Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
1998-09-25
2002-12-17
Cuneo, Kamand (Department: 2827)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C361S776000, C228S180500, C257S780000
Reexamination Certificate
active
06495773
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to wire bonding methods, semiconductor devices, capillaries for wire bonding and methods of forming ball bumps, and, more particularly, to a wire bonding method, a semiconductor device, a capillary for wire bonding and a method of forming ball bumps which provide an efficient and reliable wire bonding.
Generally, semiconductor elements provided in a semiconductor device are electrically connected, using a wire, to a lead serving as an external connection terminal. Such a wire is disposed by a wire bonding unit to extend between a pad formed on a semiconductor element and an inner lead of the lead.
The number of wires disposed is equal to the number of pads formed on the semiconductor element. With increasingly higher degree of integration, an increasing number of pads are provided in semiconductor elements. As a result, the number of wires disposed between the pad and the inner lead becomes increasingly large.
Improvement in productivity and reduction of size are major requirements in manufacturing a semiconductor device. Reduction of time needed to wire-bond individual wires lends itself to improvement in productivity, while lowering of a loop height of wires lends itself to reduction of size.
2. Description of the Prior Art
FIGS. 1 and 2
show wires disposed according to the conventional wire bonding.
FIGS. 3A-3C
and
FIGS. 4A-4C
show a detailed wire bonding method.
FIG. 1
shows a wire
1
disposed using the most generally practiced wire bonding method. Referring to
FIG. 1
, the wire
1
is disposed between a pad
2
a
formed in a semiconductor element
2
and an inner lead
3
a
of a lead frame
3
.
In order to dispose the wire
1
between the pad
2
a
and the inner lead
3
a,
the wire
1
is first bonded to the pad
2
a
(first bonding). In this case, the ball bonding as shown in
FIGS. 3A-3C
is employed in the first bonding.
As shown in
FIGS. 3A-3C
, a metal wire forming the wire
1
is led through a capillary
7
provided in a wire bonding unit. Referring to
FIG. 3A
, a ball-shaped part
4
is formed at the head of the wire
1
by spark discharge. Then, as shown in
FIG. 3B
, the ball-shaped part
4
is pressed against the pad
2
a
using the capillary
7
and bonded therewith using the ultrasonic welding process.
Then, the wire
1
which extends to the inner lead
3
a
is formed by a guiding process effected by guiding by drawing the capillary
7
in a direction indicated in FIG.
3
C. The wire
1
is bonded to the inner lead
3
a
(second bonding). In this case, the stitch bonding process shown in
FIGS. 4A-4C
is employed in the second bonding.
Referring to
FIG. 4A
, the end of the capillary
7
is guided by drawing to a bonding position of the inner lead
3
a.
Subsequently, as shown in
FIG. 4B
, the end of the capillary
7
is pressed against the inner lead
3
a
so that the wire
1
is bonded to the inner lead
3
a
in the ultrasonic welding process. The portion of the wire
1
which is pressed by the capillary
7
is deformed as a result of the capillary
7
being pressed against the inner lead
3
a.
Thereafter, the capillary is guided upward. While the capillary is guided upward, a clamper
8
provided in the wire bonding unit holds the wire
1
fixed. Therefore, the wire
1
is cut at the portion thereof that is deformed and has lost the mechanical strength.
As has been described, according to the most generally practiced conventional wire bonding method, the wire
1
is provided between the pad
2
a
of the semiconductor element
2
and the inner lead
3
a
of the lead frame
3
as shown in FIG.
1
through the first bonding that uses the ball bonding process and the second bonding that uses the stitch bonding process (hereinafter, such a method of providing the wire
1
is referred to as forward bonding).
A description will now be given, with reference to
FIGS. 5A-6B
, of the characteristic of the ball bonding process and the stitch bonding process.
FIG. 5A
is a perspective view showing how the wire
1
is bonded to the pad
2
a
using the ball bonding process, and
FIG. 5B
is a top view of the bonded area. As has been described already, the ball bonding process is performed such that the ball-shaped part
4
is formed in the wire
1
using spark discharge and then bonded to the pad
2
a.
As a result of such a process, the bonded area at the bottom of the ball-shaped part
4
has a circular shape in a top view and resides within the pad
2
a.
FIG. 6A
is a perspective view showing how the wire
1
is bonded to the inner lead
3
a
using the stitch bonding process, and
FIG. 6B
is a top view of the bonded area. As shown in the figures, since the wire
1
is pressed by the capillary
7
according to the stitch bonding, the wire
1
is flattened into a bonded portion
9
having a relatively wide area. For comparison,
FIGS. 6A and 6B
include broken lines indicating the pad
2
a.
Characteristically, the stitch bonding process requires a wider bonding area as compared to the ball bonding process.
In the forward bonding described above, the wire
1
is guided upward after being bonded to the pad
2
a
and then bonded to the inner lead
3
a.
Hence, the height of the loop formed by the wire
1
is relatively high with respect to the upper major surface of the semiconductor element
2
.
In the construction shown in
FIG. 1
, the wire
1
is higher in level than the upper major surface of the semiconductor element
2
by a height H. Accordingly, the forward bonding has a problem in that it is difficult to make the semiconductor device thin.
FIG. 2
shows a wire
5
provided according to a bonding method proposed to eliminate the problem resulting from the forward bonding. In
FIG. 2
, those components that are the same as the components of
FIG. 1
are designated by the same reference numerals.
As shown in
FIG. 2
, the wire
5
is provided such that the ball-shaped part
4
is formed at the end of the wire
5
jutting out of a capillary using spark discharge. The ball-shaped part
4
is then pressed against the inner lead
3
a
of the lead frame
3
so as to be welded thereon according to the ball bonding process.
The capillary is then guided upward to a level slightly higher than the upper major surface of the semiconductor element
2
and then guided horizontally so that the wire
5
is guided to a position above the pad
2
a.
The capillary is then pressed against the pad
2
a
so that the wire
5
is bonded to the pad
2
a
using the stitch bonding process. More specifically, a ball bump
6
formed of a gold or the like is provided in advance on the pad
2
a,
and the wire
5
is bonded to the ball bump
6
.
It will be noted that the bonding order of the bonding method shown in
FIG. 2
is different from that of the bonding method shown in
FIG. 1
in that wire is bonded to the inner lead
3
a
in the first bonding and then bonded to the pad
2
a
(more specifically, to the ball bump
6
on the pad
2
a
) in the second bonding. The method for providing the wire
5
will be referred to as a backward bonding.
According to the backward bonding, wire
5
is first bonded to the inner lead
3
a
which is lower in level than the upper major surface of the semiconductor element
2
. The wire
5
is then guided upward to reach the same height as the upper major surface of the semiconductor element
2
. Thereafter, the capillary is guided horizontally so that the wire
5
can be bonded to the pad
2
a
(more specifically, the ball bump
6
).
As shown in
FIG. 2
, the wire
5
provided between the pad
2
a
and the inner lead
3
a
is bent to form an approximately right angle (an inverted L shape) so that the loop formed by the wire
5
is lower than that of the construction shown in FIG.
1
. Accordingly, it is possible to make the semiconductor device thin.
While the backward bonding described above ensures that the semiconductor device produced has a relatively small height because the loop-formed by the wire
5
is relatively low, it has a problem in that the ball bu
Kasai Junichi
Nomoto Ryuji
Sato Mitsutaka
Tsuji Kazuto
Armstrong Westerman & Hattori, LLP
Cuneo Kamand
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