Abstract:
Disclosed is apparatus to enhance thermal energy transfer from a heater to a DUT in IC handler systems. The pick-up head of an IC handler system is made of metal blocks in maximal thermal contact, and further includes an electrically resistive and thermally conductive layer. The electrically resistive layer provides ESD protection to the DUT. The preferred apparatus uses a collapsible billows suction cup to secure, pick-up, and align DUTs.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to automated test equipment for testing integrated circuits (ICs), and in particular to apparatus and methods for optimizing thermal energy transfer between self-aligning pick-up heads and DUTs in multiple-device IC handler systems. 
   BACKGROUND 
   Packaged integrated circuits (ICs) are typically tested prior to sale. Testing is typically carried out using automatic test equipment (ATE) that includes an IC test signal generator (IC tester), a test fixture (e.g., a socket) for transmitting electrical signals from the IC test signal generator to an IC device-under-test (DUT), and handler equipment that moves the DUTs between shipping, temperature-soaking, and test-fixture trays. The temperature-soaking tray is used for heating or cooling DUTs before testing. The handler also applies heat to the packaged ICs for high-temperature tests designed to identify heat-intolerant parts. 
     FIG. 1  depicts a handler system  100  used to move packaged IC DUTs between shipping, shuttle, temperature-soaking, and test-fixture trays collectively labeled tray  120 . Handler system  100  includes an arm  110  that moves DUTs between trays. Arm  110  may be an input arm or a test arm, depending on the target trays to which the DUTs are to be moved. An input arm moves DUTs between shipping, temperature-soaking, and shuttle trays. A test arm moves DUTs between the shuttle and tester trays. 
   Handler system  100  includes vertically movable frames (referred to as “hands”)  115 , and each hand  115  supports one device pick-up head  130 . Each device pick-up head  130  includes a base structure  120  held by an associated hand  115  and a movable portion  125  that transmits a vacuum pressure used to secure and pick-up a DUT during movement from one location to another. Specifically, to move DUTs between trays, arm  110  is moved horizontally and stationed over a tray, and then hands  115  are lowered until the movable portion  125  of each device pick-up head makes contact with the upper surface of the respective DUT. Next, vacuum pressure is transmitted to the device pick-up heads to secure the DUTs, and hands  115  are moved upward from the tray, thereby lifting the DUTs. Input arm  110  is then moved horizontally and stationed over the receiving tray, and hands  115  are lowered until the DUTs contact the tray. The vacuum pressure is then released, and a brief positive pressure (puff) is transmitted to each device pick-up head to release the DUTs. 
     FIG. 2  is a cross-sectional side view showing a simplified device pick-up head  130  similar to device pick-up heads  130  of  FIG. 1 . Pick-up head  130  is mounted on handler system  100  of  FIG. 1  in an embodiment in which handler system  100  is adapted to test BGA-packaged DUTs. 
   Device pick-up head  130  includes a rigid (e.g., aluminum) base structure  210 , a movable portion  220 , an adjustment collar  230 , and a spring  240  adapted to bias movable portion  220  away from base structure  210 . Base structure  210  includes an opening  212  and a hole  214  that slidably receives movable portion  220 . Base structure  210  also includes a spring mounting structure  216  adapted to hold the upper portion of spring  240 . Opening  212  slidably receives movable portion  220  such that lower surface  223  of portion  220  faces away from opening  212 . A shaft  224  extends upward from the base of movable portion  220  through hole  214 , and a narrow connection tube  226  extends from the upper end of shaft  224 . A central passage  228  extends through the base of movable portion  220 , shaft  224 , and connection tube  226  to transmit vacuum force from a conventional vacuum source (not shown). Applying vacuum force via central passage  228  secures BGA DUTs to lower surface  223  of base  220 . 
   Adjustment collar  230  sets the distance from top surface  217  to lower surface  223  to a predetermined distance T to equalize distances T 1 , T 2 , and T 3  ( FIG. 1 ). Equalizing these distances ensures that during pick-up, each DUT is lifted up by the respective head. Manual adjustment of distance T for each test run increases the time required for each test. 
   During high-temperature tests, the DUTs are first soaked in a temperature-soaking tray until the DUTs attain a target temperature. The DUTs are then moved onto a tester tray for the high-temperature tests. A heat source (not shown) applied to top surface  217  of base structure  210  maintains the temperature of the DUTs at the target value. The applied heat is transmitted to the DUTs through air in opening  212  and the material of movable portion  220  in each pick-up head. Unfortunately, the series combination of air, movable portion  220 , and the small contact area between base structure  210  and movable portion  220  forms an ineffective thermal conductor. Thus it takes a considerable time to heat the DUTs to the target temperature, and it is difficult to maintain the DUTs at the target temperature. 
   SUMMARY 
   The present invention addresses the need for test engineers to quickly and efficiently raise a packaged IC to a test temperature and to maintain the IC at the test temperature during high-temperature testing. In accordance with one embodiment, a pick-up head of an IC handler system includes one or more thermally conductive layers (e.g., aluminum) sandwiched between a thermal energy generator and a device-under test (DUT). An electrically resistive layer disposed between the thermally conductive layer and the DUT limits current passing between the DUT and the pick-up head, and therefore prevents harmful electrostatic discharge current from damaging the DUT. In one embodiment, a collapsible billows suction cup picks up the DUT and maintains the DUT in contact with the electrically resistive layer. 
   The claims, and not this summary, define the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  (prior art) illustrates a simplified conventional ATE arrangement. 
       FIG. 2  (prior art) is a cross-sectional side view showing a simplified integrated circuit handler system pick-up head utilized to pick-and-place BGA-packaged ICs in the ATE arrangement shown in  FIG. 1 . 
       FIG. 3  is a cross-sectional side view showing an integrated circuit handler system according to an embodiment of the present invention. 
       FIG. 4  depicts, in cross-section, a pick-up head  400  for a handler system adapted in accordance with an embodiment of the invention adapted for use with QFP (quad fine pitch) IC packages, one of which is shown as QFP  402 . 
   

   DETAILED DESCRIPTION 
     FIG. 3  depicts a pick-up head  300  for an integrated circuit (IC) handler system adapted in accordance with one embodiment of the invention. AS is conventional, pick-up head  300  is used to pick up DUTs, and to thermally condition DUTs during thermal testing. 
   Pick-up head  300  includes a thermal energy source  305  and a collapsible billows suction cup  335 . Thermal energy source  305  includes a thermal energy generator  320  (a heater), a thermally conductive layer  322 , and an electrically resistive layer  307 . Thermal energy generator  320  is in thermal contact with a first surface  327  of thermally conductive layer  322 . In one embodiment, thermally conductive layer  322  includes two aluminum blocks, a work press  325  and a blade pack  326 . In other embodiments, work press  325  and blade pack  326  are formed from a single piece of material. Thermally conductive layer  322  preferably exhibits a thermal conductivity of between 40 and 450 W/mK. 
   The lower surface of blade pack  326 , a second surface  328  of thermally conductive layer  322 , is in thermal contact with the top surface  309  of electrically resistive layer  307 , a third surface of thermal energy source  305 . Finally, the lower surface  308  of electrically resistive layer  307 , a fourth surface of thermal energy source  305 , can be in thermal contact with the upper surface of a packaged integrated circuit. 
   The arrangement and composition of structures  320 ,  325 ,  326 , and  307  make up source  305 , which is part of an efficient thermal energy transfer system that optimally conducts thermal energy between the thermal energy generator  320  and a DUT. Also, the system maximizes thermal surface contact area between work press  325  and blade pack  326 , further enhancing thermal conduction. 
   Thermally conductive layer  322  has an aperture  330  that extends through electrically resistive layer  307 , blade pack  326 , and work press  325 . In the current embodiment, aperture  330  does not extend through work press  325 . A retraction mechanism including a central passage  340  runs from the upper end  331  of aperture  330  to a vacuum source (not shown) through work press  325  and thermal energy generator  320 . Central passage  340  transmits vacuum force to billows suction cup  335  during a pick-up operation. Billows suction cup  335  attaches to central passage  340  by a first end  331  and extends through aperture  330 . A second end  329  of collapsible billows suction cup  335  protrudes from the lower side of resistive layer  307  when vacuum force is absent. The extent to which suction cup  335  protrudes is set to the same value for all the pick-up heads in handler system  100  ( FIG. 1 ) at manufacture. 
   During operation, handler system  100  is lowered towards the DUTs until second end  329  of each suction cup  335  makes contact with the upper surface of the respective DUT. Second end  329  of each suction cup  335  forms a vacuum seal with the upper surface of the respective DUT when vacuum force is applied. Billows suction cups  335  retract into their respective apertures  330 , and thereby secure DUTs against surfaces  308 . 
   At full retraction, the second end  329  of suction cup  335  is coplanar with lower surface  308  of electrically resistive layer  307 ; hence the top surface of each DUT is in thermal contact with the respective heat source  305 . Each DUT is thus automatically aligned with each head. 
   In one embodiment, work press  325  and blade pack  326  are aluminum, a good thermal and electrical conductor. Electrically resistive layer  307  is preferably thermally conductive and electrically resistive. In one embodiment, electrically resistive layer  307  is anodized aluminum. 
   In the current embodiment, anodized aluminum is the material of choice for electrically resistive layer  307  because the blade pack  326  is aluminum. During anodization of blade pack  326 , aluminum and oxygen atoms combine to form an anodized aluminum layer that adheres to the aluminum surface forming an electrically resistive layer  307 . Resistive layer  307  provides excellent thermal energy conduction (coefficient of thermal conductivity of about 50 W/mK) between DUTs and thermally conductive layers  322 . Equally important, resistive layer  307  slowly conducts electrostatic charge away from DUTs to prevent ESD damage. In one embodiment, resistive layer  307  exhibits an electrical resistance of between one megohm and one terohm. 
     FIG. 4  depicts, in cross-section, a pick-up head  400  for a handler system adapted in accordance with an embodiment of the invention adapted for use with QFP (quad fine pitch) IC packages, one of which is shown as QFP  402 . This type of package differs from the BGA package of  FIG. 3  in that leads  403  of QGP  402  extend from the sides. 
   Head  400  is similar to head  300  of  FIG. 3 , like-numbered elements being the same or similar. In this embodiment, blade pack  326  is modified to include an inverted “ledge”  405  upon which is mounted a rectangular frame  410 . In operation, frame  410  presses against leads  403  to ensure contact between leads  403  and tester pins (not shown). Frame  410 , formed of PEEK in one embodiment, is attached to blade pack  326  using screws (not shown) or any other appropriate connection means. (PEEK is a thermally stable thermoplastic with excellent chemical and fatigue resistance.) The height of frame  410  is selected so the lower surface  308  of electrically resistive layer  307  contacts the upper surface of QGP  402  when frame  410  contacts leads  403 . 
   While the present invention has been described in connection with specific embodiments, variations of these embodiments will be obvious to those of ordinary skill in the art. For example, implementation of the invention is not limited to aluminum, but may be implemented using materials with appropriate thermal characteristics (e.g., steel, copper). In other embodiments, the resistive layer can be made from a material other than the material of the work press or blade pack. In such case, an intermediate layer may be required to adhere the resistive layer to the blade pack. Further, the retractable suction cup and corresponding retraction force can be of a different type, including a suction cup attached at the end of an up-down movable mechanical system. In this case, a DUT is secured by lightly pressing the suction cup on the DUT to create a vacuum seal during a downward movement; pick-up and alignment are achieved by the upward movement. Those of skill in test equipment design can adapt the present invention for use in different IC test methodologies. Therefore, the spirit and scope of the appended claims should not be limited to foregoing description.