Abstract:
A method and apparatus is provided for attaching a cooling structure to the surface of an integrated circuit (IC). The attachment of the cooling structure, for example a heat sink, to the IC requires that certain pressure is applied, usually by connecting the cooling structure to a Printed Circuit Board (PCB). However, excess pressure may damage the ball grid array (BGA) that connects the IC to the PCB. Attachment of a cooling structure to the IC package substrate is provided without support from the PCB. In one embodiment, shock absorbers are also attached to the cooling structure and the PCB to prevent undesirable vibration of the heat sink mass from affecting the IC.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims priority from U.S. provisional patent application Ser. No. 60/663,225, filed on Mar. 21, 2005, the entirety of which is incorporated herein by this reference thereto. 

   BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The invention relates to integrated circuits. More particularly, the invention relates to a method and apparatus for attaching a cooling structure to an integrated circuit. 
   2. Description of Prior Art 
   Heat-sink-attachment and thermal-interface-design options are based, on performance considerations, in addition to cost effectiveness. These performance considerations include functional considerations such as thermal properties, and reliability considerations such as mechanical and environmental properties. In addition, the ease and cost of assembly and disassembly of the heat sink (HS) attachment structure are important. 
   A typical state of the art attachment of a HS or a heat spreader to the backside of a high-power chip physically interconnects the HS structure to the system&#39;s substrate structure, namely the printed circuit board (PCB). Such a design approach is used when there is a need to produce a high enough pressure at the thermal interface between the HS and the chip. Such pressure is often needed for a satisfactory thermal performance of the interface. 
   Also known in the art are attachments of an HS to a package substrate (PS), rather than to the PCB. Designs of this type are not intended and, in many cases, are not even supposed to produce high pressure at the HS/chip interface. Such HS-to-package attachment designs in the current art are acceptable if a relatively low pressure, for example a pressure in the range of 5-20 psi, can ensure a satisfactory thermal management of the integrated circuit (IC) device. 
   However, it is becoming a more frequent requirement in the industry that a high interfacial pressure of, for example in the range of 70 psi or higher, is needed to produce and control a satisfactory thermal contact. This occurs for example, in the case where a carbon nano-tube (CNT) based HS is used and the tips of the CNTs require high pressure to bend sufficiently and provide the necessary thermal contact. Such a HS is discussed in U.S. patent application Ser. No. 10/925,824, System and Method Using Self-Assembled Nano Structures in the Design and Fabrication of an Integrated Circuit Micro-Cooler, assigned to common assignee and which is herein incorporated by reference thereto for all that it contains. In such case the HS is typically attached to the PCB. This is usually done by using screw-based elements, with or without springs, or flat-spring-based structural elements. This approach can produce a very high pressure at the HS/chip interface providing the necessary pressure to achieve the thermal interface required. However, this approach does suffer from the shortcomings that are discussed below. 
   As shown in  FIGS. 1   a  and  1   b  chip  110 , connected in a flip-chip (FC) position, is soldered to a package substrate  140  through solder joints  120 . A metal frame  130  mounted on top of the substrate  140  further secures the chip  110 . The objective of such a reinforcement is to increase the flexural rigidity of the substrate  140  so that it does not bend as a result of the elevated temperature, typically is the range of 220° C. to 280° C. or so, applied to the system during the reflow soldering process. As is known, such a process is an essential part of the technology that is currently used to surface-mount IC packages on PCBs. The solder joints  120  are coupled to ball grid array (BGA) solder joints  150  through the substrate  140 , the BGA solder joints  150  being soldered to the PCB  160 . The BGA material is typically the most vulnerable part of the BGA package structure. Using prior art solutions, when a screw-based design is used to mount an HS on a package that is further attached to the PCB  160 , it is not only the thermal interface that experiences elevated pressure, but also the BGA solder joints  150 . This circumstance, favorable from the standpoint of the thermal performance of the device, can have a detrimental effect on the reliability of the BGA solder material, both on a short-term basis because of the excessive static overload, and on a long-term basis because of the significant mechanical loading added. During the system manufacturing, for example, during the reflow soldering process, and operation, for example, power cycling conditions, such a mechanical loading superimposes the thermally induced loading. The thermally induced loading is caused by the change in temperature in the structure in question which is fabricated of dissimilar materials. In addition, temperature gradients are experienced. A significant mechanical pre-stressing may worsen the mechanical performance. That is, the adhesive and/or cohesive strength of the BGA solder material is affected. In some cases, the elevated tensile forces that are applied to the PCB  160  as a result of the mounting of a HS to it can lead to local mechanical, also referred to as physical, damage of the PCB  160  and can result in electrical opens and/or shorts. 
   Therefore, due to the limitations of prior art solutions, it would be advantageous to provide an HS mounting structure, such that the BGA solder joints and the PCB do not experience the high pressure applied at the thermal HS/chip interface. 
   SUMMARY OF THE INVENTION 
   A method and apparatus is provided for attaching a cooling structure to the surface of an integrated circuit (IC). The attachment of the cooling structure, for example a heat sink, to the IC requires that certain pressure is applied, usually by connecting the cooling structure to a Printed Circuit Board (PCB). However, excess pressure may damage the ball grid array (BGA) that connects the IC to the PCB. Attachment of a cooling structure to the IC package substrate is provided without support from the PCB. In one embodiment, shock absorbers are also attached to the cooling structure and the PCB to prevent undesirable vibration of the heat sink mass from affecting the IC. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plan view ( FIG. 1   a ) and a section view of ( FIG. 1   b ) of a chip mounted to a PCB (prior art); 
       FIG. 2  is a plan view ( FIG. 2   a ) and a section view ( FIG. 2   b ) of a first heat sink mounting platform in accordance with the disclosed invention; 
       FIG. 3  is a plan view ( FIG. 3   a ) and a section view ( FIG. 3   b ) of a second heat sink mounting platform in accordance with the disclosed invention; and 
       FIG. 4  is a plan view ( FIG. 4   a ) and a section view ( FIG. 4   b ) of a third heat sink mounting platform having shock absorbers. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The disclosed invention addresses the limitations of the prior art by excluding the ball grid array (BGA) solder joints, for example the BGA solder joints  150  shown in  FIG. 1 , from exposure to a high axial pressure. Regardless of the actual mounting pressure of a heat sink (HS) to the back side of a chip, for example chip  110 , this pressure is not transmitted to the BGA solder joints  150 . Specifically, there is disclosed a structural approach that enables the direct attachment of an HS to the package substrate (PS) of a chip, for example PS  140 , by using a robust mounting platform (MP). In accordance with an embodiment of the disclosed invention, the MP comprises a piece of hardware that can be two- or three-dimensional and that can be any of several very different designs. A three-dimensional version can be used, for example, for a juxtaposed/multilevel package design. The MP provides sufficient real estate and structural strength so that an HS, or a heat-spreader, can be attached to it. The MP is attached to the PS  140  along PS  140  edges, as disclosed below. Such an attachment is facilitated because the PS is typically reinforced by a metal frame, for example the metal frame  130 . The metal frame  130  provides a structural foundation for the successful mounting of the MP. However, if an IC package does not have a metal frame or a PS reinforcement, but the PS is still robust enough to provide a reliable support for the MP, the disclosed invention may be equally applicable. 
   A typical total thickness of the PS and the metal frame mounted on top of PS is about 2.6 millimeters. This thickness is sufficient to fasten the MP reliably. 
     FIG. 2   a  and  2   b  provide plan ( FIG. 2   a ) and section ( FIG. 2   b ) views where and exemplary and non-limiting schematic  200  of a first heat sink mounting platform (MP)  210  that is screw-based. The MP  210  is a frame that encompasses an IC from its sides. On at least two opposite sides of the MP  210  there are fastening screws  220  designed to fasten the MP  210  to the PS  140  and/or the metal frame  130 . Fastening screws  220  may directly connect to the PS  140  and/or the metal frame  130 . However, springs such as spring element  230 , may be used as well as other elements to enable such attachment. In other embodiments of the disclosed invention, a flat spring (not shown), clamp (not shown), clip (not shown) or other fastening element may be used to secure the attachment of the MP  210  to the PS  140  and/or metal frame  130 . The fastening elements, regardless of type or structure, withstand a very high in-plane load before buckling or otherwise failing. The MP  210  provides ample real-estate to support an HS (not shown) and to apply the desired pressure to the back of the chip  110 , without the pressure being transferred to the BGA solder joints  150 . 
   Similarly, a flat-spring-based attachment (FSA)  300  is shown in  FIGS. 3   a  and  3   b , the principle of operation is similar to those discussed with respect to  FIGS. 2   a  and  2   b  and therefore, not repeated here. An MP attached directly to a PS, is further capable of relieving the BGA solder joints. For example, the BGA solder joints  150 , and/or the PCB are relieved from excess loading. In this embodiment, the fastening screws may be equipped with a torque limiter that limits the amount of pressure a fastening screw applies to a spring, or directly to the PS and/or metal frame. 
   The curvilinear end-spring-elements (ESEs) elongated in the direction of the package edge, are attached to the screws at their tips, for example as shown in  FIG. 2 , provide good mechanical contact on the surface of the elements. These contacts can be reinforced, if necessary, by thin strips of a soft metal or by a metal type Velcro®, to maximize the friction at the interface between the ESEs and the PS, and/or the metal frame. The adhesion forces are due to the reaction of the stiff and robust frame structure of the MP to the elongation of the screws during MP mounting and the resulting deformation of the ESEs. 
   In the embodiment shown in  FIGS. 3   a  and  3   b , the reaction force is provided initially by the curved springs  330 . The springs may be manufactured from initially flat spring-metal strips. The required spring design is predetermined by an appropriate calculation, for example by predictive modeling, aimed at the evaluation of the forces that such a spring imposes on the structure after it is deployed during the mounting process. The edges of the flat springs can be preliminarily bonded, or otherwise attached, to the MP, and released during the mounting process, after the frame is put onto the package structure. For instance, a low temperature melting solder, for example Indium based, can be used to bond the ends of the flat springs to the MP frame, and can then be released by heating up the system after the FSP frame is put onto the package frame. Another option is to use strings that are cut off after the FSP frame is put onto the package frame. 
     FIGS. 4   a  and  4   b  provide plan ( FIG. 4   a ) and section ( FIG. 4   b ) views  400  of a third heat sink mounting platform having a shock absorber. The HS (not shown) is a mass placed over the chip and may be susceptible to the impact of a variety of forces. These forces may result in vibrations amplified by the HS mass and may potentially cause damage to the BGA solder joints  150 . Therefore, in one embodiment of the disclosed invention the shock absorbers  432 , having for example springs  434 , mount the MP  410  to the PCB  160 . The MP structures disclosed in  FIG. 2  or  3  may be used to affix the MP to the PS or the metal frame. The MP  410  is modified by having the necessary hooks, for example protruding holes  440  to enable the attachment of the shock absorber  432 . Vibrations are restrained through the operation of the shock absorber mechanism thereby preventing damage to BGA solder joints  150 . 
   The invention disclosed herein provide various advantages over prior art solutions, that include, but are not limited to the examples set forth below. A successful attachment of the HS to the PS and/or the metal frame can be achieved without any change in the existing package structure. The ability to mount, remove, and replace the HS without damaging the package, the HS itself, or the PCB are important considerations in HS design and mounting technology, and these considerations are addressed in the invention disclosed herein. The MP is a mechanical attachment and hence is easy to install, repair, and use. Furthermore, no epoxies or other chemicals are used which shortens the production time and overcomes ergonomic problems. The thermal performance achieved using the disclosed MP is superior to prior art solution because significant pressure can be applied to mount the HS on top the chip  110  without harming the integrity of the BGA solder joints. Furthermore, the disclosed invention can be used regardless of whether an additional interface material, for example, thermal grease or phase changing material, is or is not used for improved thermal performance of the HS-to-chip interface. 
   Although the invention is described herein with reference to preferred embodiments, one skilled in the art will readily appreciate that other applications may be substituted for those set forth herein without departing from the spirit and scope of the present invention. Specifically, the particular mounting of the MP to the PS can be achieved by using a screw-based approach/platform (SBP) and a flat-spring-based (FSP) approach, Velcro-type attachment, and others, all to be considered to be within the framework of the disclosed invention. Furthermore, while fastening screws are shown to be opposite each other, embodiments designed to provide a balanced force holding the MP in place are also envisioned and are specifically included within the spirit of the disclosed invention. 
   Accordingly, the invention should only be limited by the Claims included below.