Die size-increasing integrated circuit leads and thermally...

Details Die size-increasing integrated circuit leads and thermally... Die size-increasing integrated circuit leads and thermally...

1999-05-14

2001-05-01

Lee, Eddie C. (Department: 2815)

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

Lead frame

C257S692000, C257S696000, C257S783000

Reexamination Certificate

active

06225683

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of integrated circuit (I.C.) leadframes, and particularly to leadframes designed to provide improved heat dissipation and/or increased maximum die size.
2. Description of the Related Art
A I.C. package's “leadframe” includes the leads that extend from the package, the structure that supports the I.C. die, and any structures employed to hold these elements together. Many types of leadframes have been developed and are in use, including “lead-under-paddle” (LUP) and “chip-on-lead” (COL) leadframes, for example.
Plan view and corresponding cross-sectional views of a standard 8-lead “mini small outline package” (MSOP) leadframe are shown in
FIGS. 1
a
and
1
b
, respectively. As used herein, a “standard” package is one for which a set of dimensions has been defined and adopted by the I.C. industry. An I.C. die
10
is on a “paddle”
12
, and is secured to the paddle with some type of adhesive
14
. The ends of the package's leads
16
which are to be connected to the die are arrayed around, but physically separate from, the paddle
12
, and the interconnections between the die and the leads are provided by wire bonds
18
. The ends of the leads that will be encapsulated within the package are referred to herein as the “inner” portions of the leads, with the portions that extend out from the package referred to as the “outer” portions.
When operating, an I.C. die generates heat, and many leadframes include features to aid in dissipating the heat produced by the die. For example, in the leadframe of
FIG. 1
, the paddle
12
is often made of metal, and the adhesive
14
is thermally conductive, so that some of the heat generated by the die is conducted to the paddle. The physically separated paddle
12
and leads
16
are held in a desired proximity to each other with a mold compound
20
(not shown in
FIG. 1
a
for clarity), which forms the exterior of the I.C. package
22
. When heat is generated by I.C. die
10
, it is conducted to the paddle as noted above, and then to the leads
16
through the mold compound
20
. Unfortunately, molding compounds in common use have a very low thermal conductivity. This increases the thermal resistance between the die and the leads, and limits the amount of heat that can be dissipated by the die. The limited heat dissipation capabilities of such a package in turn limit the power that can be consumed by the I.C. die, as well as the uses to which the die can be put. Poor heat dissipation can also accelerate the failure rate of the die's circuitry.
Wire bonds are usually “wedge bonded” to the package leads, typically accomplished by pressing a gold wire bond to a small silver-plated area (the “wedge bond shelf”) of a copper lead. However, to provide good anchoring of the package leads, the inner portions which include the bond shelves should extend well into the molding. This reduces the maximum size of the paddle—and consequently the maximum die size—i.e., the die's surface area—that can be accommodated in a given standard package size.
One approach to these problems is found in U.S. Pat. No. 5,541,446 to Kierse. Here, an I.C. die is affixed directly to the package leads, and the paddle is eliminated. This arrangement helps to reduce the package's thermal resistance, but does nothing to address the die size area lost to the bond shelves. Furthermore, with the package encapsulating only the leads and the die, it may be difficult to ensure the coplanarity of the package leads.
SUMMARY OF THE INVENTION
A new “paddle-under-lead” (PUL) leadframe is presented that greatly reduces the adverse impacts of the problems noted above. The PUL leadframe provides much improved heat dissipation, while allowing a larger die size to be accommodated within a standard I.C. package size. A new bifurcated inner lead design is also presented. The bifurcated leads are suitable for use with a variety of leadframe types, including the new PUL leadframe, and further increase allowable die size.
The construction of a PUL leadframe has the inner portions of an I.C. package's leads extended along and affixed to the top of a paddle; each of the inner portions includes a bond shelf. A complete I.C. package is formed by affixing an I.C. die to the top of the inner leads, and interconnecting wire bonds between the die's bonding pads and the leadframe's bond shelves. Because the die is affixed directly to the leads, heat generated by the die is efficiently conducted out of the package via the package's leads. The paddle is preferably metal, and thermally conductive adhesives are preferably used to affix die to leads and leads to paddle. When so configured, the paddle serves as a heat spreader and heat sink and further enhances the package's ability to dissipate heat.
The PUL leadframe increases the size of a die that can be accommodated in a standard package size when compared with prior art leadframes. The leadframe's inner leads are affixed to the paddle, rather than separated from it as is done conventionally. As a result, a larger die size can be accommodated within the same standard package size. Affixing the inner lead sections to the paddle provides good anchoring for the leads, and also serves to avoid coplanarity problems during die attach. The paddle can be made of a conductive material such as copper, and by connecting a wire bond between die and paddle can serve as a ground or power plane for the die.
The invention's novel inner leads bifurcate the inner portions of an I.C. package's leads into upper and lower sections, with the upper section serving as a wedge bond shelf and the lower section downset from the upper section. Conventionally, a downset lead's bond shelf is located in series between the downset and the outside of the package; because both downset and bond shelf must be accommodated within specified package dimensions, the maximum die size is necessarily reduced. The use of the described bifurcated leads avoids this consequence by enabling the downset and the bond shelf to occupy parallel spaces, thereby recapturing the space that is normally lost to the downset. The bifurcated leads can be used with a number of leadframe types, including, for example, the PUL leadframe described herein, LUP leadframes, and COL leadframes.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.


REFERENCES:
patent: 5252855 (1993-10-01), Ogawa et al.
patent: 5436500 (1995-07-01), Park et al.
patent: 5541446 (1996-07-01), Kierse
patent: 5963433 (1999-10-01), Kim
patent: 6075283 (2000-06-01), Kinsman et al.
patent: 4-91458 (1992-03-01), None

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