Active solid-state devices (e.g. – transistors – solid-state diode – Housing or package – With provision for cooling the housing or its contents
Reexamination Certificate
1998-05-28
2001-01-30
Potter, Roy (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Housing or package
With provision for cooling the housing or its contents
C257S713000, C257S727000, C257S731000
Reexamination Certificate
active
06181006
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to the field of integrated circuit packages and, in particular, to mounting arrangements for securing an integrated circuit package to a heat sink.
BACKGROUND
Integrated circuits (“ICs”) have many uses in industries ranging from communications to consumer electronics. By way of example, a power transistor IC is formed by fabricating one or more transistor cells on a silicon wafer, commonly referred to as a transistor “chip”. The transistor chip is attached to an isolating layer, normally a ceramic substrate, which is thermally, but not electrically, conductive. The ceramic substrate is itself attached to a thermally conductive mounting flange. A protective cover is secured to the flange, covering the substrate and transistor chip, thereby forming a power transistor IC “package.”
Various electrically conductive (e.g., thin metal) leads may be attached to, and extend away from the package, in order to connect common terminals of the transistor chip to other circuit elements located, e.g., on an adjacent printed circuit (“PC”) board. For example, with a bipolar junction type power transistor, respective electrical leads attached to the package are connected to a base, emitter and collector of the transistor chip.
Because the power transistor package generates a significant amount of heat during operation, the bottom surface of the package mounting flange is normally directly secured to a metallic heat sink underlying the PC board. For example, a single layer PC board has a layer of dielectric material between respective top and bottom conductive surfaces, wherein the bottom surface acts as a reference ground. This bottom surface is connected, usually with screws or solder, to the underlying metal heat sink, so that the bottom surface and the heat sink have the same ground potential with respect to any circuit elements attached to the top surface of the PC board.
There are several known techniques for securing an IC package to a heat sink surface. For example, as illustrated in
FIG. 1
, an exemplary IC package
20
may be secured to a heat sink
22
by a solder connection
28
between the bottom surface of the package mounting flange
26
and the surface of the heat sink
22
.
While this approach is relatively simple, the solder weld material
28
will invariably have a different thermal expansion coefficient than the respective (typically metal) mounting flange
26
and heat sink
22
. As a result, the bond between the mounting flange
26
and heat sink
22
will weaken or even be destroyed by the thermal expansion stress between the respective layers, especially when subjected to repeated changes in temperature during each use of the IC package
20
. Further, the presence of the intervening bonding material layer
28
may lesson the effectiveness of the heat conduction between the flange
26
and heat sink
22
. A still further disadvantage with this approach is that, in order to remove the IC package
20
for repair or replacement, the entire heat sink
22
must be heated to break the solder bond
28
, thereby causing any other solder bonds on the same heat sink
22
to be weakened.
Referring to
FIG. 2
, as an alternative to using a solder connection, the IC package
20
may be secured to the heat sink
22
with a pair of screws
24
through openings located on respective ends of the mounting flange
26
. Referring to
FIGS. 3 and 4
, still another technique for securing an IC package to a heat sink is to insert one or more screws
30
into, so as to be protruding above, the surface of the heat sink
22
. A resilient metal strip
32
is extended from the screw(s)
30
and is shaped so as to apply a clamping force upon the cover of the IC package
20
, thereby distributing a substantially centered force that “secures” the mounting flange
26
against the heat sink
22
.
Still another method for securing an IC package to a heat sink is disclosed and described in pending U.S. patent application Ser. No. 08/956,193, entitled, “Mounting Arrangement For Securing An Integrated Circuit Package To A Heat Sink,” which is fully incorporated herein by reference for all it teaches. As taught therein, and as illustrated in
FIG. 5
, a top surface
52
of a protective cover
50
of an IC component package
40
is provided with a centered-protrusion
54
. A resilient retaining-spring
46
formed into a ribbon-like shape having opposing ends
56
and
58
that extend from a curvelinear bottom surface
60
is provided with an opening
62
sized to mate with the centered protrusion
54
.
To mount the IC package
40
to a heat sink
42
, the retaining-spring opening
62
is compressively mated onto the package cover protrusion
54
as the mounting flange
45
of the IC package
40
is inserted between substantially parallel walls
44
and
48
protruding from the heat sink
42
, such that the opposing retaining-spring ends
56
and
58
extend away from the package cover
50
at substantially the same, albeit reverse angles. The walls
44
and
48
are distanced from each other just so as to cause moderate compression of the opposing retaining-spring ends
56
and
58
toward each other as the flange
45
is inserted against the heat sink
42
.
The walls
44
and
48
are each provided with a respective plurality of notches
64
and
68
, which extend substantially parallel to the heat sink
42
in a “ratchet-type” relief pattern. Once the mounting flange
45
is pressed against the heat sink
42
, the opposing spring ends
56
and
58
are retained in place by the respective wall notches
64
and
68
. In this manner, the spring
46
applies a retaining force against the package cover
50
, thereby securing the mounting flange
45
against the heat sink
42
, as indicated by the arrow
70
.
With any of the above-illustrated methods for securing an IC package to a heat sink, once the IC package is secured to the heat sink, electrical leads extending from the package (not shown in
FIGS. 1-5
) must be connected to respective conductive surface leads or areas, e.g., located on an adjacent PC board attached to the heat sink.
By way of illustration, referring to
FIG. 6
, the mounting flange
86
of an IC package
80
is mounted on a heat sink
82
via a conventional solder weld
84
. A single layer PC board
88
is also secured to the heat sink
82
, e.g., by screws (not shown) adjacent both sides of the package
80
. The PC board includes a metal top surface
90
, a layer of dielectric material
92
, and a metal bottom surface
94
, respectively, wherein the bottom surface
94
and attached heat sink
82
collectively act as a reference ground with respect to circuit elements (not shown) attached to the top surface of the PC board
88
. Respective leads
96
and
98
extend from opposite sides of the package
80
and are connected to corresponding conductive paths formed on the top surface
90
of the PC board
88
via respective solder welds
100
and
102
.
As with the problems of using a solder weld connection between the respective package flange (
28
) and heat sink (
22
) described above in conjunction with
FIG. 1
, the solder weld connections
100
and
102
are also prone to problems caused by different thermal expansion coefficients between the solder material, the conductive surface
90
, and the respective (metal) leads
96
and
98
. In particular, the solder material can crystallize after repeated heating and cooling, causing welds
100
and
102
to weaken and/or fail, with the respective leads
96
and
98
lifting and separating from the surface
90
of the PC board
88
.
Thus, it would be desirable to provide improved arrangements for securing an IC component package to a heat sink, whereby solder-welds are eliminated.
Further, it is desirable to provide for as much thermal conduction of heat from the respective IC package and electrical leads to the heat sink as possible.
SUMMARY OF THE INVENTION
The present invention provides improved arrangements for securing an IC component package to a heat sink in a manner which provide
Ahl Bengt
Leighton Larry C.
Moller Thomas W.
Ericsson Inc.
Lyon & Lyon LLP
Potter Roy
LandOfFree
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