Active solid-state devices (e.g. – transistors – solid-state diode – Housing or package – With provision for cooling the housing or its contents
Reexamination Certificate
2000-09-20
2004-03-30
Pham, Long (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Housing or package
With provision for cooling the housing or its contents
C257S692000
Reexamination Certificate
active
06713864
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a heat-dissipation-enhanced semiconductor package. More particularly, the present invention is directed to a heat-dissipation-enhanced semiconductor package for reducing the probability of delamination occurring, and the number of steps in the manufacturing process.
2. Description of the Related Art
For a semiconductor package, the problem of heat dissipation is a very important issue. A semiconductor package with bad heat dissipation may not just produce errors, but may also reduce product reliability and greatly increase manufacturing cost.
FIG. 1
shows the DHS (Drop-in Heat Sink) structure of a semiconductor package disclosed in U.S. Pat. No. 5,225,710. The package's structure comprises: a die a pad
14
; a die
12
, which is attached to a first surface
141
of the die pad
14
with a suitable adhesive
15
, such as a silver paste; a plurality of leads
13
electrically connected to an active surface
121
of the die
12
by a plurality of bonding wires
17
, such as gold wires; a heat sink
16
and an encapsulant
11
. The die pad
14
and the plurality of leads
13
are all a part of a leadframe and are placed inside an upper mold
18
during manufacture. The heat sink
16
is located inside a lower mold
19
, and one surface of the heat sink
16
contacts the bottom of the lower mold
19
with points
161
and
162
. Another surface of the heat sink is attached to the second surface
142
and the die pad
14
.
An encapsulant
11
is injected to fill the molding cavity left by the package structure when the upper mold
18
and lower mold
19
are closed. The heat generated by the die
12
in the DHS package is characteristically dissipated from the die pad
14
, through the heat sink
16
attached to the die pad
14
and then to the atmosphere.
FIG. 2
shows the EDHS (Exposed Drop-in Heat Sink) structure of a semiconductor package disclosed in U.S. Pat. No. 5,381,042. The difference between the EDHS structure and the DHS structure is that in the EDHS structure a heat sink
21
with a flat bottom is directly exposed to the bottom of the semiconductor package, unlike the heat sink
16
of the DHS structure, which contacts the bottom of the semiconductor package through the points
161
and
162
. The exposed surface drop-in heat sink
21
provides a larger contact area than the drop-in heat sink
16
to dissipate heat. Therefore, heat dissipation in the EDHS structure is more effective than that in the DHS structure. However both the DHS and the EDHS structures have the following disadvantages:
1. During the DHS
16
and EDHS
21
manufacturing processes, the heat sink is put inside the lower mold
19
first, and the die pad
14
then aligned to the heat sink. In other words, an extra step is added, which increases the cycle time of the manufacturing process, and thus reduces the throughput.
2. The DHS
16
and EDHS
21
packages are covered by the encapsulant
11
, but both heat sinks have a CTE (Coefficient of Thermal Expansion) that is different from that of the encapsulants. Therefore, when temperature changes cause expansion and shrinking, the effect of this thermal stress on the contact surface between the heat sink and the encapsulant will cause delamination of that contact surface. Moreover, in both examples, because the amounts of encapsulant
11
inside the upper mold
18
and lower mold
19
are not the same, the package structure will also be warped after it cools, due to the different amounts of shrinking produced by the different amounts of encapsulant. Moisture in the atmosphere can enter the package through cracks caused by either delamination or warping; therefore, the reliability of the delaminated or warped semiconductor package is reduced.
3. When the encapsulant
11
is injected into the closed mold, the heat sink
21
is attached by four tie bars on the diagonals of the leadframe (not shown). As the strength of the four tie bars is not necessarily great enough to hold the heat sink
21
in place, some flashed encapsulant will be left on the bottom of the semiconductor package after encapsulation. The manufacturing cost of these packages is also increased by the additional deflashing step required to correct that.
4. In use, the known semiconductor packages described above provide heat dissipation paths which extend from the die
12
, through the die pad
14
, to the heat sinks
16
or
21
, and finally to the atmosphere. These heat dissipation paths are too limited because the plurality of leads are not used for dissipating the heat, reducing the efficiency of their heat dissipation.
5. Finally, thin products such as some consumer ICs, where the thickness of the package (“P” in
FIGS. 1 and 2
) is less than 1.00 mm, the thickness of the space in the lower mold is less than 0.45 mm, as shown in
FIGS. 1 and 2
. Therefore, the heat sinks
16
or
21
shown in
FIGS. 1 and 2
cannot be put inside such a thin package.
SUMMARY OF THE INVENTION
The present invention provides a semiconductor package which eliminates the need to put a heat sink inside the lower mold before the package is encapsulated. A semiconductor package constructed in accordance with the present invention reduces the delamination caused by a difference in the CTE's of the encapsulant and the heat sink. The invention eliminates the need for a deflashing process and permits a plurality of leads to be used for dissipating the heat generated by the die. The invention provides a semiconductor package that is suitable for a thin product, such as a TQFP (Thin Quad Flat Package) or a TSOP (Thin Small Outline Package).
In a particular embodiment of a semiconductor package in accordance with the present invention, only the contents of the upper mold is encapsulated. The heat sink is attached to the die pad and to a part of the plurality of leads with a thermally conductive and electrically insulating adhesive glue.
In accordance with the invention, the thickness of the heat sink is adjustable to suit the user's demands. In particular, in accordance with the invention, the heat sink is not limited by the thickness specification of the lower mold in and, therefore, the present invention is more advantageous for manufacturing thin products.
Since the width of the heat sink covers both the die pad and a part of the plurality of leads, the heat generated by the die can be dissipated not only to the atmosphere but also through the heat and the leads sink to the printed circuit board attached to the leads. Furthermore, because of the width of the heat sink, there is no need for precise alignment between the die pad and the heat sink. There is also no need for the tie bars of the leadframe to exert pressure on the heat sink, and therefore the cycle time of the manufacturing process is reduced and throughput is increased.
Moreover, because the heat sink according to the present invention is not encapsulated inside the lower mold, but attached to some of the leads with an adhesive glue instead, even though the CTEs of the heat sink and encapsulant or the leadframe and encapsulant are not the same, the encapsulant will not be cracked or delaminated when structural expansion and shrinking occurs. Thus, the reliability of products using the semiconductor package is increased.
Finally, because the heat sink is not encapsulated, unsightly flash left when encapsulating the upper mold will be hidden when the heat sink is attached to the die pad in accordance with the present invention. This eliminates the need for a deflashing step in the manufacturing process.
A first embodiment of the semiconductor package for enhancing heat dissipation in accordance with the present invention comprises a die, a leadframe, an encapsulant and a heat sink. The leadframe includes a die pad having a first surface that the die is attached to and a plurality of leads electrically connected to an active surface of the die. The encapsulant is used to seal said die and leadframe. The heat sink is attached to the second surface of the d
Farahani Dana
Ladas & Parry
Pham Long
Siliconware Precision Industries Co. Ltd.
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