Abstract:
A thermal interface material comprises a phenyl ester and a thermally conductive filler. The material optionally contains an epoxy resin derived from nutshell oil or an epoxidized dimer fatty acid.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of International Patent Application No. PCT/US2010/055924 filed Nov. 9, 2010, which claims priority to U.S. Provisional Patent Application No. 61/261,152 filed Nov. 13, 2009, the contents of both of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to a thermally conductive material that is utilized to transfer heat from a heat-generating electronic device to a heat sink that absorbs and dissipates the transferred heat. 
       BACKGROUND OF THE INVENTION 
       [0003]    Electronic devices containing semiconductors generate a significant amount of heat during operation. The level of heat generated is related to the performance of the semiconductor, with less highly performing devices generating lower levels of heat. In order to cool the semiconductors, which must be cooled in order to obtain appreciable performance, heat sinks are affixed to the device. In operation, heat generated during use is transferred from the semiconductor to the heat sink where the heat is harmlessly dissipated. In order to maximize the heat transfer from the semiconductor to the heat sink, a thermally conductive material, known as a thermal interface material (TIM), is utilized. The TIM ideally provides intimate contact between the heat sink and the semiconductor to facilitate the heat transfer. 
         [0004]    There are various types of TIMs currently used by semiconductor manufacturers, all with their own advantages and disadvantages. For those semiconductors generating relatively lower levels of heat than high performing semiconductors, a preferred thermal solution is the use of a thermal gel containing aluminum as the conductive material. These materials provide adequate thermal conductivity (3 to 4 W/m-K), but they can be susceptible to delamination under stress. 
         [0005]    Thus, it would be advantageous to provide a thermal interface material that is easy to handle and apply, yet also provides a highly adequate thermal conductivity and reliable performance. 
       SUMMARY OF THE INVENTION 
       [0006]    This invention is a composition for use as a thermal interface material in a heat-generating, semiconductor-containing device. 
         [0007]    In one embodiment, the composition comprises aluminum metal particles and a phenyl ester. In another embodiment, the composition further comprises an epoxidized dimer fatty acid. In a third embodiment, the composition further comprises an epoxy resin derived from nutshell oil. In all embodiments, a catalyst is optional. The metal particles are substantially devoid of added lead. The presence of the phenyl ester as the main resin component makes the composition more flexible, thus preventing cracking and increasing the contact between the heat sink and the semiconductor. Thus, the presence of the phenyl ester acts to inhibit thermal degradation and consequently works to keep the thermal impedance stable over time. 
         [0008]    The use of the epoxidized dimer fatty acid, and in some embodiments additionally of the epoxy resin derived from nut oil, provides an optimum range of modulus for the thermal interface material. These epoxies form a gel-like or tacky mass that physically keeps the solder particles connected and in place within the thermal interface material, thus keeping the thermal impedance stable over time. 
         [0009]    In another embodiment, this invention is an electronic device containing a heat-generating component, a heat sink and a thermal interface material according to the above description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0010]      FIG. 1  is a side view of an electronic component having a heat sink, a heat spreader, and thermal interface material. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    The thermal interface material of the present invention may be utilized with any heat-generating component for which heat dissipation is required, and in particular, for heat-generating components in semiconductor devices. In such devices, the thermal interface material forms a layer between the heat-generating component and the heat sink and transfers the heat to be dissipated to the heat sink. The thermal interface material may also be used in a device containing a heat spreader. In such a device, a layer of thermal interface material is placed between the heat-generating component and the heat spreader. and a second layer of thermal interface material is placed between the heat spreader and the heat sink. 
         [0012]    In one embodiment, the phenyl esters are selected from the group consisting of 
         [0000]    
       
                 
         
             
             
         
       
     
         [0013]    propionate diacetate 
         [0000]    
       
                 
         
             
             
         
       
     
         [0014]    bisphenol A diallyl diacetate 
         [0000]    
       
                 
         
             
             
         
       
     
         [0015]    dimer diacetate 
         [0000]    
       
                 
         
             
             
         
       
     
         [0016]    mono-functional acetate 
         [0017]    and 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    tetra-functional acetate. 
         [0018]    The phenyl ester will be present in the composition within a range of 5 to 35 weight percent based on the total weight of the composition. 
         [0019]    The epoxidized dimer fatty acids are the reaction products of dimer fatty acids and epichlorohydrin. In one embodiment, the epoxidized dimer fatty acid has the following structure in which R is a 34 carbon chain represented as C 34 H 68 : 
         [0000]    
       
                 
         
             
             
         
       
     
         [0020]    It is commercially available from CVC Chemical in New Jersey. 
         [0021]    The epoxy resin derived from nutshell oil comprises one or both of the following structures: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0022]    These resins are commercially available from Cardolite Corporation in New Jersey. Either the monofunctional epoxy or the difunctional epoxy or a blend of any ratios is equally effective within the TIM composition. 
         [0023]    The use of a catalyst for the epoxy functionality is optional, but any catalyst known in the art suitable for polymerizing or curing epoxy functionality may be used. Examples of suitable catalysts include peroxides and amines. When present, the catalyst will be used in an effective amount; in one embodiment, an effective amount ranges from 0.2 to 2% by weight of the composition. 
         [0024]    Aluminum metal particles are typically used in thermal interface materials due to their lower cost compared to solder or silver, although silver particles may also be present. An exemplary aluminum metal powder is commercially available from Toyal America in Illinois. In one embodiment the metal powder has an average particle size of about 1-10 microns. In one embodiment, the metal powder will be present in the composition in a range from 50 to 95 weight percent of the total composition. 
         [0025]    In one embodiment illustrated in  FIG. 1 , an electronic component  10  utilizing two layers of thermal interface materials comprises a substrate  11  that is attached to a silicon die  12  via interconnects  14 . The silicon die generates heat that is transferred through thermal interface material  15  that is adjacent at least one side of the die. Heat spreader  16  is positioned adjacent to the thermal interface material and acts to dissipate a portion of the heat that passes through the first thermal interface material layer. Heat sink  17  is positioned adjacent to the heat spreader to dissipate any transferred thermal energy. A thermal interface material is located between the heat spreader and the heat sink. The thermal interface material  18  is commonly thicker than the thermal interface material  15 . 
       EXAMPLES 
       [0026]    Compositions were prepared to contain the components in weight percent shown in the below Table. The inventive samples are identified as A, B, C, and D. The comparative samples are identified as E, F and G. They all consist of a liquid reactive mixture of polymer resins and aluminum powder. 
         [0027]    The TIM compositions were tested for thermal conductivity by measuring the resistance within a TIM composition disposed between a silicon die and a copper plank. The silicon die was heated and the heat input measured using a combination of a voltage and current meter. The heat traveled through the TIM to the copper heat sink, and the temperature on the heat sink was read by a thermocouple. Resistance was calculated from these values. 
         [0028]    The results are reported in the Table and show that the inventive compositions containing the phenyl ester, compared to the comparative compositions, exhibited stable and lower thermal impedance, especially after the reliability tests of baking and thermal cycling. Low thermal impedance is needed for heat dissipation, and it is also important that thermal impedance remains stable over time, thereby assuring a longer life for the ultimate device in which it is used. 
         [0029]    The results further show that the inventive compositions containing the phenyl ester exhibited a lower modulus that did not increase after exposure to high temperature. Low modulus is needed so that the compositions remain soft and flexible, which results in better thermal conductivity. This is in contrast to the comparative compositions, which all showed a significant increase in modulus after high temperature baking. These comparative compositions exhibited high thermal degradation, becoming hard and brittle, which ultimately would result in interfacial delamination of the TIM to its substrate. 
         [0000]    
       
         
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 SAMPLE ID AND COMPOSITION IN PERCENT BY WEIGHT 
               
             
          
           
               
                 COMPONENT 
                 A 
                 B 
                 C 
                 D 
                 E 
                 F 
                 G 
               
               
                   
               
             
          
           
               
                 Epoxidized 
                   
                   
                 2.5 
                   
                   
                 14.5 
                 7.25 
               
               
                 nutshell oil 
               
               
                 Epoxidized 
                 5 
                 5 
                 2.5 
                   
                 14.5 
                   
                 7.25 
               
               
                 dimer fatty 
               
               
                 acid 
               
               
                 X-Diacetate 
                 14.9 
                 15 
                 14.9 
                 14.9 
               
               
                 phenyl ester 
               
               
                 ECE861 
               
               
                 2-Phenyl-4- 
                 0.1 
                   
                 0.1 
                 0.1 
                 0.5 
                 0.5 
                 0.5 
               
               
                 methyl 
               
               
                 imidazole 
               
               
                 Aluminum 
                 80 
                 80 
                 80 
                 80 
                 80 
                 80 
                 80 
               
               
                 powder 
               
             
          
           
               
                 VISCOSITY (at room temperature) (kcps) 
               
             
          
           
               
                 Cone-and- 
                 100000 
                 100000 
                 90000 
                 50000 
                 28000 
                 17000 
                 24000 
               
               
                 Plate @ 5 RPM 
               
             
          
           
               
                 THERMAL IMPEDANCE (taken at room temperature after conditions stated) 
               
               
                 (C · cm 2 /Watt) 
               
             
          
           
               
                 Before cure 
                 0.224 
                 0.22 
                 0.24 
                 0.24 
                 0.22 
                 0.21 
                 0.22 
               
               
                 Cured at 150 C. 
                 0.18 
                 0.17 
                 0.18 
                 0.17 
                 0.2 
                 0.2 
                 0.2 
               
               
                 for 1 hr 
               
               
                 Baked at 150 C. 
                 0.2 
                 0.19 
                 0.2 
                 0.19 
                 0.4 
                 0.36 
                 0.4 
               
               
                 for 100 hrs 
               
             
          
           
               
                 MODULUS (taken at room temperature after conditions stated) (Pa) 
               
             
          
           
               
                 Cured 
                 35000 
                 25000 
                 31000 
                 29000 
                 25000 
                 500 
                 1500 
               
               
                 100 hrs at 
                 44000 
                 28000 
                 37000 
                 33000 
                 160000 
                 50000 
                 85000 
               
               
                 125 C. 
               
               
                 100 hrs at 
                 45000 
                 30000 
                 40000 
                 35000 
                 350000 
                 210000 
                 240000 
               
               
                 150 C. 
               
               
                 100 hrs at 
                 38000 
                 24000 
                 30000 
                 30000 
                 125000 
                 23000 
                 60000 
               
               
                 121 C. and 
               
               
                 100% RH 
               
               
                 125 cycles 
                 35000 
                 27000 
                 33000 
                 28000 
                 100000 
                 15000 
                 50000 
               
               
                 from - 55 C. 
               
               
                 to 125 C. 
               
               
                 MSL L3 260 C. 
                 35000 
                 30000 
                 36000 
                 34000 
                 250000 
                 150000 
                 200000