Semiconductor device manufacturing: process – Packaging or treatment of packaged semiconductor – Making plural separate devices
Reexamination Certificate
1997-07-10
2002-11-12
Talbott, David L. (Department: 2827)
Semiconductor device manufacturing: process
Packaging or treatment of packaged semiconductor
Making plural separate devices
C438S118000, C438S122000, C438S123000, C438S124000, C438S127000, C257S675000
Reexamination Certificate
active
06479323
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a semiconductor device having a heat sink attached to a lead frame and a method for making the same. More particularly, the present invention relates to a method and structure for attaching a heat spreader or heat slug to a lead frame.
BACKGROUND OF THE INVENTION
FIG. 1
a
is a top view of a semiconductor device
100
which includes semiconductor die
101
, die attach adhesive
102
, heat slug
104
, bonding wires
105
-
106
, molding compound
107
, lead frame
108
(which includes leads
111
and
112
) and double-sided polyimide tape
109
. Elements which are encased in molding compound
107
are shown in dashed lines in
FIG. 1
a
.
FIG. 1
b
is a cross sectional view of semiconductor device
100
along plane
1
b
—
1
b
of
FIG. 1
a
.
Heat slug
104
is used to dissipate heat generated within device
100
during operation of device
100
. Heat generated within semiconductor die
101
is conducted through die attach adhesive
102
to heat slug
104
. Heat slug
104
is typically a metal, such as copper or aluminum, which has a high thermal conductivity. Heat slug
104
conducts heat to the environment external to device
100
and thereby prevents heat build-up in the vicinity of die
101
. As illustrated in
FIG. 1
b
, the bottom portion of heat slug
104
extends through molding compound
107
such that this portion is exposed to the outside environment. A heat spreader (not shown) is similar to heat slug
104
, except that a heat spreader is totally encapsulated by molding compound
107
(i.e., is not exposed to the outside environment). Heat spreaders and heat slugs are hereinafter generically referred to as heat sinks.
Polyimide tape
109
is used to connect heat sink
104
to lead frame
108
. Polyimide tape
109
is typically positioned around the perimeter of the upper surface of heat sink
104
. The leads of lead frame
108
(including leads
111
and
112
) are positioned on polyimide tape
109
, such that polyimide tape connects heat sink
104
and lead frame
108
. After heat sink
104
is connected to lead frame
108
, die
101
is attached to central area of the upper surface of heat sink
104
using die attach adhesive
102
. Polyimide tape
109
holds heat sink
104
and lead frame
108
in a fixed relationship when molding compound
107
is formed around the various elements of semiconductor device
100
.
FIG. 2
is a cross sectional view of tape
109
, which includes polyimide layer
201
and adhesive layers
202
and
203
. Polyimide tape
109
is thermally conductive and electrically insulating. As a result, heat received by heat sink
104
is conducted through polyimide tape
109
to lead frame
108
. However, because polyimide tape
109
is electrically insulating, polyimide tape
109
does not short the various leads of lead frame
108
.
To attach heat sink
104
to lead frame
108
, adhesive layer
202
is placed on heat sink
104
, thereby fixing polyimide tape
109
to heat sink
104
. Lead frame
108
is then positioned on adhesive layer
203
, thereby fixing polyimide tape
109
to lead frame
108
. Lead frame
108
and heat sink
104
are then clamped together and polyimide tape
109
is heated, thereby curing adhesive layers
202
and
203
and attaching lead frame
108
to heat sink
104
. Lead frame
108
must be placed on adhesive layer
203
shortly after adhesive layer
202
is placed on heat sink
104
to prevent adhesive layer
203
from becoming contaminated by exposure to the environment.
Polyimide tape
109
is relatively expensive because adhesive layers
202
and
203
must be laminated onto each side of polyimide layer
201
. Moreover, if polyimide tape
109
is improperly clamped between heat sink
104
and lead frame
108
, gaps may exist between adhesive layer
202
and heat sink
104
and/or between adhesive layer
203
and lead frame
108
. As a result, poor adhesion may exist between heat sink
104
and lead frame
108
. Further contributing to this problem, polyimide tape
109
has a tendency to warp, thereby promoting a sub-optimal connection between lead frame
108
and heat sink
104
. Additionally, polyimide layer
201
must have a minimum thickness, typically more than one mil, to form a film. As polyimide layer
201
becomes thicker, the ability of polyimide tape
109
to conduct heat is reduced.
U.S. Pat. No. 4,783,428 issued to Kalfus discloses another method for coupling a lead frame to a heat sink. In Kalfus, a first layer of thermally conductive epoxy is screen printed onto a heat sink. The first layer of epoxy is cured and a second layer of thermally conductive epoxy is screen printed on the first layer of epoxy. A lead frame is then placed on the second layer of epoxy and the second layer of epoxy is cured to attach the lead frame to the heat sink. The lead frame must be connected to the heat sink soon after the second layer of epoxy is screen printed to prevent contamination of the second layer of epoxy.
It would therefore be desirable to have a method and structure for attaching a lead frame to a heat sink which overcomes the above described short-comings of polyimide tape. It would also be desirable if this method and structure provided a heat sink which (1) includes the materials necessary to connect the heat sink to the lead frame, and (2) can be conveniently stored for long periods of time until the heat sink is to be connected to the lead frame.
SUMMARY
Accordingly, the present invention provides a semiconductor device which includes a heat sink and a lead frame. A fully cured layer of thermally conductive epoxy is connected to the heat sink and a layer of thermoplastic is connected to the fully cured epoxy layer. The lead frame is connected to the layer of thermoplastic. Both the epoxy layer and the thermoplastic layer are applied in paste form, thereby substantially eliminating the adhesion problems previously associated with the use of polyimide tape
109
.
The epoxy and thermoplastic layers can be formed in a variety of ways. In one embodiment, the fully cured epoxy layer is formed on the heat sink and the thermoplastic layer is formed over the fully cured epoxy layer. The resulting structure can advantageously be stored until the heat sink is connected to the lead frame. In another example, the fully cured epoxy layer is formed on the heat sink and the thermoplastic layer is formed on the lead frame. Again, the resulting structures can be stored (separately) until the heat sink is to be connected to the lead frame.
In another embodiment, the fully cured epoxy layer is connected to the lead frame (instead of the heat sink) and the thermoplastic layer is connected to the heat sink (instead of the lead frame). Again, the epoxy layer is connected to the thermoplastic layer. In this embodiment, the epoxy and thermoplastic layers can be formed in a variety of different ways. In one example, the fully cured epoxy layer is formed on the lead frame and the thermoplastic layer is formed over the fully cured epoxy layer. Alternatively, the fully cured epoxy layer is formed on the lead frame and the thermoplastic layer is formed on the heat sink. Again, the resulting structures can be stored until the heat sink is to be connected to the lead frame.
In accordance with another embodiment of the invention, a heat sink structure includes a thermally conductive heat sink, a fully cured first layer of thermally conductive epoxy located over the heat sink, and a partially cured second layer of thermally conductive B-stage epoxy located over the first layer of epoxy. The first and second layers of epoxy are applied in paste form and the resulting heat sink structure can be stored until the heat sink structure is connected to a lead frame. When the heat sink structure is to be connected to the lead frame, these structures are positioned such that the partially cured second epoxy layer is contacting the lead frame. The second epoxy layer is then fully cured, thereby connecting the heat sink structure to the lead frame.
In a variation, the fully cured epoxy layer is formed
Lo Randy H. Y.
Mekdhanasarn Boonmi
Tracy Daniel P.
Chambliss Alonzo
Guth Michael A.
Klivans Norman R.
National Semiconductor Corporation
Skjerven Morrill LLP
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