JP2003303845A - Semiconductor device and wire bonding method - Google Patents

Abstract

[57] The present invention aims to improve the power cycle resistance by reducing the thermal stress generated at the bonding interface of the wire in response to the demand for thickening the bonding wire accompanying the increase in capacity of the power semiconductor device. A connection electrode formed on a semiconductor chip 2 using a bonding wire 4 and an external electrode 3 connected to the electrode
In the semiconductor device having an assembly structure in which the two are connected to each other, the loop portion 4-3 of the bonding wire 4 connected across the electrodes is caused by the difference in thermal expansion accompanying the power cycle and the heat cycle. As a means for reducing the stress generated in the wire, a thin portion 4a having a flat cross-sectional shape along the loop portion is partially or continuously formed to enhance the flexibility of the wire loop, whereby the wire joint portion 4-1, The stress acting on the 4-2 joint interface is reduced to prevent the occurrence of wire peeling and to improve the power cycle resistance.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor device for implementing an IGBT module or the like in which a connecting electrode on a semiconductor chip and another electrode, for example, an external lead terminal are interconnected by a bonding wire, and the wire thereof. Regarding the bonding method.

[0002]

2. Description of the Related Art In the above-mentioned semiconductor device, ultrasonic wire bonding is generally adopted as a method of electrically connecting a connection electrode on an IGBT semiconductor chip and an electrode of an external lead terminal. By the way, in recent years, the operating current of the power semiconductor device has tended to increase as the capacity of the semiconductor device increases, and accordingly, the bonding wire (aluminum wire) is made thicker than that of the wire currently used. It is necessary to use a wire with a large wire diameter.

FIG. 12 shows a conventional semiconductor device, and FIG. 13 shows the structure of a conventional ultrasonic wire bonder applied to wire bonding of the semiconductor device, taking the above-mentioned IGBT module as an example. First, in FIG.
1 is an insulating substrate, 2 is a semiconductor chip (IGBT) solder-mounted on the substrate 1, 3 is an electrode of an external lead terminal (hereinafter, referred to as an external electrode), 4 is a connection electrode of the semiconductor chip 2 and the external electrode 3. Bonding wire (aluminum round wire with a circular cross section) connected between these bonding wires 4
Is bonded using the ultrasonic wire bonder described below. In the figure, 4-1 is a wire bonding portion at a first bonding point bonded to the electrode surface of the semiconductor chip 2, 4-2 is a wire bonding portion at a second bonding point bonded to the external electrode 3, 4
Reference numeral -3 indicates a wire loop portion formed between the wire joining portions 4-1 and 4-2, and reference numeral 5 indicates a soldering portion.

In the ultrasonic wire bonder shown in FIG. 13, 6 is a support arm of the bonder, 7 is an ultrasonic horn,
Reference numeral 8 is a wire cutter, 9 is a wire clamp, 10 is a wire holder, 11 is a bonding tool (wedge tool), 12 is a bonding tool holder, and 13 is a table on which a semiconductor device assembly is mounted. In the ultrasonic wire bonder having the above structure, the wire 4 is supplied from the wire supply unit (not shown) to the bonding tool 11 through the wire holder 10. Here, the bonding tool 1
1 is moved onto the semiconductor chip 2 which is the first bonding point (or the table 13 is moved to the bonding position while the arm 6 is fixed), the wire clamp 9 and the bonding tool 11 are operated, and the tip of the wire 4 is moved. Is pressed against the electrode surface of the semiconductor chip 2 to apply pressure, and in this state, ultrasonic vibration generated from an ultrasonic vibration unit (not shown) is applied to the bonding tool 11 via the ultrasonic horn 7. As a result, the bonding portion 4-1 of the wire 4 is bonded to the connection electrode surface of the semiconductor chip 2. Then, the wire 4 is fed so as to form the arc-shaped loop portion 4-3, and the bonding tool 11 is moved to the electrode 3 of the external terminal, which is the second bonding point, where the same bonding operation as described above is performed. The bonding portion 4-2 of the wire 4 is bonded to the external electrode 3, and finally the wire 4 is cut. As a result, the semiconductor chip 2 and the external electrode 3 are interconnected by the bonding wire 4.

[0005]

By the way, in the above-mentioned power semiconductor device applied to the operation control of the motor and the like, since the drive current is repeatedly turned on and off in accordance with the motor control, a severe power cycle and heat cycle are added. It will be. In addition, there is a difference in linear expansion coefficient between the semiconductor chip 2, the external electrode 3, and the wire 4, and a difference in linear expansion coefficient at the bonding interface between the semiconductor chip 2 and the wire 4 due to repeated rapid temperature changes during operation. The resulting thermal stress is generated, and particularly when the wire diameter of the wire 4 is increased, the rigidity of the wire itself is also increased, so that the thermal stress at the bonding interface is remarkably exhibited. For this reason,
Damage such as peeling of the wire at the joint surface between the semiconductor chip 2 and the wire 4 and cracking of the semiconductor chip is likely to occur, which results in a reduction in the resistance to power cycles and heat cycles and a decrease in the reliability of the semiconductor device. There is a problem.

On the other hand, as a countermeasure for the above-mentioned problem, conventionally, JP-A-9-27522 or JP-A-11-3 is used.
As disclosed in Japanese Patent No. 30134, when a wire is bonded to an electrode surface by using an ultrasonic wedge bonding tool, the wire bonding portion is preformed into a flat shape and then pressed against an electrode of a semiconductor chip. By performing ultrasonic bonding with a wire, a wire bonding method that increases the bonding area between the wire and the electrode to increase the heat cycle and power cycle resistance without applying an excessive pressing load to the semiconductor chip is available. Are known.

However, according to the knowledge obtained by the inventors through a wire bonding test, etc., when a thick aluminum wire having a wire diameter of φ400 μm and φ500 μm is used as the bonding wire, the wire is formed by the above preforming method. It has been confirmed that there is a limit to the improvement in the power cycle withstanding ability even if the joining area between the electrode and the electrode is increased, and a trouble occurs such that the wire joining portion is easily peeled off.

That is, the bonding wire forms an arc loop between the first bonding point and the second bonding point, and the thermal stress generated due to the difference in thermal expansion between the wire and the bonding partner electrode is applied to the loop portion. Although the flexibility is used to absorb it, the rigidity of the loop portion increases as the wire diameter increases, so the thermal stress cannot be completely absorbed by the flexibility of the loop itself, and the stress is applied to the joints at both ends. Cause wire peeling.

The present invention has been made in view of the above points, and an object thereof is to meet the demand for thicker bonding wires with the increase in capacity of a semiconductor device, power cycle resistance,
An object of the present invention is to provide a semiconductor device improved so as to further improve reliability and a wire bonding method thereof.

[0010]

In order to achieve the above object, according to the present invention, a connecting electrode formed on a semiconductor chip by using a bonding wire and other parts such as an external lead terminal connected to the electrode. In a semiconductor device having an assembled structure in which electrodes are interconnected, the loop portion of the bonding wire connected across the electrodes, that is, extending in an arc shape between a first bonding point and a second bonding point. As a means for reducing the stress generated in the wire, a thin portion having a flat cross-section along the loop portion is formed in the wire loop portion (claim 1), and a specific embodiment thereof is a thin portion of the wire. Is formed at one or a plurality of locations along the loop portion (claim 2), or the thin-walled portion of the wire is continuously formed over substantially the entire length of the wire loop (claim 3). Can.

As described above, by forming a thin portion having a flat cross section along the loop direction in the loop portion of the wire whose both ends are joined to the first bonding point and the second bonding point, the wire is formed in the thin portion. The cross-section coefficient is smaller, and the flexibility of the loop portion is increased as compared with the case where the loop portion is formed by a round wire having a circular cross-sectional shape. Therefore, by connecting the electrodes of the semiconductor device to each other by using this bonding wire, even if a wire having a large wire diameter is used as the bonding wire, the thermal stress generated in the wire due to the severe power cycle and heat cycle is prevented. It can be absorbed by the loop itself. As a result, an excessive stress is not applied to the bonding interface of the wire, a high power cycle resistance is secured, and the reliability of the semiconductor device can be improved.

According to the wire bonding method of the present invention applied to the semiconductor device, the wire supplied to the bonding tool is bonded to the electrode surface of the connection partner at the first bonding point and the second bonding point. Here, with respect to the wire supplied to the bonding tool, a thin-walled portion having a flat cross-sectional shape along the loop direction at a portion corresponding to a loop portion between the bonding portion of the first bonding and the bonding portion of the second bonding. Is formed and bonding is performed (claim 4). Specifically, a round wire having a circular cross section is pressed by applying a pressing force on the wire feeding path to the bonding tool, and the wire has a flat cross section. Then, it is plastically deformed to form a thin portion (claim 5).

As an embodiment of this case, the thin-walled portion of the wire is locally formed by dividing it into one or a plurality of locations on the wire loop (claim 6), or is continuously formed over substantially the entire length of the wire loop. Can be formed (claim 7). Further, when the thin portion is formed, a pressing force is applied while heating the wire to form the thin portion (claim 8).
As a result, it is possible to easily perform the plastic deformation processing of the thin portion while the wire is softened without causing work hardening and cracking.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to FIG.
~ It demonstrates based on the Example shown in FIG. In the drawings of the embodiments, members corresponding to those in FIGS. 12 and 13 are designated by the same reference numerals, and detailed description thereof will be omitted. First, FIGS. 1 to 4 show an embodiment of a semiconductor device according to claims 1 to 3 of the present invention. The assembly structure of the semiconductor device shown in FIGS. 1 and 2 is basically the same as that of FIG. 12, except that the bonding wire 4 connected between the semiconductor chip 2 and the external electrode 3 is not connected to the loop portion 4-3. As described below, a thin portion 4a having a flat cross-sectional shape along the loop direction is formed. here,
In the embodiment of FIG. 1, the thin portion 4a is formed at two locations along the shape of the loop portion 4-3, whereas in the embodiment of FIG. 2, the thin portion 4a is substantially the loop portion 4-3. It is formed continuously over the entire area. The cross-sectional shape of the original round wire of the bonding wire 4 is shown in FIG. 1 (b), and the cross-sectional shape of the thin portion 4a is shown in FIG. 1 (c) and FIG. 2 (b). It is preferable that the cross-sectional change from the portion to the thin portion is not discontinuous but transitions gently.

3 and 4 show the bonding wire 4
Is an embodiment of an example in which stitch bonding is performed along a wiring path between the semiconductor chip 2 and the external electrode 3. In the illustrated example, each loop portion of the wire 4 stitch-bonded is shown over substantially the entire length thereof. A thin portion 4a similar to that of the second embodiment is formed. By forming the thin-walled portion 4a having a flat cross section in the loop portion 4-3 of the bonding wire 4 as described above, the cross-section coefficient of the thin-walled portion 4a against bending of the wire in the loop direction is reduced and the flexibility of the wire is reduced. As a result, due to the difference in thermal expansion coefficient between the semiconductor chip 2, the external electrode 3, and the bonding wire 4, the thermal stress generated in the bonding wire 4 due to the power cycle and the heat cycle is absorbed by the flexibility of the thin portion. ,
The stress applied to the bonding interface of the wire is reduced.

In this case, it is preferable that the thickness of the thin portion 4a is not more than half of the original wire diameter (circular cross section), and the wire diameter of the wire (aluminum wire) 4 in the configuration of FIG. φ
When the wire length between the first bonding point and the second bonding point is 400 μm, and the wire length is 40 mm, the thin portion 4a having a thickness of 100 μm is formed by dividing the loop portion 4-3 into two parts. Due to the difference between the coefficient of linear expansion of the tip 2 and the external electrode 3 and the wire 4, the bonding portion 4 of the wire 4
-1 reduces the maximum stress generated at the bonding interface between the semiconductor chip 2 and the semiconductor chip 2 by about 20% as compared with a wire that is a round wire without forming the thin portion 4a. Further, by continuously forming the thin portion 4a on the loop portion 4-3 of the wire 4 as shown in FIG. 2 to FIG. 4, the thin portion 4a is generated at the wire bonding interface under the same wire diameter and length conditions as described above. It is possible to reduce the maximum stress to about 50%. In addition, in the embodiment of FIG. 1, the thin portion 4a is formed in the loop portion 4-3 of the wire 4 at two locations, but the same effect can be obtained by setting it at one location or at three or more locations. Is obtained.

Next, FIG. 5 shows an analysis result obtained by simulating the stress applied to the bonding wire by the inventors. That is, on the basis of the shape of the wire shown in FIG. 5 (a), the wires having the structures shown in (b) to (d) ((b), (c), (d) are shown in FIG. 12 and FIG. 1, corresponding to the bonding wire shown in FIG. 2) is used as a test model, and the stress generated in each part (IV) set on the wire is analyzed under a simulated condition in which a power cycle is applied to the semiconductor device. The comparison results are summarized in the table in Fig. 5 (e). The numerical values in the table of FIG. 5 (e) are comparative values with the stress generated in the part I of the test model shown in FIG. 5 (b) as 100.

As can be seen from the results of this analysis, the bonding wire 4 was left as a round wire in the test model shown in FIG.
Assuming that the stress generated at the bonding interface (site I) between the wire and the connection electrode is 100, the stresses generated at the bonding interface (site I) are 75 and 45, respectively, in the sample models of (c) and (d). Reduce. As a result, in the above-described semiconductor device shown in FIGS. 1 to 4, it is possible to prevent the occurrence of peeling of the wire bonding portion and ensure a high power cycle resistance.

Next, as described above, the bonding wire 4
The wire bonding method of the present invention, which is performed by forming the thin portion 4a on the inner surface of the thin film portion 4a, will be described based on the embodiment of the ultrasonic bonder described below. First, FIG. 6 shows the entire structure of an ultrasonic bonder used in the wire bonding method of the present invention. This bonder is basically the same as the conventional structure shown in FIG.
A thin-walled portion molding unit 14 is additionally provided on the way of the supply path of the wire 4, and a bonding portion of the first bonding and a bonding portion of the second bonding are connected to the wire supplied to the tool using the molding unit 14 as described later. A thin portion having a flat cross-sectional shape is pressed and formed at the wire portion corresponding to the loop portion between the and.

Next, the structure of a specific embodiment of the thin portion molding unit 14 is shown in FIGS. Here, the thin portion forming unit of FIG. 7 is applied when the thin portion 4a is dispersedly formed in the loop portion 4-3 of the wire 4 as shown in FIG. 1, and the thin portion forming unit of FIG. The unit has a thin-walled part 4a on the loop part of the wire 4 as shown in FIG.
It is applied when forming continuously.

That is, the thin portion molding unit 14 of FIG.
Then, a movable upper molding die 14a and a fixed lower molding die 14b are arranged above and below the wire supply passage so that the upper molding die 14a has an upper surface as an inclined surface and is pressed against the wedge shape. The cam 14c is moved forward and backward by the drive motor 14d. When the cam 14c is moved forward while the wire is passed between the upper molding die 14a and the lower molding die 14b, the upper molding die 14a descends and presses the wire against the lower molding die 14b. The thin portion 4a is formed by plastically deforming to a flat cross section.

Here, the processing position of the thin portion 4a formed on the wire 4 extends between the joining portion 4-1 of the first bonding and the joining portion 4-2 of the second bonding as shown in FIG. When the wire 4 fed out from the wire supply unit and supplied to the bonding tool 11 passes through the thin portion molding unit 14, the timing is set in the area corresponding to the loop portion 4-3 Then, the thin portion forming unit 14 is driven to form the thin portion 4a.

On the other hand, in the embodiment of the thin portion molding unit shown in FIG. 8, the molding upper roller 14e is attached to the roller holder 14a-1 corresponding to the molding upper mold 14a in FIG. The lower molding roller 14f faces each other across the wire supply passage, and the roller 14f is connected to the roller driving motor 14g via the gear mechanism as a driving side roller. With such a configuration, when the wire is sandwiched between the upper molding roller 14e and the lower molding roller 14f, the upper molding roller 14e is lowered and pressed against the wire by the operation of the drive motor 14d, as described in FIG. The wire is flattened to form the thin portion 4a. When the lower molding roller 14f is rotated by operating the roller driving motor 14g in this state, the wire 4 is fed forward, and at the same time, the thin portion 4a is continuously formed along the wire. Thereby, as shown in FIG. 2, or FIG. 3 and FIG. 4, the thin portion 4a can be continuously formed in the substantially entire length region of the loop portion 4-3 of the bonding wire 4. In addition, by intermittently performing the pressurizing operation of the upper molding roller 14f with respect to the wire feeding operation of the lower molding roller 14f, as shown in FIG. It is possible to disperse and form the thin portion 4a.

Next, an ultrasonic wire bonder applied to the wire bonding method according to the eighth aspect of the present invention is shown in FIG.
Further, an embodiment of the thin portion molding unit mounted on the bonder will be described with reference to FIGS. The thin-walled molding unit of this embodiment is additionally equipped with a preheating unit and a heater for heating the bonding wire in addition to the molding unit of the molding unit described with reference to FIGS. 7 and 8, and the bonding wire passing through this molding unit. Heat is applied to soften and the thin portion is pressure-molded in this state.

That is, in FIG. 9, a heater is incorporated into the thin-walled portion forming unit 14 arranged on the supply path of the wire 4 as described later, and a preheating unit 15 having an electric heater built therein is provided in front of the unit 14. Equipped. Further, in the embodiment of the thin-walled molding unit shown in FIG. 10, the upper molding die 14a of the molding unit shown in FIG.
The lower molding die 14b is equipped with a molding plate 14h that also serves as a heat transfer plate, an electric heater 14i, a heat insulating material 14j that surrounds the heater, and a temperature sensor 16.
To heat the molding plate 14h. Further, the temperature of the heater 14h is measured by the temperature sensor 16 and fed back to the heating power supply unit to control so as to maintain a predetermined heat generation temperature.

Then, when the wire 4 which has been heated to the preheated state is fed into the thin portion molding unit 14 via the preheating unit 15 described above, the upper molding die is driven by a motor as in the molding unit of FIG. 14a descends to pinch the wire 4 between the forming plates 14h and the heater 14i.
The thin portion 4a is press-molded with the wire 4 softened by the heating. When the wire 4 is an aluminum wire, the thin-walled portion 4a can be easily press-molded without causing work hardening and cracking of the wire 4 by setting the heating temperature of the wire to about 250 ° C to 400 ° C. .

Further, in the thin portion molding unit of the embodiment shown in FIG. 11, the electric heater 14i and the heat insulating material 1 are added to the construction of FIG.
4j, a temperature sensor 16, and a heat transfer plate 14k interposed between the electric heater 14i, the upper molding roller 14e, and the lower molding roller 14f. Reference numeral 14m is a transmission gear that rotationally drives the upper molding roller 14e in synchronization with the lower molding roller 14f.

With this structure, when the heater 14i is energized from a heating power source unit (not shown), the heat transfer plate 1
4k is transferred to heat the upper molding roller 14e and the lower molding roller 14f to a high temperature. The temperature control of the heater 14i in this case is performed in the same manner as the unit of FIG. Then, the preheating unit 1 shown in FIG.
When the wire 4 is fed into the thin-walled portion forming unit 14 in a preheated state via 5, the roller holder 14a-1 of the forming upper roller 14e is lowered by the motor drive, and the forming upper roller 14e and the forming lower roller 14e are lowered. Roller 14f
The wire 4 is sandwiched between them. When the molding lower roller 14f is rotated by driving the roller driving motor 14g in this state, the molding upper roller 14e rotates in synchronization with this, and the wire passing therethrough is heated to the softening temperature and thinned along the wire. The part 4a is continuously formed. In addition,
When a thin portion is formed at a predetermined portion corresponding to the loop portion of the wire 4, the drive cam 14c that has pressed the upper molding roller 14e downwardly moves backward, and a return spring (not shown) causes a holder 14a-1 for the upper molding roller. Moves to the raised position, and a series of press forming steps for the wire 4 is completed.

[0029]

As described above, according to the present invention, in a semiconductor device assembled by connecting a connecting electrode on a semiconductor chip and an electrode of another component with a bonding wire,
By forming a thin portion with a flat cross-sectional shape in the loop portion of the bonding wire, the flexibility of the loop portion is increased,
Therefore, even when a wire having a large diameter is used as the bonding wire, the loop itself can effectively absorb the thermal stress generated in the wire due to the power cycle and the heat cycle applied to the semiconductor device. As a result, it is possible to prevent peeling of the wire bonded on the semiconductor chip and provide a highly reliable semiconductor device capable of withstanding a severe power cycle.

Further, according to the wire bonding method of the present invention, the thin portion can be intermittently or continuously formed in the loop portion of the wire on the way of supplying the wire to the bonding tool. By heating the wire at the time of forming the portion, it is possible to easily perform the plastic deformation processing of the thin portion while the wire is softened without causing work hardening and cracking.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an embodiment of a semiconductor device according to the present invention, (a) is a side view of an assembled state after wire bonding, and (b) and (c) are views in the direction of arrow (a). -B, c-
Wire cross section of c

2A and 2B are configuration diagrams of another embodiment of the semiconductor device according to the present invention, in which FIG. 2A is a side view of an assembled state in which wire bonding is performed, and FIG. 2B is a wire taken along line bb in FIG. Cross section

FIG. 3 is a configuration diagram of an embodiment of a semiconductor device according to the present invention in which wires are stitch bonded.

FIG. 4 is a configuration diagram of another embodiment of a semiconductor device according to the present invention in which wires are stitch-bonded.

FIG. 5 is an explanatory diagram showing an analysis result of stress generated in a bonding wire, (a) is a wire shape diagram of a test model,
(b) to (d) are wire structure diagrams of each test model, (e) is (b)
~ (D) Table of analysis results showing the stress generated in each part of the wire as a comparative value for each test model

FIG. 6 is a configuration diagram of a main part of a first embodiment of an ultrasonic wire bonder applied to the wire bonding method of the present invention.

7 is a sectional view of the configuration of an embodiment of the thin portion molding unit in FIG.

FIG. 8 is a sectional view showing the configuration of another embodiment of the thin-walled portion forming unit in FIG.

FIG. 9 is a configuration diagram of essential parts of a second embodiment of an ultrasonic wire bonder applied to the wire bonding method of the present invention.

FIG. 10 is a sectional view showing the configuration of an embodiment of the thin portion molding unit in FIG.

FIG. 11 is a sectional view showing the configuration of another embodiment of the thin-walled portion molding unit in FIG.

FIG. 12 is an assembly structure diagram of a conventional semiconductor device to which wire bonding is applied.

13 is a configuration diagram of a main part of a conventional ultrasonic wire bonder applied to wire bonding of the semiconductor device of FIG.

[Explanation of symbols]

2 substrates 3 external electrodes 4 Bonding wire 4-1 and 4-2 wire joints 4-3 Wire loop part 4a Thin part 7 ultrasonic horn 8 wire cutter 9 wire clamp 10 wire holder 11 Bonding tool 14 Thin wall molding unit 14a Upper mold 14b Molding lower mold 14e Molding upper roller 14f Lower forming roller 14i heater 15 Preheating unit

Claims (8)

[Claims] 1. A semiconductor device assembled by connecting a connecting electrode on a semiconductor chip and an electrode of another component with a bonding wire, wherein a thin portion having a flat cross section is formed in a loop portion of the bonding wire. A semiconductor device characterized by the above. 2. The semiconductor device according to claim 1, wherein the thin portion of the wire is formed at one or a plurality of locations along the wire loop. 3. The semiconductor device according to claim 1, wherein the thin portion of the wire is formed continuously over substantially the entire length of the wire loop. 4. A wire bonding method applied to a semiconductor device according to claim 1, wherein a bonding tool is used and the wire supplied to the tool is bonded to the electrode surface at the first bonding point and the second bonding point. First of all for the wire supplied to the bonding tool
A wire bonding method, characterized in that a thin portion having a flat cross-sectional shape is formed at a portion corresponding to a loop portion between a bonding joint portion and a second bonding joint portion to perform bonding. 5. The wire bonding method according to claim 4, wherein a round wire having a circular cross section is plastically deformed into a flat cross section by press working to form a thin portion on the way of supplying the wire to the bonding tool. A wire bonding method characterized by: 6. The wire bonding method according to claim 4, wherein the thin-walled portion of the wire is locally formed at one or more locations on the wire loop. 7. The wire bonding method according to claim 4, wherein the thin portion of the wire is formed continuously over substantially the entire length of the wire loop. 8. The wire bonding method according to claim 4, wherein the wire is heated and pressed to form a thin portion.