Electricity: electrical systems and devices – Housing or mounting assemblies with diverse electrical... – For electronic systems and devices
Reexamination Certificate
1999-08-30
2001-06-19
Picard, Leo P. (Department: 2835)
Electricity: electrical systems and devices
Housing or mounting assemblies with diverse electrical...
For electronic systems and devices
C361S704000, C361S719000, C257S718000, C257S719000, C024S457000, C248S505000
Reexamination Certificate
active
06249436
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 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 causes 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 is presented. The heat sink assembly includes a heat sink, a circuit board and a package, the package including an electronic component such as an integrated circuit. The heat sink includes a plurality of torque bars and a plurality of locking features, where each torque bar of the plurality of torque bars has a different locking feature of the plurality of locking features. The package is located between the heat sink and the circuit board and attached to a die attach area of the circuit board.
The heat sink assembly further includes a heat sink retainer having a plurality of ends, each end of the plurality of ends being secured in a different locking feature of the plurality of locking features. Tension in the retainer imparts torque on the heat sink which causes the heat sink to impart a downward, e.g., first, force on the package. This downward force creates the thermal contact between the heat sink and the package.
Of importance, the retainer presses the heat sink down uniformly on the package. Thus, the heat sink assembly in accordance with the present invention avoids damage caused by the application of uneven force distribution on the package of conventional heat sink assemblies.
Further, the downward force applied to the package by the heat sink is countered by an equal upward, e.g., second, force applied by the base section of the retainer on the circuit board. Since the base section is located directly opposite the die attach area, the upward force is transferred from the base section directly back to the heat sink 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 of conventional heat sink assemblies.
Further, the retainer is designed to pass through the circuit board at a distance from the die attach area. This allows greater utilization of the die attach area, e.g., allows more traces and/or vias to be located on or in the circuit board at the die attach area, compared to a case where a retainer passes through the die attach area.
In one embodiment, a heat sink retainer includes a plurality of ends, a base section and a plurality of spring elements, where each spring element of the plurality of spring elements integrally connects a different end of the plurality of ends to the base section. In accordance with this embodiment, the locking features of the heat sink are notches on the torque bars. The ends of the retainer are moved laterally away from the package to snap the ends into the notches thus securing the retainer to the heat sink. Of importance, tension in the retainer securely presses the ends into the notches.
Also in accordance with the present invention, a method of pressing a heat sink into thermal contact with a package is presented. The method includes attaching the package to a die attach area of a substrate such as a circuit board. The heat sink is positioned such that the package is located between the heat sink and the substrate. A
Chervinsky Boris L.
Gunnison McKay & Hodgson, L.L.P.
Hodgson Serge J.
Picard Leo P.
Sun Microsystems Inc.
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