Abstract:
A fixture for supporting a plurality of semiconductor chips during the thermal cycling of the chips, including a fluid-permeable bottom screen, a chip-cavity-defining plate supported against a top surface of the bottom screen, a lower attaching mechanism for attaching the chip-cavity-defining plate to the top surface of the bottom screen, and a removable fluid-permeable top screen attached to a top surface of the chip-cavity-defining plate to cover the plurality of holes and chips therein.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to carriers for chip-scale devices, also referred to as wafer scale packaging (WSP) devices or as WSP chips, and also relates to techniques for rapid, efficient thermal testing and/or thermal cycling of WSP chips.  
         [0002]     Thermal testing and/or cycling of a batch of WSP chips ordinarily is accomplished by placing a large number of WSP chips in a conventional plastic carrier, placing the carrier in a thermal chamber, and either heating the chamber and/or passing a heated gas or liquid medium through the chamber. For temperature cycling, typically the carrier and the WSP chips therein are alternately subjected to “hot baths” and “cold baths” of gas or liquid medium to provide rapid thermal ramp-up times and thermal ramp-down times. A typical liquid used for this purpose is “FLUORINERT”, which is commercially available from 3M Corporation. A typical inert gas used as a thermal medium is nitrogen.  
         [0003]     One prior art chip carrier, part number H20-130-2462-C02 available from Entregris Corporation, is shown in  FIG. 1 .  
         [0004]     The Entregris chip carrier product of  FIG. 1  has the shortcoming that it does not allow fluid thermal medium to flow through the carrier and come in direct contact with the chips being carried. The Entregris chip carrier therefore has very long thermal ramp-up and ramp-down times, which adds substantially to the cost of thermal stress cycling procedures. Typically, five-minute temperature ramping times or less are desirable in thermal cycling, between, for example, −55 degrees Celsius (C.°) to +125 C.° or even as high as +150 C.°. Another shortcoming of the Entregris chip carrier product of  FIG. 1  is that the plastic material, which is manufactured under the trade mark FLUOROWARE, does not tolerate high temperatures. Another shortcoming is that the plastic material out-gases at temperatures slightly above room temperature, which may deleteriously affect the performance of chips in the carrier. The plastic is composed of carbon-impregnated petro-chemical materials, and the plastic usually is coated by a layer of anti-static material. Consequently, heating the plastic carrier results in release of free ionic gases. The out-gassing tends to cause electronic charge and plastic residues to be deposited on the chip surfaces. This often causes errors in circuit operation of the chips, resulting in loss of the chips during functional testing thereof.  
         [0005]     Other conventional chip carriers typically are also made of plastic material. None of the unknown chip carriers are well-suited for supporting WSP chips during the thermal testing and/or thermal cycling that usually is a requirement for a semiconductor manufacturer to meet the “qualification” standards for each product that most large customers require to be met before they will purchase the product.  
         [0006]     There are additional reasons that cause conventional fixturing mechanisms and devices, such as the above described Entregris chip carrier, to be unsuitable for performing thermal stress test sequences and thermal cycling on small devices such as WSP chips. Presently available fixturing mechanisms such as chip support trays do not adequately support WSP chips under test, and do not allow proper flow of gas or liquid thermal mediums around the WSP chips to be thermally tested or thermally cycled.  
         [0007]     Also, the thermal mass of the prior art chip support fixturing devices or trays is so large that it greatly reduces the rate at which the WSP chips attain the desired temperatures. This has prevented the desired amount of thermal shock specified by the above-mentioned qualification standards from being applied to the WSP chips, because most of the thermal energy from the thermal medium is being transferred between the thermal medium and the prior art carriers, rather than between the thermal medium and the chips. Furthermore, most of the thermal energy involved in the thermal cycling, has been wasted.  
         [0008]     Also, the prior art plastic chip carriers tend to warp or be physically deformed due to mismatches in temperature expansion coefficients of the materials, and the resulting stretching, flexing, etc. of the materials when subjected to increased temperatures may interfere with the ability of the carriers to adequately hold the WSP chips, and may displace them from the carrier cavities in which the WSP chips are intended to be supported. Such displacement of a WSP chip may result in damage to it while it is in a thermal testing or thermal cycling chamber. The damage may include chipping of edges of the chip and/or damage to the chip metallization (especially to solder bumps that are used for external electrical contact to the chip metallization), causing rejection and loss of the chip at the functional testing stage.  
         [0009]     Thus, there is an unmet need for a fixturing mechanism capable of reliably containing and supporting WSP chips and like to be tested, wherein the fixturing mechanism allows a thermal gas or liquid medium to readily and uniformly flow around the WSP chips under test.  
         [0010]     There also is an unmet need for a thermal stress fixture that does not damage WSP chips therein.  
         [0011]     There also is an unmet need for a thermal stress fixture that allows fast temperature ramp-up and fast temperature ramp-down during thermal stress cycling.  
         [0012]     There also is an unmet need for a thermal stress fixture that avoids waste of thermal energy during thermal stress testing and/or thermal cycling.  
         [0013]     There also is an unmet need for a thermal stress fixture that avoids damage to semiconductor chips due to out-gassing of substances from materials of which the thermal stress fixture is composed.  
       SUMMARY OF THE INVENTION  
       [0014]     Accordingly, is an object of the invention to provide a fixturing mechanism and method that are capable of reliably containing and supporting WSP chips and like to be tested that also allow a thermal gas or liquid medium to directly contact the WSP chips under test and readily and uniformly flow around the WSP chips under test.  
         [0015]     It is another object of the invention to provide a thermal stress fixture that does not damage WSP chips therein.  
         [0016]     It is another object of invention to provide a thermal stress fixture that allows fast temperature ramp-up and fast temperature ramp-down during thermal stress cycling.  
         [0017]     It is another object of the invention to provide a thermal stress fixture that avoids waste of thermal energy during thermal stress testing and/or thermal cycling of semiconductor chips.  
         [0018]     It is another object of invention to provide a thermal stress fixture that avoids damage to semiconductor chips due to out-gassing of substances from materials of which the thermal stress fixture is composed.  
         [0019]     Briefly described, and in accordance with one embodiment, the present invention provides a fixture for supporting a plurality of semiconductor chips during the thermal stressing and/or cycling of the chips, including a gas-permeable and liquid-permeable bottom screen, a chip-cavity-defining plate supported against a top surface of the bottom screen, a lower attaching mechanism for attaching the chip-cavity-defining plate to the top surface of the bottom screen, and a removable gas-permeable and liquid-permeable top screen attached to a top surface of the chip-cavity-defining plate to cover the plurality of holes and chips therein. In the described embodiment, the fixture ( 100 ) includes a fluid-permeable bottom screen ( 20 ), a chip-cavity-defining plate ( 22 ) disposed against a top surface of the bottom screen ( 20 ), the chip-cavity-defining plate having a plurality of holes ( 24 ) therein, a fluid-permeable top screen ( 40 ), and a removable mounting flange ( 30 ) attached to a bottom surface of the top screen ( 40 ) for holding the top screen against a top surface of the chip-cavity-defining-plate ( 22 ) to cover the plurality of holes ( 24 ) and chips ( 10 ) therein. The top screen, bottom screen and the plurality of holes in the chip-cavity-defining plate form a plurality of cavities for containing a plurality of semiconductor chips, respectively. In the described embodiment, a bottom surface of the chip-cavity-defining plate ( 22 ) is adhesively attached to the top surface of the bottom screen ( 20 ), and a top surface of the mounting flange is adhesively attached to a bottom surface of the top screen. The top screen and bottom screen are composed of pre-tensioned stainless deal screen mesh.  
         [0020]     According to the method of the invention, the semiconductor chips ( 10 ) are thermally cycled by supporting them in a the fixture, wherein the fixture has very low thermal mass. The semiconductor chips ( 10 ) are placed in various cavities ( 24 ) defined by the holes ( 24 ) in the bottom screen ( 20 ) and the chip-cavity-defining plate, and a subassembly including the top screen ( 40 ) and the chip-cavity-defining plate ( 22 ) is placed on a subassembly including the bottom plate and the chip-cavity-defining plate to cover the cavities ( 24 ) and the chips ( 10 ) therein. The fixture ( 100 ) with the chips ( 10 ) therein is placed in a thermal cycling device ( 50 ). The semiconductor chips are thermally stressed and/or thermally cycled by passing a fluid thermal medium of a predetermined temperature through the top screen ( 40 ), around the semiconductor chips ( 10 ), and through the bottom screen ( 20 ).  
         [0021]     A plurality of fixtures ( 100 ) are made by adhesively attaching bottom surfaces of a plurality of chip-cavity-defining plates ( 22 ) to a surface of taut pre-tensioned fluid-permeable screen material stretched over a tensioning frame to form a plurality of bottom subassemblies having chip cavities into the which semiconductor chips can be placed. The top surfaces of a plurality of mounting flanges ( 30 ) are adhesively attached to a surface of the taut pre-tensioned fluid-permeable screen material to form a plurality of top subassemblies which can be aligned with and attached to the bottom subassemblies, respectively, to provide covers over the cavities and semiconductor chips therein during the thermal cycling.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]      FIG. 1  is an exploded view of prior art fixture for supporting a batch of WSP chips or the like.  
         [0023]      FIG. 2A  is a three-dimensional exploded view of a WSP thermal stress fixture of the present invention.  
         [0024]      FIG. 2B  is an enlarged three-dimensional sections view of a portion of the WSP fixture of  FIG. 2A  showing a WSP chip within a cavity of the fixture and also showing a flow path of thermal fluid medium through the fixture and directly contacting the WSP chip.  
         [0025]      FIG. 3  is a generalized diagram of a thermal testing chamber containing a plurality of loaded WSP fixtures of  FIG. 2A , and also showing flow of thermal fluid medium through the WSP fixtures.  
         [0026]      FIG. 4  is a diagram illustrating a thermal cycle produced by the thermal testing chamber of  FIG. 3 .  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]     Referring to the exploded view of  FIG. 2A , WSP thermal stress fixture  100  of the present invention includes a generally rectangular fine mesh stainless steel bottom screen  20  which functions as the bottom of fixture  100 . Stainless steel bottom screen  20  can be composed of stainless steel pre-tensioned mesh. In the described embodiment, screen  20  is composed of stainless steel screen material manufactured according to specification number SS 101-10, available from Microscreen, Inc. of South Bend, Ind. A generally rectangular tray  22  having an array of WSP chip cavities  24  therein is disposed on the upper surface of bottom screen  20 . Each chip cavity  24  is in the form of a round hole that extends to bottom screen  22 , which forms a bottom of each chip cavity  24 . Tray  22  can be composed of 6061-T6 or equivalent of aluminum material, and can have a thickness of 40 mils (millimeters). Alternatively, the chip cavities  24  can be elliptical or rectangular.  
         [0028]     Tray  22  includes a pair of clearance openings  25  along each of its four edges, and a pair of screws  26  extends through the clearance holes  25 , respectively, and through corresponding clearance holes  29  through bottom screen  20  which are respectively aligned with clearance holes  25  of tray  22 . The threaded portions of screws  26  engage threaded holes  27  in four tabs  28  located on the bottom surface of bottom screen  20 . Screws  26  thus hold tray  22  against the upper surface of bottom screen  20 .  
         [0029]     A generally rectangular mounting flange  30  is disposed around the upper edge surfaces of tray  22 . Mounting flange  30  can be composed of the same aluminum material as tray  22  and can have the same thickness as tray  22 . A generally rectangular top screen  40  composed of the same stainless steel mesh as bottom screen  20  is disposed on the upper surface of frame  30 . A clearance hole  32  extends through the central portion of each side of frame  30 . Four screws  34  extend upward through a hole  35  in each of the four tabs  28 , through the four holes  32  of frame  30 , respectively, and through corresponding holes  41  in the edges of top screen  40 . Four knurled nuts  37  engage the threads of screws  34  and draw top screen  40  and frame  30  against the subassembly including tray  22  and bottom screen  20 .  
         [0030]      FIG. 2B  shows a section view of the fixture  100 , including one of the cavities  24  and a chip  10  loosely placed in cavity  24  of tray  22 . Chip  10  rests on the top surface of bottom screen  20 . However, the top surface of chip  10  does not touch the bottom surface of top screen  40 . A top subassembly  30 , 40  composed of top screen  40  and mounting flange  30  is tightly held by screws  34  and nuts  37  against the bottom subassembly  20 , 22  composed of bottom screen  20  and tray  22  so that the bottom surface of mounting flange  30  is pressed against the upper surface of bottom screen  20 . Arrows  33  show the flow paths of gas thermal medium which rapidly ramps the WSP chip up to the desired thermal stress temperature and later rapidly ramps the WSP chip down to the desired lower thermal stress temperature.  
         [0031]     The above-mentioned stainless steel screen material is shipped by the manufacturer tightly pre-tensioned over a tensioning frame. To construct the bottom subassembly  20 , 22 , a suitable glue or adhesive, such as EPOTEK B9114-2 glue, is applied to the bottom surface of the trays  22 , which are then placed on the taut screen material while it is still tightly stretched on the tensioning frame. After curing for 24 hours at +25 degrees Celsius followed by 2 hours at +150 degrees Celsius followed by 30 minutes at +200 and degrees Celsius, the screen material is cut along the edges of the trays  22 , and the four tabs  28  are attached to the bottom edges of each bottom subassembly  20 , 22  by means of small screws  26  extending through clearance holes  25  of tray  22  into threaded holds  27  in tabs  27 . Four screws  34  are threaded through holes  35  in the four tabs  28  and extend upward alongside the outer edges of the tray  22  to complete bottom subassembly  20 , 22 . Alternatively, however, clips could be used instead of all the above mentioned screws, and other adhesive material, such as latex rubber compound, could be used instead of glue.  
         [0032]     Similarly, the top subassembly  30 , 40  is formed by applying the adhesive to the top surfaces of a number of frames  30  and placing them on the taut framed screen material. After curing, the top screen  40  of each top subassembly  30 , 40  is cut along the outer edges of its mounting flange  30 . Using a vacuum pencil (not shown), individual WSP chips can ( FIG. 2B ) are loaded into the various cavities  24  of bottom subassembly  20 , 22 . Top subassembly  30 , 40  is then placed so that the four screws  34  are aligned with the clearance holes  32  and  41 . Top subassembly  30 , 40  then is lowered onto bottom subassembly  20 , 22  and the nuts  37  are threaded on to the portions of screws  34  extending above the top screen  40  and tightened. After the thermal cycling process, the top subassemblies  30 , 40  are removed, and the WSP chips are removed from the chip cavities  24 .  
         [0033]      FIG. 3  is a diagram of a thermal stress chamber  50 . Thermal stress chamber  50  includes a thermally insulated hot chamber  53  and a thermally insulated cold chamber  52  defined by a thermally insulated housing  51 . The thermal stress fixtures  100  are placed in a chamber  60  of a movable carriage  55  which can be rapidly moved back and forth between a lower cold chamber  52  and an upper hot chamber  53  in order to subject WSP chips within the thermal stress fixtures  100  to thermal stress cycles having the temperature profile shown in  FIG. 4 . Access to cold chamber  52  is through a movable, thermally insulated door  57 , and access to hot chamber  53  is through a movable, thermally insulated door  56 . View ports  56 A and  57 A are provided in doors  56  and  57 , respectively. Movable carriage  55  moves up and down as indicated by arrows  77  in response to a pneumatic cylinder  74  controlled by a controller  44 . Pneumatic cylinder  74  includes a vertically movable piston  73  that moves up and down as indicated by arrows  76 . A cable  70  has one end connected to the top of movable carriage  55 . Cable  70  passes over idler pulleys  71  and  72 , and its second end is connected to the upper end of piston  73 . Air flow control is controlled by controller  44  to adjust the amount of liquid nitrogen that flows through refrigeration elements  58  to maintain a preset cold temperature in cold chamber  52  in response to a thermal sensor (not shown) in cold chamber  52 . A controller  44  controls the amount of power delivered to heating elements  54  in hot chamber  53  to maintain a preset hot temperature in hot chamber  53  in response to a thermal sensor (not shown) in hot chamber  53 . A number of the thermal stress fixtures  100  loaded with chips  10  are manually placed on a shelf  61  in chamber  60  of movable carriage  55 .  
         [0034]     The top  55 A of movable carriage  55  includes a peripheral lip  64  that engages a corresponding surface of a ledge  62 , 68  to form a “door” that maintains a thermal seal between hot chamber  53  and cold chamber  52  when movable carriage  55  is lowered all the way into cold chamber  52 . Similarly, the bottom  55 B of movable carriage  55  includes a peripheral lip  66  that engages a corresponding surface of ledge  62 , 68  to form another door that maintains a thermal seal between hot chamber  53  and cold chamber  52  when movable carriage  55  is raised all the way into hot chamber  53 . The ramping times that the thermal stress fixtures and the WSP chips therein experience is a function of the thermal mass and other properties of the two chambers  52  and  53 . The controller  44  can cause movable carried  55  to move from one chamber to the other hand seal the two chambers from each other in approximately 7 seconds. There is a small fan (not shown) in each chamber that keeps the thermal medium, such as nitrogen, moving so that it flows through the thermal stress fixtures  100  and provides rapid three minute ramping times between the temperature extremes that are preset as inputs to controller  44 . Thermal stress chamber  50  is commercially available from Blue M Corporation.  
         [0035]     Thermal stress chamber  50  includes a controller  44  that allows the upper temperature, the lower temperature, and the number of cycles to be manually set.  FIG. 4  shows the profile of a typical thermal stress cycle produced by thermal stress chamber  50  of  FIG. 3 , wherein the lower temperature is −65 degrees Celsius, the upper temperature is +125 or +150 degrees Celsius, and the number of cycles is typically between 500 and 1000. The profile of a typical thermal stress cycle, shown in  FIG. 4 , begins at −65 degrees Celsius, and ramps up to +125 degrees Celsius in three minutes, remains at +125 degrees Celsius for a “dwell time” of approximately 20 minutes, and then ramps down to −65 degrees Celsius in three minutes, and remains at that temperature for a dwell time of 20 minutes.  
         [0036]     The structure of the described embodiment of the invention is relatively simple and is easily fabricated using readily available materials. No complex machining/forming operations are required, nor is any special tooling required in order to produce the described WSP chip support fixture. The low thermal mass and rapid thermal transfer characteristics of the described fixtures result in short temperature ramp-up and temperature ramp-down times. Furthermore, by varying the depths and/or diameters of the cavities  24 , various WSP chips can be thermally tested and/or thermally cycled using the same fixturing equipment, including support fixtures, chip loading/unloading equipment, etc.  
         [0037]     Thus, the invention provides a simple, economical way to restrain and protect small chips, chip-scale devices, and the like under test conditions during thermal cycling in either or both gas and liquid thermal test mediums. The invention provides minimal restriction of the thermal fluid medium flow around the WSP chips, thereby enhancing the thermal transfer process due to lack of restriction by providing rapid, thermal transfer between the WSP chips and the medium, and also provides a substantial reduction in the thermal mass of the fixture which allows rapid thermal ramp-up and ramp-down times.  
         [0038]     While the invention has been described with reference to several particular embodiments thereof, those skilled in the art will be able to make various modifications to the described embodiments of the invention without departing from its true spirit and scope. It is intended that all elements or steps which are insubstantially different from those recited in the claims but perform substantially the same functions, respectively, in substantially the same way to achieve the same result as what is claimed are within the scope of the invention. For example, a the thermal stress fixture  100  of the present invention might be used in a commercially available “purge and surge” single thermal chamber system instead of the system shown in  FIG. 3  in order to subject the WSP chips to a temperature cycling profile similar to that shown in  FIG. 4 .