Abstract:
A clamping system decouples the clamping forces in an electrical circuit assembly coupled to a heatsink. A heatsink clamping assembly applies controllable and predictable force on the electrical circuit assembly including an integrated circuit device (“chip”). The applied force is controlled to effectively ensure intimate contact between the chip and the heatsink to facilitate efficient chip cooling. The force applied to the chip is decoupled from the much higher force required to clamp the electrical interposer interconnect structure between the electrical circuit assembly and the printed circuit board.

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. patent Ser. No. 09/618,980, filed on Jul. 19, 2000, and which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a system for attaching a heatsink to an integrated circuit device, and more particularly, to a system for clamping a heatsink to an integrated circuit device that applies controllable force onto the integrated circuit device. 
     2. Description of the Related Art 
     The use of integrated circuits is becoming more prevalent every day. Integrated circuits (ICs) are used in a multitude of different devices from household appliances to computer applications. However, ICs are also rather fragile. They are generally thin pieces of silicon on which circuits are constructed. These IC “chips” are subject to corrosion, environmental damage, physical shocks, and other damage mechanisms. For this reason, IC chips are packaged using a variety of different materials and package styles to protect them from possible damage during transportation and use. 
     Conventional protective packaging is generally a plastic or ceramic material used as a base for the IC chip and serving as a means of expanding (“fanning out”) the electrical connections of the IC chip. The connections between the IC chip and the package are typically accomplished using wire bonds or, in the case of “flip chip,” solder balls. In a “flip chip” arrangement, the top of the IC chip is flipped over face down onto the base package. Solder balls placed between the face of the chip and the base package provide electrical connections between the chip and the base package. Additionally, a lid may be attached over the IC to provide protection for the chip. The choice of protective packaging will be determined by various factors including the parameters of the chip itself, the IC application, and the packaging material cost. 
     In addition to providing chip protection, a component is also needed to allow the chip and package to make electrical connections to other devices. These connection components function as electrical components, with circuits that connect the chip to the Printed Circuit Board (PCB) or other device to which the chip is attached. 
     There are several different types of connection components allowing a chip and package to make electrical connections to a PCB. The selection of the appropriate electrical connection component will depend to a large extent upon the particular design of the chip itself, the number of connections required, and the size of the package. For example, for connections with a chip encased in a protective package that is no larger than 32 mm square (1.59 in 2 ) area, an array of solder balls (a ball grid array package or “BGA”) may be used to make the electrical connections. Similarly, for packages no larger than 42 mm square (2.73 in 2 ) area, an array of solder columns (a column grid array package or “CGA”) may be used to make the electrical connections. Both the ball and column grid arrays can be directly soldered to a PCB. 
     Both the ball and column grid array methods have a size limitation stemming from the long-term reliability of the connection. This limitation is mainly a function of the coefficient of thermal expansion (CTE) mismatch that typically exists between the IC package construction material and the PCB construction material. As a package gets larger the outermost connections from the center of the package may be stressed beyond their yield point as the IC chip heats the package. Therefore, for larger chips with a high lead count, or for more durable chip package systems, it is preferable to use an interposer component. An interposer provides an array of compressible contact points, such points being comprised typically of a polymer with imbedded electrically conductive metal or comprised of a metal spring structure. The required electrical connection is accomplished through contact pressure rather than a rigid bond. By being a compliant connection the interposer thus effectively decouples heat expansion stresses that can occur between the package and the PCB. However, when using an interposer to provide an electrical connection between a packaged IC and a PCB, considerable pressure must be applied to ensure a good low resistive electrical contact connection is achieved. 
     Certain IC chips also require the capability to dissipate a large amount of heat energy. For example, some high-powered chips may give off over 100 watts of heat energy. For such high-powered chips, the cooling provided by ambient air is not sufficient to prevent the chip from overheating. An additional component for removing the heat from the chip is required. A heatsink is commonly attached to an IC chip package, with a thermal interposer material in-between, in order to provide superior heat dissipation. The thermal interposer provides good thermal conductivity between the device and heatsink. In a conventional system, the heatsink may be attached, with a thermal interposer material in-between, to the package lid protecting the chip, or if no lid is used, directly to the chip itself. 
     The entire assembly consisting of the heatsink, the thermal interposer, the chip, the base package, and the electrical connection components, is clamped together to ensure proper electrical connections and heat transfer capabilities. Significant clamping force (often exceeding several hundreds of pounds) is required to clamp the chip-package-interposer-PCB arrangement tightly enough to ensure a proper electrical connection through the interposer. The heatsink-to-chip connection does not require a similar clamping force to provide heat transfer capability. However, because the entire assembly is clamped together when a conventional heat sink structure is used, all components are subjected to the same clamping force. This clamping force could damage the chip itself, even though a lid may be used, since the lid could compress into the underlying chip. 
     In the conventional packaged chip and heatsink arrangement where the chip is covered by a lid, the heatsink and thermal interposer do not contact the chip directly. Heat must pass through the air layer or other conductive layer between the chip and the lid, as well as through the lid itself, and the thermal interposer, before being dissipated by the heatsink. Such an arrangement makes it difficult to effectively cool certain high-powered chips. A preferable arrangement is to provide contact directly between the heatsink and the chip itself. This arrangement provides for superior heat transfer properties. However, without a lid present to absorb some of the compressive forces, extreme care must be taken to ensure that the chip is not crushed in this situation due to the clamping force required for the rest of the assembly. 
     Thus a high-powered chip packaging assembly that includes both a heatsink and an electrical interposer has two different, competing clamping force needs. Significant clamping force is required for achieving proper connections in the chip-package-electrical interposer-PCB portion of the assembly. However, significantly less clamping force is desirable between the chip-thermal interposer-heatsink assembly, to avoid damaging the chip. Thus, there is a need for a system that decouples the clamping forces between these two sections of the overall assembly. 
     SUMMARY OF THE INVENTIONS 
     The present invention provides a system that decouples the clamping force in an electrical circuit assembly coupled to a heatsink. A heatsink clamping assembly applies controllable and predictable force on the electrical circuit assembly including a packaged microelectronic integrated circuit device (“chip”). The applied force is controlled to effectively ensure intimate contact between the chip and the heatsink to facilitate efficient chip cooling. The force applied to the chip is decoupled from the much higher force required to clamp the interposer interconnection between the electrical circuit assembly and the printed circuit board. There are certain instances where the base of the heat sink may be a hollow chamber or heat pipe structure, and as such could be damaged if the full clamping force were required to be imposed. 
     In one embodiment, a heatsink clamping assembly comprises an electrical circuit assembly electrically connected to a printed circuit board (PCB). The electrical assembly includes an electrical circuit. A backing plate coupled to studs contacts the PCB, and the studs extend through apertures in the PCB. A clamp plate with a window contacts the edges of the electrical assembly while allowing the electrical circuit to pass through the window. The studs pass through the clamp plate, and a first pair of clamp nut assemblies clamps the electrical circuit assembly and PCB between the backing plate and the clamp plate. 
     A heatsink contacts a thermal interposer on top of the electrical circuit, and resides slightly above the clamp plate. The studs extend through the heatsink. A second pair of clamp nut assemblies connects the heatsink to the backing plate. The force used upon the first and second pairs of clamp nut assemblies may differ, thereby decoupling the two forces. 
     In one embodiment, the electrical circuit assembly comprises an electrical circuit, a package electrically coupled to the electrical circuit, and an electrical interposer electrically coupled to the package. The electrical interposer provides an electrical connection to the PCB. In one embodiment, the electrical circuit is an integrated circuit flip chip. 
     The features and advantages described in the specification are not all-inclusive, and particularly, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims hereof. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter, resort to the claims being necessary to determine such inventive subject matter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is an exploded three-dimensional view of the elements of a clamping assembly for an integrated circuit with a heatsink in an embodiment of the present invention. 
     FIG. 1B is an exploded three-dimensional view of the elements of a clamping assembly for an integrated circuit with a heats ink in another embodiment of the present invention. 
     FIG. 2 is a cut-away view of the layers of a clamping assembly for an integrated circuit with a heatsink in an embodiment of the present invention. 
     FIG. 3 is a top view of a heatsink and two vertical sliced views of the heatsink in an embodiment of the present invention. 
     The figures depict a preferred embodiment of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to several embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever practicable, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     FIG. 1A is an exploded view of the components in an embodiment of a clamping assembly  100 . Clamping assembly  100  includes an electrical circuit assembly  111 , a printed circuit board (“PCB”)  130 , a backing plate  140 , a clamp plate  150 , a thermal interposer  114 , and a heatsink  160 . Clamping assembly  100  has a first clamped group of components and a second clamped group of components, wherein the first and second groups may be clamped together using different amounts of force. 
     The first clamped group of components includes the electrical circuit assembly  111 , the PCB  130 , the backing plate  140  and the clamp plate  150 . Electrical assembly  111  includes an electrical circuit  110 ; however, no clamping force is applied to the electrical circuit  110  within the first clamped group of components. Backing plate  140  is connected to two studs  142 A and  142 B. PCB  130  includes two apertures  133 A and  133 B, dimensioned to allow the studs  142  to pass through. Clamp plate  150  includes two apertures  153 A and  153 B, dimensioned to allow the studs  142  to pass through. Clamp plate  150  also includes a clamp plate window  156 , which runs through the clamp plate  150 , and is dimensioned to fit around the electrical circuit  110  and prevent the electrical circuit  110  from being subjected to the clamping force applied to the first clamped group of components. A pair of clamp nut assemblies  152 A and  152 B clamp onto the studs  142  above the clamp plate  150 , and may be adjusted to provide the first clamped group of components with the desired clamping force. 
     The electrical circuit assembly  111  and the PCB  130  must be mated together tightly in order to assure a good electrical connection between their components. Typically, a force of approximately 60-80 grams per contact (0.13-0.18 pounds per contact) is required to assure proper electrical contact. This translates into a stud  142  clamping force of approximately 90-120 kilograms force (190-260 pounds force). Because the electrical circuit  110  passes through the clamp plate window  156 , this force is not applied to the electrical circuit  110 . 
     The second clamped group of components clamps the electrical circuit  110  to the heatsink  160 , with the thermal interposer  114  in-between. The heatsink  160  includes two apertures  163 A and  163 B, dimensioned to allow the studs  142  to pass through. A pair of clamp nut assemblies  162 A and  162 B clamps onto the studs  142  above the heatsink  160 , and may be adjusted to provide the second clamped group of components with the desired clamping force. 
     The clamp plate  150  is dimensioned to approximately match the thickness of the electrical circuit  110 . Thus, when the heatsink  160  is clamped on top of the clamp plate  150 , the heatsink  160  contacts both the clamp plate  150  and the electrical circuit  110 , with the thermal interposer  114  in-between, and provides a thermal conducting connection for removing heat from the electrical circuit  110 . Maintaining a proper thermal conducting connection requires significantly less clamping force than the electrical connection between the PCB  130  and the electrical circuit assembly  111 . A stud  142  clamping force of approximately 4.5-6.8 kilograms force (10-15 pounds force) is applied to the second clamping group to provide a good thermal conduction connection between the heatsink  160  and the electrical circuit  110 . 
     The backing plate  140  and the clamp plate  150  are composed of a rigid material. In one embodiment, the backing plate  140  and the clamp plate  150  are composed of steel. Heatsink  160  is composed of any highly thermally conductive material, for example, aluminum, copper, or even some impregnated polymer material. In some cases, a combination of materials are used, for example: aluminum for the heat sink fin structure and copper for a heat sink base or heat sink base heat pipe. The studs  142  and nut clamping assemblies  152  and  162  are composed of a rigid material capable of carrying a significant clamping force. The dimensions of the PCB  130 , backing plate  140 , clamp plate  150 , thermal interposer  114 , and heatsink  160  are dependent upon the size of the electrical circuit  110  and the electrical circuit assembly  111 . The PCB  130 , backing plate  140 , clamp plate  150  and heatsink  160  are all dimensioned to have a surface area large enough to accommodate apertures surrounding the electrical circuit assembly  111  to allow the studs  142  to pass through for clamping. The studs  142  are dimensioned to be sufficiently long enough to pass through the PCB  130 , electrical circuit assembly  111 , clamp plate  150 , thermal interposer  114 , and heatsink  160  and connect to the clamp nut assemblies  152  and  162 . 
     FIG. 1B is an exploded view of the components in another embodiment of a clamping assembly  102 . Clamping assembly  102  similarly contains a first and a second clamped group of components as described regarding clamping assembly  100 . However, in clamping assembly  102 , the electrical circuit is an integrated circuit (“IC”) chip  110 . The electrical circuit assembly  111  is a package  112  and an electrical interposer  120 . Clamping assembly  102  decouples the force applied to the first clamped group of components (package  112 , electrical interposer  120 , PCB  130 , backing plate  140 , and clamp plate  150 ) from the force applied to the second clamped group of components (IC chip  110 , chip thermal interposer  114 , and heatsink  160 ). 
     Package  112  holds and protects the IC chip  110  and provides electrical connections to the chip  110 . The electrical interposer  120  provides an electrical connection between the IC chip  110  connections (through the package  112 ) and the PCB  130 . Clamping assembly  102  further includes a chip thermal interposer  114 , which provides a thermal conducting layer between the chip  110  and the heatsink  160 . 
     Clamping assembly  102  is designed for the particular needs of chips that have the following characteristics: (1) they are high-wattage chips, typically greater than 100 watts, and (2) they have high electrical interconnect counts, requiring relatively large chips, typically 20 mm square (0.62 in 2 ) in area or larger. High-wattage chips typically generate too much heat to use a lidded package configuration, and it is preferable to attach a heatsink directly onto the back of the chip through a thermal interposer. Large chips require large packages, and as discussed previously, an electrical interposer  120  is more suitable than ball or column grid arrays for providing larger IC chip packages with electrical connections to a PCB. 
     Thus IC chip  110  is typically a high-wattage chip emitting approximately 100 watts or more, and is typically 20 mm square or larger in size. However, it will be evident to one of skill in the art that the clamping assembly of the present invention may be used with smaller IC chips of lower wattage. In order to further protect the IC chip  110 , and improve the heat transfer capabilities between the IC chip  110  and the heatsink  160 , clamping assembly  102  includes the chip thermal interposer  114 . Chip thermal interposer  114  may be a layer of a conductive material such as silicone grease or a thermal epoxy that coats the IC chip  110 . 
     The IC chip  110  is attached to the package  112 . The package  112  may be one of a number of different types of IC chip packages. In one embodiment, a “flip chip” type of IC chip  110  and package  112  is used. In a “flip chip,” the IC chip  110  is flipped over onto the package  112  so that the bond pads on the top of the IC chip  110  are directly above the top of the package  112 , which contains a footprint of the IC chip  110  electrical connections. The bond pads between the IC chip  110  and package  112  have solder balls bonded to them that form physical and electrical connections between the IC chip  110  leads and the package  112 . 
     In one embodiment, package  112  is composed of a ceramic material. In another embodiment, package  112  is composed of a plastic or laminate material. The size of the package  112  is determined by the size of the IC chip  110 . Package  112  is dimensioned to be larger in area than the IC chip  110 , allowing a portion of the package  112  to provide surface contact with the clamp plate  150  as the IC chip  110  passes through the clamp plate window  156 . In one embodiment including an IC chip of approximately 20 mm square, the package size is approximately 45 mm square (3.14 in 2 ) area or larger. It will be evident to one of skill in the art that a smaller package may be used if a smaller IC chip is used. 
     The electrical interposer  120  provides an electrical connection between the package  112  and the PCB  130 . The electrical interposer  120  consists of a sheet of carrier material that captures an array of compressible conductive buttons that have pieces of electrically conductive material embedded within them to connect leads on the package  112  to leads on the PCB  130 . In one embodiment, the membrane material is a polyamide film. In another embodiment, a ceramic or polymer frame contains an array of spring-like structures typically made of gold-plated or silver-plated beryllium, copper, molybdenum, or similar metals. The electrical interposer  120  is sized to be as large or slightly larger than the package  112 . Thus, for a package size of approximately 45 mm square, the electrical interposer size will be approximately 50 mm square. The dimensions of the PCB  130 , backing plate  140 , clamp plate  150  and heatsink  160  are dependent upon the size of the IC chip  110 , the package  112 , and the electrical interposer  120 . 
     In one embodiment, the heatsink  160  includes a heat pipe that thermodynamically connects to the IC chip  110  through the chip thermal interposer  114 . A heatsink  160  including a heat pipe typically provides superior heat transfer properties compared to the heatsink alone. A heat pipe is a block of a conductive metal, such as copper, with a hollow cavity inside. The cavity is airtight and maintained at a partial vacuum, and contains a fluid selected for its boiling temperature. Water is often selected for the fluid, as are various alcohols. Heat contacting one side of the heat pipe vaporizes the water within the partial vacuum, and the vapor transfers heat to the heat sink side of the heat pipe where it condenses. In this manner, heat is spread over the base of the heat sink very efficiently, as the heat pipe wicks heat away from the IC chip  110  and transfers it to the heatsink  160 . 
     A heat pipe is hollow and therefore somewhat fragile. However, because the clamping assembly  102  decouples the clamping force applied to the heatsink  160  from the clamping force applied to the other components, a low clamping force loading on the heat pipe may be achieved as necessary. 
     FIG. 2 is a cut-away view of the layers of another embodiment of a clamping assembly  104  for an electrical circuit with a heatsink. The heatsink of clamping assembly  104  includes a heat pipe  200 . The backing plate  140  and studs  142  are shown separated from the rest of the clamping assembly  104 . When clamping assembly  104  is fully assembled, the backing plate  140  contacts the PCB  130  and forms the bottom layer of the clamping assembly  104 . 
     Moving from the bottom to the top of the clamping assembly  104 , the following components comprise a first group of clamped components: the backing plate  140  contacts the PCB  130 ; the PCB  130  contacts the electrical interposer  120 ; and the electrical interposer  120  contacts the package  112 . The top surface of package  112  that is external to the chip also contacts the clamp plate  150 . The package  112  is attached to the IC chip  110 ; however, the IC chip  110  passes through the clamp plate window and therefore does not contact the clamp plate  150 . The IC chip  110  is therefore not included in the first clamped group of components. 
     The studs  142  are connected to the backing plate  140  and pass through apertures in the PCB  130  and clamp plate  150 . The first group of clamped components between the clamp plate  150  and the backing plate  140  are clamped together through a first clamping assembly that connects onto the pair of studs  142 A and  142 B. The first clamping assembly consists of a pair of clamping pressure transfer bushings  258 A and  258 B, a pair of clamp springs  256 A and  256 B, and a pair of clamp nuts  254 A and  254 B. 
     Moving from the top of the first clamped group of components to the top of the entire clamping assembly  104 , the following components comprise a second group of clamped components: the IC chip  110  contacts the chip thermal interposer  114  (for example, a thin layer of grease or silicon); the chip thermal interposer  114  contacts a heat pipe  200 ; the heat pipe  200  contacts a bonding material layer  205 ; and the bonding material layer  205  bonds the heat pipe  200  to a heatsink. The heatsink includes a heatsink bottom plate  266 , a set of heatsink fins  264 , and a heatsink top plate  262 . 
     The studs  142  pass through apertures in the heat pipe  200 , the heatsink bottom plate  266  and the heatsink top plate  262 . The second group of clamped components is clamped together through a second clamping assembly that connects onto the pair of studs  142 A and  142 B. The second clamping assembly consists of a pair of inserts  270 A and  270 B, a pair of spring washers  272 A and  272 B, and a pair of clamp nuts  274 A and  274 B. 
     FIG. 3 is a top view of a heatsink that requires 4 studs. FIG. 3 includes two different vertical sliced views through the heatsink, in an embodiment of the present invention. View  301  includes a heat pipe  200 , a heatsink bottom plate  266 , a set of heatsink fins  264 , and a heatsink top plate  262 . View  302  is a vertical slice through the heatsink along the apertures  163 A and  163 B, illustrating the apertures  163  in the heatsink top plate  262 , the heatsink bottom plate  266 , and the heat pipe  200  that allow the studs to pass through the heatsink. View  302  also illustrates that the heat sink top plate  262  includes a set of recessed cavities  263 A and  263 B around each aperture  163 , allowing the heat sink clamping assemblies to be recessed into the heatsink itself. 
     In FIG. 3, the heatsink is shown to include four apertures ( 163 A, B, C and D) dimensioned to allow a set of four studs to pass through the heatsink for use in clamping together the clamping assembly. It will be evident to one of skill in the art that the clamping assembly may be designed to have different numbers of studs used for clamping. For example, in one embodiment, there is a pair of two studs. In another embodiment, a set of four studs is used. The number of apertures through the heatsink, PCB and clamp plate component of the clamping assembly corresponds to the number of studs used. 
     In one embodiment, the clamping assembly includes several different IC chips. In this embodiment, a single large heatsink removes heat from several different IC chips. The clamp plate contains several windows corresponding to the number of IC chips, or one large window to encompass all the chips, or any combination of windows between, dimensioned to allow an IC chip to pass through and contact the heatsink directly. The clamping assembly components are dimensioned to accommodate the area of several IC chips as well as the apertures used to allow the studs to pass through the clamping assembly. 
     Although the invention has been described in considerable detail with reference to certain embodiments, other embodiments are possible. As will be understood by those of skill in the art, the invention may be embodied in other specific forms without departing from the essential characteristics thereof For example, different types of electrical circuit assemblies may be used within the invention. Additionally, a heat pipe may be included in the heatsink assembly. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variations as fall within the spirit and scope of the appended claims and equivalents.