Abstract:
A system and method of attaching a heat sink to an integrated circuit chip includes providing a compliant material for constraining the heat sink&#39;s mechanical motion while simultaneously allowing for thermal expansion of the heat sink.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of, and claims priority from, commonly-owned, co-pending U.S. patent application Ser. No. 11/761,234, filed on Jun. 11, 2007. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED-RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
     Not Applicable. 
     FIELD OF THE INVENTION 
     The invention disclosed broadly relates to the field of cooling of semiconductor chips, and more particularly relates to attachment of heat sinks to semiconductor chips. 
     BACKGROUND OF THE INVENTION 
     The performance of integrated electronics chips has increased dramatically over recent years. This increased performance has been achieved in part by increasing the chip operating frequency which has resulted in greater chip power (Watts) and chip power density (Watts/cm2). This has increased the need for efficient thermal power management to conduct the heat away from the chip to the ambient surroundings using for example heat sinks, fans, vapor chambers, liquid coolers and other means to cool the chips to maintain an acceptable operating temperature. Today&#39;s powerful processors generate so much heat that chips will thermally overheat if the thermal cooling solution is not operational even for a short period of time. A heatsink is a device that is attached to the microprocessor chip to keep it from overheating by providing a thermal conduction path of the heat generated by the chip to the ambient environment by moving air over the heat sink. Basic heat sink structures have a heat spreader which makes thermal contact with the chip via an interface of a thermally conductive adhesive and fins which provide a large surface area to transfer the heat to the ambient air environment. Typically a fan is used to provide an air flow over the fins to optimize the heat transfer from the heat sink to the ambient air. 
     Most commercially available computers incorporate a heat sink directly attached to the chip. This combination of the chip and heat sink is often referred to as a “chip package.” The basic design of a chip package is shown in  FIG. 1  in which a heatsink  102  is mounted on a chip  120 . The heatsink  102  shown is a conventional passive metal heat sink with fins. The chip  120  makes thermal contact with the heat sink  102  through a thermal interface material  111 . The chip  120  is attached to a chip carrier  122  which has a pin grid array and interfaces to an electrical socket  110  which is mounted onto a printed circuit board  125 . The heat sink  102  is secured to the chip  120  by a frame  112  and mounting screws  116  in order to inhibit horizontal and vertical movement of the heat sink as would occur under external forces, including shock and vibration of the system.  FIG. 2  shows the top view of the chip package of  FIG. 1 . 
     Clearly this design is meant to stabilize and constrain the heatsink  102  and it is effective in doing so. The problem inherent in this design, however, is that the rigid assembly results in deformation of the entire package due to differences in the coefficient of thermal expansion (CTE) between the heatsink  102  and the chip package assembly. The need to constrain the mechanical motion of the heat sink  102  due to external forces (shocks) requires a rigid, non-compliant attachment which unfortunately results in package deformation. Contributing to this problem is the rigidity and non-compliance inherent in heatsinks, which are usually metal structures. Currently produced heatsinks fail to provide for the structural stresses and strains generated during the operation of the electronic device (the chip  120 ). Therefore, there is a need for a solution that overcomes the above shortcomings of the prior art. 
     SUMMARY OF THE INVENTION 
     Briefly, according to an embodiment of the invention, a system and method of attaching a heat sink to an integrated circuit chip includes providing a compliant material for constraining the heat sink mechanical motion while simultaneously allowing for thermal expansion of the heat sink; and providing at least one mechanical limit stop disposed between the heat sink and a frame. Additionally, the invention provides for placing the compliant material between the heat sink and the at least one mechanical limit stop. Further horizontal constraint pads are positioned between the heat sink and the at least one mechanical limit stop. Vertical constraint pads can be positioned between the heat sink and the at least one mechanical limit stop. 
     According to another embodiment of the present invention, a structure for attaching a heat sink to an integrated circuit chip includes a set of ball bearings positioned to allow motion of the heat sink in the X and Y directions while constraining motion in the Z direction. The ball bearings are attached using braces with each ball bearing positioned at the corner sidewalls of the heat sink such that force applied to the ball bearing from the heat sink will prevent mechanical movement of the heat sink in a vertical direction. 
     According to another embodiment of the present invention, a structure for attaching a heat sink to an integrated circuit chip includes a servo control system. The servo control system includes a voice coil motor to actuate the heat sink. Further, at least one gap sensor creates a position signal between the heat sink and a fixed frame. 
     According to another embodiment of the present invention, an attachment structure for attaching a heat sink to an integrated circuit chip includes: a platform for the heat sink; a plurality of horizontal limit stops including compliant material for constraining mechanical motion of the heat sink while allowing for thermal expansion of the heat sink in a chip package, wherein each horizontal limit stop is positioned on the platform such that the compliant material makes contact with the heat sink and the chip; and a plurality of vertical limit stops including compliant material, wherein each vertical limit stop is positioned on the platform such that the compliant material makes contact with a bottom surface of the heat sink and the chip. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To describe the foregoing and other exemplary purposes, aspects, and advantages of the present invention, we use the following detailed description of exemplary embodiments of the invention with reference to the drawings, in which: 
         FIG. 1  is an illustration of a cross-section view of a basic chip package design with a passive heatsink, according to the known art; 
         FIG. 2  is an illustration showing the top view of the basic chip package of  FIG. 1 , according to the known art; 
         FIG. 3   a  is a side view of a chip package assembly according to an embodiment of the present invention; 
         FIG. 3   b  is a top view of the chip package assembly according to an embodiment of the present invention; 
         FIG. 3C  is a detailed view of the corner of the chip package assembly according to an embodiment of the present invention; 
         FIG. 4  is a 3D view of the chip package assembly according to an embodiment of the present invention; 
         FIG. 5   a  is a side view of an illustration of a chip package assembly with ball bearings, according to an embodiment of the present invention; 
         FIG. 5   b  is a top view of an illustration of a chip package assembly with ball bearings, according to an embodiment of the present invention; 
         FIG. 6  is a close-up cut-away view of one of the ball bearings of  FIG. 5 , according to an embodiment of the present invention; 
         FIG. 7  is a side view of a chip package assembly using non-contact voice coil motors, according to an embodiment of the present invention; 
         FIG. 8  is a top view of the assembly of  FIG. 7 , according to an embodiment of the present invention; 
         FIG. 9  is an exploded view of a voice coil motor, according to an embodiment of the present invention; and 
         FIG. 10  shows a diagram of the servo control system of  FIG. 7 , according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     We describe an attachment method for a heat sink, according to an embodiment of the present invention. As will be shown, the present embodiment changes the mechanical boundary conditions of the heat sink to allow slowly varying relative motion while still providing mechanical support for shock inputs. This is accomplished by changing the method of heat sink attachment, such that mechanical motion is limited under shock but provides compliance for thermal expansion. As such this will reduce the heat sink/package mechanical interaction due to the mismatch of the coefficients of thermal expansion (CTE) for those materials. A CTE mismatch occurs when the heat sink material experiences thermal expansion at a different rate than that of the frame. This is one of the main causes of package deformation. 
     Referring now in specific detail to the drawings, and particularly  FIG. 3   a  there is shown a side view of a chip package assembly  300  with attached heat sink  302 . According to an embodiment of the present invention, pads  312  and  314  are fabricated from a highly damped elastomeric material such as those commercially available as C-1105 from EAR Specialty Composites. These materials are also viscoelastic in that they exhibit both the properties of a viscous liquid which “flows” at slow deformation speeds and an elastic solid at higher speeds. These materials have a frequency dependent elastic modulus which increases at higher frequencies, thus becoming stiffer if the load changes quickly. 
       FIG. 3   b  shows a top view of the chip package assembly  300 . The elastomeric material is used in the X and Y pads  314  and  318  at the corner mounts of the heat sink  302  to control motion of the heat sink  302  in the X and Y directions as well as the Z pads  312  at the bottom side of the heat sink  302  corners to control motion in the Z direction. 
     As shown in  FIG. 3   b  the X and Y pads  314  and  318  are disposed at the corner mounts, positioned between the heat sink  302  and the horizontal limit stops  316 . Pads  312  are also positioned between the heat sink  302  and the vertical stops  308 . The viscoelastic material is sufficiently rigid that it limits mechanical motion in the presence of shocks; yet it provides compliance sufficient to handle the thermal expansion mismatch of the heat sink/package  300 . The positioning of the pads will reduce the effects of shock from X, Y, and Z forces exerted on the heat sink  302 . Positioning the pads  314  at the bottom only will limit the effects from a Z force shock only. 
     The key advantages of employing the pads  312 ,  314 ,  318  at the corner mounts and the bottom of the heat sink  302  are: 1) they allow mechanical motion from thermal expansion; and 2) they restrict mechanical motion due to shock. The key aspects of the pads are the viscoelastic properties of the material used and the positioning of the pads with respect to the heat sink  302 . 
       FIG. 3   c  presents a detailed view of one corner of the chip assembly package  300  of  FIGS. 3   a  and  3   b . This view shows the corner of the heat sink  302  which is abutted by pads  314  and  318  which may be, for example, attached to the limit stop  316  and the heat sink  302  with an adhesive glue. When a force F 2  is applied to the heat sink  302 , pad  318  is compressed. However, as the elastic modulus of the pad  318  is frequency dependent, the restoring force would depend upon the frequency of the applied force. For slowly varying forces such as would occur with thermal expansion, pad  318  would be soft, but for higher frequency forces the pad  318  would be very stiff. This allows the heat sink  302  to expand due to temperature changes, but provides constraint of the heat sink  302  for high frequency forces. Note that pad  314  experiences a shear force during the applied force F 2  and allows movement of the heat sink  302  both for thermal and high frequency forces. 
     For a force F ( 320 ) in the X, Y plane the pad  314  would experience a force F 1 =F cos(θ) and pad  318  would experience a force F 2 =F sin(θ). Each pad would respond as described above. 
     As the package may experience a force in any arbitrary direction, the heat sink  302  can experience a force which has components in the X, Y and Z planes. As shown in the three-dimensional (3D) view of  FIG. 4 , the pads in the Z direction will compress when a force has a downward Z component. The clamp  304  holds the center of the heat sink  302  in the Z direction and applies a downward bias force on the pads  312  which prevents the heat sink from lifting off the chip  320  when there is an upward Z component. To minimize the deflection of the pad  312  to the bias force a higher modulus elastomer may be deployed or the pad thickness may be reduced. In one example the dimension of the pads may measure 5 mm by 5 mm and have a thickness of 1 mm. 
     Another embodiment is shown in  FIG. 5   a  in which ball bearings  504  allow the heat sink  502  to move in a horizontal direction while limiting motion in the vertical direction.  FIG. 5   b  illustrates how the horizontal motion is impeded by pads  514  secured to horizontal stops  516 . The pads  514  are viscoelastic as shown in  FIG. 4 . The ball bearings  504  are secured by braces  506  attached to the horizontal stops  516 . Note that these bearings  504  are only at the bottom, not the sides. 
       FIG. 6  shows a close-up view of one of the ball bearings  504 . The arrows encircling the ball bearing  504  indicate how the ball bearing  504  can rotate, or spin, while remaining in a fixed position. The heat sink  502  is in contact with the top portion of the ball bearing  504 . A slight horizontal motion of the heat sink  502  will produce a swiveling of the ball bearing  504 . The horizontal stops  516  with the pads  514  attached will constrain the heat sink  502  from excessive movement. 
     It should be understood that what has been discussed and illustrated serves to provide examples of the possible embodiments within the spirit and scope of the invention; they should not be construed to limit the invention. One with knowledge in the art, after following the discussion and diagrams herein, can employ any viscoelastic material having the same properties as C-1105 bearings from EAR, or flexures properly positioned at the corner mounts as discussed above to provide the advantages of a reduction in package deformation while allowing for limited mechanical motion due to thermal expansion. 
     Another approach to limit mechanical motion in the presence of shocks and/or vibrations while allowing for slow thermal expansion is to deploy active servo control of the heat sink. H. Newton, Newton&#39;s Telecom Dictionary, 22 nd  Edition, Copyright© 2006 Harry Newton, defines a servo as: “Servo: short for servomechanism. Devices which constantly detect a variable, and adjust a mechanism to response to changes.” 
     Another embodiment of the present invention is shown in  FIG. 7  wherein active servo control is employed to constrain the movement and/or expansion of a heat sink  702 . Voice coil motors are used to actuate the heat sink  702 .  FIG. 7  shows one example of a voice coil motor  728  which controls the X motion of one corner of the heat sink  702 . Each voice coil motor includes: a voice coil  726  mounted onto the heat sink  702  and a magnetic circuit consisting of permanently affixed magnets  720  and  722 , with flux return paths and mechanical assembly to hold the magnets in place  724 . The servo method of heat sink constraint differs from the previously described embodiments in that there may be no actual contact made between the heat sink  702  and the board  744 . This is indicated in  FIG. 7  by the gaps  799 . 
       FIG. 8  shows a top view of the assembly of  FIG. 7  with Z direction voice coil motors  710  and  712 .  FIG. 8  also shows the voice coils for the X and Y directions,  724 ,  726  and  734  and  736 , in opposite corners, which are part of the voice coil motor assembly. For example  726  is the voice coil for voice coil motor  728  as shown in  FIG. 7 . 
     Gap sensors  735 ,  737 ,  725 ,  727  measure the location of the heat sink  702  edge to a fixed frame in the X and Y directions. Similarly, gap sensors  704  and  706  measure the location of the heat sink  702  to the frame  744  in the Z direction. One example of gap sensors may include proximity sensors using well known capacitance or eddy current measurement methods. The capacitance between two plates is proportional to 1/d, where d is the gap between the plates, thereby the gap can be measured by measuring C and computing 1/C. The voice coil motor and gap sensors are used in a servo loop to control the location of the heat sink  702  relative to the frame  744 . 
     As shown in  FIG. 8  two vertical axis voice coil motors  710  and  712  are disposed in opposite corners of the top frame  744  to maintain the Z height of the heat sink  702  relative to the frame  744 . For example, a Z position signal Z gap  704  is compared to a Z gap target and the difference between the Z gap target and Z gap  704  will create an error signal as shown in  FIG. 10  which is input to the servo controller Gc which produces a signal to control the current to the physical plant Gp which includes Z voice coil motors  710  and heat sink  702 . The current applied to Z voice coil motor  710  will produce a force on the heat sink  702  to actuate it in the +Z or −Z direction until the Z gap value is equal to the target value. Similarly a second servo loop using Z gap  706  would be running in parallel, which for example may have a Z gap target  706  equal to the Z gap  704  target  704 , to maintain the heat sink  702  parallel to the frame  744 . 
     To maintain the X and Y position of the heat sink  702 , horizontal axis voice coils  724 ,  726  are deployed in one corner of the heat sink  702  and voice coils  734  and  736  are deployed in the opposite corner of the heat sink  702 . These voice coils are part of a voice coil motor assembly, an example of which is shown in  FIG. 7  as  728 . A position signal from the difference of Gap X=Xgap  735 −Xgap  725  can be generated by measuring the gap in the X direction using Xgap sensors  735  and  725  and taking the difference between the two signals. 
     Similarly, by monitoring the gap in the Y direction using Y gap sensors  737  and  727  a position signal can be generated from the difference of Gap Y=Ygap  737 −Ygap  727 . These signals are input to the servo control system as shown in  FIG. 10 . For example, GapX would be compared to a GapX target, which for example may have a value of zero such as would occur when Xgap  735  is equal to X gap  725  and the heat sink  702  is centered with respect to the center of the frame  744 . 
     The difference between the GapX and Gap X target will create an error signal as shown in  FIG. 10  which is input to the servo controller, Gc, which produces a signal to control the current to Gp, the physical plant, which includes the voice coil motor and heat sink  702 . The current applied to the voice coils  726 ,  736  to produces a force on the heat sink  702  to actuate it in the +X or −X direction until the GapX value is equal to the Gap X target value. 
     Referring to  FIG. 9  there is shown an exploded top view of voice coil motor (VCM)  728  located in the right quadrant of  FIG. 8 . This VCM produces a motion of the heat sink  702  in the X direction when a current is applied to the voice coil  726 . The VCM is comprised of permanent magnets  720  and  722 , each of which is made of two magnets with reverse polarity. The magnets  720  and  722  and flux return plates  721 ,  723  are held in place by a non-magnetic mechanical fixture  724 . When a current passes through the coil  726 , the coil experiences a force in the +X or −X direction dependent on the direction of the current and transfers that force to the heat sink. Similarly a current passing through voice coil  736  applies a force in the X direction on the opposite corner of the heat sink  702 . 
     The coils  726  and  736  are attached to the heat sink  702  and using the servo control system the heat sink  702  will remain centered with respect to the frame  744  in the X direction as previously described while allowing thermal expansion of the heat sink  702 . Similarly, when using the servo control system with voice coils  724  and  734 , the same control of the heat sink  702  in the Y direction can be achieved. In the Z direction, the gap  799  between the heat sink  702  and the frame  744  will be held to a predetermined target value, such that the heat sink  702  remains parallel to the frame  744 . 
     Therefore, while there have been described what are presently considered to be the preferred embodiments, it will be understood by those skilled in the art that other modifications can be made within the spirit of the invention. Solutions which combine elements of the described solutions including using mechanical and servo control systems are also possible.