Abstract:
A captive shoulder nut for a electronic module heat sink has a threaded nut portion and a tubular portion extending therefrom. A ramped section in the exterior wall of the tubular portion leads to a retaining groove adjacent the bottom face of the nut portion. A compression spring is slipped over the tubular portion to ride over the ramped section and be retained at one end in the retaining groove. The tubular portion is suitable to be inserted through a straight hole, whereof the inner diameter of the spring is larger than the hole. The free end of the tubular portion is suitable to be flared to a larger diameter than the hole, thereby establishing a captivation.

Description:
BACKGROUND OF THE INVENTION 
   In assembling electronic components and modules, inserts, spacers and standoffs have been often used. The attachment of components and parts has been accomplished by screws, spring clips, clamps and other such devices. In a chassis for holding electronic components, space for the manual manipulation of parts and tools often is an issue. 
   Captive screws and captive fasteners are devices used to fasten two components together, where the fastener remains with one of the components when loosened. Typically a captive screw is “caught” by the component it remains with by a flange, a ferrule, a spring clip or the like, which structure prevents the total removal of the captive fastener from that component. The usefulness of captive fasteners is that they do not get lost or fall out of the associated component before and during assembly. 
   This feature has become very useful in the assembly and removal of components associated from electronic module boards, peripheral component interconnect boards (PCI boards), and printed circuit boards (PC boards), and in the environment of the chassis for housing these boards. 
   Modern large scale integrated (LSI) circuits, microprocessors, microchips and other integrated circuit devices (IC chips) generate a substantial amount of heat, especially when operating at very high frequencies. Such heat generation can amount to 10&#39;s of watts and even 100&#39;s of watts of heat per hour. It has become imperative to mount heat sinks on these IC chips to dissipate as much heat as possible. In such instances the heat sink is mounted to the board or to a mounting frame which in turn is mounted to the board on which the IC chip is also mounted. 
   Spring clips have been used to hold heat sinks to IC chips on PC boards. However, these clips are sensitive to vibration, often interfere with the heat transfer fins on the heat sink and are often hard to positively snap into place and to release. 
   Captive screws have provided and improvement over heat sink clips. Two or four captive screws are used and engage respective flanged corners of a heat sink. These captive screws have threaded ends which usually engage a threaded ferrule or threaded bushing mounted into a hole through the PC board. They also require a ferrule or bushing though the heat sink&#39;s flange through which they extend. 
   A captive screw may use a slip ring, annular flange, or projecting shoulder positioned on the captive screw at a location below the heat sink flange&#39;s surface. This projecting structure prohibits the captive screw from being withdrawn out of the heat sink and thereby holds the fastener captive on the heat sink. Captive screws are generally driven (tightened and loosened) by tool engagement with their head. Typically, captive screws have Phillips, slotted, or TORX heads requiring appropriate screw drivers. 
   Oftentimes a sheet of compressible elastomeric heat transfer polymeric material is used between the top surface of the IC chip and the bottom of the heat sink. This heat transfer interface material takes up for any surface irregularities in the mating IC chip and heat sink. 
   Captive screws for IC chip heat sinks with heat transfer polymeric sheeting have incorporated spring tie-down designs where the tie-down force exerted by the captive screw is governed by the spring force of a compresses spring. This structure permits the heat sink to “float”, i.e., move through expansion and contraction as the IC chip temperature changes. 
   As the chassis for electronic modules is made smaller with a smaller foot print, and as more boards are crowded into tightly spaced racks in a chassis, the size and position of heat sink tie-down screws, including captive screws, becomes an issue. Moreover, Phillips, slotted and even TORX heads can round out with poor tool alignment. The use and installation of board mounted receiving threaded bushings or ferrules and of heat sink mounted ferrules adds to the cost of the securement hardware. Alignment of the heat sink assembly when aligning the heat sink screws with the board mounted receiving threaded bushing or ferrules generally requires two hands and some lateral movement. This lateral movement can jeopardize the integrity of the printed circuit coating on the board, and miss-align the interface polymer heat transfer pad. This makes the removal and reinstallation of heat sinks in tight quarters difficult. 
   To improve the ease of alignment of a heat sink and to assure proper positioning thereof, permanent board mounted studs have been proposed. These studs can be threaded for securing a fastener thereto and can also act as alignment pins during the removal and the reinstallation of a heat sink. What is desired is a nut-type structure for use with board mounted studs for securing a heat sink to an IC chip. 
   The objective of the present invention is to provide a captive nut structure for use with IC chip heat sinks, which captive nut structure remains with the heat sink when it is removed. 
   A second objective is to provide spring tie-down which will permit the heat sink to float with temperature changes. 
   A third objective is to minimize the manufacturing costs of the captive nut structure and to minimize the number of components thereof 
   SUMMARY OF THE INVENTION 
   The objectives of the present invention are realized in a heat sink nut assembly that is captive to the heat sink. This nut assembly includes a spring for exerting a tie-down force on the heat sink, and also includes an enlarged section to captivate it to the heat sink. 
   While this nut assembly was designed for heat sinks for PCI and PC board electronic components and modules, it is equally applied to holding other structures. 
   The nut assembly includes a threaded hexagonal nut portion and a tubular portion extending therefrom. A ramped section, adjacent to the tubular portion&#39;s connection to the nut portion, flares outwardly as it approaches the nut portion, and is terminated to form an annular shoulder thereby establishing a retaining groove in the tubular portion adjacent the nut portion. 
   A compression spring has its ends each formed into a flat circular loop. The inside diameter of the compression spring is large enough to slide over the tubular portion. The upper end of the spring is slid over the ramped section to be retained in the retaining groove. 
   An advantage of the present invention is that it permits the use of threaded studs to hold the heat sink. The previous male screw is eliminated. Therefore, the studs act as a locator and alignment means for assembly, and the possibility of damage to the PC board due to a male screw is eliminated. The present invention has fewer parts than a captive screw assembly. The ferrule and/or threaded bushing are eliminated. The structure takes up less space (has a smaller footprint) than a captive screw with a ferrule. This permits more space on a heat sink base pad for fin area. 
   The assembly of the nut and spring is slipped though a straight hole in the flange of the heat sink. The spring, which will not pass through the hole, extends between the bottom of the nut portion of the assembly and to top face of the heat sink flange. The assembly is then compressed and the free end of the tubular portion is flared over to extend outwardly greater than the hole size. A special flaring tool may be required to repeat the same final end diameter on each tubular portion of a new captive nut assembly. 
   The flared end acts to captivate the nut assembly, as well as to provide a shoulder stop to provide the correct spring force. When a heat sink carrying the captive nut assemblies is mounted onto a PCI board or a PC board, the flared end provides a stop against the board to establish a correct spring force hold down on the flanges of the heat sink. The springs also permit the heat sink to float with temperature affected changes in dimensions. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features, advantage and operation of the present invention will become readily apparent and further understood from a reading of the following detailed description with the accompanying drawings, in which like numerals refer to like elements, and in which: 
       FIG. 1  is a side elevation cross-section of the captive nut assembly holding a heat sink to a PC board; 
       FIG. 2  is an exploded perspective assembly view of the captive nut assembly prior to installation and flaring; 
       FIG. 3  is a longitudinal cross-sectional view of the captive nut assembly with the spring assembled on the retaining groove; 
       FIG. 4   a  is a perspective view of the spring; 
       FIG. 4   b  is a first side view of the spring; 
       FIG. 4   c  is a second side view of the spring take at a 90 degree rotation from the first side view; 
       FIG. 4   d  is a first end view of the spring; 
       FIG. 4   e  is an opposite end view of the spring; 
       FIG. 5   a  is a perspective view of the nut and tubular members; 
       FIG. 5   b  is a side view of the nut and tubular members; 
       FIG. 5   c  is a tubular member end view of  FIGS. 5   a  and  5   b:    
       FIG. 5   d  is a nut member end view of  FIGS. 5   a  and  5   b;    
       FIG. 6 . is a side view for a alternate embodiment for  FIG. 5   b  where the tubular member has an annular relief groove adjacent the free end thereof; and 
       FIG. 7  is a cross-sectional view of the t alternate embodiment of  FIG. 6 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is a captive shoulder nut assembly having a spring tie-down for mating with a stud on a PCI board (peripheral component interconnect board) or a PC board (printed circuit board) for holding a heat sink to an IC device (integrated circuit device or microchip) or other electronic module. A PC board  11 ,  FIG. 1 , has a microchip, integrated circuit (IC)  13  mounted to the surface thereof. A plurality of threaded studs  15  are mounted to the PC board  11  to be upstanding about the periphery of the IC  13 . These studs  15  are usually two or four, and are used for mounting a heat sink  17  to the board  11  over and in contact with the IC  13 . 
   Integrated circuit  13  carries a heat transfer compressible interface pad  19  on the top surface thereof for mating with the heat sink base plate  21  and for taking up for any irregularities in the base plate  21  or the top surface of the IC  13 . 
   Straight holes  23  extend through the flanges  25  which extend about the periphery of the base plate  21 . Each hole  23  is to be mated to an outwardly extending stud  15 . The studs  15  act as positioning pins when mounting the heat sink  17  to the board  11  and over the IC  13 . 
   Each captive shoulder nut assembly  27  is screwed onto a stud  15  to tie-down the flange  25  with the force of a compressed spring  29  carried thereon. The tie-down force exerted on the flange  25  by the spring  29  is affected by the stand-off distance “A” and the resultant compressed length “B” of the spring  29 . A tubular sleeve  31  is connected to a threaded nut  33  which is tightened down onto the stud  15  thereby driving the sleeve  31  into contact with the top surface of the PC board  11 . The free end of the sleeve  31  is flared outwardly  35  to a larger diameter than the hole  23 . This flared free end  35  captivates the sleeve  31 , the spring  29  and the nut  33  to the flange  25  of the heat sink  17 . The flared end  35  also forms a stop against the PC board  11  top surface. 
   The narrow profile (small footprint) of the captive nut assembly  27  permits a smaller flange  25  than previously thereby yielding a larger area for heat sink fins  37 . The upper end of the spring  29  is held to the nut  33  and sleeve  31  at a retaining groove described below. 
   The captive shoulder nut assembly prior to installation on a heat sink or other device is shown in  FIG. 2 . The assembly has two components. The first component is the nut  33  and tubular sleeve  31  extending from one face thereof. The second component is the spring  29 . The nut  33  and sleeve  31  are made as one machined component. However, alternately, a shoulder nut may have a tubular sleeve pressed on or crimped on a projecting barrel-like shoulder on the nut. This would permit the nut and sleeve to be made of two dissimilar materials. 
   The nut  33  and sleeve  31 ,  FIG. 2 , are machined from a single stock material, which may be made of type  304  stainless steel, or of brass or bronze or alloys of any of these. The spring  29 ,  FIG. 2 , may be made of type  302  stainless steel or of spring wire. 
   The free end  37  of the sleeve is chamfered  37  and then has a section of its interior wall which undercut into a counter sunk bore  39 . The undercut of the counter sunk bore  39  creates a thinner wall thickness in the region of the sleeve  31  to be flared out. The counter sunk bore  39  terminates in the shoulder  51 , shown in  FIG. 2  and  FIG. 3 . This shoulder  51  is the transition between the counter sunk bore  39  section and the other portion of the tubular sleeve  31 .  FIG. 3  shows a longitudinal cross-section of the assembled components of  FIG. 2 . As seen, the inside diameter  41  of the tubular sleeve portion  31  is sufficiently large to pass over the stud  15 . Only the nut  33  is threaded  43 . The inside diameter  41  of the tubular sleeve portion  31  is shown to increase only in the undercut bore section  39  after the shoulder  51 . Also shown in  FIG. 3  are a ramped section  45  and a retaining groove  47 , which are discussed below. The external wall of the tubular sleeve  31  below the ramped section  45  remains a uniform cylindrical wall as shown in  FIGS. 2 and 3 . 
     FIGS. 4   a  through  4   e  show various views of the shape of the compression spring  29 , which is helically wound with flat circular ends.  FIGS. 5   a  through  5   d  show various views of the nut  33  and counter sunk tubular sleeve  31 . A ramped section  45  in the exterior wall of the tubular sleeve  31  leads to a retaining groove  47  adjacent the bottom face of the nut,  FIGS. 3 ,  5   a  and  5   b . The length of the retaining groove  47  is slightly larger that the diameter of the wire from which the spring  29  is constructed. The depth of the retaining groove  47  is sufficient to hold the end circular loop of the spring  29 ,  FIG. 3 . 
   The angle “C”,  FIG. 5   b , of the ramped section  45  is sufficient to spread the end of the spring  29  in order to position it into the retaining grove  47 . Angle “C” can be about 10 degrees. The ramped section  45 ,  FIGS. 2 ,  3 ,  5 A,  5 B,  6  and  7 , extends increasingly linearly outwardly about the cylindrically shaped tubular sleeve  31  as it approaches the bottom of the nut  33  and the groove  47 . As seen, the ramped section  45  is frustoconical in shape, i.e., the shape of a truncated cone. The conical wall surface of this ramped section  45  extends furthest outwardly, i.e., has the largest diameter, in the direction of the groove  47  whereupon it truncates at the groove  47 ,  FIGS. 5A ,  5 B so that the groove extends on the outer wall of the tubular sleeve  31  between the truncated end of the ramped section  47  and the bottom of the nut  33 . 
   The captive shoulder nut assembly is captivated onto a heat sink  17  with the following steps. The spring  29  is slid onto the sleeve  31  with its leading end pushed over the ramped section  45  and into the retaining groove  47 . This assembly is then inserted though a hole  23  in a heat sink to partially compress the spring  29  and permit the leading end of the sleeve  31  to project below the flange  25  of the heat sink sufficiently for a flaring tool to grasp the sleeve  31  above the end of the counter sunk section  39 . The flaring tool then flares out the counter sunk section  39  to a diameter greater than the diameter of the hole  23 . The heat sink is then mounted on the studs  15  and the nut  33  is tightened until the flared free end of the sleeve  31  stops against the top surface of the PC board,  FIG. 1 . 
   As an alternative to the under cut or counter sunk section  39  in the sleeve  31 , an external groove  49  can be cut in the external surface of the free end of the sleeve  31 ,  FIGS. 6 and 7 . This external annular groove  49  can be V-shaped. During the flaring operation of the assembly steps, the flaring tool is clamped inboard of the V-groove  49  and the wall of the sleeve  31  is flared outwardly. 
   Many changes can be made in the above-described invention without departing from the intent and scope thereof. It is therefore intended that the above description be read in the illustrative sense and not in the limiting sense. Substitutions and changes can be made while still being within the scope and intent of the invention and of the appended claims.