Ultra-thin composite package for integrated circuits

Details Ultra-thin composite package for integrated circuits Ultra-thin composite package for integrated circuits

1994-08-26

2001-02-06

Saadat, Mahshid (Department: 2815)

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

Housing or package

With contact or lead

C257S666000, C257S704000, C257S701000, C257S710000, C257S708000, C257S787000

Reexamination Certificate

active

06184575

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to ultra-thin packaging for integrated circuits. More particularly, it relates to composite packaging that utilizes a metallic base in combination with a molded plastic cap and in some embodiments, a thinned lead frame.
BACKGROUND OF THE INVENTION
The trend in the electronics industry today is toward higher circuit board densities creating a need for smaller and thinner integrated circuit packages. As the industry gets increasingly into such products as memory cards, smart cards, and Personal Computer Memory Card International Association (PCMCIA) cards, package thickness becomes a critical issue for these and other applications.
The semiconductor industry is presently producing one millimeter Quad Flat Pack (QFP) packages and the demand is growing for even thinner packages. It is becoming increasingly difficult to reduce package thickness substantially more using conventional plastic package structures. This is because plastic packages must inherently contain sufficient material to retain its structural rigidity.
FIG. 1
shows a prior art Quad Flat Pack plastic package
2
with lead frame
4
, die
6
, die attach pad
8
, and wire bonds
10
that are encapsulated in a molded plastic
12
package. Conventional lead frames are typically 6 mils thick, although thinner lead frames have been used.
FIG. 2
shows a lead frame assembly
20
prior to encapsulation where the die attach pad
8
is stamped from the same material as the lead frame
4
and supported by corner support arms attached to the lead frame not shown. The plastic material beneath the die attach pad
8
is at least approximately 6 mils thick which brings the thickness of the plastic base to in the range of about 18 to 22 mils. A thin package using this conventional plastic structure is generally about 40 mils thick (about 1 mm).
Another problem with the thicker or larger packages is that they impart larger residual stress on the die due to differing thermal expansion rates of the package material (usually plastic) and lead frame from that of the die. Another disadvantage is that larger packages are more likely to contain moisture after the preheat stage in the manufacturing process, which is not sufficient to drive off all the moisture, thus increasing the possibility of package cracking. What is needed is a thin package structure that avoids these problems and can be used in the applications mentioned above.
SUMMARY OF THE INVENTION
The present invention permits the reduction of package thickness while maintaining structural rigidity by using a composite package structure and in some embodiments, a modified lead frame. A package in accordance with one embodiment of the present invention includes a metal base having a cavity that receives a die, a lead frame electrically connected to the die, and a molded plastic cap that cooperates with the base to encapsulate the die. An electrically insulating layer is positioned between the top surface of the metal base and the lead frame to electrically isolate the lead tips. The metal base is thinner than those used in conventional plastic structures and has a cavity formed therein to receive an integrated circuit die. Also, the cap used is thinner than those of conventional plastic structures which contributes to a overall thinner package.
The use of the metal base provides overall package robustness and gives the necessary support that permits the lead frame to retain its stiffness and integrity. The metal base material can be constructed out of a number of materials that remain structurally rigid at reduced thicknesses. By way of example, materials such as anodized aluminum, Copper Kovar Copper (CKC), or many of the other metals used in the microelectronics packaging industry can be used.
In another embodiment, an even thinner package is achieved by using a modified lead frame in conjunction with the metal base. The lead frame has a multi-thickness configuration where an inner portion or lead tip region (i.e. principally the portion of the lead frame that is under the cap) has a smaller thickness than an outer portion of the lead frame that is outside the cap. The outer portion, (i.e. principally the portion not under the cap), is formed into legs that attach and electrically connect the package to the circuit board.
In a method of the invention, a lead frame is attached to a top surface of a metal base using an adhesive material that electrically insulates the lead frame from the base. A die is inserted into a cavity in the top surface of the metal base and attached thereto. The die is then electrically connected to the lead frame and a plastic cap is formed over the metal base to encapsulate the die and a portion of the lead frame. In a preferred embodiment, a central portion of the lead frame is thinned prior to the attaching step such that the central portion of the lead frame is thinner than an outer portion of the lead frame, wherein the plastic cap covers at least a majority of the thinned central portion of the lead frame.
In addition to providing a thinner package, the present invention has the advantages of reducing package cracking due to moisture and enhancing thermal performance of the package due to the higher thermal conductivity of the metal base. Improved thermal performance of packages increases the operational life of the chip and permits the use of efficient less costly cooling technologies.


REFERENCES:
patent: 4707724 (1987-11-01), Suzuki et al.
patent: 4711700 (1987-12-01), Cusack
patent: 5103292 (1992-04-01), Mahulikar
patent: 5155299 (1992-10-01), Mahulikar et al.
patent: 447884 (1991-09-01), None
patent: WO94/11902 (1994-05-01), None

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