Abstract:
A thermal interface assembly and method for forming a thermal interface between a microelectronic component package and a heat sink having a total thermal resistance of no greater than about 0.03° C.-in 2 /W comprising interposing a thermal interface assembly between an microelectronic component package and heat sink with the thermal interface assembly comprising a thermal interface material having phase change properties and a sealing member selected from the group consisting of an o-ring and/or shim in an arrangement such that the thermal interface material is shielded from the atmosphere when the microelectronic component package is operational.

Description:
FIELD OF INVENTION 
   This is a Continuation-In-Part of patent application Ser. No. 10/090,540 filed on Mar. 1, 2002 now U.S. Pat. No. 6,761,928 which is in turn a division of U.S. patent application Ser. No. 09/513,483 filed Feb. 25, 2000 now U.S. Pat. No. 6,372,997 and relates to a thermal interface assembly and method for establishing a thermal interface with low contact thermal resistance between a microelectronic component package and a heat sink. 

   BACKGROUND OF THE INVENTION 
   Microelectronic components, such as semiconductors, generate substantial heat which must be removed to maintain the component&#39;s junction temperature within safe operating limits. Exceeding these limits can change the performance characteristics of the component and/or damage the component. The heat removal process involves heat conduction through an interface material from the microelectronic component to a heat sink. The selection of the interface material and the thermal resistance at the interface between the heat generating component (e.g. silicon ic chip) and the heat sink controls the degree of heat transfer. As the demand for more powerful microelectronics increase so does the need for improved heat removal. 
   The thermal resistance between the microelectronic component package and the heat sink is dependent not only upon the intrinsic thermal resistance of the interface material but also upon the contact interface thermal resistance formed at the junction between the interface material on each opposite side thereof and the microelectronic component and heat sink respectively. One known way to minimize contact thermal resistance at each interface junction is to apply high pressure to mate the interface material to the microelectronic package and heat sink. However, excessive pressure can create detrimental and undesirable stresses. Accordingly, the application of pressure is generally limited so as not to exceed 100 psi and preferably below about 20 psi. 
   It is also known to use a thermal grease or paste as the thermal interface material or to use a thin sheet composed of a filled polymer, metallic alloy or other material composition having phase change properties. A material having phase change properties is characterized as having a viscosity responsive to temperature with the material being solid at room temperature and softening to a creamy or liquid consistency as the temperature rises above room temperature. Accordingly, as the microelectronic component heats up the material softens allowing it to flow to fill voids or microscopic irregularities on the contact surface of the microelectronic component and/or heat sink. Excess phase change material is squeezed out allowing the opposing surfaces between the microelectronic component and heat sink to physically come closer together as the phase change material melts thereby reducing the thermal resistance between them. 
   Since the microelectronic package and heat sink do not generally have smooth and planar surfaces a relatively wide and irregular gap may form between the surfaces of the microelectronic component and heat sink. This gap can vary in size from less than 2 mils up to 20 mils or greater. Accordingly, the interface material must be of adequate thickness to fill the gap. The use of thermal grease, paste or phase change materials cannot presently accommodate large variations in gap sizes. In general as the thickness of the interface material increases so does its thermal resistance. It is now a preferred or targeted requirement for a thermal interface material to have a total thermal resistance, inclusive of interfacial contact thermal resistance, in a range not exceeding about 0.03° C.-in 2 /W at an applied clamping pressure of less than 100 psi and preferably less than about 20 psi. 
   SUMMARY OF THE INVENTION 
   A thermal interface assembly has been discovered in accordance with the present invention comprising a thermal interface material consisting of a material having at least one layer of a low melting alloy composition having phase change properties in combination with a sealing member selected from the group consisting of an o-ring and/or a shim for placement between a microelectronic component package and a heat sink in an arrangement such that the sealing member surrounds the thermal interface material to form a sealed thermal interface assembly possessing low contact interfacial thermal resistance when the microelectronic component package is operational. 
   The present invention further includes a method for forming a thermal interface between a microelectronic component package and a heat sink comprising the steps of interposing a thermal interface assembly between said microelectronic component package and heat sink comprising a thermal interface material having phase change properties and a sealing member selected from the group consisting of an o-ring and/or a shim in an arrangement with the sealing member surrounding the thermal interface material such that the thermal interface material is shielded from the atmosphere when the microelectronic component package is operational. 
   The thermal interface material may consist of a single material element preferably in the form of a foil possessing a thickness of from 0.0001 to 0.050 inches, and more preferably of a thickness of less than about 2 mils, and a composition consisting essentially of a low melting alloy having phase change properties or a multi-layer material structure having two or more layers, each of high thermal conductivity with at least one of the layers consisting essentially of a low melting alloy having phase change properties and with the other layer(s) representing a carrier layer(s) preferably of metal composition. High thermal conductivity for purposes of the present invention shall mean a thermal conductivity of above at least 10 W/m-K. The preferred class of high thermal conductivity metal carrier layer(s) should be selected from the transition elements of row  4  in the periodic table in addition to magnesium and aluminum from row  3  and alloys thereof. 
   The preferred multi-layer material structure for the thermal interface assembly of the present invention comprises three layers having an intermediate solid core of a high thermal conductivity metal or metal alloy and a layer on each opposite side thereof composed of a material having phase change properties. A material having phase change properties shall mean for purposes of the present invention a low melting metal or metal alloy composition having a melting temperature between 40° C. and 160° C. The preferred low melting material alloy of the present invention should be selected from the group of elements consisting of indium, bismuth, tin, lead, cadmium, gallium, zinc, silver and combinations thereof. An optimum low melting alloy composition of the present invention comprises at least between 10 wt %-80 wt % indium and 20 wt %-50 wt % bismuth with the remainder, if any, selected from the above identified group of elements. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other advantages of the present invention will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings of which: 
       FIG. 1  is a cross sectional view showing one embodiment of the thermal assembly of the present invention disposed between a conventional heat sink and a conventional cpu microelectronics package; 
       FIG. 2  is a cross sectional view of another embodiment of the thermal assembly of the present invention; 
       FIG. 3  is a cross sectional view of yet another embodiment of the thermal assembly of the present invention shown disposed between a conventional heat sink and, a conventional microelectronics mounting surface using both a flexible shim in addition to an o-ring for shielding the thermal interface material from the atmosphere; 
       FIG. 4  is a cross sectional view of a two layer thermal interface material in accordance with the present invention with one layer having phase change properties; and 
       FIG. 5  is a cross sectional view of a three layer thermal interface material in accordance with the present invention having two opposing layers with phase change properties on opposite sides of a metallic core. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The thermal interface assembly of the present invention comprises a thermal interface material  10  and a sealing member  12  adapted for placement between a heat sink  14  and a microelectronic component package  16  in an arrangement such that the sealing member  12  surrounds the thermal interface material  10  leaving an open space  32  as shown in  FIG. 3 . The sealing member  12  forms a seal for isolating the thermal interface material from the atmosphere, as is shown in  FIGS. 1-3  respectively, when the microelectronic component package  16  is operational, i.e., when electrical power is applied across the microelectronic component package  16  such that the temperature of the microelectronic component package  16  rises above room temperature. 
   The thermal interface material  10  may consist of a single element preferably in the form of a foil or a multi-layer material structure with at least one layer composed of a low melting metal or metal alloy composition (hereinafter “LMA”) having phase change properties and having at least one other layer composed of a high thermal conductivity material. The LMA alloy is characterized by a viscosity which is responsive to temperature such that the LMA alloy composition will be solid at room temperature but will soften i.e., begin to melt as the temperature rises above room temperature. The LMA layer should possess a thickness of from 0.0001 to 0.050 inches with a preferred thickness of less than about 2 mils. For purposes of the present invention, an LMA alloy composition should have a melting temperature of less than 157° C. and should preferably be of a composition selected from the group of elements consisting of indium, bismuth, tin, lead, cadmium, gallium, zinc, silver and combinations thereof. An optimum LMA alloy composition of the present invention comprises at least between 10 wt %-80 wt % indium and 20 wt %-50 wt % bismuth with the remainder, if any, selected from the above identified group of elements. An example of one suitable LMA alloy composition would comprise indium at 51 wt %, tin at 16.5 wt % and bismuth of about 32.5 wt %. This composition melts at approximately 61° C. 
   It was discovered in accordance with the present invention that the LMA alloy composition is susceptible to oxidation in the presence of air and that oxidation of the LMA alloy reduces its thermal conductivity and diminishes its thermal performance. It has further been discovered that such oxidation can be prevented by surrounding the thermal interface material  10  with a sealing member  12  to shield the thermal interface material  10  from the atmosphere when the microelectronic component package  16  is operational. It should be understood that the sealing member  12  does not necessarily have to form a completely hermetic seal. Accordingly, the sealing member  12  may be composed of any material composition which will not readily transmit air or oxygen to the thermal interface material  10 . As such, the sealing member  12  may allow a relatively slow diffusion of air or oxygen. In the latter case, the rate of oxidation of the thermal interface member  10  will still be significantly slowed down after the microelectronic component package  16  becomes operational relative to the rate of oxidation of the thermal interface member  10  without the use of the sealing member  12 . 
   The sealing member  12 , as shown in  FIG. 1 , may be composed of any soft, pliable, compliant organic material and may be represented by an O-ring or a shim or a combination thereof which will effectively seal off the thermal interface material  10  from a continuous supply of fresh air from the atmosphere when interposed between a heat source and heat sink and can be a pre-formed sealing member or a “form-in-place” sealing material. An example of an acceptable preformed sealing material would be a soft silicone rubber gel such as “Silguard 527” a trademark product of the Dow Corning Corporation. 
     FIG. 2  is an alternative embodiment of the present invention in which the sealing member  12  is composed of an O-ring seated in a cavity or groove  20  formed in the heat sink  14 . The design, size and composition of the O-ring should be such that the O-ring will be squeezed down approaching a thickness at least as thin as the bond-line of the LMA alloy after squeeze-out when the microelectronic component package  16  is operational, in order to minimize thermal resistance. It should be understood that it is customary for the heat sink  14  to be clamped to the cpu package  16 . The clamping pressure further causes the O-ring to seal off the area between the base of the heat sink  14  and the opposing surface (which may be the lid) of the cpu package  16 . Although the use of a cavity or groove  20  formed in the heat sink  14 , as is shown in  FIG. 2 , is optional it is preferred since it accommodates the placement of the O-ring and assures that the O-ring will squeeze down to minimize bond-line thickness. In this way the sealing member  12  is forced into compression to seal off the space between the mated surfaces of the cpu package  16  and the heat sink  14 . 
   An alternative embodiment using both a shim and O-ring for shielding the thermal interface material  10  from the atmosphere when placed between a conventional heat source and heat sink is shown in  FIG. 3 . This embodiment of the invention is preferably intended for a heat source having a conventional microelectronics mounting surface  30  which supports a conventional microelectronics component  29 . In this embodiment the flexible shim comprises a flexible shim support  27  and a flat flexible shim section  28  extending outwardly from the shim support  27 . The flexible shim support  27  has a donut-like configuration with an opening  31  which permits mounting the flexible shim support  27  about the microelectronics component  29 . The flat flexible shim section  28  extends outwardly from the shim support  27  to form a support platform for mounting an O-ring  12  between the heat sink  14  and the flat flexible shim section  28 . 
   As explained above the thermal interface material  10  may include one or more additional layers of high thermal conductivity. Any metallic high thermal conductivity material may be used having a thermal conductivity of above at least 10 W/m-K inclusive of any of the transition elements of row  4  in the periodic table in addition to magnesium and aluminum of row  3  and their alloys. However, a foil sheet of either aluminum or copper as the carrier layer is preferred.  FIG. 4  shows a thermal interface material  10  formed of a two layer metallic structure consisting of a solid metal or metal alloy sheet  22  of high thermal conductivity, designated a carrier layer, and a superimposed low melting alloy sheet  23  possessing phase change properties. An alternative and preferred three layer arrangement is shown in cross section in  FIG. 5  consisting of an intermediate carrier layer  24  equivalent in composition to the carrier layer  22  of  FIG. 4  and two opposing layers  25  of a low melting alloy equivalent in composition to the low melting alloy layer  23 . The low melting alloy layers  23  or  25  may be laminated over the entire planar surface of the high thermal conductivity layered sheets  22  and  24  respectively or may be laminated over less than the entire planar surface of the high thermal conductivity layered sheets  22  and  24  leaving an exposed border (not shown) of any desired geometry.