JP2000058745A - Power semiconductor module - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the joining strengths between Al wires and Au wires by deciding the thicknesses of plated Au films which are the external electrodes to which the Al wires are joined within a specific range. SOLUTION: A power semiconductor element 9 is mounted on a metallic base 1 through a heat sink 4 and a resin substrate mounted with a semiconductor element 8 is stuck to the base 1 through an insulating layer 2. External electrodes 5 each of which is composed of a plated Ni film 13 and an Au film 12 are formed on the base 1 and substrate 3 and electrically connected to the pads of the semiconductor elements 9 and 8 through Au wires 6 and Al wires 7. The thicknesses of the plated Au films 12 are decided within the range of <=0.2 μm or >=0.9 μm. Therefore, the reliability of the moisture resistance of the joints between the Al wires 7 and Au wires 6 can be improved and a highly reliable IPM can be obtained, because the corrosion resistance of the intermetallic compound formed on the phase boundary is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a power semiconductor module such as an intelligent power module (IPM) in which a driver IC, a power semiconductor element and the like are modularized.

[0002]

2. Description of the Related Art Power electronics, including power devices, are being actively developed for the purpose of miniaturization, high performance, high functionality, high reliability, and improvement of user-friendliness. In recent years, IPMs (Intelligent Power Systems) with built-in drive circuits, various control and protection circuits, alarm functions, etc.
Module) has also been remarkably developed.

[0003] The structure of the IPM is roughly divided into a substrate on which a circuit portion for providing various functions as described above is formed, and a power semiconductor element for applying a current for rotating a motor is mounted. Substrates are mixed and modularized.

FIG. 5 is a schematic sectional view of a conventional IPM.
The power semiconductor element 9 fixed to the heat sink 4 is mounted on the metal base 1, and the resin substrate 3 to which the semiconductor element 8 covered by the resin mold 10 is connected is fixed on the metal base 1 via the insulating layer 2. The pads on the power semiconductor element and the external electrodes 5 formed on the resin substrate 3 are electrically connected by an Al wire 7 having a diameter of 300 μm. Also, the semiconductor element 8 in the mold
A formed on the upper pad and the lead frame 11
The u-plated external electrode is connected by an Au wire 6 having a diameter of 30 μm. In recent years, attempts have been made to mount an unmolded semiconductor element (hereinafter, referred to as a bare chip) directly on a resin substrate in order to reduce the size of the IPM. In this structure, an external electrode of an Au plating film to which the Au wire and the Al wire are connected is formed on the resin substrate. Generally, an external electrode is formed by forming a Ni plating layer on a wiring film of Cu or the like, and then forming an Au plating film on the outermost surface. The Ni plating film is applied to prevent diffusion of Cu to the Au surface during heating in the module manufacturing process. The electrical connection between the Au and Al wire pads and the external electrodes is made by a ball bonding method in which a ball is formed at one end of the wire by discharge in the case of an Au wire. This is performed by an ultrasonic bonding method. The Au plating film is used for the external electrode because the connection reliability of the Au wire is high and the solderability of the element is excellent.

[0005]

In a conventional IPM in which a bare chip is mounted on a resin substrate, external electrodes of an Au plating film are formed on the resin substrate as described above. It is desirable to make the Au plating film as thin as possible in consideration of cost. However, the thinner the Au plating film, the lower the reliability of bonding to the Au wire. Therefore, it is necessary to select a thickness as short as possible to ensure the reliability of bonding with the wire, and the thickness is about 0.5 to 0.8 μm.

On the other hand, an Au plating film as an external electrode to which an Al wire is connected is also formed on the resin substrate.
In the case of the l-wire, the bonding property is excellent even if the Au plating film is thin. Therefore, a thick Au plating film as in the case of the Au wire is not necessary.
It is formed to the same thickness as the electrode part in the case of a u wire. However, this thickness is selected in consideration of only the bonding property with the Au wire, and does not consider the moisture resistance reliability of the bonding portion between the Al wire and the Au plating film. That is, the conventional IPM has a problem that the moisture resistance reliability of the bonding interface between the Al wire and the Au plating film is extremely low, and the wire breaks after long-term use.

FIG. 6 shows that an A1 wire having a diameter of 300 μm is ultrasonically bonded to an Au plating film having a thickness of 0.5 μm and then exposed to high temperature and high humidity in order to evaluate the moisture resistance reliability of the wire bonding portion. 4 shows the peel strength of the wire joint portion over time. It can be seen that the peel strength decreases significantly in a short time.

FIG. 7 is a schematic cross-sectional view of the interface of the wire bonding portion after the moisture resistance reliability test is performed. Cracks are observed at the interface.

SUMMARY OF THE INVENTION The present invention has been made in consideration of the above problems, and has as its object to improve the bonding strength of both wires in a power semiconductor module in which Al wires and Au wires are mixed.

[0010]

The means for achieving the above object will be described below while showing the results of a study by the present inventors.

When an Al wire is connected to the Au-plated electrode and the bonding interface after being left in a high-temperature and high-humidity state for a long time is observed in detail, an intermetallic compound of Au and Al is formed. Although it progresses in the intermetallic compound, most of the progress has progressed at the interface between the intermetallic compound and Au or Ni. From these observations, it was found that corrosion was caused by intermetallic compounds and A
It is thought to be due to the potential difference between u and Ni,
Since the potential difference between Au and the intermetallic compound is considered to be smaller than that of Ni, the cause can be estimated to be based on the potential difference between the intermetallic compound and Ni.

It is generally said that at the junction between Au and Al, an intermetallic compound is easily formed even at a low temperature.
Therefore, an intermetallic compound is formed even in a high-temperature and high-humidity test at a heating temperature of 100 ° C. or less. The intermetallic compound is A
Since it is formed by the diffusion of u atoms and Al atoms, an intermetallic compound having different ratios of Au atoms and Al atoms depending on the heating temperature and heating time is formed. It is considered that these intermetallic compounds have different mechanical properties and chemical properties. Therefore, it is considered that the intermetallic compound formed at the junction between the conventional Au plating electrode and the Al wire has extremely low corrosion resistance.

As described above, the intermetallic compound has a different composition depending on the heating temperature and time.
The inventors considered that a compound having a different composition was formed depending on the thickness of the Au plating film. Therefore, in order to find an electrode of an Au plating film having excellent moisture resistance reliability,
After the same Al wire as that of the related art was ultrasonically bonded to an electrode having a different thickness of the Au plating film, a high temperature and high humidity test was performed.

FIG. 8 shows the peel strength after the high-temperature and high-humidity test as Au.
It is the result plotted against plating thickness. When the plating film has a thickness in the range of 0.2 μm or more to 0.9 μm or less,
The peel strength was remarkably reduced, and 0.2 μm or less and 0.9 μm
It does not decrease when the thickness is more than m.

According to the above study, the conventional 0.5 to 0.8 μm
In the case of the thickness of the Au plating film in the range of m, it became clear that the moisture resistance reliability of the joint with the Al wire was extremely low. Therefore, if the thickness of the Au plating film is set to 0.2 μm or less or 0.9 μm or more, the moisture resistance reliability is not impaired.

The reason why the corrosion resistance can be varied depending on the thickness of the plating film is that the type of the intermetallic compound formed at the joint interface varies depending on the thickness. That is, when the plating film thickness is extremely thin, not more than 0.2 μm, an intermetallic compound in which the ratio of Au atoms to Al atoms is extremely small is formed. On the other hand, when the thickness of the plating film is 0.9 μm or more, Au atoms are sufficiently supplied, so that a compound in which the ratio of Al atoms to Au atoms is in a stable state is considered to be formed. On the other hand, when the thickness is in the range of 0.2 μm or more to 0.9 μm or less, it can be estimated that an intermetallic compound having significantly lower corrosion resistance than the intermetallic compound in the case described above is formed. Therefore, it is necessary to select the thickness of the Au plating film of the metal base of the IPM and the external electrode on the resin substrate to be 0.2 μm or less or 0.9 μm or more.

Although the thickness of the Au plating film is preferably selected to be 0.2 μm or less in consideration of cost, it is difficult because the bonding property with the Au wire is low. Therefore, a method for ensuring the bondability even at a thickness of 0.2 μm or less was studied.

FIG. 9 shows an example in which an electrode having a different thickness of an Au plating film is formed on a resin substrate, and an Au wire having a diameter of 30 μm is formed on the electrode when the semiconductor element is mounted on the resin substrate or when only the electrode is used. 7 is a plot of the peel strength after bonding under the same bonding conditions with respect to the film thickness. Regardless of the presence or absence of the element, the peel strength decreases as the film thickness decreases. Further, the peel strength is lower when the element is mounted than when the element is not mounted, regardless of the film thickness.
The difference between the two was considered to be due to the process of bonding the semiconductor element to the resin, and the composition of the two electrodes was analyzed using an Auger electron spectrometer.

FIG. 10 compares the composition ratios of the elements at this time. Comparing the amount of Au atoms which has the greatest influence on the bonding property, it is significantly lower when the device is mounted than when no device is mounted. Furthermore, when the device is mounted, the ratio of C atoms is high, that is, the ratio of Au atoms is reduced due to contamination by C atoms and the like by the resin substrate and the adhesive in the device bonding process, and the bondability is considered to be reduced. Can be

From this study, it is considered that the bonding property can be improved by preventing the contamination in the element bonding process or by removing the contaminant. Therefore,
As a method of removing contaminants by physical means, ion cleaning by irradiating Ar ions was attempted.

FIG. 11 shows the results of comparing the bondability of Au wires in the case where the semiconductor element is adhered, the element is adhered, and ion cleaning is performed. In ion cleaning, a resin substrate having an external electrode is placed in a chamber in a high vacuum atmosphere, Ar is sealed, a high voltage is applied at a pressure of about 10 ⁻³ Torr to ionize Ar, and Ar ions are applied to the external electrode. Irradiation method. The peel strength in the case of performing the ion cleaning is remarkably improved at any Au film thickness as compared with the case where the element is still bonded.

FIG. 12 shows the result of analysis of the composition of both electrodes at this time. By performing ion cleaning, contaminant elements such as C are removed, and the ratio of Au atoms is increased. Contaminant elements can be removed by sputtering Ar atoms as in the case of ion cleaning.

On the other hand, as a method of improving bonding properties other than cleaning, a ball formed at the tip of an Au wire is bonded to a pad on a semiconductor element and a wire is bonded to an external electrode. There is a reverse bonding method of bonding a ball to an electrode. According to this method, a ball having a larger volume than a wire is joined to the external electrode, so that the ball can be deformed with a high load and a large ultrasonic output, so that the joining property can be improved.

FIG. 13 shows a semiconductor device mounted on a resin substrate.
The peel strength at each plating film thickness when an Au wire is bonded by a reverse bonding method on an electrode having a different thickness of the Au plating film is compared with a case where bonding is performed by a conventional method. The reverse bonding method shows higher peel strength at all plating film thicknesses than the conventional method, and a highly reliable peel strength can be obtained even at a film thickness of 0.2 μm or less.

On the other hand, in the conventional bonding method, it is also effective to select an electrode having a different plating film thickness or plating material depending on the wire to be bonded. That is, the thickness of the Au plating film of the external electrode to which the Au wire is joined is selected from the range of 0.4 to 0.8 μm as in the conventional case, and the thickness of 0.2 μm of the present invention is applied to the external electrode to which the Al wire is joined. The conventional problem can be solved by setting the Au plating thickness to be equal to or less than 0.9 μm. Further, the problem can be solved by using a Ni plating film instead of the Au plating film for the external electrode to be bonded to the Al wire.

[0026]

FIG. 1 shows an IPM according to an embodiment of the present invention.
1 is a schematic sectional view of FIG. The configuration is such that a power semiconductor element 9 is mounted on a metal base 1 via a heat sink 4, and a resin substrate 3 on which the semiconductor element 8 is mounted is an insulating layer 2.
Is adhered on the metal base 1 via the. External electrodes 5 formed of a Ni13 / Au12 plating film are formed on the metal base 1 and the resin substrate 3, and these external electrodes and the pads of the power semiconductor element 9 and the semiconductor element 8 are each made of Au having a diameter of 30 μm. Electrical connection is made with the wire 6 and the Al wire 7 having a diameter of 300 μm. The thickness of the Ni plating film 13 of the external electrode is 5 μm.
The thickness of the u plating film 12 is 0.2 μm. Since the surface of the Au plating film is cleaned by the ion cleaning method, the bonding strength with the Au wire is high even if the thickness is 0.2 μm, and the moisture resistance reliability of the bonding portion between the Al wire and the Au plating film is high. And a highly reliable IPM can be realized. The same effect can be obtained not only by the ion cleaning method but also by the sputtering method or the like for cleaning the surface of the Au plating film.

Further, in this embodiment, the thickness of the Au plating film of the external electrode is 0.2 μm. However, if the thickness is 0.8 μm or more, highly reliable bonding with the Au wire can be performed without cleaning treatment. A highly reliable IPM having no problem in the moisture resistance reliability of the Al wire joint can be realized.

FIG. 2 is a schematic sectional view of an IPM according to an embodiment of the present invention. In the configuration, a power semiconductor element 9 is mounted on a metal base 1 via a heat sink 4, and a resin substrate 3 on which the semiconductor element 8 is mounted is adhered on the metal base 1 via an insulating layer 2. On the metal base 1 and the resin substrate 3, Ni13 /
External electrodes 5 formed of an Au12 plating film are formed, and these external electrodes and the pads of the power semiconductor element 9 and the semiconductor element 8 are electrically connected by an Au wire 6 having a diameter of 30 μm and an Al wire 7 having a diameter of 300 μm, respectively. A connection has been made. The thickness of the Ni plating film 13 of the external electrode to which the Au wire is connected is 5 μm.
Has a thickness of 0.5 μm. The thickness of the Ni plating film 13 of the external electrode to which the Al wire is connected is 5 μm.
The thickness of the u plating film 12 is 0.2 μm. The thickness of the Au plating film of the external electrode to which the Au wire is connected is 0.5 μm
Therefore, the bonding strength with the Au wire is high, and the thickness of the Au plating film of the external electrode to which the Al wire is connected is 0.2.
Since the thickness is μm, it is possible to realize a highly reliable IPM with high moisture resistance reliability of the joint. In this embodiment, the thickness of the Au plating film of the external electrode to which the Al wire is connected is 0.2 μm.
m, but even when the thickness is 0.9 μm or more, it is possible to realize an IPM with high moisture resistance reliability at the junction.

FIG. 3 is a schematic sectional view of an IPM according to an embodiment of the present invention. In the configuration, a power semiconductor element 9 is mounted on a metal base 1 via a heat sink 4, and a resin substrate 3 on which the semiconductor element 8 is mounted is adhered on the metal base 1 via an insulating layer 2. An Ni13 / Au12 plating film and external electrodes 5 formed of Ni13 are formed on the metal base 1 and the resin substrate 3, and these external electrodes and the pads of the power semiconductor element 9 and the semiconductor element 8 each have a diameter of 30 μm. And an Al wire 7 having a diameter of 300 μm. Ni plating film 1 of external electrode to which Au wire is connected
3 is 5 μm, and the thickness of the Au plating film 12 is 0.5 μm.
μm. The external electrode to which the Al wire is connected is a Ni plating film 13 having a thickness of 5 μm without an Au plating film.
It is formed only with. Since the thickness of the Au plating film of the external electrode to which the Au wire is connected is 0.5 μm, Au
Since the external electrode to which the Al wire is connected is a Ni plating film 13 having a high bonding strength with the wire, the Al joint has high moisture resistance reliability and a highly reliable IPM can be realized.

FIG. 4 is a schematic sectional view of a power semiconductor module according to an embodiment of the present invention. The configuration is such that a power semiconductor element 9 is mounted on a metal base 1 via a heat sink 4 and a resin substrate 3 on which the semiconductor element 8 is mounted.
Are adhered on the metal base 1 via the insulating layer 2.
Ni13 / Au12 on metal base 1 and resin substrate 3
External electrodes 5 formed of a plating film are formed, and these external electrodes and the pads of the semiconductor element 8 have a diameter of 30 μm.
A reverse bonding method in which a ball formed at one end of a wire is bonded to the external electrode 5 with an Au wire 6 of m m, and the pad of the power semiconductor element 9 and the external electrode 5 have a diameter of 300 mm.
Electrical connection is made by the ultrasonic bonding method of the μm Al wire 7. The thickness of the Ni plating film 13 of the external electrode is 5 μm, and the thickness of the Au plating film 12 is 0.2 μm. Although the thickness of the Au plating film of the external electrode to which the Au wire is connected is 0.2 μm, the bonding strength with the Au wire is high because the Au wire is connected by the reverse bonding method. It is possible to realize a highly reliable IPM in which the wire joint has high moisture resistance reliability.

[0031]

According to the present invention, a highly reliable power semiconductor module can be provided.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing an IPM module according to an embodiment of the present invention.

FIG. 2 is a schematic sectional view showing an IPM module according to one embodiment of the present invention.

FIG. 3 is a schematic sectional view showing an IPM module according to one embodiment of the present invention.

FIG. 4 is a schematic sectional view showing an IPM module according to one embodiment of the present invention.

FIG. 5 is a schematic sectional view showing a conventional IPM module.

FIG. 6 is a diagram showing the moisture resistance reliability of a conventional external electrode and Al wire connection.

FIG. 7 is a schematic cross-sectional view of a conventional interface between an external electrode and an Al wire joint after a moisture resistance reliability test.

FIG. 8 shows A at the connection between the Au plating electrode and the Al wire.
The figure which shows the relationship between the thickness of a u plating film, and humidity resistance reliability.

FIG. 9 is a diagram showing the bonding property between an Au wire and an Au plating film in a case where a semiconductor element is mounted on a resin substrate and in a case where the semiconductor element is not mounted on the resin substrate in relation to a thickness of the plating film.

FIG. 10 is a view comparing the composition of an Au plating film when a semiconductor element is mounted on a resin substrate and when the semiconductor element is not mounted.

FIG. 11 is a view showing the bondability between an Au wire and an Au plating film in a case where a semiconductor element is mounted on a resin substrate and cleaning is performed, and in a case where cleaning is not performed, in relation to a thickness of the plating film.

FIG. 12 is a view comparing the composition of an Au plating film when a semiconductor element is mounted on a resin substrate and cleaning is not performed;

FIG. 13 is a view showing the bondability when a semiconductor element is mounted on a resin substrate and an Au wire is bonded to an Au plating film by a conventional bonding method and a reverse bonding method in relation to the thickness of the plating film.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Metal base, 2 ... Insulation layer, 3 ... Resin substrate, 4 ... Heat sink, 5 ... External electrode, 6 ... Au wire, 7 ... Al wire,
8 semiconductor element, 9 power semiconductor element, 10 mold resin, 11 lead frame, 12 Au plating film, 13 Ni plating film.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kazuya Takahashi 7-1-1, Omikacho, Hitachi City, Ibaraki Prefecture Inside the Hitachi Research Laboratory, Hitachi, Ltd. (72) Inventor Toshiaki Morita 7-1 Omikacho, Hitachi City, Ibaraki Prefecture No. 1 F term in Hitachi Research Laboratory, Hitachi, Ltd. (reference) 5F044 AA03 FF04 FF05

Claims (4)

[Claims] A semiconductor element in which a pad on the semiconductor element and an external electrode are electrically connected by an Au wire;
In a power semiconductor module in which a semiconductor element in which a pad on a semiconductor element and an external electrode are electrically connected by a l wire is mixed, the external electrode is formed of an Au film having an average thickness of 0.2 μm or less. A power semiconductor module, comprising: A semiconductor element in which a pad on the semiconductor element and an external electrode are electrically connected by an Au wire;
In a power semiconductor module in which a semiconductor element in which a pad on a semiconductor element and an external electrode are electrically connected by a l-wire is mixed, the external electrode is formed of an Au film having an average thickness of 0.9 μm or more. A power semiconductor module, comprising: 3. A semiconductor device in which a pad on a semiconductor device and an external electrode are electrically connected by an Au wire;
In a power semiconductor module in which a pad on a semiconductor element is electrically connected to an external electrode by a l-wire, the external electrode connected to the Au wire has a thickness of 0.4 to 0.8 μm. Au film and Al
A power semiconductor module, wherein an external electrode to which a wire is connected is formed of an Au film having an average thickness of 0.2 μm or less or 0.9 μm or more. 4. A semiconductor device in which a pad on a semiconductor device and an external electrode are electrically connected by an Au wire;
In a power semiconductor module in which a semiconductor element in which a pad on a semiconductor element is electrically connected to an external electrode by a l-wire is mixed, the external electrode connected to the Au wire has an average thickness of 0.4 to 0. Au with a thickness of 0.8 μm
A power semiconductor module, wherein an external electrode to which the film and the Al wire are connected is formed of a Ni film.