Patent Publication Number: US-2023133965-A1

Title: Semiconductor Device With Unbalanced Die Stackup

Description:
BACKGROUND 
     A semiconductor memory package may include a plurality of semiconductor memory dies and a controller packaged together on a substrate and encapsulated in a mold compound. The memory dies may be disposed in adjacent stacks, with the memory dies in each stack being electrically coupled to the substrate with respective pluralities of bond wires. The substrate includes communication lines that route electrical signals between the bond wires and the controller and to external connections. 
     As semiconductor memory packages continue to increase in storage capacity and decrease in size, the routing of electrical signals between the bond wires and the controller becomes more complicated. In addition, rearranging the memory dies within the package to address routing issues often creates wasted space, which can decrease the storage capacity of the package. 
     SUMMARY 
     The present disclosure describes a semiconductor memory package having unbalanced die stacks arranged in a configuration that increases storage capacity without increasing the size of the package. 
     In one aspect, a semiconductor memory package includes a substrate, a first stack of memory dies, and a second stack of memory dies. The first stack of memory dies is electrically coupled to a top layer of the substrate and includes a first number of memory dies. The second stack of memory dies is electrically coupled to the top layer of the substrate and includes a second number of memory dies greater than the first number of memory dies. An upper surface of the first stack of memory dies and an upper surface of the second stack of memory dies are coplanar or substantially coplanar. The semiconductor memory package further includes a controller mounted on and electrically coupled to the top layer of the substrate. The controller is included in the first stack of memory dies. The first stack of memory dies includes a bottom memory die mounted on the controller and a top memory die having an upper surface that is coplanar or substantially coplanar with the upper surface of the second stack of memory dies. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing summary, as well as the following detailed description, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the present disclosure, there are shown in the drawings embodiments which are presently preferred, wherein like reference numerals indicate like elements throughout. It should be noted, however, that aspects of the present disclosure can be embodied in different forms and thus should not be construed as being limited to the illustrated embodiments set forth herein. The elements illustrated in the accompanying drawings are not necessarily drawn to scale, but rather, may have been exaggerated to highlight the important features of the subject matter therein. Furthermore, the drawings may have been simplified by omitting elements that are not necessarily needed for the understanding of the disclosed embodiments. 
         FIG.  1    is a cross-sectional side view of a portion of a semiconductor memory package including a substrate, a controller, and two stacks of memory dies in accordance with some implementations. 
         FIG.  2    is a cross-sectional side view of a portion of a semiconductor memory package including a substrate, a controller, and two unbalanced stacks of memory dies in accordance with some implementations. 
         FIG.  3    is a cross-sectional side view of a portion of a semiconductor memory package including a substrate, a controller, and two unbalanced stacks of memory dies with one stack having bond wires on both sides in accordance with some implementations. 
         FIG.  4    is a cross-sectional side view of a portion of a semiconductor memory package including a substrate, a controller, and two unbalanced stacks of memory dies with one stack including adjacent memory dies sharing bond wires in accordance with some implementations. 
         FIGS.  5 - 6    are detailed and enlarged views of the top surfaces of the semiconductor memory package as shown in any of  FIGS.  2 - 4    in accordance with some implementations. 
         FIGS.  7 - 9    are cross-sectional side views of the portions of semiconductor memory packages respectively shown in  FIGS.  2 - 4    with an additional bridge memory die in accordance with some implementations. 
     
    
    
     DETAILED DESCRIPTION 
     The present subject matter will now be described more fully hereinafter with reference to the accompanying Figures, in which representative embodiments are shown. The present subject matter can, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided to describe and enable one of skill in the art. 
       FIG.  1    is a cross-sectional side view of a portion of a semiconductor memory package  100  including a substrate  104 , a controller  106 , and two stacks  110  and  120  of semiconductor memory dies  108 , all encapsulated in a mold compound  102 . Mold compound  102  may include, for example, an epoxy molding compound (EMC) or other encapsulant material known in the art. 
     Package  100  may be any type of semiconductor device, such as a system-in-package (SiP). In one nonlimiting example, package  100  is a storage device (e.g., a secure digital (SD) card or a MultiMediaCard (MMC)) and the memory dies  108  are NAND memory dies. 
     Substrate  104  is both a mechanical base support of package  100  and an electrical interface that provides access to the memory dies  108  housed within the package. The electrical interface includes a plurality of metal layers within the substrate, including at least one layer for routing data using conductive (e.g., copper) traces, a ground layer, and/or a power layer. The plurality of metal layers include at least (i) a top layer in electrical contact with controller  106  and bond wires  112  and  122 , and upon which controller  106  and other elements are mounted, and (ii) a bottom layer in electrical contact with solder balls  103 , through which signals are routed between memory dies  108  and circuit elements outside package  100 . 
     Controller  106  may be an application-specific integrated circuit (ASIC), or any other type of controller. Controller  106  is electrically coupled to the top layer of substrate  104 . In some implementations, controller  106  may be mounted using flip chip mounting. 
     Controller  106  includes at least one bonding pad on a first side A and at least one bonding pad on a second side B opposite the first. As used herein, “first side A” refers to a side of controller  106  that is adjacent to, is closest to, or is otherwise neighboring a first die stack (e.g., die stack  110 ), and “second side B” refers to a side of the controller  106  that is adjacent to, is closest to, or is otherwise neighboring a second die stack (e.g., die stack  120 ). In some implementations, controller  106  includes four bonding pads, with two on each side. In general, controller  106  may include any number of bonding pads on the first side A and/or the second side B. 
     Spacers  132  and  134  allow die stacks  110  and  120  to be mounted closer to the center of package  100 , thus allowing for reduced x-y dimensions of the package. 
     Semiconductor memory dies  108  may be integrated circuits configured for data storage. In some implementations, memory dies  108  are Not AND (NAND) flash memory dies, each comprising a plurality of NAND memory cells. In other implementations, memory dies  108  may each comprise a plurality of other types of memory cells, such as Not OR (NOR) memory cells. 
     The use of two die stacks in a side-by-side configuration (one on each side of the controller  106 ) allows more memory dies  108  to be electrically coupled to the controller  106  than if there was a single die stack, without expanding the package size in the x-y directions and/or the z-direction. Such a configuration optimizes routing between the bond wires ( 112  and  122 ) and channels on both sides of the controller  106 . For example, bond wires  112  coupled to memory dies in stack  110  may be coupled to communication lines in substrate  104  that are routed to one or more bonding pads on side A of controller  106 , and bond wires  122  coupled to memory dies in stack  120  may be coupled to communication lines in substrate  104  that are routed to one or more bonding pads on side B of controller  106 . Here the term channel refers to an external connection to the controller, such as a memory die bonding pad, and the term communication line refers to metal traces within the substrate that are used to route signals between the memory dies  108  and the controller  106  and external devices (not shown). 
     To conserve even more space in the x-direction and shrink the package without decreasing storage capacity, controller  106  may be moved to one of the die stacks, and the other of the die stacks may be supplemented with one or more additional memory dies  108 . In some such implementations, one of spacers  132  or  134  may be replaced with controller  106 , while the other one of spacers  132  or  134  may be replaced by the one or more additional memory dies  108 . Having one or more additional memory dies  108  allows for increased storage capacity. As a result, there are an unequal number of memory dies  108  in each of the two die stacks. Such a configuration may be referred to as an unbalanced stackup of memory dies, a semiconductor memory package having unbalanced die stacks, or unbalanced stacking of memory dies. Example implementations of such a configuration are described below with reference to  FIGS.  2 - 9   . 
       FIG.  2    is a cross-sectional side view of a portion of a semiconductor memory package  200  including a substrate  104 , a controller  106 , and two unbalanced stacks  210  and  220  of memory dies  108  in accordance with some implementations. 
     Die stack  210  includes controller  106 , which is electrically coupled to and mounted on substrate  104 , and a first stack of memory dies  108  mounted on the controller  106 . In some implementations, no spacer is present in die stack  210  or between memory dies  108  and substrate  104 . According to the present disclosure, the controller  106  is mounted on and electrically coupled to the top surface of the substrate  104 . In some implementations, the controller  106  may be a flip-chip die, coupled to the substrate  104  with solder balls (not shown). In some implementations, the controller  106  may be a wire-bond die, coupled to the substrate  104  with bond wires. In some implementations, a WEF (Wire Embedded Film) may separate the controller  106  and the memory die  108  that is mounted on top of the controller  106 . 
     Each memory die  108  in stack  210  is electrically coupled to substrate  104  with bond wires  212 . In this example, there are four memory dies  108  in stack  210 . In other implementations, there may be any number of memory dies in stack  210 , from a minimum of one memory die to a maximum of N memory dies, where N represents the number of memory dies it would take for the stack to reach the top of the usable area within the package. 
     Die stack  220  includes a second stack of memory dies  108  mounted on substrate  104 . Each memory die  108  in stack  220  is electrically coupled to substrate  104  with bond wires  222 . In some implementations, no spacer is present in die stack  220  or between memory dies  108  and substrate  104 . In some implementations, memory dies  108  of die stack  220  are not mounted on controller  106 . In this example, there are six memory dies  108  in stack  220 . In other implementations, there may be any number of memory dies in stack  220 , such as M memory dies, where M represents the number of memory dies it would take for the stack to reach the top of the usable area within the package. For example, M may equal N+P, where N is the number of memory dies in stack  210 , and P is the number of memory dies that may be added to stack  220  to bring the height of the stack up to the top of the usable area within the package. In the example depicted in  FIGS.  2   , N=4, P=2, and M=6. Stated another way, by moving controller  106  to stack  210 , and eliminating the spacers  132 ,  134  ( FIG.  1   ), stack  220  may be supplemented with, for example, two additional memory dies  224  and  226 . Other integers for N, P, and M may be used for other implementations, depending on package height requirements and the thickness of the memory dies  108 . By having more memory dies in stack  220  (e.g., memory dies  224  and  226 ), package  200  has increased storage capacity without compromising on the outline (size) of the package. For example, the overall height (dimension in the z-direction) of semiconductor memory package  200  may be the same as the overall height semiconductor memory package  100  while having more memory dies and storage capacity. 
     In the presently preferred embodiments, the top surfaces  211  and  221  of the die stacks  210  and  220  are coplanar or substantially coplanar. Stated another way, the heights of the die stacks  210  and  220  from the substrate  104  are the same or substantially equal. This maximizes storage capacity for a given volume of internal package space. More details regarding this feature are discussed below with reference to  FIGS.  5 - 6   . 
     In the configuration depicted in  FIG.  2   , the memory dies in stack  210  may send and receive signals to and from one or more bonding pads on side A of controller  106  with bond wires  212  and conductive (e.g., copper) traces in the substrate  104  in the area to the left of the controller in the figure, and the memory dies in stack  220  may send and receive signals to and from one or more bonding pads on side B of controller  106  with bond wires  222  and conductive (e.g., copper) traces in the substrate  104  in the area to the right of the controller in the figure. As storage capacity (e.g., the number of memory dies) increases, the number of bonding pads (and corresponding channels) of the controller tends to increase, which causes routing complexity to increase. Thus, in some implementations, the memory dies  108  may be implemented in other configurations that optimize routing of bond wires and substrate traces, as described below with reference to  FIGS.  3 - 4   . 
       FIG.  3    is a cross-sectional side view of a portion of a semiconductor device package  300  including a substrate  104 , a controller  106 , and two unbalanced stacks  310  and  320  of memory dies  108  with one stack ( 320 ) having bond wires  322  and  328  on both sides in accordance with some implementations. 
     Die stack  310  includes controller  106 , which is electrically coupled to and mounted on substrate  104 , and a first stack of memory dies  108  mounted on the controller  106 . In some implementations, no spacer is present in die stack  310  or between memory dies  108  and substrate  104 . Each memory die in stack  310  is electrically coupled to substrate  104  with bond wires  312 . In this example, there are four memory dies  108  in stack  310 . In other implementations, there may be any number of memory dies in stack  310 , from a minimum of one memory die to a maximum of N memory dies, where N represents the number of memory dies it would take for the stack to reach the top of the usable area within the package. 
     Die stack  320  includes a second stack of memory dies  108  mounted on substrate  104 . In some implementations, no spacer is present in die stack  320  or between memory dies  108  and substrate  104 . In some implementations, memory dies  108  of die stack  320  are not mounted on controller  106 . A first subset of memory dies in stack  320  are electrically coupled to substrate  104  with bond wires  322 , and a second subset of memory dies in stack  320  (memory dies  324  and  326 ) are electrically coupled to substrate  104  with bond wires  328 . Bond wires  322  and  328  are coupled to opposite sides of the stack. This may be implemented by rotating memory dies in the second subset  180  degrees, so their bond wire hookups are disposed on the opposite side of the stack compared to the bond wire hookups for memory dies in the first subset. In this example, there are six memory dies  108  in stack  320 , with four memory dies in the first subset and two memory dies in the second subset. In other implementations, there may be any number of memory dies in stack  320 , such as M memory dies, where M represents the number of memory dies it would take for the stack to reach the top of the usable area within the package. For example, M may equal N+P, where N is the number of memory dies in stack  310 , and P is the number of memory dies that may be added to stack  320  to bring the height of the stack up to the top of the usable area within the package. In the example depicted in  FIGS.  3   , N=4, P=2, and M=6. Other integers for N, P, and M may be used for other implementations, depending on package height requirements and the thickness of the memory dies  108 . In addition, any number of memory dies may be included in the first and second subsets of memory dies, with the numbers (two and four) depicted in the figure being illustrative. 
     In some implementations, memory dies in stack  310  send and receive signals to and from one or more bonding pads on side A of controller  106  with bond wires  312 , memory dies in the first subset of stack  320  send and receive signals to and from one or more bonding pads on side B of controller  106  with bond wires  322 , and memory dies in the second subset of stack  320  (e.g., memory dies  324  and  326 ) send and receive signals, using bond wires  328 , to and from (i) one or more bonding pads on side A of controller  106 , or (ii) one or more bonding pads on side B of controller  106  not being used for communications with memory dies in the first subset of stack  320 . For example, memory dies in the first subset of stack  320  may send and receive signals to and from a first bonding pad on side B of controller  106  using bond wires  322 , and memory dies in the second subset of stack  320  may send and receive signals to and from a second bonding pad on side B of controller  106  using bond wires  328 . 
     By rotating the second subset of memory dies so that the corresponding bond wires  328  are directed towards the center of the package, there may be fewer routing constraints associated with the second die stack  320 . For example, if by adding memory dies  324  and  326  to stack  320  increases routing complexity to an extent that package size may need to be increased to accommodate a larger substrate, the additional memory dies (e.g.,  324  and  326 ) may be rotated so that their bond wires  328  and associated routing in the substrate do not conflict or otherwise interfere to as great an extent with bond wires  322  and associated routing on the other side of the stack. 
     In addition, by rotating the second subset of memory dies so that the corresponding bond wires  328  are directed towards the center of the package, the length of the communication lines in the substrate that correspond to the bond wires of the lower memory dies in stack  320  may be reduced. Specifically, rather than routing data from controller  106  all the way to bond wires  222  at the edge of substrate  104  (as depicted for memory dies  224  and  226  in  FIG.  2   ), such data may be routed to bond wires  328  towards the center of the package (as depicted for memory dies  324  and  326  in  FIG.  3   ). This decreases the length of the traces in substrate  104  used for routing data between controller  106  and the bottom two memory dies in stack  320 , which shortens the channel length (the length of the metal traces between bond wires and bonding pads) for these memory dies, thereby reducing impedance issues and cross talking on the channel. 
     To maximize storage capacity for a given volume of internal package space, in some implementations, the top surfaces  311  and  321  of the die stacks  310  and  320  are coplanar or substantially coplanar. Stated another way, the heights of the die stacks  310  and  320  from the substrate  104  are the same or substantially equal. More details regarding this feature are discussed below with reference to  FIGS.  5 - 6   . 
       FIG.  4    is a cross-sectional side view of a portion of a semiconductor memory package  400  including a substrate  104 , a controller  106 , and two unbalanced stacks  410  and  420  of memory dies  108  with one stack including adjacent memory dies  424  and  426  sharing bond wires and routing material in accordance with some implementations. 
     Die stack  410  includes controller  106 , which is electrically coupled to and mounted on substrate  104 , and a first stack of memory dies  108  mounted on the controller  106 . In some implementations, no spacer is present in die stack  410  or between memory dies  108  and substrate  104 . Each memory die in stack  410  is electrically coupled to substrate  104  with bond wires  412 . In this example, there are four memory dies  108  in stack  410 . In other implementations, there may be any number of memory dies in stack  410 , from a minimum of one memory die to a maximum of N memory dies, where N represents the number of memory dies it would take for the stack to reach the top of the usable area within the package. 
     Die stack  420  includes a second stack of memory dies  108  mounted on substrate  104 . In some implementations, no spacer is present in die stack  420  or between memory dies  108  and substrate  104 . In some implementations, memory dies  108  of die stack  420  are not mounted on controller  106 . The memory dies in stack  420  are electrically coupled to substrate  104  with bond wires  422 . A subset of memory dies in stack  420  (memory dies  424  and  426 ) are aligned and stacked on top of each other, thereby reducing the amount of space required in the x-direction required by stack  420 , which reduces the channel length (the length of the metal traces between bond wires and bonding pads) for the bottom memory dies in the stack. In some implementations, memory die  424  and memory die  426  are vertically disposed with no horizontal offset relative to each other. Memory dies  424  and  426  may be sized and positioned such that, for example, neither memory die  424  nor memory die  426  extends past the other in an x- and/or y-direction. The smaller channel length reduces impedance and cross-talking issues as described above with reference to  FIG.  3   . The memory dies in the vertically stacked subset (memory dies  424  and  426 ) share bond wires and routing material (e.g., in an area between the memory dies). Stated another way, each memory die  424  and  426  includes (i) memory cells for supporting storage functions of the memory die, and (ii) routing material for supporting combined electrical coupling to substrate  104  for the instant memory die and a neighboring memory die using bond wires  422 . 
     There may be any number of memory dies in stack  420 , such as M memory dies, where M represents the number of memory dies it would take for the stack to reach the top of the usable area within the package. For example, M may equal N+P, where N is the number of memory dies in stack  410 , and P is the number of memory dies that may be added to stack  420  to bring the height of the stack up to the top of the usable area within the package. In the example depicted in  FIGS.  4   , N=4, P=2, and M=6. Other integers for N, P, and M may be used for other implementations, depending on package height requirements and the thickness of the memory dies  108 . In addition, any number of memory dies may be included in the subset of vertically stacked memory dies, with the numbers (two) depicted in the figure being illustrative. 
     In some implementations, memory dies in stack  410  communicate with (send and receive signals to and from) one or more bonding pads on side A of controller  106  with bond wires  412 , and memory dies in stack  420  communicate with (send and receive signals to and from) one or more bonding pads on side B of controller  106  with bond wires  422 . 
     By combining a subset of memory dies (e.g.,  424  and  426 ) in a vertical stack, there may be fewer bond wires  422  and, therefore, fewer routing constraints associated with the second die stack  420 . For example, if by adding memory dies  424  and  426  to stack  420  increases routing complexity to an extent that package size may need to be increased to accommodate a larger substrate, the additional memory dies (e.g.,  424  and  426 ) may be combined so that they are electrically coupled to a single set of bond wires  422  for two memory dies in the subset. 
     To maximize storage capacity for a given volume of internal package space, the top surfaces  411  and  421  of the die stacks  410  and  420  are coplanar or substantially coplanar. Stated another way, the heights of the die stacks  410  and  420  from the substrate  104  are the same or substantially equal. More details regarding this feature are discussed below with reference to  FIGS.  5 - 6   . 
       FIGS.  5 - 6    are detailed and enlarged views of the top surfaces of the semiconductor memory package as shown in any of  FIGS.  2 - 4    in accordance with some implementations. Specifically, surface  502  in  FIGS.  5 - 6    may correspond to surface  211 ,  311 , or  411  in  FIGS.  2 - 4   , and surface  504  in  FIGS.  5 - 6    may correspond to surface  221 ,  321 , or  421  in  FIGS.  2 - 4   . Likewise, die stack A in  FIGS.  5 - 6    may correspond to die stack  210 ,  310 , or  410  in  FIGS.  2 - 4   , and die stack B in  FIGS.  5 - 6    may correspond to die stack  220 ,  320 , or  420  in  FIGS.  2 - 4   . 
     In  FIG.  5   , each die stack is the same height. As such, the top memory dies in each stack, or more specifically, top surfaces  502  and  504  are coplanar. As used herein, the term coplanar means two surfaces lying in the same plane (depicted as plane P). 
     In  FIG.  6   , each die stack is substantially the same height. As such, the top memory dies in each stack, or more specifically, top surfaces  502  and  504  are substantially coplanar. As used herein, substantially the same height and substantially coplanar mean the height difference (depicted as offset Δ) between the top surfaces  502  and  504  of the top memory dies in each stack is less than the thickness H of either of the top two memory dies. In some implementations, for example, the die stacks are configured such that Δ is equal to or less than half of thickness H. If the height difference (Δ) is greater than thickness H, then another memory die could have been added to the lower stack to take advantage of the available interior space of the package and optimize storage capacity. 
       FIGS.  7 - 9    are cross-sectional side views of the portions of semiconductor memory packages respectively shown in  FIGS.  2 - 4    with an additional bridge memory die in accordance with some implementations. 
     In some implementations, an additional memory die ( 702 ,  802 ,  902 ) may be mounted on top of the two memory dies stacks ( 210 / 220 ,  310 / 320 ,  410 / 420 ) such that a first portion of the additional memory die is mounted on one stack and a second portion of the additional memory die is mounted on the other stack. In some implementations, the additional memory die may be added when the two die stacks have the same height (as depicted in  FIG.  5   ). 
     In  FIGS.  7 - 9   , package  700  ( FIG.  7   ) corresponds to package  200  ( FIG.  2   ), package  800  ( FIG.  8   ) corresponds to package  300  ( FIG.  3   ), and package  900  ( FIG.  9   ) corresponds to package  400  ( FIG.  4   ). The additional memory die ( 702 ,  802 ,  902 ) bridges the two stacks, and may be referred to as a bridge memory die. Based on package geometry and available internal space, the additional memory die may be added in order to further increase storage capacity of the package without compromising on the outline (size) of the package. 
     In the implementations described above with reference to  FIGS.  2 - 9   , the silicon ratio may be maintained across each die stack. Stated another way, the ratio of the amount of silicon in each die stack may be balanced. For example, the amount of silicon in stack  210  may be substantially the same (e.g., within 10%) as the amount of silicon in stack  220  for package  200 . By maintaining substantially the same silicon ratio across each stack in a pair of stacks, stresses on the package are uniform, which increases robustness of the package. Further benefits of a balanced silicon ratio include even mold flow, reduced warpage, and uniform thermals across the package. 
     In the implementations described above with reference to  FIGS.  2 - 9   , the wire pitch of the bond wires may be maintained across each die stack. For example, the wire pitch for bond wires  212  of stack  210  may be substantially the same (e.g., within 10%) as the wire pitch for bond wires  222  of stack  220  for package  200 . By maintaining substantially the same wire pitch across each stack in a pair of stacks, uniformity of the routing throughout the package may be maintained, which increases robustness of the package. 
     In the implementations described above with reference to  FIGS.  2 - 9   , the memory dies in each stack may have different thicknesses. For example, one or more base memory dies (on the bottom of a stack closest to the substrate) may be thicker than one or more ceiling memory dies (on the top of a stack). In another example, a middle memory die may be configured to be a transition memory die between bottom memory dies and top memory dies. Such a middle memory die may have more wire bonds than the other memory dies in the stack, causing the middle memory die to be thicker. In some implementations, the thicknesses of memory dies may vary between 60 microns (e.g., for a bottom memory die) and 40 microns (e.g., for a top memory die). In other implementations, memory dies having other thicknesses (greater or less than 60 microns, or greater or less than 40 microns) may be used. For implementations in which memory dies in a stack have different thicknesses, the different thicknesses can be mixed and matched so that the two stacks in a given package are the same height or substantially the same height, thereby maximizing storage capacity for a given package size. 
     Thus, in order to maximize storage capacity for a semiconductor memory package without increasing the package size, the package may include a substrate (e.g.,  104 ), a first stack of memory dies (e.g.,  210 ,  310 , or  410 ), and a second stack of memory dies (e.g.,  220 ,  320 , or  420 ). The first stack of memory dies is electrically coupled to a top layer of the substrate and includes a first number of memory dies. The second stack of memory dies is electrically coupled to the top layer of the substrate and includes a second number of memory dies greater than the first number of memory dies. An upper surface of the first stack of memory dies (e.g.,  211 ,  311 , or  411 ) and an upper surface of the second stack of memory dies (e.g.,  221 ,  321 , or  421 ) are coplanar or substantially coplanar (e.g., as described above with reference to  FIGS.  5 - 6   ). 
     In some implementations, the semiconductor memory package further includes a controller (e.g.,  106 ) mounted on and electrically coupled to the top layer of the substrate. In some implementations, the controller is included in only one of the die stacks. For example, the controller is included in the first stack of memory dies. 
     In some implementations, the first stack of memory dies includes a bottom memory die mounted on the controller and a top memory die having an upper surface (e.g.,  211 ,  311 , or  411 ) that is coplanar or substantially coplanar with the upper surface (e.g.,  221 ,  321 , or  421 ) of the second stack of memory dies. 
     In some implementations, the controller includes a first bonding pad disposed on a first side of the controller (e.g., side A) and a second bonding pad disposed on a second side of the controller (e.g., side B) opposite the first side of the controller, and the first stack of memory dies is electrically coupled to the first bonding pad of the controller with a first plurality of bond wires (e.g.,  212 ,  312 , or  412 ). 
     In some implementations, the second stack of memory dies is electrically coupled to the second bonding pad of the controller with a second plurality of bond wires (e.g.,  222 ). 
     In some implementations, a first subset of the second stack of memory dies (e.g., the top four memory dies in stack  320 ) is electrically coupled to the second bonding pad of the controller with a second plurality of bond wires (e.g.,  322 ), a second subset of the second stack of memory dies (e.g., the bottom two memory dies in stack  320 ) is electrically coupled to the first bonding pad of the controller with a third plurality of bond wires (e.g.,  328 ), and the second plurality of bond wires and the third plurality of bond wires are disposed on opposite sides of the second stack of memory dies. 
     In some implementations, a first subset of the second stack of memory dies (e.g., the top four memory dies in stack  320 ) is electrically coupled to the second bonding pad of the controller with a second plurality of bond wires (e.g.,  322 ), a second subset of the second stack of memory dies (e.g., the bottom two memory dies in stack  320 ) is electrically coupled to the second bonding pad of the controller (or to a third bonding pad of the controller on side B) with a third plurality of bond wires (e.g.,  328 ), and the second plurality of bond wires and the third plurality of bond wires are disposed on opposite sides of the second stack of memory dies. 
     In some implementations, a first subset of the second stack of memory dies (e.g., the top four memory dies in stack  320 ) is electrically coupled to the top layer of the substrate with a first plurality of bond wires (e.g.,  322 ), a second subset of the second stack of memory dies (e.g., the bottom two memory dies in stack  320 ) is electrically coupled to the top layer of the substrate with a second plurality of bond wires (e.g.,  328 ), and the first plurality of bond wires and the second plurality of bond wires are disposed on opposite sides of the second stack of memory dies. 
     In some implementations, the first stack of memory dies (e.g.,  310 ) is vertically stacked in a first diagonal direction (e.g., towards the upper right of  FIG.  3   ), a first subset of the second stack of memory dies (e.g., the top four memory dies of stack  320 ) is vertically stacked in a second diagonal direction opposite the first diagonal direction (e.g., towards the upper left of  FIG.  3   ), and a second subset of the second stack of memory dies (e.g., the bottom two memory dies of stack  320 ) is vertically stacked in the first diagonal direction (e.g., towards the upper right of  FIG.  3   ). 
     In some implementations, the upper surface (e.g.,  502 ) of the first stack of memory dies and the upper surface (e.g.,  504 ) of the second stack of memory dies are offset by a distance (e.g., Δ) that is smaller than a thickness (e.g., H) of a top memory die of the first stack of memory dies or a top memory die of the second stack of memory dies. 
     In some implementations, the first stack of memory dies has a first vertical height with respect to the top layer of the substrate (e.g., the distance in the z-direction between substrate  104  and surface  211 ,  311 , or  411 ), the second stack of memory dies has a second vertical height with respect to the top layer of the substrate (e.g., the distance in the z-direction between substrate  104  and surface  221 ,  321 , or  421 ), and a difference between the first vertical height and the second vertical height is smaller than a thickness (e.g., H) of a top memory die of the first stack of memory dies or a top memory die of the second stack of memory dies. 
     In some implementations, at least two adjacent memory dies (e.g.,  424  and  426 ) of the second stack of memory dies are vertically disposed with no horizontal offset, an internal routing layer is vertically disposed between data storage portions of the at least two adjacent memory dies, and the at least two adjacent memory dies are electrically coupled to the top layer of the substrate with a single layer of bond wires (e.g., bond wires  422  connected to memory die  424 ). 
     In some implementations, the semiconductor memory package further includes a memory die (e.g.,  702 ,  802 , or  902 ) mounted on top of the first stack of memory dies and the second stack of memory dies. 
     In some implementations, the first stack of memory dies is mounted on a controller (e.g.,  106 ), the first stack of memory dies and the controller have a first silicon ratio, and the second stack of memory dies has a second silicon ratio substantially equal to (e.g., within 10% of) the first silicon ratio. 
     In some implementations, the first stack of memory dies has a first bond wire pitch, and the second stack of memory dies has a second bond wire pitch substantially equal to (e.g., within 10% of) the first bond wire pitch. 
     In some implementations, the controller is an application-specific integrated circuit (ASIC) and the first stack and second stack include respective pluralities of NAND memory dies. 
     In another implementation, a semiconductor memory package includes a substrate (e.g.,  104 ), a controller (e.g.,  106 ), a first stack of memory dies (e.g.,  210 ,  310 , or  410 ), and a second stack of memory dies (e.g.,  220 ,  320 , or  420 ). The controller is mounted on and electrically coupled to a top layer of the substrate. The first stack of memory dies is electrically coupled to the top layer of the substrate and includes (i) a bottom memory die mounted on the controller and (ii) a top memory die having a first vertical distance (in the z-direction) from a top layer of the substrate. The second stack of memory dies is electrically coupled to the top layer of the substrate and includes (i) a bottom memory die mounted on a top layer of the substrate (e.g.,  226 ,  326 , or  426 ) and (ii) a top memory die having a second vertical distance (in the z-direction) from the top layer of the substrate. The second stack includes more memory dies than the first stack, and the first vertical distance and the second vertical distance are substantially equal (e.g., as described above with reference to  FIGS.  5 - 6   ) 
     In another implementation, a semiconductor memory package includes a substrate (e.g.,  104 ), a controller (e.g.,  106 ), a first stack of memory dies (e.g.,  310 ), and a second stack of memory dies (e.g.,  320 ). The controller is mounted on and electrically coupled to a top layer of the substrate. The first stack of memory dies is mounted on the controller and includes first bonding means for electrically coupling the memory dies (e.g.,  312 ) in the first stack to a top layer of the substrate. The second stack of memory dies is mounted on the top layer of the substrate and includes second bonding means for electrically coupling the memory dies (e.g.,  322 ,  328 ) in the second stack to the top layer of the substrate. The second stack includes more memory dies than the first stack, the first bonding means are disposed about one side of the first stack, and the second bonding means are disposed about two opposite sides of the second stack. 
     In some implementations, a first plurality of the second bonding means (e.g.,  322 ) electrically couple a first subset of memory dies in the second stack (e.g., the top four memory dies in stack  320 ) to a bonding pad on a first side (e.g., side B) of the controller, and a second plurality of the second bonding means (e.g.,  328 ) electrically couple a second subset of memory dies in the second stack (e.g., the bottom two memory dies in stack  320 ) to a bonding pad on a second side of the controller (e.g., side A) opposite the first side. 
     In some implementations, a first plurality of the second bonding means (e.g.,  322 ) electrically couple a first subset of memory dies in the second stack (e.g., the top four memory dies in stack  320 ) to a first bonding pad on a first side of the controller (e.g., bonding pad  1  on side B), and a second plurality of the second bonding means (e.g.,  328 ) electrically couple a second subset of memory dies in the second stack (e.g., the bottom two memory dies in stack  320 ) to the first bonding pad, or to a second bonding pad on the first side of the controller (e.g., bonding pad  1 , or bonding pad  2 , on side B). 
     It will be appreciated by those skilled in the art that changes could be made to the exemplary embodiments shown and described above without departing from the broad inventive concepts thereof. It is understood, therefore, that this invention is not limited to the exemplary embodiments shown and described, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the claims. For example, specific features of the exemplary embodiments may or may not be part of the claimed invention and various features of the disclosed embodiments may be combined. The words “right”, “left”, “lower” and “upper” designate directions in the drawings to which reference is made. The words “inwardly” and “outwardly” refer to directions toward and away from, respectively, the geometric center of the ball grid array having a multi-surface trace interface. Unless specifically set forth herein, the terms “a”, “an” and “the” are not limited to one element but instead should be read as meaning “at least one”. 
     It is to be understood that at least some of the figures and descriptions of the invention have been simplified to focus on elements that are relevant for a clear understanding of the invention, while eliminating, for purposes of clarity, other elements that those of ordinary skill in the art will appreciate may also comprise a portion of the invention. However, because such elements are well known in the art, and because they do not necessarily facilitate a better understanding of the invention, a description of such elements is not provided herein. 
     Further, to the extent that the methods of the present invention do not rely on the particular order of steps set forth herein, the particular order of the steps should not be construed as limitation on the claims. Any claims directed to the methods of the present invention should not be limited to the performance of their steps in the order written, and one skilled in the art can readily appreciate that the steps may be varied and still remain within the spirit and scope of the present invention.