Wire bonded device with ball-shaped bonds

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 bump

6

must be formed on the pad

2

a

using a gold or the like.

The ball bump

6

serves as a buffer for preventing the pressure of the capillary from being exerted on the semiconductor element

2

directly when the wire

5

is bonded to the pad

2

a

using the stitch bonding process. The ball bump

6

also has a function of improving the resistance of the wire bonding.

If the wire

5

is directly bonded to the pad

2

a

using the stitch bonding process without using the ball bump

6

, the semiconductor element

2

may be damaged due to the pressure exerted by the capillary. Insufficient bonding of the wire

5

to the pad

2

a

may cause the wire

5

to be removed from the pad

2

a.

For this reason, it is necessary to provide the ball bump

6

on the pad

2

a

in the conventional backward bonding.

The conventional backward bonding has a problem in that a process of providing the ball bump

6

is needed in addition to the process of providing the wire

5

, adding to the number of processes in the wire bonding, and in that the efficiency in producing the semiconductor device is unsatisfactory.

The ball bonding process employed in forming the ball bump

6

is such that a gold ball is formed at the end of a gold wire and then bonded to the pad

2

a,

after which the gold wire is cut. In this process of forming the gold ball, a pressure is exerted on the semiconductor element

2

.

As has been described, the bonding of the wire

5

to the ball bump

6

involves a pressure exerted on the semiconductor element

2

. Therefore, the semiconductor element

2

undergoes the pressure on two occasions during the wire bonding process, that is, when the ball bump

6

is formed and when the wire

5

is bonded. Thus, even if the ball bump

6

is provided, a possibility remains that the semiconductor element

2

is damaged during the whole wire bonding process.

As has been described with reference to

FIGS. 5A-6B

, the stitch bonding process requires a wider bonding area as compared to the ball bonding process. Therefore, it is impossible to bond the wire

5

on the upper major surface of the semiconductor element

2

with a high density in the backward bonding. The stitch bonding process is not opted for if the semiconductor element

2

has pads

2

a

disposed thereon with a high density.

For this reason, only the forward bonding shown in

FIG. 1

can be applied to the high-density semiconductor element

2

. Accordingly, the semiconductor element

2

has a relatively large size for the aforementioned reason.

As has been described, the backward bonding is useful for the semiconductor element

2

that does not have pads

2

a

disposed thereon with a high density. However, an upper surface

6

a

(the surface to which the wire

5

is bonded) of the ball bump

6

is rugged because it is the cut surface of the gold wire. Therefore, the bonding strength that exists between the wire

5

and the ball bump

6

is not satisfactory so that the reliability of the wire bonding is not satisfactory.

Further, as shown in

FIG. 8

, if an especially large bump

6

b

is produced on an upper surface of the ball bump

6

, the wire

5

is bonded at a position displaced from the center of the ball bump

6

. An edge part

5

a

characterized by a low bonding strength and produced as a result of the wire

5

deformed flat by the capillary

7

resides on the end of the upper major surface of the ball bump

6

. The wire

5

may be cut at a portion adjacent to the edge of the upper major surface of the ball bump

6

.

It is thus an object of the present invention to provide a semiconductor device having a wire loop with an L shape or curved shape. In this present invention, the semiconductor device includes a semiconductor element; a lead; a wire connecting the semiconductor element and the lead, wherein the lead is directly bonded to the wire using a first ball-shaped part formed in the wire. An electrode formed in the semiconductor element is directly bonded to the wire using a second ball-shaped part formed in the wire. The wire forms a loop having a curved shape and including a vertical portion that extends vertically from a position where the wire is bonded to the lead, and a horizontal portion that extends horizontally from a position where the semiconductor element is bonded to the wire.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a wire bonding method, a semiconductor device, a capillary for wire bonding and a method of forming ball bumps in which the aforementioned problems are eliminated.

Another and more specific object of the present invention is to provide a wire bonding method, a semiconductor device, a capillary for wire bonding and a method of forming ball bumps which provide an efficient bonding process, reduction in damage to connected bodies, and reduction in the height of semiconductor elements.

In order to achieve the aforementioned objects, the present invention provides a wire bonding method comprising:

a first bonding process for forming a first ball-shaped part in a wire and bonding the first ball-shaped part to a first connected member;

a ball-shaped part forming process for guiding the wire away from the first connected member so as to form a predetermined loop and forming a second ball-shaped part in a predetermined position in the wire; and

a second bonding process for bonding the second ball-shaped part to a second connected member.

According to the wire bonding method of the present invention, a damage is prevented from occurring in a semiconductor element when the wire is bonded to the second connected member because the second ball-shaped part functions in the same manner as a ball bump of the backward bonding process. Since the second ball-shaped part is formed in the wire, no additional process separate from the sequence of the wire bonding process is necessary in order to form the ball bump. Accordingly, the wire bonding process can be carried out efficiently. The process for bonding the second ball-shaped part to said second connected member is a so-called ball bonding process and requires a relatively small area. Hence, a wire bonding process can be securely carried out in a semiconductor element characterized by a high density with which electrodes are provided.

Preferably, in the ball-shaped part forming process, the second ball-shaped part is formed while the wire remains uncut. According to this aspect of the present invention, it is possible to position the second ball-shaped part on the pad in a reliable manner. Accordingly, the second ball-shaped part can be properly bonded to the pad.

Preferably, at least the second ball-shaped part is formed by spark discharge. According to this aspect of the present invention, no modification to the existing wire bonding unit is necessary because the spark discharge process is normally practiced to form the first ball-shaped part.

In another preferred embodiment, the wire is guided vertically upward after the first ball-shaped part is bonded to the first connected member; and the wire is then guided horizontally, forming a right angle, whereupon the second ball-shaped part is formed and bonded to the second connected member. According to this aspect of the present invention, the wire provided between the first connected member and the second connected member is low. Therefore, the height of the semiconductor device produced can be kept low.

In order to attain the aforementioned objects, the present invention provides a wire bonding method comprising the steps of:

providing a first wire according to the wire bonding method as claimed in claim 1;

forming a third ball-shaped part in a second wire and bonding the third ball-shaped part to the second connected member; and

guiding the second wire away from the second connected member so as to form a loop above the first wire, and stitch-bonding the second wire to the first connected member.

According to this wire bonding method, the second wire can be easily provided above the first wire because the loop formed,by the first wire is low. Accordingly, it is possible to provide wires with a high density and to prevent the first and second wires from interfering each other.

In another preferred embodiment, the first connected member is embodied by a lead frame, and the second connected member is embodied by a semiconductor element. Accordingly, the semiconductor device can be thin and the production efficiency thereof is improved as compared to the conventional method.

In still another preferred embodiment, the first connected member and the second connected member are embodied by a semiconductor. Therefore, the multi-chip module produced according to the present invention can be thin and the production efficiency thereof is improved as compared to the conventional method.

In yet another preferred embodiment, the wire is embodied by a thin gold wire, and a ball bonding process is employed to bond at least the second ball-shaped part to the second connected member. According to this aspect of the present invention, a damage is prevented from being exerted on the second connected member.

In order to attain the aforementioned objects, the present invention also provides a semiconductor device comprising:

a semiconductor element;

a lead; and

a wire connecting the semiconductor element and the lead;

wherein

the lead is directly bonded to the wire using a first ball-shaped part formed in the wire,

an electrode formed in the semiconductor element is directly bonded to the wire using a second ball-shaped part formed in the wire, and

the wire forms a loop having an L shape and including a vertical portion that extends vertically from a position where the wire is bonded to the lead, and a horizontal portion that extends horizontally from a position where the semiconductor element is bonded to the wire.

According to the semiconductor device of the present invention, a damage exerted on the semiconductor element is reduced because the electrode formed on the semiconductor element is directly bonded to the wire using the second ball-shaped part formed in the wire. By providing the wire so as to form a L-shaped loop, it is possible to reduce the height of the semiconductor device.

In order to attain the aforementioned objects, the present invention also provides a wire bonding capillary for use in a bonding process for bonding a wire to a connected member, wherein an end of a main body of the capillary includes a projection. According to the wire bonding capillary of the present invention, the end portion projecting from the main body of the capillary acts to bond the wire to the ball bump and the portion surrounding the projecting end portion acts to press the ball bump.

Preferably, a bonding part for bonding the wire to the connected member is formed at the end of the main body of the capillary, and a pressurizing part for pressing a ball-shaped part formed by the bonding part in the connected member is formed adjacent to the bonding part. Accordingly, the process for bonding the wire to the connected member and the process for welding the ball-shaped part formed in the connected member can be carried out using the same capillary. Hence, the forming of the ball bump can be carried out efficiently.

In order to attain the aforementioned objects, the present invention also provides a ball bump forming method using the wire bonding capillary as claimed in claim 10, the ball bump forming method comprising:

a bonding process for forming the ball-shaped part in the wire and bonding the ball-shaped part to the connected member using an end portion of the wire bonding capillary;

a ball bump forming process for forming a ball bump on the connected member by cutting the wire by drawing the wire bonding capillary away from a position where the wire is bonded to the connected member; and

a shaping process for turning an upper major surface of the ball bump flat by causing a portion of the wire bonding capillary which portion surrounds the end portion to press the ball bump.

Further, the present invention provides a ball bump forming method using the wire bonding capillary as claimed in claim 11, the ball bump forming method comprising:

a bonding process for forming the ball-shaped part in the wire and bonding the ball-shaped part to the connected member using the bonding part provided in the wire bonding capillary;

a ball bump forming process for forming a ball bump on the connected member by cutting the wire by drawing the wire bonding capillary away from a position where the wire is bonded to the connected member; and

a shaping process for turning an upper major surface of the ball bump flat by causing the pressurizing part provided in the wire bonding capillary to press the ball bump.

According to the ball bump forming method of the present invention, the upper major surface of the ball bump formed in the connected member can be turned flat. Thus, the quality of bonding that exists between the ball bump and the wire can be improved, resulting in a highly resistant bonding. The bonding process, the ball bump forming process and the shaping process can be effected in a successive manner using the same capillary. Hence, the ball bump forming process according to the present invention is highly efficient.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with t accompanying drawings, in which:

FIG. 1

shows a conventional forward bonding;

FIG. 2

shows a conventional backward bonding;

FIGS. 3A-3C

explain ball bonding;

FIGS. 4A-4C

explain stitch bonding;

FIG. 5A

is a perspective view showing how a wire is bonded to a pad using a ball bonding process;

FIG. 5B

is a top view showing a bonding position;

FIG. 6A

is a perspective view showing how a wire is bonded to an inner lead using a stitch bonding process;

FIG. 6B

is a top view showing a bonding position;

FIG. 7

illustrates a problem with the conventional backward bonding;

FIG. 8

illustrates a problem with the conventional backward bonding;

FIG. 9

shows how a first ball-shaped part is formed;

FIG. 10

shows how the first ball-shaped part is bonded to a lead frame;

FIG. 11

shows how the wire is guided to a pad;

FIG. 12

shows how a second ball-shaped part is formed;

FIG. 13

shows how the second ball-shaped part is bonded to the pad;

FIG. 14

shows a state in which the second ball-shaped part is bonded to the pad;

FIG. 15

gives a comparison between a loop formed by a wire provided according to a wire bonding method of the present invention and a loop formed by a wire provided according to a conventional wire bonding method;

FIG. 16

is a lateral sectional view of a semiconductor device according to the present invention;

FIG. 17

illustrates a variation of the wire bonding method of the present invention;

FIG. 18

shows an example in which the wire bonding method of the present invention is applied to the multi-chip module;

FIG. 19A

is a front view of a capillary according to the present invention;

FIG. 19B

is a lateral sectional view showing a portion near an end of the capillary on an enlarged scale;

FIG. 20

shows a method for forming a ball bump, specifically showing a gold ball being formed;

FIG. 21

shows a state in which the gold ball is bonded to a pad;

FIG. 22

shows a state in which the ball bump is formed;

FIG. 23

shows a process for shaping the ball bump;

FIG. 24

shows a shaped ball bump;

FIG. 25

shows a wire bonding process for bonding a wire to the ball bump, specifically explaining a method for forming a ball-shaped part in the wire;

FIG. 26

explains a method for bonding the ball-shaped part to a lead frame;

FIG. 27

shows a state in which the wire is guided toward the ball bump;

FIG. 28

shows a state in which the wire is being bonded to the ball bump; and

FIG. 29

shows a state in which the wire has been bonded to the ball bump.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description will now be given, with reference to

FIGS. 9-29

, of the embodiments of the present invention.

FIGS. 9-14

are diagrams which explain the wire bonding method according to a first embodiment of the present invention. The description given below of the first embodiment assumes that the wire bonding method is applied to the semiconductor production process. However, the application of the first embodiment is not limited to the semiconductor production process.

A description will be given, with reference to

FIG. 9

, of constituting elements used in the wire bonding. Referring to

FIG. 9

, a pad

11

is provided on a semiconductor element

10

. The semiconductor element

10

is mounted on a lead frame

12

. An inner lead

13

of a lead

22

(see

FIG. 16

) that constitute the lead frame

12

lies in the neighborhood of the semiconductor element

10

.

A capillary

14

and a spark electrode

15

constitute a wire bonding unit. The capillary

14

and the spark electrode

15

are provided in the conventional wire bonding unit and are not new features added to the construction of the wire bonding method according to the first embodiment.

A thin gold wire that embodies a wire

16

is inserted through the capillary

14

. The capillary

14

is translatable by a translation unit (not shown). An ultrasonic oscillation is connected to the capillary

14

so that the wire

16

and a connected member (the pad

11

and the inner lead

13

, in the case of the first embodiment) are welded together by ultrasonic welding owing to ultrasonic oscillation generated by the ultrasonic oscillator.

The spark electrode

15

is also translatable by a translation unit (not shown).

A description will now be given of the wire bonding method according to the first embodiment. As shown in

FIG. 9

, a first ball-shaped part

17

is formed at the end of the wire

16

inserted through the capillary

14

and projecting downward from the end of the capillary

14

. The first ball-shaped part

17

is formed as a result of spark discharge induced by the spark electrode

15

.

When the first ball-shaped part

17

has been formed at the end of the wire

16

projecting from the end of the capillary

14

, the capillary

14

is translated toward the inner lead

13

by the translation unit so that the first ball-shaped part

17

is pressed against the inner lead

13

, as shown in FIG.

10

. The ultrasonic oscillator is then driven to generate ultrasonic oscillation so that the capillary

14

acts to weld the first ball-shaped part

17

to the inner lead

13

according to the ultrasonic welding process (first connected member as described in the claims). As a result, the wire

16

is ball-bonded to the inner lead

13

. The above-described part of the wire bonding will be referred to as a first bonding process.

When the wire

16

(the first ball-shaped part

17

) has been bonded to the inner lead

13

, the capillary

14

is translated vertically upward to a level slightly higher than the upper major surface of the semiconductor element

10

, and then translated horizontally so that the capillary

14

is located above the pad

11

formed on the upper major surface of the semiconductor element

10

. While the capillary

17

is being translated, the wire

16

continues to be fed to the capillary

14

.

By translating the capillary

14

as described above, the wire

16

forms a loop having a inverted L shaped such that the loop includes a vertically extending part which projects vertically upward from the inner lead

13

and a horizontally extending part which extends horizontally.

When the wire

16

is guided by drawing to a position above the pad

11

, the spark electrode

15

is translated below the capillary

14

. As shown in

FIG. 12

, a second ball-shaped part

18

is formed by spark discharge in the wire

16

that is fed outside the capillary

14

, in a process that will be referred to as a ball-forming process. The second ball-shaped part

18

is formed by spark discharge at an intermediate position in the wire

16

that is guided away from the first ball-shaped part

17

.

Subsequently, the capillary

14

is translated downward so as to press the second ball-shaped part

18

against the pad

11

. At the same time, the ultrasonic oscillator is driven to generate ultrasonic oscillation. Therefore, as shown in

FIG. 13

, the second ball-shaped part

18

is bonded to the pad

11

(second connected member as described in the claims) using the capillary

14

. This part of the wire bonding will be referred to as a second bonding process.

When the second ball-shaped part

18

has been bonded to the pad

11

, the capillary

14

is translated upward to a position shown in FIG.

14

. While the capillary

14

is translated being upward, the wire

16

is prevented from being fed through the lower end of the capillary

14

, using a clamper (not shown), so that the wire

16

is cut at the bonding position as the capillary

14

is translated upward. This completes one sequence of the bonding process for the wire

16

.

The first ball-shaped part

17

is then formed again, after which the processes shown in

FIGS. 9-13

are repeated. In this way, bonding of the wire

16

to the pad

11

and the inner lead

13

is carried out on a continuous basis.

According to the wire bonding method described above, when the wire

16

is bonded to the pad

11

, the second ball-shaped part

18

formed in the wire

16

has the same function as the ball bump

6

(see

FIG. 2

) in the backward bonding. The second ball-shaped part

18

functions as a shock absorber for preventing the semiconductor element

10

from being damaged when the wire

16

is bonded to the pad

11

.

Since the second ball-shaped part

18

is formed in the wire

16

, no additional process for forming the ball bump

6

is necessary aside from the main wire bonding process. Thus, it is possible to carry out wire bonding process in a simple and efficient manner.

The second ball-shaped part

18

is bonded to the pad

11

in the ball bonding process. As shown in

FIG. 5B

, the bonded area is circular in shape in a top view and relatively small size. Because the ball bonding requires a relatively small area, the wire bonding process with respect to the semiconductor element

10

having a large number of pads

11

disposed with a high density is possible.

In the process shown in

FIG. 12

for forming the second ball-shaped part

18

, the wire

16

remains connected to the capillary

14

. For this reason, in the second bonding process shown in FIG.

13

, it is possible to position the second ball-shaped part

18

on the pad

11

in an accurate manner. Therefore, it is ensured second ball-shaped part

18

is reliably bonded to the pad

11

.

Further, by using the wire bonding method according to the first embodiment, it is possible to maintain the loop formed by the wire

16

relatively low.

FIG. 15

shows, for comparison, both the wire

16

provided by using the wire bonding method according to the embodiment described above and the wire

1

provided by using the conventional wire bonding method (forward bonding).

The wire

1

provided according to the conventional forward bonding forms a loop extending to the level higher than the upper major surface of the semiconductor element

10

, as shown in FIG.

15

. In contrast, the wire bonding according to the first embodiment employs the backward bonding in that the wire

16

is first connected to the inner lead

13

and then the wire

16

is guided by drawing to form a loop having an inverted L shape so as to be connected to the pad

11

. Hence, the loop of the wire

16

is relatively low.

While the wire bonding according to the first embodiment is configured such that the wire

16

is provided in a process similar to that of the conventional backward bonding, an advantage of the wire bonding according to the first embodiment is that fewer processes are required in order to form the wire

16

having a loop as low as the loop formed by the conventional backward bonding process, because the second ball-shaped part

18

is formed directly in the wire

16

in a spark discharge process that uses the spark electrode

15

.

FIG. 16

shows a semiconductor device

20

according to a second embodiment of the present invention. In the semiconductor device

20

, the wire

16

is provided according to the wire bonding method described above.

FIG. 16

shows an example in which the wire bonding method described above is applied to a semiconductor device having a plastic package.

The semiconductor device

20

generally comprises the semiconductor element

10

, the wire

16

, the lead

22

, a sealing resin

21

and the like. The semiconductor element

10

is mounted on a stage

23

that forms a part of a lead frame. The wire

16

is disposed between the pad

11

formed on the upper major surface of the semiconductor element

10

and the inner lead

13

of the lead

22

, using the wire bonding method described above.

The sealing resin

21

is constructed to seal therein the semiconductor element

10

and the wire

16

so as to protect the semiconductor element

10

and the wire

16

. Since it is necessary to provide the sealing resin

21

so as to cover at least the wire

16

, the sealing resin

21

becomes thick if the loop formed by the wire

16

is high, resulting in a relatively large size of the semiconductor device.

However, the loop formed by the wire

16

produced according to the wire bonding method of the first embodiment is low so that the thickness of the resin package formed of the sealing resin

21

is small. Accordingly, the semiconductor device

20

is satisfactorily small in height.

FIG. 17

shows a variation of the wire bonding method of the present invention.

FIG. 17

shows an example in which the wire bonding method of the present invention is applied to a wire connection structure in which the first wire

16

is provided above a second wire

24

.

Recently, as semiconductor elements come to have increasingly higher density, the number of pads provided on the semiconductor elements becomes large. For example, the construction as shown in FIG.

17

, in which two rows of pads

11

and

25

are provided on the semiconductor element

10

, are available (in a top view, the pads

11

and

25

are usually provided in two straight lines or arrayed in a zig zag manner).

In such a construction, it is necessary to have the wire

16

extending from the pad

11

on the semiconductor element

10

and the wire

24

extending from the pad

25

on the semiconductor element

10

. Hence, the wire connection structure in which the first wire

16

and the second wire

24

are provided with a vertical separation is required.

In the example shown in

FIG. 17

, the semiconductor element

10

is mounted on a die bonding layer

28

provided in a substrate

27

. The substrate

27

is a stacked substrate comprising a first wiring layer

29

, a first insulating layer

30

, a second wiring layer

31

and a second insulating layer

32

are built, in the stated order from the top, on a base member

33

. The pad

11

is electrically connected to the second wiring layer

30

by the first wire

16

. The pad

25

is electrically connected to the first wiring layer

29

by the second wire

24

.

In the above-described construction, the first wire

16

is provided using the wire bonding method of the present invention, and the second wire

24

is provided using the conventional forward bonding process. As has been described, the loop formed by the wire

16

provided using the wire bonding method of the present invention is lower than the loop formed by the wire

24

provided using the forward bonding process. For this reason, it is ensured that, even if the wire

24

is provided above the wire

16

, the wires

16

and

24

are prevented from interfering with each other and causing a short circuit.

In a similar conventional construction, the overall height of the loops formed by the wires is significantly high because a second wire must be provided at a certain distance above a first wire provided in the forward bonding process and already forming a relatively high loop. By employing the wire bonding method according to the present invention to provide the first wire

16

, the overall height of the loops formed by the wires

16

and

24

can be limited.

FIG. 18

shows an example where the wire bonding method of the present invention is applied to a multi-chip module (MCM). In the multi-chip module shown, a plurality of semiconductor elements including semiconductor elements

10

and

35

are provided on a substrate

37

.

As shown in

FIG. 18

, the wire bonding method of the present invention can be applied not only to connecting the semiconductor element

10

to the inner lead

13

but to connecting the semiconductor element

10

to the semiconductor element

35

directly in a multi-chip module configuration. Other conceivable applications of the wire bonding method of the present invention include connecting a plurality of leads to each other using a wire. Thus, the wire bonding method of the present invention can be applied to various configurations in which a wire is used to connect members.

The backward bonding process described with reference to

FIG. 2

is useful to keep the height of the loop formed by the wire

5

low, if the pads

2

a

to which the wire is stitch-bonded are not placed with a high density. However, as shown in

FIGS. 7 and 8

, the conventional method of forming the ball bump

6

has a disadvantage in that the upper major surface

6

a

of the ball bump

6

is rugged, or the bump

6

b

is produced thereon. This disadvantage hinders the backward bonding process from being practiced. If the upper major surface

6

a

can be finished flat when the ball bump

6

is formed, the backward bonding process can be advantageously employed.

A description will now be given, with reference to

FIGS. 19A-24

, of a method of forming a ball bump in which the upper major surface

6

a

of the ball bump

6

can be made flat. In

FIGS. 19A-24

, those components that are the same as the components shown in

FIGS. 9-14

are designated by the same reference numerals and the description thereof is omitted.

FIGS. 14A and 14B

show a capillary

40

used in the method of forming a ball bump according to a third embodiment of the present invention.

FIG. 11A

is a front view of the entirety of the capillary

40

, and

FIG. 14B

is an enlarged lateral sectional view of a lower end of the capillary

40

.

The feature of the capillary

40

is that a projection is provided at the lower end of a capillary main body

41

having a cylindrical shape. Specifically, a bonding part

42

is formed to project from a center of the end of the capillary main body

41

. A flat surface that lies adjacent to the bonding part

42

constitutes a pressurizing part

43

. An insertion hole

44

through which the wire

16

is inserted is formed at the center of the capillary

40

.

The bonding part

42

is used to bond a ball-shaped part

50

to the pad

11

as described below. The pressurizing part

43

is used to shape a ball bump

51

formed in the pad

11

as described below.

Referring to

FIGS. 19A and 19B

, the dimensions of the capillary

40

is such that, for example, L

1

=9.525 [mm], L

2

=1.588 [mm], L

3

=0.15-0.25 [mm], L

4

=0.2-0.4 [mm], and L

5

=0.2-0.3 [mm]. The shape of the capillary

40

is not limited to the one shown in

FIGS. 19A and 19B

.

The capillary

40

having the above-described construction is a simple variation of the conventional capillary

7

(see

FIGS. 3A-4C

) in that only the end of the capillary is modified. The bonding part

42

and the pressurizing part

43

can be easily formed by machining the capillary

40

(for example, by a cutting process). Therefore, providing the bonding part

42

and the pressurizing part

43

does not invite a rise in the cost for producing the capillary

40

.

A description will now be given, with reference to

FIGS. 20-24

, of the method of forming the ball bump using the capillary

40

. Although the illustrated shape of the capillary

40

shown in

FIGS. 20-24

is different from that of

FIGS. 19A and 19B

, the illustrations designate the same entity throughout.

When the wire

16

is inserted through the capillary

40

, the ball bump

51

is formed on the pad

11

such that the ball-shaped part

50

is formed in that part of the wire

16

that projects downward from the end of the bonding part

42

. Specifically, the ball-shaped part

50

is formed by spark discharge using the spark electrode (not shown in

FIG. 20

) in a process similar to the one shown in FIG.

9

.

When the ball-shaped part

50

is formed at the end of the wire

16

that projects from the bonding part

42

, the capillary

40

is lowered by a translating unit to approach the pad

11

. As shown in

FIG. 21

, the bonding part

42

formed in the capillary

40

presses the ball-shaped part

50

against the pad

11

.

Subsequently, ultrasonic oscillation is generated by an ultrasonic oscillator so that the bonding part

42

formed in the capillary

40

acts to weld the ball-shaped part

50

on the pad

11

(connected member as described in the claims). As a result, the ball-shaped part

50

is ball-bonded to the pad

11

. This part of the wire bonding process will be referred to as a bonding process.

When ball-shaped part

50

is bonded to the pad

11

, the capillary

40

is raised. Since a clamper (not shown) prohibits the wire

16

from being fed, the wire

16

is cut at a position above the ball-shaped part

50

. As shown in

FIG. 22

, the ball bump

51

is formed. This part of the wire bonding process will be referred to as a ball bump forming process.

Immediately after the ball bump forming process, an upper major surface

51

a

of the ball bump

51

is rugged and bumpy. Therefore, as has already been described, the backward bonding process cannot be carried out properly using the ball bump

51

at this stage.

According to the third embodiment, the ball bump

51

is shaped after the ball bump forming process.

In the shaping process, the capillary

40

is translated horizontally as shown in

FIG. 23

to a position at which the pressurizing part

43

formed in the capillary

40

is directly opposite the ball bump

51

.

The capillary

40

is used to provide the wire

16

between the pad

11

and the inner lead

13

. For this reason, the capillary

40

is configured to be movable both vertically and horizontally. Therefore, the above-described process of translating the capillary

40

does not require any additional construction of the wire bonding unit.

While the pressurizing part

43

is directly opposite the ball bump

51

, the capillary

40

is lowered. As shown in

FIG. 23

, the pressurizing part

43

presses the ball bump

51

.

Since the pressurizing part

43

has a flat surface, the rugged and bumpy upper major surface

51

a

of the ball bump

51

is turned into a flat surface by the pressure provided by the pressurizing part

43

, when the ball bump forming process is completed. Subsequent to this process, the capillary

40

is raised again so that the pressurizing part

43

is separated from the ball bump

51

. As a result of this, the ball bump

51

having its upper surface

51

a

turned flat is formed, as shown in FIG.

24

. This part of the wire bonding process is referred to as a shaping process.

As has been described, according to the ball bump forming process of the third embodiment, the upper surface

51

a

of the ball bump

51

can be turned flat by pressing and shaping the upper surface

51

a

of the ball bump

51

using the pressurizing part

43

of the capillary

40

. It is to be appreciated that, in the subsequent backward bonding process, the ball bump

51

and the wire

16

can be bonded more properly and securely than with the conventional technology.

By using the capillary

40

as shown in

FIGS. 19A and 19B

having the bonding part

42

and the pressurizing part

43

formed at its end, the bonding process, the ball bump forming process and the shaping process can be performed consecutively. Accordingly, the process of forming the ball bump

51

can be effected efficiently.

A description will now be given, with reference to

FIGS. 25-29

, of a method of performing the backward bonding between the ball bump

51

formed on the pad

11

and the inner lead

13

. In

FIGS. 25-29

, those components that are the same as the components of

FIGS. 9-14

are designated by the same reference numerals and the description thereof is omitted.

In order to connect the ball bump

51

(pad

11

) and the inner lead

13

using the wire

16

, the ball-shaped part

17

is formed in the part of the wire

16

which projects from the bonding part

42

of the capillary

40

, as shown in FIG.

25

. The ball-shaped part

17

is formed by inducing spark discharge in the spark electrode

15

.

When the ball-shaped part

15

is formed at the end of the wire

16

, the capillary

40

is translated by the translation unit to approach the inner lead

13

. As shown in

FIG. 26

, the capillary

40

presses the ball-shaped part

17

against the inner lead

13

. Subsequently, an ultrasonic oscillator generates ultrasonic oscillation so that the bonding part

42

causes the ball-shaped part

17

welded on the inner lead

13

(first bonding using the ball bonding process).

When the wire

16

(ball-shaped part

17

) is bonded to the inner lead

13

, the capillary

40

is translated vertically upward to a level which is slightly higher than the upper major surface of the semiconductor element

10

, and then translated horizontally so that the capillary

40

is above the ball bump

51

. As a result of this translation, the wire

16

forms a L-shaped loop including a vertical portion which projects vertically upward from the inner lead and a horizontal portion which extends horizontally, as shown is FIG.

27

.

When the wire

16

is guided by drawing to a position above the ball bump

51

, the capillary

40

is lowered so that the bonding part

42

causes the wire

16

to be pressed against the ball bump

51

, and the ultrasonic oscillator generates ultrasonic oscillation. Thus, as shown in

FIG. 28

, the bonding part

42

causes the wire

16

to be bonded to the ball bump

51

in a ultrasonic welding process (second bonding using the stitch bonding process).

Since the upper major surface

51

a

of the ball bump

51

is flat, it is ensured that the wire

16

is properly and securely bonded to the ball bump

51

. The wire bonding process according to the above-described configuration offers an improved degree of reliability as compared to the conventional technology.

When the wire

16

is bonded to the ball bump

51

(pad

11

), the capillary

40

is raised, resulting in a state shown in

FIG. 29. A

clamper (not shown) prevents the wire

16

from being fed while the capillary is being raised so that the wire

16

is cut at the bonding position during the upward translation of the capillary

40

. This completes one sequence of the backward bonding process for the wire

16

.

Thereafter, the ball-shaped part

17

is formed again as shown in FIG.

25

. Then, the above-described processes shown in

FIGS. 25-29

are repeated. In this way, the wire bonding process for providing the wire

16

between each pair of the ball bump

51

and the inner lead

13

is carried out on a continuous basis.

It will be appreciated that, in the above-described process, the capillary

40

used in the ball bump forming process is also used in the backward bonding process so that it is possible to form the ball bump

51

and the wire

16

in a successive manner, resulting in an improvement in the efficiency with which the entire wire bonding process is carried out. Since the wire bonding unit is used both for the purpose of forming ball bumps and effecting the backward bonding, the entire wire bonding system can have a simple configuration.

The present invention is not limited to the above described embodiments, and variations and modifications may be made without departing from the scope of the present invention.

Number	Date	Country	Kind
7-083726	Apr 1995	JP
8-72309	Mar 1996	JP

Number	Name	Date	Kind
4142288	Flammer et al.	Mar 1979	A
4732313	Kobayashi et al.	Mar 1988	A
4875618	Hasegawa et al.	Oct 1989	A
5151559	Conru et al.	Sep 1992	A
5295619	Takahashi et al.	Mar 1994	A
5328079	Mathew et al.	Jul 1994	A
5431329	Hasegawa et al.	Jul 1995	A
5476211	Khandros	Dec 1995	A
5874354	Heitzer et al.	Feb 1999	A
5976964	Ball	Nov 1999	A
6040984	Hirakawa	Mar 2000	A

Number	Date	Country
60-245237	Dec 1985	JP
61-280626	Dec 1986	JP
2-94533	Apr 1990	JP
5-6893	Jan 1993	JP
6-140454	Apr 1994	JP

Wire bonded device with ball-shaped bonds

Information

Patent Number

Date Filed

Date Issued

Inventors

Original Assignees

Examiners

Agents

CPC

US Classifications

Field of Search

US

International Classifications

Abstract

Description

Claims

Priority Claims (2)

Parent Case Info

US Referenced Citations (11)

Foreign Referenced Citations (5)