Abstract:
A system and method for electrically and thermally coupling adjacent IC packages to one another in a stacked configuration is provided. A flex circuit is inserted in part between ICs to be stacked and provides a connective field that provides plural contact areas that connect to respective leads of the ICs. Thus, the flex does not require discrete leads which must be individually aligned with the individual leads of the constituent ICs employed in the stack. The principle may be employed to aggregate two or more contact areas for respective connection to leads of constituent ICs but is most profitably employed with a continuous connective field that provides contact areas for many leads of the ICs.

Description:
TECHNICAL FIELD  
       [0001]     This invention relates to stacking leaded integrated circuit devices. More particularly, this invention relates to stacks of leaded integrated circuits and associated flex circuitry.  
       BACKGROUND  
       [0002]     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 which exhibit a set of leads extending from at least one lateral side of a typically rectangular package.  
         [0003]     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 leaded packages with fine-pitched leads emergent from one or both sides of a package. A common package for flash memory is a fine pitch thin small outline package commonly known as the TSOP. Flash circuitry in TSOP packaging typically differs from common TSOP-packaged DRAMs in that flash TSOPs typically exhibit fine pitch leads emergent from the shorter pair of the lateral sides of the package while DRAM TSOPs typically exhibit leads emergent from the longer pair of sides of the package.  
         [0004]     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 rail bus systems that interconnect the leads of stacked leaded IC devices by use of rails. The present assignee also owns, 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.  
         [0005]     Many other techniques have been developed that use various means for interconnecting the leads of the stacked devices. For example, U.S. Pat. No. 4,696,525 to Coller et al. teaches 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 heat insulators between adjoining upper and lower (major) package surfaces, which can inhibit the module&#39;s overall ability to dissipate heat.  
         [0006]     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  
       [0007]     The present invention provides a system and method for electrically and thermally coupling adjacent IC packages to one another in a stacked configuration. A flex circuit having an interconnective pattern is inserted between ICs to be stacked. A part of the flex circuit emerges from between the ICs and provides a connective field that provides plural contact areas that connect to respective leads of the ICs. Thus, the flex does not require discrete leads which must be individually aligned with the individual leads of the constituent ICs employed in the stack. The principle may be employed to aggregate two or more contact areas for respective connection to leads of constituent ICs but is most profitably employed with a continuous connective field that provides contact areas for many leads of the ICs. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1A  depicts one embodiment of a flex circuit that may be employed in accordance with a preferred embodiment of the present invention.  
         [0009]      FIG. 1B  depicts an embodiment of a flex circuit in accordance with an embodiment of the present invention.  
         [0010]      FIG. 2  depicts an enlargement of the area marked “A” in  FIG. 1B .  
         [0011]      FIG. 3  depicts another aspect of a flex circuit in accordance with a preferred embodiment of the present invention.  
         [0012]      FIG. 4  depicts an enlargement of the area marked “B” in  FIG. 3 .  
         [0013]      FIG. 5  is an exploded view of a stacked module devised in accordance with a preferred embodiment of the present invention.  
         [0014]      FIG. 6  is a perspective view of a stacked module devised in accordance with a preferred embodiment of the present invention.  
         [0015]      FIG. 7  is a depiction of an enlarged area marked “C” from  FIG. 6  above.  
         [0016]      FIG. 8  is a side view of a stacked module in accordance with a preferred embodiment of the present invention.  
         [0017]      FIG. 9  depicts an enlarged view of the area marked “D” in  FIG. 8 .  
         [0018]      FIG. 10  depicts an enlarged view of a part of the area marked “E” in  FIG. 9 .  
         [0019]      FIG. 11  is a view of a stacked module in accordance with a preferred embodiment of the present invention.  
         [0020]      FIG. 12  is another side view of a stacked module in accordance with an alternative preferred embodiment of the present invention.  
         [0021]      FIG. 13  is depicts an enlarged view of the area marked “F” in  FIG. 12 .  
         [0022]      FIG. 14  is a perspective view of a stacked module in accordance with an alternative preferred embodiment of the present invention.  
         [0023]      FIG. 15  is a side view of a stacked module in accordance with another alternative preferred embodiment of the present invention.  
         [0024]      FIG. 16  is a depiction of an enlarged view of the area marked “H” in  FIG. 15 .  
         [0025]      FIG. 17  is a perspective view of a stacked module in accordance with another alternative preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0026]      FIG. 1A  depicts a cross-sectional view of a part of one embodiment of a flex circuit that may be employed in accordance with a preferred embodiment of the present invention. Depicted flex circuit  12  is an exemplar flex circuit embodiment that may be employed in accordance with the present invention in stacked modules comprised of leaded packaged integrated circuits. Depicted exemplar flex circuit  12  exhibits at least one connective field  19  which before inclusion in a module, typically will exhibit a loop-like or arcuate shape but may exhibit other shapes. An arcuate shape such as the one depicted as an example is typically deformed to be more adapted with the shape of the leads of the constituent ICs of a module in compliance with a preferred embodiment of a method for devising a stacked IC module in accordance with the present invention. The cross-section of  FIG. 1A  further depicts adhesive  18  disposed along at least a part of flex circuit  12  for adherence to ICs when flex circuit  12  is employed in a module.  
         [0027]      FIG. 1B  is another depiction of a flex circuit  12  that may be employed to advantage in accordance with some preferred embodiments of the present invention. Flex circuit  12  is comprised of two layers, one of which is a substrate layer  16  and the other of which layers is a conductive layer  14 L which provides plural contact areas  14  of the more than one connective fields  19  as shown. Plural contact areas  14  are held in spaced apart relation by their adherence on substrate layer  16 . Typically, connective field  19  is configured in a loop-like or arcuate configuration. When flex circuit  12  is disposed between ICs in a module, the leads of the ICs then make contact with connective field  19  and typically deform connective field  19  through compression to encourage adaptation of the shape of connective field  19  to the configuration of the leads. Thus, the arcuate configuration of connective field  19  improves contact between the contact areas  14  with the respective leads of the constituent ICs of the module. Connective field  19  could, however, be devised in any shape so long as it exhibits plural contact areas  14  for contact with at least two respective leads from an integrated circuit (packaged) of the module to be created. As another example, connective field  19  could be more of a folded structure that exhibits plural contact areas  14  spaced apart laterally as part of a contiguous structure to allow the single structure of the flex circuit to be used to position a plurality of contact areas adjacent to a plurality of package leads in typically a one-to-one correspondence but it is advantageous in typical embodiments to have connective field  19  take a natural arcuate shape before flex circuit  12  is included in a module. Further the connective field need not be a closed loop. By having plural contact areas  14 , connective field  19  allows plural contact areas  14  to be aligned with their respective leads in a stacked module thus avoiding the more difficult task of aligning individually each discrete lead from a flex circuit with the associated lead from ICs of the stacked module.  
         [0028]     Flex circuit  12  may have more than two layers but is shown with layers  14 L and  16  as an efficient and simple construction that may be employed in devising modules in accordance with preferred embodiments of the present invention. As those of skill will appreciate, flex circuitry with more than two layers may readily be employed particularly where especially complex electrical connections are required. Adhesive  18  is shown on flex circuit  12  and an optional form  17  is shown within connective field  19 . Adhesive  18  may also be disposed on ICs  20  and/or  22  in addition to or instead of on flex circuit  12  when constructing a module in accordance with some embodiments of the present invention. Optional form  17  is preferably an elastomer to provide a ready preformed shape for configuration of a connective field  19  and encourage compressive forces to enhance contact between contact areas  14  and leads of ICs  20  and  22 .  
         [0029]     Flex circuit  12  is shown with substantially planar portion  12 A that resides between ICs in a stacked module devised in accordance with some embodiments of the present invention. An optional portion  12 B of flex circuit  12  is also resident between ICs in a stacked module and typically resides beneath a part of portion  12 A.  
         [0030]     Conductive layer  14 L and connect areas  14  are preferably comprised of copper although any conductive material may be employed for such purposes. A more preferred copper layer would be thin and ductile copper deposited upon substrate layer  16  that is preferably a polyimide. An etched copper or other conductive material may also be used as a conductive layer  14 L but a conductive layer  14 L deposited on a substrate would be preferred.  
         [0031]      FIG. 2  depicts an enlargement of the area marked “A” in  FIG. 1 . Contact areas  14  are shown in spaced apart relation on substrate layer  16 . Thin film adhesive  18  is shown on flex circuit  12 . Those of skill will recognize that thin film adhesive  18  is not required but a thin film adhesive is a well understood method for adhering flex circuit  12  in module  10  as later depicted and preferred adhesives will have thermal conductivity properties to enhance the thermal conduction between the ICs in a module.  
         [0032]      FIG. 3  depicts another aspect of exemplar flex circuit  12  in accordance with a preferred embodiment of the present invention. As shown in  FIG. 3 , flex circuit  12  exhibits connective field  19  and a part of flex circuit  12  identified as  12 B is disposed under upper portion  12 A of flex circuit  12  in the depicted embodiment of flex circuit  12 . Other flex circuits may be employed with preferred modules in accordance with alternative embodiments of the present invention where an overlap of a part  12 A and  12 B is not present and an example of which construction will be later shown.  
         [0033]      FIG. 4  depicts an enlargement of the area marked “B” in  FIG. 3  showing in more detail a part of connective field  19  and the contact areas  14  that provide connective facility between the flex circuit and the leads of constituent ICs of a module devised in accordance with a preferred embodiment.  
         [0034]      FIG. 5  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 upper leaded IC  20  and lower leaded IC  22 . The present invention may be employed with other circuitry besides and in addition to flash memory including, just as non-limiting examples, DRAMs, FPGAs, system stacks that include logic and memory and communications or graphics modules. It should be noted therefore, that there the depicted profile for ICs  20  and  22  is not a limitation and that upper and lower ICs  20  and  22  respectively need not be TSOPs or TSOP-like and the packages employed need not have only one die or even leads emergent from two sides. For example, a module in accordance with embodiments of the present invention may employ ICs  20  and  22  that have dual die within each package and may exhibit ICs that have leads emergent from only one side of the package. Further, a module  10  in accord with the present invention need not have only 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.  
         [0035]     Flex circuit  12  is disposed between ICs  20  and  22  and force (represented by Fo+ and/or Fo in the Fig.) with or without heat is applied to bring together the components flex circuit  12  and ICs  20  and  22 . As a result, connective field  19  is preferably compressed to have conformity with the configurations of the leads of the constituent ICs thus improving contact between contact areas  14  and respective leads. Those of skill will recognize that flex circuit  12  may be constructed in two pieces with one piece for each of connective fields  19  and such a construction, although less than preferred, should be understood to be within the scope of the present invention. Further, identified adhesive  18  may be applied to the ICs  20  and/or  22  in addition to or in place of its disposition on flex circuit  12  but that use of a thin file adhesive on flex circuit  12  is preferred for efficient construction.  
         [0036]     When ICs  20  and  22  are brought together with flex circuit  12  between, portions of contact areas  14  exposed around parts of connective fields  19  are preferably disposed in contact with leads  24  of ICs  20  and  22  and, in preferred embodiments, contact between leads  24  and appropriate contact areas  14  is preferably realized while connective field  19  deforms in compliance with the configurations of the constituent IC leads. Solder as shown in later Figs. is then preferably employed between leads and contact areas  14 . A module could be built in accordance with an embodiment in which no solder was employed and compression between leads  24  and flex circuit  12  was the sole realization of the contact between contact areas  14  and respective leads  24  but as those of skill will recognize, such a construction would not be preferred. Further, a module in accordance with an alternative embodiment could be devised in which the contact areas  14  do not touch the respective leads but await realization of electrical contact with solder or other conductive attachment. Other forms of bonding other than solder between contact areas  14  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.  
         [0037]      FIG. 6  is a perspective view of a module  10  showing ICs  20  and  22  in stacked disposition with connective field  19  of flex circuit  12  having contact areas  14  in contact with respective ones of the leads of ICs  20  and  22 . ICs  20  and  22  have four sides  31 ,  33 ,  35 , and  37  with leads  24  emergent from at least one of said sides with the particular configuration of IC shown having leads emergent from two opposite sides of the respective package. Upper surface  29  and lower surface  30  of the ICs are also identified in  FIG. 6 . Those of skill will recognize that the identified “sides” need not be perpendicular to respective upper and lower surfaces  29  and  30  of ICs  20  and  22 .  
         [0038]      FIG. 6  illustrates that, in devising a module in accordance with the present invention, some embodiments may be constructed with the assistance of a mechanical fixture  25  and fixture  25  and module  10  may be brought together (see force FA) to correlate connective field  19  of flex circuit  12  with leads  24  of ICs  20  and  22  so as to align leads  24  and contact areas  14  of connective field  19  of flex circuit  12  by pushing on either leads or the flex as the case may be. As those of skill will recognize, embodiments of the present invention assist in avoidance of what would otherwise be difficult registration and correlation between discrete leads from a flex circuit and individual leads  24  of ICs  20  and  22 .  
         [0039]      FIG. 7  shows greater detail of the area marked “C” in  FIG. 6  and shows that ICs  20  and  22  are in thermal contact through adhesive layers  18  and parts of flex circuit  12  that lie between ICs  20  and  22 . Connective field  19  of flex circuit  12  about optional form  17  shows contact areas  14  in contact with leads  24  of ICs  20  and  22  as substrate layer  16 , which is preferably a dielectric layer such as polyimide for example, is shown between contact areas  14  and preferably holds contact areas  14  in spaced apart relation. There is shown a one-to-one correspondence between individual contact areas  14  and individual respective leads of ICs  20  and  22  but in some more unusual instances, a wider contact area  14  may be in contact with two or more side-by-side leads  24  of an IC if the electrical connections require such a configuration.  
         [0040]      FIG. 8  is a side view of a module  10  devised in accordance with a preferred embodiment of the present invention. Module  10  includes ICs  20  and  22  and illustrates flex circuit  12  in part between said ICs  20  and  22  with thin film adhesive layers  18 . Flex circuit  12  is shown with connective field  19  about form  17 . Examples of module  10  in a variety of different profiles may be devised in compliance with the present invention and the methods of the present invention modules  10  may be devised to present profiles in compliance with a variety of application requirements.  
         [0041]      FIG. 9  is an enlarged depiction of a cross-section through the area marked “D” in  FIG. 8 . Flex circuit  12  is shown disposed with portion  12  A between ICs  20  and  22 . Flex circuit  12  emerges from the ICs to provide connective field  19  which has as earlier shown, plural contact areas  14  for connection to leads  24 . In the depicted embodiment, flex circuit  12  after transit through connective field  19  then re-enters between ICs  20  and  22  as portion  12 B which is in the depicted embodiment disposed beneath portion  12 A. Solder  26  is shown in  FIG. 9  and is employed to secure electrical connection between respective contact areas  14  and respective leads  24 .  
         [0042]      FIG. 10  is an enlarged depiction of the area marked “E” in  FIG. 9  and illustrates in cross-section exemplar flex circuit  12  with conductive layer  14 L and substrate layer  16 .  
         [0043]      FIG. 11  illustrates an end view of module  10  illustrating connective field  19  along leads  24  of ICs  20  and  24 . Those of skill will recognize that this is an end on view if ICs  20  and  22  are typical flash TSOPs while if the stacked module  10  is devised with TSOP DRAMs, the depiction of  FIG. 11  is more an exemplification of a side view of module  10 .  
         [0044]      FIG. 12  is a side view of a module  10  devised in accordance with an alternative preferred embodiment of the present invention. In depicted module  10  of  FIG. 12 , flex circuit  12  exhibits a connective field  19  that provides more area for contact areas  14  to be in contact with leads  24  of ICs  20  and  22  when ICs  20  and  24  are disposed about flex circuit  12 . This is shown in greater detail in  FIG. 13  which is an enlarged depiction of the area marked with “F” in  FIG. 12 .  
         [0045]      FIG. 13  illustrates connective field  19  of flex circuit  12  (without use of a form  17 ) and by extension, contact areas  14  disposed in proximal compression with respective leads  24  of upper IC  20  and lower IC  22 . It will be noted that in the depicted embodiment, flex circuit  12  emerges from between ICs  20  and  22  as shown with respect to earlier illustrated embodiments and then re-enters between ICs  20  and  22 .  
         [0046]      FIG. 14  is a perspective view of a module  10  that illustrates the continuous nature of connective field  19  across more than one contact area for connection with more than one lead  24  thus avoiding the use of separated leads to realize connections between respective leads of ICs  20  and  22  which as those of skill will appreciate, leads to manufacturing advantages in avoidance of the need for fine registration between individual fine pitch leads from ICs  20  and  22  and corresponding individual fine pitch leads from flex circuitry for interconnection of the constituent ICs.  
         [0047]      FIG. 15  illustrates a stacked module  10  in accordance with an alternative preferred embodiment of the present invention. Unlike earlier depicted embodiments, the embodiment of module  10  depicted in  FIG. 15  employs flex circuit  12  to provide connective field  19  but does not exhibit a return of a part  12 B of flex circuit  12  to the area in between ICs  20  and  22  but instead, a portion of flex circuit  12  identified as portion  12 C is adhered to the bottom surface  30  of lower IC  22  with adhesive  18  as shown in greater detail in  FIG. 16 .  
         [0048]      FIG. 16  is an enlarged depiction of the area marked “H” in  FIG. 15 . As illustrated, flex circuit  12  emerges from between IC  20  and IC  22  and exhibits connective field  19  but is disposed so as to be proximal to the lower side of feet  28  of respective leads  24  of ICs  20  and  22 . In the depicted embodiment, portion  12 C of flex circuit  12  is disposed along a portion of lower surface  30  of lower IC  22 .  
         [0049]      FIG. 17  is a perspective view of a stacked module  10  in accordance with an alternative preferred embodiment of the present invention as shown in earlier  FIGS. 15 and 16 .  
         [0050]     Although the present invention has been described in detail, it will be apparent to those skilled in the art that many embodiments taking a variety of specific forms and reflecting changes, substitutions and alterations 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.