Active solid-state devices (e.g. – transistors – solid-state diode – Lead frame
Reexamination Certificate
2000-06-13
2002-01-15
Chaudhuri, Olik (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Lead frame
C257S669000, C257S724000
Reexamination Certificate
active
06339252
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed toward an improved plastic package for a semiconductor integrated circuit device and towards leadframes and methods for making such a package.
BACKGROUND OF THE INVENTION
Semiconductor integrated circuit devices, which are typically formed from silicon or gallium arsenide, are conventionally enclosed in plastic packages. The elements of such a package include a metal leadframe, an integrated circuit device, bonding material to attach the integrated circuit device to the leadframe, bond wires or other connectors that conductively connect metal pads on the integrated circuit device to individual leads of the leadframe, and a hard plastic encapsulant material that covers the other components and forms the exterior of the package.
FIG. 1
is a conventional metal leadframe
10
. Although not shown, several identical leadframes
10
are formed in an array from a single sheet of metal and each leadframe of the array is processed in parallel.
Leadframe
10
includes a central metal die pad
11
(shown by horizontal hatching) and radiating metal leads
12
adjacent to die pad
11
. An opposite end of each lead
12
is connected to the peripheral frame of the leadframe (not shown). A dam bar (not shown) also crosses and connects the leads. The frame and dam bar are removed during a trim and form step. The portion of each lead
12
that is within the dashed line is plated with silver. The perimeter of die pad
11
also may be plated. A ring of an adhesive polyimide tape
13
(shown by diagonal hatching) crosses and connects each of the leads
12
at a midpoint and serves to stabilize the leads during processing. Four tie bars
14
support die pad
11
and connect it to the peripheral dam bar and frame (not shown). Five leads
15
are connected to die pad
11
for support. Die pad
11
is downset, as indicated by downset marks
16
on tie bars
14
and leads
15
.
An alternative prior art embodiment of a die pad shown is in FIG.
2
. Metal die pad
20
is surrounded by a ring
21
. Ring
21
is connected by sixteen metal connectors
22
to die pad
20
. Tie bars
23
connect die pad
20
to the remainder of the peripheral frame of the leadframe (not shown). This configuration facilitates spacing of the leads (not shown) and provides an encapsulant locking feature.
In a completed package, the integrated circuit device is mounted on the die pad. Typically, integrated circuit devices require connection to a power voltage source, sometimes denoted “Vdd,” and a ground voltage source, sometimes denoted “Vss.” A lead of the package typically is dedicated to supply Vdd. Sometimes, a second dedicated lead provides Vss. The use of dedicated leads for power and ground voltages, however, limits the flexibility of the package because the user of the package, for example, a chip manufacturer or computer manufacturer, cannot vary which leads are to be used for connection to input and output (“I/O”) signals and which leads are to be used for connection to Vdd and Vss. Sometimes, a second dedicated lead provides Vss. The use of dedicated leads for power and ground voltages, however, limits the flexibility of the package because the user of the package, for example, a chip manufacturer or computer manufacturer, cannot vary which leads are to be used for connection to input and output (“I/O”) signals and which leads are to be used for connection to Vdd and Vss.
SUMMARY OF THE INVENTION
The present invention includes a package made from a leadframe having an inventive design. The package overcomes the limitations of prior art packages by allowing all leads of the package to be used for either I/O signals or power or ground leads. This allows a common package design to be used even when the location of the power voltage, ground voltage, and I/O signal inputs vary among different electronic devices.
A leadframe within the present invention includes a central metal die pad. A metal ring surrounds the die pad. The metal ring is spaced a small distance apart from the die pad. A nonconductive connector is attached between the die pad and the surrounding ring and holds the die pad and the ring together. An example nonconductive connector is a polyimide adhesive tape. A plurality of leads extend from the periphery of the leadframe toward the die pad and ring. The leads terminate adjacent to the ring. The ring is between the ends of the leads and the die pad. None of the leads directly connect to the ring. In an alternative embodiment, one or more of the leads is directly connected to the ring and provides support for the ring. Depending on the application of the package, a bypass capacitor or decoupling capacitor is connected between the die pad and the ring and filters the input power voltage.
The present invention also includes a method for making the leadframe described above. Step
1
of the method provides a metal sheet. Step
2
patterns the metal sheet to form the die pad, the surrounding ring, and the leads. At this initial stage, a temporary metal connector or a plurality of temporary metal connectors also are formed from the metal sheet and connect the die pad to the ring. Step
3
connects a nonconductive connector, such as polyimide adhesive tape, between the die pad and ring. A plurality of nonconductive connectors, such as two or four segments of tape, may be used to connect the die pad and ring. Step
4
removes the temporary metal connector(s) so that only the nonconductive connector(s) holds the die pad to the surrounding ring. Step
4
can be accomplished using several methods, including punching, etching, or laser cutting methods. Optionally, the tips of the leads and the periphery of the die pad are plated with another metal, such as silver, to facilitate connection to bond wires. Further, a bypass, decoupling, or filter capacitor may be connected between the die pad and the surrounding ring. Capacitor connection may be accomplished using a variety of methods, such as conductive adhesive or soft soldering methods.
A package made from the above-described leadframe includes the metal die pad, surrounding metal ring, nonconductive connector(s), and leads. An integrated circuit device is on the die pad. The leads are connected by conductors, e.g., bond wires, to conductive pads on the integrated circuit device. One of the leads is connected by a bond wire to the ring. Typically, the ring is connected to the lead that is attached to a power voltage input Vdd. The ring in turn is connected by a bond wire to a pad on the integrated circuit device for inputting the power voltage to the integrated circuit device. The die pad is isolated from the ring and is floating. In an alternative embodiment, a lead that attaches to a ground voltage is connected by a bond wire to the die pad. A capacitor (filter, bypass, or decoupling) may be connected between the ring and the die pad. Hardened encapsulant material covers the entire structure, including the nonconductive connectors and the integrated circuit device.
A method of making the above package also is within the present invention. Step
1
of the method provides a leadframe having the features described above, including a central die pad, a surrounding ring separated from the die pad, a nonconductive connector holding the die pad to the ring, and radiating leads. Step
2
attaches an integrated circuit device to the die pad. Step
2
is accomplished using industry standard practices utilizing conductive or nonconductive organic epoxies or polyimides, as well as soft solders and conductive adhesive tapes. Step
3
installs conductive connectors between the integrated circuit device and the leads, between one or more of the leads and the ring, and between the ring and the integrated circuit device. Step
3
is accomplished using industry standard practices, such as wire bonding using thermosonic, thermo-compression, or ultrasonic wedge methods. Step
4
applies an encapsulant material over the entire structure. Typically, industry standard thermoset epoxies are used. These include Epoxidized ortho uresol
Bancod Ludovico
Choi Yeon Ho
Crowley Sean T.
Kham Nhun Thun
Niones Eulogia A.
Amkor Technology Inc.
Chaudhuri Olik
Date Celisa K.
Ha Nathan W.
Parsons James E.
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