Active solid-state devices (e.g. – transistors – solid-state diode – Housing or package – With provision for cooling the housing or its contents
Reexamination Certificate
2001-07-19
2002-08-13
Williams, Alexander O. (Department: 2826)
Active solid-state devices (e.g., transistors, solid-state diode
Housing or package
With provision for cooling the housing or its contents
C257S713000, C257S667000, C257S717000, C257S774000, C257S780000, C257S738000, C257S704000, C257S710000
Reexamination Certificate
active
06433420
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to semiconductor packages, and more particularly, to a semiconductor package with a heat sink having air vents.
BACKGROUND OF THE INVENTION
A ball grid array (BGA) semiconductor package is the mainstream in packaged products for providing high density of electronic components and electronic circuits mounted therein with sufficient I/O connections, and accordingly array-arranged solder balls connected to the I/O connections are increased in number and density. As a result increase in heat generated in operating the semiconductor package can be expected, and the generated heat then requires to be dissipated effectively, otherwise, the reliability and life-time of the semiconductor package may be detrimentally affected.
Besides, in the foregoing semiconductor package, materials used for making a substrate for mounting a semiconductor chip thereon and for making an encapsulant for encapsulating the semiconductor chip are also critical in consideration of the heat dissipation. For example, in a plastic BGA (PBGA), ceramic BGA (CBGA) or tape BGA (TBGA) semiconductor package, the substrate is widely made of a resin or ceramic material having the heat dissipating efficiency much poorer than a metallic material. Further, the encapsulant is made of a molding resin having a sufficiently small coefficient of conductivity approximately only 0.8 w/m□k, which makes the heat generated from the semiconductor chip with electron circuits and electronic elements mounted thereon not able to be dissipated to the atmosphere effectively, and thus the reliability of the semiconductor chip is degraded. Therefore, a BGA semiconductor package having a heat sink of god thermal conductivity is disclosed in U.S. Pat. Nos. 5,851,377 and 5,977,626 for improving the heat dissipating efficiency.
Illustrated in
FIG. 1
is the conventional semiconductor package
2
having the heat sink
23
. A semiconductor chip
21
is attached to a substrate
20
by means of an adhesive
212
such as silver paste, and then is electrically connected to the substrate
20
through gold wires
22
. Subsequently, the heat sink
23
is mounted at pre-determined positions on the substrate
20
in no contact with the semiconductor chip
21
and the gold wires
22
positioned below a flat portion
230
of the heat sink
23
, wherein the flat portion
230
downwardly extends to form supporting members
231
for strongly supporting the flat portion
230
on the substrate
20
. Thereafter, a molding process is performed to form an encapsulant
24
for encapsulating the semiconductor chip
21
, the gold wires
22
and the heat sink
23
, while the flat portion
230
of the heat sink is partially exposed to the outside of the encapsulant
24
. Finally, with the implantation of solder balls
25
, the semiconductor package
2
is completed in fabrication.
The fabricated semiconductor package
2
is of a thickness approximately 1.17 mm, wherein, excluding the thinness of the semiconductor chip
21
, the adhesive
212
and the heat sink
23
, a 0.54 mm gap d is remained for filing the encapsulant
24
therein, that is, the heat generated from the semiconductor chip
21
needs to pass a 0.54 mm-long path through the encapsulant
24
before reaching the heat sink
23
to be dissipated to the atmosphere. However, as the encapsulant
24
is rather poor in thermal conduction, which makes the heat dissipating efficiency reduced, and further results in decrease in the reliability of the semiconductor chip. Therefore, in the fabrication of the semiconductor package, it tends to shorten the heat dissipating path i.e. the gap d, so as to improve the heat dissipating efficiency and reduce the overall thickness of the semiconductor package as well as save the packaging cost.
However, the shortening for the heat-dissipating path is restricted in accordance with the application of the heat sink embedded in the above semiconductor package, and accordingly problems are possibly generated.
First, during the molding process, as shown in
FIG. 2
, a molten resin
34
flows into a runner
36
formed by molds
37
and a substrate
30
clamped by the molds
37
, and then through an injecting gate
38
into a mold cavity
372
. As the molten resin
34
flows from the injecting gate
38
into the relatively wider mold cavity
372
, the resin flow rate increases; then while the molten resin
34
further flow to a gap
39
between a semiconductor chip
31
and a heat sink
33
, the flow rate slows down since the gap
39
is narrower than the mold cavity
372
. This results in an unstable resin flow rate as the molten resin
34
flows faster at the outside of the gap
39
than inside of the gap
39
, as illustrated as dotted lines in a resin flow curve of FIG.
3
. Due to the unstable flow rate, air in the gap
39
can not be escaped through an air channel
373
, and is trapped in the gap
39
to form voids. The void formation will increase the heat resistance, and accordingly the heat dissipating efficiency will be reduced. Moreover, a popcorn effect will also be generated as proceeding subsequent processes for fabricating the semiconductor package under a high temperature due to the void formation, and thus the semiconductor package will be damaged by the popcorn effect and the relability thereof is definitely degraded.
In addition, further in the semiconductor package with the heat sink tightly bonded to the encapsulant, a thermal stress is generated due to temperature variations in a reflow solder process and a temperature cycle of a reliability test, according to the difference in coefficient of thermal expansion (CTE) between the heat sink and the encapsulant. This makes the heat sink and the encapsulant differently deformed, resulting in warpage and delamination for degrading the reliability of the semiconductor package.
SUMMARY OF THE INVENTION
A objective of the present invention is to provide a semiconductor package with a heat sink having taper air vents for ventilating air in a mold cavity, and for avoiding voids formed in an encapsulant in a gap between a semiconductor chip and the heat sink during a molding process. Moreover, the semiconductor package of the invention is capable of dispersing a thermal stress on the encapsulant and the heat sink for preventing warpage and delamination from occurrence, so as to assure the reliability of the semiconductor package.
According to the foregoing and other objectives, the semiconductor package of the present invention comprises: a substrate having a first surface and a second surface; a semiconductor chip having a first surface and a second surface, while the second surface of the chip is attached to the first surface of the substrate; a plurality of first conductive elements (i.e. gold wires) for electrically connecting the chip to the substrate; an embedded heat sink mounted on the first surface of the substrate and having a flat portion and a plurality of supporting members, while the supporting members are used to support the flat portion of the heat sink to be positioned above the chip without contacting the first conductive elements; a plurality of second conductive elements (i.e. solder balls) mounted on the second surface of the substrate for electrically connecting the chip to an external device; and an encapsulant for encapsulating the chip, the first conductive elements and the heat sink while the flat portion of the heat sink is partially exposed to the outside of the encapsulant.
The invention is further characterized in forming at least one taper air vent in the flat portion of the heat sink, wherein the taper air vent has a wide opening facing the semiconductor chip and a narrow opening exposed to the outside of the semiconductor package. During a molding process, air in a gap between the heat sink and the chip allows to be escaped through the taper air vents so as to prevent voids from forming in the encapsulant. As the result, the semiconductor package can be improved in heat dissipating efficiency, and a popcorn effect can be prevented f
Lai Yu Ting
Yang Chung Hsien
Corless Peter F.
Edwards & Angell LLP
Jensen Steven M.
Siliconware Precision Industries Co. Ltd.
Williams Alexander O.
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