Abstract:
The present invention provides an improvement on the use of flexible circuit connectors for electrically coupling IC devices to one another in a stacked configuration by use of the flexible circuit to provide the connection of the stacked IC module to other circuits. Use of the flexible circuit as the connection of the IC module allows the flexible circuit to provide strain relief and allows stacked IC modules to be assembled with a lower profile than with previous methods. The IC module can be connected to external circuits through the flexible circuit connectors by a variety of means, including solder pads, edge connector pads, and socket connectors. This allows for IC devices to occupy less space then with previous methods, which is beneficial in modules such as memory modules with multiple, stacked memory devices.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 11/330,307, filed Jan. 11, 2006, which is hereby incorporated by reference herein. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates to stacking leaded integrated circuit devices and, in particular, to stacks and stacking integrated circuits in leaded packages. 
       BACKGROUND 
       [0003]    A variety of systems and techniques are known for stacking packaged integrated circuits. Some techniques are devised for stacking chip-scale packaged devices (CSPs) while other systems and methods are better directed to leaded packages such as those that exhibit a set of leads extending from at least one lateral side of a typically rectangular package. 
         [0004]    Memory devices are packaged in both chip-scale (CSP) and leaded packages. However, techniques for stacking CSP devices are typically not optimum for stacking leaded devices. Although CSP devices are gaining market share, in many areas, integrated circuits continue to be packaged in high volumes in leaded packages. For example, the well-known flash memory integrated circuit is typically packaged in a leaded package with fine-pitched leads emergent from one or both sides of the package. A common package for flash memory is the thin small outline package commonly known as the TSOP typified by leads emergent from one or more (typically a pair of opposite sides) lateral sides of the package. thin small outline package commonly known as the TSOP typified by leads emergent from one or more (typically a pair of opposite sides) lateral sides of the package. 
         [0005]    The assignee of the present invention, Staktek Group L.P., has developed a wide variety of techniques, systems and designs for stacks and stacking with both leaded and CSP devices. In leaded package stacking, Staktek Group L.P. has developed, for example, U.S. Pat. No. 6,572,387 issued Jun. 3, 2003 and U.S. patent application Ser. No. 10/449,242 published as Pub. No. 2003/0203663 A1 which disclose and claim various techniques and apparatus related to stacking leaded packages. 
         [0006]    Many other techniques have been developed for interconnecting the leads of the stacked devices. For example, U.S. Pat. No. 4,696,525 to Coller et al. purports to teach a socket connector for coupling adjacent devices in a stacked configuration to one another. The socket has external conductors that interconnect leads from like, adjacent devices to one another. Sockets, however, are limited in several respects. They are not versatile in their ability to implement complex interconnections. In addition, such sockets, which have relatively thick, plastic bodies, act as thermal insulators between upper and lower package surfaces, and inhibit the module&#39;s overall ability to dissipate heat. 
         [0007]    Although the art has many techniques for stacking leaded devices, a new system and method for stacking leaded package devices is a welcome development. Accordingly, the present application discloses improved systems and methods for electrically and thermally coupling adjacent integrated circuit devices in stacked modules. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides a system and method for stacks and stacking leaded package ICs packages. A flex circuit is disposed between leaded ICs to be stacked. In a preferred embodiment, leads of constituent leaded IC packages are configured to allow the lower surface of the leaded IC packages to contact the surface of the flex circuitry that provides connection between an upper and lower leaded IC package. In an optional embodiment, a part of the flex circuit emerges from between the leaded ICs and provides a connective facility for connection to external or application environments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is an exploded view of a stacked module devised in accordance with a preferred embodiment of the present invention. 
           [0010]      FIG. 2  is a side view of a stacked module devised in accordance with a preferred embodiment of the present invention. 
           [0011]      FIG. 3  is a plan view of one side of a flex circuit in accordance with an embodiment of the present invention. 
           [0012]      FIG. 4  is a plan view of another side of a flex circuit in accordance with an embodiment of the present invention. 
           [0013]      FIG. 5  depicts the area marked “A” in  FIG. 2 . 
           [0014]      FIG. 6  is a side view of a stacked module in accordance with an alternative preferred embodiment of the present invention. 
           [0015]      FIG. 7  is a plan view of a stacked module in accordance with an alternative preferred embodiment of the present invention. 
           [0016]      FIG. 8  is a plan view of a stacked module in accordance with another alternative preferred embodiment of the present invention. 
           [0017]      FIG. 9  is a plan view of a stacked module in accordance with another alternative preferred embodiment of the present invention. 
           [0018]      FIG. 10  is a plan view of another side of a stacked module in accordance with another alternative preferred embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1  is an exploded view of an exemplar stacked module  10  devised in accordance with a preferred embodiment of the present invention. Exemplar module  10  is comprised of leaded ICs  20  and  22  each having upper and lower sides or surfaces  23  and  25 , respectively, and lateral sides S 1  and S 2  which, as those of skill will recognize, may be in the character of edges or sides and need not be perpendicular in aspect to the upper and lower surfaces  23  and  25 . Leads  24  are emergent from sides S 1  and S 2 . In a preferred embodiment, leads  24  are deflected to remain within the space defined by planes PL and PU defined by lower surfaces  25  and  23  respectively of the respective ICs to allow the lower surfaces  25  of each of the respective leaded packaged ICs to be in contact with the respective surfaces  15  and  17  of flex circuit  12  when the ICs are connected to the flex. In this disclosure, contact between the lower surface  25  of a leaded IC and the surfaces of flex circuit  12  includes not only direct contact between surface or side  25  and flex but shall include those instances where intermediate materials such as adhesive is used between the respective leaded IC and flex. 
         [0020]    The present invention may also be employed with circuitry other than or in addition to memory such as the flash memory depicted in a number of the present Figs. Other exemplar types of circuitry that may be aggregated in stacks in accordance with embodiments of the invention include, just as non-limiting examples, DRAMs, FPGAs, and system stacks that include logic and memory as well as communications or graphics devices. It should be noted, therefore, that the depicted profile for ICs  20  and  22  is not a limitation and that upper and lower leaded ICs  20  and  22  respectively need not be TSOPs or TSOP-like and the packages employed may have more than one die or leads emergent from one, two, three or all sides of the respective package body. For example, a module  10  in accordance with embodiments of the present invention may employ leaded ICs  20  and  22  that have more than one die within each package and may exhibit leads emergent from only one side of the package. In such cases, adhesives will typically be employed between the IC and flex circuit. Further, a module  10  in accord with the present invention need not have two ICs as the invention may be employed to devise a stacked module  10  with two or more ICs as those of skill will understand after appreciating this disclosure. Further, techniques disclosed herein may be employed to stack a leaded IC in a leaded-CSP combination stack. 
         [0021]    In the depicted preferred embodiment, flex circuit  12  (e.g., “flex”, “flex circuitry”, “flexible circuit” etc.) is disposed between leaded ICs  20  and  22  and exhibits a first side  15  having two pluralities of connective sites  34  and  36  adapted for connection to a leaded IC and, in this embodiment, another optional plurality of connective sites  32 . Flex circuit  12  also exhibits a second side  17  having two pluralities of connective sites  44  and  46 . Those of skill will recognize that flex circuit  12  may be comprised from traditional flexible circuitry or, in some of the alternative embodiments, what is sometimes called rigid-flex may be employed. Such rigid flex exhibits rigid areas and flexible areas to provide an interconnection function required of flex circuit  12  in the present invention. 
         [0022]    Pluralities  34  and  36  and  44  and  46  of connective sites are adapted for connection to the leads of leaded packages IC  20  and IC  22 , respectively, each of which has a plurality of peripheral sides, individual ones of which sides are identified as S 1  and S 2 . Optional third plurality of connective sites  32  is adapted for connection of module  10  to an external circuit or environment. 
         [0023]    Plural leads  24  are emergent from at least one of the plural sides of the ICs and typically, a plurality of leads  24  is emergent from one of the plural sides of each of the ICs  20  and  22  and a second plurality of leads  24  is emergent from another one of the plural sides of each of ICs  20  and  22 . Leaded ICs  20  and  22  are connected to flex circuit  12  through the leads  24  of leaded ICs  20  and  22 . As those of skill will recognize, many techniques exist for connecting the leads of leaded ICs  20  and  22  to the connective sites. Such techniques include, as a non-limiting example, use of solder or other conductive attachment. Other forms of bonding other than solder between the connecting sites and leads  24  may also be employed (such as brazing or welding for example) but soldering techniques are well understood and adapted for use in large scale manufacturing. 
         [0024]      FIG. 2  depicts a side perspective view of a stacked module  10  devised in accordance with a preferred embodiment of the present invention. As depicted, lower side  25  of each of leaded ICs  20  and  22  are adjacent to sides  15  and  17  respectively, of flex circuit  12 . To realize the adjacent and, preferably, contact (touching) relationship between the lower side  25  of a selected leaded IC and the respective flex circuit side, leads  24  typically require modification or reconfiguration which is preferably performed before mounting of the leaded IC to flex circuit  12 . Those of skill will note that a preferred method for reconfiguration of leads  24  comprises use of a jig to fix the position of body  29  of the respective leaded IC and, preferably, support the lead at the point of emergence from the body at sides S 1  and S 2  of leaded ICs  20  and  22  before deflection of the respective leads toward the upper plane PU to confine leads  24  to the space between planes PL and PU of the respective leaded IC as earlier shown in  FIG. 1 . This is because typically, leaded ICs such as TSOPs are configured with leads that extend beyond the lower plane PL. In order for the lower surface  25  of the respective leaded packaged ICs to contact (either directly or through an adhesive or thermal intermediary) the respective surfaces of the flex circuit, the leads  24  must be typically reconfigured. 
         [0025]      FIG. 3  depicts a plan view of side  15  of the flex circuit. As depicted, side  15  exhibits three pluralities of connective sites,  32 ,  34 , and  36 , each comprised of individual connective sites  32 C,  34 C, and  36 C, respectively. First and second pluralities  34  and  36  are adapted for connection to leaded IC  20  through leads  24 , with optional plurality of connective sites  32  being adapted for connecting module  10  to an external circuit or environment. 
         [0026]      FIG. 4  depicts a plan view of side  17  of flex circuit  12 . As depicted, side  17  exhibits two pluralities of connective sites  44  and  46  respectively, each comprised of multiple connective sites  44 C and  46 C, respectively, these sites being adapted for connection to leaded IC  22  through leads  24 . 
         [0027]      FIG. 5  depicts the area identified by “A” in earlier  FIG. 2 . As depicted, the standard lead shape is modified or reconfigured to reduce the profile X of module  10  as lower surfaces  25  of leaded ICs  20  and  22  are adjacent to and, preferably, in contact with surfaces  15  and  17 , respectively, of flex circuit  12 . Profile X is the distance between respective upper planes PU 20  and PU 22 . Leads  24  of leaded ICs  20  and  22  are preferably configured to allow leaded ICs  20  and  24  to be in either direct or indirect (through intermediary adhesive for example) contact with flex  12 . 
         [0028]    Leads  24  of leaded ICs  20  and  22  employed in an exemplar module  10  are shown in contact with connective sites  34 C and  44 C, for example, while lower surface  25  of the leaded ICs  20  and  22  are in contact with the respective sides  15  and  17  of flex circuit  12 . 
         [0029]      FIG. 6  depicts an exemplar module  10  having connective sites  32  for connection to an external circuit or environment. Those of skill will recognize that when a third plurality of connective sites such as the depicted reference  32  are employed, they may be disposed on either side  15  or  17  of flex circuit  12 . In this depiction, adhesive  33  is shown between lower surfaces  25  and respective sides of flex circuit  12 . 
         [0030]      FIG. 7  illustrates that, in devising a module in accordance with the present invention, some embodiments may be constructed where connective sites  32  take the form of edge connector pads for connection with an edge connector such as, for example, those typically found in computer applications for memory expansion. 
         [0031]      FIG. 8  illustrates a plan view of an exemplar module  10  in accordance with an alternative preferred embodiment of the present invention.  FIG. 8  employs a socket connector as the third plurality of connective sites  32  for connecting stacked module  10  to an external circuit or environment. 
         [0032]      FIG. 9  illustrates a module  10  in accordance with an alternative preferred embodiment of the present invention, showing alternative arrangements of the pluralities of connection sites on side  15  of the flex circuit. In the depiction of  FIG. 9 , the first and second pluralities of connective sites are oriented in a first direction while the third plurality of connective sites for connection of the circuit module to an application environment are oriented in a direction perpendicular to the orientation of the first and second pluralities of connective sites. 
         [0033]      FIG. 10  is a plan view of another side of the stacked module depicted in  FIG. 9 , sharing alternative arrangements of the pluralities of connective sites on side  17  of the flex circuit. 
         [0034]    It will be seen by those skilled in the art that many embodiments taking a variety of specific forms and reflecting changes, substitutions, and alternations can be made without departing from the spirit and scope of the invention. Therefore, the described embodiments illustrate but do not restrict the scope of the claims.