Semiconductor device manufacturing: process – Packaging or treatment of packaged semiconductor – Including adhesive bonding step
Reexamination Certificate
1999-12-01
2001-11-20
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Packaging or treatment of packaged semiconductor
Including adhesive bonding step
C438S106000
Reexamination Certificate
active
06319755
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of making an integrated circuit package, and in particular to a process for attaching a conductive strap to an integrated circuit die and a leadframe.
BACKGROUND OF THE INVENTION
A typical integrated circuit package includes an integrated circuit die attached to a leadframe, the leadframe being the backbone of a typical molded plastic package. Leadframes serve first as a die support fixture during the assembly process, and are subsequently electrically connected to the die bond pads after die-attach by wirebonding. After molding, the leadframe becomes an integral part of the package. The package includes external terminals (e.g., leadfingers) for power and signal distribution. In addition, the package may provide for heat dissipation.
As is well known in the art, high-resistance and thermally inefficient integrated circuit devices significantly reduce system reliability and electrical efficiency. One of the factors contributing to such inefficiency involves the use of wirebond connections between the package's leadfingers and the die. Such wirebonds contribute a large percentage of the overall package resistance. In addition, wirebond cratering and a so-called “purple plague” phenomena associated with wirebonds affect device reliability. “Purple plague” is a phenomenon when gold (the typical material for bond wires) is combined with an aluminum top metal on the die. When gold is combined with an aluminum top metal, intermetallic formations may result, which can cause open circuits.
Another contributing factor of device inefficiency is inadequate heat dissipation. For example, heat from a power MOSFET (Metal-Oxide-Semiconductor Field-Effect-Transistor) device on a printed circuit board (PCB) typically is removed through a plate that forms the drain-lead connection on the back of the die. The plate conducts heat to the PCB through a conductive connection. With smaller devices and higher power, such heat dissipation may not be adequate in some applications.
To improve device resistance and heat dissipation in power MOSFET applications while keeping a small package size, a copper strap has been developed to eliminate the wirebonds associated with the source inputs of the power MOSFET device. By replacing the wirebonds connecting the source to the leadframe with a solid copper strap, a highly conductive (both thermally and electrically) path between the leadframe and the die is created. The reduction in thermal and electrical resistance allows for, e.g., less paralleling of devices, smaller chips and package outlines and higher reliability. Circumventing wirebonding also allows a decrease in assembly time and elimination of the cratering and purple plague phenomena associated with wirebonds.
In addition, the copper strap creates a low-resistance parallel path with the die's top surface, further reducing package resistance. Heat dissipation also is improved because the copper strap provides two thermal-dissipation avenues - one through the source leadfingers to the PC board and the other through a top surface of the package, since the copper strap is routed close to the top of the mold compound encapsulant and thus facilitates heat radiation from the top surface of the package.
FIG. 1
illustrates a flowchart of a conventional assembly process for a power MOSFET package employing a copper strap.
FIGS. 2
,
3
A,
3
B,
4
A,
4
B,
5
A and
5
B show various stages during the assembly process. The process starts in Step
10
.
Referring to
FIGS. 1 and 2
, Step
12
loads a leadframe
50
into a typical die attach machine. Step
14
dispenses a conductive adhesive epoxy
56
onto a leadframe pad
52
of leadframe
50
. Step
16
picks a die
58
from a wafer tape (not shown) and places die
58
on top of conductive epoxy
56
on leadframe pad
52
(
FIGS. 3A and 3B
) using a conventional method. Step
18
loads the die-bonded leadframe strips into transport magazines and then unloads the transport magazines from the die attach machine. Step
20
loads the transport magazines into a conventional curing oven for a first curing step, wherein conductive epoxy
56
is cured.
Subsequent to curing conductive epoxy
56
, Step
22
of
FIG. 1
removes the magazines of die bonded leadframe strips from the oven and loads the leadframes into a solder paste dispensing system. Referring to
FIGS. 4A and 4B
, Step
24
dispenses a solder paste
60
onto a top surface of die
58
and input leadfingers
54
. Step
26
of
FIG. 1
singulates a copper strap
62
from a reel or a matrix reel. The copper strap
62
is then picked and placed onto the solder paste
60
on die
58
and input leadfingers
54
.
FIGS. 5A and 5B
show top and side views of copper strap
62
on die
58
and input leadfingers
54
.
Step
28
of
FIG. 1
loads the leadframe units into a reflow oven for a second curing step wherein solder paste
60
is cured. Referring to
FIG. 5B
, a problem associated with Step
28
is that, after solder paste
60
curing, the bond line thickness BLT1 at the bottom of copper strap
62
is uncontrolled. The leadframe units are then placed into another transport magazine in preparation for subsequent steps. Once the transport magazine is full, the units are unloaded from the oven in Step
30
. The process ends in Step
32
.
The process of
FIG. 1
requires the use of both an epoxy and solder paste, and accordingly requires two curing steps, namely, an epoxy cure for die attach and a solder paste cure for copper strap attach. Artisans will appreciate that the cost of packaging an integrated circuit die depends, in part, on the materials used and the number of steps of the packaging process.
Therefore, what is needed is a less costly and more efficient method of packaging an integrated circuit die. In particular, what is needed is a more efficient method of attaching a conductive strap that allows electrical connection between a leadframe and an integrated circuit die.
SUMMARY OF THE INVENTION
The present invention lowers material costs and improves the efficiency of an integrated circuit die packaging process where a conductive strap is used to interconnect a leadframe and an integrated circuit die. The present invention also improves the reliability of the package.
An embodiment of a method within the present invention includes applying a conductive first adhesive on a leadframe pad of a leadframe. A conductive second adhesive is applied on a member of the leadframe that is connected to one or more input leadfingers. A first surface of an integrated circuit die is placed on the leadframe pad through the first adhesive. A conductive third adhesive is applied on second surface of the integrated circuit die opposite the first surface. A conductive strap is placed on the second surface of the integrated circuit die through the third adhesive and on the leadframe member through the second adhesive. The first, second and third adhesives are simultaneously cured, which permanently attaches the die to the leadframe pad and the conductive strap to the die and the leadframe.
In one embodiment, the first, second and third adhesives are conductive epoxy materials. The first, second and third adhesives may be the same material, or may be different materials provided that they can cure simultaneously.
In one embodiment, the integrated circuit die comprises a power MOSFET having a source input conductively connected to input leadfingers via the conductive strap, a gate input connected to a leadfinger by a wire bond or other conductor, and a drain output conductively connected to the leadframe pad.
In one embodiment, the second adhesive is applied prior to placement of the die on the first adhesive. In an alternative embodiment, the second adhesive is applied subsequent to placement of the die on the first adhesive.
By replacing solder paste with a conductive adhesive (e.g., silver-filled epoxy) to electrically connect the conductive strap to the die and the leadframe, only one curing step is needed, thereby simplifying the packaging proces
Amkor Technology Inc.
Chen Bernice B.
Nelms David
Nhu David
Parsons James E.
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