Abstract:
A technique for enhancing thermal coupling between a device and a thermally conductive material includes using porous portions to draw fluid through conduits. Capillary action then draws fluid from the porous portion into a space between the device and the thermally conductive material to provide a fluid layer between the device and the thermally conductive material.

Description:
TECHNICAL FIELD 
     The present invention pertains to device fixtures and, more particularly, to cooled device fixtures. 
     BACKGROUND 
     Many semiconductor devices, such as, for example, radio frequency (RF) semiconductor devices, are manufactured in factories including equipment for individually testing the electrical performance of each device. One such test is commonly referred to as a burnout test in which heat dissipation and thermal conditions under which the device is operated may be extreme and may lead to device failure if the device is not adequately cooled during testing. Because devices that are individually tested tend to sell for relatively high prices, any yield degradation caused by testing directly impacts the profit of the company. For example, for every $100 RF device damaged at test, the company will not realize the $100 of revenue from the sale of that device. 
     Presently, when devices are individually tested, each device is placed in a specially designed cooling fixture including a conduction-cooled heat sink that may have an associated fan. After the device has been placed on the cooling fixture, it is clamped into place to prevent movement of the device and to allow the device under test to conduct heat to the cooling fixture. It is not uncommon for the device to be clamped into the cooling fixture with a clamp force of as much as 30 pounds (lbs.), which can lead to unintended damage of potentially fragile parts or structures inside the device. Additionally, the 30 lb. force can be unwieldy and difficult to control. 
     Because the device and the test fixture are not perfectly planar, there exists a small gap between the bottom face of the device being tested and the top of the cooling fixture when the device is placed on the cooling fixture. For example, the gap may be due to surface roughness and features on each of the mating interfaces. The air gap between the device under test and the cooling fixture inhibits thermal conduction between the device and the fixture, thereby preventing the device from easily coupling its heat to the cooling fixture and resulting in device heating that may result in increased device die temperature. Accordingly, to enhance the thermal conduction path between the device and the cooling fixture, a thin layer of thermally conductive grease such as, for example, Wakefield grease is commonly applied to the contact surface of the device before the device is clamped into place on the cooling fixture. Such grease is a non-water soluble thermal conductor. While the Wakefield grease aids in thermal conduction, grease thickness and air pockets in the grease may lead to inconsistent or unpredictable thermal conduction during device testing. 
     After testing of the device is complete, the clamp holding the device to the fixture is released and the device is manually removed from the fixture using equipment such as tweezers. An operator then uses cotton swabs and a methanol based solvent to remove the grease from the device that has been tested and the device is placed into a sorting bin representative of the electrical characteristics of the device. Care must be taken to ensure that all grease residue is removed from devices because, once purchased, devices are commonly soldered into place as parts of systems or subsystems. Failure to remove absolutely all of the Wakefield grease residue from the device would contaminate the soldering process, thereby yielding cold solder joints, poor bonding and potentially open circuits. In practice, however, some of the grease residue will always remain on the device. Whether such residue affects manufacturing processes depends on the quantity of residue. 
     As will be readily appreciated, the foregoing process requires manual labor to apply the Wakefield grease to the device to be tested and to remove the grease from the device after testing is complete. Because certain devices are 100 percent tested (i.e., each device leaving the factory is tested) the manual labor costs associated with device testing could be considerable. In fact, while electrical testing of devices may require on the order of 50 seconds of testing time, the manual labor associated with applying the Wakefield grease to the device before testing and removing the same from the device after testing may equal the test time, thereby doubling the process time for testing a device. Accordingly, not only is the use of the Wakefield grease expensive in terms of manual labor costs, it is expensive in terms of product throughput time. Furthermore, some grease residue will always remain on the device, which could affect the processing of the device by the purchaser. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exemplary isometric view of a test fixture; 
     FIG. 2 is an exemplary assembly view of the reservoir, coupler and fixture insert of FIG. 1; 
     FIGS. 3-5 are exemplary plan, side elevational and end elevational views, respectively, of the fixture insert of FIGS. 1 and 2; 
     FIGS. 6-8 are exemplary plan, side elevational and end elevational views, respectively, of the reservoir of FIGS. 1 and 2; 
     FIGS. 9-11 are plan, side elevational and end views of the coupler of FIGS. 1 and 2; 
     FIGS. 12 and 13 are exemplary elevational views of a device disposed on the fixture insert of the foregoing drawings; and 
     FIG. 14 is an exemplary flow diagram illustrating one manner in which the components of the foregoing drawings may be used to test devices. 
    
    
     In the following description, common reference numerals refer to common structures of features. 
     DETAILED DESCRIPTION 
     Turning now to FIG. 1, a test fixture  10  generally includes a block  12  on which first and second test circuits  14 ,  16  may be fastened and in which a fixture insert  18  may be installed. The test fixture  10  may also include input/output ports  22  that may be coupled to the first and second test circuits  14 ,  16  to provide signals thereto or to receive signals therefrom. A heat sink  24 , such as a finned heat sink, may also be mounted to the block  12  to enable the block  12  to more rapidly dissipate heat that may be generated by a device under test. In general, both block and finned heat sinks may be fabricated from good thermal conductors such as copper, aluminum and the like. The test fixture  10  may also include a coupler  30  that is fastened between the fixture insert  18  and a reservoir  32  adapted to hold a fluid, such as, for example, distilled water. As described below in further detail, the fluid may be used to cool a device placed on the fixture insert  18 . 
     For ease of explanation, only the fixture insert  18 , the coupler  30  and the reservoir  32  are shown in FIG.  2 . The fixture insert  18  includes first and second porous portions  34 ,  36  that are inserted into apertures or slots in the fixture insert  18 . The fixture insert  18  also includes a conduit  38  in fluid communication with each of the slots and the porous portions  34 ,  36 . Threaded bores  40 ,  42  are provided in the fixture insert  18  to accommodate screws that fasten the coupler  30  to the fixture insert  18 . A first O-ring (not shown) may be used to seal a conduit  44  in the coupler  30  to the conduit  38  of the fixture insert  18 . The coupler  30  also includes threaded bores  46 ,  48 , or any other suitable features, to accommodate screws that fasten the reservoir  32  to the coupler  30 . The reservoir  32 , which is adapted to hold a liquid such as, for example, distilled water, also includes a conduit  50  in fluid communication with the conduit  44  of the coupler, thereby putting the reservoir  32  in fluid communication with the porous portions  34 ,  36 . A second O-ring (not shown) may be provided between the reservoir  32  and the coupler  30  to seal the junction between the conduit  50  of the reservoir  32  and the conduit  44 . 
     Optionally, a wicking member may be disposed within the vertical portion of the conduit  44  to aid the capillary effect in wicking the fluid from the reservoir  32  up to the conduit  38  of the fixture insert  18 . For example, a threaded shaft of a bolt having an outer diameter smaller than the diameter of the conduit  44  may be placed within the conduit  44  to enhance the capillary effect. 
     In one exemplary embodiment, the fixture insert may have dimensions of 1.25 inches by 3 inches and the slots or apertures for receiving the porous portions  34 ,  36  may have dimensions of 0.5 inches by 0.1 inches. Additionally, the radius of the conduit  38  and the fixture insert  18  may be 0.07 inches and the conduits  44 ,  50  may be similarly sized. 
     The porous portions  34 ,  36  may be separately milled and inserted into the fixture insert  18  by a friction or interference fit or by any other suitable methods including adhesives or mechanical fasteners. The porous portions  34 ,  36  may be fabricated from, for example, sintered metals such as titanium, brass, copper, stainless steel or other metals that will not react or corrode when exposed to the reservoir fluid, which may be, for example, distilled water or any other non-residue fluid. Alternatively, the porous portions  34 ,  36  could be fabricated from screen material, metal cloth, plastic or any other suitable synthetic or natural that would act as a wick. 
     In operation, due to the capillary effect, fluid from the reservoir  32  passes through the conduit  50  to the conduit  44  and from the conduit  44  to the conduit  38 . Upon reaching the conduit  38 , the porous portions  34 ,  36  wick the water from the conduit  38  up to the top faces of the porous portions, which are disposed substantially co-planar with the top face of the fixture insert  18 . Because it is a capillary effect that draws the fluid from the reservoir  32  to the porous portions  34 ,  36 , the reservoir  32  may be located at a position lower than the fixture insert  18 . Additionally, while the fixture insert  18  is shown in FIGS. 1 and 2 as being horizontally oriented with the faces of the porous portions  34 ,  36  facing upwards, the test fixture  10  and its fixture insert  18  may be oriented at any suitable angle and the capillary effect that draws the water from the reservoir  32  to the porous portions  34 ,  36  will continue to operate. Furthermore, even if the faces of the porous portions  34 ,  36  are oriented to be downwards, water will not leak from the porous portions  34 ,  36  due to the capillary effect and the surface tension of the fluid within the porous portions  34 ,  36 . Accordingly, the arrangement of the reservoir  32 , the coupler  30  and the fixture insert  18  shown in FIGS. 1 and 2 is merely exemplary and other arrangements of these components is contemplated. In fact, it is possible to eliminate the coupler  30  in favor of directly connecting the reservoir  32  to the fixture insert  18 . 
     To this point the operation of the capillary effect to wick water from the reservoir  32  through the coupler  34  and up to the upper faces of the porous portions  34 ,  36  has been described. However, with reference to FIGS. 12 and 13, a secondary capillary effect is described, whereby water from the porous portions  34 ,  36  is wicked across an interface between the fixture insert and a device under test  60 , which may be, for example, a semiconductor device or any other device. As shown in FIG. 12, the interface between the device under test  60  and the fixture insert  18  is shown as being partially filled with fluid  62 . Although FIG. 12 shows the fluid  62  as occupying only a portion of the interface between the device under test  60  and the fixture insert  18 , it will be readily understood that such an illustration is merely for instructional purposes and, in practice, the fluid from the porous portions  34 ,  36  would wick across the entire interface between the device under test  60  and the fixture insert  18  as shown in FIG.  13 . 
     Accordingly, the second capillary effect, which wicks water between the device under test  60  and the fixture insert  18  aids in conducting heat from the device under test  60  into the fixture insert  18 , which in turn passes heat to the block  12  that includes the heat sink  24 . Additionally, the elimination of the thermal grease leaves the device  60  free from residue of non-water soluble thermal conductor, such as Wakefield grease. 
     Preliminary testing reveals that the use of the disclosed cooling technique can increase product test yield by as much as 10 percent and may save 30 percent on pre and post-preparation costs. Additionally, the use of the disclosed technique may enhance the die thermal transfer performance during device testing by as much as 45 percent over the use of the Wakefield grease technique. Further, the use of the disclosed technique may enhance package thermal conduction by as much as 47 percent over the use of the Wakefield grease technique. It has been estimated that the cost savings of the disclosed technique may be several hundreds of thousands of dollars across many product lines that are presently tested using the Wakefield grease technique. 
     Referring to FIG. 14, a test process  70  is shown. The test process  70  begins at block  72  during which a device is selected to be tested. The selection process may include a robotic arm (not shown) lifting a device to be tested from a tray using vacuum force to retain the device on the robotic arm. After the device to be tested has been selected at block  70 , control passes to block  74 , at which point the selected device is placed on the fixture insert with an appropriate amount of applied force. The applied force may be on the order of 5 lbs. and may, in fact, be provided by the robotic arm that selected the device in block  72  described above. Relevant to the prior disclosure of the capillary action carried out by the conduits  38 ,  44  and  50  and the interface between a device under test  60  and the porous portions  34 ,  36  of the fixture insert  18 . After the device is placed on the fixture insert  18  with the appropriate force at block  74 , the interface between the device  60  and the fixture insert  18  is filled with water by capillary action, thereby aiding the heat sinking of the device to the fixture insert  18 . 
     After the device  60  has been placed on the fixture insert, block  76  carries out electrical performance testing, which may include any number of standardized electrical test or any other suitable tests. After the completion of electrical performance testing, control passes from block  76  to block  78  at which point the device  60  is removed from the fixture insert  18 . 
     Block  78  may be carried out by the same robotic arm that was used in block  72  and block  74  to select the device and to place the appropriate amount of force on the device  60  during testing. After the device  60  has been removed from the fixture insert  18  at block  78  the device  60  is dried at block  80 . Drying may be accomplished by any suitable means, such as, for example, dabbing the device  60  on an absorbent cloth or material or by heating the device  60  to cause the fluid to evaporate. After the device  60  is dried at block  80 , the device  60  is binned at block  82  based on the results of the electrical performance testing carried out by block  76 . 
     As will be readily appreciated from a review of FIG.  14  and its attendant description, the entire test process  70  has been described as automated and as not requiring human intervention. Of course, this is not necessarily required and human intervention could be used at any point in the test process  70 . As will be further appreciated, the elimination of the Wakefield grease in the testing process eliminates the need to manually clean the tested device with any potentially hazardous chemicals thereby eliminating the cleaning step and the exposure of personnel to such chemicals. Additionally, the elimination of the cleaning step in favor of the drying step of block  80  reduces the time to execute the test process and yields a residue-free device after testing. 
     While the foregoing description is pertinent to cooling devices while they are being tested, it will be readily appreciated that the teachings and principles included herein are not strictly limited to device testing and may be applied to other situations that test situations. For example, it would be possible to use devices embodying the teachings disclosed herein to cool devices when devices are used in a circuit in the field. 
     Additionally, although certain apparatus constructed in accordance with the teachings of the invention have been described herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all embodiments of the teachings of the invention fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.