Abstract:
The base of a heat sink may be selectively plated with a solder wetting material and soldered to an integral heat spreader also selectively plated with gold. In another embodiment, the solder may be applied in the form of an insert made up of an electrical heating wire sandwiched between indium foil which acts as solder when heated by the intervening wire.

Description:
BACKGROUND  
       [0001]     This invention relates generally to techniques for removing heat from integrated circuits.  
         [0002]     Integrated circuits may develop considerable amounts of heat during operation. This heat build up may adversely affect the electronic device using those components, the components themselves, and other surrounding components.  
         [0003]     Thus, it is desirable to dissipate heat from electronic components as effectively as possible. To this end, conventionally, a heat sink is positioned over an integrated circuit package. The heat sink may include fins. The electronic device may include a fan which blows air over the heat sink in some cases.  
         [0004]     The interface between the heat sink and the integrated circuit may be facilitated by having an integral heat spreader. The integral heat spreader may be thermally coupled to the heat sink base. A thermal interface material may be utilized between the heat sink base and the integral heat spreader to improve the heat transfer characteristics from the integrated circuit to the heat sink. Ideally, the thermal interface material reduces the resistance to heat transfer.  
         [0005]     Thus, there is a need for better ways to couple integrated circuits through integrated heat spreaders to heat sinks. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is an enlarged, cross-sectional view of one embodiment of the present invention;  
         [0007]      FIG. 2  is an exploded perspective view corresponding to  FIG. 1 ;  
         [0008]      FIG. 3  is a top plan view of the heat sink base shown in  FIG. 2  in the course of manufacture;  
         [0009]      FIG. 4  is a cross-sectional view through the integral heat spreader shown in  FIG. 2  in the course of manufacture;  
         [0010]      FIG. 5  is a perspective view of a heating device in accordance with one embodiment of the present invention;  
         [0011]      FIG. 6  is an enlarged, cross-sectional view of another embodiment of the present invention; and  
         [0012]      FIG. 7  is a side elevation of a process for making the device shown in  FIG. 5  in accordance with one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]     Referring to  FIG. 1 , the integrated circuit assembly  10  may include a socket  12  to be attached to a printed circuit board (not shown). The socket  12  may have catches  14  to engage slots in a heat sink  24 . The heat sink  24  may include a base  28  and upstanding fins  23  extending away therefrom.  
         [0014]     The substrate  16  may be situated over the socket  12 . A semiconductor integrated circuit  20  may be plugged into the socket  12 . The circuit  20  may be partially surrounded by an integrated heat spreader  18  designed to aid in the transfer of heat from the integrated circuit  20  to the heat sink  24 .  
         [0015]     The interface between the integrated heat spreader  18  and the base  28  of the heat sink  24  may include a pair of solder wetting layers  26  and  27 . The layer  26  may be initially secured to the integral heat spreader  18  and may be formed by selective coating. Likewise, the layer  27  may be initially formed on the base  28  of the heat sink  24  and may be selectively coated thereon.  
         [0016]     Thus, as shown in  FIG. 2 , the integral heat spreader  18  has its layer  26  and the lower surface  50  of the second level heat sink or heat sink base  28  of the heat sink  24  has a selectively coated layer  27  formed thereon. Thus, when the heat sink  24  is positioned on the integral heat spreader  18 , the layers  26  and  27  may be bonded by the solder or thermal interface material  29 .  
         [0017]     Referring to  FIG. 3 , selective deposition of the layer  27  on the surface  31  of the base  28  of the heat sink  24  may be accomplished by masking off the regions not corresponding to the shadow of the integral heat spreader, leaving an opening  33  through which the lower surface  30  appears and a surrounding mask  34 . In one embodiment, the mask  34  may be formed of rubber or similar plating masking materials. Once the heat sink  28 , other than the region  32 , is appropriately protected, the heat sink  24  may be exposed in a gold bath to plate the exposed region  30  with the layer  27 .  
         [0018]     Similarly, as shown in  FIG. 4 , the lower and side surfaces of the integral heat spreader  18  may be covered with a mask  34 . Then the integral heat spreader  18  may be exposed in a gold bath to form the gold layer  26 .  
         [0019]     In both cases, the mask  34  may be removed from the heat sink  24  and the integral heat spreader  18  prior to combination of the heat sink  24  to the integral heat spreader  18 .  
         [0020]     Other selective deposition techniques may be utilized as well. For example, a rubber mask may be pressed against the part to be plated and metal may be electroplated or electrolessly plated on surfaces not protected by the mold. The metal may be sprayed on the part. Sputtering may also be used.  
         [0021]     By selective plating on the heat sink base  28  and integral heat spreader  18  top surface, improved thermal performance can be achieved without unnecessarily plating solder wetting material over the entire bottom surface of the heat sink base and the entire surface of the integral heat spreader.  
         [0022]     The layers  26  and  27  may be formed of material that wets the solder (such as indium solder) used to bond the heat sink  28  to the heat spreader  18 . The layers  26  and  27  may be formed of gold, silver, indium, or tin, to mention a few examples. Advantageously, the layers  26  and  27  are formed of a material that does not significantly oxidize.  
         [0023]     Gold, as one example, is known to have very good wetting characteristics with thermal interface materials, such as indium solder thermal interface material. Gold may improve the reliability of the interface between the heat sink  24  and the integral heat spreader  18 . By controlling the amount of gold and its extent to only the shadow of the integral heat spreader  18  on the base  28 , extra gold, which would wet the thermal interface material  29 , is avoided.  
         [0024]     Typically, nickel is plated on the integral heat spreader and the base of the heat sink. If the layers  26  and  27  were not formed of a solder wetting material, the solder bond would be weaker.  
         [0025]     In some embodiments, a dissimilarity is achieved between the wetting characteristics of the selectively plated heat sink area and the non-selectively plated heat sink area, which is generally nickel. As a result, solder or other thermal interface material easily wets and spreads over the selectively plated area. However, the non-selectively plated area will not wet as easily and will, thus, act as a barrier to the further spreading of the solder thermal interface material  29 . In some embodiments, by retaining the thermal interface material  29  in the desired area, less thermal interface material may be utilized, pump-out may be reduced, resulting in reliability improvements, and the thermal interface material may be directed to fully fill the gold plated area, improving thermal performance in some embodiments.  
         [0026]     Thermal performance may be improved both before and after thermal cycling with a thermal interface material such as indium solder when used with gold plated surfaces. In some embodiments, the gold provides a consistent, robust bonding surface that nickel cannot offer.  
         [0027]     By selectively coating a solder wetting material, such as gold, the amount of such material that is utilized is reduced. For example, in some embodiments, only 30 percent of the entire heat sink base may be coated.  
         [0028]     In addition, solder thermal interface material has a thermal performance with gold plating that is much less sensitive to fan heat sink attach force and polymer thermal interface materials. This is due to the filling of the solder and the formation of an intermetallic bond between the gold and the solder thermal interface material. As a result, the attach force has minimal impact on thermal performance. This may enable a reduction in fan-to-heat sink attach force and the resulting reduction in board bending issues.  
         [0029]     Referring to  FIG. 5 , a solder insert  30  may include embedded wire  35 . The insert  30  can be placed between a second level heat sink or base  28  and an integral heat spreader  18  of an electronic package  10   a  as shown in  FIG. 6 . Electrical current can be applied to the insert  30 , and the heating wire  35  liquefies the surface layers  32  of solder thermal interface material. After solidification, in some embodiments, the resulting solder bond line may provide an excellent thermal link between the second level heat sink  28  and the electric package  10   a.    
         [0030]     The second level heat sink  28  may be clamped by catches  14  to a socket  12  as described previously. A substrate  16 , a die  20 , and an integral heat spreader  18  may be mounted over the socket  12 . Selectively plated layers  27  and  26 , as described previously, may be provided.  
         [0031]     Referring to  FIG. 7 , in one embodiment, the insert  30  may be formed as a sandwich of wire  35  and the layers  32  that may be formed of solder thermal interface foil. The three layers may be joined by rotating rollers  40  and  42  with adhesive application to join the wire  35  to the foil layers  32 . The sheet  48  then may be cut to size to form individual inserts  30 .  
         [0032]     In operation, when electrical current is applied to the wire  35 , the layers  32  may be melted. The heating wire  35  may be formed of kanthal or tungsten, in one embodiment of the present invention. In another embodiment, indium foil layers  32  may be attached to the gold layers  26 ,  27  on an integral heat spreader  18  and the second level heat sink  28  by cold forming. Indium foil layers may also be attached to gold-free surfaces such as nickel surfaces. Thereafter, the insert  30  may be placed between the heat sink  24  and the integral heat spreader  18  in assembled condition to melt the foil layers  32  and to reflow the solder. It may be desirable to coat the wire  35  with an electrically insulating layer (not shown), such as a polymer, including epoxy or colloidal silica in advance. In one embodiment, the insulating layer only needs to withstand the melting point of indium, which is 171° C.  
         [0033]     In some embodiments, an efficient way of melting the solder thermal interface material in place is provided. In this way, it is not necessary to heat the entire setup, including the integrated circuit die  20 , which may be damaged by the heating. It also allows easy heat sink attachment in the assembled state. There is no need to preheat the second level heat sink or the assembly in an oven in some embodiments. Liquid metal will flow into all of the small interfaces between the integral heat spreader and the second level heat sink, ensuring good thermal contact in such embodiments. In some embodiments, the insert  30  enables the heat sink  24  to be removed and reworked when needed.  
         [0034]     Using indium as a thermal interface material, rather than polymer, may reduce the thermal resistance of the second level heat sink by approximately one-third. This may allow the use of extruded aluminum technology for the second level heat sink, avoiding the use of copper and other more expensive second level heat sinks. The presence of the heating wire can act as a spacer to control the second level thermal interface bond line, reducing the tendency of the solder to be squeezed out of the bone line.  
         [0035]     In accordance with another embodiment of the present invention, the insert can be utilized with a polymer solder hybrid. The polymer solder hybrid needs to be reflowed before use to melt the indium in the hybrid. The heating elements can also be used to cure or crosslink the polymer in the polymer solder hybrid. The insert may also be used to cure crosslinked conventional polymer second level thermal interface materials, thereby reducing pump-out issues associated with non-crosslinked thermal interface materials.  
         [0036]     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.