Metal fusion bonding – Process – Plural joints
Reexamination Certificate
2000-03-02
2001-08-07
Dunn, Tom (Department: 1725)
Metal fusion bonding
Process
Plural joints
C228S180100
Reexamination Certificate
active
06270000
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wire bonding method for connecting a first bonding point and a second bonding point with a wire.
2. Prior Art
Various wire bonding methods have been proposed in producing semiconductor devices. The most general method is illustrated in FIG.
2
.
First, in step (a), a ball
2
a
is created by electrical discharge of an electric torch
3
on a wire
2
that extends from the lower end of a capillary
1
. Afterward, the electric torch
3
is moved in the direction shown by the arrow. As shown in step (b), the capillary
1
is then moved to a point above the first bonding point
4
a
. In step (c), the capillary
1
is lowered, and the ball
2
a
at the tip end of the wire
2
is subjected to a first bonding to the first bonding point
4
a.
Afterward, in step (d), the capillary
1
is raised. Then, in step (e), the capillary
1
is moved to a point above the second bonding point
5
a
, which is located on a lead
5
. Next, in step (f), the capillary
1
is lowered, and the wire
2
is subjected to a second bonding to the second bonding point
5
a
. After the capillary
1
is raised to a predetermined position, a damper
6
is closed; and the capillary
1
and damper
6
are raised together so that the wire
2
is tail-cut from the attachment root of the second bonding point
5
a
in step (g).
One wire connection is thus completed.
Examples of this wire bonding method are disclosed in Japanese Patent Application Laid-Open (Kokai) No. S57-87143 and Japanese Patent Application Publication (Kokoku) No. H1-26531.
In Japanese Patent Application Laid-Open (Kokai) No. S57-87143, after finishing bonding to a first bonding point, a capillary is raised for a distance that is equal to the length required for bonding to a second bonding point, and then the capillary is moved along a circular path to the second bonding point or a point adjacent to the second boning point.
In Japanese Patent Application Publication (Kokoku) No. H1-26531, after the completion of bonding to a first bonding point, a capillary is moved in the opposite direction from the second bonding point, and then the capillary is moved along an oblique upward path and then oblique downward path so as to be brought to a second bonding point.
FIGS.
3
(
a
)and
3
(
b
)show the end of capillary in step (f) of FIG.
2
and the end of the wire in step (g) of FIG.
2
.
As seen from step (f) and FIG.
3
(
a
), the wire
2
is pressed against the second bonding point
5
a
on the lead
5
; and in step (g), the wire
2
is tail-cut from the attachment root thereof. As a result, a projection
2
c
which is bent laterally remains at the tip end of the tail
2
b
(i.e., the portion of the wire
2
that extends out to the capillary
1
) as shown in FIG.
3
(
b
). As seen from FIG.
3
(
a
), the projection
2
c
is formed when the wire
2
is pressed against the lead
5
; and when the wire is thus pressed against the lead
5
, impurities
7
on the surface of the lead
5
adhere to the undersurface of the projection
2
c.
If the ball
2
a
is formed at the end of the wire with impurities
7
adhering thereto, the impurities
7
remain on the ball
2
a
, resulting in that the discharge efficiency changes, and abnormal balls are formed at the end of the wire. Furthermore, it is difficult to form a small ball. When the volume of the ball
2
a
formed is large enough to absorb the volume of the projection
2
c
, eccentricity would not occur in the ball
2
a
even if the projection
2
c
extends laterally at the end of the wire. However, when the ball
2
a
formed is small and is of the size close to the volume of the projection
2
c
, then the formed ball
2
a
is eccentric as shown in FIG.
3
(
c
). In this regard as well, the formation of a small ball is difficult.
In recent years, so as to accomplish a reduction in the cost of dice (semiconductor elements), there has been a tendency to reduce the size of dice and to use a fine pitch bonding so that the spacing of pads is narrow and as small as approximately 50 to 60 microns. In conventional wire bonding methods, however, it is difficult to accomplish a formation of extremely small balls that are free of eccentricity as described above on the consistent basis. As a result, the conventional methods are not sufficiently able to comply with fine pitch bonding.
SUMMARY OF THE INVENTION
Accordingly, the object of the present invention is to provide a wire bonding method which accomplishes a consistent formation of balls at the end of a bonding wire that are subjected to first bonding to a first bonding point as an extremely small ball.
The above object is accomplished by unique steps of the present invention for a wire bonding method in which a first bonding is performed on a first bonding point by bonding a ball formed at the tail end of a wire, after which the wire is delivered from a capillary and a second bonding is performed on a second bonding point so that the first bonding point and second bonding point are connected by the wire; and in the present invention, after the bonding to the second bonding point is done and the bonded ball is separated from the wire, a secondary ball is formed at the tail end of the wire extending from the tip end of the capillary by performing a secondary discharge with an electric torch, this secondary ball is ball-bonded to an arbitrary point other than the second bonding point, then the bonded secondary ball is separated from the tail end of the wire extending from the tip end of the capillary, and a primary ball formed is on this tail end by performing a primary discharge with the electric torch, so that the thus formed ball is bonded to the next first bonding point.
In the present invention, the diameter of the secondary ball is set to be a large value, while the diameter of the primary ball is set to be a small value.
In other words, the diameter of the secondary ball is 1.7 times the diameter of the wire or greater, and the diameter of the primary ball is 1.5 times the diameter of the wire or less.
REFERENCES:
patent: 5295619 (1994-03-01), Takahashi et al.
patent: 5395037 (1995-03-01), Takahashi et al.
patent: 5981371 (1999-11-01), Yamazaki et al.
patent: S57-87143 (1982-05-01), None
patent: H1-26531 (1989-05-01), None
Dunn Tom
Kabushiki Kaisha Shinkawa
Koda & Androlia
Pittman Zidia
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