Captive shoulder nut assembly

A captive shoulder nut assembly, to hold a heat sink onto a PC board, has a nut member with a threaded portion and a tubular portion extending therefrom. The nut member mates with an upstanding threaded stud. A biasing compression spring rides over the nut with an end coil loop secured to the nut. The nut's tubular portion is suitable for insertion through a straight hole in the heat sink, with the spring remaining above the hole and operating against the heat sink. The free end of the tubular portion can be flared to a larger diameter than the hole, thereby captivating it to the heat sink and permitting movement by the nut through the hole while acting against the spring force. A stop shoulder on the nut and a projecting shoulder on the stud define the length travel of the nut onto the stud and establishes a pre-load compression force on the spring. A self-centering structure included for the nut to engage the stud.

BACKGROUND OF THE INVENTION

This invention relates to heat sinks and especially to fasteners and nut assemblies use to attach or tie-down heat sinks to printed circuit boards and/or microelectronic circuit “chips”.

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, fasteners, and tools is often an issue.

Captive fasteners, such as captive screws and captive nuts are devices used to fasten two components/objects together, where the fastener remains with one of the components when loosened. Typically included with these fasteners is a tie-down spring which permits the alignment and other movement between the two fastened objects.

The captive fastener is “caught” to a component/object by a flange, a ferrule, a spring clip or the like. This retaining structure prevents the total removal of the captive fastener from that component.

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's of watts and even 100'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 circuit board, or mounted to a mounting frame, which in turn is mounted to the circuit board on which the IC chip is also mounted.

Spring clips have been used to secure heat sinks, but are sensitive to vibration. They often interfere with the heat transfer fins on the heat sink and are often hard to positively snap into place and to release.

Captive fasteners have provided and improvement over heat sink clips. Two or four captive fasteners, such as screws or nuts, are used to engage respective flanged corners of a heat sink. These captive fasteners have threaded ends which usually engage a threaded ferrule or a threaded bushing, or a threaded screw/bolt mounted into a hole through the PC board.

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 thereby providing the greatest positive surface contact.

Captive fasteners for IC chip heat sinks, with heat transfer polymeric sheeting, have incorporated spring tie-downs. The tie-down force exerted on the heat sink is the total of the spring forces of the compressed springs. This structure permits each heat sink to “float”, i.e., to move through expansion and contraction phases as the IC chip temperature changes.

The chassis (i.e., chip size) of microprocessor and electronic modules is becoming smaller with smaller footprints. As more boards are crowded into tightly spaced cases or into tightly spaced racks in a chassis, the size and position of heat sink tie-down screws, including captive fasteners, becomes an issue. The alignment of the heat sink during its mounting over an (integrated circuit) IC chip requires an ease of alignment of the captive fasteners with the board mounted receiving members (an easy operation in the alignment of the fasteners). This can require, generally, two hands and some lateral movement. This lateral movement can jeopardize the integrity of the printed circuit coating, and can create a missalignment of the interface polymer heat transfer pad on the IC. There is also an issue with the tightened fasteners and the tie-down force exerted by the spring.

What is desired is a self-aligning nut-type structure for use with board mounted studs for securing a heat sink to an IC chip.

What is also desired is to provide this nut structure captivated to the heat sink base.

What is further desired is to provide this nut structure with a spring tie-down, whereof the nut can float on the heat sink base and the spring exerts the tie-down force,

What is also further desired is to provide this nut structure with a positive adjustment for setting the spring force to a consistent predetermined spring force.

SUMMARY OF THE INVENTION

The objectives of the present invention are realized in a captive nut assembly for securing a heat sink base to a PC board over an IC chip. A plurality of captive nuts are used to hold the heat sink, with each nut engaging an upstanding threaded screw, i.e., a bolt or a threaded stud. Each bolt or stud in turn is mounted in the PC board and becomes a part of the entire assembly. A compression spring mounted onto a nut exerts a tie-down pressure on the heat sink base to drive/bias it towards the IC chip and PC board. The captive nut is captivated to the heat sink base and is allowed to float up and down within a range of movement through a respective hole in the heat sink base under the movement of the spring.

A nut includes an internal stop shoulder adjacent its internal threads. An upstanding stud includes a chamfered leading end, a threaded section thereafter, and a projecting shoulder below, i.e., following, the threaded section. Each nut is tightened onto its respective stud until the projecting stud shoulder and the nut stop shoulder engage. This engagement defines the travel distance of the nut onto the stud and sets the preload force on the tie-down spring. By fully tightening each nut the heat sink is uniformly held onto the IC chip.

Factors which are considered when calculating the tie-down force of the springs to hold the heat sink to the IC chip include the size and length of the spring, the spring (material) strength and flexure characteristic, the coil spacing and its force verses movement characteristics, and the preload compression distance. The preload compression distance is how far the nut is permitted to travel until the assembly is tight and fully seated. This in turn is a function of the length of the threads on the nut, the location of the nut's stop shoulder, and the location of the studs projecting shoulder.

Each captive nut is T-shaped with a head portion and a shank portion. The head portion has a larger outside diameter than the shank portion. The compression spring is mounted to extend about the shank portion.

A peripheral rib extends like an external ring on the outside wall of the shank portion adjacent the head portion. This forms an undercut about the shank where the end coil loop of the spring can be entrapped and held to abut the bottom face of the head. This permits the spring to be pre-assembled onto the T-shaped nut and carried thereon during the further assembly of the invention.

The head member can have a Phillips, a slotted, an Allen wrench, a TORX, a hex head or other type of top face shape for receiving a tightening tool. The shank portion may be a solid dowel-type member with a bore drilled or otherwise formed longitudinally into it from its leading end. The bore is threaded to engage the threads of an upstanding PC board mounted stud. A section of the bore, outboard of the threads is counter-bored to a larger inside diameter to form the nut's stop shoulder.

This counter-bored section of the shank forms a tubular structure which is integral with and a continuous part of the rest of the nut. This tubular section has a thinner wall thickness than the threaded section.

The tip of the free end of the tubular section is further counter-bored to an even larger inside diameter. This tip, free end is flared once the shank of the nut is inserted through the hole in the base of the heat sink to captivate the nut and spring sub-assembly to the heat sink base. The second, or larger counter-bore creates the thinnest wall section and thereby controls the length of the tubular section which is flared. This in turn, affects the shape and size of the flare.

Each PC board mounted threaded stud has a press fit anchor-type head and carries an adjacent peripheral groove which with the head anchors the stud into the PC board. The stud can have a plain cylindrical rod section extending from the mounting head. This section is a solid, dowel-type member. The leading upward end of the dowel section has its outside diameter reduced to form the projecting shoulder which engages the nut stop shoulder. The reduced (inside diameter) I.D. portion is threaded to mate with the nut threads.

The flaring of the nut, and its successively step-in receiving sections provide a self-centering function for mating the nut and stud.

The structure of the T-shaped nut and the structure of the PC board mounted upstanding stud can be varied and still provide the operation and function of the present invention. Departures in the shape of the threaded stand-off stud may also be used and still provide the operation and function of the present invention. What is required is that the nut movement is stopped on the stud prior to the stud bottoming out on the threads of the nut, or conversely, the nut bottoming out on the base end of the stud. This movement stop will allow the threads be turned further, once the stop is reached, to provide a torque on the threads which slightly deforms them and establishes a pressure to provide a positive tightened assembly which resists loosening under vibration.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a captive shoulder nut assembly having a nut member including a spring tie-down. The nut member passes through a plate on a heat sink and mates with and threaded stud mounted on a PCI board (peripheral component interconnect board) or a PC board (printed circuit board). The heat sink is thereby held to an IC device (integrated circuit device or microchip) or other electronic module mounted on the board with a pre-determined spring force.

A captive shoulder nut assembly11,FIG. 1, is suitable for holding a heat sink13onto a PC board15over an electronic module (IC module)17mounted to the PC board15. The assembly11has a nut member19which is T-shaped. The head portion21of the nut19leads to a cylindrical-shaped shank portion23. The shank portion23is a solid, dowel-like structure with a bore25extending longitudinally down its centerline.

The bore25has a threaded section extending from the root of the bore25. A first counter-bored section29outboard of the threaded section27provides a first enlarged inside diameter section29leading into the threaded section27. A second, further counter-bored section31is outboard of the first counter-bored section29and provides a second further enlarged inside diameter section31leading into the first counter-bored section29

Each counter-boring successively reduces the wall thickness. At the second counter-bored section31the wall thickness forms a tubular section which is capable of being flared at the end33thereof. This succession of structural changes lends to the nut self-centering onto a mating stud47discussed below. The lead-in diameter to the threaded portion27of the bore25is reduced in a succession of steps.

A ramped section forms an external rib or ring35on the shank portion23adjacent the head portion21. This space between the underside of the head21and the ring35defines a retaining groove37in the outside of the nut member19just below the face of the head portion21. A biasing compression spring39is positioned over the shank portion23to have an end coil loop thereof secured within the retaining groove37. The tubular portion31as well as the first counter-bored section29and the threaded section25of the nut are suitable to be inserted through a straight hole41in the base43in the heat sink13. The inner diameter of the spring39is larger than the diameter hole41and thereby the spring remains above the hole41and operates against the heat sink base43.

The second counter-bored tubular, outermost section31is flared at its end33with a flaring tool after the nut19and spring39sub-assembly is mounted in the hole41to extend partially below the heat sink base43. The sub-assembly is thusly made captive to the heat sink base43with the nut being movable upward and downward in the hole41against the spring force of the biasing spring39.

The change in inside diameter between the threaded section27and the first counter-bored section29forms a stop shoulder45within the nut19at the inboard end of the first counter bore section29.

The nut member19is capable of mating with an upstanding threaded stud47attached to the PC board15at its underlay backing49. Each threaded stud47has a press fit anchor-type head51which is press fit into the backing49after the stud47is pushed through a hole53in the board. A peripheral groove55adjacent the head51deforms the underlay backing49material and anchors the stud47into the PC board.

The stud47,FIG. 5, can have a plain cylindrical rod section57extending from the mounting head51. This section57is a solid, dowel-type member. The leading upward end of the dowel section has its outside diameter reduced to form the projecting shoulder59which engages the nut stop shoulder45. The reduced I.D. portion is threaded to form a threaded section61of the stud47to mate with the nut threads27. The outermost lead end of the threaded section61is chamfered63.

The top of the head21portion of the T-shaped nut member19carries a Phillips socket65for a Phillips driver.

The details of the T-shaped nut19, the compression spring39, and the board mounted stud47are further shown inFIG. 2,FIGS. 3a-3c, andFIG. 4.

The invention provides: (1) that the nut movement is stopped on the stud prior to the stud bottoming-out on the threads of the nut, or conversely, the nut bottoming out on the base end of the stud; (2) the movement stop allows the threads be turned further, once the stop is reached, to provide a torque on the threads which slightly deforms them and establishes a pressure to affect a locked-thread assembly which resists loosening under vibration; (3) an alignment lead-in to the threaded bore of the nut is effected by a succession of increasingly larger under cuts leading to the free end of the nut; (4) that the hold-down spring rides with the nut during assembly; and (5) that the free-end or outboard undercut section has a wall which can be flared outwardly to capture the nut onto the heat sink base.

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.