Heat dissipating assembly

A heat dissipating assembly includes a heat sink (10) having a chassis (12), a back plate (30) and a plurality of fasteners (40). Each fastener includes a screw cap (42), a spring (44) and a sleeve (46). The screw cap and the sleeve are movably connected together. The screw cap includes a head (420) and a tube (422) extending from the head. The tube includes a first cavity (424) and a second cavity (426). The back plate forms a plurality of posts (34) passing through the chassis and threadedly engaging in the first cavities of the tubes. A spring (44) is compressed between the first cavity of the tube and a bottom wall of the sleeve, for providing appropriate downwardly pressing force on the chassis of the heat sink against a CPU (22) mounted on a PCB (20).

DESCRIPTION

1. Field of the Invention

The present invention relates generally to a heat dissipating assembly for a computer, and more particularly to a heat dissipating assembly having fasteners for mounting a heat sink to a printed circuit board.

2. Description of Related Art

During operation of an electronic component such as a computer central processing unit (CPU), a large amount of heat is often produced. The heat must be quickly removed from the CPU to prevent it from becoming unstable or being damaged. Typically, a heat sink is attached to an outer surface of the CPU to absorb the heat from the CPU. The heat absorbed by the heat sink is then dissipated to ambient air.

Conventionally, a heat sink comprises a heat spreader and a plurality of fins extending from the heat spreader. The heat spreader is fastened on a printed circuit board by extending screws or bolts through holes defined in the printed circuit board.

Referring toFIG. 5, a conventional heat dissipating assembly900is shown, which comprises a heat sink92and fasteners94.

The heat sink92comprises a heat spreader920and a plurality of fins922extending from the spreader920. The spreader920has a bottom mating surface (not labeled) for thermally engaging with a heat generating electronic device such as a CPU (not shown) mounted on a printed circuit board (not shown). The heat spreader920forms four ears924extending outwardly from four corners thereof. Each ear924defines a hole926, for permitting passage of the fastener94therethrough. An annular step portion928is formed by the ear924in the hole926, for supporting a spring96thereon.

The fastener94comprises a circular head940, an elongated shaft942extending from the head940and terminating with a threaded bottom end944. The spring96is dimensioned to loosely fit around the shaft942, and contacts the annular step portion928when installed, for exerting a spring force downwardly on the ear924when the fastener94is threadedly fastened to a retainer (not shown) attached to the printed circuit board. An annular recess946is defined in a middle of a circumferential periphery of the shaft942. A washer98is received in the recess946to mount the fastener94to the ear924of the spreader920after the recess946of the shaft942and the threaded bottom end944passing through the hole926to reach a position below the ear924.

In assembly, the spring96is fitted around the shaft942. The shaft942is extended through the hole106of ear924of the spreader920. The spring96is compressed between the head940and the annular step portion928. The washer98snaps in the recess946of the shaft942when the recess946reaches the position below the ear924.

However, the spring96directly contacts the spreader920such that the spring96is prone to scrape the spreader920during the compression of the spring96to a predetermined position in which the threaded bottom end944can have a threaded engagement with the retainer. The scraping between the spring96and the spreader920may form metal scraps, which if fall on the printed circuit board will affect the electrical characters of the electronic components on the printed circuit board and can increase electrical resistance of the components, or shorten the components, whereby the performance of the electronic components deteriorates, or even the electronic components break down.

In view of the above, a new heat dissipating assembly which overcomes the above-mentioned disadvantages is desired.

SUMMARY OF INVENTION

Accordingly, what is needed is a heat dissipating assembly having a fastener for fastening a heat sink to a printed circuit board, wherein during operation of the fastener, risk of scraping between a spring and the heat sink to form metal scraps which may cause short circuit of electronic components on the printed circuit board is prevented.

According to an embodiment of the present invention, a heat dissipating assembly comprises a heat sink having a chassis, a back plate and a plurality of fasteners. Each fastener comprises a screw cap, a spring and a sleeve. The screw cap comprises a head and a tube extending from the head. The tube comprises a first cavity and a second cavity. The back plate forms a plurality of posts passing through the chassis and threadedly engaging in the first cavities of the tubes, respectively. A spring is compressed between the screw cap and the sleeve, and exerts a downwardly pressing force on the sleeve against the chassis whereby the chassis is pushed by the springs to have an intimate contact with a CPU mounted on a printed circuit board through which the posts extend upwardly.

DETAILED DESCRIPTION

Reference will now be made to the drawing figures to describe a heat dissipating assembly100in accordance with a preferred embodiment of the present invention in detail.

Referring toFIG. 1, the heat dissipating assembly100comprises a heat sink10, a backplate30and a plurality of fasteners40. The heat dissipating assembly100removes heat from a CPU22mounted on a printed circuit board (PCB)20. Four through holes24are defined in the PCB20respectively adjacent to four corners of the CPU22.

The heat sink10comprises a chassis12, and a plurality of fins14extending upwardly from the chassis12. An ear16is extended horizontally and outwardly from each of four corners of the chassis12. A fixing hole160is vertically defined through each ear16. Each fixing hole160corresponds to one of the through holes24of the PCB20.

The back plate30mounted on an underside of the PCB20comprises a cross-shaped base32. An aperture320is defined in a central portion of the base32. The aperture320is formed for reducing weight of the back plate30. Four posts34extend upwardly from four respective distal ends of the base32, for extending through the through holes24of the PCB20and through the fixing holes160of the heat sink10. Each post34terminates with a threaded top end340. A collar342is formed on a circumferential periphery of the post34and abuts against a bottom of the ear16, for protecting the ear16of the heat sink10from being deformed downwardly by the fastener40with an excessive pressure and protecting the PCB20from being damaged.

Referring toFIG. 2andFIG. 3, each fastener40is made of insulative material such as plastic, and comprises a screw cap42, a spring44and a sleeve46. The screw cap42comprises a circular head420having a cross slot (not labeled) therein so that a screwdriver (not shown) can be used to apply a rotation force to the screw cap42. The screw cap42forms an elongated tube422extending downwardly from the head420. The tube422comprises a first cavity424and a second cavity426communicating with the first cavity424. The first cavity424is located above the second cavity426. The second cavity426has a larger diameter than the first cavity424, for receiving the spring44therein. The tube422has a thread (not labeled) in the cavity424, for engaging with the threaded top end340of a corresponding post34of the back plate30. A flange428is formed at a bottom end of a circumferential periphery of the tube422. The flange428has a bevel (not labeled) for facilitating insertion of the tube422into the sleeve46. The sleeve46defines a chamber460for receiving the spring44. The first cavity424, the second cavity426and the chamber460of the sleeve46cooperatively form a room for the fastener40. The room receives the post34and the spring44therein. The sleeve46comprises a cylindrical wall462and a floor464extending inwardly from a bottom of the cylindrical wall462. A plurality of spaced hooks468is circumferentially formed at an inner surface of the wall462, for engaging with the flange428of the tube42. The combination of the flange428of the tube422and the hooks468of the sleeve46acts as an interlocking means to provide a movable connection between the screw cap42and the sleeve46. The floor464defines an opening466, for permitting the passage of the post34into the fastener40.

Also referring toFIG. 4, in assembly of the heat dissipating assembly100, the springs44are partly received in the chambers460of the sleeves46, respectively. The sleeves46are then brought to be assembled with the tubes422of the screw caps42by to inserting the flanges428of the tubes422into tops of the sleeves46until the hooks468of the sleeves46lock with the flanges428of the tubes422, whereby the screw caps42are movably connected with the sleeves46. Simultaneously, the springs44are partly received in the second cavities426of the tubes422. Accordingly, the springs44are totally received in the chambers460of the sleeves46and the second cavities426of the screw caps42. A bottom end of the spring44abuts against the floor464of the sleeve46and a top end abutting a portion of the screw cap42around the thread in the first cavity424. The springs44exert spring forces downwardly on the ears16via the bottom walls464of the sleeves46when the fasteners40are threadedly engaged with the posts34of the back plate30such that the risk of direct contact of the springs44and the ears16which may cause metal scraps to be formed between the springs44and the ears16is prevented. The fasteners40are thereby assembled.

The posts34of the back plate30are extended in turn through the through holes24of the PCB20and the fixing holes160of the ears16. The heat sink10is placed on the CPU22mounted on the PCB20. The fasteners40are assembled to the posts34of the back plate30by extending the threaded top ends340of the posts34through the openings466of the floors464and centers of the springs96to reach bottoms of the first cavities424, respectively. Then the screwdriver is used to rotate the screw caps42until the threaded top ends340are tightly threadedly engaged with the threads in the first cavities424of the screw caps42. The floors464of the sleeves46are pushed by the springs44to depress the ears16of the chassis12of the heat sink10. Thus, the chassis12of the heat sink10can have an intimate contact with the CPU22to thereby absorb heat from the CPU22. The threaded engagement of the threaded top ends340of the posts34with the threads in the first cavities424of the tube422provides a firm connection of the heat sink10to the PCB20. The heat sink10is thereby secured to the PCB20. In this position, the chassis12of the heat sink10is attached on the CPU22. The springs44are compressed between the first cavities424of the tubes422and the floors464of the sleeves46, whereby the springs44provide appropriate pressing forces on the ears16of the heat sink10toward the CPU22. Thus, the heat sink10is intimately and evenly contacted with the CPU22.