Patent Application
20150221626
The present invention relates to a power semiconductor module such as an IGBT module, and more particularly to a power semiconductor module in which a wiring is formed of a wire or ribbon on a plate-like conductive member disposed on a semiconductor device.
A power semiconductor module such as an IGBT module handles a large current of several tens to several hundreds amperes per semiconductor device, which involves large heat generation of semiconductor devices. In recent years, further downsizing of the power semiconductor module is demanded, and the heat generation density tends to be increasing more and more. The semiconductor device composed of Si or SiC is connected with another device or an electrode by means of a wire, a ribbon, or the like composed of copper or aluminum. However, since there is a difference in thermal expansion rate between the semiconductor device and the wiring material, there is a problem in that a bonding portion is broken due to thermal fatigue during repetition of switching operations (ON and OFF operations for energization).
Therefore, as a technique for improving the reliability of wiring connection, PTL 1 discloses a power semiconductor module having a structure in which a heat-diffusing metal plate is connected on a semiconductor chip by means of solder, and the heat-diffusing metal plate and a wiring pattern on an insulating substrate are connected by means of a thin metal (ribbon) having a thickness of about 100 to 200 μm. It is described in PTL 1 that the heat-diffusing metal plate provides an effect of equalizing the heat in the semiconductor chip in which the temperature is elevated at the central portion. Similarly, as a technique for improving the reliability of wiring connection using a conductive metal plate, there is PTL 2. PTL 2 presents a solution from the viewpoint of stress buffer, in which two metal plates having a thermal expansion coefficient intermediate between a wiring member and a semiconductor device are used to eliminate a connecting portion having a large difference in thermal expansion coefficient.
That is, if a material having a proper thermal expansion coefficient is used, the heat-diffusing metal plate connected on the semiconductor chip via a bonding material is a dominant connection reliability improving means that can equalize the temperature distribution of the chip and reduce the thermal stress of a wiring bonding portion.
[PTL 1] JP-A-2013-197560
[PTL 2] JP-A-2012-28674
However, the heat-diffusing metal plate having the conventional structure is not optimized for a layout in a planar direction along the chip surface in view of heat diffusion and bonding layout. For example, in an IGBT device (insulated gate bipolar transistor), an emitter electrode (main electrode) and a gate electrode (control electrode) are formed in a front surface electrode, and a collector electrode (main electrode) is formed in a back surface electrode. A gate current only flows for a short time in turning on and off, and the amount of the current that instantaneously flows is only about one several tenth to one several hundredth of the current that flows between the emitter and the collector. Therefore, the amount of heat generation of the gate electrode is small relative to that of the emitter electrode, so that especially the emitter electrode has to be efficiently cooled. However, in the conventional structures described in PTLs 1 and 2, although the conductive member connected on the emitter electrode and the gate wiring are described, they are only placed side by side. Therefore, there is a problem in that a detailed layout on the chip for efficiently cooling a high-heat-generating portion is not optimized.
Also in a MOSFET device (insulated gate field-effect transistor) in which a source electrode (main electrode) and a gate electrode (control electrode) are formed in a front surface electrode and a drain electrode (main electrode) is formed in a back surface electrode, the amount of current that flows between the main electrodes is larger than the amount of current that flows through the gate electrode, and therefore, a high-heat-generating portion and a low-heat-generating portion are included. How to efficiently cool the high-heat-generating portion is a common problem to transistor devices.
It is an object of the invention to provide a power semiconductor module that can efficiently cool a high-heat-generating portion of a transistor device and has excellent connection reliability of a wiring bonding portion.
One configuration of the invention for solving the problem is directed to “a power semiconductor module including: a heat sink; a circuit board connected to the heat sink via a bonding material and formed with a wiring on a front surface of an insulating substrate; a transistor device including a main electrode and a control electrode formed on one surface and a back surface electrode formed on the other surface, the back surface electrode being connected to the circuit board via a bonding material; a first conductive member bonded to the main electrode via a bonding material; and wire or ribbon-shaped connection terminals electrically connecting the first conductive member and the control electrode with another device or the circuit board, wherein the control electrode is disposed at a corner portion of the main electrode, and the first conductive member has a shape in which the first conductive member is cut out at a portion above the control electrode”. When the structure is adopted, in which the control electrode is disposed at the corner portion of the main electrode, the first conductive member is connected to the main electrode, and the first conductive member has the shape in which the first conductive member is cut out at the portion above the control electrode, the main electrode as a high-heat-generating portion relative to the control electrode can be covered in maximum area with the conductive member continuously and without a hole or groove, a heat conduction path is not interrupted in the conductive member, and thus a high heat-equalizing effect is obtained.
According to the invention, it is possible to provide a power semiconductor module that can efficiently cool a high-heat-generating portion of a transistor device and has excellent connection reliability of a wiring bonding portion.
Hereinafter, examples will be described with reference to the drawings.
A transistor device 5 is bonded on the wiring pattern 3 via a bonding material 4. For the bonding material, solder, or a sintering paste of fine particles of silver or copper is used. A paste of fine particles of silver oxide or copper oxide from which silver or copper is generated by reduction can also be used. According to the kind of the bonding material, silver or nickel plating is applied to the wiring pattern 3 for improving the wettability of the bonding material or ensuring bonding strength. For the transistor device 5, Si or SiC is used as a material, and an IGBT (Insulated Gate Bipolar Transistor) or MOSFET (Metal Oxide Semiconductor Field-Effect Transistor) is used as the kind of the device. Here, a main electrode 6 and a control electrode 7 are formed on a front surface of the transistor device 5 as in
In an IGBT device, an emitter electrode (main electrode) and a gate electrode (control electrode) are formed in a front surface electrode, and a collector electrode (main electrode) is formed in a back surface electrode. In a MOSFET device, a source electrode (main electrode) and a gate electrode (control electrode) are formed in a front surface electrode, and a drain electrode (main electrode) is formed in a back surface electrode. For example, for using an IGBT device as an inverter device, the inverter device is configured as a module in which a plurality of IGBT devices and diode devices are combined together. In
Here, a conductive member 10 is connected on the main electrode 6 via a bonding material 9. Connection terminals 11 composed of aluminum, copper, or a clad material of aluminum and copper are connected on the conductive member 10 using an ultrasonic bonding machine, and connected with the wiring pattern 3 on the circuit board, another semiconductor device, or the like. In the embodiment of the invention, the control electrode 7 is disposed at a corner portion of the main electrode 6. Moreover, the conductive member 10 is designed to be located closest to the inside of a guard ring so that the main electrode 6 of the transistor device on the front surface electrode side is covered in maximum area, but the conductive member has a structure in which a cutout is provided only above the control electrode 7. A connection terminal 12 composed of aluminum, copper, or a clad material of aluminum and copper is connected on the control electrode 7 using an ultrasonic bonding machine, and connected with the wiring pattern 3 on the circuit board.
The main electrode 6 and the control electrode 7 are covered with a thin film having a thickness of several micrometers, such as aluminum, nickel, gold, silver, or copper, for bonding with the conductive member 10 or ultrasonic bonding with the connection terminal 12. For the material of the bonding material 9, solder, a sintering paste of fine particles of silver or copper, or the like, is used similarly to the material of the bonding material 4.
In the case of using, for the conductive member 10, a material having a thermal conductivity higher in a direction horizontal to the electrode surface of the transistor device than in a direction vertical to the electrode surface, before heat generated by the device is conducted to a wiring such as a wire or ribbon at the upper portion, the heat is diffused in the plane of the conductive member 10 along the chip surface, and a favorable heat-equalizing effect is obtained. Therefore, the wire or ribbon does not peel off due to elevation of temperature only at a specific portion of the chip, so that the wiring connection reliability is improved in the entire chip. For example, a composite material of metal (copper, aluminum, or the like) and a graphite fiber having thermal conductivity anisotropy such as of 20 W/mK in a certain plane and 2000 W/mK in the orthogonal direction of the plane can be used. Moreover, it is further preferable to use a material obtained by stacking layers having different thermal conductivities, such as a clad material of copper/invar/copper. One reason for this is that since the thermal conductivity of invar (iron-nickel alloy) is 13 W/mK, which is lower than 400 W/mK of copper, the heat generated by the transistor device is less likely to be conducted to the upper portion, and the heat conducts through copper along the device surface and is equalized. Another reason is that the thermal expansion rate can be adjusted to a preferable value intermediate between Si or SiC (3 to 5 ppm/K) and the wiring material (Al about 23 ppm/K and Cu about 16 ppm/K) due to the ratio of copper (about 16 ppm/K) and invar (about 1 ppm/K), and thus thermal stress can be reduced. For example, by setting the ratio of copper/invar/copper to 1:1:1, a thermal expansion rate of about 11 ppm/K is obtained, making of a connecting portion of materials having a large thermal expansion difference can be avoided, and both the wiring connection reliability and the connection reliability of the conductive member to the chip can be improved.
Although, in the power semiconductor module, a wire having a diameter of 200 to 500 μm or a ribbon having a thickness of 100 to 300 μm is used for allowing large current to flow into the main electrode, it is particularly preferable to use a clad material of copper/invar/copper having a total thickness of 1 mm or more so that the stress strain of a connecting portion between the wiring and the clad material of copper/invar/copper does not overlap the stress strain of a connecting portion between the clad material of copper/invar/copper and the transistor device. Also in terms of relaxing an impact on the chip when cutting the wire or ribbon, it is preferable to use a clad material having a thickness of 1 mm or more. As the material of the conductive member 10, a clad material of copper/molybdenum/copper can also be used other than the clad material of copper/invar/copper. In the example of
Next, an advantageous effect obtained due to the fact that the control electrode 7 is located at the corner portion of the main electrode 6 and that the conductive member 10 has a structure in which the conductive member is cut out at a portion above the control electrode 7 will be described. In a MOSFET device, a gate electrode insulated by an oxide film (SiO2) exists, and capacitive components are included. These are referred to as a gate-source capacitance and a gate-drain capacitance. Then, turning on or off is performed with a gate voltage, and a gate current flows at the time of turning on or off. This current, which is used for charging and discharging the gate-source capacitance and the gate-drain capacitance, is small and only one several tenth of the amount of current that flows between the source and the drain. An IGBT device structurally contains the MOSFET, in which a gate current flows only instantaneously and the amount of current is small. As described above, since the current that flows through the control electrode (gate electrode) is smaller than that of the main electrodes (emitter electrode-collector electrode or source electrode-drain electrode) and flows only for a short time, the control electrode is a low-heat-generating portion and the main electrode is a high-heat-generating portion. Actually, the main electrode 6 occupies the most part of a front surface electrode 8 in terms of area in many cases, and a temperature difference of 20 to 30° C. occurs even in the main electrode 6 depending on a driving temperature. Although the conductive member 10 is effective for heat equalization, a high heat-equalizing effect is obtained when the control electrode 7 is disposed at a corner portion and the main electrode 6 is continuously covered with the conductive member 10. Conversely, it is not preferable to dispose the control electrode at the chip center because a heat conduction path is interrupted.
Moreover, when the conductive member is connected on the main electrode but the conductive member is not connected to the control electrode as shown in
According to the example, the electrodes and the conductive member bonded to the electrode can be configured to have the most excellent chip heat equalization in consideration of the presence of a difference in heat generation amount between the main electrode and control electrode of the transistor device, so that it is possible to provide a power semiconductor module having high wiring connection reliability when operating at a high temperature.
In this example, an example of a power semiconductor module in which a conductive member is also provided on the control electrode 7 will be described using
In this configuration, a conductive member 22 is bonded to the control electrode 7 via a bonding material 21. For the bonding material 21, solder, or a sintering paste of fine particles of silver or copper is used, similarly to the bonding material 9. Similarly to the conductive member 10, in the case of using, for the conductive member 22, a material having a thermal conductivity higher in the direction horizontal to the electrode surface of the transistor device than in the direction vertical to the electrode surface, before heat generated by the device is conducted to a wiring such as a wire or ribbon at the upper portion, the heat is diffused in the plane of the conductive member along the chip surface, and a favorable heat-equalizing effect is obtained. Therefore, the wire or ribbon does not peel off due to elevation of temperature only at a specific portion of the chip, so that the wiring connection reliability is improved in the entire chip. It is particularly preferable to use a material obtained by stacking layers having different thermal conductivities, such as a clad material of copper/invar/copper. A connection terminal 23 composed of aluminum, copper, or a clad material of aluminum and copper is connected to the conductive member 22 using an ultrasonic bonding machine, and connected with the wiring pattern on the circuit board, another semiconductor device, or the like. Because of the presence of the conductive member 22, the chip is less likely to be damaged by impacts caused by ultrasonic bonding and cutting, and thus connection is possible by means of a copper ribbon having a width of 1 mm or more.
In this configuration, the control electrode 7 is located at the corner portion of the main electrode 6, the main electrode 6 is covered with the conductive member 10 not having a hole or groove that interrupts a heat conduction path, and therefore, a high heat-equalizing effect is obtained. Moreover, when the connection terminal 23 is ultrasonic-bonded to the conductive member 22 on the control electrode 7, the conductive member 10 is not an obstacle unlike the configuration in
In this example, an example of a power semiconductor module including a plurality of transistor devices and a plurality of diode devices mounted therein and having an excellent heat-equalizing property in the entire module will be described.
In general, power devices handle a plurality of transistors and a plurality of diodes while making the transistors and diodes into a module to realize downsizing, an improvement in mountability, and the like. When a wiring pattern of a circuit board and an device, or devices are connected by means of a connection terminal such as a wire, cutting of the wire on a chip means exertion of an impact on the chip with the cutter of the ultrasonic bonding machine, and therefore, the cutting on the chip is avoided in many cases. Therefore, a wiring pattern of a circuit board and a transistor, the transistor and a diode, and the diode and the wiring pattern of the circuit board are usually connected continuously by means of an aluminum wire having a diameter of 0.4 to 0.5 mm using a wedge bonding type ultrasonic bonding machine.
On the other hand, in the progress of higher capacity and higher heat generation density of power semiconductor modules, copper has attracted attention as a wire material. Since copper has a thermal conductivity and an electrical conductivity higher than those of aluminum, and has a thermal expansion rate close to that of Si or SiC, copper is advantageous also in terms of stress. As described above, a copper-containing wire or ribbon such as of copper or a clad of copper and aluminum is a dominant connection reliability improving means that has an excellent heat transfer property, equalizes the temperature distribution of the entire module, and simultaneously can reduce the thermal stress of a wiring bonding portion. However, since copper is harder than aluminum, copper is less likely to be crashed in ultrasonic bonding, a high load and high power have to be applied, and thus the chip is likely to be damaged. Moreover, when the wiring pattern of the circuit board and the transistor, the transistor and the diode, and the diode and the wiring pattern of the circuit board are continuously connected as in the conventional case, the connection has to be a straight connection because copper is hard and thus less likely to be bent in the chip surface direction. Therefore, the number of copper wirings or the wiring direction is limited.
In contrast, when a conductive member is provided on the electrode of a transistor or diode as in Example 1 or 2, the conductive member acts as an impact buffer in ultrasonic bonding. Therefore, connection using the copper-containing wire or ribbon or cutting thereof is possible on the chip surface, and it is possible to adopt a wiring method with an excellent heat-equalizing property in the entire module. This will be specifically described using the drawings.
In
In the conventional example of
According to this example as described above, a circuit board and a conductive member, and the conductive member and another conductive member can be connected by means of copper-containing wire or ribbon-shaped connection terminals that are independent of each other. This independent wire connection makes it possible to attach a copper-based wire (ribbon) between a transistor and a diode in a direction greatly different from a copper-based wire (ribbon) direction between a circuit board and a transistor device. Therefore, even when the electrode pattern and the chip position are misaligned, a reduction in the number of copper-based wires (ribbons) can be avoided. This greatly contributes to heat equalization in the entire module.
In this configuration, the control electrode is located at the corner portion of the main electrode, the main electrode is covered with the conductive member not having a hole or groove that interrupts a heat conduction path, and therefore, a high heat-equalizing effect is obtained. Moreover, when the connection terminal 62 is ultrasonic-bonded, the conductive member 34 is not an obstacle. Therefore, since it is unnecessary for the conductive member 34 to keep a distance from a bonding portion of the connection terminal 62 by reducing the size of the conductive member 34, a high heat-equalizing effect due to the conductive member 34 is obtained. Further, the wiring pattern 3 and the conductive members 32 and 34 are connected by means of copper ribbons that are independent of each other in the module of this example, and the ribbons can be attached more densely than when using a continuous linear copper ribbon. Therefore, a heat-equalizing effect is obtained also in the entire module, and the connection reliability of the connection terminal is improved.
As described above, the circuit board and the conductive member, and the conductive member and another conductive member are connected by means of connection terminals that are independent of each other, and stitch bonding on the conductive member on the transistor device is eliminated. Therefore, the transistor devices can be connected by means of a copper-containing material having an excellent heat transfer property, and heat conduction paths extend vertically and horizontally, which greatly contributes to heat equalization in the entire module.
The embodiments of the invention have been specifically described using the examples. However, the invention is not limited to the configurations of the examples, and can be variously modified within the scope not departing from the gist of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-015049 | Jan 2014 | JP | national |
