Abstract:
A chip package including a chip extension for containing thermal interface material (TIM) and improves chip cooling, and a related method, are disclosed. In particular, the chip package includes a chip, a cooling structure coupled to the chip via a TIM, and a chip extension may be thermally coupled to an outer edge of the chip. A TIM placed between the chip and the cooling structure is contained during thermal cycling by the chip extension such that void formation at the edge of the chip, which can move between the chip and cooling structure, is suppressed. The chip extension also improves lateral heat dissipation by providing a greater thermal contact area between the cooling structure and the chip and, if needed, the substrate at a much lower cost than using larger die with lower production unit output from a wafer.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Technical Field  
         [0002]     The present invention relates generally to chip packages, and more particularly, to a chip package including a chip extension for containing thermal interface material and improving chip cooling.  
         [0003]     2. Related Art  
         [0004]     The drive for increasing chip performance (higher operating frequencies) is resulting in increased chip power, and the reduction of circuit size is increasing chip power density. Chip leakage power is aggravated by shrinking device size, and is strongly affected by operating temperature. As a result, as chips continue to become smaller, there is a need to continue enhancing chip cooling capability.  
         [0005]     One common cooling mechanism is to thermally connect chips to a cooling structure, such as a lid or heat sink, via a thermal interface material (TIM). Commonly used TIMs include thermal pastes, thermal adhesives, and phase change materials; and less commonly used TIMs include liquid metals and solders.  FIG. 1  shows an illustrative chip package  8  including a chip  10  coupled to a substrate  12 , which is in turn coupled to a card  14  via a conventional grid array  16 . A cooling structure  18  is thermally coupled by a TIM  20  to chip  10 , and cooling structure  18  is coupled to substrate  12  via polymer adhesive  22 . In this example, cooling structure  18  is in the form of a lid.  
         [0006]     There is a need to improve the reliability of cooling structures  18  that utilize compliant TIMs  20  (i.e., thermal paste). One problem with conventional structures is caused by the relatively high viscosity of high performance thermally conductive pastes. High performance thermal pastes are designed to have high viscosity at elevated temperature to prevent the materials from readily flowing (i.e., sagging) off of chip  10  during normal operation. The high viscosity is also due to their high solids loading, which enhances thermal performance. Unfortunately, the high viscosity also results in “paste pumping,” which refers to the situation in which the TIM is pumped into and out of the gap between chip  10  and cooling structure  18 . In particular, as shown by the arrows in  FIG. 1 , as power is applied to and removed from chip  10 , package  8  heats and cools, i.e., it thermally cycles. During these thermal transients there is often relative movement of cooling structure  18  toward and away from back side  24  of chip  10 . This movement is caused by the materials coefficient of thermal expansion and temperature differences that arise during device operation. When cooling structure  18  moves toward back side  24  of chip  10 , the space for TIM  20  above chip  10  decreases and some of the paste is squeezed out the side of the gap between chip  10  and cooling structure  18 . When cooling structure  18  and chip  10  move in opposite directions, i.e., away from one another, the gap increases. As the gap increases, surplus TIM  20  from around the gap flows back into the gap, maintaining the thermal integrity of the structure, while entrapped gas, typically ambient air, can enter the paste. Air moving into the gap tends to form pockets referred to as voids  26 . These voids  26  have much lower thermal conductivity than TIM  20 , causing chip  10  temperature to rise, and further increasing power dissipation, usually because of device leakage current. These voids  26  tend to grow with additional cycling, further degrading the cooling, degrading device reliability, and increasing the power consumption.  
         [0007]     Another problem with conventional structures is that, in most high power flip chip packages, device cooling by heat transfer to and through substrate  12  is nearly negligible. As a result, virtually all the heat must be removed from a back side  24  (non-circuit side) of chip  10 . Semiconductor devices are produced in massive quantities on a single wafer. Typically, a prototype device design is produced in a die size that is later reduced in size to increase the number of devices on a processed wafer. This chip ‘shrink’ increases the density of the power on the device since the body size is physically smaller for the same power consumption. Silicon used for devices has good thermal conductivity and will spread the heat created by the active devices to the backside of the die as well as laterally across the die surface. Specific regions of the device can become much hotter, often because these regions are where the die cores are located. Initial builds of devices on large die have the advantage of providing lateral heat spreading from these ‘hot spots’. Thus, decreasing the die size improves the die count on each wafer but also reduces the lateral heat spreading of the silicon.  
         [0008]     In view of the foregoing, there is a need to contain TIMs when the cooling structure separates during thermal cycling, and to improve lateral heat transfer from the chip to reduce the heat flux without impacting the number of die that can be produced on a wafer.  
       SUMMARY OF THE INVENTION  
       [0009]     The invention includes a chip package including a chip extension for containing thermal interface material (TIM) and improving chip cooling, and a related method. In particular, the chip package includes a chip, a cooling structure coupled to the chip via a TIM, and a chip extension, which may be thermally coupled to an outer edge of the chip. A TIM placed between the chip and the cooling structure is contained during thermal cycling by the chip extension such that void formation at the edge of the chip, which can move between the chip and cooling structure, is suppressed. The chip extension also improves lateral heat dissipation by providing a greater thermal contact area between the cooling structure and the chip and, if needed, the substrate at a much lower cost than using larger die with lower production unit output from a wafer.  
         [0010]     A first aspect of the invention is directed to a chip package comprising: a chip; a cooling structure coupled to the chip via a thermal interface material; and a chip extension thermally coupled to at least one outer edge of the chip.  
         [0011]     A second aspect of the invention includes a chip package comprising: a substrate; a chip mounted to the substrate; a cooling structure coupled to the chip via a thermal interface material; and a thermally conductive chip extension thermally coupled to at least one outer edge of the chip and to the substrate, the chip extension also thermally coupled to the cooling structure via the thermal interface material.  
         [0012]     A third aspect of the invention is related to a method of containing a thermal interface material in a chip package during thermal cycling and improving heat dissipation, the method comprising the steps of: providing a chip extension adjacent to the chip; and placing the thermal interface material between a cooling structure and the chip and the chip extension, whereby the chip extension contains the thermal interface material during thermal cycling and provides a thermal contact area between the cooling structure and the chip.  
         [0013]     The foregoing and other features of the invention will be apparent from the following more particular description of embodiments of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The embodiments of this invention will be described in detail, with reference to the following figures, wherein like designations denote like elements, and wherein:  
         [0015]      FIG. 1  shows a prior art chip package including voids created by thermal cycling.  
         [0016]      FIG. 2  shows one embodiment of a chip package according to the invention.  
         [0017]     FIGS.  3 A-B show a detail of alternative embodiments of a chip extension according to the invention.  
         [0018]     FIGS.  4 A-B show alternative embodiments of a chip extension according to the invention.  
         [0019]     FIGS.  5 A-D show plan views of alternative embodiments of a chip extension according to the invention. 
     
    
     DETAILED DESCRIPTION  
       [0020]     The invention includes a chip package including a chip extension that contains thermal interface material (TIM) and improves chip cooling, and a related method. With reference to the accompanying drawings,  FIG. 2  shows one embodiment of a chip package  108  according to the invention. Chip package  108  includes a chip  110  electrically connected to a substrate  112  by solder connections  113  and under-fill material  115 . Substrate  112  is in turn electrically connected to a card  114  via a conventional grid array  116 , e.g., ball grid array (BGA), column grid array (CGA), land grid array (LGA) or pin grid array (PGA). A cooling structure  118  is thermally coupled by a TIM  120  to chip  110  to provide a thermal path from the chip to cooling structure  118 . TIM  120  may include any now known or later developed thermal interface material such as a thermal paste, liquid, phase change material and other materials. Cooling structure  118  is also coupled to substrate  112  via a conventional adhesive  122 , e.g., a polymer or solder. In the embodiment illustrated, cooling structure  118  is in the form of a lid, but it could be any other conventional structure such as a heat sink or heat spreader. Likewise, although an adhesive  122  is shown between lid  118  and substrate  112 , an elastomeric gasket, O-ring, metal seal or similar contact configuration can be used, for instance, held in place between the lid and substrate  112  by an externally applied compressive load.  
         [0021]     In order to improve the reliability of TIM  120  in cooling structure  108 , that utilizes compliant TIMs (i.e., thermal paste), the invention includes a chip extension  150  thermally coupled to at least one outer edge  152  of chip  110 . “Thermally coupled” indicates that heat can be transferred from chip  110  to chip extension  150 , either directly or indirectly. Chip extension  150  is also coupled to cooling structure  118  via TIM  120 . In one embodiment, chip extension  150  includes silicon, copper, aluminum, copper-molybdenum, graphite, aluminum-silicon-carbide, tungsten carbide, aluminum nitride, beryllia, or diamond based composites, such that it is thermally conductive. As a result, chip extension  150  also provides an additional thermal path from chip  110  to cooling structure  118  and thus improves chip cooling while not degrading the main thermal path through a back side  124  (non-circuit side) of chip  110 . In addition, chip extension  150  may also enlarge the thermal path from back side  124  of chip  110  to cooling structure  118 . Chip extension  150  may also prevent hot spots within chip  110 , especially if the maximum power is near outer edge  152  of chip  110 . As a result of the improved cooling, chip  110  can operate at higher frequencies/power with improved reliability. It should be recognized, however, that where the additional thermal path or enlarged path is not desired, chip extension  150  does not need to be thermally conductive. In this case, chip extension  150  enhances thermal reliability by facilitating the flow of the TIM back into the gap above the chip, to reduce the potential of void formation during thermal cycling of the assembly.  
         [0022]     In any event, chip extension  150  extends laterally away from chip  110  such that it creates an apparent increase in chip  110  size, moving the origin of the entrapped air further away from the hot spots of chip  110 . Accordingly, chip extension  150  aids in containing TIM  120  in a larger area gap, and especially high viscosity TIMs, between chip  110  and cooling structure  118 . Cooling structure  118  may include an enlarged pedestal  130  to accommodate chip extension  150 , however, this may not be necessary in all cases. In operation, chip extension  150  prevents formation of voids  26  ( FIG. 1 ) in TIM  120 , thus ensuring better thermal conductivity over the lifetime of operation. As illustrated, in one embodiment, an adhesive  160  fills a region between chip  110  and chip extension  150 , and may provide thermal coupling between chip  110  and chip extension  150 . Adhesive  160  can be thermally conductive if desired to transfer heat from chip  110  to chip extension  150 . Typical thermally conductive materials for adhesive  160  may include, for example, silver or aluminum filled epoxies, low and high temperature solders, filled silicone rubbers and other commercially available materials. It should be recognized that the size of the region between chip  110  and chip extension  150  may vary such that chip extension  150  is fairly close to chip  110  or relatively distant from chip  110 . In the latter case, a moat of adhesive  160  would exist between chip  110  and chip extension  150 . Depending on the application and material for extension attachment, a very small gap would be desirable for minimal thermal resistance at this interface, while creating an extension to only reduce paste voiding, the gap can be larger since the feature is mainly a mechanical join. Thus, the gap can range from less than one mil to perhaps as much as 50 mils or more, respectively. If an electrically conductive adhesive is used and solder connections  113  on chip  110  underside could potentially be shorted by adhesive bridging between contacts, chip  110  can have an electrically insulating underfill  115  or a barrier of insulating materials can be created around chip  110  to prevent electrically conductive adhesive intrusion.  
         [0023]     Chip extension  150  may include a variety of alternative shapes and structures as shown, for example, in enlarged FIGS.  3 A-B,  4 A-B and  5 A-D. FIGS.  3 A-B show a detail of a chip extension  250  according to one embodiment. In particular, chip extension  250  may include a shape feature  154  adjacent to a lower surface  156  of chip  110  to prevent dislocation of chip extension  250  during thermal cycling. Shape feature  154  may have, for example, a tapered chamfer shape (left side FIGS.  3 A-B), a notched step shape (right side of FIGS.  3 A-B) or any other shape useful for reducing stresses that may arise during adhesive or component joining or thermal cycling. As also illustrated in FIGS.  3 A-B, chip  110  and chip extension  250  can have co-planar surfaces  124  and  258 , respectively, adjacent to cooling structure  118  (only a portion shown and TIM not shown).  FIG. 3A  does not include underfill under chip extension  250 , while  FIG. 3B  shows an underfill material  162 , which may be different than adhesive  160 . An underfill material  162  can be introduced under chip extension  250  to either support the chip extension during assembly to cooling structure  118  or provide additional heat transfer to substrate  112 . The chip extension underfill material  162  can be thermally conductive or thermally insulative.  
         [0024]     FIGS.  4 A-B show various alternative embodiments of a chip extension  350  in which the chip extension may be ramped or tapered as it extends away from chip  110  to minimize stresses at abrupt edge discontinuities that can form voids. This ramped or tapered edge also allows for more TIM  120  volume capacity. Although, only a substantially triangular ( FIG. 4A ) and substantially trapezoidal shape ( FIG. 4B ) have been illustrated, the ramp may take a variety of other forms, e.g., curvilinear. FIGS.  4 A-B both show an underfill material  362  introduced under chip extension  350  to either support the chip extension during assembly to cooling structure  118  or provide additional thermal transfer to substrate  112 . The chip extension underfill material  362  can be thermally conductive or thermally insulative. In  FIG. 4A , underfill material  362  under chip extension  350  is different than adhesive  160  or underfill  115 .  
         [0025]     FIGS.  5 A-C illustrate plan views of chip  110  and various embodiments of chip extensions  450  that include at least two discontinuous portions  470 . Chip extensions  450  are shown as segments since this would be the most economical usage of highly thermal conductivity materials. If chip  110  is to be underfilled, openings at the corners of the extensions can be provided to allow access to the corners of chip  110  for underfill introduction. As shown in  FIG. 5D , if desired, a full ‘picture frame’  452  extension shape can also be used and the spacing between chip  110  and chip extension  450  adjusted to maximize the gap for adhesive filling or minimize it for better heat conduction. It should be recognized that the various embodiments described above may be combined in any desired fashion.  
         [0026]     In one embodiment, the attachment of the chip extension(s) would be accomplished after chip  110  has been attached. Ideally, the chip extension is attached to chip  110  outer edges to produce coplanar surfaces with back side  124  of chip  110 . One method to accomplish this would include attachment of the chip extension to a chip that has been turned upside down on a non-stick flat support surface. With back side  124  of chip  110  on the surface, at least one of the outer edges of chip  110  could be coated with adhesive and the chip extensions driven against the chip outer edges to create the desired gap. After curing the chip extension assembly would be removed from the support surface. As described above, if desired, an underfill material  162 ,  362  can be introduced under the chip extension to either support the chip extension during assembly to cooling structure  118  or provide additional thermal transfer to substrate  112 . This feature can be introduced during or after the chip extension is attached. The chip extension underfill material can be thermally conductive or thermally insulative.  
         [0027]     The invention also includes a method of containing TIM  120  in chip package  108  during thermal cycling and improving heat dissipation. The method includes providing a chip extension  150 ,  250 ,  350 ,  450  adjacent to chip  110 , and placing TIM  120  between cooling structure  118  and chip  110  and chip extension  152 . As noted above, chip extension  150  provides additional space for retaining TIM  120  during thermal cycling and additional heat spreading path(s) from chip  110 .  
         [0028]     While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the embodiments of the invention as set forth above are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention as defined in the following claims.