Wire bonding method

A wire bonding method comprising the steps of raising a capillary after bonding a ball formed on the tip end of the wire extending from the tip end of a capillary to a first bonding point, moving the capillary to a position located at an upward inclination in the direction of a second bonding point, further moving the capillary to a position located at an upward inclination in the opposite direction from the second bonding point, then moving the capillary to a position located at an upward inclination in the direction of the second bonding point, and then lowering the capillary so that the wire is bonded to the second bonding point by the capillary, thus producing a stable low-height M-shaped wire loop between the first and second bonding point.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a wire bonding method in which a wire 
passing through a capillary is connected to a first bonding point and 
second bonding point by the capillary, and more particularly, to an 
M-shape wire loop formation method. 
2. Prior Art 
Japanese Patent Application Laid-Open (Kokai) No. H10-189641 discloses, in 
its column 4, line 22-column 5, line 24 and in FIGS. 2 and 3, an 
improvement of a trapezoidal wire loop which is disclosed, for example, in 
FIG. 2 of Japanese Patent Application Publication (Kokoku) No. H5-60657. 
In this improved wire loop, a part of the upper linear portion of the 
trapezoidal wire loop is formed in a downward bow shape, so that the wire 
loop is in substantially an M-shape. Below, this loop shape will be 
referred to as an M-shaped loop. 
The M-shaped loop will be described in detail below with reference to FIGS. 
4 through 6. 
As shown in FIG. 4, the M-shaped wire loop is formed when upper surface 
(first bonding point) of a pad 2a on a semiconductor chip 2 mounted on a 
lead frame 1 and the upper surface (second bonding point) of a lead 1a on 
the lead frame 1 are connected by a wire 3, and it consists of a neck 
height part 31, a trapezoid length part 32 and an inclined part 33. Kinks 
3a and 3b are formed at both ends of the trapezoid length part 32, and a 
kink 3c is formed in the trapezoid length part 32 so that the shape of the 
trapezoid length part 32 is bowed downward. This M-shaped loop is formed 
by the capillary that is moved as shown in FIG. 5 by the steps shown in 
FIG. 6. 
As shown in steps (a) of FIG. 6, the capillary is lowered with a clamper 
(not shown) which holds the wire 3 maintained in a closed state, and a 
ball formed on the tip end of the wire is bonded to the first bonding 
point A. Afterward, the capillary 4 is raised to point B while the wire 3 
is delivered. Next, as shown in step (b), a reverse operation is performed 
in which the capillary 4 is moved horizontally to point C in the opposite 
direction from the second bonding point G. As a result, a first kink 3a is 
formed in a portion of the wire 3. The wire 3 delivered in the process 
from point A to point C forms the neck height part 31 shown in FIG. 5. 
Next, in step (c), the capillary 4 is raised to point D1, delivering the 
wire 3. Afterward, in step (d), the capillary 4 is moved to point D2 in 
the direction of the second bonding point G. Then, in step (e), the 
capillary 4 is raised to point D while the wire 3 is delivered. As a 
result of these steps (d) and (e), a second kink 3c is formed in the wire 
3. The length of wire delivered in the operation from point D1 to point D2 
(i.e., the length from the first kink 3a to the second kink 3c) forms the 
first horizontal wire part 34 shown in FIG. 5. 
Next, in step (f), the capillary 4 is moved in the opposite direction from 
the second bonding point G; in other words, a second reverse operation is 
performed so that the capillary 4 is moved horizontally to point E. As a 
result of this operation from point C to point E, a third kink 3b is 
formed in the wire 3. The wire 3 delivered at this time forms the second 
horizontal wire part 35 shown in FIG. 5. Next, in step (g), the capillary 
4 is raised to point F1 while an amount of wire 3 that will form the 
inclined part 33 shown in FIG. 5 is delivered; and then the clamper (not 
shown) is closed. When the clamper is closed, no wire 3 is delivered even 
if the capillary 4 is subsequently moved. Next, in step (h), the capillary 
4 is moved horizontally to point F in the direction of the second bonding 
point G. Further, in steps (h) and (i), the capillary 4 is positioned at 
the second bonding point G by being caused to perform a circular-arc 
movement or by being caused to perform a circular-arc movement and then 
lowered; as a result, the wire 3 is bonded to the second bonding point. 
In the prior art described above, the track of the capillary 4 is 
complicated, as shown in FIGS. 5 and 6. Since the amount of movement of 
the capillary 4 is large, a considerable amount of time is required for 
bonding. Furthermore, as a result of the reverse operation in step (b), 
the wire 3 is bent from the root (first bonding point A); and then the 
wire 3 is pulled in the direction of arrow in FIG. 7 so that the root 
portion of the wire 3 (at the first bonding point A) is returned to its 
original position in steps (h) through (i), and damage such as cracking, 
breaking, etc. may occur in the neck part 30 of the wire 3 as shown in 
FIG. 7. 
Furthermore, in the M-shaped wire loop, since the elasticity of the wire 
loop shape as a whole is large, there may be cases in which the kinks 3a, 
3b and 3c are pulled and extended in steps (g) through (i), resulting in 
that the neck height part 31 extending from the first bonding point A to 
the first kink 3a cannot return to the original perpendicular position; 
and this causes differences in the wire loop shape and in the height Ha of 
the kink 3a. In some cases, the height Hb of the kink 3b becomes higher 
than the height Ha as a result of such fluctuations in the height Ha, or 
as a result of rebound due to plastic deformation of the wire 3 at the 
time of bonding to the second bonding point G or of the mold flow that 
occurs in subsequent processes following wire bonding, etc. As a result, 
the height Hb of the kink 3b is approximately 200 to 400 .mu.m. Thus, it 
is difficult to form a wire loop with a stable overall height of 200 .mu.m 
or less. 
SUMMARY OF THE INVENTION 
In view of the above, the object of the present invention is to provide a 
wire bonding method for forming an M-shaped wire loop in which the track 
of the capillary is simple and the amount of movement of the capillary is 
small so that the bonding time can be short, and in which damages to the 
neck part of the wire can be avoided. 
The second object of the present invention is to provide a wire bonding 
method which allows a stable formation of a low wire loop in which the 
overall height of the wire loop is 200 .mu.m or less. 
The above objects are accomplished by unique steps taken in a wire bonding 
method in which a wire that passes through a capillary is connected to a 
first bonding point and second bonding point by the capillary; and in the 
present invention, [a] the capillary is raised after a ball formed on the 
tip end of the wire extending from the tip end of the capillary is bonded 
to the first bonding point, [b] the capillary is then moved to a position 
located at an upward inclination in the direction of the second bonding 
point, [c] the capillary is then moved to a position located at an upward 
inclination in the opposite direction from the second bonding point, [d] 
the capillary is then moved to a position located at an upward inclination 
in the direction of the second bonding point, and then [e] the capillary 
is lowered so as to bond the wire to the second bonding point.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
One embodiment of the present invention will be described with reference to 
FIGS. 1 and 2. 
First, in step (a) of FIG. 1, which is the same as in a conventional 
method, the capillary 4 is lowered with a clamper (not shown) that holds 
the wire 3 maintained in a closed state, and the ball formed on the tip 
end of the wire is bonded to the first bonding point A. Afterward, the 
capillary 4 is raised to point B, delivering the wire 3. 
Next, the process that characterizes the present invention is performed. 
As shown in step (b), the capillary 4 is moved from point B to point C, 
which is located at an upward inclination in the direction of the second 
bonding point G (process from point B to point C in FIG. 2). It goes 
without saying that the amount of movement and the angle at which the 
capillary is raised in this case are set so that no damage occurs to the 
neck part 30 of the wire 3 (see step (e)). As a result, a first kink 3a is 
formed in a portion of the wire 3. The wire 3 delivered in the process 
from point A to point B forms the neck height part 31 shown in FIG. 4. 
Next, in step (c), the capillary 4 is moved to point D which is located at 
an upward inclination in the opposite direction from the second bonding 
point G (process from point C to point D in FIG. 2), delivering the wire. 
In other words, the capillary 4 is moved obliquely upward in the direction 
of the first bonding point A. As a result of the steps (b) to (c), a 
second kink 3c is formed in the wire 3. The (length of) wire that is 
delivered in the operation from point B to point C (i.e., the length from 
the first kink 3a to the second kink 3c) forms the first horizontal wire 
part 34 shown in FIG. 4. 
Next, in step (d), the capillary 4 is moved to point E, which is located at 
an upward inclination in the direction of the second bonding point G 
(process from point D to point E in FIG. 2), delivering the wire. As a 
result of the steps (c) to (d), a third kink 3b is formed in the wire 3. 
The (length of) wire that is delivered from point C to point D (i.e., the 
length from the second kink 3c to the third kink 3b) forms the second 
horizontal wire part 35 shown in FIG. 4. The (length of) wire 3 that is 
delivered in the step (d) forms the inclined part 33 shown in FIG. 4. 
Afterward, the clamper (not shown) is closed. When the clamper is closed, 
no wire 3 is delivered even if the capillary 4 is subsequently moved. 
Next, in step (e), the capillary 4 is caused to perform a circular-arc 
movement from point E or is caused to perform such a circular-arc movement 
and is then lowered, so that the capillary 4 is positioned at the second 
bonding point G, thus bonding the wire 3 (process from point E to point G 
in FIG. 2) thereto. 
The operation from point E to the second bonding point G has no direct 
connection with the gist of the present invention; therefore, it goes 
without saying that the operation that is similar to the operations 
disclosed in the above-described Japanese Patent Application Publication 
(Kokoku) No. H5-60657 and Japanese Patent Application Laid-Open (Kokai) 
No. H10-189641 can be performed for the point E to point G, or any various 
other types of operations may be performed. 
In the above description, the track of the capillary 4 from point B to 
point C, the track from point C to point D and the track from point D to 
point E are straight lines as shown in FIG. 2. However, in cases where it 
is difficult to deliver the wire 3 due to the diameter, hardness or 
elasticity of the wire 3 or the shape or condition of the capillary 4, 
etc., it is possible to set the track of the capillary 4 so as to be a 
curve that passes through points B, C, D and E, e. g., a splined curve, 
etc., as shown in FIG. 3, instead of using the linear track shown in FIG. 
2. 
Though it depends upon the distance and height difference between the first 
and second bonding points A and G and other factors, the amount of wire 
delivered in step (d) or from point D to E is set to be the largest 
compared to the amount of wire delivered in other steps. 
In conventional methods, as shown in FIGS. 5 and 6, an extremely large 
number of steps (a) through (i) are performed, and in addition, the amount 
of movement of the capillary 4 is large. In the present invention, on the 
other hand, as shown in FIGS. 1 and 2, the number of steps required is 
small ((a) through (e)), and the amount of movement of the capillary 4 is 
also small. Accordingly, the bonding time can be greatly shortened. 
Furthermore, in the prior art shown in FIG. 6, the neck height part 31 
which is bent by the reverse operation performed in step (b) is, by the 
steps from (h) to (i) shown in FIG. 6, brought back to its original 
position by being pulled in the pulling direction of the wire shown in 
FIG. 7. However, in the present invention, such a sequential movement 
(which is from the reverse operation to the operation that brings the neck 
height part 31 to its original position) is not taken. Therefore, the wire 
is less likely to be damaged compared to the prior art. 
In the present invention, the first kink 3a and the neck height part 31 are 
formed in the steps (a) and (b) when the capillary 4 is raised to point B 
after bonding to the first bonding point A, and the capillary 4 is raised 
to point C which is located at an upward inclination in the direction of 
the second bonding point G. Accordingly, the height Ha of the kink 3a can 
be stabilized, and the wire loop shape can be also stabilized. As a 
result, whichever is higher of the heights Ha and Hb of the wire loop as a 
whole can be reduced to as low as 100 to 200 .mu.m in height. 
As seen from the above, in the present invention, the capillary is raised 
after a ball formed on the tip end of the wire extending from the tip end 
of the capillary is bonded to the first bonding point, the capillary is 
then moved to a position located at an upward inclination in the direction 
of the second bonding point, the capillary is then moved to a position 
located at an upward inclination in the opposite direction from the second 
bonding point, the capillary is further moved to a position located at an 
upward inclination in the direction of the second bonding point, and then 
the capillary is lowered so that the wire is bonded to the second bonding 
point. Accordingly, the track of the capillary is simple and the amount of 
movement of the capillary is small, and the bonding time can be shortened. 
Also, no damage occurs to the neck part of the wire. Moreover, a low wire 
loop shape in which the height of the wire loop as a whole is 200 .mu.m or 
less can be stably formed.