Semiconductor package with heat dissipating structure

Details Semiconductor package with heat dissipating structure Semiconductor package with heat dissipating structure

2003-03-24

2004-02-24

Clark, Sheila V. (Department: 2815)

Active solid-state devices (e.g., transistors, solid-state diode

Lead frame

With heat sink means

C257S796000, C257S706000

Reexamination Certificate

active

06696750

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to semiconductor packages with heat dissipating structures, and more particularly, to a semiconductor package having a heat sink whose surface is exposed to outside of a chip-encapsulation encapsulant so as to improve heat dissipating efficiency of the semiconductor package.
BACKGROUND OF THE INVENTION
Generally in semiconductor packages, a molding resin or encapsulation compound used for encapsulating chips is mostly a material having poor thermal conductivity such as epoxy resin, making heat produced from operation of the chips not able to be effectively dissipated via the molding resin to the atmosphere, which may thereby degrade reliability of the semiconductor packages especially for those mounted with highly integrated chips therein.
For solving the above heat dissipation problem, it is desired to incorporate a heat sink or heat spreader in a semiconductor package and expose a surface of the heat sink to outside of an encapsulant that encapsulates a chip mounted in the semiconductor package by which heat produced from the chip can be directly dissipated via the exposed surface of the heat sink to the atmosphere, thereby effectively enhancing heat dissipating efficiency. Such a package structure with an exposed heat sink is taught by U.S. Pat. Nos. 5,796,159, 5,598,034 and 5,608,267 in which the heat sink is attached by means of an adhesive to and below a die pad or leads of a lead frame.
However, the foregoing attachment between the heat sink and the lead frame via the adhesive would undesirably make fabrication processes more complex and cost-ineffective to implement. Accordingly, U.S. Pat. Nos. 5,328,870, 5,381,042, 5,444,602, 5,489,801 and 5,530,295 disclose a semiconductor package not having to adhere a heat sink to a lead frame; this type of package structure is named EDHS-QFP (exposed drop-in heat sink-quad flat package) generally fabricated by procedural steps illustrated in
FIGS. 4A
to
4
D. Referring to
FIG. 4A
, an encapsulation mold
20
is prepared for use in a molding process and composed of an upper mold
21
and a lower mold
22
, each of the upper and lower molds
21
,
22
having a cavity
210
,
220
respectively. Then, a heat sink
23
is placed in a drop-in manner into the cavity
220
of the lower mold
22
, with a bottom surface
230
of the heat sink
23
abutting against a bottom wall of the cavity
220
of the lower mold
22
.
Referring to
FIG. 4B
, a die-bonded and wire-bonded lead frame
24
is stacked on the heat sink
23
. A chip
25
is mounted on a surface of a die pad
240
of the lead frame
24
and electrically connected to inner leads
241
of the lead frame
24
via a plurality of bonding wires
26
, allowing a surface of the die pad
240
, opposite to the surface mounted with the chip
25
, to be in contact with a top surface
231
of the heat sink
23
.
Subsequently, referring to
FIG. 4C
, when a mold engaging process is performed to engage the upper mold
21
with the lower mold
22
, outer leads
242
of the lead frame
24
are clamped between the upper and lower molds
21
,
22
, and the chip
25
and bonding wires
26
formed on the lead frame
24
are received within the cavity
210
of the upper mold
21
. With the upper mold
21
being coupled to the lower mold
22
, a resin injecting process is performed by which a resin material such as epoxy resin is injected into the cavities
210
,
220
of the upper and lower molds
21
,
22
to form an encapsulant
27
that hermetically encapsulates the chip
25
, the bonding wires
26
, the heat sink
23
, and the die pad
240
and inner leads
241
of the lead frame
24
, allowing these encapsulated elements to be protected against damage from external moisture or contaminant. As there is no adhesive applied between the heat sink
23
and the die pad
240
, during mold engagement, the die pad
240
and tie bars (not shown) connected thereto are adapted to provide a downward pressing force to press the die pad
240
toward the heat sink
23
; in particular, the die pad
240
is usually spaced apart from the bottom wall of the cavity
220
of the lower mold
22
by a distance smaller than a thickness of the heat sink
23
by about 2-3 mils, whereby the tie bars are tensioned and rebound to allow the die pad
240
to abut against the heat sink
23
. Such a method to first place the heat sink
23
in the cavity
220
and then stack the die pad
240
on the heat sink
23
without having to beforehand adhere the heat sink
23
to the die pad
240
via an adhesive can thereby simplify fabrication processes and reduce production costs.
Then, by removing the encapsulation mold
20
from the lead frame
24
, the encapsulant
27
is completely formed on the lead frame
24
, with the bottom surface
230
of the heat sink
23
and the outer leads
242
being exposed to outside of the encapsulant
27
, as shown in FIG.
4
D. The exposed heat sink
23
facilitates dissipation of heat produced from the chip
25
, and the exposed outer leads
242
can be bent or deformed to be input/output (I/O) ports of the semiconductor package, which are electrically connected to an external device such as printed circuit board (PCB, not shown) so as to allow the chip
25
to operate via the external device.
However, the above EDHS-QFP structure still renders significant drawbacks. As no adhesive is applied between the heat sink and the die pad, such an interface may be considered delaminated and undesirably forms a gap between the heat sink
23
and the die pad
240
. This gap is normally small and not permeable for the resin material used to fabricate the encapsulant
27
, as shown in
FIG. 5A
, thereby forming a delaminated interface between the heat sink
23
and the die pad
240
with air residing in the gap. Since air has poor thermal conductivity of 0.024 W/M° C., heat produced from the chip
25
and transmitted through the die pad
240
, the gap (air), and the heat sink
23
would undesirably increase thermal transfer resistance and degrade heat dissipating efficiency.
On the other hand, if the small gap between the heat sink
23
and the die pad
240
may be partly filled with the resin material and leaves a void
28
formed in the middle of the gap, as shown in
FIG. 5B
; as a result, a central portion of the die pad
240
lacks support and the chip
25
may easily crack (as indicated by arrows in the drawing) due to impact from a flow of the resin material especially in a step of building up a packing pressure of the molding process, thereby seriously damaging quality and yield of fabricated products. This chip-cracking situation is more severe for relatively thinner chips. As chips are developed toward high integration and low profile, the larger a chip, the thinner the chip is (for example, even having a thickness smaller than 10 mils), and correspondingly a larger die pad is required, which makes it even harder to completely fill the gap between the heat sink and the die pad and thereby easily leads to void-induced chip cracks. Besides, similarly, air of poor thermal conductivity residing in the void
28
may increase thermal transfer resistance and degrade heat dissipating efficiency as to transmit heat from the chip
25
to the heat sink
23
for dissipation.
Therefore, the problem to be solved herein is to provide a semiconductor package which can prevent chip cracks, improve heat dissipating efficiency and reduce fabrication costs thereof.
SUMMARY OF THE INVENTION
An objective of the present invention is to provide a semiconductor package with a heat dissipating structure in which a resin material forms a supporting member between a die pad of a lead frame and a heat sink to provide proper support for the die pad, so as to prevent cracks of a chip mounted on the die pad during a molding process, thereby assuring yield and reliability of fabricated products.
Another objective of the invention is to provide a semiconductor package with a heat dissipating structure in which a bottom surface of a heat sink is exposed to outside of an encapsula

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