Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
2003-03-17
2004-09-07
Chervinsky, Boris (Department: 2835)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C361S707000, C361S709000, C165S080300, C165S185000, C174S016100, C174S016300, C257S718000, C257S727000
Reexamination Certificate
active
06788538
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to semiconductor device packaging and, in particular, to a structure and method for removably attaching a heat sink to an electronic package which is, even more particularly, a surface mount package.
BACKGROUND OF THE INVENTION
In order to safely operate modem computers and other electronic devices, thermal energy must be removed from components which generate large amounts of heat and which cannot dissipate the heat fast enough to avoid harmful consequences. As the art moves towards smaller, higher-power integrated circuits such as static random access memory (SRAM) integrated circuits, heat transfer from the integrated circuit package (IC package) becomes increasingly difficult and more important. (The term “IC package” includes the heat-generating integrated circuit as well as the packaging surrounding the integrated circuit.) As chip density and the speed of IC chips increase, chips often require high-performance surface mount packages assembled very close to each other on circuit cards. These packages generate heat and, because they are packed very close to each other, they become hot.
One conventional method used to dissipate the accumulating heat of an electronic device such as an IC package is to force air over the device, thereby removing the heat by convection. This method has been substantially improved by attaching a heat dissipating device, also known as a heat sink, to the electronic device. The heat sink is designed to dissipate heat at a significantly greater rate than the electronic device alone. A heat sink typically has projections such as pins or fins, exposing a greater surface area to the flow of air for greater dissipation of heat.
When the heat sink is placed in thermal contact with the electronic device, the electronic device transfers heat by conduction to the heat sink. The heat to sink then dissipates accumulating heat by convection to the ambient environment. This method has become standard in the computer industry for dissipating the large amounts of heat generated by the new generation of computer processors.
Of importance, the heat sink must be reliably attached to the electronic device. In the case of IC packages, such attachment must not undesirably is stress or damage the IC package or the circuit board to which the IC package is connected. One conventional technique is to employ a thermally conductive adhesive which bonds the heat sink to the IC package. Thermally conductive adhesives do not adhere well to plastic IC packages, however, resulting in an unacceptably high incidence of bond failure between the plastic IC package and the heat sink. Further, once the heat sink is bonded with the thermally conductive adhesive, it is difficult to remove the heat sink from the IC package without causing damage to the IC package, the heat sink, or the circuit board. Still further, it is desirable to have a removable heat sink to readily allow chip repair, rework, or replacement. Accordingly, it is preferable to avoid the use of thermally conductive adhesives altogether.
Other methods have been used to secure the heat sink to the electronic device, including the use of clips to fasten the assembly together. It is preferred that any attachment method allow for quick and easy installation and removal of the heat sink while providing a secure attachment during operation and normal handling. Electronic devices must be removed and replaced from time to time; therefore, the heat sink must be easy to remove and install without sacrificing reliability or effectiveness. Clips offer the advantages of being relatively inexpensive, simple, operational over a multiple number of assembles and disassembles, and reasonably secure so that the heat sink does not disconnect or dislodge from the electronic device.
A wide variety of different clips are known. Two-piece dips are disclosed by U.S. Pat. No. 5,617,292 issued to Steiner. As Steiner teaches, however, previous two-piece clips used to fasten heat sinks generally lack sufficient rigidity and strength, particularly at the junction between the two parts of the clip, and separation of the assembly is a potential risk. In general, two-piece clips are constructed so that the intersection of the separate leg and the leaf spring member forms a loose hinge about which rotation at least to some degree in several directions is possible. Thus, the force holding the assembly together is exerted substantially entirely in a direction normal to the electronic component. The hinge mechanism is relatively weak and prone to failure, particularly under prolonged use and shock and vibration. Holding the necessary tolerances during manufacturing imposes additional difficulty in producing two-piece clips.
One-piece clips avoid some of the drawbacks inherent in two-piece clips. Such clips generally extend over the heat sink and attach at each end to the electronic device or its socket at specially provided ports or bosses. Clips of this design require modification of the electronic device or its socket and may deleteriously affect performance of those components. To avoid these problems, other clips attach removable heat sinks directly to the circuit board to which the IC package is connected.
For example, Tseng discloses, in U.S. Pat. No. 6,043,984 assigned to Intel Corporation, a removable heat sink that uses clips or fasteners to attach the heat sink to an IC package. The IC package is mounted to a substrate such as a circuit board. The clip has four L-shaped ears that are inserted through clearance holes in the substrate and corresponding attachment holes in the heat sink. The four L-shaped ears extend from a center plate portion. The L-shaped ears are bent during insertion through the holes. Once beyond the attachment holes of the heat sink, the L-shaped ears snap back so as to permit the clip to exert a spring force that pushes the heat sink into the integrated circuit package.
As described in U.S. Pat. No. 6,154,365, also assigned to Intel Corporation, the clip taught by Tseng has several drawbacks. From one electrical assembly to the next, the spring force from the L-shaped ears is not consistent due to the variations within part manufacturing tolerances. Moreover, the L-shaped ear technique does not permit a technician to adjust the spring force from the L-shaped ears so as to evenly distribute this force over the surface of the substrate. Without an even distribution of force, the substrate is more likely to bend which may cause the substrate or the integrated circuit within the integrated circuit package to crack. When attached directly to the IC package in the manner disclosed, the heat sink exerts undue force on the IC package which can damage and ultimately destroy the IC package.
Post-type fastening members are also used to secure a heat sink to an electronic device.
FIG. 1
is a side view of an electronic device
8
which includes a heat sink
10
directly attached by post-type fastening members
12
A to a circuit board
14
. Located between the heat sink
10
and the circuit board
14
is an IC package
16
which generates heat during use. The IC package
16
is typically electrically connected to the circuit board
14
by one or more circuit interconnections, e.g., solder, which are not illustrated in
FIG. 1
for purposes of clarity. The fastening members
12
A urge the heat sink
10
towards the circuit board
14
and down onto the IC package
16
to make the thermal contact between the heat sink
10
and the IC package
16
.
Although providing the force necessary to make the thermal contact between the heat sink
10
and the IC package
16
, the fastening members
12
A cause the heat sink
10
to press unevenly on the IC package
16
. In particular, the IC package
16
acts as a pivot between the heat sink
10
and the circuit board
14
so that the end
10
A of the heat sink
10
is urged away from the end
14
A of the circuit board
14
as indicated by the arrows
18
. This causes the force exerted by the heat sink
10
on the IC package
16
to be gr
Gibbs Ronald T.
Smith Grant M.
Chervinsky Boris
RatnerPrestia
Rode Lise A.
Starr Mark T.
Unisys Corporation
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