Active solid-state devices (e.g. – transistors – solid-state diode – Housing or package – With provision for cooling the housing or its contents
Reexamination Certificate
1999-08-30
2003-09-09
Clark, Sheila V. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Housing or package
With provision for cooling the housing or its contents
C257S718000, C257S727000, C257S706000
Reexamination Certificate
active
06617685
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to the packaging of electronic components in electronic devices. More particularly, the present invention relates to the removable attachment of a heat sink to an integrated circuit package mounted on a circuit board in an electronic device.
BACKGROUND OF THE INVENTION
As the art moves towards smaller higher power integrated circuits such as SRAMs, heat transfer from the integrated circuit package (IC package) becomes increasingly difficult and more important. As used herein, the term “IC package” includes the heat generating integrated circuit as well as the packaging surrounding the integrated circuit.
One conventional technique to remove heat from an IC package is to employ a finned heat sink which is placed in thermal contact with the IC package. In this manner, heat generated by the IC package is conducted to the heat sink and then dissipated to the ambient environment.
Of importance, the heat sink must be reliably attached to the IC package in a manner which does not undesirably 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. However, thermally conductive adhesives do not adhere well to plastic IC packages 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. Yet, it is desirable to have a removable heat sink to readily allow chip repair, rework and/or replacement. Accordingly, it is desirable to avoid the use of thermally conductive adhesives altogether.
One prior art removable heat sink uses clips and/or fasteners to attach the heat sink directly to the IC package. However, when attached in this manner, the heat sink exerts undue force on the IC package which can damage and ultimately destroy the IC package. To avoid this problem, other removable heat sinks are attached directly to the circuit board to which the IC package is connected.
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
in accordance with the prior art. Located between heat sink
10
and circuit board
14
is an IC package
16
which generates heat during use. IC package
16
is typically electrically connected to circuit board
14
by one or more circuit interconnections, e.g., solder, which are not illustrated in
FIG. 1
for purposes of clarity. Fastening members
12
A urge heat sink
10
towards circuit board
14
and down onto IC package
16
to make the thermal contact between heat sink
10
and IC package
16
.
Although providing the force necessary to make the thermal contact between heat sink
10
and IC package
16
, fastening members
12
A cause heat sink
10
to press unevenly on IC package
16
. In particular, IC package
16
acts as a pivot between heat sink
10
and circuit board
14
so that end
10
A of heat sink
10
is urged away from end
14
A of circuit board
14
as indicated by arrows
18
. This causes the force exerted by heat sink
10
on to IC package
16
to be greater at side
16
A of IC package
16
than at side
16
B. This uneven force distribution can damage and even crack IC package
16
. Further, this uneven force distribution can create a gap between side
16
B and heat sink
10
resulting in poor heat transfer between IC package
16
and heat sink
10
. Alternatively, or in addition to, this uneven force distribution can cause circuit interconnection failure near side
16
B of IC package
16
. As those skilled in the art understand, these conditions can ultimately cause failure of device
8
.
To avoid these drawbacks, it has become known in the art to attach both sides of heat sink
10
to circuit board
14
. As an example, second post type fastening members
12
B illustrated in dashed lines in
FIG. 1
can be employed. This tends to equalize the force exerted by heat sink
10
on both sides
16
A and
16
B of IC package
16
. However, this also causes the ends
14
A,
14
B of circuit board
14
to be pulled up by fastening members
12
B,
12
A, respectively, relative to the die attach region
14
C of circuit board
14
to which IC package
16
is attached. This bending force, indicated by arrows
20
, causes warpage of circuit board
14
such that circuit board
14
is displaced to a position
22
. Over time, this warpage can cause device
8
to fail, e.g., from failure of circuit interconnections between IC package
16
and circuit board
14
.
Accordingly, the art needs a heat sink assembly which results in even force application to the IC package and avoids circuit board warpage.
SUMMARY OF THE INVENTION
In accordance with the present invention, a heat sink assembly for an electronic device is presented. The assembly includes a retainer having a body with a first body edge and a second body edge, the second body edge being opposite and removed from the first body edge, a finger extending from the body along the first body edge and at least one leg extending from the body along the second body edge. The assembly further includes a heat sink having a base and fins extending from the base, where slits in the fins define a trench. In the finished assembly, an upper package which includes an electronic device is located between a substrate such as a circuit board and the heat sink, and the retainer holds the heat sink in place.
When attached to the heat sink, tension in the retainer causes the finger to apply a downward vertical force on the heat sink. The heat sink, in turn, presses downwards towards the upper package. Of importance, by locating a finger tip of the finger at the center of the heat sink, the heat sink presses down uniformly on the upper package.
In one embodiment, the upper package is attached to an upper surface of the circuit board, and a lower package which includes an electronic device is attached to a lower surface of the circuit board directly opposite the upper package. In accordance with this embodiment, the retainer includes two legs which are symmetric with respect to the finger, and each leg applies an equal upward force on the lower package. In this manner, a heat sink assembly in accordance with the present invention avoids the application of uneven force distribution on the upper and lower packages and the associated damage and ultimate device failure caused by use of conventional heat sink assemblies.
Further, the vertical upward force is transferred from the legs directly back to the heat sink through the lower package, the circuit board and the upper package without imparting any bending force on the circuit board. In this manner, a heat sink assembly in accordance with the present invention avoids circuit board warpage and the associated ultimate device failure associated with conventional heat sink assemblies.
In one embodiment, the body of the retainer is adjacent an edge of the circuit board so that the retainer passes around the circuit board instead of through the circuit board. The allows greater utilization of the circuit board, e.g., allows more traces and/or vias to be located on or in the circuit board, compared to the case where a retainer passes through the circuit board.
Also in accordance with the present invention, a method of assembling a heat sink assembly includes locating an upper package having an electronic device between a substrate such as a circuit board and a heat sink. A retainer is then positioned so that a finger of the retainer is aligned with a trench of the heat sink. The finger is then slid into the trench to secure the heat sink in place with the retainer.
While aligning the finger of the retainer with the trench of the heat sink, legs of the retainer are also aligned to slide around the circuit board. Thus,
Clark Sheila V.
Gunnison McKay & Hodgson, L.L.P.
Hodgson Serge J.
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