Abstract:
A microelectronic package comprises a substrate, a electronic chip mounted on the substrate, a thermal interface material, a spring clip, and a retention frame. The thermal interface material is located between the electronic chip and the slug and is capable of thermally coupling the electronic chip to a slug without curing. The spring clip is located between the retention frame and the slug. In the assembled microelectronic package, the retention frame caps the substrate, and the spring clip applies a constant force to the slug to ensure reliable and continuous thermal contact between the electronic chip and the slug.

Description:
FIELD 
     This invention relates to the packaging of circuits, and more particularly to the packaging of microelectronic circuits. 
     BACKGROUND 
     As the density of circuit elements, such as resistors, capacitors, and transistors, is increased in a microelectronic circuit, heat generated by the circuit elements during the operation of the circuit increases. This heat conducts through the chip on which the microelectronic circuit is fabricated and must be efficiently removed from the chip. Failure to efficiently remove the heat from the chip can result in catastrophic failure of the microelectronic circuit. 
     FIG. 1 is a cross-sectional view of prior art microelectronic circuit package  100 . Microelectronic circuit package  100  comprises substrate  103 , encapsulated solder balls  106 , electronic chip  109 , thermal interface material  112 , integrated heat spreader  115 , and sealant material  118 . Substrate  103  is a fragile sandwich of conductors and insulators formed on a ceramic base. Encapsulated solder balls  106  provide a conductive path for signals routed between electronic chip  109  and substrate  103 . Thermal interface material  112  is a cured epoxy and is capable of transmitting heat from electronic chip  109  to integrated heat spreader  112 . Integrated heat spreader  115  is a complex machined device that efficiently dissipates heat into the air and is secured to substrate  103  by sealant material  118 . The bond provided by sealant material  118  between integrated heat spreader  115  and substrate  103  hermetically seals the space enclosed by substrate  103  and integrated heat spreader  115 . 
     Unfortunately, there are several problems with microelectronic package  100 . First, integrated heat spreader  115  is a complex machined device that is expensive to manufacture. Second, thermal interface material  112  is a cured epoxy that takes a significant amount of time to cure, which slows the manufacturing process, and increases the cost of the package. 
     Other problems with microelectronic package  100  arise as the heat flow in electronic chip  109  increases. First, the mismatch between the coefficients of thermal expansion of integrated heat spreader  112 , thermal interface material  112 , and electronic chip  109 , causes electronic chip  109  to tear away from integrated heat spreader  112  and generate voids and uneven heat flow in thermal interface material  112 . Second, the mismatch between the coefficients of thermal expansion of heat spreader  112  and substrate  103  cause integrated heat spreader  112  to tear away from substrate  103 , which breaks sealant material  118 , destroying the hermetic seal and structurally damaging the fragile conductors and insulators of substrate  103 . 
     For these and other reasons there is a need for the present invention. 
     SUMMARY 
     An electronic chip package comprises an electronic chip, a slug thermally coupled to the electronic chip, and a mechanism capable of exerting a constant force on the slug. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a prior art microelectronic circuit package. 
     FIG. 2 is an exploded perspective view of some embodiments of the microelectronic circuit package of the present invention. 
     FIG. 3 is a perspective view of one embodiment of a slug of the present invention. 
     FIG. 4 is a perspective view of one embodiment of a spring clip of the present invention. 
     FIG. 5 is a perspective view of an alternate embodiment of a spring clip of the present invention. 
     FIG. 6 is a block diagram of one embodiment of a computer system suitable for use in connection with the present invention. 
    
    
     DESCRIPTION 
     The illustrative microelectronic circuit package  200  shown in the accompanying drawings is particularly suited to the packaging of high performance microprocessors. FIG. 2 is an exploded perspective view of microelectronic circuit package  200  comprising chip-slug unit  203 , packaging assembly  206 A, and packaging assembly  206 B. 
     Chip-slug unit  203  comprises electronic chip  209 , thermal interface material  212 , and slug  215 . Electronic chip  209  includes microelectronic circuits that produce heat during their normal operation. An advantage of microelectronic package  200 , which is capable of removing heat from electronic chip  209 , is that it is not limited to use in connection with a particular chip substrate material or a particular circuit technology. Some chip substrate materials that are suitable for use in connection with the present invention include silicon, germanium, gallium arsenide, silicon-on-sapphire, and silicon-on-insulator. Some circuit technologies that are suitable for use in connection with the present invention include bipolar junction transistor (BJT) technologies and metal-oxide-semiconductor (MOS) technologies. In one embodiment, electronic chip  209  is a microprocessor chip fabricated in a MOS technology. Alternatively, electronic chip  209  is an application specific integrated circuit (ASIC) fabricated in more than one technology or a memory chip fabricated in a MOS technology. Another advantage of microelectronic package  200  is that a single packaging technology is suitable for packaging a variety of electronic chips that make up a complex electronic system. 
     Slug  215  has a plurality of edges  216  and is fabricated from a material having high thermal conductivity, such as copper or a copper alloy. One function of slug  215  is to remove heat from electronic chip  209 , so the greater the thermal conductivity of slug  215 , the more efficiently slug  215  removes heat from electronic chip  209 . Slug  215  preferably has a shape that is easy to manufacture and that efficiently conducts heat away from electronic chip  209 . In one embodiment, slug  215  is a single thermally conductive plate, such as a square copper plate. Preferably, slug  215  has the shape of two stacked plates. In one embodiment, slug  215  has the shape of a plurality of stacked square or rectangular plates, with each of the stacked plates having a perimeter that is less than the plate below. FIG. 3 shows slug  215  having the shape of two stacked square plates, where the top plate perimeter  303  is less than the bottom plate perimeter  306 . Slug  215  is not limited to square or rectangular plates. Plates in the shape of a polygon, such as triangular, octagonal, or hexagonal plates, or circular plates are also suitable for use in the fabrication of slug  215 . An advantage that flows from fabricating slug  215  as a plurality of stacked plates is that it is fabricated using simple and inexpensive manufacturing operations. In one embodiment, slug  215  is fabricated using a single punch operation. Alternatively, slug  215  is fabricated by an extrusion process. In still another embodiment, slug  215  is fabricated using a simple stamping operation. 
     In assembling chip-slug unit  203 , thermal interface material  212  is sandwiched between electronic chip  209  and slug  215  to thermally couple the heat flowing in electronic chip  209  to slug  215 . An advantage of the present invention is that an uncured material is suitable for use as thermal interface material  212 . Using an uncured material for thermal interface material  212  reduces the time and cost of manufacturing microelectronic package  200 . In one embodiment, thermal interface material  212  is a two-phase material. In an alternate embodiment, thermal interface material  212  is a two-component flexible epoxy adhesive. Thermal interface materials are manufactured in a variety of forms, which allows the designer of chip-slug unit  203  to tune the thermal interface between the electronic chip  209  and the slug  215 . For example, if a thermal tape does not provide a sufficient gap between electronic chip  209  and slug  215 , then a thermal gel may be used. In one embodiment, thermal interface material  212  is a thermal tape. In an alternate embodiment, thermal interface material  212  is a thermal mesh. In still another alternate embodiment, thermal interface material  212  is a thermal gel. 
     Packaging assembly  206 A comprises substrate  218  and encapsulated solder balls  221 , and packaging assembly  206 B comprises retention frame  224 , spring clip  227 , and retention frame attachment feature  230 . 
     Substrate  218  is not limited to a particular material. Any material that is suitable for use as a substrate in a microelectronic package is suitable for use in connection with the fabrication of substrate  218 . Preferably, the material selected for substrate  218  provides a solid structural base for mounting electronic chip  209 , provides a solid electrical base for fabricating conductive structures to route signals from electronic chip  209  through solder balls  221  to the pins of substrate  218 , and is chemically inert. In one embodiment, substrate  218  is fabricated from a ceramic. Alternatively, substrate  218  is fabricated from a plastic. Preferably, substrate  218  is fabricated from an organic material. The solder balls included in encapsulated solder balls  221  are fabricated from solder comprising tin or a tin alloy, and are encapsulated in an epoxy. 
     Retention frame  224  caps and protects substrate  218  and electronic chip  209  from physical damage. Retention frame  224  is preferably fabricated from a material that is inexpensive and easily shaped. In one embodiment, retention frame  224  is fabricated from a plastic. In an alternate embodiment, retention frame  224  is fabricated from steel or stainless steel. Retention frame  224  is shaped as a cap to cover and protect the top and sides of the assembly of chip-slug unit  203  and substrate  218 . In one embodiment, retention frame  224  has a top surface  232 , and top surface  232  has a center cut hole  233  for exposing slug  215  to the air outside retention frame  224  for the purpose of dissipating heat. In one embodiment, slug  215  is capable of partially extending through hole  233 . Any manufacturing process capable of shaping metal parts is suitable for fabricating retention frame  224 . In one embodiment, retention frame  224  is a machined part. Preferably, retention frame  224  is formed by a stamping operation. Retention frame attachment feature  230  is capable of attaching or coupling retention frame  224  to substrate  218 . In one embodiment, retention frame attachment feature  230  is an attachment mechanism, such as a screw, pin, or clip. 
     Spring clip  227  exerts a constant force on slug  215  when chip-slug unit  203  is assembled with packaging assembly  206 A and  206 B. By exerting a constant force on slug  215 , a constant thermal coupling is maintained between electronic chip  209  and slug  215 . In one embodiment, spring clip  227  is fabricated as an integral part of retention frame  224 . In an alternate embodiment, spring clip  227  is fabricated separately from retention frame  224  and attached to retention frame  224  in a separate manufacturing operation. Spring clip  227  is preferably fabricated from a material that is capable of maintaining a constant force after deformation. In one embodiment, spring clip  227  is fabricated from plastic. In an alternate embodiment, spring clip  227  is fabricated from steel. Preferably, spring clip  227  is fabricated from stainless steel. FIG. 4 is a perspective view of one embodiment of spring clip  227  having a u-shape. FIG. 5 is a perspective view of one embodiment of spring clip  503  fabricated as a single piece of stainless steel. 
     In the assembly of one embodiment of microelectronic circuit package  200 , encapsulated solder balls  221  are mounted on substrate  218 . Substrate  218  provides electrical coupling from encapsulated solder balls  221  to the pins of the substrate  218 . The electrical coupling is provided by fragile layers of conductors and insulators embedded in substrate  218 . An advantage of the present invention is that the assembly of microelectronic circuit package  200  is fabricated in a way that avoids damaging the fragile layers of conductors and insulators during heating and cooling cycles of electronic chip  209 . Chip-slug unit  203  is mounted on encapsulated solder balls  221 . Chip-slug unit  203  comprises electronic chip  209 , slug  215 , and thermal interface material  212  sandwiched between electronic chip  209  and slug  215 . Electronic chip  209  is electrically coupled to substrate  218  through encapsulated solder balls  221 . Electronic chip  209  is thermally coupled to slug  215  through thermal interface material  212 . Retention frame  224  fits over chip-slug unit  203 , and attaches to substrate  218 . Spring clip  227  is located on an inner surface of retention frame  224  and contacts chip-slug unit  203  along the perimeter of slug  215 . After the assembly of chip-slug unit  203  and packaging assembly  206 A and packaging assembly  206 B, retention frame  224  caps substrate  218 , and spring clip  227  exerts a constant force on slug  215  in order to maintain thermal coupling between electronic chip  209  and slug  215 . 
     FIG. 6 is a block diagram of one embodiment of computer system  600  including system board  603 , electronic chip package  606 , system bus  609 , and peripheral devices  612 . Electronic chip package  606  includes a substrate, a processor mounted on the substrate, a slug thermally coupled to the processor, and a retention frame including a spring clip. The spring clip is capable of exerting a force on the slug to maintain thermal coupling between the slug and the processor. Electronic chip package  606  is mounted on system board  603  and peripheral devices  612  are coupled to system board  603  through system bus  609 . Although specific embodiments have been illustrated and described herein, it will be appreciated by those of skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is intended that this invention be limited only by the claims and the equivalents thereof.