Abstract:
A system for burn-in testing of integrated circuits employs a cooling module with an aperture that accommodates a standard size holder for various chips, the holder being placed in the mouth of the aperture, in contact with a flexible seal. When the module is raised to make contact from below with a socket on a test board, the seal confines the cooling fluid and contacts on the upper surface of the holder are pressed against a set of corresponding contacts on the test board.

Description:
BACKGROUND OF INVENTION 
   The field of the invention is burn-in testing of integrated circuits. In particular, the invention relates to apparatus for placing a test chip in a test fixture and cooling the chip during the test. 
   Standard procedure in the integrated circuit industry has been to conduct a test called a burn-in test, in which a circuit is operated at an elevated temperature and/or voltage for a period of time, while being tested for proper operation. 
   Operation at an elevated temperature and/or voltage stresses the circuit through the differential thermal expansion of various components and also accelerates various failure mechanisms in the transistors and interconnects that combine to make up the integrated circuit. 
   For example, an integrated circuit will have many thousands or millions of joints where dissimilar conductors meet. Heating and cooling cycles will put stress on these joints. The failure of any one joint through separation of the two components may cause the circuit to fail. 
   As technology has evolved, the operating temperatures and heat dissipation of circuits have increased. At the same time, competition has exerted pressure on manufacturers to reduce costs. Expensive methods of temperature control such as are employed in mainframe computers are not commercially practical in the field of consumer electronics or other price-sensitive markets. 
   Driven by industry demands to produce computer die with increasingly dense circuitry on larger die dimensions, a need for advanced means to cool the chip during the burn in operation has been revealed. 
   Present methods of transferring heat to and from the chip are done by means of direct contact with a chilled copper block. This method is limited by the contact area between the die and block. For the best results, it is desirable to make 100% contact between die surface and the chilled block, however, matching the surface profiles between the chilled block and die to make this a reality is not possible. There will therefore be variation in the amount of contact between one chip and another and therefore variation in thermal resistance resulting in variation of test temperature between one chip and another. 
   Potential damage to the die interconnect joints, test sockets and boards exist with current methods and devices used in placing and holding the block against the chip. 
     FIG. 3  illustrates a typical test apparatus used at present. 
   At the bottom of the Figure, a test table  10  supports the structure. The test system rests on table  10  on legs  22  that support board holder  20 . Board holder  20  holds printed circuit board  110 , which may be a production board or a special test board that holds one or more chips being tested. 
   Socket  115  provides a defined location for the chip and electrical contacts in conventional connections. 
   Substrate  117  holds the chip being tested, denoted with numeral  120 , and provides a standard interface with the test board. 
   Electrical power and a test pattern of signals will be delivered to chip  120  through connections in substrate  117  and test board  110 . 
   Cooling of the chip during the test is provided by a unit denoted generally by numeral  210  and having an electrical heater  220  and cooling fluid entering on flexible tube  232  and exiting on tube  234 . These tubes typically carry cooling water and must be insulated from contact with the chip or with the voltage on the heating wire. An electrical ground connection  222  is provided to ensure that electrical leakage does not bias the chip itself, which is conventionally designed to operate with its outer surface at ground. 
   At the top of the figure, block  240  denotes a transfer mechanism, typically a hydraulic cylinder with a piston, that forces the bottom surface of heatsink  210  against the top surface of chip  120 . 
   There is the problem of incomplete contact that will vary from chip to chip and therefore produce different operating temperatures during the test. 
   In addition, the application of force to improve contact carries the danger of damage to the chip. 
   SUMMARY OF INVENTION 
   The invention relates to apparatus for conducting burn-in tests with control over temperature during the test. 
   A feature of the invention is a cooling mechanism that employs a non-conductive fluid in direct contact with the chip being tested. 
   Another feature of the invention is insertion of the chip in a test circuit board from below. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  shows a perspective view of an apparatus according to the invention. 
       FIG. 2A  shows a side view of the apparatus of FIG.  1 . 
       FIG. 2B  shows a detail of FIG.  2 A. 
       FIG. 3  shows a view of a prior art apparatus. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows an overall perspective view of an apparatus according to the invention, in which a set of chip holders  117  (also referred to as substrates) on a carrier  50  on the left front of the Figure are presented to a material handling system (referred to as a pick and place system) denoted generally by the numeral  400 . The handling system transfers a holder to a location system  200  that raises it to engage contacts on the lower side of test board  110 . 
   Optionally, a board stiffener  112  supplies mechanical strength to support board  110  to resist deflection from the upward force supplied by the location system  200 . 
   Carrier  50  contains a number of substrate locations  52  that hold a substrate  117  that interfaces with the chips being tested. 
   Arm  450  of the pick and place system  400  sequentially picks up the next substrate and chip and places it in the locating apparatus that places the substrate into a socket on a test board  110  similar to that used in the prior art. At the end of the test, the pick and place system removes the tested chip and proceeds to the next one. The material handling system is shown in partially pictorial and partially schematic fashion. Those skilled in the art will appreciate that various types of commercially available handling systems may used in systems according to the invention. 
     FIG. 2A  shows a side view of the apparatus. Location system  200  is shown receiving a substrate  117  having a chip  120  projecting from its lower side. As will be discussed below, the arrangement with the test chip being positioned on the underside of the test board has advantages. 
   After arm  450  is removed, system  200  will raise the substrate  117  up until the pins  118  (shown in  FIG. 2B ) on the top surface engage with corresponding sockets on the test board. 
   Illustratively, one of the functions of the substrate  117  is to present a standard holder contact arrangement to the test board, with internal interconnect wiring not shown being arranged to accommodate various pin-outs on the different chips being tested. In that case, there will be different models of holder  117  with an integrated circuit contact pattern adapted to engage different integrated circuit contacts and a standard pattern of pins or other contacts  118  to engage the test board. 
   Pins  118  on the substrate  117  engage the test socket  115  from below. 
   Referring now to  FIG. 2B , there is shown in cross section a detail of FIG.  2 A. At the top, a substrate  117  has pins  118  projecting upward as it rests on flexible seal  510 . Seal  510  is located on a shelf in chamber  210 . Optionally, the seal could be on the lower surface of substrate  117  and rest against the flat shelf in chamber  210 . Preferably, the individual receiving contacts in socket  115  on the test board  110  are designed according to the zero insertion force principle, in order to reduce the force needed to be applied to the IC holder to make proper electrical contact. 
   Another method and common practice for contacting I/O pads is to have commercially available pogo pins or spring probes embedded into the test socket in an array that corresponds to the land grid array of the substrate. In this case, pins  118  would not be attached or needed on the substrate. 
   Pogo pins are used in current applications and would be the preferred method with this invention. 
   The system  200  raises up the temperature control module denoted generally with the numeral  500  that contains the fluid reservoir  210  that has an aperture  230  at the top that receives the substrate  117  and supports the flexible seal that confines the cooling fluid. 
   After the system  200  has raised the temperature control module  500  into position, it continues to apply force upward in order to maintain pressure on seals  510  that combine with substrate  117  to close the top of reservoir  230  and contain the non-conductive cooling fluid. In operation, the fluid enters in line  232  and exits in line  234 , surrounding chip  120 . Temperature sensors  235  within substrate  117  or immersed within the fluid within substrate  117  will detect the temperature of the coolant near the chip  120 . That information is sent to a control system  240  that executes an algorithm to cool or heat the fluid as required to maintain the temperature of chip  120  within a specified range. 
   The test program is then executed under control of controller  125 , at the end of which, the substrate is ejected from board  110  by conventional mechanical means such as a pin pressing on the surface opposite the chip. 
   The fluid is drained from the aperture and the pick and place system removes the substrate and places the next one on module  500 . The fluid need not be drained entirely from the system, so long as the pressure is reduced so that the fluid does not leak out while the seal is broken. 
   The sequence of operations is: 
   Insert a set of chips into corresponding substrates  117  and place them chip-down in holder  50 ; Repeat for the number of test chips:
     1) Pick up the nth substrate and place it in the top of aperture  230  of module  500 ;   2) Raise module  500  to engage the pins on the upper surface of substrate  117  with the test board  110 ; In the case with product not having pins  118 , the substrate I/O pads will contact spring probes embedded within the test socket.   3) Apply upward pressure to module  500  to seal the fluid aperture;   4) Fill the fluid aperture  230  with fluid and bring the chip to the desired temperature;   5) Execute the test program;   6) Drain the cooling fluid;   7) Eject the substrate from the test board;   8) Retract the cooling module;   

   Go to Step 1 and repeat. 
   It is an advantageous feature of the invention that the test board  110  is maintained upside down; i.e. with the socket on the bottom surface. It does not matter if other electronics that may be part of the test system are placed on the upper surface of board  110 . Leaks from the coolant fluid will not puddle up on the board surface, possibly damaging the electronics. Debris, e.g. from the substrate such as capacitors will not become lodged between the board and the upward-moving cooling module. 
   The material handling system  400  may maintain the substrates in the orientation in which they are presented, as shown in  FIG. 2A , or it may invert them. For example, the substrates may be presented on carrier  50  with the chips up, then rotated 180 degrees to be placed on the cooling module  500  as shown in FIG.  2 B. 
   The mechanism that lifts the cooling module into position may be hydraulic or may be operated by an electric motor, as is convenient. 
   While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced in various versions within the spirit and scope of the following