Patent Publication Number: US-6661660-B2

Title: Integrated vapor chamber heat sink and spreader and an embedded direct heat pipe attachment

Description:
CROSS REFERENCE 
     This application is a divisional of application U.S. Ser. No. 09/746,554, filed on Dec. 22, 2000, entitled “An Integrated Vapor Chamber Heat Sink and Spreader and an Embedded Direct Heat Pipe Attachment,” currently pending. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally to the field of electronic devices and, in particular, the present invention relates to thermal management of electronic devices. 
     BACKGROUND 
     The current trends in microprocessor design are to increase their power, decrease their size, and increase their speed. This results in higher power in a smaller, faster microprocessor. Another trend is towards lightweight and compact electronic devices. As microprocessors become lighter, smaller, and more powerful, they also generate more heat in a smaller space, making thermal management a greater concern than before. 
     The purpose of thermal management is to maintain the temperature of a device within a moderate range. During operation, electronic devices dissipate power as heat. The heat must be removed from the device; otherwise, it will get hotter and hotter until it fails, reducing its service life. Short of failure, electronic devices run slowly and dissipate power poorly at high temperatures. 
     Naturally, heat moves from the device to the surrounding air and warms up the air by convection. The temperature at the surface of a heat-generating device is called the junction temperature. Heat is generated at the junction and must move from the junction to the surrounding or ambient air. Unfortunately, there is always some resistance to heat transfer, called thermal resistance. Basically, it is not easy to move heat from the device into the surrounding air. In fact, air is a rather good thermal insulator. Lowering the thermal resistance from the junction to the ambient air increases the power dissipation. To lower this thermal resistance, heat sinks are used. 
     An Integrated Vapor Chamber Heat Sink and Spreader 
     Current thermal designs do not have a sufficiently low thermal resistance to efficiently dissipate the heat generated by the new high power electronic devices. One such design for desktop and server computers is shown in FIG.  15 . Two layers of thermal interface material  1508 ,  1510  between the die  1504  and the heat sink  1516  contribute a significant portion of the total thermal resistance. Also, the long distance between the die  1504  and the heat sink  1516  contributes to the high thermal resistance. There is a need for a new thermal design with a lower thermal resistance that can efficiently dissipate heat for high power electronic devices. 
     If the heat sink  1516  were put directly in contact with the die  1504 , the thin lower wall of the heat sink  1516  would not have enough area available for heat transfer. Consequently, it would increase the thermal resistance internal to the heat sink  1516  and inefficiently dissipate heat. There is a need for a new thermal design that puts a heat sink directly in contact with the die and overcomes the problem of high internal thermal resistance. 
     Heat spreading is another problem introduced by putting a heat sink directly in contact with a die. Often there are “hot spots” on the die. Hot spots are spatial variations of power dissipation that increase the local temperature and cause malfunctions. Current thermal designs, such as the one shown in FIG. 15 have a heat spreader  1506  with inefficient heat spreading. There is a need for a new thermal design for desktop and server computers that eliminates the separate heat spreader, puts a heat sink directly in contact with the die, and spreads heat more uniformly. 
     An Embedded Direct Heat Pipe Attachment 
     Current designs for new high power mobile electronic devices, such as telephones, radios, laptop computers, and handheld devices do not efficiently dissipate the heat generated by these devices. One such design is shown in FIG.  16 . The total thermal resistance is too high for effective power dissipation. One reason is that the heat pipe  1612  is too far away from the die  1604 . Another reason is that the spreader plate  1608  lies between the heat pipe  1612  and the die  1604 . There is a need for a new thermal design with low thermal resistance for effective power dissipation in mobile devices that embeds a heat pipe in a heat spreader and puts it in direct contact with the die. 
     In the prior art, applying forces  1614  at the corners of the spreader plate  1608  produces unbalanced loads that sometimes cause the spreader plate  1608  to tilt in various ways as it presses down on the thermal interface material  1606 . This leads to large variations in the bond line thickness of the thermal interface material. These bond line thickness variations increase thermal resistance to an unacceptable level and reduce product reliability. There is a need for a new thermal design with central point loading over the center of the die resulting in uniform thickness of the thermal interface material and decreasing thermal resistance. 
     If a heat pipe is put in direct contact with the die, it must be protected from caving in under the pressure of the point load. There is a need for a new thermal design that embeds a heat pipe in a heat spreader so that the heat pipe is protected. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An Integrated Vapor Chamber Heat Sink and Spreader 
     FIG. 1 shows a perspective view of one embodiment of an integrated heat sink and spreader. 
     FIG. 2 shows a cross-section view of the integrated heat sink and spreader in FIG.  1 . 
     FIG. 3 shows a perspective view of a vapor chamber heat sink in one embodiment of the integrated heat sink and spreader in FIG.  2 . 
     FIG. 4 shows a cross-section view of one embodiment of a vapor chamber heat sink. 
     FIG. 5 shows a cross-section view of a hollow vapor chamber base in one embodiment of the vapor chamber heat sink in FIG.  4 . 
     An Embedded Direct Heat Pipe Attachment 
     FIG. 6 shows a perspective view of one embodiment of an arrangement for pressing a heat-generating item against a substrate. 
     FIG. 7 shows a cross-section view of the arrangement in FIG.  1 . 
     FIG. 8 shows an exploded view of a heat sink, a heat pipe, and a heat-spreading plate in one embodiment of the arrangement in FIG.  7 . 
     FIG. 9 shows a perspective view of a heat pipe and a heat-spreading plate in one embodiment of the arrangement in FIG.  7 . 
     FIG. 10 shows another perspective view of a heat pipe and a heat-spreading plate in one embodiment of the arrangement in FIG.  7 . 
     FIG. 11 shows a cross-section view of one embodiment of an embedded direct heat pipe attachment. 
     FIG. 12 shows an exploded view of a heat sink, a heat pipe, and a spreader plate of the embedded direct heat pipe attachment in FIG.  11 . 
     FIG. 13 shows a cross-section view of one embodiment of an electronic assembly. 
     FIG. 14 shows a flow chart of one embodiment of a method of assembling an embedded direct heat pipe attachment. 
     Prior Art 
     FIG. 15 shows a cross-section view of a prior art heat sink and spreader. 
     FIG. 16 shows a cross-section view of a prior art heat pipe and heat spreader. 
    
    
     DETAILED DESCRIPTION 
     In the following detailed description of the invention reference is made to the accompanying drawings which form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the inventions may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the inventions. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present inventions. 
     An Integrated Vapor Chamber Heat Sink and Spreader 
     An integrated heat sink and spreader for thermal management is described herein. The integrated heat sink and spreader efficiently dissipates the heat generated by high power electronic devices, such as desktop and server computers. 
     FIG. 1 shows a perspective view of one embodiment of an integrated heat sink and spreader  100 . An integrated heat sink and spreader  100  for cooling an item  102  comprises a vapor chamber heat sink  104  and a plurality of heat-radiating fins  110 . The item  102  is any heat-generating item, such as a processor for a desktop or server computer. 
     FIG. 2 shows a cross-section view of the integrated heat sink and spreader  100  in FIG.  1 . As shown in FIG. 2, the vapor chamber heat sink  104  is defined by a thinner first wall  106  and a thicker second wall  108 , the thicker second wall  108  being engageable with the item  102  in efficient heat transferring relationship. An efficient heat transferring relationship is one where the orientation and relative sizes are such that most of the heat generated by the item  102  is transferred and the thermal resistance is low. One efficient heat transferring relationship is direct thermal contact with the item  102  through a thin layer of thermal interface material. Some examples of thermal interface material are: solder, air, helium, polymer adhesive, silicone grease, silicone rubber, and thermal paste. A plurality of heat-radiating fins  110  are attached to the thinner first wall  106  of the integrated heat sink and spreader  100 . The heat-radiating fins  110  provide extended surfaces for heat transfer to the surrounding air. The fins  110  may be any type, including plate fins, serrated fins, pin fins, or disc fins. The fins  110  may be attached to the thinner first wall  106  with solder, air, helium, polymer adhesive, silicone grease, silicone rubber, thermal paste, or the like. 
     The integrated heat sink and spreader  100  may be either active or passive. Active heat sinks consist of a heat sink with a fan mounted directly to the heat sink. In an active heat sink, the fan blows air on the fins and base of the heat sink and provides cooling via air impingement. The use of active heat sinks is widespread in desktop computers. Passive heat sinks, on the other hand, are cooled by air flow across the heat sink fins. The air flow is usually provided by one or more system fans and may sometimes be ducted from the fan face to the heat sink. Passive heat sinks with or without ducted air flow are used widely in workstation and server computers. In addition, the integrated heat sink and spreader  100  may be an extruded heat sink, a folded-fin heat sink, an integrated vapor-chamber heat sinks, or any other type of heat sink. 
     FIG. 3 shows a perspective view of a vapor chamber heat sink  104  in one embodiment of the integrated heat sink and spreader  100  in FIG.  2 . In one embodiment, the thicker second wall  108  is at least twice as thick as the thinner first wall  106 , as shown in FIG.  3 . For example, the thicker second wall  108  may have a thickness of about 2 to 3 millimeters and the thinner first wall  106  may have a thickness of about 1 to 1.5 millimeters. 
     In one embodiment, the integrated heat sink and spreader  100  (shown in FIG. 2) has a thicker second wall  108  with a height  116  and a base surface area defined by a width  112  and a length  114 , as shown in FIG.  3 . The base surface area is large enough to spread heat substantially uniformly across the base surface area. Also, the base surface area is engageable with the item  102  (shown in FIG.  2 ). As shown in FIG. 3, the height  116  of the thicker second wall  108  is small enough to efficiently transfer heat. Together, the base surface area and height  116  minimize total thermal resistance. For example, the integrated heat sink and spreader  100  may have a width of at least about 5 centimeters and a length of at least about 6 centimeters, resulting in a base surface area of about 5×6=30 centimeters. In one embodiment, the base surface area is at least as large as the surface area of the item  102  engageable with the base surface area. For example, the base surface area may be the size of the footprint of the item  102 . Advantageously, the base surface area is small enough for mobile electronic devices and, at the same time, large enough to increase the heat spreading and cooling without increasing the total thermal resistance. In general, the total thermal resistance is given by ΣR=Σ(L/(kA)), where L is height, k is thermal conductivity, and A is the effective area. Thermal resistance is usually measured from the junction at the surface of the item  102  to the ambient air. Preferably, the integrated heat sink and spreader minimize total thermal resistance, including an optimal base surface area and corresponding height  116 . Given the equation, there are a range of acceptable shapes and sizes that will minimize total thermal resistance. 
     FIG. 4 shows a cross-section view of one embodiment of a vapor chamber heat sink  400 . A vapor chamber heat sink  400  for conducting heat away from an item  402  mounted to a substrate  418  comprises a hollow vapor chamber base  404  and a plurality of fins  410 . Thermal interface material  420  is interposed between the item  402  and the hollow vapor chamber base  404 . The hollow vapor chamber base  404  has a chamber  416 . The hollow vapor chamber base  404  has a thinner first wall  406  and a thicker second wall  408 . The plurality of fins  410  are bonded to the thinner first wall  406  to form a heat sink. The thicker second wall  408  has a surface area contactable with the item  402  that is sufficiently large to spread the heat generated by the item  402 . 
     FIG. 5 shows a cross-section view of a hollow vapor chamber base  404  in one embodiment of the vapor chamber heat sink  400  shown in FIG.  4 . As shown in FIG. 5, the hollow vapor chamber base  404  includes a fluid under pressure within a chamber  416 , an evaporator  411 , a condenser  412 , and a wick  414 . The evaporator  411  is associated with the thicker second wall  408  (shown in FIG. 4) and vaporizes the fluid. The condenser  412  is associated with the thinner first wall  406  (shown in FIG. 4) and condenses the fluid. The wick  414  returns the fluid to the evaporator  411 . The wick may be placed anywhere that provides a return path from the condenser  412  to the evaporator  411  and is a design decision. 
     A typical vapor chamber heat sink consists of an evaporator  411 , an adiabatic section, and a condenser  412 . Fluid vaporizes in the evaporator  411  and condenses in the condenser  412 . In an electronic device, the evaporator  411  is placed in contact with a heat-generating item, and the condenser  412  is cooled by forced convection. Since the evaporation and condensation temperatures are identical, an ideal heat pipe would move heat from the hot to the cold regions with negligible temperature drops. When a vapor chamber and fins are combined, the resulting heat sink consists of a hollow vapor chamber base that functions like a heat pipe. Typical heat sink thermal resistances of 0.2 to 0.4° C./Watt can be expected using a vapor chamber with fins heat sink at an air flow rate of 15 to 20 cfm, where cfm is the volumetric flow rate of a liquid or gas in cubic feet per minute. 
     Referring back to FIG. 4, in one embodiment, the vapor chamber heat sink  400  further comprises a top surface of the item  402  integrated with the hollow vapor chamber base  404 , a bottom surface of the item  402  attached to a substrate  418 , and a layer of thermal interface material  420  interposed between the item  402  and the hollow vapor chamber base  404 . The thickness of the thermal interface material  420  is highly exaggerated in FIG.  4  and other figures. Thermal interface material is usually a thin layer of material that produces intimate, poreless thermal contact. The substrate  418  is any kind of carrier, such as a circuit board, a motherboard or a test board. 
     In one embodiment, the thermal resistance between the item  402  and the vapor chamber heat sink base  404  is less than about 0.26° C./Watt. In one embodiment, the vapor chamber heat sink base  404  is capable of efficiently cooling an item  402  having a power of at least 190 Watts. With uniform heating, a numeric simulation indicated that the present invention was capable of handling 190 Watt, while the prior art was only capable of handling 130 Watt. The thermal resistance of the present invention between the die and the vapor chamber heat sink was about 0.26° C./Watt, while the thermal resistance of the prior art was about 0.38° C./Watt. The calculation (190 Watt-130 Watt)/130 Watt=0.46 shows about a 50% increase in power handling capacity. 
     Referring to both FIGS. 4 and 5, in one embodiment, a heat sink  400  for controlling the temperature of a heat-producing item  402 , comprises a heat pipe  404  and a plurality of heat-dissipating fins  410 . The heat pipe  404  includes a thinner first wall  406  and a thicker second wall  408 . The thicker second wall  408  is contactable with the item  402  in efficient heat-transferring relationship. The walls define a chamber  416 . The chamber  416  has a vaporizing region  411  proximate the thicker second wall  408 , and a condensing region  412  proximate the thinner first wall  406 . The exterior of the thicker second wall  408  has a size and topography relative to the item  402  and a sufficient thickness to efficiently absorb and spread heat from the item  402  and to efficiently apply such absorbed and spread heat to the vaporizing region  411 . The plurality of heat-dissipating fins  410  are in efficient heat-transferring relationship with the exterior of the thinner first wall  406 . The fins  410  have a cumulative surface area sufficiently large to efficiently dissipate heat transferred to the fins  410  through the thinner first wall  406  from the condensing region  412 . In one embodiment, the thicker second wall  408  is at least twice as thick as the thinner first wall  406 . In one embodiment, the item  402  has an exposed surface. The exterior of the thicker second wall  408  is adapted to contact the exposed surface of the item  402  in efficient heat-transferring relationship. The area of the exterior of the thicker second wall  408  is sufficiently larger than the area of the item&#39;s exposed surface to effect the spreading of heat transferred from the item&#39;s exposed surface to the exterior of the thicker second wall  408 . This spreading of heat is done efficiently throughout the thicker second wall  408  and from there to the vaporizing region  411 . 
     The integrated vapor chamber heat sink and spreader has many advantages over prior art thermal designs, such as the one shown in FIG. 15, including efficient heat dissipation for high power microprocessors and lower total thermal resistance. The integrated vapor chamber heat sink has less thermal resistance than prior art thermal designs by eliminating a layer of thermal interface material and a pedestal. Heat is more efficiently dissipated, since there is a smaller distance from the heat-generating item to the heat sink. Additionally, more area is available for heat transfer, decreasing the thermal resistance internal to the heat sink. About 50% more power is dissipated and up to 190 Watts of power can be dissipated efficiently. Also, thermal resistance is reduced by placing the heat sink directly in thermal contact with the heat-generating item and increasing the effective area of heat transfer. As a result, there is more uniform heat spreading across the heat sink. 
     It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. 
     An Embedded Direct Heat Pipe Attachment 
     An embedded direct heat pipe attachment is described herein. The embedded direct heat pipe attachment effectively dissipates the heat generated by small and thin high performance electronic devices, such as telephones, radios, laptop computers, handheld computers, and other mobile applications. 
     FIG. 6 shows a perspective view of one embodiment of an arrangement  600  for pressing a heat-generating item against a substrate while ensuring that a compressible and easily damaged heat pipe  606  is not damaged. A portion of the heat pipe  606  is embedded in a spreader plate  608 . Preferably, the heat pipe  606  has a thin profile less than 2 millimeters. In one embodiment, a heat sink  620  is attached a portion of the heat pipe  606 . 
     FIG. 7 shows a cross-section view of the arrangement  600  in FIG.  1 . The heat pipe  606  is capable of being thermally coupled to the heat-generating item  602 . In one embodiment, thermal interface material  628  thermally couples the heat pipe  606  to the heat-generating item  602 . The heat-generating item  602  may be a high power microprocessor for a telephone, radio, laptop computer, handheld device or any other high power electronic component. In one embodiment, a heat sink  620  is attached an end portion  622  of the heat pipe  606 . 
     FIGS. 8-10 show detailed features of various elements of the arrangement  600  in FIG.  7 . FIG. 8 shows an exploded view of a heat sink  620 , a heat pipe  606 , and a heat-spreading plate  608 . FIGS. 9 and 10 show top and bottom perspective views of a heat pipe  606  and a heat-spreading plate  608 . The arrangement  600  (shown in FIG. 7) comprises an essentially incompressible heat-spreading plate  608 , a groove  614  (shown in FIG.  8 ), and facilities  618  (shown in FIG. 7) for applying a force  616  (shown in FIG.  7 ). The essentially incompressible heat-spreading plate  608  has a first surface  610  (shown in FIG. 8) engageable with the heat-generating item  602  (shown in FIG. 7) and a second surface  612  (shown in FIG. 10) opposed to the first surface  610  (shown in FIG.  8 ). The heat-spreading plate may be a copper shell or the like. As shown in FIG. 8, the groove  614  is formed in the first surface  610  of the heat-spreading plate  608  for receiving a first end portion  621  of the heat pipe  606 . The groove  614  has a depth which is substantially the same as or slightly greater than the thickness of the heat pipe  606 . 
     In FIG. 7, a force  616  applied to the second surface  612  (shown in FIG. 10) of the heat spreading plate  608  presses the heat-generating item  602  against the substrate  604  and the force  616  has limited compressive effect on the heat pipe  606 . The facilities  618  for applying the force  616  to the second surface  610  (shown in FIG. 9) of the heat-spreading plate  608  presses the heat-generating item  602  against the substrate  604 . The force  616  is directed substantially at the center of the heat-generating item  602 . In one embodiment, a portion of the heat-spreading plate  608  is in contact with the heat-generating item  602  on at least two sides of the heat pipe  606  adding extra protection from damage. Thus, the heat pipe  606  embedded in the heat-spreading plate  608  is protected from caving in under the pressure of the force  616  or other damage. 
     FIG. 9 shows a perspective view of a heat pipe  606  and a heat-spreading plate  608  in one embodiment of the arrangement  600  in FIG.  7 . In FIG. 9, the heat pipe is shown embedded into the groove  614  (shown in FIG. 8) of the heat-spreading plate  608 . In one embodiment, the arrangement  600  (shown in FIG. 7) further comprises means for bonding  624  the first end portion  621  (shown in FIG. 8) of the heat pipe  606  into the groove  614  (shown in FIG. 8) so that an exposed surface  626  of the heat pipe  606  is substantially even with the first surface  610  of the heat-spreading plate  608 . As shown in FIG. 7, the heat pipe  606  is capable of being thermally coupled to the heat-generating item  602 . The means for bonding  624  may be solder, epoxy, brazing or the like. 
     By embedding the heat pipe  606  in the heat-spreading plate  608  and putting it in direct contact with a heat-generating item  602 , the thermal resistance is low enough to effectively dissipate power for high power mobile computers. This is an advantage over the prior art, such as that shown in FIG.  16 . Also, the thermal resistance is lowered by decreasing the amount of solder  1610  around the heat pipe  1612  in the prior art and moving the heat pipe  1612  closer to the heat-generating item  1604 . 
     In FIG. 7, one embodiment of the arrangement  600  further comprises thermal interface material  628  interposeable between the heat pipe  606  and the heat-generating item  602 . The thermal interface material is capable of thermally coupling the heat pipe  606  to the heat-generating item  602 . Some examples of thermal interface material are: solder, air, helium, polymer adhesive, silicone grease, silicone rubber, and thermal paste. In one embodiment, the facilities  618  for applying the force  616  directed substantially at the center of the heat-generating item presses the thermal interface material  628  into a layer of substantially uniform thickness. In the prior art, shown in FIG. 16, comer loading caused unbalanced loads which caused tilt between the heat-generating item  1604  and the heat-spreading plate  1608  which lead to large variations in bond line thickness of the thermal interface material  1606 . The substantially uniform thickness of the thermal interface material  628  decreases thermal resistance over the prior art. 
     FIG. 11 shows a cross-section view of one embodiment of an embedded direct heat pipe attachment  1100 . FIG. 11 shows an embedded direct heat pipe attachment  1100  for providing low thermal resistance for cooling a heat-generating component  1101  in a mobile electronic device. The heat-generating component  1101  is mounted to a carrier  1102 , such as a circuit board. The embedded direct heat pipe attachment  1100  comprises a heat pipe  1102 , thermal interface material  1104 , a spreader plate  1106 , and bonding means  1112 . In one embodiment, a spring plate  1116  applies a point load  1118  substantially at the center of the embedded direct heat pipe attachment  1100 . 
     FIG. 12 shows an exploded view of a heat sink  1126 , a heat pipe  1102 , and a spreader plate  1106  of the embedded direct heat pipe attachment  1100  in FIG.  11 . The heat pipe  1102  has at least one exposed surface  1103 . The exposed surface  1103  is substantially flat and capable of being thermally coupled to the heat-generating component  1101  (shown in FIG.  11 ). In FIG. 11, the thermal interface material  1104  thermally couples the heat pipe  1102  to the heat-generating component  1101 . In one embodiment, the bonding means is selected from the group consisting of solder and epoxy. In FIG. 12, the spreader plate  1106  has a surface  1108 , shown in FIG.  12 . The surface  1108  is substantially flat except where it defines a recess  1110  capable of receiving all but the exposed surface  1103  of the heat pipe  1102 . In one embodiment, the heat pipe  1102  includes a first end portion  1120 , a surface opposite the exposed surface (not shown), a first side  1122  of the end portion  1120  and a second side opposite the first side (not shown). In one embodiment, the bonding means is applied to the first end portion  1120  only on the first  1122  side and second side (not shown) of the first end portion  1120 . In another embodiment, the heat pipe  1102  is a remote heat exchanger which includes a second end portion  1124  opposite the first end portion  1120  and a heat sink  1126  thermally coupled to the second end portion  1124 . The heat pipe may be long enough so that heat can be directed towards a fan or other air flow located a distance from the heat-generating item. 
     FIG. 13 shows a cross-section view of one embodiment of an electronic assembly. An electronic assembly  1300  comprises a substrate  1302 , a die  1304 , a heat pipe  1306 , a spreader plate  1308 , a subassembly  1310 , thermal interface material  1312 , and a plate  1314 . The die  1304  has a top and is mounted on the substrate  1302 . The spreader plate  1308  defines a recess capable of receiving the heat pipe  1306 . The subassembly  1310  includes the heat pipe  1306  bonded into the recess of the spreader plate  1308  so that the subassembly  1310  is capable of being thermally coupled directly to the die  1304 . Thermal interface material  1312  for thermally coupling the die  1304  to the subassembly  1310  puts the heat pipe  1306  inn direct contact with the die  1304 . The plate  1314  applies a point load  1316  substantially at the center of the subassembly  1310 . In one embodiment, the total height of the electronic assembly  1300  is minimized. For example, the heat pipe may have a height of about 2 millimeters and the total height may be about 4.5 to 5 millimeters. A small stack height is advantageous in thin, mobile devices, such as telephones, radios, laptop computers, handheld devices. A small stack height provides a compact design as well as decreased thermal resistance. 
     In one embodiment, the spreader plate  1308  spreads the pressure form the point load  1316  so that the heat pipe  1306  is not deformed and the thermal interface material  1310  is pressed into a very thin layer of substantially uniform thickness. In one embodiment, the subassembly  1310  is thermally coupled to the die  1304  so that about 80% of the heat from the die  1304  is conducted away by the heat pipe  1306  and about 20% of the heat is conducted away by the spreader plate  1308 . The spreader plate  1308  may be in thermal contact with the die  1304  around the heat pipe  1306 . 
     In one embodiment, a thermal resistance at the point where the heat pipe  1306  and the die  1304  engage one another is less than about 0.8° C./Watt. The electronic assembly  1300  reduces the thermal resistance from the die  1304  to the heat pipe  1306  by about 26% over the prior art, shown in FIG.  16 . The present invention has a uniform power dissipation capacity of about 27 Watts, while that of the prior art was only about 23 Watts. The junction to heat pipe thermal resistance was 0.8° C./W for the present invention and 1.12° C./W for the prior art. The uniform power dissipation was measured with a heat pipe to ambient air thermal resistance (θ ha  of 1.1° C./W, a die temperature of 100° C. and an ambient air temperature of 50° C. Among its many advantages, the present invention offers about 26% lower thermal resistance and increased power handling capacity over the prior art. 
     FIG. 14 shows a flow chart of one embodiment of a method of assembling an embedded direct heat pipe attachment. The method  1400  comprises: forming a heat-pipe-shaped slot into a spreader plate  1402 , placing a heat pipe inside the slot  1404 , bonding all but one surface of the heat pipe into the slot to create a subassembly  1406 , mounting a die on a substrate  1408 , placing a thermal interface material on a top surface of the die  1410 , placing the subassembly on the top surface of the thermal interface material  1414 , and placing a plate on the top surface of the subassembly  1416 . In one embodiment, the method further comprises machining the top surface of the thermal interface material to create a flat surface  1412 . In another embodiment, the method further comprises applying force downward from the top of the plate to create a point load substantially at the center of the top surface of the subassembly  1418 . 
     It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.