Abstract:
The present invention relates to a method and apparatus that minimizes shock/vibrational motion in interposer sockets. The ability to control shock/vibration can ensure successful operation and substantially increase socket lifetime. The present invention discloses a device for maintaining a heat sink in a desired relationship to a mounting base while limiting the transmission of shock and vibrational motion to and from the heat sink includes a fastener extending from the mounting base, a spring compressed between the fastener and the heat sink, and a damper compressed between the fastener and the sink wherein the fastener maintains the spring and the damper in a compressed state such that the spring and the damper bear on said heat sink.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    The subject matter of the present application is related to the subject matter of co-pending application entitled “Dynamic Isolating Mount for Processor Packages,” incorporated herein by reference.  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not applicable.  
         BACKGROUND OF THE INVENTION  
         [0003]    1. Field of the Invention  
           [0004]    The present invention generally relates to the use of vibration isolation in mounting an electrical component on a circuit board. More particularly, the present invention relates to a tunable vibration damper for a microprocessor package.  
           [0005]    2. Background of the Invention  
           [0006]    For many decades, circuit boards, such as those used in computers, have been manufactured by attaching electrical components to the board. In some cases, the components are soldered directly to the board. Although generally satisfactory, soldering a component directly to the board makes it difficult and costly to change that component should it be desired or necessary to replace one component with another. A microprocessor, for example, may have hundreds of connections that, should the processor fail, must be desoldered. A new processor, with its hundred of connections must then be attached to the board. Further, this process must occur without damaging the other components mounted on the circuit board. Even if the processor has not failed, it still might be desired to replace it, for example, a new and improved version of the processor is made available.  
           [0007]    For these and other reasons, “interposer” sockets have become available. Although defined in various ways, an interposer socket is a socket to which a chip (i.e., a microprocessor) is mated. The socket is then mated to the circuit board or to a socket soldered to the circuit board. Advantageously, an interposer docket does not require solder either to be mated to the board (or other socket) or to the electrical component mounted on it. Instead, a lever or other mechanism is engaged to hold the interposer socket to the circuit board.  
           [0008]    As technology has progressed, some chips (i.e., microprocessors) have become more powerful and accordingly consume more electrical power. This increase in power usage causes the chips to become hotter and larger heat sinks are required to dissipate the increased thermal load. Mounting a large chip with a heat sink in an interposer socket may be problematic in the face of shock/vibration loads.  
           [0009]    For example, motion caused by a fan, opening and closing cabinet doors in a rack of computers, seismic activity, and vibration induced by adjacent equipment may cause interposer sockets to fail and become intermittent. Obviously, this failure may cause the electrical component contained in the interposer to cease functioning as intended.  
           [0010]    For successful operation and prevention of premature socket failure, the source of the vibration should be eliminated. If this is impossible or difficult, then a vibration isolation device should be used at or near the socket to minimize the potential for the socket to fail.  
         BRIEF SUMMARY OF THE INVENTION  
         [0011]    The present invention relates to a method and apparatus that minimizes shock/vibrational motion in interposer sockets. The ability to control shock/vibration can ensure successful operation and substantially increase socket lifetime.  
           [0012]    In accordance with a preferred embodiment of the present invention, a device for maintaining a heat sink in a desired relationship with a mounting base while limiting the transmission of shock and vibrational motion to and from the heat sink includes a fastener extending from the mounting base, a spring compressed between the fastener and the heat sink, and a damper compressed between the fastener and the heat sink wherein the fastener maintains the spring and the damper in a compressed state such that the spring and the damper bear on said heat sink.  
           [0013]    In accordance with another preferred embodiment of the present invention, a method for limiting shock/vibrational motion of a microprocessor includes placing a damping material in parallel with a clamping spring, wherein the damping material is in the form of a sleeve that surrounds the clamping spring.  
           [0014]    In accordance with yet another preferred embodiment of the present invention, a method for limiting shock/vibrational motion of a microprocessor comprising placing a damping material in series with a clamping spring, wherein the damping material is in the form of a viscous liquid inside the capscrew, similar to a shock absorber.  
           [0015]    These and other aspects of the present invention will become apparent upon studying the following detailed description, figures and claims.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings in which:  
         [0017]    FIGS.  1  is a schematic diagram of a system in accordance with a preferred embodiment of the present invention; and  
         [0018]    [0018]FIG. 2 is a schematic diagram of a spring-dashpot model. 
     
    
     NOTATION AND NOMENCLATURE  
       [0019]    Certain terms are used throughout the following description and claims to refer to particular system components. As one skilled in the art will appreciate, computer companies may refer to a given component by different names. This document does not intend to distinguish between components that differ in name but not function. In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ” Also, the term “couple” or “couples” is intended to mean either an indirect or direct electrical connection. Thus, if a first device “couples” to a second device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections. To the extent that any term is not specially defined in this specification, the intent is that the term is to be given its plain and ordinary meaning.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    Because of the desire to mount CPUs in area array interposer sockets, a device is needed to ensure that vibrational motion is kept to a minimum. The present invention provides a positive means for controlling vibration through an energy dissipation device, or damper. Additionally, this damper may be tuned to the specific system mass and stiffness.  
         [0021]    The preferred embodiment of the invention is described below in the context of a processor chip and heat sink combination mounted on a circuit board with an interposer socket. It should be noted, however, that the chip need not be a processor nor is the heat sink required. Broadly, the invention is useful to reduce vibration for any type of component mounted to a circuit board.  
         [0022]    Referring initially to FIG. 1, system  100  includes a backing plate  10  with a plurality of recesses  20 , a PWB  30 , interposer socket  40 , interposer columns  45 , a landgrid array (LGA) or processor  42 , a heat sink  55 , and at least one tunable vibration damping assembly  60 . When combined, interposer columns  45  and processor  42  are referred to as CPU assembly  50 . Interposer socket  40  is mounted on PWB  30  and CPU assembly  50  is received in socket  40  in the conventional manner. As is known in the art, a plurality of interposer columns  45  connects PWB  30  to processor  42 . Heat sink  55  is mounted on CPU assembly  50  in such a manner as to achieve sufficient thermal contact therewith. If desired, a material such as thermal grease (not shown) can be used at the interface to enhance the flow of heat from CPU assembly  50  into heat sink  55 .  
         [0023]    According to a preferred embodiment, heat sink  55  is held in place by at least one, and preferably a plurality of tunable vibration damping assemblies  60 . Each tunable vibration damping assembly  60  preferably comprises a standoff  90 , a helical coil spring  120 , a damping sleeve  102 , and a capscrew  110 . Instead of a helical coil spring, any type of Belleville spring can be used. Each standoff  90  is mounted on or pressed into backing plate  10  and passes through a corresponding hole  57  in the base of heat sink  55 . Standoffs  90  each preferably comprise a generally cylindrical member having a threaded internal bore  92 . Each capscrew  110  includes a head  112  and a male threaded body  114  sized to threadingly engage bore  92 .  
         [0024]    Coil spring  120  preferably has an inside diameter that is larger than the outside diameter of standoff  90  and an outside diameter that is smaller than the diameter of head  112 . Similarly, damping sleeve  102  preferably has an inside diameter that is larger than the outside diameter of coil spring  120  and an outside diameter that is smaller than the diameter of head  112 . The damping sleeve may also be constructed in such a manner as to be integral with the coil spring.  
         [0025]    Prior to the engagement of capscrew  110  with standoff  90 , coil spring  120  and damping sleeve  102  are placed in concentric arrangement around standoff  90 . Both coil spring  120  and damping sleeve  102  are preferably somewhat longer than the extension of standoff  90  above the base of heat sink  55  when they are in their natural, or non-compressed states. Thus, when it is desired to assemble one of the present tunable vibration damping assemblies  60 , capscrew  110  is threaded into bore  92  of standoff  90  and tightened until head  112  engages and then compresses spring  120  and sleeve  102 . Capscrew  110  is tightened until spring  120  and sleeve  102  are compressed a desired amount and thus apply a desired amount of force to heat sink  55 . In this manner, a tunable damping force is applied to heat sink  55  to maintain it in good thermal contact with the CPU while simultaneously damping vibrations or shocks that would otherwise be transmitted from the PWB to the CPU.  
         [0026]    In order to describe the operation of the present invention, reference will be briefly made to FIG. 2, a schematic of a simple spring-dashpot system. FIG. 2 includes a Kelvin element comprising a linear spring in parallel with a viscous damper. Kelvin model  200  includes a spring component  210 , a dashpot component  220 , and a fixed origin  230 . In a Kelvin model, spring component  210  functions according to Hookean elastic behavior. For example, when a force is applied to spring  210  it deforms by an amount that is directly proportional to the applied force. The classical solid behavior is given by Equation 1.  
           F=kx   (1) 
         [0027]    F is force (stress), x is the extension distance (strain), and k is the proportionality constant. This constant is also called a modulus. The deformation is reversible when the stress is removed. However, if stress is continuously applied, a Hookean solid does not deform any further; it shows no time-dependant deformation.  
         [0028]    Dashpot, or damper  220 , functions according to Newtonian viscous behavior. For example, the applied force (stress) is proportional not to the distance (strain), but rather to the rate of strain. This classical liquid behavior is given by Equation 2.  
           F=kdx   (2) 
         [0029]    where F is force, dx is the rate of extension (strain), and k is the proportionality constant. In shear this equation is written: 
         τ=ηγ  (3) 
         [0030]    and the proportionality constant η is viscosity. The damping material continues to deform as long as force is applied. The deformation is not reversible; when the force is removed, the damping material ceases to deform.  
         [0031]    It should be understood that the only material that exhibits true Newtonian viscous behavior is a viscous liquid. In reality, a “viscous” solid displays viscous and elastic behavior. However, for explanatory purposes only, in the current invention, coil spring  120  is assumed to display purely elastic behavior and damper  102  is assumed to display purely viscous behavior.  
         [0032]    When these two components are combined, the viscoelastic behavior of the system can be modeled using the elastic and viscous elements in parallel; the strain of the two elements in parallel is the same and the total stress is the sum of the stress in the two elements. As the load is applied, the viscous element resists deformation but slowly deforms, transferring the applied stress to the elastic element. Thus, the deformation of this two-element model is limited by the extensibility of the elastic element. When load is removed, the “transient creep” strain is recovered.  
         [0033]    More specifically, this model exhibits a “delayed elastic” or viscoelastic response to applied loads. After sudden imposition of a shear stress, spring  210  will eventually reach the expected strain, but is retarded in doing so by dashpot  220 . Dashpot or damper  102  of the present invention, accordingly prevents spring  120  from reaching its expected strain, thus limiting vibrational motion.  
         [0034]    In order to ensure that vibrational motion is minimized in the present application, damper  102  should possess the following properties. It should be resistant to temperatures below 130° C., possess a loss factor of at least 0.010, and be easily manufactured by companies such as Sorbothane. Examples of such materials include, but are not limited to, rubbers, silicones, and neoprenes.  
         [0035]    The simple Kelvin model described above describes a simple spring-dashpot system which is useful to understanding the following model which more accurately models the behavior of vibration damping assembly  60 .  
         [0036]    The present spring-mass-dashpot system preferably includes four clamping springs with total spring constant K (lb/in), a CPU and heat sink with mass W/g (lb-sec 2 /in), and four damping sleeves, each having a damping value C (lb-sec/in). The magnification factor of a single degree of freedom spring-mass-damper system can be determined according to Equation 4: 
           X/X   0 =1/[{1−(ω/ω n ) 2 } 2 +{2ξ(ω/ω n )} 2 ] ½   (4) 
         [0037]    where:  
         [0038]    X is the amplitude of vibration (in),  
         [0039]    X 0  is static deflection, or F 0 /K (in),  
         [0040]    ω is frequency of excitation (rad/sec),  
         [0041]    ω n  is natural frequency, or [Kg/W] ½  (rad/sec),  
         [0042]    ξ is a damping factor,=C/C 0 ,  
         [0043]    C 0  is critical damping,=2Wω n /g (lb-sec/in),  
         [0044]    k is the spring constant of one clamping spring (lb/in),  
         [0045]    K is the total clamping spring constant, or nK (lb/in),  
         [0046]    n is the number of clamping springs, in this case 4,  
         [0047]    W is the weight of heat sink (lb),  
         [0048]    g is a gravitational constant, or 386 in/sec, and  
         [0049]    F 0  is the total static clamping force applied (lb).  
         [0050]    For a resonant, lightly damped system ξ=0.01, and Equation 4 becomes X/X 0 =50. However, for a resonant, critically damped system ξ=1, and Equation 4 becomes X/X 0 =0.5. Thus, the vibration amplitude of the heat sink can be reduced by a factor of 100 or more.  
         [0051]    The critical damping value of the four damping sleeves can be determined by Equation 5: 
           C   0 =2 Wω   n =2 [KW/g]   ½   (5) 
         [0052]    Thus, Equation 5 defines the amount of damping necessary in the four damping sleeves to provide a critically damped system.  
         [0053]    Critical damping refers to zero amplitude for a damped oscillator; the body returns back to its equilibrium position at an optimum rate. Critical damping is desirable because vibrational oscillations cease, preventing intermittent motion. By tuning the damping sleeves  102  to equal approximately twice the product of the mass weight and natural frequency, critical damping is obtained.  
         [0054]    In order to tune a damper, a critical damping value is mathematically projected, similar to that shown in Equation 5. A material possessing a damping value equal to a fraction of the projected critical damping value is then employed as the damper. For example, according to Equation 5, if the weight of the heat sink is 0.10 lb and the natural frequency of the system is 500 rad/sec, then the critical damping value, C 0  is 100 lb-rad/sec, because C 0 =2Wω n . If four damping sleeves are used, each sleeve should possess a damping factor C of approximately 25 lb-rad/sec.  
         [0055]    The damper may be produced in the form of a sleeve (e.g., sleeve  102  in FIG. 1), a viscous liquid inside the capscrew, or any form capable of damping the clamping spring  120 . Additionally, the damper may be part of a Kelvin system as described above (e.g., spring and dashpot in parallel) or as part of a Maxwell system (e.g., spring and dashpot in series) and the spring(s) and damper(s) need not necessarily be positioned adjacent to each other.  
         [0056]    It should be understood that the damping assemblies and systems described herein may be used in a computer system including a chassis, a system board, and an input device. In a preferred embodiment, the input device is either a mouse or a keyboard.  
         [0057]    The above discussion is meant to be illustrative of the principles and various embodiments of the present invention. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.