Abstract:
Integrated circuit packaging with improved thermal transmission from the integrated circuit heat source to the exterior of the packaging. Improved packaging employs a compressive interposer which allows for greater manufacturability of the packaged integrated circuit parts. Additionally different shaped compressive interposers are described.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to the fabrication of semiconductor devices, and relates more particularly to packaging of semiconductor devices and more particularly thermally enhancement of packaging of semiconductor devices.  
       BACKGROUND OF THE INVENTION  
       [0002]     Integrated circuits (ICs) generate heat as an undesirable byproduct during their use. This heat byproduct is a significant design consideration both in the design of the IC and the design of products using the IC. One strategy of addressing the heat design consideration is the design of the integrated circuit to generate less heat in the first place. Another strategy of addressing heat is in how the IC is driven during use. Another common strategy is to cool the IC. Many different tactics have been used to cool the IC. One tactic is to convectively cool the IC by applying moving air over the IC. One of the convective tactics has applied heat exchangers filled with liquids or gasses that pull off the heat generated by the IC. A more common tactic to cool IC&#39;s has been to attach a heat sink with radiant cooling fins to the IC to wick away the heat generated by the IC. Frequently, the finned heat sink is combined with a fan, and then radiates off the heat or combined with a fan convectively transfers heat.  
         [0003]     In the vast majority of implementations of the tactics described above, the ICs have been housed in mechanically designed housings commonly called IC packaging. The packaging provides two primary functions: to physically and electrically insulate the IC while at the same time providing easy electrical contact to the IC. Since the primary purpose of the packaging is to insulate and protect the IC, the packaging has a tendency to inhibit the release of heat energy. One prior art tactic to address this design issue is to include a heat spreader inside the packaging or a heat slug which has a surface exposed on the outer surface of the package.  
         [0004]      FIG. 1  illustrates a cross-section of a prior art surface mount IC package employing a heat slug. This example is a thermally enhanced plastic ball grid array (TE PBGA) package. The IC package  10  is a housing for an IC  12  with an active surface  14  and a non-active surface  16  which is affixed via an adhesive layer  18  to a substrate  20 . The active surface  14  of the IC  12  is electrically connected via wire  22  to mounting surfaces  24  on the substrate  20  which contains traces  26  which are typically eventually electrically connected to solder balls  28 . Though not shown in  FIG. 1 , these solder balls  28  are the means by which the packaged IC  10  is surface mounted to provide electrical connection to an electronic circuit board (not shown).  
         [0005]     In the IC package shown the heat slug  30  is non-electrically mounted via mounts  32  on the substrate  20  over the IC  12  and then surrounded by the molding compound  34 . The slug is typically manufactured out of copper, aluminum, or steel. In the case of the TE PBGA the slug is approximately 300 μm thick and made of copper. In some prior art IC packages the heat slug  30  remains exposed as shown in  FIG. 1  so that it can have better transfer heat out of and away from the IC and its packaging. Other prior art IC packages employ a heat spreader fully encased in the molding compound. The heat slug configuration provides marginal beneficial effects. However, the benefits of heat slugs  30  configured in this way have limited efficiency and effectiveness and are problematic to manufacture.  
         [0006]     Part of the reason for its limited effectiveness is that the slug is too far from the IC  12  which acts as a heat source. The packages illustrated in  FIG. 1  are typically rated to handle two to three watts (2W-3W). Additionally, the space between slug  30  and IC  12  is typically filled with molding compound  34 . Because of low thermal conductivity, the molding compound acts to thermally insulate the IC  12 . Molding compound is a very poor thermal conductor. For a comparison of the thermal conductivity of a typical molding compound like G760 with other materials see the following table:  
                                         TABLE 1                                   Material   Thermal Conductivity (W/mK)                                        Silicon   148.0           Copper   386.0           Aluminum   222.0           37-63 Solder   50.7           molding compound   0.7˜0.9                      
 
         [0007]     This lack of proximity and thermal insulation limits the efficiency of removing the heat from the IC  12 . Trying to configure the slug  30  closer to the IC exacerbates the manufacturability problem of using the slug  30  because it can interfere with the wire leads  22  to the IC  12  and the substrate circuit  26  and can increase the difficulty of forming the molding compound  34  around the IC  12  and slug  30 .  
         [0008]     Another limitation of the prior art heat slugs is the requirement for mounts  32 . These mounts are required to maintain distance between the slug  30  and the IC  12 . Another drawback of these prior art heat slugs  30  is that the mounts  32  require landing areas (not shown) on the substrate that must be accommodated when designing the substrate circuits  26 .  
         [0009]      FIG. 2  is an illustration of another prior art IC package  40 . In this package an interposer layer  42  rests between the IC  12  and the heat slug  44 . It is common to have conductive adhesive layer  46  between IC  12  and interposer layer  42  and to have conductive adhesive layer  48  between the interposer layer  42  and the heat slug  44 .  
         [0010]     Although the package in  FIG. 2  has better thermal properties, it is problematic to manufacture. If the accumulated vertical dimensions are on the larger side of acceptable tolerances, when the device is clamped in the mold tool, the IC may be crushed or damaged. If the dimensions are adjusted to avoid crushing of the die, mold compound will tend to mold over the top of the spreader. This will lead to poor thermal performance. The tolerance accumulated from thicknesses of components and adhesive layers will cause the manufacturing process to vary between crushing damage and poor molding.  
         [0011]     There is thus a need in the art for removing heat from an IC that is more efficient and/or easier to manufacture and therefore more effective. Based on a preliminary analysis, it is believed that the improvements to IC packaging described below will produce a doubling of the wattage capacity.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The following is a brief description of the drawings and should be considered in conjunction with the preceding and following detailed description:  
         [0013]      FIG. 1  (prior art) is an illustration of a prior art IC package with a heat slug;  
         [0014]      FIG. 2  (prior art) is an illustration of an improved prior art IC package with a non-compressive interposer between the IC and heat slug;  
         [0015]      FIG. 3  is an illustration of an embodiment of a Z-shaped, compressive interposer between the IC and a heat slug;  
         [0016]      FIG. 4  is an illustration of an embodiment of a Z-shaped, compressive interposer without a heat slug;  
         [0017]      FIG. 5  is an illustration of an embodiment of a Z-shaped, compressive interposer affixed to the active surface of the IC;  
         [0018]      FIG. 6  is an illustration of a four-leaf, Z-shaped compressive interposer in a flip chip package;  
         [0019]      FIG. 7  is an illustration of an embodiment of a Z-shaped compressive interposer;  
         [0020]      FIG. 8  is an illustration the embodiment of a Z-shaped compressive interposer which is illustrated in  FIG. 7  prior to shaping for assembly;  
         [0021]      FIG. 9  is an illustration of a four-leaf Z-shaped compressive interposer in a flip chip package prior to shaping for assembly;  
         [0022]      FIG. 10  is an illustration of a the compressive interposer in  FIG. 9  after folding;  
         [0023]      FIG. 11  is an illustration of and embodiment of an accordion-shaped, compressive interposer;  
         [0024]      FIG. 12  is an illustration of the embodiment of the accordion-shaped, compressive interposer which is illustrated in  FIG. 11  prior to shaping for assembly;  
         [0025]      FIG. 13  is an illustration of an embodiment of a C-shaped, compressive interposer as found in the package;  
         [0026]      FIG. 14  is an illustration of the embodiment of the C-shaped, compressive interposer which is illustrated in  FIG. 13  prior to shaping for assembly;  
         [0027]      FIG. 15  is an illustration of a four-leaf, compressive interposer before it is folded;  
         [0028]      FIG. 16  is a top view illustration of the four-leaf, compressive interposer of  FIG. 15  in a top view after it is folded;  
         [0029]      FIG. 17  is a side view illustration of the four-leaf, compressive interposer of  FIG. 15  in a side view after it is folded;  
         [0030]      FIG. 18  is an illustration of an alternative embodiment of a compressive interposer;  
         [0031]      FIG. 19  is an illustration of the embodiment of the compressive interposer which is illustrated in  FIG. 18  prior to shaping for assembly;  
         [0032]      FIG. 20  is an illustration of an alternative embodiment of a compressive interposer;  
         [0033]      FIG. 21  is an illustration of the embodiment of the compressive interposer which is illustrated in  FIG. 20  prior to shaping for assembly;  
         [0034]      FIG. 22  is an illustration of a combination of different shaped compressive interposers;  
         [0035]      FIG. 23  is an illustration of a multi-chip IC package; and  
         [0036]      FIG. 24  is an illustration of a compressive standoff for supporting the compressive interposer over the substrate.  
     
    
     DETAILED DESCRIPTION  
       [0037]      FIG. 3  illustrates an IC package  50  with improved heat transmission characteristics. The IC  12  in package  50  is thermally connected to the heat slug  36  by means of a shaped compressive interposer  52 . In the embodiment illustrated in  FIG. 3 , the compressive interposer  52  has a Z-shaped compressive section described in greater detail below.  
         [0038]     The core of the IC package  50  is the IC  12 . The IC  12  is also the primary source of heat which must be dissipated. Typically IC&#39;s are a silicon based structure of complex manufacture with many fine details and are relatively fragile. This is one of the reasons they must be enclosed in a package. A typical IC at the time of authorship is 50-360 μm thick. It is usually configured in a package with its active surface  14  up and is affixed to a substrate  20  which is similar to a circuit board in that it contains circuitry  26  that routes the connections made by means of connectors  22  to surface-mount solder ball  28  locations. The substrate  20  could be rigid. Currently typical rigid substrates range in thickness from approximately 200 μm to a more standard 360 μm or 560 μm. The substrate could also be a film substrate approximately 50 μm to 100 μm thick. The embodiment illustrated in  FIG. 3  employs a non-compressive interposer  68 . A typical non-compressive interposer  68  might be approximately 50 μm to 200 μm in thickness and typically a blank silicon die to match the coefficient of expansion of the IC  12 . The non-compressive interposer  68  is typically bonded to the IC with a thermally conductive adhesive (not shown) that is not electrically conductive to avoid shorting the active side  14  of the IC  12 .  
         [0039]     In the embodiment illustrated in  FIG. 3  the compressive interposer  52  is mounted on top of the non-compressive interposer  68 . In this case a silver filled adhesive (not shown) can be used because there is no risk of electrical conductance and a silver filled adhesive has higher thermal conductance properties. In the embodiment illustrated the compressive interposer is comprised of a shaped 150 μm copper sheet. In other embodiments other materials and thicknesses and shapes may be used. The choice of materials and material thicknesses depends on a number of factors. Some of these factors include how much heat needs to be transmitted and at what rate, the flexibility of the material, the mechanics of the clamping and molding process, and the configuration of the package. Generally the overall requirements are that: the material is thermally conductive, the structure is deformable during the molding process, and provides enough surface area on the top and bottom to collect and transmit the heat generated by the IC  12  to an external heat sink (not shown).  
         [0040]     The purpose of the heat slug  36  is to act as a heat sink and to provide a thermal connectivity path for other heat transfer devices (not shown) when the IC package is used on circuit boards (not shown) for devices requiring an IC. Many suitable materials may be used as the heat slug including, but not limited to, silicon, aluminum, steel, copper and other heat transmissive materials. It is desirable that the coefficient of expansion of the slug  36  material be closely matched with the coefficient of expansion of its surrounding materials to avoid failure caused by thermal cycling of the package  50 .  
         [0041]     In this embodiment the top section  52  of the compressive interposer makes thermal contact with the heat slug  36 . While in some embodiments a thermally conductive adhesive or paste may be placed between the compressive interposer  52  and the heat slug  36 , it is preferable that this material be soft or at least flexible. The reason for this preference is to allow for variation in the position of the compressive interposer  52  relative to the heat slug  36  during manufacture. The center section  54  of the compressive interposer  52  makes thermal contact with a non-compressive interposer  68 . In this interface a thermally conductive adhesive (not shown) may be used. The non-compressive interposer  68  in-turn is thermally mounted on the active surface  14  of the IC  12 . Between these layers a non-electrically conductive, thermally conductive adhesive (not shown) is typically used. This non-compressive interposer layer can be made of any thermally conductive material. However, since it is in close proximity to the active surface of the IC, it is preferable that this material is a non-electrically conductive material such as a blank silicon die. In the embodiment shown in  FIG. 3  the non-compressive interposer layer is a blank silicon die to keep electrical separation from the compressive interposer  52 . Controlled thermal expansion materials such as molybdenum, invar-copper-invar or copper-invar-copper structures may be preferable in some embodiments depending on cost and need.  
         [0042]     During operation the heat generated by the IC  12  is transmitted from the IC  12  through the non-compressive interposer  68  to the center section  54  of the compressive interposer  68  and up to the top section  52  via a compressive section  64  of the compressive interposer  52  including bends  62  and  60  of the compressive interposer  52 . The primary importance of the compressive nature of the compressive section  64  is during manufacture of the package  50 . Specifically, the compressive nature of the interposer  52  allows compressive forgiveness while still maintaining thermal contact between the IC  12  and heat slug  36 . A secondary purpose of the compressive nature of the compressive section  64  is that it is believed that this flexibility will allow for greater tolerance to differences in the coefficient of expansion of adjacent materials during thermal cycling. A suitable material for the compressive interposer  52  has high thermal conductivity and is flexible in the desired shape. Examples of suitable materials include copper, aluminum and steel and flexible ceramics. The choice of material and thicknesses and compressive section details with this and other configurations described herein depend on the needs of a particular package. Although  FIG. 3  illustrates a shaped compressively deformable interposer other types of compressive interposers would also be suitable in other applications. For example the compressive interposer could be wire mesh or engineered materials with microstructures like thermally conductive foams. The salient characteristics of the materials are that they provide good thermal conductance and are compressively deformable during the IC packaging process.  
         [0043]      FIG. 4  is an illustration of an IC package  51  similar to the package in  FIG. 3  with the exception that it is lacking a heat slug/spreader  36  of  FIG. 3 . In this embodiment the outer surface  58  of the compressive interposer  52  is exposed to provide direct contact with external heat transfer devices (not shown).  
         [0044]      FIG. 5  is an illustration of an IC package  53  similar to the IC package  51  of  FIG. 4 . The difference in the package illustrated in  FIG. 5  is the absence of the non-compressive interposer  68  in  FIG. 4 . In this embodiment, the center section  54  of the compressive interposer  52  is affixed directly to the active surface  14  of the IC  12  via on electrically non-conductive, thermally conductive adhesive (not shown). In one embodiment the adhesive takes the form of a thin tape. Thermally conductive adhesives are widely available in many forms including tape forms and softer paste forms. Both electrically conductive and electrically non-conductive adhesives that suit the purposes described herein are widely available.  
         [0045]      FIG. 6  illustrates an IC package  70  with a flipped chip. In this package the active surface  14  of the IC  12  is down. It (the IC  12 ) is electrically attached to the substrate  20  via solder balls  29 . It should be noted that the IC package  70  shown in  FIG. 6  employs a four-leaf, compressive interposer  71 . Three (3) compressive sides  84  can be seen in  FIG. 6 . This embodiment of a compressive interposer is described in greater detail below.  
         [0046]      FIG. 7  and  FIG. 8  illustrate the compressive interposer  52  of  FIG. 3 ,  FIG. 4 , and  FIG. 5  after they are bent into shape. The top section  58  will be in thermal contact with the heat slug  36  in  FIG. 3  and  FIG. 5  and exposed to the outside of the package in  FIG. 4 . The center section  54  will be in thermal contact with the non-compressive interposer  68  in  FIG. 3  and  FIG. 4  and with the IC in  FIG. 5 . Fold lines  60  and  62  create the compressive section  64  which allows for compressive give during the manufacturing process and during thermal cycling.  FIG. 8  also illustrates two additional features that are only shown in this embodiment but may be applied to any other embodiment as well. Flow holes  66  allow the flow of molding compound during manufacture. Locking features  69  help to hold the compressive interposer  52  in place during its life.  
         [0047]      FIG. 9  and  FIG. 10  illustrate another embodiment of a compressive interposer  71 . This compressive interposer is similar to the two-leaf interposer  52  in  FIG. 7  and  FIG. 8 . Though the illustrated interposer is only shown in  FIG. 6 , it would apply to the other package configurations as well. The center section  74  is affixed to the IC  12  or non-compressive interposer (not shown) depending on the package design. The compressive sections  84  fold up so that the second fold line  80  gets closer to the center of the center section  74  of the compressive interposer  71  along fold lines  82  and then the top sections  72  fold back down along fold lines  80  to a horizontal position in a plane parallel to the plane of the center section  74 . These top sections are then either affixed to a heat slug (not shown) exposed to the outside of the package (not shown) or are exposed to the outside of the package (not shown) ready to receive external heat transfer systems (not shown).  
         [0048]      FIG. 11  illustrates an alternative embodiment of the compressive interposer  102 . In this embodiment the compressive section of the compressive interposer  102  is formed by multiple bends  104 ,  106  and  108  and straight subsections  112  and  114  forming an accordion type configuration. It is believed that this configuration may allow the bottom section  110  of the interposer  102  to remain flat on the IC or non-compressive interposer  116 . Whether it is the IC or non-compressive interposer  116  depends on the configuration of the rest of the package and whether a flip chip configuration is desired. For example for a flip chip a non-compressive interposer may not be used. On the other hand if the active surface of the chip is up it might be desirable for the bottom surface  120  of a non-compressive interposer  116  to be facing the active surface of the IC rather than the compressive interposer, particularly if the compressive interposer is made of an electrically conductive material.  FIG. 11  further illustrates the use of two interposers  102 .  
         [0049]      FIG. 12  illustrates one of the compressive interposers  102  of  FIG. 11  prior to being folded for assembly. Top section  102  will be exposed or thermally contacting a heat slug (not shown). Bottom section  110  will be contacting the non-compressive interposer or IC  116 . The compressive section will be formed using straight sections  112  and  114  and bend lines for bends  104 ,  106  and  108 .  
         [0050]      FIG. 13  illustrates an alternative embodiment of a compressive interposer  132  for the present invention. In this embodiment the compressive section  134  is C-shaped.  FIG. 14  illustrates the top section  132  which will be exposed or in thermal contact with a heat slug and bottom section  136  which will be in thermal contact with the IC or non-compressive interposer  116  with a bottom IC facing surface  120 . It also illustrates the lines that represent the beginning and end of the C-shaped curvature of the compressive section  134 .  
         [0051]      FIG. 15 ,  FIG. 16  and  FIG. 17  illustrate an embodiment of a compressive interposer  250  which combines elements of an accordion-shaped, compressive section with a Z-shaped, compressive section.  FIG. 15  illustrates the compressive interposer  250  before it is folded.  FIG. 16  illustrates the compressive interposer  250  after it is folded.  FIG. 17  illustrates how the compressive section is folded into accordion-like shape. The advantage of this design over the design illustrated in FIGS.  9  and  FIG. 10  is that there is less of a bottle neck in the compressive sections and it creates more surface area in the top sections  252  to affix to a heat slug (not shown) or exposed outside the package (not shown) to affix to an external heat transfer device (not shown).  
         [0052]     To fold the compressive interposer illustrated in  FIG. 15 , first fold up the top sections  252  along bend  260 . Then, fold back the sides of compressive section  264  so that their centers fold along fold lines  266  while at the same time folding along fold lines  262  to get an accordion-shape as illustrated in  FIG. 17 . The triangular shaped section  265  becomes the center of the Z-shape section of the design illustrated in  FIG. 15 .  
         [0053]      FIG. 18  and  FIG. 19  illustrate an alternative embodiment of the C-shaped spring leaf, interposers illustrated in  FIG. 12  and  FIG. 13 . In this embodiment the interposers are formed from a series of interleafed springs with C-shaped compressive sections  144  &amp;  148 . In this embodiment the IC  12  or non-compressive interposer is not completely covered by the compressive interposer  142  leafs  146  &amp;  150 . While only three leaves are shown and two labeled to simplify the illustration and description respectively other embodiments could have any number of leafs.  
         [0054]      FIG. 20  and  FIG. 21  illustrate an alternative embodiment of the C-shaped spring leaf spacers illustrated in  FIG. 18  and  FIG. 19 . In this embodiment the interposers are formed from a series of interleaved springs with C-shaped compressive sections  164  &amp;  168 . In this embodiment IC  12  or non-compressive interposer  116  is completely covered by the compressive interposer  162 .  
         [0055]      FIG. 22  illustrates an alternative embodiment of an interposer comprised of a series of compressive sections  180 ,  182 , and  184 . These interposers may be of the same type or configuration or may be of different types or configurations. In  FIG. 22 a  C-Shaped compressive section  182  is sandwiched between two accordion-shaped compressive sections  180  and  184 . In this case the compressive sections are welded, spot welded, brazed, soldered or otherwise thermally attached to a heat slug or to an interposer top sheet  122  as shown in  FIG. 22 .  
         [0056]      FIG. 23  is an illustration of a multi-chip IC package  200 . In this package the substrate can support multi ICs. Two IC&#39;s  202  and  204  can be seen in  FIG. 23 —each with its own compressive interposer(s)  216  and  218 . In the embodiment shown the interposer  218  for one IC  204  is formed with a punch press to form the C-shape spring leaves. While the interposer  216  for the other IC  202  is brazed/welded to a top section  212  of the interposer  210  which has an exposed top section. The individual compressive, thermally conductive interposer can be made or assembled together with stress release feature  220  as shown in  FIG. 23 .  
         [0057]      FIG. 24  illustrates a feature that can be included in any of the compressive interposer embodiments described above. Specifically it illustrates a compressive section  224  between the top section  212  and of the compressive interposer  210  and a mount section  226  which rests on the substrate  20 . These features may be used for extra support or to help register the location of the compressive interposer within the package during manufacture. This feature can be combined with any of the other embodiments described herein  52 ,  71 ,  100 ,  130 ,  250 ,  140 ,  160 ,  122  or embodiments not specifically described herein.  
         [0058]     Although a focus is made on shaped compressive interposers other forms of shaped and unshaped compressive interposers are also contemplated as with the scope and spirit of the innovation.  
         [0059]     The above description of the present invention is illustrative, and is not intended to be limiting. It will thus be appreciated that various additions, substitutions and modifications may be made to the above described embodiments without departing from the scope of the present invention. Accordingly, the scope of the present invention should be construed in reference to the appended claims.