invention. 
     
    
     FIELD 
       [0002]    The present invention relates generally to thermal testing of a device under test. More particularly, the present invention relates to a thermal head adapter used with a precision temperature forcing system. 
       BACKGROUND 
       [0003]    A precision temperature forcing system (PTFS) provides a low-cost means to thermally test a device under test (DUT). The thermal head of a PTFS is designed for coplanar positioning of the bottom edges of its thermal cap and glass shroud. This usually involves pressure sealing the bottom edges of the thermal cap and shroud directly to the host printed board (PB) of the DUT. A compressible gasket allows for the thermal cap and shroud to seal against the PB. The air nozzle is retractable against a spring for sealing the bottom edge of the thermal cap to the PB. 
         [0004]    The thermal cap is attached to the air flow nozzle of the PTFS thermal head and directs temperature controlled air directly onto the DUT and then out through its vent holes into the shroud area. The thermal cap is intended to direct air flow onto the DUT and minimize the air volume directly around the DUT, reducing the air flow rate necessary to force the DUT to the target temperatures. 
         [0005]    However, standard conductive or nonconductive silicone rubber thermal caps accommodate only a limited range of component sizes, wherein the component is a direct-mounted DUT or a DUT mounted in a test socket. When a DUT or its test socket is too large to fit inside a standard thermal cap, or the thermal cap cannot be retracted far enough to seal to the top of the DUT or its test socket, the thermal cap can be omitted. However, once the thermal cap is omitted the entire shroud air volume must be forced to the target temperatures. The extra thermal load slows down temperature transition times and also requires higher air flow rates, which can cause condensation and icing issues at extended cold temperatures. 
         [0006]    In an attempt to solve this problem, the PB area around the large test socket is built up using a material such as electrostatic discharge (ESD) foam to raise the shroud footprint up to the top of the test socket. With this configuration, the thermal cap can seal to the top the test socket. However, only a small portion of the DUT body surface is exposed to the forced air, resulting in a rather poor thermal transfer between the forced air and the DUT. Additionally, this built-up footprint requires a large “keep out” area around the DUT so that the ESD foam may properly seal to both the thermal head shroud and host PB. 
         [0007]    Finding a material suitable for adapting to a larger than standard DUT is also problematic. The material must be pliable and compressible to provide a good air seal. Conductive and nonconductive silicone foam rubber sheets are compatible with the temperature ranges but they are very expensive and the nonconductive foam presents electrostatic discharge (ESD) issues. Either conductive silicone foam rubber or standard electrostatic discharge foam can cause electrical leakage currents across exposed PB surface solder pads and circuit traces. Typical standard electrostatic discharge foam, however, has a tendency to deform, shrink and become brittle with multiple temperature cycles. This leads to air leakage which can result in condensation and icing issues. Characterization and production testing requires a durable and reliable solution for thermal testing a DUT or a test socket containing a DUT that is larger than standard thermal cap sizes. This is especially challenging when a DUT must be tested over a wide temperature range (e.g. −55° C. to +125° C.). 
       SUMMARY 
       [0008]    In accordance with the present invention, a thermal head adapter for testing a device under test (DUT) is provided. This thermal head adapter can accommodate a large DUT or a test socket containing a DUT and will improve durability and reliability for thermally testing a large DUT or a DUT test socket while requiring a much smaller printed board (PB) footprint. 
         [0009]    The thermal head adapter interfaces the PB to the shroud. The thermal head adapter comprises a first section with a first substantially circular perimeter and a second section with a second perimeter. The shroud is pressure sealed onto an upper surface of first section, and the base of the second section is pressure sealed to the PB. The perimeter of the first section is greater than the perimeter of the second section. The upper surface of the first section may comprise ridges that effectively form a substantially circular moat-like structure to capture fallen condensation from the shroud walls. A drain may take the liquid within the boundary of the ridges to a desired location outside of the thermal head adapter. A nitrogen port may be located within the first section and may carry dry nitrogen gas from an outside source into the shroud. The thermal head adapter has a cavity that runs through both the first section and the second section, allowing for the placement of the thermal head adapter over a DUT or a DUT test socket. Flexible foil heaters with integral temperature sensors may be bonded to the exterior of the base near the PB interface and to the exterior opposite the thermal head shroud footprint. The heaters maintain the surface temperature of the thermal head adapter above the dew point to prevent condensation from moist room air. 
         [0010]    This thermal head adapter allows for all forced air to flow down through the precision temperature forcing system&#39;s air nozzle and thermal cap, and go directly onto the DUT or the exposed portion of the DUT in a test socket, out the thermal cap vent holes and into the shroud area. This minimizes the thermal load required to force the DUT to the proper temperature, since the thermal cap no longer needs to be omitted for a larger than standard DUT or a DUT test socket. The additional advantages associated with this are improved reliability and reduction of cost and schedule associated with DUT temperature testing. Without condensation and icing issues, long thermal cycles can be automated and unmanned. 
         [0011]    The PB footprint size is also minimized. This frees up PB space for other components around the DUT and/or a smaller PB. Additionally, the thermal head shroud and PB interfaces are displaced vertically from each other using the adapter, allowing for each to be independently optimized. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Various embodiments are described herein with reference to the following drawings. Certain aspects of the drawings are depicted in a simplified way for reason of clarity. Not all alternatives and options are shown in the drawings and, therefore, the invention is not limited in scope to the content of the drawings. In the drawings: 
           [0013]      FIG. 1  is a perspective view of a thermal head adapter according to one embodiment of the invention; 
           [0014]      FIG. 2  is a top view of a thermal head adapter of  FIG. 1  placed over a DUT test socket; 
           [0015]      FIG. 3  is a cross-sectional A-A view of the thermal head adapter and DUT test socket of  FIG. 2  and its host PB; 
           [0016]      FIG. 4  is a cross-sectional view of an alternative thermal head adapter and DUT test socket embodiment; and 
           [0017]      FIG. 5  depicts a precision temperature forcing system (PTFS) with its thermal cap pressure sealed to the top of the DUT test socket and its shroud pressure sealed to the top of the thermal head adapter of  FIG. 1  in the operating position. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1  depicts a perspective view of a thermal head adapter  100  according to one embodiment of the present invention. Thermal head adapter  100  is provided for use with a precision temperature forcing system (PTFS) for interfacing a printed board (PB) with a DUT or a DUT test socket to the thermal cap and shroud of the thermal head of the PTFS. 
         [0019]    Thermal head adapter  100  comprises a first section  110  and a second section  120 . First section  110  comprises an upper surface  112  and a lower surface  114 . A shroud (not shown) of a PTFS is pressure sealed to upper surface  112 . A first ridge  116  is formed along the perimeter of upper surface  112  of first section  110 . If the perimeter of first section  110  is circular in shape, first ridge  116  may be circular in shape as well, as shown in  FIG. 1 . A second ridge  118  is formed on upper surface  112  at a substantially uniform distance from the first ridge  116 . Second ridge  118  has a smaller perimeter than first ridge  116 , as shown in  FIG. 1 . Thermal head adapter  100  also includes a cavity  130 , which is the space between DUT test socket  190  and the inner wall of both first section  110  and second section  120 . 
         [0020]    First section  110  and second section  120  may be manufactured as a single piece. Alternatively, first section  110  and second section  120  may be manufactured as separate pieces, and may either be permanently or removably affixed to each other. First section  110  and second section  120  may be made from a molded plastic. The material used to manufacture first section  110  and second section  120  is preferably compatible with the temperature range of the precision temperature forcing system (e.g. −90° C. to +225° C.). The material used for first section  110  and second section  120  should at least be compatible with the temperature ranges used while testing the DUT, e.g. −55° C. to +125° C. 
         [0021]    First ridge  116  and second ridge  118  may be molded out of the same piece of material as first section  110  and second section  120 . First ridge  116  is at a height superior to upper surface  112 . Second ridge  118  is at a height superior to upper surface  112 . First ridge  116  and second ridge  118  may be the same height. Alternatively, first ridge  116  and second ridge  118  may be different heights. 
         [0022]    Condensation from moist room air may form on the exterior walls of the shroud. This condensation may then fall down the exterior walls of the shroud and compile as a liquid on upper surface  112 . A first region  132  bounded by first ridge  116 , upper surface  112 , and second ridge  118  is formed to contain the fallen liquid. Condensation that lands on upper surface  112  would fall within first region  132 . When affixing the shroud to upper surface  112 , the shroud may be placed at any location on upper surface  112  within first region  132 . First ridge  116  and second ridge  118  should be a height sufficient to contain the condensation that falls on upper surface  112  from the shroud and direct it towards a drain  160  (not shown). 
         [0023]    A second region  134  comprises the portion of upper surface  112  that is located between second ridge  118  and cavity  130 . 
         [0024]    Second section  120  comprises at least one sidewall  122  and a base  124 . The perimeter of base  124  of second section  120  is less than the perimeter of first section  110 . Base  124  is pressure leaded against the PB. 
         [0025]    Cavity  130  is sized to accommodate a DUT or a DUT test socket. Cavity  130  runs through both first section  110  and second section  120 . Cavity  130  of thermal head adapter  100  may be aligned with a DUT or a DUT test socket, wherein the test socket is already set up on the PB. 
         [0026]      FIG. 2  is a top view of thermal head adapter  100  according to one embodiment of the invention.  FIG. 2  shows the thermal head adapter  100  placed over a DUT test socket  190  mounted to a host PB  192 . A thermal cap of the thermal head (not shown) will be pressure sealed to the top of DUT test socket  190  in operation. Because the top surface of DUT test socket  190  may not be perfectly flat, a test socket interface  140  may be placed on top of DUT test socket  190  to provide a smooth, flat surface for a good air seal with the bottom edge of the thermal cap of the thermal head. Test socket interface  140  may be a plate with a flat top surface  142  and a hole  144 . Test socket interface  140  may be rectangular-shaped. Alternatively, test socket interface  140  may comprise other shapes as well. Hole  144  should align with the exposed portion of the DUT in DUT test socket  190  to allow for maximum airflow onto the DUT. The material used to manufacture test socket interface  140  is preferably compatible with the temperature range of the precision temperature forcing system (e.g. −90° C. to +225° C.). The material used for test socket interface  140  should at least be compatible with the temperature ranges used while testing the device under test, e.g. −55° C. to +125° C. 
         [0027]      FIG. 3  is a cross-sectional view of section A-A of thermal head adapter  100  and a DUT test socket  190  mounted to host PB  192  from  FIG. 2 . In  FIG. 3 , the cross-section A-A illustrates the component parts of thermal head adapter  100  and how they interface with the DUT test socket  190  and host PB  192 . 
         [0028]    A foam gasket  126  may be affixed within a hollowed-out portion of base  124 . Foam gasket  126  would allow for compression when thermal head adapter  100  is either manually or mechanically pressed down onto host PB  192 , enabling a compression-seal of thermal head adapter  100  to the host PB  192 . 
         [0029]    In one aspect of the invention, the thermal management system may utilize dry nitrogen for forcing air temperature within the shroud closer to room temperature. For this situation, a port  150  extends from a port inlet  152  through a port outlet  154  on upper surface  112 . Port outlet  154  is located within second region  134  on upper surface  112 . Port  150  is provided for injecting dry nitrogen into the shroud area. Within the shroud area, the injected room temperature dry nitrogen mixes with the DUT exhaust air to bring shroud air closer to room temperature. A plug  156  may be inserted into port  150  when port  150  is not in use. Alternatively, port  150  may be located in second section  120 . 
         [0030]    A drain  160  extends from a drain inlet  162  on upper surface  112  through a drain outlet  164 . Drain inlet  162  of drain  160  is located within the first region  132  on upper surface  112 . Condensation forms on the outside walls of the shroud and falls to first region  132  on upper surface  112 , and flows through drain inlet  162 , exiting through drain outlet  164  into an appropriate device, such as a tube or a container. Drain  160  allows for condensation to be properly removed from thermal head adapter  100 . 
         [0031]    A plurality of heaters  170  may be attached to lower surface  114  of first section  110  or a sidewall of the at least one sidewall  122  of second section  120 .  FIG. 3  shows heaters  170  on both lower surface  114  and sidewall  122 . Alternatively, a single heater may be used in some embodiments. The plurality of heaters  170  may be flexible foil heaters. The plurality of heaters  170  may be bonded to the surfaces of thermal head adapter  100 . The plurality of heaters  170  may have integral temperature sensors that are bonded to the exterior sidewall  122  of the base second section  120  near the PB interface and to the exterior lower surface  114  of first section  110  opposite the thermal head footprint. The plurality of heaters  170  would provide a means of keeping the outside surface of thermal head adapter above the dew point of room air to prevent condensation and icing. The plurality of heaters  170  may be Minco Flexible Thermofoil™ heaters. 
         [0032]      FIG. 4  is a cross-sectional view depicting an alternative embodiment of thermal head adapter  400 . In this embodiment, an upper shroud interface  410  and a lower PB interface  420  are separate pieces that are removably affixed to one another at a common circular interface  430 . The common circular interface has a gasket seal to affix upper shroud interface  410  to lower PB interface  420 . Thus, various sized lower PB interfaces are interchangeable for use with the same upper shroud interface  410 . Various sized upper shroud interfaces may be interchangeable for use with the same lower PB interface  420 . For example, if the testing situation requires a smaller lower PB interface, a smaller sized PB interface may be selected from a range of various sized PB interfaces and sealed onto upper shroud interface  410 . If a subsequent testing procedure requires a larger sized lower PB interface  420 , the previous lower PB interface may be removed and a larger sized lower PB interface  420  may be sealed onto upper shroud interface  410 . This embodiment allows for various combinations of upper shroud interfaces  410  and lower PB interfaces  420  to be assembled to meet various thermal testing applications. 
         [0033]    In operation, a DUT test socket  190  is affixed (e.g. soldered) to PB  192 , as shown in  FIG. 5 . Cavity  130  of thermal head adapter  100  is then aligned with DUT test socket  190  so that when thermal head adapter  100  is placed over the DUT test socket  190  and onto PB  192 , the DUT test socket  190  rests within cavity  130 . Thermal head adapter  100  may be pressed down onto the PB  192 , compressing foam gasket  126  within base  124  so that base  124  is pressure sealed to the PB  192 . A shroud  194  of a thermal head is placed onto upper surface  112  within first region  132 . A thermal cap  196  of a thermal head is placed onto test socket interface  140 . 
         [0034]    Once the DUT is in position in the DUT test socket  190  and is ready for testing, forced air from the thermal head flows through hole  144  and/or cavity  130 , and onto the exposed portion of the DUT. The majority of the forced air exits the vent holes of the thermal cap. Some of the forced air flows through the test socket and across the DUT, the air then moves out the sides and bottom of the test socket, exiting the test socket and flowing up into the shroud area. The air eventually flows out of the shroud through thermal head vents (not shown). The air that enters the shroud may be cool or cold air. To warm this cold air, room temperature dry nitrogen gas may be injected via port  150  into the shroud. The room temperature dry nitrogen gas mixes with the cold thermal cap and DUT test socket  190  exhaust air to reduce the temperature differential across the shroud walls and condensation and icing on its exterior walls from moist room air. 
         [0035]    If moist room air against the outside walls of the shroud is cooled to its dewpoint, condensation may form on the outside walls of the shroud. When this condensation falls down the outside wall of the shroud, it will land as a liquid in first region  132  on upper surface  112  of thermal head adapter  100 . The liquid then enters drain inlet  162  and flows through drain  160  to exit through drain outlet  164 . The capturing and draining of the condensed liquid keeps moisture from entering the DUT test socket  190  and damaging the device under test (DUT), its host PB  192  or other components or test equipment. 
         [0036]    Once the testing of the DUT is finished, thermal cap  196  and shroud  194  of the thermal head are mechanically lifted off the test socket interface  140 , and upper surface  112  of first section  110 . 
         [0037]    Thermal head adapter  100  vertically displaces the shroud and the PB interfaces so that each can be independently optimized. Because of the thermal head adapter&#39;s design, the PB footprint size can be minimized. Minimization of the PB footprint size frees up space for other components to be placed around the DUT test socket  190 . 
         [0038]    Variations and modifications of the present invention will be obvious to those skilled in the art and it is intended to cover in the appended claims all such modifications and equivalents. The entire disclosures of all references, applications, patents, and publications cited above, are hereby incorporated by reference.