Abstract:
An assembly, including a tool for measuring an applied force and its centroid relative to the center of the tool. A method of measuring and adjusting a force and its centroid applied to a semiconductor chip in a socket by an abutting heat sink consisting of the steps of inserting the tool in the socket, applying a heat sink on said tool, measuring the applied force and its centroid with respect to the center of the tool, adjusting the heat sink until the centroid of the applied force is substantially aligned with the center of the tool, removing the heat sink and tool, from the socket, substituting a semiconductor chip for the tool and reapplying the heat sink whereby the centroid of the force applied by said heat sink is substantially aligned with the center surface of the semiconductor chip in said semiconductor device.

Description:
FIELD OF INVENTION  
         [0001]    This invention relates generally to tools and methods for measuring the force applied to a surface and more particularly to a tool that is capable of simultaneously measuring both the total force applied to the surface and the centroid of the force so applied and to a method of using the measured total force and centroid to alter either or both the measured total force and the position of the centroid of the force.  
         BACKGROUND OF INVENTION  
         [0002]    It is well known that semiconductor devices must be and are extensively tested. Many such tests are usually performed after the semiconductor device has been partially completed. One such test is the process known as semiconductor device test and burn-in. This process is a widely known and used testing process for measuring the expected life of a semiconductor device. Typically the semiconductor device is tested after the semiconductor chip is mounted on a module base but before a module cap has been applied over the chip. To perform this test and burn-in, respective uncapped semiconductor devices are placed in respective sockets on a so-called Burn-In-Board (BIB) provided with a plurality of such sockets. The filled Burn-In-Board is then placed in an oven and heated while being electrically connected to a suitable test apparatus so that the semiconductor devices can be tested while being subjected to various electrical and temperature conditions.  
           [0003]    Because the operation of the semiconductor device can, in itself, produce heat in the chip and because of convection currents in the oven, the temperature gradients across the surface of any one each chip can vary significantly from those across the surface of any other chip although all the chips are identical and are being subjected to the same electrical and/or oven temperature conditions. Without assurance that each chip is meeting the same temperature conditions, only an estimate of what can be expected in actual operation can be realized. Therefore, it is desirable that the temperature range and distribution across the entire surface of each chip under going the test be known, be closely controlled. To try and control the temperature range and distribution across the surface of each chip under going the test the prior art placed heat sinks against the surface of the chips.  
           [0004]    It has been recognized that thermal contact between the heat sink and the device under treatment will be improved when increased force is applied to the device by the heat sink. However, increasing the force beyond certain limits may cause damage to the device without improving the temperature distribution or gradient. Therefore a means to measure and optimize the force applied by the heat sink to the chip has long been sought by the prior art.  
           [0005]    Today, the presently available, state of the art burn-in equipment attempts to solve these problems by using substantially planar heat sinks that are provided with spring loading and gimbal action. Such heat sinks may also be coupled, for example to a microprocessor temperature controller, a temperature sensing means, heating and cooling means. Heat sinks similar to those used in Burn-In-Boards are also used in module handlers in which the semiconductor may under go additional more extensive electrical tests one module at a time.  
           [0006]    Although the start of the art heat sinks have provided improved results, there still is no assurance that all the devices being tested are subjected to identical conditions even when an optimum force is applied, by the heat sink, to the chip.  
         SUMMARY OF THE PRESENT INVENTION  
         [0007]    It has now been found, by the present inventors, that failure of the prior art to solve the above problem occurs because each respective heat sink in the burn-in board can fail to be positioned correctly with respect to the entire surface of its respective chip. For example even with the spring loaded, gimbal action heat sinks of the prior art, a heat sink may be improperly positioned in a socket such that it is not in contact with the surface of the underlying chip or in contact with only a single corner or edge of the chip surface. The result of this inaccurate positioning of the heat sink with respect to the surface of the chip will be large temperature gradients across the chip surface. Another result, for example, may be that the microprocessor controller will be unable to maintain proper chip temperature.  
           [0008]    This inaccurate positioning of the heat sink can be caused, for example, by a twist in one of the electrical connectors or in the cooling or heating supply lines connected to the heat sink which can be so slight that a visual inspection would not uncover any error in the position of the heat sink.  
           [0009]    The present inventors have now discovered that good thermal contact between the chip under test and the heat sink is a function of both the total force applied by the heat sink to the chip and the position of the centroid of the applied force with respect to the center of the chip.  
           [0010]    The present invention is thus directed to a mechanism that will consistently and accurately provide for and permit the proper positioning of a heat sink with respect to a semiconductor device in a test socket by measuring not only the total amount of force applied by a heat sink to an underlying surface but also by measuring the centroid of the applied force, i.e., total amount of force applied by the heat sink, with respect to the center of the underlying surface.  
           [0011]    With these measurements, the position of the heat sink, in the socket, can be adjusted until that the total force applied by the heat sink is substantially equal to the desired applied force and that the centroid of the applied force is substantially aligned with the center of the underlying chip surface.  
           [0012]    The present invention is thus directed to a mechanism capable of simultaneously measuring both the total force applied to the surface and the centroid of the force so applied between a heat sink and an underlying chip surface.  
           [0013]    The present invention is also directed to a method of measuring both the total force applied to the surface of a semiconductor device and the centroid of the applied force and repositioning the heat sink so that the total applied force and/or its centroid can be adjusted such that the total applied force is within a desired range and that the centroid of the applied force is substantially aligned with the center of the surface of the semiconductor chip.  
           [0014]    More particularly, the present invention is an assembly, including an upper and a lower plate with a plurality of load cells therebetween for measuring the applied force and determining its centroid, with respect to the center of the surface of the upper plate.  
           [0015]    If the surface of the heat sink is not substantially parallel with the surface of the upper plate, the centroid of the force, applied by the heat sink, will cause the amount of load registered by at least one of the three load cells to be different from that registered by the other two load cells. In such an instance the position of the heat sink is adjusted until the registered load of each of the load cell becomes substantially equal. Substantial equality between the load cell measurements means that the centroid of the force applied by the heat sink is substantially positioned over the center of the assembly. At this time the heat sink and assembly can be removed from the Burn-In-Board socket. A semiconductor device, having a chip thereon whose center is substantially identical to the center of the top plate of the assembly, is now substituted, in the socket, for the assembly and the heat sink applied to the surface of the semiconductor device. Because the heat sink is replaced in the socket, over the semiconductor device in exactly the same position as it was prior to the removal of the assembly, one can be assured that the heat sink will apply substantially the identical force to the semiconductor device as it applied to the assembly and that the centroid of the applied force is aligned with the center of the semiconductor device substantially the same as it was applied to the surface of the assembly.  
           [0016]    The bottom plate of the assembly is positioned in a semiconductor device socket so that the top plate of the assembly will be in the same position as the surface of the chip mounted on a semiconductor device will be when the semiconductor device is placed in the socket. Thus the heat sink will contact the chip surface exactly as it contacted the top plate of the assembly thus assuring that the force in a manner identical to the semiconductor chip surface so that the force applied by the heat sink against the top plate of the assembly will be measured by the 3 load cells permitting the location of the center of the force to be determined with respect to the center of any semiconductor device later positioned in the socket.  
           [0017]    The present invention thus provides a means of improving uniformity of the thermal contact between each heat sink and the semiconductor device it is in contact with regardless of the thermal interface used.  
           [0018]    These objects, features and advantages of the present invention will become further apparent to those skilled in the art from the following detailed description taken in conjunction with the accompanying drawings wherein: 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 is a partially sectioned view of an oven containing a burn-in-board having a plurality of chip sockets electrically connected to a test apparatus,  
         [0020]    [0020]FIG. 2 is a sectional view of a semiconductor device positioned in a socket of a burn-in-board with a heat sink positioned thereon,  
         [0021]    [0021]FIG. 3 is a sectional view of the assembly of the present invention positioned in a socket of a burn-in-board with a heat sink positioned thereon,  
         [0022]    [0022]FIG. 4 is an exploded view of the assembly of the present invention, and  
         [0023]    [0023]FIG. 5 depicts the positions of the centered of the load as measured on a plurality of assemblies. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]    A full appreciation of the features and advantages of the present invention can best be gained by reference to the drawings and more particularly to the FIGS. 1 through 6 where FIG. 1 is a partially sectioned view of an oven containing a burn-in-board having a plurality of chip sockets electrically connected to a test apparatus, FIG. 2 is a sectional view of a semiconductor device positioned in a socket of a burn-in-board with a heat sink positioned thereon, FIG. 3 is a sectional view of the assembly of the present invention positioned in a socket of a burn-in-board with a heat sink positioned thereon, FIG. 4 is an exploded view of the assembly of the present invention and FIG. 5 depicts the positions of the centroid of the load as measured on a plurality of assemblies.  
         [0025]    [0025]FIG. 1 is a partially sectioned view of an oven  10  containing a burn-in-board  12  having a plurality of semiconductor device sockets  14  each of which is electrically connected to a test apparatus  16 . Each semiconductor device socket  14 , as shown in FIG. 2, is well known to the art and is adapted to accept a semiconductor device  18  comprised of a insulating base  22  provided with a plurality of pins  23  coupled via suitable wiring patterns to various active and inactive devices, i.e., transistors, resistors, capacitors and the like, formed in a semiconductor chip  24 . In the present instance the active face  25  of the semiconductor chip  24 , i.e. the face containing the active and inactive devices, is positioned against the base  22  while its back or inactive face  26  is exposed. When the semiconductor device  18  is complete a cover or heat sink (not shown) is typically placed over the chip  24  and secured to the base  22 .  
         [0026]    To perform a burn-in test each semiconductor device  18  that is to be tested is left uncovered and plugged into a respective burn-in socket  14  on a burn-in-board  12  and a respective heat sink  28  is placed against the exposed back or inactive face  26  of each respective semiconductor device as shown in FIG. 2. When all the sockets  14  on the board are filled with devices and have heat sinks disposed thereon, the burn-in-board  12  is inserted in the oven, connected to the tester  16  and the oven  10  heated. The heat sink  28  is provided with a base  29  adapted to lie flat on the back  26  of the semiconductor chip. Typically the heat sink  28  is also provided a temperature sensing unit  30  wired to a suitable temperature measuring device (not shown) and with an internal cooling coil (not shown) connected to a suitable source of coolant via cooling lines  31 . The heat sink is thus designed so that the lower surface of the base  29  will substantially cover and contact the back surface face  26  of the chip  24  so that the chip  24  can be maintained at a selected temperature thereby prevent overheating of the chip and premature failure of the chip. Since such heat sinks are well known to the art and commercially available, further description of these heat sinks is not deemed necessary.  
         [0027]    At times however, because of problems, which may not be readily visible, such as a kink or a twist in one of the wires or cooling lines coupled to the heat sink, the lower base  29  of the heat sink is not properly seated on the back of the chip and cannot provide the desired or expected cooling of the chip such that either the chip overheats or there is a wide variation in the temperature across the surface of the semiconductor chip.  
         [0028]    Tests, performed by the present inventors on a 15 millimeter square chip running at 100 watts, determined that practicing burn-in using the prior art techniques where the heat sinks are placed against the chip surface without measuring either the total force applied to the surface of a semiconductor device or the centroid of the applied force or repositioning the heat sink as taught by the present invention could result in temperature variations of up to over 100 degrees Celsius across the face of the chip.  
         [0029]    However when the total force applied to the surface of a semiconductor device or the centroid of the applied force was measured on the same 15 millimeter square chips and, as taught by the present invention, the heat sinks were repositioned so that the total applied force fell within the desired range of 10 to 15 pounds and the centroid of the applied force was aligned to within 1.5 mm of the center of the surface of the semiconductor chip the temperature variations across the surface of the chips fell to less than 40 degrees Celsius.  
         [0030]    The present invention is thus designed to avoid this problem of temperature variation across the face of the chip and, as shown in FIGS. 3 and 4, uses an assembly  40  that can be inserted in each socket  14  in the burn-in-board  12  in lieu of the semiconductor device  8 . The assembly  40 , as will be further described below, is designed to provide information that will permit both the measurement of the total force applied by the base  29  and its centroid.  
         [0031]    The assembly  40  of the present invention, thus permits corrections to be made to the seating of the heat sinks  28  prior to the insertion of actual semiconductor devices in the burnin-board sockets so when actual semiconductor devices are inserted in the Burn-in-Board sockets that the correct total force will be applied thereto and that the centroid of the applied force will be satisfactorily positioned with respect to the center of the chip. In the event that a heat sink base  29  cannot be adjusted in a socket such as to apply either the correct amount of total force or to properly position the centroid of the applied force with respect to the center of a chip placed in the socket, the socket will be deemed defective and left empty of a semiconductor device during testing.  
         [0032]    As more fully shown in FIG. 4 the assembly  40  comprises first and second parallel plates  42  and  43 , separated by three triangularly spaced load cells  45 ,  46  and  47  each of which is coupled by a respective lead  45   b,    46   b  and  47   b  to a piece of test equipment  50  that is designed or programmed to measure the load applied to each of the cells  45 ,  46 , and  47  and calculate the centroid of the applied force with respect to the center of the upper plate  42 . Because the assembly  40  is to placed into a respective burn-in socket  14  on a burn-in-board  12  in lieu of a semiconductor device, the assembly  40  is designed to be no thicker than the semiconductor device for which it is being substituted. However it should be understood that the lower plate  43  of the assembly  40  is as large as the device insulating base  22 . It should also be understood that the upper plate  42  of the assembly may be considerably larger than the chip  24  on the semiconductor device that is to be tested.  
         [0033]    Once the assembly  40  is placed in the burn-in-board socket  14  a respective heat sink  28  is inserted into the socket  14  so that its lower surface  29  abuts the exposed top surface  42   a  of the upper plate  42 . That is the heat sink  28  is placed against this top surface, in the same manner that it would be placed against the back surface of a semiconductor device.  
         [0034]    The load cells  45 ,  46  and  47  are located in recesses  45   a,    46   a  and  47   a  formed in the lower plate  43  and the top plate  42  is secured thereon. To assure that the centroid of the applied force will be accurately measured it is necessary that the load cells  45 ,  46  and  47  be positioned beyond the perimeter of the chip that will be tested in the burn-in-board. To this end, the upper plate  42  of the assembly is also substantially larger than the chip it is being substituted for. This is particularly shown in FIG. 5 where the load cells are indicated at the points of the triangle formed by the load cells and the size of chip  25  is indicated by the dotted insert.  
         [0035]    The load cells, in FIG. 5, are positioned at the apexes of an equilateral triangle centered over the chip position. This layout has the advantage that when the load is perfectly centered, the three load cells will measure identical forces. It should also be understood that the triangle formed by the three load cells need not be an equilateral triangle. It should also be understood that more than three load cells can be used and that they can be arranged in other than triangular form, for example, the device could have four load cells with each cell positioned beneath a respective one of each of the  4  corners of the top plate  42 .  
         [0036]    As shown in FIG. 4, the top plate  42  is slightly smaller than the lower plate  43  but substantially larger than the chip for which it is being substituted. This top plate  42  is provided with a flange  42   a  so that it can be loosely secured in a square ring  49  by a rabbet  49   a  let into the lower inner edge of the ring  49 . In this way when the ring  49  is secured to the lower plate  33 , the upper plate  42  although restrained by the ring is still free to move within the ring  49  under the stimulus of a force applied by the heat sink base  29  contacting the upper plate  42 . It should be clear that the plate  42  fits loosely within the ring  49 . When the base  29  of the heat sink contacts the upper plate  42 , the upper plate  42  is forced against the load cells  45 ,  46  and  47  so that each load cell produces a signal that is proportional to the amount of force applied thereto. If the base  29  is tilted with respect to the upper plate  42  each of the load cell will experience a different loading. As is well known to the art, the centroid of the load can readily be calculated using the principal that the summation of the moments about any axis is equal to zero. Generally the moments are summed about the device X axis and Y axis. Thus, the location of the centroid is easily calculated. This has been repeated for a large number of heat sinks in a burn in oven with the results shown in FIG. 5.  
         [0037]    [0037]FIG. 5 depicts the positions of the centroid of the load with respect to where the center  52  of a chip  51  would be located as measured on a plurality of assemblies before any corrective action in positioning of the heat sinks was undertaken. In this FIG. 5, the cells  45 ,  46  and  47  are indicated in phantom. The perimeter of a nine millimeter square chip  51  also indicated in phantom.  
         [0038]    It is to be noted that one centroid  53  is located outside the perimeter of the chip  60  indicating that a chip placed in this socket would be contacted by only one edge of the heat sink unless corrective action was undertaken.  
         [0039]    Ideally any corrective action would result in the centroid of the applied force being positioned exactly on the center of the chip. However it has been found, for all practical purposes, that if the corrective action of repositioning the heat sink results in the centroid becoming relocated to within 3.0 millimeters of the center of a 9 mm chip that satisfactory burn-in results can be realized.  
         [0040]    The present invention thus provides a simple, cost effective method of assuring that the force applied by a heat sink to a semiconductor chip positioned in a burn-in-board will be substantially located on the center of the chip to be tested independent of the chip&#39;s design.  
         [0041]    It will be obvious to one skilled in the art that the present invention is well suited for optimizing heat sink contact in many types of module test or module burn-in equipment or with modules that have covers or lids or plastic encapsulations. Some types of test or burn in equipment have the ability to adjust for variations in device thickness by altering the location of the heat sink, in this case it may be desirable to make the assembly thicker then the device. The invention is equally well suited for use with any type of heat sink such as air or liquid cooled and passive or actively controlled.  
         [0042]    This completes the description of the preferred embodiment of the invention. Since changes may be made in the above construction without departing from the scope of the invention described herein, it is intended that all the matter contained in the above description or shown in the accompanying drawings shall be interpreted in an illustrative and not in a limiting sense. Thus other alternatives and modifications will now become apparent to those skilled in the art without departing from the spirit and scope of the invention as set forth in the following claims.