Abstract:
A Flash memory card is disclosed comprising a substrate, a Flash memory die on top of the substrate, a controller die on top of the Flash memory die, and an interposer coupled to with the controller die and on top of the Flash memory die wherein the interposer results in substantial reduced wire bonding to the substrate. The interposer can surround or be placed side by side with the controller die. A system and method in accordance with the present invention achieves the following objectives: (1) takes advantage of as large of a Flash memory die as possible, to increase the density of the Flash card by reducing the number of wire bond pads on the substrate and enabling insertion of the largest die possible that can fit inside a given card interior boundary; (2) more efficiently stacks Flash memory dies to increase density of the Flash card; and (3) has a substantially less number of bonding wires to the substrate as possible, to improve production yield.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Under 35 U.S.C. § 120 the present application is a continuation of U.S. patent application Ser. No. 11/452,805, filed Jun. 14, 2006, entitled “FLASH MEMORY CARD, which is a continuation-in-part of and claims the benefit of priority to U.S. patent application Ser. No. 11/237,283, filed Sep. 27, 2005, entitled “FLASH MEMORY CARD,” a portion of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to Flash memory and more specifically to a high density Flash memory card. 
       BACKGROUND OF THE INVENTION 
       [0003]    A Flash memory card application is driven mainly by consumer electronics products, such as a digital still camera (DSC), cell phone, PDA or MP3 player, toward a smaller form factor and high density. As the form factor becomes smaller while the density requirement grows higher, a need is created for innovative approaches to include as many Flash components into a limited space within the boundary of a particular Flash memory card. 
         [0004]    Conventional approaches to pack more density into a Flash memory card are accomplished through wire bonding of one or more Flash memory dies and a Flash controller. A Flash memory die is usually much larger than a Flash controller die in physical size. The Flash memory dies and Flash controller are individually wire-bonded onto a substrate of the Flash memory card. 
         [0005]    The substrate usually has interconnecting traces that serve to connect signals among the Flash controller, Flash memory dies and Flash card interface. The substrate is then molded with resin or covered with external casing and made into a finished Flash memory card. 
         [0006]    For example, if there are approximately 40 signals and pads on the controller and approximately 20 signals and pads on each Flash memory die, placement constraints are created if both the Flash controller die and Flash memory dies are to be interconnected through wire bonding to the base substrate. Typically 60 pads on the substrate are required for one Flash memory die design and 20 more pads for each additional Flash memory die that is stacked. Therefore, the Flash memory die sizes have to be smaller to leave spaces for the wire bond pads. Furthermore, production yield problems are created due to the necessity of staggering bonding wires from multiple layers of dies. 
         [0007]    Accordingly, what is needed is a system and method for providing a high density, small form factor Flash memory card which addresses the above-identified issues. The system should be easy to implement, cost effective and adaptable to existing systems. The present invention addresses such a need. 
       SUMMARY OF THE INVENTION 
       [0008]    A Flash memory card is disclosed comprising a substrate, a Flash memory die on top of the substrate, a controller die on top of the Flash memory die, and an interposer coupled to with the controller die and on top of the Flash memory die wherein the interposer results in substantial reduced wire bonding to the substrate. The interposer can surround or be placed side by side with the controller die. A system and method in accordance with the present invention achieves the following objectives: (1) takes advantage of as large of a Flash memory die as possible, to increase the density of the Flash card by reducing the number of wire bond pads on the substrate and enabling insertion of the largest die possible that can fit inside a given card interior boundary; (2) more efficiently stacks Flash memory dies to increase density of the Flash card; and (3) has a substantially less number of bonding wires to the substrate as possible, to improve production yield. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  shows Flash memory card architecture. 
           [0010]      FIG. 2  shows a top view of wire-bonded substrate of a Flash memory card (prior art). 
           [0011]      FIG. 3  shows a cross section view of wire-bonded substrate of a Flash memory card (prior art). 
           [0012]      FIG. 4  shows a top view of staggered bonding wires from multiple layers of dies (prior art). 
           [0013]      FIG. 5  is a cross section view of staggered bonding wires from multiple layers of dies (prior art). 
           [0014]      FIG. 6  is a top view of an on-die-interposer having a center cut out to accommodate Flash controller die. 
           [0015]      FIG. 7  is a cross-section view of an on-die interposer having a center cut out to accommodate Flash controller die. 
           [0016]      FIG. 8  is a cross-section view of stacking two Flash memory dies design. 
           [0017]      FIG. 9  is a side view of stacking two Flash memory dies design where the serpentine interposer is used. 
           [0018]      FIG. 10  is a cross section view of stacking three Flash memory dies design. 
           [0019]      FIG. 11  is a side view of stacking three Flash memory dies design where the serpentine interposer is used. 
           [0020]      FIG. 12  is a relation curve between pad distance and loop height. 
           [0021]      FIG. 13  is a side view of stacking two Flash memory dies design where the extended tab interposer is used. 
           [0022]      FIG. 14  is a side view of stacking three Flash memory dies design where the extended tab interposer is used. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    The present invention relates generally to Flash memory and more specifically to a high density Flash memory card. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiments and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features described herein. 
         [0024]      FIG. 1  shows a conventional Flash memory card  10 . The Flash memory card  10  comprises a Flash card interface  11 , a Flash memory controller  12  and one or more Flash memory dies  13 . The Flash card interface  11 , usually in the form of connector or gold finger contacts, serves as a communication channel to the host devices such as a digital still camera (DSC), cell phone, PDA, MP3 player or PC. The Flash controller  12  controls the on-card Flash memory  13  and responds to requests from the host devices through Flash card interface  11 . 
         [0025]    In the manufacturing process, the Flash controller  12  and Flash memory  13  are mounted on a piece of substrate that has a Flash card interface  11  built-in and interconnecting traces in-between the controller  12  and the Flash memory  13 . The Flash controller  12  and Flash memory  13  can be in either bare die form or in a packaged form. The invention particularly focuses on the application where at least two bare die semiconductor components are used and the two components have different die sizes. 
         [0026]    The conventional Flash memory card that utilizes die form components tends to adopt one of two approaches in the manufacturing process, called the side-by-side approach and the stacking approach. Both approaches will be described herein below. 
         [0027]      FIG. 2  shows the top view of a conventional layout of a larger Flash memory die  23 , and a smaller controller die  24  on one side of the substrate  22  in a side-by-side approach.  FIG. 3  shows a cross-sectional view of the same arrangement shown in  FIG. 2 . 
         [0028]    In the side-by-side approach shown in  FIG. 2 , a Flash controller  24  and a Flash memory die  23  are placed side-by-side on a substrate  22 . Wires  18 ,  20  and  38  are bonded from both dies on pads  19 , 27  and  40  to the pads  25 , 26  and  35  on the substrate  22  below, respectively. The substrate  22  needs to have about 30 pads to accommodate Flash controller  24  connection. An additional twenty  20  pads are required to accommodate Flash memory die  23  connection. As seen in  FIG. 3 , the substrate  22  is laminated with internal traces  34 ,  36  and  37  to connect among the Flash controller  24 , the Flash memory die  23  and the Flash card interface  33 . It is a relatively simple process in manufacturing with a side-by-side approach. However, this severely limits the die size of Flash memory and Flash controller usable for a given Flash memory card  21 , because a total of 50 bonding wires and pads are required on the substrate  22 . 
         [0029]      FIG. 4  shows the top view of a conventional layout of a larger Flash memory die  123 , and a smaller Flash controller die  124  on one side of the substrate  122  in a stacking approach.  FIG. 5  shows a cross-sectional view of a conventional layout shown in  FIG. 4 . 
         [0030]    In the stacking approach of  FIG. 4 , a Flash controller die  124  is placed on top of the Flash memory die  123  on a substrate  122 . Wires  118 ,  120  and  138  are bonded from both dies on pads  119 ,  127  and  140  to the pads  125 ,  126  and  135  on the substrate below respectively. Due to multi-layer stacking, staggered wire bonding is necessary. The substrate  122  needs to have about 30 pads to accommodate Flash controller  124  connection. An additional 20 pads are required to accommodate Flash memory die  123  connection. As seen in  FIG. 5 , the substrate  122  is laminated with traces  134  to connect among the Flash controller  124 , the Flash memory die  123  and the Flash card interface  133 . This creates a limitation in Flash memory die size, as staggered wire bonding needs more space on the substrate  122  to distribute pads and wires. Space to accommodate a total of 50 additional bonding wires and pads is required on the substrate. Because the Flash controller is placed on top of the Flash memory die, there is less restriction in Flash controller die size compared to that of the side-by-side approach, where a larger controller die would reduce the size of the Flash as both of them compete for the same, common space available on the substrate. 
         [0031]    The present invention addresses the restrictions of Flash memory die size in both conventional side-by-side and stacking approaches. The present invention further saves perimeter wire-bond space and simplifies wire-bonding complexity on the substrate. Consequently, larger die sizes containing more memory capacity can be used in either the side-by-side or stacking assembly. 
         [0032]    A system and method in accordance with the present invention addresses the above-mentioned problems to achieve the following objectives: (1) Enable placement of the largest Flash memory die possible inside a given geometry to maximize the density of the Flash card by reducing the number of wire bond pads on the substrate. (2) Being able to efficiently stack Flash memory dies if necessary, to increase density of the Flash card. (3) Having as little bonding wires to the substrate as possible, to improve production yield. 
         [0033]    To describe the features of the present invention in more detail, refer now to the following description in conjunction with the accompanying Figures. 
         [0034]      FIG. 6  shows the top view of the Flash memory card in accordance with the present invention. The layout comprises a larger Flash memory die  223 , a center cutout interposer  242  and a Flash controller die  224  on top of the substrate  222 .  FIG. 7  shows a cross-sectional view of the layout of  FIG. 6 . 
         [0035]    In this embodiment an interposer  242  is on top of a Flash memory die  223  on a substrate  222 , as shown in  FIG. 6 . The interposer  242  is a type of material similar to the substrate  222  that has bonding pads and is pre-fabricated with connecting traces  243  and  244 . It can also be a thin, I-metal layer flex circuit made with copper/polyimide. The interposer  242  size is just large enough to allow its bonding pads  227  to be located next to the exposed Flash die pads  226 . The interposer  242  has bonding pads  227 ,  225  conveniently fabricated to be close to the corresponding pads  226 ,  219  of the Flash memory die  223  below, and the Flash controller die  224  in the center, respectively, as shown in  FIG. 6  and  FIG. 7 . It also has corresponding pads  240  that are conveniently fabricated to be able to connect the pads  235  on the main substrate  222  below the Flash memory die. These wires  238  from the interposer  242  are the only ones required to connect to the substrate  222  and are in turn connected to Flash card interface  233  through traces  234  on the substrate  222 , as shown in  FIG. 7 . 
         [0036]    Unlike the conventional approaches, the present invention shifts almost all wire bonding connection from die to substrate  222  below to interposer  242  in the middle of Flash card  221 . This substantially reduces the pad space required on the substrate  222  by changing the number of pads on the substrate from in the order of 50 to less than 10, a saving of 80%. This saving in real estate on substrate  222  enables placement of a substantially larger Flash memory die previously not possible due to the space reserved for the bonding pads. Further, with the usage of interposer  242 , the Flash controller die  224  does not need to be placed directly on top of substrate  222 . There is, therefore, less restriction on the Flash controller die size, which has the same benefit as that of the conventional stacking approach. Typically, this invention improves the ratio of Flash memory die size to actual Flash memory card size from about 62% to 90% or higher. It allows for a larger Flash memory die, and therefore a higher density die to be used on the same Flash memory card  221  design. 
         [0037]    In order not to introduce extra height while stacking Flash controller die  224  on the interposer  242 , that is, sitting on top of Flash memory die  223 , the interposer  242  has a cut-out  228  in the middle area large enough to accommodate the Flash controller die  224 , as shown in  FIG. 6  and  FIG. 7 . The Flash controller die  224  sits like an island directly on top of the Flash memory die  223  instead of on the interposer  242 . 
         [0038]    If stacking of more Flash memory dies is necessary, then a flexible circuit on-die interposer  342  can be used, as shown in  FIG. 8  and  FIG. 9 . Note that the flexible interposer  342  can be folded in a serpentine fashion to expand more stacking of Flash memory dies  323 ,  423  and  523 , if necessary, as in  FIG. 10  and  FIG. 11 . 
         [0039]    If stacking of more Flash memory dies is necessary, then an extended tab interposer  344  can be used, as shown in  FIG. 13 . Additional wire bonding  346  is used to connect interposer  344  and  343 . If the extended tab interposer  344  is not stiff enough to perform wire bonding process, a stand-off  347  can be added for additional support. Note that the extended tab interposers  348 ,  349  and wire bonding  351 ,  352  can be used to expend more stacking of Flash memory dies  323 , 423  and  523 , if necessary, as in  FIG. 14 . It is optional to use stand-off  353  and  354  for additional support for wire bonding process. 
         [0040]      FIG. 8  shows the cross sectional view of the layout in accordance with the present invention with two Flash memory dies  323  and  423 , a flexible circuit on-die interposer  342  and a Flash controller die  324  on top of the substrate  322 .  FIG. 9  shows a side view of the layout of  FIG. 8 . 
         [0041]    A spacer  345  is necessary between the Flash controller die  324  and the Flash memory die  423  stacking above, as in  FIG. 8  and  FIG. 9 . An additional spacer  445  is required if more stacking of Flash memory die  523  is needed, as shown in  FIG. 10  and  FIG. 11 . 
         [0042]      FIG. 10  shows a cross sectional view of the layout in accordance with the present invention with three Flash memory dies  323 ,  423  and  523 , a flexible circuit on-die interposer  342  and a Flash controller die  324  on top of the substrate  322 .  FIG. 11  shows a side view of the layout of  FIG. 10 . 
         [0043]    The height of the spacer  345  and  445  is less than that of conventional stacking Flash memory card design. The reason is that spacer height is a function of the wire bonding loop height. The lower the loop height, the lower the spacer height is required. The loop height is also a function of the lateral distance between the two bonding pads, as shown in  FIG. 12 . 
         [0044]    As shown in  FIG. 12 , the distance between pad A and pad B is DI  154 . The distance between pad A and pad C is  02   153 . The wires  151  and wire  152  are for these two pairs of pads respectively. Their corresponding loop heights are HI  156  and H 2   155  respectively. The shorter the lateral distance DI  154 , the lower the loop height HI  156  is necessary. Due to the use of the interposer  342 , the corresponding pads  327  and  326  between interposer  342  and Flash memory dies  523 ,  423  and  323  can be right next to each other physically, as shown in  FIG. 11 . The corresponding pads  319  and  318  between interposer  342  and Flash controller die  324  can be right next to each other physically, as shown in  FIG. 10 . It therefore has the shortest wires possible between any corresponding pads and thus creates the lowest deterministic loop height possible. The net effect of lower spacer  345  and  445  results in more stacking height possible for Flash memory dies  323 ,  423  and  523  and achieves more density on Flash memory card  321  as a consequence. 
       Alternate Exemplary Embodiment 
       [0045]    One alternative embodiment is to place the controller unit to the middle of the Flash memory die and use direct die-to-die wire bonding even without the need for an on-die interposer. 
         [0046]    Yet another alternate embodiment is to form an on-die redistribution layer on the Flash memory in lieu of the on-die interposer. The only difference is that a redistribution layer can be fabricated on the Flash memory wafer, while the interposer is normally applied to the individual Flash memory die during substrate subassembly. 
         [0047]    Another alternate embodiment is to use flexible substrate instead of flexible on-die interposer. 
       Advantages Over Prior Art 
       [0048]    Unlike the conventional approach that brings all bonding wires from pads on Flash controller and Flash memory dies to the substrate, a Flash memory card in accordance with the present invention utilizes an on-die interposer, as shown in  FIG. 6 , to reduce the number of bonding wires. The interposer sits on top of the Flash memory die with bonding pads and traces to connect among Flash controller die, Flash memory die and Flash card interface, as shown in  FIG. 7  and  FIG. 8 . Since the Flash card interface signals are limited in number (less than 10) and are usually controlled by the Flash controller, they are the only signals that need to be wire-bonded to the substrate. This will dramatically reduce wire-bonding pads on the substrate from about 50 to less than 10, a reduction by 80%. 
         [0049]    The bonding wire length in this invention is more uniformly distributed with the on-die interposer than that of the conventional approach. No bonding wires have to cross over other wires as with prior art in Flash memory die stacking situation, as shown in  FIG. 4 . 
         [0050]    Use of on-die interposer for wire-bond interconnect between the Flash memory die and Flash controller effectively reduces wire-bond space on the substrate and allows largest possible Flash memory die to be used on the Flash memory card, as illustrated in the die size shown in  FIG. 4  (prior art) and  FIG. 6  (present invention). 
         [0051]    Use of an on-die imposer for direct wire-bond to save perimeter wire-bond space on the substrate. 
         [0052]    Use of center-cutout on-die interposer to accommodate Flash controller die. It reduces overall height and allows more density as shown in  FIG. 9 . 
         [0053]    Use of flexible circuit on-die interposer to allow expandable Flash memory die stacking. 
         [0054]    The present invention also reduces bonding wire length through convenient interposer pads nearby. It further reduces the loop height of bonding wire, allowing the use of spacers having less thickness. As a consequence, more stacking of Flash memory dies can be achieved in the same Flash memory card. 
         [0055]    Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.