Abstract:
The present invention is directed toward apparatus and methods of testing and assembling bumped die and bumped devices using an anisotropically conductive layer. In one embodiment, a semiconductor device comprises a bumped device having a plurality of conductive bumps formed thereon, a substrate having a plurality of contact pads distributed thereon and approximately aligned with the plurality of conductive bumps, and an anisotropically conductive layer disposed between and mechanically coupled to the bumped device and to the substrate. The anisotropically conductive layer electrically couples each of the conductive bumps with a corresponding one of the contact pads. In another embodiment, an apparatus for testing a bumped device having a plurality of conductive bumps includes a substrate having a plurality of contact pads distributed thereon and substantially alignable with the plurality of conductive bumps, and an anisotropically conductive layer disposed on the first surface and engageable with the plurality of conductive bumps to electrically couple each of the conductive bumps with a corresponding one of the contact pads. Alternately, the test apparatus may also include an alignment device or a bumped device handler. In another embodiment, a method of testing a bumped device includes engaging a plurality of contact pads with an anisotropically conductive layer, engaging the plurality of conductive bumps with the anisotropically conductive layer substantially opposite from and in approximate alignment with the plurality of contact pads, forming a plurality of conductive paths through the anisotropically conductive layer so that each of the conductive bumps is electrically coupled to one of the contact pads, and applying test signals through at least some of the contact pads and the conductive paths to at least some of the conductive bumps.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to apparatus and methods of testing and assembling bumped die and bumped devices using an anisotropically conductive layer, suitable for testing, for example, flip chip die, chip scale packages, multi-chip modules, and the like.  
         BACKGROUND OF THE INVENTION  
         [0002]    Bumped die and other bumped devices are widely used throughout the electronics industry. As the drive toward smaller electronics continues, the pitch (or spacing) of solder bumps on such bumped devices continues to decrease. The increasingly finer pitches of the solder bumps on bumped die and bumped devices raise concerns about the reliability of these devices. These concerns are being addressed by testing.  
           [0003]    A die (or chip) is typically tested during the manufacturing process to ensure that the die conforms to operational specifications. Solder bumps (or balls) are then formed on bond pads of the die using a solder deposition device, such as a solder ball bumper. The solder bumps are typically formed with a height of from 25 μm to 75 μm. The bumped die are then tested by placing conductive test leads in contact with the solder bumps on the die, applying a test signal to the bumps via the test leads, and determining whether the bumped die responds with the proper output signals. If the bumped die tests successfully, it may be installed on a printed circuit board, a chip scale package, a semiconductor module, or other electronics device.  
           [0004]    [0004]FIG. 1 is a cross-sectional view of a bumped die  10  engaged with a test carrier  20  in accordance with the prior art. In this typical arrangement, the bumped die  10  includes a substrate  12  with a plurality of bond pads  14  thereon. A solder bump  16  (or other suitable conductive material) is formed on each of the bond pads  14 . The test carrier  20  has a plurality of contact pads  22  thereon, each of the contact pads  22  being electrically coupled with a test lead  24 . For testing of the bumped die  10 , the solder bumps  16  engage the contact pads  22  of the test carrier  20 , and the appropriate test signals are applied to the bumped die  10  through some of the test leads  24 . Output signals from the bumped die  10  are monitored through other test leads  24  to determine whether the bumped die  10  is functioning to specifications. Test carrier apparatus of the type shown in FIG. 1 for testing unpackaged die are described in U.S. Pat. No. 5,519,332 to Wood et. al., incorporated herein by reference.  
           [0005]    Testing of the bumped die  10  generally includes four levels of testing. A first or “standard probe” level includes the standard tests for gross functionality of die circuitry. A second or “speed probe” level includes testing the speed performance of the die for the fastest speed grades. A third or “bum-in die” level involves thermal cycling tests intended to drive contaminants into the active circuitry and to detect early failures. And a fourth or “known good die (KGD)” level includes testing to provide a reliability suitable for final products.  
           [0006]    To ensure proper transmission of the test signals and output signals, the solder bumps  16  may be temporarily connected with the contact pads  22  by reflowing the bumps, thereby soldering the bumps to the contact pads. After the testing is complete, the solder bumps  16  may be reflowed to disconnect the bumps from the contact pads. Connecting and disconnecting the solder bumps  16  from the contact pads  22 , however, involve time consuming processes and may damage the solder bumps  16  or the contact pads  22 .  
           [0007]    Another problem with soldering the solder bumps  16  to the contact pads  22  is that the coefficient of thermal expansion (CTE) of the bumped die  10  may be appreciably different from the CTE of the test carrier  20 . During bum-in die testing, the bumped die  10  and test carrier  20  are placed in a bum-in oven and subjected to temperature cycling (e.g. −55° C. to 150° C.) for a time period of from several minutes to several hours or more. Due to the different CTE of the bumped die  10  and the test carrier  20  and the rigidity of the solder connections, significant stresses may develop throughout the components. These stresses may result in delamination or other damage to the bumped die  16  or the test carrier  20 , and may degrade or damage the connection between the solder bumps  16  and the bond pads  14 .  
           [0008]    An alternate approach to soldering is to simply compress the solder bumps  16  into engagement with the contact pads  22 . Ideally, only a small compression force is needed to engage the solder balls  16  against the contact pads  22  so that tests may be conducted. Methods and apparatus for testing die in this manner are fully described in U.S. Pat. No. 5,634,267 to Farnworth and Wood, incorporated herein by reference. The applied compression force, however, must be kept to a minimum because larger forces may damage the circuitry of the bumped die  10  or the test carrier  20 .  
           [0009]    A problem common to both the solder reflow and the compression force methods of engagement is that the solder bumps  16  are not uniformly shaped. As shown in FIG. 1, the solder bumps  16  are usually of different heights. Using typical manufacturing methods and solders, the nominal variation between the tallest and shortest bumps (shown as a distance d on FIG. 1) is presently about 10% of the average solder ball height. Therefore, when the bumped die  10  is placed on the test carrier  20 , the shorter solder bumps may not touch the corresponding contact pads. In some cases, especially for very fine pitch solder bumps, the gaps between the shorter solder bumps and the contact pads may be too large to overcome using solder reflow (because of the small volume of solder in each bump) or by using compression force (because of possible damage to the bumped die).  
           [0010]    The variation in solder bump height also creates uncertainty in the final assembly of electronics components that include bumped devices. As the number of bumps on the bumped device increases, the failure rate of the assembled package increases due to solder bump non-uniformity.  
           [0011]    [0011]FIG. 2 is a partial cross-sectional view of the bumped die  10  of FIG. 1 engaged with another conventional test carrier  40 . The test carrier  40  includes a test substrate  42  having a plurality of pockets  44  disposed therein. As shown in FIG. 2, the pockets  44  have sloping sidewalls  46 , and a pair of contact blades  48  project from opposing sidewalls  46  into each pocket  44 . Conductive test leads  50  are formed on the test substrate  42 , including on the sidewalls  46  and contact blades  48  of the pockets  44 .  
           [0012]    During testing, the solder bumps  16  at least partially engage the pockets  44  of the test carrier  40  with the sharp contact blades  48  partially penetrating the solder bumps  16 . The solder bumps  16  may also contact the sloping sidewalls  46  of the test carrier  40 . Thus, the desired electrical connection between the solder bumps  16  and the test leads  50  may be achieved despite the variation in the solder bump height.  
           [0013]    Although the test carrier  40  having pockets  44  with contact blades  48  addresses solder bump height variation, testing solder bumps with the test carrier  40  has several disadvantages. For example, because the contact blades  48  penetrate the solder bumps  16 , the solder bumps may be cracked, chipped, or otherwise damaged by the contact blades. The solder bumps  16  may also become stuck to the contact blades  48 , requiring additional time and effort to disengage the bumped die  10  from the test carrier  40 . Furthermore, the test carrier  40  with the plurality of pockets  44  is relatively costly to fabricate and more difficult to maintain than alternative test carriers having flat contact pads.  
           [0014]    [0014]FIG. 3 is a partial cross-sectional view of the bumped die  10  of FIG. 1 engaged with another prior art test carrier  60 . In this example, the test carrier  60  includes a test substrate  62  having a plurality of pedestals  64  formed thereon. Test leads  66  are disposed on the test substrate  62 , each test lead  66  terminating in a contact pad  68  on the top of each pedestal  64 . A plurality of projections  69  project from each contact pad  68 . Apparatus for testing semiconductor circuitry of the type shown in FIG. 3 are more fully described in U.S. Pat. No. 5,326,428 to Farnworth et. al., U.S. Pat. No. 5,419,807 to Akram and Famworth, and U.S. Pat. No. 5,483,741 to Akram et. al., which are incorporated herein by reference.  
           [0015]    To conduct a test of the bumped die  10 , the solder bumps  16  engage the contact pads  68  so that the sharp projections  69  at least partially penetrate the solder bumps  16 . The projections  69  may be properly sized to penetrate into the taller solder bumps, allowing the shorter solder bumps to at least contact the projections of the corresponding contact pad  68 .  
           [0016]    One of the drawbacks of testing bumped die using the carrier  60  having projections  69  is that the projections (like the contact blades  48  described above) may damage the solder bumps  16 . Furthermore, the projections  69  are relatively expensive to manufacture, particularly when the projections must be sized to account for a nominal 10% variation in the solder bump height.  
         SUMMARY OF THE INVENTION  
         [0017]    The present invention is directed toward apparatus and methods of testing and assembling bumped devices using anisotropically conductive layers. In one aspect of the invention, a semiconductor device comprises a bumped device having a plurality of conductive bumps formed thereon, a substrate having a plurality of contact pads distributed thereon and approximately aligned with the plurality of conductive bumps, and an anisotropically conductive layer disposed between and mechanically coupled to the bumped device and to the substrate. The anisotropically conductive layer electrically couples each of the conductive bumps with a corresponding one of the contact pads, providing electrical contact between the conductive bumps and the contact pads despite variation in conductive bump height, and without damaging the conductive bumps.  
           [0018]    In another aspect, an apparatus for testing a bumped device having a plurality of conductive bumps includes a substrate having a plurality of contact pads distributed thereon and substantially alignable with the plurality of conductive bumps, and an anisotropically conductive layer disposed on the first surface and engageable with the plurality of conductive bumps to electrically couple each of the conductive bumps with a corresponding one of the contact pads. Alternately, the test apparatus may also include an alignment device. In another aspect, the test apparatus may include a bumped device handler. The test apparatus provides for rapid and efficient engagement, testing, and disengagement of the bumped device.  
           [0019]    In another aspect of the invention, a method of forming a semiconductor device includes providing a bumped device having a plurality of conductive bumps formed thereon, providing a substrate having a plurality of contact pads distributed thereon, forming an anisotropically conductive layer between the conductive bumps and the contact pads, approximately aligning the plurality of conductive bumps with the plurality of contact pads, and engaging the plurality of conductive bumps and the plurality of contact pads with the anisotropically conductive layer to electrically couple each of the conductive bumps with a corresponding one of the contact pads.  
           [0020]    In yet another aspect of the invention, a method of testing a bumped device includes engaging a plurality of contact pads with an anisotropically conductive layer, engaging the plurality of conductive bumps with the anisotropically conductive layer substantially opposite from and in approximate alignment with the plurality of contact pads, forming a plurality of conductive paths through the anisotropically conductive layer so that each of the conductive bumps is electrically coupled to one of the contact pads, and applying test signals through at least some of the contact pads and the conductive paths to at least some of the conductive bumps. Alternately, the method further includes at least partially curing the anisotropically conductive layer. The method advantageously reduces the time, effort and expense involved in connecting and disconnecting the conductive bumps from the contact pads, reduces the potential for damage to the conductive bumps or the contact pads, and accommodates variation in the heights of the conductive bumps.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a cross-sectional view of a bumped die engaged with a test carrier in accordance with the prior art.  
         [0022]    [0022]FIG. 2 is a partial cross-sectional view of the bumped die of FIG. 1 engaged with an alternate embodiment of a test carrier in accordance with the prior art.  
         [0023]    [0023]FIG. 3 is a partial cross-sectional view of the bumped die of FIG. 1 engaged with another embodiment of a test carrier in accordance with the prior art.  
         [0024]    [0024]FIG. 4 is a partial cross-sectional view of the bumped die of FIG. 1 engaged with a test carrier in accordance with an embodiment of the invention.  
         [0025]    [0025]FIG. 5 is a partial cross-sectional view of the bumped die of FIG. 1 engaged with a test carrier in accordance with an alternate embodiment of the invention.  
         [0026]    [0026]FIG. 6 is a partial cross-sectional view of the bumped die of FIG. 1 engaged with a test carrier in accordance with another alternate embodiment of the invention.  
         [0027]    [0027]FIG. 7 is a partial cross-sectional view of the bumped die of FIG. 1 engaged with a test carrier in accordance with yet another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]    The following description is generally directed toward apparatus and methods of testing and assembling bumped die and bumped devices using anisotropically conductive layers. Many specific details of certain embodiments of the invention are set forth in the following description and in FIGS.  2 - 7  to provide a thorough understanding of such embodiments. One skilled in the art, however, will understand that the present invention may have additional embodiments, or that the present invention may be practiced without several of the details described in the following description.  
         [0029]    Throughout the following discussion, apparatus and methods in accordance with the invention are described in relation to the testing and assembly of bumped die. It is understood, however, that the inventive apparatus and methods may be used to test and assemble any number of bumped devices, including chip scale packages, chip modules, or any other bumped devices. To simplify the following discussion, however, the inventive apparatus and methods are described in relation to testing and assembly of bumped die with a test carrier or a printed circuit board, allowing the reader to focus on the inventive aspects.  
         [0030]    [0030]FIG. 4 is a partial cross-sectional view of the bumped die  10  of FIG. 1 engaged with a test carrier  100  in accordance with an embodiment of the invention. In this embodiment, the test carrier  100  includes a test substrate  102  having a plurality of contact pads  104  coupled with a plurality of test leads  106 . An anisotropically conductive layer  160  having conductive particles  162  distributed in a suspension material  164  is formed on the test substrate  102  and contact pads  104 .  
         [0031]    The anisotropically conductive layer  160  is formed such that electrical resistance in one direction through the layer  160  differs from that measured in the other directions. Typically, electrical conductivity is provided in one direction (e.g. the “z” direction) while high resistance is provided in all other directions. The conductivity in the one direction may be pressure sensitive, requiring that the material be compressed in that direction to achieve the desired conductivity.  
         [0032]    One type of anisotropically conductive material suitable for forming the anisotropically conductive layer  160  is known as a “z-axis anisotropic adhesive.” In the z-axis anisotropic adhesive, the conductive particles  162  are distributed to a low level such that the particles do not contact each other in the xy plane. Compression of the layer  160  in the z direction, however, causes the conductive particles  162  to contact each other in the z direction, establishing an electrically conductive path. The conductive particles  162  may be formed from any suitable electrically conductive materials, such as gold, silver, or other electrically conductive elements or compounds. Similarly, the suspension material  164  may be include, for example, a thermoset polymer, a B-stage (or “pre-preg”) polymer, a pre-B stage polymer, a thermoplastic polymer, or any monomer, polymer, or other suitable material that can support the electrically conductive particles  162 .  
         [0033]    Z-axis anisotropic adhesives may be formed in a number of ways, including, for example, as a film or as a viscous paste that is applied (e.g. stenciled, sprayed, flowed, etc.) to the contact pads  104 . The anisotropically conductive adhesives may then be cured. Curing may be performed in a variety of ways, such as by subjecting the materials to certain environmental conditions (e.g. temperature, pressure, etc.), or by the removal of solvents or suitable curing compounds, or by irradiation/exposure to ultraviolet or ultrasonic energy, or by other suitable means.  
         [0034]    For example, z-axis anisotropic adhesives are commercially available in both a thermoplastic variety or a thermosetting variety. Thermoplastic anisotropic adhesives are those that are heated to soften for application to the test substrate and then cooled for curing, and include, for example, solvent-based hot-melt glue. Conversely, thermosetting anisotropic adhesives are suitable for application to the test substrate at normal ambient temperatures, and are heated for curing at temperatures from 100° C. to 300° C. for periods from several minutes to an hour or more. Suitable z-axis anisotropic adhesives include those available from A.I. Technology, Inc. of Trenton, N.J., or Sheldahl, Inc. of Northfield, Minn., or 3M of St. Paul, Minn.  
         [0035]    As best seen in FIG. 4, the anisotropically conductive layer  160  is formed on the test substrate  102 , and the bumped die  10  is positioned adjacent to the layer  160  with the solder (or conductive) bumps  16  approximately aligned with the contact pads  104 . The bumps  16  may alternately be formed of any suitable, electrically conductive material. For bumped die  10  having solder bump pitches of at least 32 μm, conventional mechanical alignment devices may be used. For finer pitches, however, more advanced optical alignment systems may be necessary, such as the type of alignment apparatus shown and described in U.S. Pat. No. 4,899,921 to Bendat et. al., incorporated herein by reference.  
         [0036]    In the test carrier  100 , the solder bumps  16  are compressed into the anisotropically conductive layer  160  prior to the curing of the layer  160  so that the solder bumps  16  become embedded in the layer  160 . The compression of the solder bumps  16  into the anisotropically conductive layer  160  compresses the conductive particles  162  into contact with each other and creates an electrically conductive path  166  between each of the solder bumps  16  and its corresponding contact pad  104 .  
         [0037]    In the test carrier  100 , the solder bumps  16  become attached to the test carrier  100  during the curing of the anisotropically conductive layer  160 . For example, in one embodiment, an anisotropically conductive layer  160  having a B stage polymer as the suspension material  164  is applied to the test carrier  100 . A bumped die  10  is pressed into the layer  160  until the solder bumps  16  are “tacked” in position, and then the bumped die  10  and test carrier  100  are placed in an oven and heated to 150° C. At this temperature, the polymer is fully crosslinked, curing the layer  160  to a hardened consistency.  
         [0038]    One or more test signals are then transmitted to the bumped die  10  through one or more of the test leads  106 , through the contact pads  104 , across the conductive paths  166 , through the solder bumps  16 , and into the bumped die  10 . Output signals from the bumped die  10  are then communicated from the solder bumps  16  back across the conductive paths  166  to the contact pads  104  and other test leads  106 , and are monitored to determine whether the bumped die  10  is functioning to the desired specifications.  
         [0039]    After testing, the bumped die  10  may be removed from the test carrier  100  by detaching the solder bumps  16  from the anisotropically conductive layer  160 . This may be accomplished in a number of ways depending upon the properties of the anisotropically conductive layer  160 , including, for example, by heating the layer  160  until it softens, or by applying solvents to dissolve the layer, or by other suitable means. After the bumped die  10  is removed, the test carrier  100  may be used to test another bumped die  10 .  
         [0040]    Alternately, FIG. 4 may represent a cross-sectional view of the bumped die  10  attached to any electronic component, such as a printed circuit board  100 . In that case, the bumped die  10  may be aligned with the contact pads  104  and attached with the anisotropically conductive layer  160  as described above, except that the bumped die  10  is not removed and remains secured to the printed circuit board  100 .  
         [0041]    Although the anisotropically conductive layer  160  is shown in FIG. 4 as being a single, continuous layer covering the entire test substrate  102 , it is not necessary that only one layer be used, or that the layer be continuous. Rather, the anisotropically conductive material may be formed on a plurality of contact pads  104  of the test carrier (or printed circuit board)  100  in a variety of patterns, including, for example, in strips covering rows of contact pads, or in a checkerboard pattern covering regions of contact pads.  
         [0042]    Furthermore, it is not necessary that the anisotropically conductive layer  160  be formed on the test carrier (or printed circuit board)  100 , but rather, the layer  160  might be formed on the solder bumps  16  of the bumped die  10 . After the layer  160  is applied to the solder bumps  16 , the test carrier  100  may be engaged with the layer to form the desired electrical connections for testing of the die.  
         [0043]    The anisotropically conductive layer  160  advantageously improves the process of testing and assembling of bumped die  10  and other bumped devices. The process of attaching (and detaching) the bumped die  10  to the test carrier (or printed circuit board)  100  using the anisotropically conductive layer  160  may be less time consuming and more economical than the prior art process of soldering (and unsoldering) the solder bumps  16  to (and from) the contact pads  104  because the rework temperatures of the anisotropically conductive layer  160  (typically 80° C. to 150° C.) may be less than the typical reflow temperature of solder (183° C.). Thus, less time and energy may be needed to bring the temperatures of the bumped die  10  and test carrier  100  up to the temperature necessary for detachment, and the potential for damaging the solder bumps  16  or the contact pads  104  may be decreased due to the reduced rework temperatures.  
         [0044]    Another advantage of the test carrier (or printed circuit board)  100  having the anisotropically conductive layer  160  is that a more flexible connection may be provided between the solder bumps  16  and the contact pads  22  than is obtained using solder. If the bumped die  10  and test carrier  100  are subjected to a large range of temperatures or repeatedly thermal cycling during the testing (e.g. burn-in tests), the flexibility of the layer  160  may relieve stresses that might otherwise occur due to the differences in the CTE of the bumped die  10  and the test carrier  100 . Depending upon the anisotropically conductive materials used, the anisotropically conductive layer  160  may advantageously expand and contract during such testing to prevent delamination or other damage to the bumped die  16  or the test carrier  100 , or to prevent damage from occurring at the connection between the solder bumps  16  and the bond pads  14 .  
         [0045]    An additional advantage of the anisotropically conductive layer  160  is that satisfactory electrical contact may be achieved between the contact pads  104  and the solder bumps  16  despite the variation in the heights of the solder bumps  16 . Because the tallest solder bumps  16  become embedded in the layer  160 , if the layer  160  is properly sized, even the shortest solder bumps  16  may be brought into contact with the layer  160  to form an electrical path  166  between the solder bumps  16  and the contact pads  104 . The anisotropically conductive layer  160  may therefore improve the electrical connection between the short solder bumps and the contact pads.  
         [0046]    The anisotropically conductive layer  160  may also reduce the compression force needed to bring the short solder bumps  16  into electrical contact with the contact pads  104 . Because the compression force is reduced, the potential for damaging the bumped die  10  or the test carrier (or printed circuit board)  100  is reduced.  
         [0047]    Yet another advantage of the anisotropically conductive layer  160  is that the solder bumps  16  of the bumped die  10  may be easily cleaned of any residual amounts of the anisotropically conductive material following testing. Some anisotropically conductive materials are commercially available that are readily dissolvable using solvents for ease of removal and cleanup. One solvent that may be suitable (depending upon the anisotropically conductive material used) is RS  816  available from Al Technology, Inc. of Princeton, N.J. Thus, the time consuming task of flux cleaning associated with traditional soldering may be avoided.  
         [0048]    [0048]FIG. 5 is a partial cross-sectional view of the bumped die  10  of FIG. 1 engaged with a test carrier  100   b  in accordance with an alternate embodiment of the invention. In this embodiment, the test carrier  100   b  includes an anisotropically conductive layer  160   b  that has a flexible outer surface  168 . The flexible outer surface  168  may be formed, for example, by at least partially curing the anisotropically conductive layer  160   b  prior to engagement with the bumped die  10 . The flexible outer surface  168  may be a resilient surface.  
         [0049]    To test the bumped die  10  using the test carrier  100   b , the die is positioned over the layer  160   b  with the solder bumps  16  approximately aligned with the contact pads  104 . The solder bumps  16  are then compressed against the flexible outer surface  168  causing localized compression of the anisotropically conductive material  160   b  in the region near each of the solder bumps  16 . The conductive particles  162  are brought into contact by the compression forces to form the conductive paths  166  between each of the solder bumps  16  and the corresponding contact pads  104 . Test signals are then transmitted to the bumped die  10  through some of the test leads  104  and the conductive paths  166 , and output signals from the bumped die  10  are transmitted from the solder bumps  16  through the conductive paths  166  to the test carrier  100   b  as previously described above.  
         [0050]    After the bumped die  10  has been tested, it is disengaged from the test carrier  100   b  by simply moving the solder bumps  16  away from the flexible outer surface  168  of the anisotropically conductive layer  160   b . If the flexible outer surface  168  of the layer  160   b  is a resilient surface, the localized compression areas near each of the solder bumps  16  will spring back to their uncompressed shape.  
         [0051]    The test carrier  100   b  having the layer  160   b  with the flexible outer surface  168  may further improve the process of testing of the bumped die  10  by reducing or eliminating the time and effort involved in detaching the solder bumps  16  from the anisotropically conductive layer  160   b . Because the solder bumps  16  are not embedded in the layer  160   b , it is not necessary to reheat the bumped die  10  or the test carrier  100   b  to the rework temperature of the anisotropically conductive layer  160   b  in order to disengage the die from the test carrier. The time, effort, and expense associated with disengaging the solder bumps  16  from the anisotropically conductive layer  160  may therefore be reduced or eliminated.  
         [0052]    Similarly, because the solder bumps  16  are not embedded in the anisotropically conductive layer  160   b , the time, effort, and expense associated with cleanup of any residual anisotropically conductive material deposited on the solder bumps  16  may also be reduced or eliminated. Depending upon the anisotropically conductive material used, the transfer of material to the solder bumps  16  may be minimized or eliminated so that the solder bumps  16  may be clean enough for immediate use after testing.  
         [0053]    [0053]FIG. 6 is a partial cross-sectional view of the bumped die  10  engaged with a test carrier (or printed circuit board)  200  in accordance with another alternate embodiment of the invention. In this embodiment, the test carrier  200  includes a test substrate  202  having a plurality of pockets  244  disposed therein. A plurality of test leads  206  are formed on the test substrate  202 , each test lead  206  terminating in a contact pad  204  that is formed within each of the pockets  244 . An anisotropically conductive layer  260  is formed on the test substrate (or printed circuit board)  202  covering the contact pads  204  and test leads  206 . The anisotropically conductive layer  260  includes a plurality of conductive particles  262  contained with a suspension medium  264 , and an outer surface  268 .  
         [0054]    In operation, the solder bumps  16  of the bumped die  10  are at least partially disposed within the pockets  244  of the test carrier  200 . The solder bumps  16  may be embedded in the anisotropically conductive layer  260  prior to the curing of the layer, or alternately, the layer  260  may be at least partially cured so that the outer surface  268  is a flexible surface and the solder bumps  16  do not penetrate the outer surface  268  or become attached to the layer  260 . In either case, a compression force may be applied to the bumped die  10  (or to the test carrier  200 ) to compress the anisotropically conductive material to form a conductive path  266  between each solder bump  16  and each contact pad  204 . Testing may then be performed on the bumped die  10 . After testing is complete, the bumped die  10  may be disengaged from the test carrier  200  in one of the ways described above. Alternately, in the case of the bumped die  10  being attached to the printed circuit board  200 , the bumped die  10  is not disengaged.  
         [0055]    The test carrier  200  having the pockets  244  and the anisotropically conductive layer  260  further improves the testing of the bumped die  10  by providing the desired electrical contact between the solder bumps  16  and the contact pads  204  without penetration of the solder bumps  16  using contact blades  48  or the like (see FIG. 2). Despite the variability of the size and shape of the solder bumps  16 , the anisotropically conductive layer  260  provides the necessary electrical contact along the conductive paths  266  between the solder bumps  16  and the contact pads  104 . Because the contact blades  48  may be eliminated, fabrication and maintenance of the test carrier  200  is simplified compared to the prior art test carrier  40  shown in FIG. 2. Also, the potential for the solder bumps  16  to be cracked, chipped, or otherwise damaged due to penetration by the contact blades  48  is eliminated.  
         [0056]    Similarly, when the bumped die  10  is engaged with the printed circuit board  200  having pockets  244  and the anisotropically conductive layer  260 , the electrical contact between the bumps  16  and the contact pads  204  is improved. As shown in FIG. 6, electrical contact between the solder bumps  16  and the sidewalls  204  is achievable over a larger contact area due to the anisotropically conductive layer  260 , providing improved electrical contact compared with the contact blades  48  of the prior art device (FIG. 2). Also, because the contact blades  48  may be eliminated, the manufacturing the pockets  244  is simplified. The pockets  244  may be formed, for example, by masking the areas surrounding the locations of the pockets  244  with a hard mask, and then etching the substrate using an etchant (e.g. KOH).  
         [0057]    [0057]FIG. 7 is a partial cross-sectional view of the bumped die  10  engaged with a test carrier (or printed circuit board)  300  in accordance with yet another embodiment of the invention. In this embodiment, the test carrier  300  includes a test substrate  302  having a plurality of pedestals  364  projecting upwardly therefrom. Test leads  306  are formed on the test substrate  302 , each test lead  306  terminating in a contact pad  304  formed on at the top of each pedestal  364 .  
         [0058]    A magnet  380  having a north pole  382  and a south pole  384  is positioned near the test substrate  302 . A plurality of magnetic flux lines  386  (only two shown in FIG. 7) emanate from the magnet  380 . An anisotropically conductive layer  360  having a plurality of conductive particles  362  and an outer surface  368  is formed on the test substrate  302 : An optical alignment system  390  (such as the type of alignment apparatus shown and described in U.S. Pat. No. 4,899,921 to Bendat et. al.) is positioned proximate the solder bumps  16  to ensure the alignment of the solder bumps  16  with the contact pads  304 . A die handler  392  is engaged with and controllably positions the bumped die  10 . Numerous types of die handlers  392  are suitable for this purpose, including, for example, those shown and described in U.S. Pat. No. 5,184,068 to Twigg et. al., U.S. Pat. No. 5,828,223 to Rabkin et. al., and the IC handlers available from Verilogic Corporation of Denver, Colo.  
         [0059]    During the formation of the anisotropically conductive layer  360 , the conductive particles  362  align with the magnetic flux lines  386  to form conductive columns along the flux lines which form a conductive path  366  between each solder bump and its corresponding contact pad. If the magnetic flux lines  386  are strong enough, some of the conductive particles  362  may be induced to protrude from the surface  368  of the layer  360  (as shown in FIG. 7). Suitable anisotropically conductive materials that form conductive paths  366  when exposed to a magnetic field include, for example, the Elastomeric Conductive Polymer Interconnect (ECPI) materials available from AT&amp;T Bell Laboratories of Murray Hill, N.J. For testing of the bumped die  10 , the solder bumps  16  may either be embedded in the anisotropically conductive layer  360  prior to the curing of the layer, or alternately, the layer  360  may be at least partially cured so that an outer surface is a flexible surface that is not penetrated by the solder bumps  16 . In either case, the solder bumps  16  are engaged with the anisotropically conductive layer  360  using the die handler  392  and the optical alignment system  390  so that each of the solder bumps  16  are electrically coupled to a corresponding one of the contacts pads  304  by at least one of the conductive paths  366 . Testing may then be performed on the bumped die  10 , and the bumped die  10  may be disengaged from the test carrier  300  in one of the ways described above.  
         [0060]    An advantage of the test carrier  300  having the pedestals  364  and the anisotropically conductive layer  360  is that the desired electrical contact between the solder bumps  16  and the contact pads  304  is provided without penetration of the solder bumps  16  using the projections  69  (see FIG. 3). Because the projections  69  may be eliminated, fabrication of the test carrier (or printed circuit board)  300  is simplified compared to the prior art test carrier  60  shown in FIG. 3. Also, the potential for the solder bumps  16  to be cracked, chipped, or otherwise damaged due to penetration by the projections  69  is eliminated.  
         [0061]    Another advantage is that the bumped device  10  may be engaged with the test carrier  300 , tested, and disengaged rapidly and efficiently. The anisotropically conductive layer  360  eliminates the time and expense associated with reflowing the solder bumps  16 , and provides the desired electrical contact despite variation in the heights of the solder bumps  16 .  
         [0062]    Although the above described embodiments of the anisotropically conductive layers have been described with specific reference to anisotropically conductive materials that form electrically conductive paths when subjected to a compression force, some anisotropically conductive materials do not require a compression force to form conductive paths. For such materials, the desired electrical contact between the solder bumps and the contact pads of the test carrier may be formed without applying a compression force.  
         [0063]    Suitable anisotropically conductive materials that do not require a compression force to form conductive paths include, for example, Elastomeric Conductive Polymer Interconnect (ECPI) materials available from AT&amp;T Bell Laboratories of Murray Hill, N.J. Conductive paths are formed in AT&amp;T Bell&#39;s ECPI materials by subjecting the materials to a magnetic field.  
         [0064]    The detailed descriptions of the above embodiments are not exhaustive descriptions of all embodiments contemplated by the inventors to be within the scope of the invention. Indeed, persons skilled in the art will recognize that certain elements of the above-described embodiments may variously be combined or eliminated to create further embodiments, and such further embodiments fall within the scope and teachings of the invention. It will also be apparent to those of ordinary skill in the art that the above-described embodiments may be combined in whole or in part with prior art apparatus and methods to create additional embodiments within the scope and teachings of the invention.  
         [0065]    Thus, although specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. The teachings provided herein of the invention can be applied to other apparatus and methods of testing and assembling bumped devices using anisotropically conductive layers, and not just to the apparatus and methods described above and shown in the figures. In general, in the following claims, the terms used should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims, but should be construed to include all apparatus and methods of testing and assembling bumped devices using anisotropically conductive layers that operate within the broad scope of the claims. Accordingly, the invention is not limited by the foregoing disclosure, but instead its scope is to be determined by the following claims.