Patent Publication Number: US-6212070-B1

Title: Zero force heat sink

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. Pat. application Ser. No. 08/687,103, filed Jul. 22, 1996, now U.S. Pat. No. 5,805,430. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to heat sink assemblies and, more particularly, to a technique and assembly arrangement wherein a heat sink is positioned in thermal contact with a substrate such as an integrated circuit chip or electronic package but does not impart any stressful forces on the substrate, thereby avoiding damage to interconnect both at the heat sink substrate interface and at the substrate underlying support interface. 
     2. Background Description 
     Modern integrated circuit chips, chip carriers, and other electronic substrates and the use of higher powers in these devices mandate a need for better heat dissipation schemes. Current industry solutions have included the use of active heat sinks or elaborate structures to improve the thermal load carrying capacity of select components; however, these approaches tend to add undesirable cost and complexity, and also reduce reliability of the final system design. 
     A simple solution appears to be the use of larger and heavier heat sinks since limitations in heat sink mass limit the range of allowable electronic package power dissipation. However, larger and heavier heat sinks have not proven effective, particularly when they are applied to surface mounted components. There are serious constraints linked to the amount of heat sink mass which can actually be supported without adversely affecting the reliability of the interconnections to surface mounted components. Large mass heat sinks cause excessive stresses to both first and second level interconnections (i.e., component to heat sink being the site of first level interconnections, and component to circuit card being the site of second level interconnections) during shipping and/or over the machine life time, resulting a degradation to interconnection geometry and subsequently package interconnection reliability. 
     In certain circumstances substituting pin through hole connections for surface mount structures is not a viable option. Therefore, it would be advantageous to provide an effective mechanism and methodology which allows using high mass heat sinks with surface mounted electronic packages and components. 
     U.S. Pat. No. 5,386,338 to Jordan et al. and U.S. Pat. No. 5,464,054 to Hinshaw et al. each show a spring clamp mechanism for securing a heat sink to a mounting frame. In operation, an electronic device package is positioned within an aperture of a mounting frame, and the heat sink is secured to the mounting frame using a spring clip which fits across the heat sink and has projecting ends which are flexed to a point underneath tab members on the mounting frame. The patents also discuss the possibility of eliminating the mounting frame and securing the projecting ends in specially designed recesses positioned outside a socket connection in a circuit board which receives the electronic package. The configurations described in Jordan and Hinshaw each require a constant spring force to be applied against the surface of the electronic package, and would be wholly inappropriate for electronic packages that use surface mount interconnections. 
     U.S. Pat. No. 4,885,126 to Polonio shows, in FIGS. 31 and 41, a packaging system wherein integrated circuit chips are housed in recessed regions in a housing that is connected to a printed circuit board using pin connections . A convection heat sink is bonded to the top of the housing forming a hermetic seal for the chips in the housing. However, like Jordan and Hinshaw, Polonio relies on a compressive spring force to maintain thermal contact between the chips in the housing and the heat sink. In Polonio, the spring members are positioned between the top of the integrated circuit chips and the bottom of the heat sink, and are compressed at the time of creating the hermetic seal. In addition, Polonio is not related to and does not address problems that arise with surface mount components. 
     U.S. Pat. No. 4,455,457, to Kurokawa discloses several different packaging arrangements for semiconductors that are designed to increase thermal dissipation. In each configuration, a chip is positioned within a recess in a housing that is equipped with pin connectors for connection to a circuit board, and a heat diffusing plate contacts the top of the chip and serves to transfer heat from the chip to a heat sink positioned on top of the housing. Kurokawa relies on an elastic bias from a metallic member positioned between the diffusing plate and the heat sink to maintain contact between the diffusing plate and the chip. The heat sink is attached directly to the heat sink. The Kurokawa reference does not discuss arrangements which would be suitable for surface mount technologies. 
     U.S. Pat. No. 5,311,402 to Kobayashi describes an integrated circuit device fitted inside a chip holder where the integrated circuit device is adhesively bonded to a cap, and where the cap fits within grooves on an underlying substrate. The Kobayashi patent is mainly directed to positioning a chip on a circuit board, and is not concerned with heat sinks or the adverse impact of a heavy element on interconnections when the heavy element is connected to a device or substrate. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to provide an electronic packaging arrangement and method which allows a heat sink to be in thermal communication with a substrate such as an integrated circuit chip or chip package without imparting stress to the substrate or its interconnections. 
     It is another object of this invention to provide a heat sink packaging arrangement and method which is adaptable to a wide range of substrates and which is not subject to exacting manufacturing or assembly tolerances. 
     It is still another object of this invention to provide a heat sink packaging arrangement which allows large heat sinks to be used in conjunction with surface mounted substrates and components that avoids stresses to both first and second level interconnect. 
     It is another object of this invention to provide a heat sink packaging arrangement that is closely matched to the thermal expansion characteristics of the substrate being cooled and the support on which the substrate is surface mounted. 
     According to the invention, a high mass heat sink is positioned and restrained in a manner wherein the mass of the heat sink is mechanically decoupled from the substrate (e.g., chip, electronic package, etc.), while at the same time the heat sink is held tightly and positioned in intimate contact with the substrate to allow for heat dissipation. A frame is mounted to a support on which the substrate is mounted, such as a printed circuit board, circuit card, or other device, and is positioned above the substrate. Preferably, the frame is made in a manner which provides sufficient clearance in the X, Y, and Z dimensions to accommodate neighboring components to the substrate which are attached to the support. The frame has an aperture in its top which is aligned with the substrate to be cooled. 
     A heat sink, which preferably has a large fan-like convection portion and a narrower slug region for dispersing the thermal energy, is connected to the frame by adhesive bonding or other means, including mechanical connections, with its narrower region extending coaxially through the aperture in contact with the substrate. Preferably, the fan-like convection region is spaced away from the top of the frame and the connection to the frame is made only in the aperture region, thereby limiting the transfer of heat to the frame. This allows the frame to have thermal expansion characteristics closely matched to the substrate being cooled and the underlying support. In a specific embodiment, the frame is comprised of two parts referred to as a plate which includes the aperture and vertical members called studs which are used to space the plate above the substrate, and the plate is made of a material selected to match the thermal expansion characteristics of the support such that it expands and contracts in the X-Y dimensions the same as the support, and the studs are made of a material selected to match the thermal expansion characteristics of the substrate such that they expand and contract in the Z dimension the same as the substrate. 
     A thermal connection is made to the substrate by depressing the heat sink against the substrate via coaxial movement through the aperture. Once in place, the heat sink is connected to the aperture. While there is a downward force on the substrate during assembly of the heat sink, this force is quickly dissipated by relaxation of the mounting members underneath the substrate. For example, with surface mount technologies such as ceramic ball grid arrays, ceramic column grid arrays, plastic ball grid arrays, plastic column grid arrays, solder balls, and solder columns, the metallic elements, such as lead/tin will undergo stress relaxation also known as “creep”. This stress relaxation will reduce the applied assembly force to very low values including zero stress such that there is reduced or zero stress on the package interconnections after assembly, and all the weight of the heat sink is supported by the frame instead of the substrate being cooled. Thermal grease or other heat transfer mediums can be positioned between the bottom of the heat sink and the top of the substrate to assist in transfer of thermal energy from the substrate to the heat sink. 
     The apertured design of the frame allows a variety of different sized substrates to be accommodated by a single frame. That is, both taller and shorter components can be fitted under the frame, and the heat sink connection to the frame can be made at varying regions on the side of the slug region of the heat sink simply by moving it coaxially through the aperture. This feature allows greatly relaxed manufacturing tolerances such as dimensions and coplanarity. The aperture itself can be fabricated with ridged regions which can either mechanically connect the heat sink to the frame or aid in the connection. Ridged members will also serve to direct the heat sink to an upright orientation and direct the heat sink downward towards the substrate. The aperture can be made in a variety of different geometries and the frame can be connected to the support by a variety of different connectors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of the preferred embodiments of the invention with reference to the drawings, in which: 
     FIG. 1 is a cross-sectional side view of a substrate surface mounted to a support with a heat sink in thermal contact with the substrate which is supported by a frame that is connected to the support; 
     FIG. 2 is a cross-sectional side view of a stud surface mounted to a support such as a printed circuit card or other device; 
     FIG. 3 is a plan view of a plate which forms part of a frame; 
     FIG. 4 is a side view of an alternative frame configuration which illustrates two alternative connectors which can be used to secure the frame to a support; 
     FIG. 5 is a side view of the frame shown in FIG. 4 mounted to a support with a heat sink attached that is in thermal contact with a substrate; 
     FIG. 6 is a plan view of the heat sink assembly shown in FIG. 4; 
     FIGS. 7 a  and  7   b  are enlarged side views of an aperture in a frame showing ridge formations in the aperture; 
     FIG. 8 is a cross-sectional side view of the heat sink assembly shown in FIG. 1 wherein a set screw is used to connect the heat sink to the frame; 
     FIG. 9 is a plan view of the plate shown in FIG. 3 showing set screws projecting into the aperture; 
     FIG. 10 is a side view of the heat sink assembly shown in FIG. 5 wherein set screws are used to connect the heat sink to the frame; and 
     FIG. 11 is a plan view of the heat sink assembly shown in FIG. 6 showing the set screws connecting the heat sink to the frame. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
     This invention pertains to decoupling those forces generated by a heat sink mass from an underlying substrate, and particularly a surface mounted substrate. Decoupling a force due to the heat sink mass from the electronic package and its interconnection structure enables and simplifies the use of larger and heavier heat sinks. The use of high mass heat sinks enables higher power dissipations for a given electronic package type. According to this invention, decoupling is accomplished by using a rigid frame to support the weight of the heat sink while maintaining the heat sink in thermal contact with the electronic package. When subjected to vibration and/or shocks during transportation or operation, the frame restrains and limits the displacement of the heat sink relative to the electronic package. Additionally, the frame transfers static and dynamic loading from the heat sink directly to the underlying printed circuit board or card. 
     FIG. 1 shows a substrate  10  positioned on a support  12 . The substrate  10  can be any of a variety of electronic components that require thermal dissipation including integrated circuit chips, electronic packages, multilayer modules, etc. The support can be any of a variety of devices including printed circuit boards or cards, ceramic or composite boards and cards, etc. Furthermore, this invention is particularly beneficial for use with substrates  10  which are surface mounted to supports  12 ; therefore, FIG. 1 depicts a surface mount interconnection  14  which can be of any of a variety of arrangements including ceramic ball grid arrays, ceramic column grid arrays, plastic ball grid arrays, plastic column grid arrays, solder balls, or solder columns. Examples of these types of surface mount interconnections are described in IBM Journal of Research and Development, Vol. 37, #5, September 1993, and R. Tummala, E. Rymaszewski,  Microelectronics Packaging Handbook.    
     While FIG. 1 depicts a surface mount interconnection  14 , it should be understood that features of this invention will be useful for all standard electronic packages and I/O configurations, such as Ball Grid Arrays, Land Grid Arrays, Quad Flat Packages (QFP), lead frame, socketed, pin-in-hole, etc. 
     A frame  16  comprised of a plate  18  and several studs  20  is positioned over the substrate  10  and preferably has sufficient clearance therefrom in the X, Y, and Z directions to accommodate any neighboring components (not shown) of the substrate  10 . The frame  16  should be sufficiently rigid to support the weight of the heat sink  22 . Suitable materials for the frame  16  may include fiberglass, fiber reinforced plastic, aluminum, phenolic, Kovare, and Delrons. The size of the frame  16  can vary widely and depends on the size of the substrate  10  to be cooled and the size of the heat sink  22  to be supported. For example, the substrate  10  could be ceramic, alumina/glass, alumina nitride or polymeric, which have the following typical dimensions 15-50 mm length, 15-50 mm width, and 1.5-20 mm height, and the heat sink material could be aluminum or copper with masses ranging up to approximately 5 kilograms (kg), and the size of the frame  16  would need to be large enough to accommodate the substrate  10  and strong enough to support the heat sink  22 . 
     FIG. 1 illustrates the preferred embodiment of the invention where the frame is comprised of a separate plate  18  and several studs  20 ; however, the frame  16 ′ could also be of unitary construction as is shown in FIG.  4 . One particular advantage of the two-piece construction is that it allows for matching the thermal expansion and contraction characteristics of both the substrate  10  and the support  12 . The plate  18  would preferably made from a material which matches the coefficient of thermal expansion of the support  12  so that it will expand or contract in the X and Y plane of the support  12  at the same rate as the support  12 . For example, if the support  12  is a printed circuit board made from fiber reinforced plastic, it will have a coefficient of thermal expansion ranging between 15 E −6 /° K and 23 E− 6 /° K. The plate  18  could be chosen to made from the same material as the support  12 , or could be metal, plastic, Kovar, or ceramic with a thermal expansion sufficiently matched to be compatible with the expansion of support  12 . Preferably, the coupled expansion of the assembly composing the substrate  10 , its attachment  14  to support  12 , and the heat sink stud or slug  24  is to be roughly equivalent to the expansion of the material selected from the studs  20 . For example, using the following elements: alumina substrate, PbSn solder, aluminum heat sink, and silicon die, an appropriate stud material would have a coefficient of thermal expansion of approximately 13 E− 6 /° K, and could be metal (e.g., Alloy 42-42% Nickel and 58% iron, Kovar) or ceramic. Because of the different characteristics which may arise between the substrate  10  and support  12 , the coefficients of thermal expansion of the plate  18  and studs  20  can be different. 
     While FIG. 4 shows a unitary construction for frame  16 ′, it should be understood that the frame  16 ′ might be made with the top  17 ′ thermally matched to the support and the mounting legs  19 ′ thermally matched to the substrate, interconnect, heat sink combination, to achieve the benefits discussed above, where the top and mounting legs would be glued or molded together to form the unitary structure. 
     FIG. 1 illustrates a preferred mounting arrangement for the heat sink  22  whereby it is joined to the frame  16  at a region on its lower “slug” portion  24  by a connection  25  positioned in an aperture in plate  18 . FIG. 3 shows the aperture  28  in plate  18 . The aperture  18  can be any geometry and will be selected to accommodate the type of heat sink  22  to be used. For example, FIG. 3 shows a round aperture, while FIG. 6 show a square aperture. Different sized substrates  10 , through a large tolerance range, can be accommodated simply by moving the slug  24  of the heat sink  22  within the aperture  18  (either coaxally or slightly non-coaxially). In the configuration shown in FIG. 1, the weight of the heat sink  22  is supported by the frame  16  which is connected to support  12 , and not by the substrate  10 . Thus, the configuration imparts very little or no stress on the interconnections (first or second level) of the substrate  12 . In addition, the fan region  23  of heat sink  22  is spaced away from the top surface of frame  16  so that heat energy supplied by the substrate  10  will not be passed to the frame  16  from the heat sink  22  except at the aperture  28 . Thus, the frame  16  will retain thermal expansion characteristics similar to the support  12 . 
     The connection  25  at the aperture  28  can be by any practical means which will transfer the weight of the heat sink  22  to the frame  16 , yet maintain the heat sink  22  in position in thermal contact with the substrate  10 . FIGS. 7 a  and  7   b  illustrate a preferred embodiment of this invention where the aperture  28  in plate  18  has a region  30  with ridges  32  formed therein, and where an adhesive material  34 , such as glue, epoxy (e.g., Hardman Epoxy #4005), or Loctites structural adhesive 324/326 is positioned in the aperture  28  between the slug  24  of heat sink  22  and plate  18 . FIG. 7 b  shows that the side wall of the slug  24  can be contoured  36  to accommodate adhesive material  34 . The ridges  32  provide adhesion enhancement of the heatsink slug  24  to the plate aperture  28 . Although not illustrated, it will be apparent to those skilled in the art that the heat sink  22  may also be mechanically connected to the frame  16  using clips, screws, and other mechanical fasteners, as well as friction fits or other mechanisms that will couple the heat sink  22  to frame  16 . 
     FIG. 2 illustrates a preferred embodiment of a stud  20  where it includes a support contacting end  36  and a plate contacting end  38 . The support contacting end  36  can be secured to support  12  by any of a variety of different techniques. For example, FIG. 2 shows a surface mounting technique where the support contacting end is secured to the surface of support  12  with a material  40  such as a solder, brazing material, or an adhesive. Alternatively, FIG. 4 shows that a screw  42  can be passed through the support  12  to secure a stud  20  or the base of a frame  16 ′ to a support, or an expansion collet  44  can be passed through the support  12  and be selectively expanded to make a connection therewith using push-down member  46  that will slide through a passage in the stud  20  or frame  16 ′. The technique for mounting the frame  16  or  16 ′ to the support  12  should be of stability sufficient to withstand vibrations and/or shocks which may be encountered during shipping and operation of the equipment which utilizes the substrate  10 . 
     With reference to FIGS. 1-3, a plate contacting end  38  of the stud  20  preferably is embossed or otherwise roughened so as to be uneven on its top surface  48  such that after the connecting screws  50  are seated in the bore holes  52 , lateral movements of the plate  18 , as would occur due to shock or vibration, will be more easily resisted, thereby maintaining the heat sink  22  centered above the substrate  10 . The studs  20  can be connected at various screw aperture sites  54  in plate  18 , and FIG. 3 illustrates connections at the four corners of the plate  18 . The positioning of the screw aperture sites  54  and the number of screw aperture sites  54  can vary considerably and can be adapted for space and position requirements of circuitry and components (not shown) underlying the plate  18  and the configuration of support  12 . Likewise, if a unitary frame  16 ′ such as that shown in FIG. 4 is used, the position of the connectors can be varied (FIG. 6, like FIG. 3 shows connectors  56  at the four corners of the frame  16 ′). 
     FIGS. 4-6 illustrate an embodiment of the invention where a square heat sink  22 ′ is positioned within a large square aperture  28 ′ in frame  16 ′. As discussed above, connection between the frame  16 ′ and heat sink  22 ′ can be made using an epoxy  34 ′ deposited in the aperture  28 ′ between the heat sink  22 ′ and frame  16 ′, or by other suitable means, including mechanical connections, which will be sufficient to support the weight of the heat sink  22 ′ and to maintain the heat sink  22 ′ in thermal contact with the substrate  10 ′. 
     Both FIG.  1  and FIG. 5 show the use of a thermal grease  58  or  58 ′or other suitable thermal medium (tapes, oils, etc.) between the substrate  10  or  10 ′ and the heat sink  22  or  22 ′. The thermal grease  58  or  58 ′is used to transfer and distribute heat more efficiently between the substrate and heat sink. Suitable thermal tape or grease mediums are commercially available from General Electric, Al Technology, Inc., Dow Corning, and Chometrics. In the preferred embodiment the heat sink  22  or  22 ′ will not be bonded to the substrate  10  or  10 ′ (e.g., the grease  58  or  58 ′ should not have an adhesive property). The lack of “bondings” assures that the heat sink is mechanically decoupled from the underlying substrate. While FIGS. 1 and 5 show the thermal grease  58  or  58 ′ as a layer it should be understood first that the grease is not required to practice the invention, and second that the grease  58  or  58 ′ would be spread very thinly across the top surface of the substrate  10  or  10 ′ and bottom surface of the heat sink  22  or  22 ′. 
     With reference back to FIG. 1, the heat sink assembly will preferably be fabricated by attaching a substrate  10  to a support  12  and attaching a frame  16  to the same support  12  at a position which overlies the substrate  12 , and where an aperture  28  in the frame  16  allows a portion of a heat sink  22  to pass therethrough and to thermally contact the substrate  10 . The heat sink  22  will be pressed downward towards the substrate with a lower portion of the heat sink moving through the aperture  28  and into thermal contact with the substrate  10 . Prior to pressing the heat sink  22  downward, a thermal grease  58  can be added between the base of the heat sink  22  and the top of the substrate  10 . In order to be useful, the heat sink  22  must make good thermal contact with the substrate  10 . The coaxial forces which electronic packages will experience is in the range of up to 1500 force grams or more. Once the heat sink  22  is pressed firmly against the substrate  10 , it should be firmly connected to the frame  16  using connection  25 , such as an epoxy glue or other suitable adhesive  34  deposited in the aperture  28  in the frame, or by mechanical fastener or the like. Thus, the weight of the heat sink  22 , which for most applications will range between 30 g and 1 kg, will be supported entirely by the frame  16 , and will also be firmly held in thermal contact with the substrate  10 . 
     The invention is particularly advantageous when surface mount interconnections  14  are used for the substrate. During the process of affixing the heat sink  22  to the frame  16 , a force will be applied to the substrate  10  and interconnection  14 . However, with surface mount interconnections the solder and/or other materials will dissipate the stress imparted through stress mechanisms. Thus, the forces generated during assembly will have been dissipated through relaxation to provide stress-free substrate attachments  14 . This stress-free state is maintained by the elements taught by this invention. 
     FIGS. 8-11 show various mechanical connector arrangements that can be used in the practice of this invention as alternatives to an adhesive bonding agent shown in FIG. 7 b . These mechanical connector arrangements will also result in stress-free substrate attachments  14  since they do not rely on continuous downward force being exerted on the frame. With reference to FIGS. 8 and 9, which are similar to FIGS. 1 and 3, respectively, it can be seen that a plurality of set screws  60  passing through the plate  18  into aperture  18  can be used to secure the heat sink  22  to the frame  16 . With reference to FIGS. 10 and 11, which are similar to FIGS. 5 and 6, respectively, it can be seen that a plurality of set screws  62 , located in the corner regions of the frame  16 ′, can be used to secure the heat sink  22 ′ to the frame  16 ′. 
     While the invention has been described in terms of its preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.