Abstract:
In a semiconductor assembly having stacked elements, discrete bumps made of a polymer such as an electrically nonconductive epoxy are interposed between the upper surface of a substrate and the lower surface of the overhanging part of an elevated element (die or package). The bumps are dimensioned to provide a clearance between the upper surface of the “bottom” substrate and the under surface of the second die or “top” package. The invention is carried out by first determining what height is required for the support; then depositing one or more of the polymer bumps at one or more suitable sites on the substrate; and placing the overhanging die or package onto the feature or features upon which it is stacked. The required height is the same as the accumulated thickness of the feature or features upon which the die or package is stacked (including the sum of thicknesses of, for example, any die, packages, spacers, adhesives layers, etc.). Suitable sites are determined according to the extent and position of the features or features upon which the die or package is stacked, the extent of the overhang, and the type and force of any stresses that may be imposed on the die or package during processing.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority from U.S. Provisional Application No. 60/686,116, filed May 31, 2005, titled “Epoxy bump for overhang die”. 
     
    
     BACKGROUND  
       [0002]     This invention relates to stacked die semiconductor packages and stacked semiconductor package modules.  
         [0003]     A conventional chip package consists of a semiconductor die affixed to a surface of a substrate and electrically interconnected to bonding pads on the substrate surface. The opposite surface of the substrate has an array of solder balls for electrical connection to, for example, a motherboard. The substrate includes, among other things, routing circuitry for mediating appropriate connection between the die and the motherboard. The die and associated interconnection parts are encapsulated with a protective molding.  
         [0004]     The die may be affixed with its “active surface” upward, and is then conventionally interconnected by so-called “wire bonding”, in which conductive wires are connected from points on the active surface of the die to the bond pads on the die attach (“upper”) surface of the substrate.  
         [0005]     To increase the capacity and performance of the package, a die may be stacked upon a die, to make a stacked die package. A portion of the active surface of the lower die (typically, for example, a peripheral marginal area) is occupied by the wire bonds (the “wire span” area), and no solid piece can be placed directly upon the wire span area. An upper die that is small enough may be placed directly upon another area of the active surface of the lower die (typically, for example, a central region). However, where an upper die is too large to fit within the available region of the lower die, a small sufficiently thick solid spacer (such as a chip of silicon or glass) may be placed directly on the available region of the lower die, and the upper die is then stacked upon the spacer, as shown in  FIG. 1A . Part of the upper die overhangs, and this part may crack or break (destroying the electronics in the die), as shown in  FIG. 1B , when the overhanging part is loaded (as, for example, during the wire bond process, when the capillary contacts the die). Or, where the die is very thin and the overhang is extensive, the overhanging portion may not crack or break, but may flex excessively during the wire bonding procedure, resulting in unacceptable bonds at the die pads.  
         [0006]     Alternatively, to increase capacity and performance, a second package may be stacked upon a die (or upon a lower package) to make a stacked package module. Where a spacer is required, or where the second package is spaced asymmetrically over the die or lower package or over the spacer, part of the upper package overhangs. The overhanging part of the upper package may crack or break or, as may be more likely, the upper package may “tilt” when the overhanging part is loaded.  
         [0007]     One way to prevent such breakage or tilt is to provide a support for the overhanging part of the upper die or package.  
         [0008]     A conventional way to support the overhang is to provide solid insulating spacers upon the lower substrate, peripheral to the lower die. Another conventional way to support the overhang is to provide solid insulating spacers upon the lower die, within the available region of the active side of the die.  
       SUMMARY  
       [0009]     This invention is directed to providing support for a die or a package that overhangs a package substrate, and thereby preventing mechanical failure (cracking or breaking) or tilt. The overhanging feature (die or package) is stacked upon one or more of a die or a package or a spacer, and has a portion that overhangs the surface of the substrate on which the stacked features are mounted. According to the invention, discrete bumps made of a polymer (such as an electrically nonconductive epoxy) are interposed between the upper surface of the substrate and the lower surface of the overhanging part of the die or package. The bumps are dimensioned to provide a clearance between the upper surface of the “bottom” substrate and the under surface of the second die or “top” substrate.  
         [0010]     The invention is carried out by first determining what height is required for the support; then depositing one or more of the polymer bumps at one or more suitable sites on the substrate; and placing the overhanging die or package onto the feature or features upon which it is stacked. The required height is the same as the accumulated thickness of the feature or features upon which the die or package is stacked (including the sum of thicknesses of, for example, any die, packages, spacers, adhesives layers, etc.). Suitable sites for the bumps are determined according to the extent and position of the features or features upon which the die or package is stacked, the extent of the overhang, and the type and force of any stresses that may be imposed on the die or package during processing.  
         [0011]     The bumps have a base where they contact the substrate, which may have a greater and lesser width. The greater width of the bump base is in some embodiments less than ten times the bump height, or in some embodiments less than four times the bump height, or in some embodiments less than two times the bump height; and or in some embodiments the greater width of the bump base is less than the bump height. In preferred bumps the base has a generally round, for example, generally circular, shape; and preferred bumps have a generally round, for example generally circular, shape in sectional views generally parallel to the substrate. Various bump shapes may be suitable. The bump may be narrower at greater distances away from the base, although the base may not necessarily be the widest part of the bump.  
         [0012]     The invention can be employed using standard equipment, such as for example a conventional die attach machine and die attach epoxy dispenser. The polymer bumps are applied during placement of the overhanging die or package, and may be made as a part of the step of dispensing the die attach adhesive onto the features upon which the die or package is stacked.  
         [0013]     In one aspect the invention features a semiconductor assembly having a semiconductor part, such as a package or a die, mounted in an elevated position in relation to an assembly substrate, in which an overhang of the elevated part (die or package) is supported by one or more polymer bumps.  
         [0014]     The stacked feature may be a die, or may be a package including a die mounted onto and electrically interconnected with, an upper package substrate. The stacked feature may be electrically interconnected to the assembly substrate by wire bonds; where the feature is a die, the wire bonds connect pads on the die with bond sites (for example on leads or bond fingers) in the assembly substrate, and where the stacked features is a package, the wire bonds connect bond sites (for example on leads or bond fingers) on the upper package substrate with bond sites (for example on leads or bond fingers) in the assembly substrate.  
         [0015]     In some embodiments the stacked feature is stacked over a first die. The first die may be affixed to a die mount region of a die attach side of the assembly substrate, with the active side facing away from the assembly substrate, and electrically interconnected to the substrate by wire bonds connecting pads on the first die with bond sites (for example on leads or bond fingers) in the assembly substrate. Or, the first die may be mounted to a die mount region of a die attach side of the assembly substrate in flip chip fashion, in which the die is mounted with the active side facing the substrate and electrical interconnection is made by conductive balls or bumps attached to pads on the die and to interconnect sites on the die attach region of the substrate.  
         [0016]     In another aspect the invention features a method for making a semiconductor assembly, by mounting a first die on an assembly substrate, mounting a die spacer using an adhesive on the first die, depositing discrete epoxy bumps on the assembly substrate, interconnecting the first die onto the assembly substrate, and mounting a second (“stacked”) die or (“top”) package upon the die spacer using an adhesive and upon the bumps. The bumps are dimensioned to provide a clearance between an upper surface of the assembly substrate and a lower surface of the second die or “top” package. Further, the second die or “top” package is interconnected onto the bottom substrate by wire bonds connecting pads on the second die (or bond sites on the “top” package substrate) with bond sites on the assembly substrate. In some embodiments the first die is electrically connected to the assembly substrate by wire bonds connecting pads on the first die with bond sites on the substrate; in other embodiments the first die is electrically connected to the assembly substrate in a flip chip manner.  
         [0017]     The invention can be useful in semiconductor packaging and, particularly, in Multi Chip Package (“MCP”) or System in Package (“SiP”) or Multi Package Module (“MPM”) package configurations. It can be used, for example, in computers, in telecommunications, and in consumer and industrial electronics. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1A  is a diagrammatic sketch in a sectional view showing a die mounted on a spacer over a lower die on a substrate, showing portions of the second die overhanging the substrate.  
         [0019]      FIG. 1B  is a diagrammatic sketch in a sectional view showing crack or breakage, or bending or bouncing of an overhanging portion of an upper stacked die as shown in  FIG. 1A , as can occur during a wire bonding operation.  
         [0020]      FIG. 2A  is a diagrammatic sketch in a sectional view showing a stacked die package, in which overhanging portions of the upper die are supported in a conventional fashion by insulating spacers.  
         [0021]      FIG. 2B  is a diagrammatic sketch in a plan view showing a stage in the construction of the conventional package of  FIG. 2A , before the upper die is mounted upon the lower die and insulating spacers.  
         [0022]      FIG. 3A  is a diagrammatic sketch in a sectional view showing a stacked die package, having four spacers mounted on the lower die, and an upper die mounted on the spacers.  
         [0023]      FIG. 3B  is a diagrammatic sketch in a plan view showing a stage in the construction of the conventional package of  FIG. 3A , before the upper die is mounted upon the four spacers.  
         [0024]      FIGS. 4A through 4F  are sketches in a sectional view showing stages in the construction of a package according an embodiment to the invention, including bumps to support a stacked die configuration.  
         [0025]      FIG. 5  is a diagrammatic sketch in a plan view showing the layout of structures on a conventional package as in  FIG. 2B .  
         [0026]      FIG. 6  is a diagrammatic sketch in a plan view showing the layout of structures on a package according an embodiment to the invention, as in  FIG. 4A through 4F .  
         [0027]      FIG. 7  is a diagrammatic sketch in a plan view showing stacked die including overhanging portions of an upper die, supported by polymer bumps according to an embodiment of the invention.  
         [0028]      FIG. 8  is a diagrammatic sketch in a sectional view showing an arrangement of a die or package mounted on a spacer over a first die, having an extensive part overhanging the substrate, supported over the substrate by polymer bumps according to an embodiment of the invention.  
         [0029]      FIG. 9  is a diagrammatic sketch in a plan view showing the arrangement of a lower die and conventional spacers beneath an upper die or package.  
         [0030]      FIG. 10A  is a diagrammatic sketch in a plan view showing the arrangement of a lower die, one conventional spacer, and a polymer bump spacer beneath an upper die or package according to an aspect of the invention.  
         [0031]      FIG. 10B  is a diagrammatic sketch in a plan view showing the arrangement of a lower die and several polymer bump spacers beneath an upper die or package according to an aspect of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0032]     The invention will now be described in further detail by reference to the drawings, which illustrate alternative embodiments of the invention. The drawings are diagrammatic, showing features of the invention and their relation to other features and structures, and are not made to scale. For improved clarity of presentation, in the FIGs. illustrating embodiments of the invention, elements corresponding to elements shown in other drawings are not all particularly renumbered, although they are all readily identifiable in all the FIGs. Terms such as “top”, “bottom”, “upper”, “lower”, “over”, “above” and the like are employed with reference to the relative positions of the various parts in the assemblies as shown in the FIGs., and it will be understood that the assemblies can be deployed in any orientation.  
         [0033]     Referring now to  FIG. 1A , there is shown a stacked die configuration having a first (“lower”) die  14  mounted on a substrate  12 , a spacer  13  mounted on the lower die  14 , and a second (“upper”) die  114 , larger than the first die  14 , mounted upon the spacer  13 . The lower die can be affixed to the substrate using a die attach adhesive, the spacer (which may be, for example, a “dummy” die or a chip of glass) can be mounted on the lower die using an adhesive such as a die attach adhesive, and the upper die can be mounted on the spacer using a die attach adhesive (the various adhesives are not shown in the FIGs.). The lower die in this example is electrically interconnected with the substrate by wire bonds (not shown in the FIG.) prior to mounting the larger upper die. Any or all of the die attach adhesives may be, for example, a die attach epoxy, or a die attach film. Because the upper die  114  in the example of  FIG. 1A  is significantly larger than the lower die  14 , and because the lower die  14  is to be electrically interconnected to the substrate  12  by wire bonding (not shown in this FIG.), the spacer  13  is required to provide clearance for the wire bond loops over the margins of the lower die. The height  11  between the downward-facing surface of the upper die  114  and the upward-facing surface of the substrate  12  is constituted by the thicknesses of the bottom die and the spacer, and the thicknesses of the various attach adhesives. The upper die  114  is much larger than the spacer  13  that the upper die is mounted upon, and large portions of the upper die project outward from the edges of the spacer, and overhang the edge of the lower die  14  and the substrate  12 , as illustrated for example by die overhang  17 .  
         [0034]      FIG. 1B  shows a detrimental effect that may result, for example, by impact of the wire bonding capillary with the die pads on the upper surface of an overhanging part of the second die during a wire bonding operation. Impact of the capillary tip  15  with the die pads on the overhanging part  117  of the die  116  may cause the overhanging part of the die to flex, as shown by the arrow  19  and bounce and/or to crack or break. Flex or bounce can result in poor wire bond interconnection to the die pads; and cracking or breakage of the die, as shown for example at  118 , can destroy the circuitry on the die.  
         [0035]      FIGS. 2A and 2B  illustrate a conventional approach to supporting an overhanging part of an upper die in a die stack. In this example a first (“lower”) die  24  is affixed, active side upward, using an adhesive onto a die attach surface of a lower substrate  22 , and the lower die is electrically connected to the substrate by wire bonds between pads on the die and wire bond sites on an metal layer in the upper layer of the substrate. Electrically insulating block- or bar-shaped spacers  25  are mounted, using an adhesive, upon the lower substrate outside the wire bond sites for interconnection of the lower die and parallel to rows of bond sites for electrical interconnection of the second (“upper”) die  224 . An insulating adhesive  23  is provided over the lower die and over the upward-facing surfaces of the spacers, and then the second (“upper”) die  224  is then mounted upon the adhesive  23  on the lower die and the spacers. The adhesive  23  has a thickness that is sufficient when cured that to provide clearance above the lower die  24  for the loop height of the wire bonds, to avoid interference by the upper die  224  with the wire bonds, and the adhesive  23  may appropriately be referred to as a “spacer adhesive”. Then the upper die is electrically interconnected to the substrate by wire bonding to the rows of wire bond pads outside the spacers.  FIG. 2B  shows stage in the construction of a package as in  FIG. 3A , having a lower die  24  and spacers  25 ,  27  mounted on a substrate  22 . The lower die  24  is electrically connected to the substrate  22  by wire bonds connecting pads on the die with bond fingers on the substrate. Spacers  25  and  27  are mounted on the substrate  22  outside (peripheral to) the bond fingers for interconnection of the lower die. Bond fingers for wire bond interconnection of the upper die (not yet placed in this FIG.) are situated outside (peripheral to) the spacers. A significant area of the substrate must be dedicated to the lower die wires, the lower die bond pads in the substrate, and to spacing among these features and between the wire bond sites and the spacers, as is evident in  FIG. 2B . Typically, for example, a separation between the lower die bond pads and the spacer must be at east about 0.5-1.0 mm; and the width of the bond fingers is about 0.4 mm; and the wires themselves may be about 1 mm long.  
         [0036]      FIGS. 3A and 3B  illustrate another conventional approach to supporting an upper die over a lower die. Here the upper die  314  is about the same size as the lower die  34 . In this approach, four smaller spacers  35  are affixed, using an adhesive, to the upward-facing (active) surface of the lower die  34 , inboard from the die pads and near the corners of the die, as shown in  FIG. 3B ; and the upper die is affixed, using an adhesive, to the upward-facing surfaces of the small spacers, as shown in  FIG. 3A . This arrangement would do nothing to help support a portion of the upper die that may extend over the substrate, in a case where the upper die is larger than the lower die.  
         [0037]     As illustrated in  FIGS. 4A through 4F , support for an overhanging upper die or package is provided according to the invention by mounting a first die  414  using an adhesive  43  on a die attach region of the die attach side of the “bottom” substrate  42  ( FIG. 4A ), mounting a spacer  46  using an adhesive  45  on the first die  414  ( FIG. 4B ), depositing discrete epoxy bumps  49  on the die attach side of the “bottom” substrate  42  ( FIG. 4C ), interconnecting the first die  414  onto the bottom substrate  42  by wire bonds  417  connecting pads on the die  414  with bond sites on the substrate ( FIG. 4D ), and mounting the second die  424  (or “top” package) upon the spacer  46  using an adhesive  47  and upon the bumps  49  ( FIG. 4E ). The bumps  49  are dimensioned to support overhanging parts of the second die or “top” package at a clearance  41  between the upper surface of the “bottom” substrate and the under surface of the second die or “top” package, The dimension for the clearance  41  is determined as the sum of the thicknesses of the lower die  414  and the spacer  46 , plus the thicknesses of the adhesives  43 ,  45 ,  47 ; as will be appreciated, the specifications for the bump height will be affected by the specifications for dimensional change, if any, in the adhesives during the curing process. Then the second die or “top” package is interconnected with the bottom substrate  42  by wire bonds  427  connecting pads on the second die  424  (or bond sites on the “top” package substrate) with bond sites on the substrate, as shown in  FIG. 4F . The sequence of steps may at some stages be different; particularly, for example, the steps giving rise to stages  4 D and  4 C can be reversed. In subsequent steps, the stacked die (or bottom die and top package), the spacer, the bumps, and the wire bond interconnects are encapsulated or molded.  
         [0038]     Any of various substrate types may be suitable for the assembly (“bottom”) substrate, providing only that the substrate have sites for electrical interconnection of the various electrical features mounted on and over it according to the invention. The assembly substrate may be, for example, a laminate with 2-6 metal layers, or a build up substrate with 4-8 metal layers, or a flexible polyimide tape with 1-2 metal layers, or a ceramic multilayer substrate. Or, for example, the substrate may be a lead frame.  
         [0039]     A comparison of a conventional configuration as in  FIG. 2B  with a configuration according to an embodiment of the invention as in  FIG. 4F  is made in  FIGS. 5 and 6 , each showing a plan view of an assembly having a first die mounted upon and electrically connected to a substrate, and a second die mounted over the first die.  FIG. 5  shows a stage in construction of a conventional package as in  FIG. 2B  after the second die has been mounted, but before the second die has been electrically connected to the substrate by wire bonds. The second die  524  and a portion of the underlying substrate  52  are visible in this view. The first die  514 , the die spacer adhesive  513 , the first die wire bonds, e.g.,  517 , and the spacers  59  are shown by dotted or broken lines, as they are obscured in this view beneath the second die  524 .  FIG. 6  shows a stage in construction of a package according to an embodiment of the invention as in  FIG. 4F  after the second die has been mounted, but before the second die has been electrically connected to the substrate by wire bonds. The second die  624  and a portion of the underlying substrate  62  are visible in this view. The first die  614 , the die spacer  616 , the first die wire bonds, e.g.,  617 , and the spacers  69  are shown by dotted or broken lines, as they are obscured in this view beneath the second die  624 . In this example, eight discrete epoxy bumps  69  are provided on the “bottom” substrate  62 . They are arranged on the “bottom” substrate at the periphery of the second die footprint, to provide maximum support for the overhang. As  FIG. 6  shows, the polymer bumps provide more free space on the substrate surface for arrangement of the wires and wire bond pads on the substrate. The choice of positions for the wire bond pads and the polymer support bumps can be adjusted according to particular design consideration for the particular die or package. Thus the package design can be more flexible.  
         [0040]      FIG. 7  shows an embodiment of the invention in which rectangular die are stacked, or a rectangular package is stacked over a rectangular die. Here the narrower dimension of the second (“top”) die or package  724  is small enough so that it can be accommodated in an area of the die that is inboard from the wire span areas of the first (“bottom”) die  714 . Accordingly they can be arranged so that no spacer is required between the die; epoxy bumps  79  on the “bottom” substrate  72  (obscured by the “top” die or package in this view) support the overhang of the ends of the upper die. As one example of this configuration, the die may be electrically interconnected with the substrate by wire bonds, e.g.,  717 ,  727  connected to die pads arranged along one edge of the die, or (as in the example shown in the FIG.) along two opposite edges of the die. The die may be memory die (of which some types are often rectangular, and significantly longer than wide). The lower die may be a flash memory, and the upper die may be a SRAM, for example; and the SRAM may be sufficiently narrow that no spacer is required between the two die. Nevertheless, because the second die (or package) is significantly longer than the width of the lower die, the significant overhang of the upper die (or package) must be supported over the underlying substrate, particularly where the upper die (or package) may be very thin and flexible, or to avoid tilt.  
         [0041]      FIG. 8  shows an embodiment in which an upper die or package  824  is asymmetrically stacked over a spacer  86  on the first (“lower”) die  814 . Epoxy bumps  89  on the “bottom” substrate  82  support the extensively overhanging (cantilevered) end of the upper die or package, preventing breakage (of a “top” die) or tilt (of a “top” die or package). The first (“bottom”) die  814  is interconnected with the substrate  82  by wire bonds  817  connecting pads on the die with bond pads on the substrate; and the second (“top”) die or package is interconnected by wire bonds  827  connecting pads on the die (or interconnect sites on the upper package substrate) with bond pads on the substrate  82 . The spacer  86  is affixed to the bottom die  814  using an adhesive  83 , and the second die or package  824  is affixed to the spacer  86  using an adhesive  87 . The bumps  89  are dimensioned to provide a clearance  81  between the upper surface of the “bottom” substrate and the under surface of the second die or “top” package. The dimension for the clearance  81  is determined as the sum of the thicknesses of the lower die  814  and the spacer  86 , plus the thicknesses of the adhesives  83 ,  87 ; as will be appreciated, the specifications for the bump height will be affected by the specifications for dimensional change, if any, in the adhesives during the curing process.  
         [0042]     The material for the epoxy bumps may be applied using a syringe, for example, or other adhesive dispense tool; or, the bumps may be applied by printing and, in this approach, the bumps may preferably be applied prior to mounting the first die (and die spacer, where required). The polymer constituting the bumps may be a “filled” epoxy, for example, or an epoxy whose rheology and viscosity characteristics permit it to hold an attitude as a bump (without collapsing) until it can be cured to hardness. Typically the epoxy of the bumps is cured following mounting of the upper die or package, in a curing step that may additionally cure one or all of the attachment adhesives. An epoxy may be used that can be cured in stages; such an epoxy may be partially cured following bump formation but prior to mounting the second die or package, and then fully cured following mounting of the upper die or package, in a curing step that may additionally cure one or all of the attachment adhesives. A stage-curable epoxy may be preferred, as this may provide improved stability during assembly steps subsequent to forming the bumps.  
         [0043]     Placement of the polymer support bumps is illustrated in particular examples in  FIGS. 9 and 10 A and  10 B. In these examples, an upper die (or package) is to be stacked over a lower die; the lower die is provided with a die spacer so that the upper die or package does not interfere with the wire bonds connecting pads on the lower die with bond sites on the substrate. In the view shown in each of these FIGs., the second (“upper”) die or package obscures all the features beneath it except a portion of the substrate that extends beyond the edge of the second die or package. Because the upper die or package is much larger than the lower die or package, in a conventional arrangement ( FIG. 9 ), first  926  and second  936  spacers are required to support the extensive portions of the upper die  924  that extend beyond the lower die  914  spacer  916  and overhang the substrate  92 . The clearance between the downward-facing side of the top die or package and the upward-facing side of the bottom substrate is determined by the thicknesses of the bottom die and the die spacer, plus the thicknesses of the adhesives by which the bottom die is attached to the substrate, the die spacer is attached to the bottom die, and the top die or package is attached to the die spacer; and the first and second spacers  926  and  936  are dimensioned, together with adhesives associated with them, to support the second die or package and to maintain the clearance of the second die or package over the substrate. In the configuration of  FIG. 10A , a spacer  1026  is provided, but there is no second spacer; instead a polymer bump  109  according to the invention is employed to support the corner of the upper die or package  1024  that would conventionally (as in  FIG. 9 ) have been supported by a second spacer; the clearance between the downward-facing side of the top package or substrate and the upward-facing side of the bottom substrate  1012  is determined by the thickness of the bottom die  1014  and the die spacer  1016  mounted on it, plus the thicknesses of the adhesives by which the bottom die is attached to the substrate, the die spacer is attached to the bottom die, and the top die or package is attached to the die spacer. In  FIG. 10B , both the first spacer and the second spacer are absent, and five epoxy bumps  109  have been placed according to the invention so as to provide support in place of the spacers.  
         [0044]     Other embodiments are within the invention.  
         [0045]     In any of the configurations described above, the overhanging feature may be either a die or a package. Particularly, for example, in a multi-package module the upper feature may be a package having lands exposed on the upward-facing side of the upper package substrate, for wire bond interconnection to the substrate; the upper package may be inverted. Examples of multipackage modules having upper packages interconnected with the module by wire bonds are disclosed, for example, in Karnezos U.S. application Ser. No. 10/632,549, filed Aug. 2, 2003; and in Karnezos U.S. application Ser. No. 10/681,572, filed Oct. 8, 2003. Examples of multipackage modules having a stacked inverted package overhanging a module substrate are shown, for example, in Karnezos et al. U.S. application Ser. No. 10/022,375, filed Dec. 23, 2004. All patents and patent applications referenced herein are hereby incorporated herein by reference.  
         [0046]     In any of the configurations described above, the lower die may be mounted onto the substrate by flip chip interconnect. In such embodiments the active side of the die faces the substrate, and interconnection is made by bumps or balls between die pads and interconnect sites on the substrate within the footprint of the die. No spacer over a flip chip lower die is necessary, inasmuch as there are no wire bonds between the die and the substrate, and the second feature may be mounted, using an adhesive, directly onto the first die. In such embodiments the clearance between the substrate and the stacked feature (that is, the height of the epoxy bumps supporting the overhanging part of the stacked feature) is determined by the thickness of the first die, together with the thickness of the interconnection (collapsed interconnect ball or bumps height) and the thickness of the adhesive by which the second feature is affixed to the first die.  
         [0047]     Details of die structures or of package structures are omitted from the drawings for simplicity of presentation. Such structures would be well understood to the skilled artisan in light of the drawings and the description herein.