Metal fusion bonding – Process – Plural joints
Reexamination Certificate
2000-07-14
2002-09-24
Elve, M. Alexandra (Department: 1725)
Metal fusion bonding
Process
Plural joints
C228S110100
Reexamination Certificate
active
06454158
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a wire bonding apparatus and a wire bonding method for a semiconductor device, and more particularly, to a wire bonding apparatus and a wire bonding method for a semiconductor device which have been improved to heighten stability of bonding stress between electrodes of a semiconductor chip and inner leads.
The term “wire bonding” indicates connection between chip electrodes and inner leads in package assembling processes of semiconductor devices. For instance, chip electrodes, such as Al electrodes, on an electrode chip and tip end portions of Ag plated inner leads are connected by means of ultrafine wires, such as Au wires, by utilizing thermal energy (temperature atmosphere of 200 to 300° C.) and ultrasonic energy; Au—Al alloys are formed by fusing Au balls at tip end portions of the Au wires with the Al electrodes and Au—Ag alloys are formed by fusing the Ag plated portions of the tip end portions of the inner leads with the Au wires.
FIG. 14
is a plan view of a wire bonding apparatus for a conventional semiconductor device and an arrangement similar to this is disclosed in Japanese Unexamined Patent Publication No. 326609/1993.
FIG. 15
is a sectional view seen from the line XV—XV as indicated by the arrow in FIG.
14
. In FIG.
14
and
FIG. 15
, a heat block
11
serves as a jig which is fitted between a heat block main body (not shown) and a lead frame
50
when performing wire bonding, and a lead frame forcing mold
13
is a jig for pressing the lead frame
50
from above to clamp the lead frame to the heat block
11
.
The lead frame
50
is so arranged that a die pad
2
, a hanging lead
8
, a tie bar
9
and a frame
10
are successively formed. The hanging bar
8
is bent at portions proximate to four corner portions of the die pad
2
at which they are in successive communication and the die pad
2
is provided on a plane lower than a place on which the inner lead
4
, tie bar
9
and frame
10
are provided (hereinafter, the die pad which is provided on a lower plane is referred to as a sunked die pad). The semiconductor chip
1
is joined to the die pad
2
through an adhesive
3
.
The heat block
11
is so arranged that a die pad mounting portion
11
a
on which the die pad
2
is mounted is in the same plane as that of an inner lead mounting portion
11
b
onto which the inner leads
4
are mounted. The die pad mounting portion
11
a
is formed with vacuum suction holes
11
c
for closely adhering the die pad
2
thereby through vacuum suction. A square concave portion
11
d
is provided between the die pad mounting portion
11
a
and the inner lead mounting portion
11
b
so that close adhesion between the die pad
2
and the die pad mounting portion
11
a
is not hindered even when the handing lead
8
is deformed to extend below of the surface of the die pad mounting portion
11
a
upon adhesion of the die pad
2
to the die pad mounting portion
11
a
through vacuum suction. At four corner portions of the concave portion
11
d
, there are further formed grooves (not shown) which extend outward of the four corner portions so that the hanging lead
8
will not interfere with mounting of the lead frame
50
.
A wire bonding method using a conventional wire bonding apparatus will now be explained. In performing wire bonding, the lead frame
50
is positioned and mounted on the heat block
11
as illustrated in
FIG. 15
, and after clamping by pinching the lead frame
50
between the frame forcing mold
13
and the heat block
11
, the die pad
2
is fixed to the die pad mounting portion
11
a
through vacuum suction to perform wire bonding thereafter.
The hanging lead
8
which forms a part of the lead frame
50
is provided in the same plane as tip end portions
4
a
of the inner leads
4
and the sunken die pad portion is in the concave portion
11
d
formed between the die pad mounting portion
11
a
and the inner lead mounting portion
11
b
. A semiconductor chip
1
is mounted on the die pad
2
which is in successive communication with the hanging lead
8
through the adhesive
3
of, for instance, Ag paste.
The die pad mounting portion
11
a
and the inner lead mounting portion
11
b
are arranged on the heat block
11
such that they are formed in the same plane. In contrast thereto, in the case of mounting the die pad
2
of the lead frame
50
having a dimension of 0.20 mm as defined by the sunken die pad onto the die pad mounting portion
11
a
, the die pad
2
is pushed upward by 0.20 mm. Since the lead frame
2
might include manufacturing errors of, for instance, ±0.05 mm, the dimension of the lead frame
50
as defined by the die pad sinking is varied in the range of 0.15 mm to 0.25 mm whereas the die pad
2
is pushed upward by the same amount.
In this condition, the hanging lead
8
and the inner leads
4
are pinched and clamped between the heat block
11
and the lead frame forcing mold
13
. The die pad
2
is then fixed onto the die pad mounting portion
11
a
by means of vacuum suction of a rear surface of the die pad
2
through the vacuum suction hole
11
c
whereupon the wire bonding process is started.
In the wire bonding process, while heat is applied to electrodes such as Al electrodes, though not shown in the drawings, which are provided at a specified electrode pitch on the semiconductor chip
1
and to tip ends of ultrafine Au wires
6
by means of the heat block
11
, pressure welding is performed while applying ultrasonic vibration to the Au balls formed at the tip ends of the Au wires
6
through electric discharge to connect the Al electrodes and the Au wires
6
. The Ag plated portions formed at the tip end portions
4
a
of the inner leads
4
and the ultrafine Au wires
6
are connected by applying heat through the heat block
11
and the frame forcing mold
13
while applying ultrasonic vibration to the ultrafine Au wires
6
.
In such a conventional wire bonding apparatus of the above-described arrangement, when detaching the frame forcing mold
13
upon completion of wire bonding, the die pad
2
and the hanging lead
8
, which had been forcibly deformed by means of the heat block
11
and the frame forcing mold
13
, are released from clamped conditions the moment the frame forcing mold
13
is detached, whereupon the die pad
2
is restored and descends by an amount it had been pushed up and the Au wires
6
are forcibly extended by an amount they were pushed up, so that there is presented a drawback that tensile force is applied to the Au wires
6
.
Further, in case manufacturing errors are present in the sunken die pad, there are presented drawbacks that deformation is applied to the hanging lead
8
arranged as to successively communicate with the die pad
2
, so that in a following process of resin sealing (not shown), the Au wires
6
are exposed on the surface of the resin when position of the semiconductor chip
1
is varied in an upward direction, and that the die pad
2
is exposed on the surface of the resin in case the position of the semiconductor chip
1
is varied in a downward direction. Such phenomena remarkably occurred in case of packages of, for instance, TSOP (Thin Small Outline Package) type or TQFP (Thin Quad Flat Package) type of especially small thickness, e.g. 1 mm.
A drawback was further presented when the semiconductor chip
1
is uniformly joined to the die pad
2
having a size substantially identical to that of the chip by means of a bonding material. The uniformly joined semiconductor chip
1
and the die pad
2
exhibit deformations such as inclinations or bowing owing to a difference in coefficients of thermal expansion.
The present invention has been made for solving the above-described problems, and it provides a wire bonding apparatus and a wire bonding method for a semiconductor device capable of decreasing an amount of restoration of the hanging lead and decreasing tensile force applied to the wires at the time of detaching the frame forcing mold.
The present invention further provides a wire bonding apparatus and
Johnson Jonathan
Mitsubishi Denki & Kabushiki Kaisha
LandOfFree
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