Patent Publication Number: US-2005139980-A1

Title: High density integrated circuit module

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
      This application is a continuation-in-part of application Ser. No. 08/774,699, filed Dec. 26, 1996 and a Continuing Prosecution Application filed Feb. 11, 1998, pending, which is a continuation of 08/497,565, filed Jun. 30, 1995, now issued as U.S. Pat. No. 5,631,193, which is a continuation of application Ser. No. 07/990,334, filed Dec. 11, 1992, now issued as U.S. Pat. No. 5,484,959. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates to a high density, integrated circuit module, which includes a plurality of vertically or horizontally stacked individual surface mount or ball-grid-array integrated circuit packages.  
      2. Brief Description of the Related Technology  
      An example of a fabrication method and apparatus for high density lead-on-package modules by laminating one or more lead frames to standard integrated circuit packages is disclosed in U.S. Pat. No. 5,484,959, assigned to the common assignee of the present invention and incorporated herein by reference. Other methods for providing high density, stacked modules are disclosed in U.S. Pat. Nos. 5,279,029, 5,367,766, 5,455,740, 5,450,959 and 5,592,364, all of which are assigned to the common assignee of the present invention and incorporated herein by reference. The general methods and apparatus disclosed in the referenced patents can be applied to the fabrication of stacked configurations comprised of individual ball-grid-array or surface mount packages. However, the characteristic lead orientation, lead shape and lead content of ball-grid-array or surface mount packages impose a different set of parameters not adequately provided for by prior methods and assemblies.  
     SUMMARY OF THE INVENTION  
      The present invention provides a novel method and apparatus for manufacturing three-dimensional, high density, integrated circuit modules from standard ball-grid-array or other surface mount integrated circuit packages which provides improved space efficiency and heat dissipation. One way to increase space efficiency is to stack individual packages. Generally speaking, higher density generates more localized heat and thus increases the need for efficient heat dissipation. Improving the thermal transfer characteristics of the individual integrated circuit packages results in better heat dissipation for the module, and improves reliability and durability.  
      The present invention provides a novel method of fabricating a three-dimensional module formed of stacked and aligned surface mount or ball-grid-array packages. Ball-Grid-Array (BGA) integrated circuit packages typically have leads that extend from the bottom surface of a rectangular solid resin casing in a two-dimensional grid pattern. The external portion of each lead finished with a ball of solder. Package leads provide electrical and thermal coupling to one or more integrated circuit dies that are embedded within the protective casing. Typically, the protective casing completely surrounds the embedded die but, in some BGA packages, the protective casing does not cover the inactive top surface of the die. Near-chip scale packages provide 1.0 mm center-to-center lead spacing. Chip scale packaging such as MICRO_BGA™ have center-to-center lead spacing of 0.5 mm. Chip scale packaging offers excellent electrical characteristics including low capacitance and thermal design.  
      Connectivity to the leads of individual packages in a module is provided by thin substantially planar lead carriers located between adjacent packages. Lead carriers are adhered to adjacent packages with a thermally conductive but electrically insulating adhesive. A lead carrier is comprised of elongated electrically and thermally conductive elements formed in one or more thin planes of conductive material that are separated by high-dielectric material. Typically, each conductive element has at least one aperture, adapted to receive and electrically couple to an individual package ball and at least one interconnect lead that extends away from the module to provide external circuit connectivity to package leads. Preferably, the lead carriers are formed from custom flexible circuits commercially available from 3M™ or other manufacturers. These well known flexible circuits are typically comprised of one or more thin layers of conductive material that are die cut and drilled to form ground planes, signal traces, pads and apertures. The conductive layers are typically embedded in and between electrically-insulating, high-dielectric material such as polyamide, polyester or teflon which results in circuits that are flexible, have dense trace, and provide accurate impedance control.  
      The present invention utilizes standard manufactured packages to form the multi-package module. Such packages typically have ball irregularities or inconsistencies, particularly ball length and solder coating variations. These variations make automated assembly problematic since the tolerances necessary to accommodate variation in ball length and excess solder, for example, do not permit the packages to be assembled within the more stringent requirements for automated assembly of the module. According to one aspect of the present invention, the leads of the ball-grid-array packages are scythed prior to assembly or as an automated step during the assembly. Scything is a method where a hot razor knife skims off a layer from the distal end of all the leads of a ball-grid-array package, reducing random excess lead length and providing a uniform, closely tolerant lead length. The step of scything allows multiple packages to be added to the module prior to a final heating step where the solder for all the packages is flowed. This method also has the advantage of increasing the minimal tolerances for positioning of ball-grid-array package on the lead carrier. An alternative method that may also be used to compensate for excess solder from the leads is to provide channels formed in the walls or edges of each aperture of the lead carrier that receives the ball so the excess solder, when heated, flows into the channels A channel is a void area in a conductive element which merges into the void area of an aperture. An edge of the channel is in close proximity to the package leads and the void area extends away from the leads. Channels take advantage of the surface tension of molten solder which will pull molten solder away from leads to fill the channel.  
      Another object of the present invention is to provide an assembly which effectively dissipates heat generated during normal operation. Efficient thermal management increases the operational life of the module, and improves reliability by eliminating the effects of elevated temperature on the electrical characteristics of the integrated circuit and packaging. When packages are not stacked, heat from the embedded integrated circuits, generated through normal operation, is primarily dissipated by convection from the package&#39;s external surfaces to the surrounding air. When modules are formed by stacking packages, the buried packages have reduced surface area exposed to the air. The use of thermally conductive adhesive facilitates the transmission of heat between adjacent packages and is an effective method of taking advantage of the exposed surfaces for removing heat from buried packages.  
      In the module of the present invention, the package leads are thermally coupled to the lead carrier and provide a path for heat from the embedded integrated circuits. Thermally conductive adhesive also facilitates transfer of heat from packages to the lead carrier.  
      In applications where it is desirable to reduce the package and module height, or where package or module warping is a concern, each package may be constructed using any of the various techniques described in U.S. Pat. Nos. 5,369,056, 5,369,058 and 5,644,161, each of which is assigned to the common assignee of the present invention and incorporated herein by reference. These patents describe methods for constructing thin, durable packages and modules with enhanced heat dissipation characteristics and minimal warpage.  
      A common application of a stacked configuration is memory modules. Most of the leads of each package are electrically connected to corresponding leads of adjacent packages. A method is required to select the individual memory package being read, written or refreshed. One method is to provide a custom manufactured lead carrier for each package. A more cost-effective method is to use a common lead carrier design with extra package interconnect leads which is then modified by clipping off or no-connecting selected interconnect leads to make each lead carrier in a stacked configuration unique. Methods for connecting a unique bit of a data word per package and for uniquely addressing each package in a stacked configuration are described in U.S. Pat. Nos. 5,279,029 and 5,371,866, both which are assigned to the common assignee of the present invention and incorporated herein by reference. While the apparatus and methods of the present invention are described herein with reference to standard, single-size packages, it will be appreciated by those of ordinary skill in the art, that those methods and apparatus are equally applicable to multiple-die packages. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a cross-sectional view of two adjacent packages of a module of the present invention;  
       FIG. 2  is a top planar view of a typical lead carrier of the present invention;  
       FIG. 3   a  illustrates the preferred embodiment for an aperture for connection with a package lead of the present invention;  
       FIG. 3   b  illustrates an alternative embodiment of an aperture for connection with a package lead of the present invention;  
       FIG. 4  illustrates a horizontally stacked module of the present invention;  
       FIG. 5  illustrates a vertically stacked module of the present invention;  
       FIG. 6  illustrates an alternative embodiment of a horizontally stacked module of the present invention; and  
       FIG. 7  illustrates an alternative embodiment of horizontally stacked module of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Other and further objects, features and advantages will be apparent from the following description of the preferred embodiments of the invention, given for the purpose of disclosure and taken in conjunction with the accompanying drawings.  
      The letter of a reference character containing numerics followed by a letter, either identifies the relative placement of the numeric reference within a stacked module or it identifies a specific embodiment.  
      Referring now to  FIG. 1 , a typical ball-grid-array package  50  is comprised of an integrated circuit  51  surrounded by an essentially rectangular solid resin casing  55 . Package leads  52  extend from the bottom surface  54  of the casing in a two-dimensional grid pattern providing electrical and thermal coupling to one or more integrated circuit die  51  that are embedded within the protective casing. The external portion of each package lead  52  includes a coating of solder having a semi-spherical shape. Typically, the protective casing  55  completely surrounds the embedded die but, in some ball-grid-array packages  50 , the protective casing  55  does not cover the inactive top surface  53  of the die. Near-chip scale packages  50  provide 1.0 mm center-to-center spacing between leads  52 . Chip scale packaging such as MICRO_BGA™ have center-to-center lead spacing of 0.5 mm. Chip scale packaging offers excellent electrical characteristics including low capacitance and thermal design.  
       FIGS. 4 through 7  show various specific embodiments of stacked module M of the present invention. The letter M designates the module M formed of a plurality of ball-grid-array packages  50 . Typically, the packages  50  are aligned as shown in  FIGS. 4, 5  and  7  where the bottom surfaces  54  of each package  50  are facing the same direction. Alternately, the packages  50  may be aligned where one or more of the packages  50  are inverted in relation to the other packages  50  as shown in  FIG. 6 . In this embodiment, the top package  50   d  is inverted with respect to the bottom package  50   e ; the top surface  53  of the top package  50   d  is in substantially full contact with the adhesive  70  on the top surface  53  of the lower package  50   e.    
      A typical application of one aspect of the present invention is shown in  FIG. 1  which illustrates a partial cross-section of any two adjacent packages  50  that comprise a module M. The internals of package  50   b  are not shown for simplicity.  FIG. 1  shows two packages  50   a  and  50   b  mounted on opposite sides of a lead carrier  60  comprised of a single thin copper plane. Interconnect leads  64  extend away from the module M to provide external circuit connectivity to package leads  52  of the top package  50   a . External connectivity may be provided in different configurations as described in detail below with reference to  FIGS. 4-7 . A typical layout of a single plane lead carrier  60  is shown in  FIG. 2 . The lead carrier  60  is made to be flexible for increased reliability and ease of assembly. A lead carrier  60  can be comprised of elongated conductive elements  65  formed from a thermally and electrically conductive thin planer material such as beryllium copper alloy C3 having a thickness of about 3 mils. Each conductive element  65  is defined to include a trace, interconnect pad, via and any other conductive feature of the lead carrier that are electrically coupled. Other preferred alloys for the lead-carrier-conductive elements  62  are full hard or hard copper alloys (110 or 197) or olin copper alloy 1094. Preferably, the lead carrier  60  is formed from custom flexible circuits from 3M™ and other manufacturers. These well known flexible circuits are typically comprised of one or more thin (1.4 mils thick) layers of conductive material that are die cut and drilled to form apertures  66 , ground planes and conductive elements  65  which include traces, mounting pads and leads. The conductive layers typically are flanked by a thin (typically 1 to 11 mill thick) layer of electrically-insulating, high-dielectric materials such at polyamide, polyester or teflon which results in circuit composites that are flexible. The material and thickness of individual layers that comprise the lead carriers  60  as well as spacing between conductive elements  65  and the width of conductive elements  65  can be precisely controlled to provide a accurate and consistent impedance control in select conductive elements  65 . Lead carriers  60  formed from custom flexible-circuits can have vias for connecting traces  65  located on different planes and conductive pads (or leads), with solder coating having footprints that are compatible with standards for ball-grid array packages  50  for electrical and mechanical coupling to a printed wiring board  80 .  
      Typically, each conductive element  65  in a lead carrier  60  has at least one aperture  66 , adapted to receive an individual package lead  52  and at least one interconnect portion  64  that extends away from the module to provide an external point of electrical connection to package leads  52 . Interconnect portions  64  preferably have a spring-like resiliency for increased reliability. Apertures  66  have about the same diameter as a package lead  52  allowing each package lead  52  to extend through the aperture  66  and for the lead carrier  60  to have substantial contact with the bottom surface  54  of a package  50 . The application of heat (about 175 degrees centigrade) that is sufficient to cause the solder comprising the package leads  52  to flow will cause the solder to adhere to a thin area of a conductive element  65  on the surface of the lead carrier  60  facing away from the package  50  that surrounds each aperture  66  to form flange  55  that provides excellent electrical and mechanical coupling between package leads  52  and the lead carrier  60 .  FIG. 3   a  illustrates the preferred semi-circle shape  66   a  for the aperture  66  where the conductive element  65  partially surrounds the package ball  52 . The semicircle shape  66   a , as opposed to a full-circle shape, enables an increased space for routing the conductive elements  65  of the lead carrier  60 .  
      The present invention utilizes standard manufactured packages  50  to form the multi-package module M. Such packages  50  typically have package lead  52  irregularities or inconsistencies, particularly, lead length and solder coating variations. These variations make automated assembly problematic since the tolerances necessary to accommodate variation in lead  52  length, for example, do not permit the packages  50  to be assembled within the more stringent requirements for automated assembly of the module M. The package leads  52  typically have excess solder that can cause electrical shorts between package leads  52 . According to one aspect of the present invention, the leads  52  of the ball-grid-array packages  50  are scythed prior to assembly or as an automated step during the assembly after the lead carrier  60  is attached. Scything is the preferred method of reducing the length by which package leads  52  extend from the package  50 . Scything is a method where a hot razor knife skims off the distal portion of all package leads  52 .  
      Referring again to  FIG. 1 , lead carriers  60  are adhered to adjacent packages  50  with a thermally conductive but electrical-insulating adhesive  70 . The adhesive  70  may be epoxy, such as Rogers Corp. R/flex 8970 which is B-staged phenolic butyryl epoxy, that may be laminated at a temperature of 130 degrees centigrade and cured at a temperature of about 175 degrees centigrade. The preferred method is to use a 2 mil thick sandwich of polyamide film  70 , such as Kapton™ which includes a 0.5 mil thick layer of adhesive on both sides (a three-layer system). A thermally conductive filled adhesive  70  may be used to enhance the transfer of heat between adjacent packages  50 , and between the packages  50  and carrier  60 .  
      Referring now to  FIG. 4 , a horizontally oriented embodiment of the present invention is illustrated. Typically, a module M is preassembled and then attached to a PWB  80  or other circuit carrying substrate. Alternately, the preassembled module M may be inserted into an integrated circuit socket.  
      In  FIG. 4 , a plurality of integrated circuit packages  50 , each with an attached lead carrier  65 , are stacked in a horizontally-oriented module M. In this configuration, each lead carrier  65  has an external interconnect portion  64  which extends from both sides of the module M to provide interconnection to an electrically and thermally conductive external interconnect structure  40 . Structure  40  provides mechanical rigidity to the module M and is adhered to the upper surface  41  of the uppermost package  50 . Structure  40  also includes circuit board interconnection portions  43  which may be formed for industry-standard socketability with an electrical socket carried in circuit board substrate  80 .  
      A vertically-oriented configuration of module M is illustrated in  FIG. 5 . In this embodiment, lead carriers  60  are formed with external interconnect portions  64  all extending to one side which requires the conductive elements  65  to be more densely spaced. In this embodiment, external interconnect portions  64  are spaced in row and column configuration for socketing or soldering to circuit connections on circuit board substrate  80 .  
      Another embodiment of module M is illustrated with reference to  FIG. 6 . In this embodiment, module M is formed in a two-high stack comprised of packages  50   e  and  50   d . In this embodiment, package  50   e  has its package leads  52  mounted directly to corresponding array of external circuit connect pads carried in substrate  80 . Upper package  50   d  is inverted with respect to package  50   e  and mounted to package  50   d  with thermally conductive adhesive layer  70 . Surface  54  of package  50   d  includes package leads  52 . A lead carrier  60 , formed as described above, is adhered to surface  54  of package  50   d  with thermally conductive adhesive  70 . External circuit interconnect portions  64  provide electrical connectivity for upper package  50   d  to circuit connection pads carried in substrate  80 .  
      Referring now to  FIG. 7 , module M is shown in an alternative three-high configuration comprised of individual packages  50   a ,  50   b  and  50   c . A lead carrier  60  is adhered to the package lead surface  54  of each package. For clarity, package leads  52  are not shown as to packages  50   a  and  50   b . Lead carriers  60  for packages  50   a  and  50   b  include external circuit connect portions  64   b  which are formed to nest together to provide mechanical rigidity and electrical and thermal conductivity for the module M. Lower package  50   c  has lead carrier  60   c  adhered to its lower surface  54  in the manner described above with thermally conductivity, electrically insulating adhesive. In this embodiment, circuit connection portions  64   a  of lead carrier  60   c  are selectively interconnected to connection portions  64   b . Package leads  52  of lower package  50   c  are connected to external circuit connections carried in substrate  80  in a standard ball-grid-array pattern.  
      According to one specific method of the present invention, a method for manufacture a module M involves the following steps: (1) mounting an adhesive  70   a  to the bottom surface  56  of a package  50 ; (2) aligning and mounting a lead carrier  60  to the adhesive  70  wherein apertures  66  receive the package leads  52 ; (3) scything of the distal end of all package leads  52 ; (4) applying heat (about 175 degrees centigrade) to cure adhesive  70  and flow the package lead  52  solder coating; (5) mounting a second thin layer of adhesive  70   b  to the lead carrier  60 ; and (6) mounting another package  50  to the adhesive  70   b , wherein the top surface  56  of the package  50  has substantial contact with the adhesive. Steps  1  though  6  are repeated for each package  50  added to the module, except steps  5  and  6  are not repeated for the last package  50 . For reliability and remanufacturability, it may be desirable to test each package  50  as it is added to the module M.  
      The preferred method replaces the steps of applying adhesive  70   a  and  70   b  with the preliminary step of applying double-sided adhesive tape  70  to both upper and lower surfaces of each lead carrier  60  prior to assembly. The step of applying heat to cause solder  53  to flow and to cure adhesive  66  after each step of mounting a package  50  is eliminated if the leads  52  of the package  50  are reduced in height prior to assembly and a thin area of the second layer of adhesive  70   b  around each aperture  66  is left void to allow the package leads  52  to form a flange when heat is applied. The module M may be assembled using a suitably formed manufacturing jig provided to hold individual packages  50  in alignment as they are stacked together with interspaced lead carriers  60  and adhesive carrying tape  70 . In this embodiment, the entire module M may be preassembled and a single heating event applied to flow the solder and cure the adhesive  70  as pressure is exerted on the module M to compress the layers.  
      Referring now to  FIG. 3   b , an alternative embodiment which compensates for excess solder steps utilizes channels  66   c  formed in each aperture  66   b .  FIG. 3   b  illustrates one shape of an aperture  66   b  with multiple channels  66   c . A channel  66   c  is a void area in a conductive element  65  which merges into the void area of an aperture  66 . When a package lead  52  is inserted into the aperture  66   b , an edge of each channel  66   c  is in close proximity to the package leads  52 . The void area of the channel  66   c  extends away from the ball  52 . When heat is applied such that the solder coating the package leads  52  becomes molten, excess solder is pulled by the inherent surface tension of molten solder to fill the voided area.  
      Communication between individual integrated circuits embedded within packages  50  and signals external to the modules are provided by various methods for implementing an external structure. Methods and apparatus of such structures are described in U.S. Pat. Nos. 5,279,029 and 5,367,766. Alternatively, the external structure  40  may be formed integral to the leads  64  extending from the lead carrier  61  as shown in  FIGS. 6 and 7 . In the embodiment shown in  FIG. 7 , the leads  64   b  are formed such to electrically and thermally connect directly to selected adjacent leads  64   b . Leads  64  may be formed with a substrate mounting portion  65  that may have a standard “gull-wing,” “J-lead” shape.  
      The foregoing disclosure and description of the invention are illustrative and explanatory of the preferred embodiments. Changes in the size, shape, materials and individual components used, elements, connections and construction may be made without departing from the spirit of the invention.