Active solid-state devices (e.g. – transistors – solid-state diode – Lead frame – With structure for mounting semiconductor chip to lead frame
Reexamination Certificate
1998-07-06
2001-05-29
Lee, Eddie C. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Lead frame
With structure for mounting semiconductor chip to lead frame
C257S676000, C257S667000, C257S783000
Reexamination Certificate
active
06239480
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates to the manufacture of semiconductor devices, and more specifically to the process of assembling an integrated circuit into a package.
2. Related Art
Laser fuses have been used in the electronics industry to repair memory elements, configure logic circuits, and customize integrated circuits such as gate arrays by selectively removing desired fuses in the device. For example, a non-functional device can be repaired by removing desired fuses, e.g., ablation by laser, to isolate defective portions of the circuitry or to substitute functionally redundant circuitry for the defective portions. Fuses can also be used to mark the device for identification of characteristics in a manner that is readable visually or electrically, e.g., serialization of the integrated circuit or how the device has been configured by the laser. An integrated circuit can be customized or configured for specific uses by altering the structure, path or electrical characteristics of the device or elements through selective removal of the fuses. It should be noted that the word “fuse” can refer to an antifuse as well as a fuse.
The fuses can be removed at various stages of the integrated circuit manufacturing process. By removing the fuses after the wafer has been sawed up into individual integrated circuits and assembled into packages, the lead time to deliver customized or configured integrated circuits, hereinafter referred to as die, can be reduced. A typical process for assembling a die into a package and configuring the die in the package includes the following basic steps:
1. deposit die-attach epoxy on a lead frame in the cavity of the package, where the cavity can be formed at the time the package is created or at a later time;
2. place the die on the epoxy and compress the die and the package together to distribute the epoxy;
3. bake the package and die to cure the epoxy;
4. attach bond wires between bonding pads on the die and lead fingers on the package;
5. configure the die with the use of a laser; and
6. seal off the top of the package, either with a lid or by filling in the cavity with mold compound.
A major factor limiting the successful application of the laser and configuration of the die in this manner is the difficulty with aligning and focusing the laser if the die does not sit evenly on the lead frame. A die
10
typically has alignment and focusing marks or targets
1
-
4
on the die's upper surface, as shown in
FIG. 1. A
typical alignment and focusing sequence for die
10
would begin with a laser scanning targets
1
though
4
one at a time and making x-axis, y-axis, and rotational corrections. The laser then returns to target
1
and makes another series of scans through to target
4
to set the focus level for the entire die
10
.
Alignment and focusing in this manner is adequate as long as the die is set evenly upon the lead frame so that the laser scanning plane is parallel with the surface of the die. In other words, as long as any non-orthogonality in the plane of the die, relative to the optics of the laser, does not cause the lasered or scanned portion of the die to fall outside of the laser spot's focus range, proper alignment and focusing can be achieved. However, when the die is not properly placed on the lead frame, the laser may be unable to perform an alignment or focus scan or to accurately ablate the fuses for proper circuit customization.
FIG. 2
shows a die
10
, which has been placed and compressed on some die-attach epoxy
20
. Epoxy
20
has been deposited on a lead frame (not shown) within a cavity of a package
21
. Several factors can cause die
10
to sit at an angle relative to package
21
, including the presence of air bubbles
22
within epoxy
20
, an uneven distribution of epoxy
20
, or an uneven placement of die
10
on epoxy
20
. This will often result in a portion of die
10
falling out of the focus range of the laser, which can lead to improper configuration of the die.
Although various leveling mechanisms are common in the industry and would be effective in re-leveling the die, these types of mechanisms are not commonly present in commercially-available lasers used for the repair and configuration of semiconductor devices. Therefore, using these mechanisms to re-level the die so that reliable configuration of the die in a package is possible would require costly and time-consuming modifications to the laser. Accordingly, it is desirable to have a method of accurately adhering a die to a package so that re-leveling the die is unnecessary, which eliminates the additional expenses required to modify the laser for re-leveling.
SUMMARY
The present invention provides a structure and method for accurately adhering a die to a package by modifying the lead frame in the package to allow excess die-attach epoxy to distribute itself into at least one cavity in the lead frame and to reduce the surface area of the lead frame supporting the die. As a result, the amount of epoxy between the die and the lead frame is reduced for less variation in the angle of the die relative to the package, and the die is provided a more rigid and stable setting to minimize the adverse effects of non-uniform epoxy distribution.
In one embodiment of the present invention, the lead frame contains an array of cavities formed in the lead frame, with the array slightly larger than the size of the die to be packaged. When the die is pressed onto the lead frame and die-attach epoxy, the excess epoxy can spread into the cavities. The cavities form a grid in the lead frame which permit the die to be well-supported, yet minimizes the surface contact area, which reduces both the chance of bubble formation and the amount of pressure required to seat the die uniformly on the lead frame. As a result, a more level positioning of the die relative to the lead frame and the package is possible.
In another embodiment of the present invention, a cavity of generally circular or oval shape is formed partially or completely through the lead frame prior to die-attach epoxy deposition. One dimension of the cavity would be slightly smaller than the longer dimension of the die to be packaged. The die-attach epoxy is deposited in the cavity, and the die pressed onto the top of the epoxy and the lead frame. The excess epoxy is forced into unfilled areas of the cavity, and if necessary, through the gaps formed between the edge of die and cavity. The die is well-supported and in direct contact with the lead frame so that the die is positioned parallel with respect to the lead frame and thus the package. The die-attach epoxy anchors the die firmly in position without affecting the position of the die relative to the lead frame because excess epoxy is not forced between the upper surface of the lead frame and the die.
The present invention permits a more consistent placement of the die in a package which results in better yield for applications where the die is laser-coded in the package.
The present invention will be more fully understood in light of the following detailed description taken together with the accompanying drawings.
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Eng Wendy
MacPherson John
Chen Tom
Clear Logic, Inc.
Cruz Lourdes
Lee Eddie C.
Skjerven Morrill & MacPherson LLP
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