Patent Publication Number: US-8110439-B2

Title: Method of stacking and interconnecting semiconductor packages via electrical connectors extending between adjoining semiconductor packages

Description:
PRIORITY DATA 
     This application is a divisional of U.S. patent application Ser. No. 11/427,695 filed Jun. 29, 2006, entitled “METHOD OF STACKING AND INTERCONNECTING SEMICONDUCTOR PACKAGES VIA ELECTRICAL CONNECTORS EXTENDING BETWEEN ADJOINING SEMICONDUCTOR PACKAGES”, which application is incorporated herein by reference in its entirety. 
     CROSS REFERENCE TO RELATED APPLICATION 
     The present application is related to U.S. Ser. No. 12/614,981, entitled “METHOD OF STACKING AND INTERCONNECTING SEMICONDUCTOR PACKAGES VIA ELECTRICAL CONNECTORS EXTENDING BETWEEN ADJOINING SEMICONDUCTOR PACKAGES”, filed Nov. 9, 2009, now U.S. Pat. No. 8,053,276. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention relate to an electronic component formed of a plurality of stacked semiconductor packages, and a method of forming the electronic component. 
     2. Description of the Related Art 
     The strong growth in demand for portable consumer electronics is driving the need for high-capacity storage devices. Non-volatile semiconductor memory devices, such as flash memory storage cards, are becoming widely used to meet the ever-growing demands on digital information storage and exchange. Their portability, versatility and rugged design, along with their high reliability and large capacity, have made such memory devices ideal for use in a wide variety of electronic devices, including for example digital cameras, digital music players, video game consoles, PDAs and cellular telephones. 
     While a wide variety of packaging configurations are known, flash memory storage cards may in general be fabricated as system-in-a-package (SiP) or multichip modules (MCM), where a plurality of die are mounted on a substrate. The substrate may in general include a rigid base having a conductive layer etched on one or both sides. Electrical connections are formed between the die and the conductive layer(s), and the conductive layer(s) provide an electric lead structure for integration of the die into an electronic system. Once electrical connections between the die and substrate are made, the assembly is then typically encased in a mold compound to provide a protective package. 
     Flash memory modules may either be portable, as in the case of a land grid array (LGA) package, or dedicated, as in the case of a ball grid array (BGA) package. Portable flash memory modules are fabricated with contact pads that allow the modules to be used as removable memory. They may be inserted into a slot in a host device, whereupon the contact pads are brought into pressure contact with a printed circuit board in the host device to allow communication between the memory module and host device. Dedicated memory modules on the other hand are soldered, or otherwise permanently affixed to the printed circuit board of a host device. 
     A cross-section of a conventional BGA package  40  is shown in  FIG. 1 . One or more memory die  20  and a controller die  22  are mounted on a substrate  24  in a stacked configuration. Generally, the substrate  24  may be formed of a rigid core  28 , of for example BT (Bismaleimide Triazine) laminate. Thin film copper layer(s)  30  may be formed on the top and bottom surfaces of the core in a desired electrical lead pattern using known photolithography and etching processes. Areas of the conductance pattern may be plated to receive solder balls  32  or other soldered contacts. The substrate may be coated with a solder mask  36  to insulate and protect the electrical lead pattern formed on the substrate. The die may be electrically connected to the substrate by wire bonds  34 . Vias (not shown) are formed through the substrate to allow electrical connection of the die through the substrate to the solder balls  32 . Once the die are electrically connected, the package may be encapsulated in a mold compound  38  to form the package  40 . The package  40  may thereafter be mounted by the solder balls  32  to a printed circuit board within a host device (not shown) in a known reflow process. 
     There is an ever-present drive to increase storage capacity within memory modules. One method of increasing storage capacity is to increase the number of memory die used within the package. In portable memory packages, the number of die which may be used is limited by the thickness of the package, which must not exceed a thickness of a standard-sized slot in the host device within which the memory module is received. 
     However, even where the thickness of a package is not limited by standard, as in a dedicated memory module, typically no more than 4 or 5 die may be stacked within a given package. The more die that are added within a package, the greater the likelihood that one or more of them will be damaged during human or automated assembly. And as the number of die goes up, package yields go down. If a single die within a package is faulty, the package must be discarded, and the good die wasted. Moreover, large numbers of die within a package draw a significant amount of current during testing and operation to power up the package. 
     It is therefore known to stack semiconductor packages together. For example, U.S. Pat. No. 6,407,448 entitled, Stackable Ball Grid Array Semiconductor Package and Fabrication Method Thereof, discloses a support structure within which a semiconductor die is seated. The support structure has metal traces formed on its bottom surface. A second layer of traces are then affixed on top of the die using adhesive, and connected to the die and support structure. Solder balls are then provided on top of the second layer of metal traces. A second package may then be stacked atop the first package by soldering the solder balls of the first package to the metal traces on the bottom surface of the second package. 
     Conventional stacked semiconductor packages have a variety of drawbacks. For example, in the design shown in the above-described U.S. Pat. No. 6,407,448, there is significant additional structure required to make the packages stackable. This significant additional structure increases the processing steps necessary to fabricate the package assembly, and adds time and expense to the fabrication process. 
     SUMMARY OF THE INVENTION 
     Embodiments of the invention, roughly described, relate to an electronic component including a plurality of stacked semiconductor packages. A first such embodiment includes an internal connector for electrically coupling the stacked semiconductor packages. A second such embodiment includes an external connector for electrically coupling the stacked semiconductor packages. 
     The first embodiment of the present invention, referred to herein as an internal lead stacked semiconductor package assembly, may be batch processed from a pair of substrate panels. The first panel may include a plurality of integrated circuit (IC) assemblies each having, for example, a plurality of memory die electrically coupled to a substrate. The second panel may include a plurality of IC assemblies each having, for example, a plurality of memory die and a controller die electrically coupled to a substrate. 
     A plurality of internal connectors may be attached to contact pads formed on a first of the IC assemblies. The internal connectors may be formed out of the material of a conventional lead frame, such as for example copper alloy 4 to 6 mils thick. The shape of the internal connectors may generally have a base portion for mating with the contact pads on the first IC assembly, a neck portion extending away from the base portion, and a head portion for being connected to a bottom of the second IC assembly. The neck portion preferably has a length sufficient to position the head portion above the uppermost die in the first IC assembly as well as any wire bonds extending therefrom. The internal connectors may be provided along one, two, three or four edges of the IC assembly. 
     The first and second IC assemblies may next be encapsulated in a mold compound in a conventional encapsulation process. The internal connectors on the first IC assembly may be completely encased within the molding compound, and thereafter the molding compound may be ground down so that a surface of the head portion of the internal connectors is exposed at the surface of the first IC assembly. 
     After the encapsulation and grinding steps, the IC assemblies may be singulated from their respective panels to define a plurality of semiconductor packages. Once cut into semiconductor packages, the packages from different panels may be joined together to form the internal connector stacked semiconductor package assembly. The packages are joined with the contact pads of the second package electrically coupled with the head portion of electrical connectors from the first package. 
     In embodiments where the internal connector stacked semiconductor package assembly is to be a BGA device, solder balls may be tacked to contact pads on the bottom of the assembly. 
     The second embodiment of the present invention is referred to herein as an external connector stacked semiconductor package assembly. IC assemblies are formed on first and second substrate panels as described with respect to the first embodiment. The IC assemblies may then be encapsulated in a mold compound, using a chase having a plurality of protrusions which define a plurality of recesses along one or more edges within a surface of the mold compound. 
     After the encapsulation, the IC assemblies may be singulated from their respective panels to define a plurality of semiconductor packages. Once cut into semiconductor packages, a plurality of external connectors may be affixed to a first of the semiconductor packages. Each external connector may in general be “C”-shaped with a base portion lying in contact with contact pads on an unencapsulated surface of the first package, a neck portion lying along and adjacent to the outer edge of the first package, and a head portion which fits into a recess in the mold compound. External connectors may be formed out of the material of a conventional lead frame, and may be provided along one, two, three or all four edges of the first semiconductor package. 
     After the external connectors are applied, the semiconductor packages may be joined together to form the external connector stacked semiconductor package assembly. The first and second packages are joined with the contact pads of the second package electrically coupled with the head portion of the external connector. In embodiments where the external connector stacked semiconductor package assembly is to be a BGA package, solder balls may be tacked to contact pads on a bottom surface of the device. 
     The internal connectors and external connectors of the above described embodiments serve to electrically couple the semiconductor packages in the assembly to each other. As would be appreciated by those of skill in the art, the conductance pattern(s) in the respective semiconductor packages may be configured in a known manner such that, once the packages are coupled together via the internal/external electrical connectors, the semiconductor die in one package are electrically coupled to the semiconductor die and/or the solder balls or other external electrical connector in the second package. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional side view of a conventional BGA package. 
         FIG. 2  is a top view of a pair of substrate panels from which a plurality of stacked semiconductor package assemblies according to the present invention may be made. 
         FIG. 3  is a flowchart of the overall fabrication process of an internal lead stacked semiconductor package assembly according to embodiments of the present invention. 
         FIG. 4  is a flowchart of a photolithography process for forming conductance patterns on the substrates used in the present invention. 
         FIGS. 5-7  are side views of a pair of integrated circuit assemblies according to the present invention in various stages of fabrication. 
         FIG. 8  is a top view of an integrated circuit assembly including internal connectors according to an embodiment of the present invention. 
         FIGS. 9-10  are side views of a pair of integrated circuit assemblies according to the present invention in various stages of fabrication. 
         FIG. 11  is a top view of an encapsulated integrated circuit assembly with the internal connectors exposed through a surface of a mold compound. 
         FIGS. 12-14  are side views of a pair of integrated circuit assemblies according to the present invention in various stages of fabrication. 
         FIG. 15  is a flowchart of the overall fabrication process for forming an external connector stacked semiconductor package assembly according to embodiments of the present invention. 
         FIG. 16  is a side view of a pair of integrated circuit assemblies according to the present invention during fabrication. 
         FIG. 17  is a cross-sectional side view of a pair of integrated circuit assemblies according to the present invention during fabrication. 
         FIG. 18  is a top view of an encapsulated integrated circuit assembly according to the present invention including recesses formed in the mold compound. 
         FIGS. 19-22  are cross-sectional side views of a pair of integrated circuit assemblies according to an embodiment of the present invention in various stages of fabrication. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments will now be described with reference to  FIGS. 2 through 22 , which roughly described, relate to a stacked semiconductor package assembly. It is understood that the present invention may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the invention to those skilled in the art. Indeed, the invention is intended to cover alternatives, modifications and equivalents of these embodiments, which are included within the scope and spirit of the invention as defined by the appended claims. Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be clear to those of ordinary skill in the art that the present invention may be practiced without such specific details. 
     A first embodiment of the present invention, referred to herein as an internal lead stacked semiconductor package assembly, will now be explained with reference to  FIGS. 2 through 13 .  FIG. 2  shows a top view of a pair of substrate panels  100   a  and  100   b . The panels  100   a  and  100   b  include a plurality of package outlines  102   a  and  102   b , respectively, at which positions individual semiconductor packages will be fabricated as explained hereinafter. It is understood that the layout of panels  100   a  and  100   b , as well as the number of rows and columns of package outlines  102   a ,  102   b  on panels  100   a ,  100   b  may vary in alternative embodiments. Moreover, the panels  100   a  and  100   b  are shown having the same configuration, and having the same number of rows and columns of package outlines as each other. It is understood that the configuration and/or the number of rows and columns on panel  100   a  may be different than that of panel  100   b  in alternative embodiments. 
     Substrate panels  100   a ,  100   b  may be a variety of different chip carrier mediums, including a PCB, a leadframe or a tape automated bonded (TAB) tape. Where substrate panels  100   a ,  100   b  are PCBs, each substrate panel may be formed of a core having a top conductive layer and a bottom conductive layer. The core may be formed of various dielectric materials such as for example, polyimide laminates, epoxy resins including FR4 and FR5, bismaleimide triazine (BT), and the like. Although not critical to the present invention, the core may have a thickness of between 40 microns (μm) to 200 μm, although the thickness of the core may vary outside of that range in alternative embodiments. The core may be ceramic or organic in alternative embodiments. 
     The conductive layers surrounding the core may be formed of copper or copper alloys, plated copper or plated copper alloys, Alloy 42 (42Fe/58Ni), copper plated steel, or other metals and materials known for use on substrate panels. The conductive layers may have a thickness of about 10 μm to 24 μm, although the thickness of the layers may vary outside of that range in alternative embodiments. 
       FIG. 3  is a flowchart of the fabrication process for forming an internal lead stacked semiconductor assembly according to embodiments of the present invention. In a step  200 , the panels  100   a  and  100   b  are drilled to define through-holes in the substrate. While step  200  is described as a single step in which both panels  100   a  and  100   b  are drilled, it is understood that panel  100   a  may be drilled at a different time and in a different process than panel  100   b . The same is true in the description of the following steps unless otherwise indicated. 
     Conductance patterns are next formed on the respective conductive layers in each panel in step  202 . In particular, as shown in the flowchart of  FIG. 4 , the surfaces of the conductive layers are cleaned in step  212 . A photoresist film is then applied over the conductive layers in step  214 . A pattern mask containing the outline of the electrical conductance pattern may then be placed over the photoresist film in step  216 . The photoresist film is exposed (step  218 ) and developed (step  220 ) to remove the photoresist from areas on the conductive layers that are to be etched. The exposed areas are next etched away using an etchant such as ferric chloride in step  222  to define the conductance patterns on the core. Next, the photoresist is removed in step  224 . Other known methods for forming the conductance pattern on the substrate panels  100   a ,  100   b  are contemplated. The conductance patterns on panel  100   a  may be the same or different than the conductance patterns on panel  100   b.    
     Referring again to  FIG. 3 , the panel may then be inspected in an automatic optical inspection (AOI) in step  232 . Once inspected, the solder mask is applied to the panel in step  234 . After the solder mask is applied, solder areas on the conductance patterns of each panel  100   a ,  100   b  may be plated with a Ni/Au, Alloy 42 or the like in step  236  in a known electroplating or thin film deposition process. In embodiments, the stacked semiconductor package assembly may be used as an LGA package including plated contact fingers. In such embodiments, the fingers may be plated first with a soft plating layer and then a hard plating layer to reduce wear on the contact fingers. Alternatively, the fingers may be plated in a single plating process. 
     The respective substrate panels may then be inspected and tested in an automated inspection process (step  240 ) and in a final visual inspection (step  242 ) to check electrical operation, and for contamination, scratches and discoloration. The substrate panels that pass inspection are then sent through the die attach process in step  244  and as described with reference to the side view of  FIG. 5 .  FIG. 5  shows an integrated circuit (IC) assembly from each of panels  100   a  and  100   b , fabricated as described above. IC assembly  110   a  shown is one of the plurality of IC assemblies being fabricated on panel  100   a , and IC assembly  110   b  shown is one of the plurality of substrate assemblies being fabricated on panel  100   b . While the IC assemblies  110   a  and  110   b  are shown in  FIGS. 5-7  and  9 - 11  as being adjacent to each other, the panels  100   a  and  100   b  including IC assemblies  110   a  and  110   b  may in fact be remote from each other until they are joined as explained hereinafter with respect to  FIG. 13 . 
     IC assembly  110   a  may include a plurality of memory die  112   a  mounted on substrate  114   a . Similarly, IC assembly  110   b  may include a plurality of memory die  112   b  mounted on substrate  114   b .  FIG. 5  shows the semiconductor die  112   a ,  112   b  stacked in an offset arrangement on the substrates  114   a ,  114   b . The offset allows electrical leads to be connected to each of the semiconductor die in the stack, at the edges of the die. Alternatively, the die could be stacked in an aligned configuration and be separated by a silicon spacer as is known in the art. The one or more die may have thicknesses ranging between 2 mils to 20 mils, but the one or more die may be thinner than 2 mils and thicker than 20 mils in alternative embodiments. 
     The die  112   a ,  112   b  may be for example flash memory chips (NOR/NAND), SRAM and/or DDT. In embodiments, each IC assembly  110   a ,  110   b  may include four memory die, though it is understood that each assembly may include more or less than that in alternative embodiments. One of the IC assemblies  110   a ,  110   b  may further include a controller chip, such as for example controller chip  116   b  on assembly  110   b , for controlling I/O for the memory chips  112   a  and  112   b . The controller chip may for example be an ASIC. Other silicon chips are contemplated in addition to or instead of the memory and controller chips described above. 
       FIGS. 5-14  are views of the internal connector stacked semiconductor package assembly in the successive fabrication stages described in the flowchart of  FIG. 3 . As shown in side view of  FIG. 5 , wire bonds  118   a ,  118   b  may be made between the die and the substrates in a step  246 . As shown in the side view of  FIG. 6 , solder paste may be added to IC assembly  110   b  in step  248  to define contact pads  120  along opposed edges of the top surface of IC assembly  110   b.    
     Referring now to  FIG. 7 , internal connectors  130  may be attached to contact pads  120  on the upper surface of IC assembly  110   b  in step  250 . Internal connectors  130  may be formed out of the material of a conventional lead frame, such as for example copper alloy or alloy 42 which is 4 mils to 6 mils thick. The type of material and the thickness of the material may vary in alternative embodiments. The shape of the internal connectors  130  may generally have a first, base portion for mating with contact pads  120 , a second, neck portion extending away from the base, and a third, head portion for being connected to a bottom of IC assembly  110   a  as explained hereinafter. The neck portion preferably has a length sufficient to position the head portion above the uppermost die in IC assembly  110   b  as well as any wire bonds extending therefrom. In  FIG. 7 , the neck of internal connector  130  is shown as being generally perpendicular to the base and head portions. It is understood that the neck portion may form oblique angles with respect to the base and head portions in further embodiments of the present invention. 
     While the internal connectors  130  are shown in  FIG. 7  as having a generally “Z”-shaped configuration, it is understood that the connectors  130  may have alternative configurations. Such additional configurations include for example either of a forward or backward “C”-shape with parallel base and head portions and a neck portion extending therebetween. In a further embodiment, the connectors  130  may be generally “I”-shaped, with the neck portion extending upward from the middle of the base portion and/or downward from the middle of the head portion. Other shapes are contemplated. 
       FIG. 8  is a top view from line  8 - 8  in  FIG. 7 . As shown therein, the internal connectors  130  and contact pads  120  are provided along opposed edges of IC assembly  110   b . In further embodiments of the present system, it is understood the contact pads  120  and internal connectors  130  affixed thereto may be provided along one edge, three edges, or all four edges of IC assembly  110   b . While five internal connectors are shown on each of the two opposed sides of IC assembly  110   b , it is understood that many more than five internal connectors may be provided in embodiments of the present invention. 
     Referring now to  FIG. 9 , IC assemblies  110   a  and  110   b  may next be encapsulated in a mold compound  132  in a step  252 . Mold compound  132  may be an epoxy such as for example that available from Sumitomo Corp. and Nitpo Denko Corp., both having headquarters in Japan. Other mold compounds from other manufacturers are contemplated. The mold compound may be applied according to various known processes, including by transfer molding or injection molding techniques to encapsulate substrates  114   a ,  114   b , die  112   a ,  112   b  and  116   b , and internal connectors  130 . 
     Referring to  FIGS. 10 and 11 , mold compound  132  on IC assembly  110   b  may next be ground down so that a surface of the head portion of the internal connectors  130  are exposed at the surface of IC assembly  110   b  (step  254 ). As an alternative to grinding mold compound  132  in step  254 , it is understood that the neck portion of internal connectors  130  may have a length so that the head portions of the internal connectors are flush with the surface of the mold compound  132  after the molding step. Alternatively, the neck portion of internal connectors  130  may have a length so that the head portions of the internal connectors protrude through the top of mold compound  132  at the encapsulation process. Thereafter, the head portions of connectors  130  may be left to protrude slightly above the surface of the mold compound, or the head portions may be ground to be flush with the surface of the mold compound. 
     Referring now to the side view of  FIG. 12 , solder paste may next be added to exposed contacts on a lower surface of IC assembly  110   a  to form contact pads  138  in step  256 . After the contact pads  138  are formed, the IC assemblies  110   a ,  110   b  may be singulated from panels  100   a ,  100   b  to form finished semiconductor packages  140   a  and  140   b  in step  260  and as indicated in  FIG. 12 . Each semiconductor package  140   a  and  140   b  may be singulated by sawing along straight lines. However, a variety of cutting methods other than sawing may be used in alternative embodiments, such as for example, water jet cutting, laser cutting, water guided laser cutting, dry media cutting, and diamond coating wire cutting. While straight line cuts will define generally rectangular or square shaped semiconductor packages  140   a ,  140   b , it is understood that semiconductor packages  140   a  and  140   b  may have shapes other than rectangular and square in further embodiments of the present invention. 
     Once cut into packages  140   a ,  140   b , the packages may be separately tested in a step  262  to determine whether the packages are functioning properly. As are known in the art, such testing may include electrical testing, burn in and other tests. 
     Referring now to  FIG. 13 , the packages  140   a  and  140   b  may be joined together in a step  264  to form internal connector stacked semiconductor package assembly  144  shown in  FIG. 13 . The packages  140   a  and  140   b  are joined with the contact pads  138  of package  140   a  electrically coupled with the head portion of electrical connectors  130  in package  140   b . The package  140   a  may be aligned to the package  140   b  for coupling using a fixture of known construction. In alternative embodiments of the present invention, it is understood that panel  100   a  may be aligned with and connected to panel  100   b . After the panels are connected, they may then be singulated. 
     In embodiments where the internal connector stacked semiconductor package assembly  144  is to be a BGA package, solder balls may be affixed to the package assembly  144 . As shown in  FIG. 14 , solder paste  142  may be applied to the grid of metal contacts on the lower surface of assembly, and the solder balls  146  may be tacked to solder paste  142  in step  266 . Where the internal lead stacked semiconductor package assembly is used as a portable device, step  266  may be omitted. Whether assembly  144  is a BGA package with solder balls  146  or an LGA package, all solder connections may be cured in a reflow process in step  268 . These solder connections include those between the packages  140   a ,  140   b  and the electrical connectors  130 , and, where applicable, the solder balls  146 . 
     The assembly  144  may be tested in a step  270 . In embodiments where assembly  144  comprises a portable semiconductor device, the assembly  144  may be encased within lids in a step  272 . When the assembly  144  is used as a dedicated device soldered to a motherboard of a host device, step  272  may be omitted. 
     A further embodiment of the present invention, referred to herein as an external connector stacked semiconductor package assembly, will be explained with reference to the flowchart of  FIG. 15 , and the views of  FIGS. 16-22 . Referring first to the flowchart of  FIG. 15 , steps  200 - 246  as described above may be repeated to provide the IC assemblies  150   a  and  150   b  shown in  FIG. 16 . As described above with respect to IC assemblies  140   a  and  140   b , assemblies  150   a ,  150   b  may include a plurality of die, such as for example for memory die  112   a  on IC assembly  150   a  and memory die  112   b  and a controller die  116   b  on IC assembly  150   b . The die may be wire bonded to a substrate  114   a  and  114   b  formed as described above. 
     Referring now to the cross-sectional view of  FIG. 17 , the IC assemblies  150   a ,  150   b  may be encapsulated in a mold compound  132  in a step  274 . As shown in  FIG. 17  and the top view of  FIG. 18 , the mold chase used in the encapsulation process may include a plurality of protrusions which define a plurality of recesses  152  along one or more edges within a surface of mold compound  132 . It is understood that the recesses  152  may be formed into the surface of the mold compound  132  after the encapsulation step in alternative embodiments. 
     While the figures show recesses  152  formed in both the IC assembly  150   a  and the IC assembly  150   b , the recesses  152  are used only in IC assembly  150   b  as explained hereinafter. In embodiments, IC assemblies  150   a ,  150   b  are encapsulated using the same molding chase, and as such recesses  152  may be formed in both IC assemblies  150   a  and  150   b . In alternative embodiments, recesses  152  may be omitted from IC assembly  150   a.    
     Referring now to the cross-sectional view of  FIG. 19 , after encapsulation, the individual IC assemblies  150   a  may be singulated from panel  100   a  in step  276  to form a finished semiconductor package  160   a . Similarly, the individual IC assemblies  150   b  may be singulated from panel  100   b  in step  276  to form finished semiconductor package  160   b . After singulation step  276 , the semiconductor packages  160   a  and  160   b  may be tested in a step  278 . As also shown in  FIG. 19 , solder paste may be applied to the bottom surfaces of semiconductor packages  160   a  and  160   b  in step  280  to define contact pads  154   a  and  154   b , respectively. 
     As seen in  FIG. 20  and described with respect to step  282 , after formation of the contact pads, external connectors  164  may be affixed to semiconductor package  160   b . Each external connector  164  may in general be “C”-shaped with a first, base portion lying in contact with contact pads  154   a , a second, neck portion lying along and adjacent to the outer edge of package  160   b , and a third, head portion which fits into a recess  152 . External connectors  164  may be formed out of the material of a conventional lead frame, such as for example copper alloy or alloy 42 which is 4 mils to 6 mils thick. The type of material and the thickness of the material may vary in alternative embodiments. In  FIG. 21 , the external connectors  164  are provided along opposed edges of IC assembly  150   b . In further embodiments of the present system, it is understood the external connectors  164  may be provided along one edge, three edges, or all four edges of IC assembly  150   b . While recesses for five external connectors  164  are shown on each of the two opposed sides of IC assembly  150   b  in  FIG. 18 , it is understood that many more than five external connectors  164  may be provided in embodiments of the present invention. 
     After the external connectors  164  are applied, semiconductor packages  160   a  and  160   b  may be joined together to form external connector stacked semiconductor package assembly  170  in a step  284  as shown in  FIG. 21 . The packages  160   a  and  160   b  are joined with the contact pads  154   a  of package  160   a  electrically coupled with the head portion of external connectors  164  in package  160   b . The package  160   a  may be aligned to the package  160   b  for coupling using a jig or other fixture of known construction. 
     In embodiments where the external connector stacked semiconductor package assembly  170  is to be a BGA package, solder balls may be affixed to package assembly  170 . As shown in  FIG. 22 , solder paste  142  may be applied to the grid of metal contacts on the lower surface of assembly, and the solder balls  146  may be tacked to solder paste  142  in step  286 . Where the external connector stacked semiconductor package assembly  170  is used as a portable device, step  286  may be omitted. Whether assembly  170  is a BGA package with solder balls  146  or an LGA package, all solder connections may be cured in a reflow process in step  288 . These solder connections include those between the packages  160   a ,  160   b  and the electrical connectors  164 , and, where applicable, the solder balls  146 . 
     The assembly  170  may be tested in a step  290 . In embodiments where assembly  170  comprises a portable semiconductor device, the assembly  170  may be encased within lids in a step  292 . When the assembly  170  is used as a dedicated device soldered to a motherboard of a host device, step  292  may be omitted. 
     The internal connectors  130  and external connectors  164  of the above described embodiments serve to electrically couple the semiconductor packages in the assembly to each other. As would be appreciated by those of skill in the art, the conductance pattern(s) in the respective semiconductor packages may be configured in a known manner such that, once the packages are coupled together via the internal/external electrical connectors, the semiconductor die in one package are electrically coupled to the semiconductor die and/or solder balls  146  or other external electrical connector in the second package. Thus, once soldered together, the package assembly may function as a single electronic component, such as for example a single flash memory device. Where the flash memory device is a dedicated component, the device may be an iNAND BGA. Where the flash memory device is a portable component, the device may be an SD LGA. Other devices are contemplated. 
     It will be evident that the semiconductor packages which are coupled together need not originate from the same substrate panel. Thus, a first substrate panel may include all identical semiconductor packages, such as for example having a controller and one or more flash memory chips. And a second substrate panel may include all identical semiconductor packages, such as for example having only flash memory chips. Packages from these respective panels may then be coupled by the internal or external electrical connectors as described above. 
     In the above described embodiments, the package assembly is comprised of two semiconductor packages of the same or similar footprint. However, it is understood that the package assembly according to either the internal or external connector configuration may include more than two semiconductor packages. Similarly, the packages within the assembly need not be the same size as each other in alternative embodiments. 
     The foregoing detailed description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. The described embodiments were chosen in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.