Abstract:
An improved thermal interface material for conducting heat away from an integrated circuit device into a heat sink is a composite material including a metal screen defining openings and a hardened structural bonding agent incorporated into the openings of the metal screen. The improved composite thermal interface material achieves outstanding bonding properties superior to conventional thermal interface materials, while also exhibiting exceptional thermal conductivity.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to processes and structures for improving heat dissipation from electronic devices, and more particularly to thermal interfaces between integrated circuit devices and heat sinks. 
       BACKGROUND OF THE INVENTION 
       [0002]    Conventional techniques for conducting heat from an integrated circuit device to a heat sink, such as an aluminum body, have generally included the use of solder joints or thermal grease to achieve the desired thermally conductive interface between the integrated circuit device and the heat sink. However, while solder joints provide good thermal conductivity, the relatively large difference between the coefficient of thermal expansion of the integrated circuit device substrate (typically silicon) and the heat sink induces fairly large stresses on the solder joints during thermal cycling of the device, leading to cracking and fracture, resulting in an undesirably short service life. While conventional thermal greases eliminate or reduce the problems associated with the mismatch between the coefficient of thermal expansion of the integrated circuit device substrate and the heat sink, thermal greases offer very limited thermal performance (i.e., they do not facilitate thermal conductivity comparable to solder joints). 
       SUMMARY OF THE INVENTION 
       [0003]    The invention involves the use of a thermal interface composite material disposed between an integrated circuit device and a heat sink, wherein the thermal interface composite material comprises a metal screen defining openings and a bonding agent incorporated into the openings of the metal screen. The thermal interface composite material provides a superior combination of bonding strength and thermal conductivity that is not achieved with conventional thermal interface materials. 
         [0004]    These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0006]      FIG. 1  is a top plan view of a compressed copper mesh useful as the metal screen in the thermal interface composite material of the invention. 
           [0007]      FIG. 2  is a cross-sectional view of an electronic component having a thermal interface composite material disposed between an integrated circuit device and a heat sink to form an electronic component in accordance with the invention. 
           [0008]      FIGS. 3A-3F  illustrate an assembly process in accordance with the invention. 
           [0009]      FIGS. 4A-4E  illustrate an alternative assembly process also in accordance with the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0010]    In accordance with the various aspects and embodiments of this invention, a thermal interface composite material comprised of a metal screen defining openings and a bonding agent incorporated into the openings is disposed between an integrated circuit device and a heat sink to provide an exceptional combination of bonding strength and thermal conductivity. 
         [0011]      FIG. 1  shows a metal screen  10  comprised of copper filaments or threads  15  that are woven into a fabric mesh defining openings  20 . The illustrated metal screen  10  has flattened upper surfaces  25  and similar flattened surfaces on the opposite side (not shown). This flattening of the opposite surfaces of the metal screen that contact the heat sink and the circuit board substrate can be achieved by compressing a conventional wire screen between two planar surfaces or platens. A desirable flattening can be achieved with a standard 100 mesh screen or sieve comprised of woven copper filaments or threads having a diameter of 0.0045 inches using a force of from about 200 psi to about 800 psi. While flattening of the upper and lower surfaces of the screen is not essential, it increases the area of contact between the metal screen and the integrated circuit device and between the metal screen and a heat sink when it is incorporated into a composite thermal interface disposed between the integrated circuit device and the heat sink. 
         [0012]    While the metal screen is most desirably provided in the form of a wire mesh screen having woven metal filaments or threads, other metal screens may be used. Examples of other suitable metal screens for use in the composite interface materials of this invention include metal screens prepared by perforating a metal foil, such as by etching, punching, or otherwise providing a plurality of openings. 
         [0013]    Copper and copper alloys are currently a preferred material for use in making or providing the metal screens used in the composite thermal interfaces of this invention because of their high thermal conductivity, low cost and malleability. However, other metals may be employed, such as nickel, silver, gold, aluminum, iron and alloys thereof. Metal screens comprised of woven metal filaments or threads which may be used include those in which the filaments have a diameter of from about 1 mil (26 micrometers) to about 50 mils (1300 micrometers) and define openings of from about 1 mil (26 micrometers) to about 50 mils (1300 micrometers). 
         [0014]    The structural bonding agent that is incorporated into the openings defined in the metal screen may be either a thermosetting resin material or a thermoplastic material. Suitable thermosetting materials that may be used for preparing the thermal interface composite materials of this invention include epoxy reins, phenolic resins, melamine-formaldehyde resins, etc. with epoxy resins being preferred. The thermosetting resin being used to prepare the composite thermal interfaces of the invention may be so-called “B-stage” resins, which refers to a stage of some thermosetting resins characterized by softening up of the resin when heated and swelling when in the presence of certain liquids. So-called “snap-cure” epoxy resins such as those disclosed in U.S. Pat. No. 5,770,706 may be utilized. Examples of thermoplastic materials that may be employed in the composite thermal interfaces of this invention include polyvinyl acetate, acrylic solvent cement (e.g., polymethylmethacrylate dissolved in methyl chloride), acrylic, toughened acrylic resins, cyanoacrylates, silicone resins, polyamines and anaerobic acrylic acid diesters. 
         [0015]    In general, the electronic components having a composite thermal interface material disposed between an integrated circuit device and a heat sink is prepared by disposing between the integrated circuit device and heat sink, a thermal interface composite material comprising a metal screen defining openings and a fluid structural bonding agent incorporated in the openings, and subsequently hardening or curing the fluid structural bonding agent. In the case of thermosetting compositions, the expression “hardening” or “curing” refers to a chemical cross-linking reaction that causes the liquid resin composition to become irreversibly converted into a solid material, which typically cannot be reconstituted in any way except by decomposition. In the case of thermoplastic materials, hardening or curing refers to either evaporation of a solvent or solidification of a molten thermoplastic material. 
         [0016]    A preferred technique for preparing an electronic component in accordance with the invention is illustrated in  FIGS. 3A through 3F . 
         [0017]    In  FIG. 3A , a metal screen  10  is placed on heat sink  30 . In the illustrated embodiment, heat sink  30  has a plate-like structure or shape. However, it should be understood that the heat sink may have other shapes, and may include fins or other structures to enhance transfer of heat from heat sink  30  to the surrounding air by convection. 
         [0018]    Thereafter, as shown in  FIG. 3B , a structural adhesive agent composition  40  is applied over metal screen  10  and spread as shown in  FIG. 3C  so that composition  40  enters into openings in metal screen  10 , and preferably fills the openings. As shown in  FIG. 3D , the integrated circuit device is placed over screen  10  impregnated with adhesive composition  40 . Pressure in then applied as suggested in  FIG. 3E , such as with a clamp, and the adhesive composition  40  impregnated into the metal screen  10  is hardened or cured. The pressure in then removed and the completed device is shown in  FIG. 3F . 
         [0019]    In an alternative assembly process, also in accordance with the invention, a screen  10  is placed on heat sink  30  as shown in  FIG. 4A , and an adhesive is applied by means of a roller  50  as shown in  FIG. 4B . The steps illustrated in  FIGS. 4C through 4E  are analogous or the same as those illustrated in  FIGS. 3D through 3F  and described above. 
         [0020]    Examples of bonding agents which may be employed in accordance with the invention are listed in Table 1. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Other Potential Bonding Agents 
               
             
          
           
               
                   
                 Material 
                 CTE 
                 Cure Cycle 
               
               
                   
                   
               
             
          
           
               
                   
                 Cookson 3090 
                 38 
                 150° C. 20 minutes 
               
               
                   
                 ShinEtsu 9030 
                 200 
                 150° C. 20 minutes 
               
               
                   
                 Loctite 3509 
                 72 
                 During solder reflow 
               
               
                   
                 No-Flow Material 
                 82 
                 During solder reflow 
               
               
                   
                 Henkel OM 360 
                 280 
                 200° C. melt 
               
               
                   
                 Loctite 214-HP 
                 80 
                 150° C. 20 minutes 
               
               
                   
                 B-stage epoxy 
                 74 
                 Varies 
               
               
                   
                   
               
             
          
         
       
     
         [0021]    The relevant material properties characterizing the strength of the adhesive bond in terms of shear force and the thermal conductivity for various known thermal interface materials (Examples 1-5) is compared with a composite thermal interface material in accordance with the invention (Example 6) comprising a copper mesh screen impregnated with an epoxy resin (Loctite 214-HP). The results are listed in Table 2 below. The composite thermal interface material of the invention exhibits outstanding thermal conductivity as compared with known thermal interface materials, and a bonding strength comparable to pure epoxy resin, which is an extremely poor thermal conductor. 
         [0000]    
       
         
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Relevant Material Properties 
               
             
          
           
               
                   
                   
                 Shear Force 
                   
               
               
                 Example 
                 Material 
                 (0 Hrs) 
                 Thermal Conductivity 
               
               
                   
               
             
          
           
               
                 1 
                 Bergquist Dove 
                 &lt;5 kg 
                 15 
                 W/m ° K 
               
               
                 2 
                 AATA Film 
                 &lt;5 kg 
                 83 
                 W/m ° K 
               
               
                 3 
                 Sn 75-Pb 
                 34 kg 
                 45 
                 W/m ° K 
               
               
                   
                 Solder 
               
               
                 4 
                 Indium Solder 
                 22 kg 
                 86 
                 W/m ° K 
               
               
                 5 
                 Epoxy 
                 86 kg 
                 .7 
                 W/m ° K 
               
               
                 6 
                 Cu Mesh 
                 72 Kg 
                 108 
                 W/m ° K (measured 
               
               
                   
                 214-HP) 
               
               
                   
               
             
          
         
       
     
         [0022]    It will be understood by those who practice the invention and those skilled in the art, that various modifications and improvements may be made to the invention without departing from the spirit of the disclosed concept. The scope of protection afforded is to be determined by the claims and by the breadth of interpretation allowed by law.