Abstract:
A device and method identify and compensate for tensile and/or shear stress due to heat-caused expansion and contraction between an integrated heat spreader and thermal interface material. This device and method may change the shape of the integrated heat spreader based upon the identification of location(s) of high tensile and/or shear stress so that additional thermal interface material may be deposited between the integrated heat spreader and a die in corresponding locations. Utilizing this method and device, heat is efficiently transferred from the die to the integrated heat spreader.

Description:
FIELD 
     The inventive subject matter relates to a device and method to control strain and tensile stress on thermal interface material in a heat spreader. More particularly, the inventive subject matter pertains to a device and method that determine stress points in thermal interface material used to transfer heat from a die to a heat spreader and design the heat spreader to optimize the thickness of thermal interface material for those stress points. 
     BACKGROUND 
     In the rapid development of computers many advancements have been seen in the areas of processor speed, throughput, communications, fault tolerance and size of individual components. Today&#39;s microprocessors, memory and other chips have become faster and smaller. However, with the increase in speed, reduction in the size of components, and increased density of circuitry found within a given chip/die, heat generation and dissipation have become more critical factors than ever. 
     FIG. 1 illustrates a die  50  placed on a substrate  30  with a finite amount of a thermal interface material (TIM)  20  placed on top of the die  50 . This TIM  20  serves at least two primary purposes. First, it acts to conduct heat from the die to the integrated heat spreader (IHS)  10 . Second, it may also provide some adhesion between the IHS  10  and die  50 . The TIM  20  may be composed of, but not be limited to, solder, a polymer containing metal, or some other substances which both act to transfer heat and provide some adhesion. During the manufacturing process the IHS  10  is pressed down upon the TIM  20  and adhesive  40 , resulting in a structure as shown in FIG.  2 . 
     As shown in FIG. 2, the IHS  10  would absorb heat from die  50  through TIM  20  and be held in place on the substrate  30  via adhesive  40 . On top of the IHS  10  a heat sink (not shown) or fan/heat sink combination (not shown) would be mounted to dissipate the heat absorbed by the IHS  10 . However, since IHS  10  and TIM  20  both experience significant tensile stress during the assembly process and due to thermal expansion and contraction when the die is powered on and off, as shown in FIG. 3, air gaps  60  form between the TIM  20  and IHS  10 . As indicated in FIG. 3, these air gaps  60  may form at the outer edges of the TIM  20  while the center portion of the TIM  20  remains in contact with the IHS  10 . 
     However, as shown in FIGS. 3 and 4, an air gap  60  may occur anywhere in the contact area between TIM  20  and IHS  10 . As illustrated in FIG. 4, an air gap  60  may form in the center of the contact area between the TIM  20  and IHS  10 , while the outer edges of the TIM  20  remain in contact with the IHS  10 . 
     As would be appreciated by one of ordinary skill in the art, these air gaps  60  shown in FIGS. 3 and 4 may form anywhere in the contact area between the TIM  20  and IHS  10  depending on the materials utilized in the IHS  10  and TIM  20  as well as the handling procedures for the IHS  10  during the manufacturing process. Further, these air gaps  60  may also form in the TIM  20  itself. It should be noted that FIGS. 3 and 4, except for the inclusion of air gaps  60 , remain unchanged from that shown in FIG.  2  and will not be discussed in further detail. 
     Since separation may occur between the TIM  20  and IHS  10 , forming air gaps  60 , as shown in FIGS. 3 and 4, due to thermal expansion and contraction, these air gaps  60  act as insulation, preventing heat being transferred from the die  50  to the IHS  10 . As heat builds up in the die  50  to higher levels, the life expectancy of the die  50  is reduced. 
     Therefore, what are needed are a device and method that can determine the stress points between the TIM  20  and IHS  10  due to thermal expansion and contraction. Further, what are needed are a device and method that may compensate for the tensile and shear stress, thereby preventing the separation of the TIM  20  and the IHS  10 . Still further, what are needed are a device and method that will provide for efficient heat transfer from the die  50  to the IHS  10 . 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and a better understanding of the inventive subject matter will become apparent from the following detailed description of exemplary embodiments and the claims when read in connection with the accompanying drawings, all forming a part of the disclosure of this inventive subject matter. While the foregoing and following written and illustrated disclosure focus on disclosing example embodiments of the invention, it should be clearly understood that the same is by way of illustration and example only and the invention is not limited thereto. Such embodiments of the inventive subject matter may be referred to, individually and/or collectively, herein by the term “invention” merely for convenience and without intending to voluntarily limit the scope of this application to any single invention or inventive concept if more than one is in fact disclosed. The spirit and scope of embodiments of the present invention are limited only by the terms of the appended claims. 
     The following represent brief descriptions of the drawings, wherein: 
     FIG. 1 is an example of an integrated heat spreader (IHS) being affixed to a die attached to a substrate; 
     FIG. 2 is an example of an assembled integrated heat spreader (IHS) and die with a thermal interface material (TIM) to conduct heat from the die to the integrated heat spreader (IHS); 
     FIG. 3 is an example of an assembled IHS and die with a TIM that has separated from the IHS to form air gaps; 
     FIG. 4 is an example of an assembled IHS and die with a TIM that has separated from the IHS to form an air gap; 
     FIG. 5 is an assembled convex IHS in an example embodiment of the present invention; 
     FIG. 6 is an assembled concave IHS in an example embodiment of the present invention; and 
     FIG. 7 is a flowchart of a process to determine the stress points in a TIM and modify the IHS to compensate for the stress points in an example embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Before beginning a detailed description of the subject invention, mention of the following is in order. When appropriate, like reference numerals and characters may be used to designate identical, corresponding or similar components in differing figure drawings. Further, in the detailed description to follow, exemplary sizes/models/values/ranges may be given, although the present invention is not limited to the same. As a final note, well-known components of computer networks may not be shown within the FIGS. for simplicity of illustration and discussion, and so as not to obscure the invention. 
     FIG. 5 is an assembled convex IHS  10  in an example embodiment of the present invention. It should be noted that FIGS. 5 and 6 are provided as merely example embodiments of the present invention. As previously discussed, with reference to FIGS. 3 and 4, depending on the material composition of the integrated heat spreader (IHS)  10  and the thermal interface material (TIM)  20 , air gaps  60  may form anywhere in the interface between the TIM  20  and IHS  10  due to separation of TIM  20  from IHS  10  caused by thermal expansion and contraction. Further, as previously discussed, air gaps may also form in the TIM  20  itself. 
     Still referring to FIG. 5, it should be noted that IHS  10  has a convex shape in which the middle portion  70  of the IHS  10  is thicker/wider than the end portions  80  of the IHS  10 . This convex shape of the IHS  10  is done in order to increase the thickness of the TIM  20  at both its respective ends  90  while allowing the middle section  100  to remain unchanged in thickness, so that the air gaps seen in FIG. 3 are less likely to materialize. This is due to the fact that as the thickness of the TIM  20  increases so does its elasticity, and therefore it can better withstand the thermal stresses causing detachment as shown in FIG.  3 . This is particularly true in the case where the TIM  20  is made of a polymer-metal combination or where the TIM  20  comprises a solder composite material. However, as previously discussed, this particular embodiment of the present invention shown in FIG. 5 is specifically designed to alleviate the detachment problems as shown in FIG.  3  and is merely provided as an example embodiment of the present invention. Those features not discussed in reference to FIG. 5 remain unchanged from those in FIGS. 2 and 3. 
     FIG. 6 is an assembled concave IHS  10  in which the center portion  70  is thinner and the end portions  80  are thicker or remain unchanged in size in an example embodiment of the present invention. This concave shape of the IHS  10  is done in order to increase the thickness of the TIM  20  in its center portion  100  while maintaining both end portions  90  at near the same size, so that the air gap seen in FIG. 4 is less likely to materialize. This is due to the fact that as the thickness of the TIM  20  increases so does its elasticity, and therefore it can better withstand the thermal stresses causing detachment as shown in FIG.  4 . This is particularly true in the case where the TIM  20  is made of a polymer-metal combination or where the TIM  20  comprises a solder composite material. However, as previously discussed, this particular embodiment of the present invention shown in FIG. 6 is specifically designed to alleviate the detachment problems as shown in FIG.  4  and is merely provided as an example embodiment of the present invention. Those features not discussed in reference to FIG. 6 remain unchanged from those in FIGS. 2 and 4. 
     FIG. 7 is a flowchart of a process to determine the stress points in a TIM  20  and to modify the IHS  10  to compensate for the stress points in an example embodiment of the present invention. Processing begins in operation  700  and immediately proceeds to operation  710 . In operation  710  the assembly package having a flat IHS  10  and using factory materials is assembled as shown in FIG.  2 . In operation  720 , the material properties and package geometry are determined. This would include determining such factors as coefficient of thermal expansion, modulus, stiffness, warpage, thickness, etc. Thereafter, in operation  730  a mechanical model is created to determine the impact of package stress on the thermal interface material (TIM)  20  during temperature cycling. This mechanical model would comprise building a statistically significant number of the packages and then cycling them through the temperature extremes that would be experienced during a normal lifetime of operation. Thereafter, in operation  740  the location in the TIM  20  that has the greatest tensile and shear stress applied there to is determined. As would be appreciated by one of ordinary skill in the art, this may be determined in a number of ways. For example, cross-sections of the assembled die  50 , substrate  30  and IHS  10  may be examined, photographed and the location of the amount of separation determined. In addition, as would be appreciated by one of ordinary skill in the art, acoustic and x-ray analysis may be used to determine the location of any separation or any air gaps that form in the TIM  20 . In operation  750 , the IHS  10  is redesigned to increase the thickness of the TIM  20  in the areas of high stress as evidenced by separation points discovered in operation  740 . In operation  760  the package is assembled with the redesigned integrated heat spreader (IHS)  10 , is manufactured in sufficient quantity to provide statistically significant data, and is then retested in temperature cycling while being compared with the modeled results. Thereafter, in operation  780  the material properties and package geometry of the IHS  10 , TIM  20  and die  50  are determined. The material properties would include, but not be limited to, the coefficient of thermal expansion, modulus, stiffness, warpage, thickness, and etc. If the material properties are within predetermined desired limits, then processing proceeds to operation  770  where processing terminates. However, if the material properties are not within the required tolerances, then processing loops back to operation  730  and repeats operations  730  through  780  until the material properties fall within specified limits. 
     The benefit resulting from the present invention is that a simple, reliable, device and method are provided for identifying and compensating for stress points that develop between an IHS  10  and TIM  20  that cause air gaps  60  to form and prevent effective heat transfer from a die  50  to an IHS  10 . This device and method compensate for tensile and shear stress due to heating-related expansion and contraction by placing larger quantities of the TIM  20  at those tensile stress points, thereby increasing the elasticity of the TIM  20  at those tensile stress points. 
     While we have shown and described only a few examples herein, it is understood that numerous changes and modifications as known to those skilled in the art could be made to the example embodiments of the present invention. Therefore, we do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.