Semiconductor device manufacturing: process – Packaging or treatment of packaged semiconductor – Metallic housing or support
Reexamination Certificate
2000-02-16
2002-11-12
Graybill, David E. (Department: 2814)
Semiconductor device manufacturing: process
Packaging or treatment of packaged semiconductor
Metallic housing or support
C438S127000
Reexamination Certificate
active
06479326
ABSTRACT:
BACKGROUND OF THE INVENTION
1. The Field of the Invention
The present invention relates to packaging of microelectronic components. More particularly, the present invention relates to improved dissipation of heat produced by microelectronic components. Additionally, the present invention relates to balancing a chip package against warpage stresses. In particular, the present invention relates to a unitary heat sink that functions as both as a die-attach paddle and an outer ring of a chip package for heat rejection that extends to the outer boundary of the chip package. In the unitary heat sink, at least one opening therein exists between the die-attach paddle and the outer boundary of the chip package.
2. The Relevant Technology
Microelectronic packages are routinely packaged in plastic molding compounds in order to reduce cost and facilitate packaging operations. Because microelectronic devices produce appreciable amounts of heat that must be removed from the device in order to ensure proper function thereof, the issue of heat removal has become increasingly important. Where the power rating of the microelectronic device becomes significantly high, plastic encapsulation usually will be replaced with either ceramic or metallic encapsulation to facilitate heat rejection. However, ceramic or metallic encapsulation is more expensive than plastic encapsulation.
A heat sink can be attached directly to a die to serve a dual function of a die-attach paddle and a heat sink. Conventional techniques have been developed to rigidly attach a heat sink to a lead frame for an integrated circuit. Several problems occur in chip packaging. One problem that occurs in using a composite heat sink is that disparities between coefficients of thermal expansion can cause destructive stresses in a package as the package cycles through heating and cooling.
As chip packaging technology follows the lead of microelectronic circuit design miniaturization, a goal is to miniaturize the chip package size to be substantially the same size as the chip itself. Lead on chip (LOC) technology allows leads to come directly to the microelectronic device without the need for wire bonding, thus, the chip package can have a smaller dimension.
FIGS. 1
a
and
1
b
illustrate a prior art method of heat management in which at least two problems exist. In
FIG. 1
a
, a semiconductor package
10
, depicted in a top plan view, illustrates LOC technology where a lead
12
makes direct contact to a chip
14
without a bonding wire. When viewed in the cross-section view of
FIG. 1
b
, package
10
reveals a die attach
16
and outer structure
18
that are substantially parallel to each other and that are connected by any of various methods such as welding or adhesive bonding. As package
10
runs through thermal cycles, because of dissimilarities in coefficients of thermal expansion between die attach
16
, adhesive bonding, if any, and outer structure
18
, destructive stresses are caused. It can also be seen that die attach
16
, outer structure
18
, and any adhesive bonding material therebetween encapsulated in a packaging plastic
26
may have antagonistic abilities to withstand thermal stresses. A first thickness
20
of packaging plastic
26
above chip
14
, and a second thickness
22
of packaging plastic
26
below die attach
16
may cause package
10
to be thermally unbalanced such that warpage and bowing may occur while in use. Although conductive heat flow into outer structure
18
may be substantial at the interface between die attach
16
and outer structure
18
, ultimate heat rejection from package
10
is poor because outer structure
18
is substantially encapsulated in packaging plastic
26
and packaging plastic
26
acts as a heat flow resistor.
FIGS. 2
a
and
2
b
illustrate prior art attempts to substantially equilibrate dissimilar stresses in packaging plastic
26
by making first thickness
20
and second thickness
22
substantially similar. Die attach
16
is downset away from outer structure
18
by a downset
24
such that first thickness
20
and second thickness
22
are substantially similar. It can be seen, however, that a second heat sink
34
, substantially externally exposed at the package lower edge
28
, is insulated from die attach
16
and outer structure
18
by packaging plastic
26
. Packaging plastic
26
acts as a thermal blanket and resists heat removal from chip
14
. Packaging plastic
26
may be required to be injected from at least two separate injection ports when a package such as package
10
depicted in either of
FIGS. 1
b
and
2
b
is being assembled. Because die attach
16
and outer structure
18
may substantially seal packaging plastic
26
in the region that forms first thickness
20
from packaging plastic
26
in the region that form second thickness
22
, a dual- pressurized, dual injection-port plastic injection molding system may be needed to properly complete formation of the body of package
10
.
FIG. 2
c
illustrates a cross-sectional view of an attempt to facilitate heat removal from chip
14
in which substantial downsetting of chip
14
to second heat sink
34
at package lower edge
28
is done wherein downset
24
extends downwardly substantially from the center line of package
10
to package lower edge
28
. One problem with this structure is that first thickness
20
of packaging plastic
26
causes substantial unbalance such that warpage and bowing of package
10
is caused when package
10
is heated during ordinary use. Additionally, a two- or three-input injection-port molding technique is required.
What is needed in the art is a heat management structure that effectively conveys heat away from a microelectronic device without the problems that arise in the prior art.
More particularly, what is needed is a heat management structure that effectively rejects heat from a microelectronic device that avoids thermal stresses caused by dissimilarities in thermal conductivities of materials. What is also needed in the art is a method of forming a chip package comprising packaging plastic that simplifies injection molding techniques over the prior art. What is also needed in the art is a heat management structure that does not cause an unbalanced package to be formed such that warpage and bowing are substantially avoided.
SUMMARY OF THE INVENTION
The present invention relates to a heat management structure within a chip package that allows for heat rejection from a chip but that avoids the prior art problems of thermal stresses caused by dissimilar thermal conductivities of a heat management structure and of creating a thermally unbalanced package due to disparate distribution of packaging plastic.
In an embodiment of the present invention a package includes a chip, leads on a chip, a die attach, a downset, a packaging plastic, and an outer structure among others. The outer structure, downset, and die attach comprise a substantially unitary article that can be made by stamping, etching, ingot casting or metal powder molding or other such unitary-article forming processes known in the art.
The complete structure of the die attach, the downset, and the outer structure is preferably made from a single piece of material such as copper or a copper alloy. When the die attach, downset, and outer structure are made by stamping, the grain structure therein, after formation by stamping and the like, would have a substantially homogenous microscopic appearance in all locations except where the downset begins next to the die attach and where the downset ends next to the outer structure.
Achieving a balanced package that substantially resists warpage and bowing during ordinary manufacture and ordinary use in the life of the package is accomplished by balancing the packaging material width with the ability of the downset to resist warpage and bowing stresses. Specific selection of materials such as the packaging material and the metal that comprises the die attach, downset, and outer structure will depend upon the specific application. The downset forms a first angl
Corisis David
Moden Walter
Graybill David E.
Workman & Nydegger & Seeley
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