FIELD OF THE INVENTION  
       [0001]     The present invention relates to microelectronic assemblies and more particularly relates to stacked microelectronic assemblies having compliant layers.  
       BACKGROUND OF THE INVENTION  
       [0002]     Semiconductor chips are commonly provided as individual, prepackaged units. A standard chip has a flat, rectangular body with a large front face having contacts for connection to the internal circuitry of the chip. Each individual chip is typically mounted to a substrate or chip carrier, which in turn is mounted on a circuit panel such as a printed circuit board. Considerable effort has been devoted towards development of so-called “multichip packages” in which several chips having related functions are attached to a common circuit panel and protected by a common package. This approach conserves some of the space which is ordinarily wasted by individual chip packages. However, most multichip packages utilize a single layer of chips positioned side-by-side on a surface of a planar circuit panel.  
         [0003]     Another space conserving design is commonly referred to as a “flip chip” package in which the front face of a semiconductor chip confronts a top surface of a circuit panel and the contacts on the chip are bonded to the circuit panel by solder balls or other connecting elements. The “flip chip” design provides a relatively compact arrangement, with each chip occupying an area of the circuit panel equal to or slightly larger than the area of the chip. As disclosed, in commonly assigned U.S. Pat. Nos. 5,148,265 and 5,148,266, the disclosures of which are hereby incorporated by reference herein, certain innovative mounting techniques offer compactness approaching or equaling that of “flip chip” packages without the reliability and testing problems commonly encountered in that approach.  
         [0004]     Another package design for saving space in electronic components is commonly referred to as a “stacked” arrangement, i.e., an arrangement where several chips are placed one atop the other. One such stacked arrangement is disclosed in commonly assigned U.S. Pat. No. 5,347,159, the disclosure of which is hereby incorporated by reference herein, wherein chips are stacked one atop the other and interconnected with one another by conductors on so-called “wiring films” associated with the chips.  
         [0005]     Another stacked arrangement is disclosed in preferred embodiments of commonly assigned U.S. Pat. No. 5,861,666, the disclosure of which is hereby incorporated by reference herein. One aspect of the invention in the &#39;666 patent provides a plurality of semiconductor chip assemblies whereby each assembly includes an interposer and a semiconductor chip mounted thereto. Each interposer also includes a plurality of leads electrically interconnecting the chip and the interposer. The assembly also includes compliant layers disposed between the chips and the interposers so as to permit relative movement of the chips and interposers to compensate for thermal expansion and contraction of the components. As is well known to those skilled in the art, semiconductor chips dissipate electrical power as heat during operation. When chips are stacked one atop the other, it is difficult to dissipate the heat generated by the chips in the middle of the stack. Consequently, the chips in such a stack may undergo substantial thermal expansion and contraction during operation. This, in turn, imposes significant mechanical stress on the interconnecting arrangements and on the mountings which physically retain the chips.  
         [0006]     Still another “stacked” arrangement is disclosed in commonly assigned U.S. Pat. No. 6,225,688, the disclosure of which is hereby incorporated by reference herein. Referring to FIGS. 1 and 2 of the &#39;688 patent, a microelectronic assembly includes a flexible substrate  10  having a wiring layer  12  and leads  14  having ends  16  extending to a plurality of attachment sites  18 . The leads  14  have connections sections configured for bonding at each attachment site. The plurality of attachment sites  18  and the ends  16  of the leads  14  extending to the attachment sites are provided at a first surface  20  of the flexible substrate  10 . The attachment sites  18  are grouped in pairs  25 A and  25 B which are spaced on the flexible substrate  10 . The flexible substrate  10  includes conductive terminals  22  accessible at the second surface  24  thereof. The conductive terminals  22  are connected with the wiring layer  12  and with at least some of the leads  14 .  
         [0007]      FIGS. 3 and 4 A show a plurality of microelectronic elements  26  assembled to the attachment sites  18 . In certain preferred embodiments, the chips are fully packaged prior to attachment to the “folding substrate.” As a result, if one chip is defectively packaged, the entire module does not need to be replaced. In one embodiment, each microelectronic element  26  preferably includes a semiconductor chip having a front face  28  with one or more electrical contacts  30  thereon. Each semiconductor chip  26  also includes a back surface  32 . Before the chips  26  are assembled, a plurality of compliant pads  31  are provided over each attachment site  18 . The compliant pads  31  define channels  35  running therebetween, such as disclosed in commonly assigned U.S. Pat. No. 5,659,952, the disclosure of which is hereby incorporated by reference herein. Next, the front face  28  of the semiconductor chip  26  is abutted against the compliant pads  31  at the attachment site  18  and the contacts  30  on the chip are aligned with the leads  14  extending to each attachment site, and the leads  14  are electrically interconnected with the contacts  30 . After the semiconductor chips  26  have been assembled to the attachment sites  18  and bonded to the leads  14 , the wiring layer  12  interconnects the semiconductor chips  26  with the conductive terminals  22  at the second surface  26  of the flexible substrate  20 .  
         [0008]     Referring to  FIG. 4B , a curable liquid encapsulant  33  is applied around at least the perimeter of the chips  26 . The encapsulant  33  flows between the front face  28  of the chip  26  and the attachment site  18 , through the channels  35  between the plurality of compliant pads  31  and around the leads  14  bonded to the contacts  30 . The encapsulant  33  is preferably cured using energy, such as heat, to provide a compliant interface between each chip  26  and the flexible substrate  10 .  
         [0009]     Referring to  FIG. 5 , the flexible substrate  10  is then folded in a gentle zig-zag or an “S” shaped pattern to stack the chips in vertical alignment with one another, whereby portions of the flexible substrate  10  overlap. During the folding step, the back surfaces  32  of paired semiconductor chips  26  are juxtaposed with one another. In order for the back surfaces  32  of the pairs of microelectronic elements  26  to be juxtaposed with one another without stretching or tearing the flexible substrate  10 , the attachment sites  18  must be spaced sufficiently apart so that there is adequate slack in the flexible substrate  10  between the paired chips  26 . The particular embodiment shown in  FIG. 5  includes a first pair  34  of semiconductor chips  26  juxtaposed back-to-back to one another and sandwiched between a first section  38  of the flexible substrate  10 . The flexible substrate  10  is then folded back over upon itself at an intermediate section  40  thereof, whereby portions of the flexible tape  10  are juxtaposed with one another. Next, the back surfaces  32  of a second pair  36  of semiconductor chips  26  are juxtaposed with one another. The final stacked assembly shown in  FIG. 5  includes the first and second pairs  34  and  36  of chips  26  whereby the first pair  34  is provided over the second pair  36  and the two pairs  34  and  36  are substantially in vertical alignment with one another. A compliant layer is preferably disposed between each chip and the attachment site on which the chip is mounted.  
         [0010]     Although the approaches set forth above offer useful ways of making stacked assemblies, still other methods would be desirable. Specifically, stacked assemblies having smaller footprints and lower silhouettes are highly desirable.  
       SUMMARY OF THE INVENTION  
       [0011]     In accordance with certain preferred embodiments of the present invention, a method of making a stacked microelectronic assembly includes providing a flexible substrate having first and second ends, the flexible substrate having a plurality of attachment sites located between the first and second ends thereof with a first one of the attachment sites located adjacent the first end of the flexible substrate, the flexible substrate including conductive terminals accessible at a surface of the flexible substrate and wiring connected to the terminals, and providing a compliant layer over the first attachment site. The method also includes assembling a plurality of microelectronic elements over the attachment sites, whereby a first one of the microelectronic elements engages the compliant layer and is movable relative to the flexible substrate, electrically interconnecting the microelectronic elements and the wiring, and folding the flexible substrate so as to stack at least some of the microelectronic elements in generally vertical alignment with one another so that the first one of the microelectronic elements engaging the compliant layer is disposed at a bottom of the stacked assembly. The stacked microelectronic elements are desirably maintained in the substantially vertical alignment so that the conductive terminals are exposed at the bottom end of the stacked assembly.  
         [0012]     In certain preferred embodiments, the wiring includes flexible leads extending to the attachment sites, and the electrically interconnecting step includes electrically connecting the microelectronic elements and the flexible leads. The wiring preferably interconnects at least some of the microelectronic elements with one another.  
         [0013]     The flexible substrate desirably includes a polymeric material having a thickness between approximately 25-75 microns. During the assembly step, the contacts on front faces of the microelectronic elements are preferably aligned with ends of the flexible leads at the attachment sites. In certain preferred embodiments, the step of providing a compliant layer desirably includes providing a plurality of compliant pads at the one of the attachment sites before the assembling step, the compliant pads defining channels therebetween. A curable liquid encapsulant may be introduced between the plurality of compliant pads and through the channels between the compliant pads. The curable liquid encapsulant may be cured to provide the compliant layer.  
         [0014]     In certain preferred embodiments, the stacking step includes grouping at least some of the microelectronic elements in pairs and juxtaposing the paired microelectronic elements with one another. Each of the microelectronic elements desirably includes a front contact bearing surface and a back surface remote therefrom, at least some of the microelectronic elements being assembled to the flexible substrate with the front contact bearing surfaces facing toward the attachment sites and the back surfaces facing away from the attachment sites. The juxtaposing step desirably includes abutting the back surfaces of the paired microelectronic elements with one another. The method may also include applying an adhesive between the back surfaces of the paired microelectronic elements before the abutting step. The adhesive may include a thermally conductive adhesive.  
         [0015]     After the flexible substrate has been folded so as to generally align the microelectronic elements in a vertical configuration, a support structure may be used for maintaining the assembly in a stacked configuration. In certain preferred embodiments, the support structure includes a bracket abutting against a top of the stacked microelectronic elements. In certain preferred embodiments, thermally conductive sheets may be provided between the back surfaces of the paired microelectronic elements for removing heat from the assembly.  
         [0016]     In other preferred embodiments of the present invention, a stacked microelectronic assembly includes a flexible substrate having a plurality of attachment sites, the flexible substrate including conductive terminals accessible at a surface thereof, wiring connected to the terminals accessible at a surface thereof, wiring connected to the terminals and flexible leads connected to the wiring and extending to the attachment sites, and a plurality of microelectronic elements assembled to the attachment sites and electrically connected to the leads. The stacked microelectronic assembly may also include a compliant layer disposed between one of the microelectronic elements and the attachment site associated therewith, whereby the one of the microelectronic elements is movable relative to the flexible substrate. The flexible substrate is preferably folded so that at least some of the microelectronic elements are stacked in substantially vertical alignment with one another, the one of the microelectronic elements being positioned at a bottom end of the stacked assembly, and a securing element maintaining the stacked microelectronic elements in substantially vertical alignment with one another, whereby the conductive terminals are exposed at the bottom end of the stacked assembly.  
         [0017]     In other preferred embodiments of the present invention, a method of making a stacked microelectronic assembly includes providing a flexible substrate having a plurality of attachment sites, the flexible substrate including conductive terminals accessible at a surface thereof and wiring connected to the terminals, and assembling a plurality of microelectronic elements over the attachment sites. The method also desirably includes electrically interconnecting the microelectronic elements and the wiring, providing an encapsulant layer between the microelectronic elements and the attachment sites, folding the flexible substrates and stacking at least some of the microelectronic elements in generally vertical alignment with one another, wherein a first one of the microelectronic elements is disposed at a bottom of the stacked assembly, and whereby a region of the encapsulant layer adjacent the first microelectronic element is more compliant than the encapsulant layer adjacent the other microelectronic elements. The method also desirably includes maintaining the stacked microelectronic elements in the substantially vertical alignment, with the conductive terminals exposed at the bottom of the stacked assembly.  
         [0018]     In yet another preferred embodiments of the present invention, a stacked microelectronic assembly includes a dielectric element having an upwardly-facing first surface and a downwardly-facing second surface and having conductive terminals exposed at the second surface, and a first microelectronic element overlying the first surface of the dielectric element. The stacked microelectronic assembly also preferably includes a second microelectronic element overlying the first microelectronic element, and a first encapsulant layer between the first microelectronic element and the first surface of the dielectric layer. The stacked microelectronic assembly also desirably includes a second encapsulant layer between the first and second microelectronic elements, whereby the first encapsulant layer is more compliant than the second encapsulant layer so that one or more of the conductive terminals underlying the first microelectronic element are movable relative to the first microelectronic element.  
         [0019]     In further preferred embodiments of the present invention, a microelectronic assembly includes a plurality of microelectronic subassemblies, each subassembly including a dielectric substrate having a top surface, a microelectronic element mounted over the dielectric substrate, whereby the microelectronic element is electrically interconnected with the dielectric substrate, and an encapsulant layer provided over the top surface of the dielectric substrate between the microelectronic element and the dielectric substrate, with the microelectronic subassemblies stacked one atop another. The encapsulant layer of the bottom one of the stacked subassemblies is more compliant than the encapsulant layers of the stacked subassemblies above the bottom subassembly.  
         [0020]     In still other preferred embodiments of the present invention, a microelectronic assembly with a basal compliant layer includes microelectronic subassemblies stacked one atop another, each subassembly having a dielectric substrate having a top surface, a microelectronic element mounted over the top surface of the dielectric substrate, an encapsulant layer between the microelectronic element and the top surface of the dielectric substrate, wherein the encapsulant layer of a bottom one of the stacked microelectronic subassemblies is more compliant than the encapsulant layers of the other microelectronic subassemblies of the stacked microelectronic assembly.  
         [0021]     In certain preferred embodiments, two or more of the stacked microelectronic subassemblies are electrically interconnected with one another. The dielectric substrate of the bottom subassembly preferably has conductive terminals accessible at a bottom surface thereof. In other preferred embodiments, at least one of the stacked subassemblies is electrically interconnected with conductive terminals at the bottom of the assembly. The encapsulant layer of the bottom subassembly may include a plurality of compliant pads.  
         [0022]     Although the present invention is not limited by any particular theory of operation, it is believed that providing a compliant layer for the bottom-most chip of a stacked assembly, while not providing a compliant layer for the remaining chips in the stack, will minimize the overall height of the stacked assembly while allowing movement between the conductive terminals at the bottom of the package and the bottom-most chip during thermal cycling.  
         [0023]     These and other preferred embodiments of the present invention will be described in more detail below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]      FIG. 1  shows a diagrammatic top view of one stage of a method of making a conventional stacked microelectronic assembly.  
         [0025]      FIG. 2  shows a side view of  FIG. 1 .  
         [0026]      FIGS. 3-5  show still further stages of a method of making a conventional stacked microelectronic assembly.  
         [0027]      FIGS. 6 and 7  show a method of making a stacked microelectronic assembly having a basal compliant layer, in accordance with certain preferred embodiments of the present invention.  
         [0028]      FIG. 8  shows a side view of a stacked microelectronic assembly having a basal compliant layer, in accordance with other preferred embodiments of the present invention.  
         [0029]      FIGS. 9 and 10  show a side view of a method of making a stacked microelectronic assembly having a basal compliant layer, in accordance with still other preferred embodiments of the present invention.  
         [0030]      FIGS. 11 and 12  shows a method of making a stacked microelectronic assembly having a basal compliant layer, in accordance with still other preferred embodiments of the present invention.  
         [0031]      FIG. 13  shows a side view of a stacked microelectronic assembly having a basal compliant layer, in accordance with another preferred embodiments of the present invention.  
         [0032]      FIG. 14  shows a side view of a stacked microelectronic assembly having a basal compliant layer, in accordance with still another preferred embodiment of the present invention.  
         [0033]      FIGS. 15 and 16  show a method of making a stacked microelectronic assembly having a basal compliant layer, in accordance with yet further preferred embodiments of the present invention.  
         [0034]      FIGS. 17 and 18  show a method of making a stacked microelectronic assembly having a basal compliant layer, in accordance with still further preferred embodiments of the present invention.  
         [0035]      FIGS. 19 and 20  show a method of making a stacked microelectronic assembly, in accordance with other preferred embodiments of the present invention.  
         [0036]      FIG. 21  shows a stacked microelectronic assembly having a basal compliant layer, in accordance with other preferred embodiments of the present invention.  
         [0037]      FIGS. 22 and 23  show a method of making a stacked microelectronic assembly having a basal compliant layer, in accordance with yet further preferred embodiments of the present invention.  
         [0038]      FIG. 24  show a stacked microelectronic assembly having a basal compliant layer, in accordance with other preferred embodiments of the present invention.  
         [0039]      FIGS. 25-27  show a method of making a stacked microelectronic assembly having a basal compliant layer, in accordance with other preferred embodiments of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0040]     As noted above, the present invention is related to providing a basal compliant layer for a stacked microelectronic assembly. In certain preferred embodiments, only the bottom microelectronic element in a stack has a compliant layer for enabling movement during thermal cycling, while the microelectronic elements above the bottom microelectronic element do not have a compliant layer. This design reduces the overall height of the stacked package, while allowing movement between the bottom-most microelectronic element and the conductive terminals of the assembly.  
         [0041]     Referring to  FIGS. 6 and 7 , in certain preferred embodiments of the present invention, a stacked assembly includes a plurality of chips  126  mounted to a flexible substrate  100 . The substrate  100  is folded to align the chips  126  in a generally vertical configuration. An adhesive  144 , such as a thermally conductive adhesive, is provided between juxtaposed back surfaces  132  of semiconductor chips  126 . The adhesive  144  bonds the back surfaces of the juxtaposed chips  126  together to provide stability to the stacked assembly. The thermally conductive adhesive  144  preferably promotes even distribution of heat in the stacked chips  126 , and thus limits the temperature rise in the hottest chips. The thermally conductive adhesive promotes conduction in the vertical direction within the stack; i.e., transfers the heat to the top and bottom of the stacked assembly for dissipation outside the assembly. To provide additional support for the assembly, a mechanical element  146 , such as a bracket, is placed over the vertically aligned chips  126  so that the bracket  146  abuts against the top  148  of the aligned chips  126 . Preferably, the bracket  146  does not include any side walls so that cooling air may freely interact with the exposed surfaces of the semiconductor chips  126 . In other embodiments, the bracket  146  may include one or more side walls having openings therein for enabling cooling air to flow therethrough. Conductive terminals  122  are exposed at the bottom of the final assembly so that the chips  126  may be electrically interconnected with an external circuit element  148 , such as a printed circuit board. A compliant layer  127  is desirably provided between bottom chip  126 A and terminals  122  to provide for relative movement of the terminals  122  and bottom chip  126 A during thermal cycling of the stacked assembly. Fusible material is preferably provided on the conductive terminals  122  for bonding the terminals  122  to conductive pads  152  located at a top surface  154  of the printed circuit board  148 .  
         [0042]      FIG. 8  shows another preferred embodiment of the present invention wherein a thermally conductive adhesive is not used between the back surfaces of the pairs of chips  226 . In this particular embodiment, the semiconductor chips  226  are assembled and electrically interconnected with a flexible substrate  210 . A basal compliant layer  227  is desirably provided between bottom-most chip  226 A and conductive terminals  222  to allow movement of the terminals  222  relative to the bottom-most chip  226 A during thermal cycling. The flexible substrate  210  is folded so that the chips  226  are stacked in vertical alignment with one another and so that the back surfaces of pairs  234 ,  236  are juxtaposed with one another. While the chips  226  are held in vertical alignment, a securing element  246 , such as the bracket described above, is placed over the top of the stack. The securing element  246  abuts against the top of the stack to maintain the stacked assembly in vertical alignment. The assembly may then be electrically interconnected with an external circuit element  248  using the methods described above.  
         [0043]      FIGS. 9 and 10  show further embodiments of the present invention whereby chips  326  are stacked in both vertical alignment and side-by-side. For example, first and second groups of microelectronic elements  325 A,  325 B are assembled to the flexible substrate  310  so that the chips  326  within the respective first and second groups  325 A,  325 B are in close proximity with one another. A compliant layer  327  is provided between chip  326 A and conductive terminals  322  to provide for movement of chip  326 A relative to terminals  322  during thermal cycling. As shown in  FIG. 10 , the flexible substrate  310  is folded over so that the back surfaces  332  of the chips  326  in the first group  325 A are adjacent the back surfaces  332  of the chips  326  in the second group  325 B. Thus, although the chips  326  in any one group are disposed side-by-side, the chips in the different groups are stacked in vertical alignment, one atop the other, to provide a stacked assembly which will save space on a circuit board. In certain preferred embodiments, the back surfaces of the chips may be in contact with one another.  
         [0044]      FIGS. 11 and 12  show still further embodiments of the present invention whereby some of the chips  426  are assembled to the first surface  420  of the flexible substrate  410  and some of the chips  426  are assembled to the second surface  424  of the flexible substrate  410 . A compliant layer  427  is provided between bottom-most chip  426 A and terminals  422  to provide for relative movement during thermal cycling. Referring to  FIG. 12 , the flexible substrate  410  is then folded in an “S”-shaped or gentle zig-zag configuration to provide a stacked assembly whereby the chips are in substantial vertical alignment with one another. The stack is maintained in vertical alignment using the thermally conductive adhesive and/or the mechanical element discussed above. Flexible metal sheets (not shown) may be placed between the microelectronic elements to transfer heat from the chips.  
         [0045]     Referring to  FIGS. 13 and 14 , in yet further preferred embodiments of the present invention, the conductive terminals can be accessible at either the first surface or the second surface of the flexible substrate. A compliant layer  527  is preferably provided between bottom-most chip  526 A in the stack and conductive terminals  522  to provide for relative movement during thermal cycling. After the chips are assembled to the flexible substrate, the flexible substrate is folded so that the conductive terminals are exposed at the bottom of the stack so the assembly may be electrically connected to an external circuit element, such as a printed circuit board.  FIG. 13  shows a flexible substrate  510  having the chips  526  assembled to both the first and second surfaces  520 ,  524  of the flexible substrate  510  and the conductive terminals  522  being accessible at the second surface  524  of the flexible substrate  510 .  FIG. 14  shows another embodiment whereby a flexible substrate  610  has chips assembled to both the first and the second surfaces  620 ,  624  of the flexible substrate  610 ; however, the conductive terminals  622  are accessible at the first surface  620  of the flexible substrate  610 . A compliant layer  627  is provided between bottom-most chip  626 A and conductive terminals  622  to provide for movement during thermal cycling. In this particular embodiment, an extra fold is provided in the flexible substrate  610  when forming the stacked assembly so that the conductive terminals  622  are exposed at the bottom of the assembly.  
         [0046]      FIGS. 15 and 16  show yet another embodiment of the present invention whereby the flexible substrate  710  includes a plurality of electrically conductive test contacts  701 . The test contacts  701  are connected to the wiring layer (not shown) and at least some of the leads (not shown) which interconnect the chips  726  with the flexible substrate  710 . The test contacts may be disposed on either the first surface  706  of the flexible substrate or the second surface  707 . Referring to  FIG. 16 , a compliant layer  727  is provided between bottom-most chip  726 A in the stack and conductive terminals  722  to provide for relative movement of chip  726 A to terminals  727  during thermal cycling. As depicted in  FIGS. 17 and 18 , the test contacts may be disposed on both the first surface  806  and the second surface  807 . As depicted in  FIG. 18 , after the flexible substrate is folded, the test contacts are preferably exposed on the top end of the assembly. By incorporating test contacts, the assembly may be tested before, during or after the assembly is connected to a larger circuit panel such as a printed circuit board. Having the test contacts disposed on the top end of the assembly facilitates this testing because the test contacts are more easily accessed. A compliant layer  827  is provided between bottom-most chip  826 A in the stack and conductive terminals  822  to provide for relative movement of chip  826 A to terminals  827  during thermal cycling.  
         [0047]      FIGS. 19 and 20  depict another embodiment of the present invention whereby the assembly includes two or more encapsulant layers  933 A,  933 B having different levels of compliancy. Encapsulant layer  933 B is preferably more compliant than encapsulant layer  933 A. The encapsulant is disposed between the face of each chip and the flexible substrate. The encapsulant is typically formed by applying a curable liquid encapsulant composition around the perimeter of the chips and then curing the composition to form cured encapsulant layers  933 A,  933 B. A plurality of complaint pads (not shown) may be disposed on the flexible substrate  910  before the liquid curable encapsulant is disposed on the flexible substrate. The curable liquid encapsulant composition is dispensed onto the flexible substrate  910  after the leads  914  are interconnected to the semiconductor chips  926  and before the substrate  910  is folded. The liquid curable encapsulant composition is desirably cured before the flexible substrate  910  is folded.  
         [0048]      FIG. 22  depicts still another embodiment of the present invention. The multi-part stacked microelectronic assembly includes a first stacked microelectronic assembly and a second stacked microelectronic assembly which is interconnected with the first stacked microelectronic assembly. The first stacked microelectronic assembly includes a plurality of vertically aligned first semiconductor chips  1026   a . The first assembly also includes a first flexible substrate  1010   a  which is disposed in a folded configuration and which has a plurality of electrically conductive first terminals  1022   b , and first wiring (not shown) including a plurality first leads  1014   a  which interconnect the first chips  1026   a  with first terminals  1022   a . The first stacked microelectronic assembly may also include a first adhesive  1009   a  disposed between the back surfaces of vertically aligned first chips  1026   a , or another first securing element for maintaining the vertical alignment of such first chips. The first assembly also includes a plurality of electrically conductive test contacts  1001   a  disposed on the first flexible substrate  1010   a . Such test contacts are electrically interconnected to first chips  1026   a.    
         [0049]     The second stacked microelectronic assembly includes a plurality of vertically aligned semiconductor chips  1026   b  and  1026   b ′. The second assembly also includes a second flexible substrate  1010   b  which is disposed in a folded configuration and which has a plurality of electrically conductive second terminals  1022   b , and second wiring (not shown) including a plurality second leads  1014   b  which interconnect the second chips  1026   b ,  1026   b ′ with second terminals  1022   b . The second assembly may also include a second adhesive  1009   b  disposed between the back surfaces of vertically aligned second chips  1026   b ,  1026   b ′ or another securing element for maintaining the vertical alignment of such second chips. The second assembly also includes a plurality of electrically conductive connection pads  1099   b  disposed on second substrate  1010   b . The first and the second stacked assemblies are interconnected to form a multi-part stacked assembly aligning and interconnecting the first terminals  1022   a  of the final assembly with the connection pads  1099   b  of the second assembly. The bottom-most encapsulant layer  1033   b ′ is more compliant than the other encapsulant layers  1033   a ,  1033   b  of the stack so as to provide for relative movement between second terminals  1022   b  and bottom-most chip  1026   b ′ during thermal cycling.  
         [0050]      FIGS. 22 and 23  show yet another preferred embodiment of the present invention whereby a support element is disposed around one or more of the microelectronic elements  1126 , with the terminals  1122  and/or test contacts  1101  disposed on an area of the flexible substrate  1110  that is greater than the foot print of the adjacent microelectronic element  1126 A,  1126 B. The terminals or test contact may be disposed in such an area because, for example, of a need to match the arrangement of terminals with the arrangement of connection pads on a printed circuit board, because the number of terminals or text contacts needed is greater than the number that can be accommodated in an area that corresponds to the foot print of the microelectronic element or because of a desire to increase the pitch, or center to center, distance between adjacent terminals or test contacts. When some of the terminals and/or contacts are disposed in an area outside the footprint of the adjacent microelectronic element, it may be desirable to incorporate one or more support elements, such as support rings  1108 , into the assembly. As depicted in  FIG. 22 , a first support ring  1108 A is disposed around the bottom-most microelectronic element  1126 A and a second support ring  1108 B is disposed around the top microelectronic element  1126 B. Such support rings may be made of any relatively rigid material such as a metal or a plastic. Metal or epoxy support rings are preferred. The support rings help to maintain the planarity of the assembly, especially when the assembly is incorporated into a larger circuit element, such as a printed circuit board, and/or when the test contacts are engaged with a testing device. In preferred embodiments, if one of the microelectronic elements is surrounded by a support element, each of the microelectronic elements is surrounded by a support element. A compliant layer  1127  is preferably provided between bottom-most chip  1126 A in the stack and conductive terminals  1122  to provide for relative movement of chip  1126 A to terminals  1127  during thermal cycling. A second compliant layer  1129  may be provided between support ring  1108 A and conductive terminals  1122 .  
         [0051]     Certain preferred embodiments of the present invention include stacked assemblies such as those disclosed in commonly assigned U.S. patent application Ser. No. 10/267,450, filed Oct. 9, 2002, the disclosure of which is hereby incorporated by reference herein. Referring to  FIG. 24 , a stacked chip assembly includes a plurality of units  1256 A- 1256 D. Each such unit includes a panel or chip carrier  1220  and a chip  1258  associated with that panel. Each such chip has a front or contact bearing surface  1260  and a rear surface  1262 . The front surface  1260  of each chip has contacts  1264  arranged in rows adjacent the center of the chip. The chip also has edges  1266  bounding the front and rear surfaces  1262 . The thickness of the chip (the dimension between the front surface  1260  and back surface  1262 ) typically is substantially smaller than the other dimensions of the chip. For example, a typical chip may be about 100-200 microns thick and may have horizontal dimensions (in the plane of the front and rear surfaces) of about 0.5 cm or more. The front surface  1260  of the chip faces towards the second surface  1230  of the associated panel  1220 .  
         [0052]     An encapsulant layer  1268 , such as a layer of adhesive, is disposed between the chip  1258  and the panel  1220  of each unit  1256 . The encapsulant layer  1268 ′ of the bottom-most unit  1256 D is preferably more compliant than the encapsulant layers  1268  of the other units  1256 A- 1256 C stacked above the bottom-most unit  1256 D. Each encapsulant layer  1268  preferably defines an aperture in alignment with the bond window. Encapsulant layer  1268  may be provided by applying a liquid or gel material between the chip and the panel at the time of assembly or by providing a porous layer such as an array of small resilient elements between the layers and injecting a flowable material into such layer as taught, for example, in certain embodiments of U.S. Pat. Nos. 5,659,952 and 5,834,339, the disclosures of which are hereby incorporated by reference herein. Preferably, however, the encapsulant layer is provided as one or more solid or semi-solid pads having substantially the same horizontal extent as the desired encapsulant layer in the final product. These pads are placed between the chip and panel during assembly. For example, the pad may be pre-assembled to the panel or to the chip before the chip is juxtaposed with the panel. Such a solid or semi-solid pad can be placed quite accurately in relation to the chip and the panel. This helps to assure that the pad does not cover terminals  1222 , even where there is only a small clearance between the nominal position of the pad edge and the terminals. Such a pad may include an uncured or partially cured layer and other adhesion-promoting features as discussed, for example, in U.S. Pat. No. 6,030,856, the disclosure of which is hereby incorporated by reference herein. Alternatively or additionally, the pad may be provided with a thin layer of a flowable adhesive on one or both surfaces, and this layer may be a non-uniform layer as described in U.S. Pat. No. 5,548,091, the disclosure of which is hereby incorporated by reference herein, to help prevent gas entrapment in the layer during assembly.  
         [0053]     The chip  1258  of each unit is aligned with the central region of the associated panel, so that the rows of contacts  1264  are aligned with the bond window  1232  in the panel. The connection section  1240  of each lead is connected to a contact  1264  of the chip. During this process, the connection section of each lead is detached from the anchor section  1244  of the lead by breaking the frangible section  1242  of the lead. This process may be performed as described in the aforementioned U.S. Pat. No. 5,489,749 by advancing a tool (not shown) such as a thermal, thermosonic or ultrasonic bonding tool into the bond window of the panel in alignment with each connection section so that the tool captures the connection section and forces it into engagement with the appropriate contact.  
         [0054]     The units are stacked one on top of the other as illustrated in  FIG. 24 . Each terminal  1222  is connected to the corresponding terminal of the next adjacent unit via a solder ball  1278 . The solder balls  1278  serve as conductive elements which join the corresponding terminals of the various units into vertical conductive buses. Each solder ball makes contact with the terminal of one unit through an aperture and with a terminal of the other unit through an aperture in the dielectric layer of the panel  1220  in that unit.  
         [0055]     Prior to assembly of the stack, the individual units can be tested in a test socket having contacts corresponding to the locations of the terminals. Typically, the solder balls are bonded to the terminals of each unit so that they project from the first surface of the panel and the unit is tested with the solder balls in place. For example, the test socket may have openings adapted to engage the solder balls. Because all of the units have terminals and solder balls in the same pattern, the single test socket can be used to test all of the units.  
         [0056]      FIGS. 25-27  show a microelectronic assembly similar to that disclosed in commonly assigned U.S. Pat. No. 5,861,666 and U.S. patent application Ser. No. 10/487,482, filed Sep. 17, 2004, the disclosures of which are hereby incorporated by reference herein. Referring to  FIGS. 25-27 , microelectronic assembly  1300  is preferably made from a number N of prefabricated subassemblies, comprising N-1 subassemblies  1310  ( FIG. 26 ) and bottom subassembly  1320  ( FIG. 25 ). Referring to  FIG. 26 , subassembly  1310  comprises a semiconductor chip  1301  having opposed surfaces  1302  and  1303 , one surface having exposed electrical contacts (not shown), and substrate  1315 , such as a flexible dielectric substrate having a first surface  1316  and a second surface  1317 . Chip  1301  is mounted on first surface  1316  of substrate  1315  and the contacts are electrically connected to conductors (not shown) on a surface of substrate  1315 . Fan-out connectors  1311 , such as high-melting solder balls, are affixed to the second surface  1317  of substrate  1315  (the side opposite chip  1301 ). Referring to  FIG. 25 , the bottom-most subassembly  1320  comprises an encapsulated microelectronic element  1301 ′, encapsulant  1304  and substrate  1325  having top surface and bottom surface  1327 . A plurality of joining units  1321  are affixed to second surface  1327  (the side opposite from microelectronic element  1301 ) of substrate  1325 . The encapsulant  1304  is preferably compliant so as to allow for relative movement between terminals  1321  and chip  1301 ′ during thermal cycling. Bottom subassembly  1320  is adapted to serve as the bottom-most unit of stack  1300  and may be affixed directly to a printed circuit board or to a second microelectronic assembly.  
         [0057]     Referring to  FIG. 27 , when subassemblies  1310  and  1320  are stacked, fan-out connectors  1311  electrically interconnect the subassemblies within the stack, thereby acting as vertical conductors. To allow stacking, fan-out connectors  1311  of each subassembly  1310  must be positioned outside of the region of substrate  1315  of the next lower subassembly occupied by chip  1301 . Typically, this requirement results in fan-out connectors  1311  of each subassembly  1310  being disposed in a peripheral region of interposer  1315 .  
         [0058]     As will be appreciated, numerous variations and combinations of the features discussed above can be utilized without departing from present invention as defined by the claims. For example, certain preferred embodiments above depict a stacked microelectronic assembly which is four chips high, however, more chips or fewer chips may be used in accordance with the chip stacking methods of the present invention. Accordingly, the foregoing description of the preferred embodiments should be taken by way of illustration rather than by way of limitation of the present invention.
A method of making a stacked microelectronic assembly includes providing a flexible substrate having first and second ends, the flexible substrate having a plurality of attachment sites located between the first and second ends thereof including a first one of the attachment sites located adjacent the first end of the flexible substrate, the flexible substrate including conductive terminals accessible at a surface of the flexible substrate and wiring connected to the terminals, providing a compliant layer over the first attachment site, assembling a plurality of microelectronic elements over the attachment sites, wherein a first one of the microelectronic elements engages the compliant layer and is movable relative to the flexible substrate, electrically interconnecting the microelectronic elements and the wiring, folding the flexible substrate and stacking at least some of the microelectronic elements in generally vertical alignment with one another so that the first one of the microelectronic elements engaging the compliant layer is disposed at a bottom of the stacked assembly, and maintaining the stacked microelectronic elements in the substantially vertical alignment, wherein the conductive terminals are exposed at the bottom end of the stacked assembly.