Heat sink and package surface design

Details Heat sink and package surface design Heat sink and package surface design

2002-07-26

2004-06-29

Chervinsky, Boris (Department: 2835)

Electricity: electrical systems and devices

Housing or mounting assemblies with diverse electrical...

For electronic systems and devices

C257S706000, C257S717000, C165S080300, C165S185000, C361S719000

Reexamination Certificate

active

06757170

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention relates to an electronic assembly that includes a thermal interface material sandwiched between a heat sink and an integrated circuit package.
BACKGROUND
Electronic devices generate heat during operation. Thermal management refers to the ability to keep temperature-sensitive elements in an electronic device within a prescribed operating temperature.
Historically, electronic devices have been cooled by natural convection. The cases or packaging of the devices included strategically located openings (e.g., slots) that allow warm air to escape and cooler air to be drawn in.
The development of high performance electronic devices, such as processors, now requires more innovative thermal management. Each increase in processing speed and power generally carries a “cost” of increased heat generation such that natural convection is no longer sufficient to provide proper thermal management.
One common method of cooling electronic devices includes thermally coupling a heat sink to the package of the electronic device. A typical heat sink includes protrusions such as fins or pins that project from a body of the heat sink. The protrusions give the heat sink a larger surface area such that the heat sink dissipates a greater amount of heat from the package into the surrounding environment. Heat sinks are fabricated from materials with high thermal conductivity in order to efficiently transfer thermal energy from the electronic device package.
FIG. 1
shows a prior art electronic assembly
6
. Electronic assembly
6
includes an integrated circuit such as die
8
that is secured within an integrated circuit package
14
. Integrated circuit package
14
is typically soldered or plugged into a motherboard on a computer. Integrated circuit package
14
includes a heat spreader
12
that is connected to a heat sink
10
. Heat sink
10
cools the integrated circuit package
14
during the operation of an electronic system that includes die
8
.
A thermal interface material
16
is sometimes used to promote an effective thermal path between heat spreader
12
and heat sink
10
. Thermal interface material
16
is typically in the form of a paste or tape.
New thermal interface materials with higher thermal conductivities are continually being developed to meet the requirements for more efficient heat removal. These improved materials are necessary to keep the next generation of processors operating at lower temperatures.
Some of the new thermal interface materials are phase-change materials. These new phase-change materials have been proven to be thermally superior to other types of thermal interface materials.
As used herein, a phase change thermal interface material is a material that changes from solid to liquid when its temperature is raised above a certain level. The phase transition temperature of thermal interface material
46
is below the operating temperature of the junction between heat sink
10
and integrated circuit package
14
but above ambient temperature such that there is a transition from solid to liquid. As thermal interface material
16
changes to a liquid, it flows into the cracks in heat sink
10
and heat spreader
12
. When thermal interface material
16
cools below a certain temperature, it turns permanently back into a solid.
Heat sink
10
is typically compressed against heat spreader
12
by adhesives, screws, and/or bolts. Another common method uses one or more clips to compress heat sink
10
against heat spreader
12
.
Compressing heat sink
10
against integrated circuit package
14
decreases the thermal impedance between integrated circuit package
14
and heat sink
10
. However, the new phase-change materials are often squeezed, or squished, out from between heat sink
10
and heat spreader
12
.
FIG. 2
shows a compressive force (designated by arrow A) applied to heat sink
10
and integrated circuit package
14
. Thermal interface material
16
tends to squish out the sides as it changes from solid to liquid, since it is sandwiched between two flat surfaces on heat sink
10
and heat spreader
12
. Larger compressive forces generate more leakage. Containing thermal interface material
16
is particularly critical when metal-based, electrically conductive, low-melting temperature alloys are used as thermal interface material
16
. The escaping thermal interface material
16
forms droplets
18
that can drip off the electronic assembly
6
onto a surface of a substrate such as a printed circuit board. If the droplets
18
fall onto the substrate, they may contaminate one or more electrical pathways.
The flat mating surfaces on heat sink
10
and heat spreader
12
come into contact as heat sink
10
engages heat spreader
12
. Therefore, electronic assembly
6
does not include the ability to maintain bond line thickness between heat sink
10
and heat spreader
12
or align heat sink
10
relative to heat spreader
12
. Maintaining bond line thickness between heat sink
10
and heat spreader
12
would be desirable because a pre-specified volume of the thermal interface material could be contained between heat sink
10
and integrated circuit package
14
.
There is a need for an electronic assembly that adequately maintains a thermal interface material between a heat sink and an integrated circuit package. In addition, any improved electronic assembly should be able to control bond line thickness between the heat sink and integrated circuit package. Interlocking the heat sink and integrated circuit at least partially together would also be desirable.


REFERENCES:
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patent: 5276586 (1994-01-01), Hatsuda et al.
patent: 5471027 (1995-11-01), Call et al.
patent: 5623394 (1997-04-01), Sherif et al.
patent: 5905636 (1999-05-01), Baska et al.
patent: 5907474 (1999-05-01), Dolbear
patent: 5985697 (1999-11-01), Chaney et al.
patent: 6218730 (2001-04-01), Toy et al.
patent: 6294408 (2001-09-01), Edwards et al.
patent: 6570764 (2003-05-01), Bhatia et al.

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