Semiconductor device

Abstract

The purpose of the present invention is to provide a technology capable of welding a terminal thicker than a circuit portion of an insulating substrate to the circuit portion without impairing insulation performance of the insulating substrate in a semiconductor device. This semiconductor device comprises: an insulating substrate having an insulating layer and a circuit portion provided on the insulating layer; a terminal having a laser irradiation surface, joined to the circuit portion by laser welding, and having a thickness greater than a thickness of the circuit portion; a semiconductor element joined to the circuit portion via a joining material and electrically connected to the terminal; a metal film formed on a surface of the terminal opposite to the laser irradiation surface and having a melting point higher than a melting point of the terminal; and a gap formed between the terminal and the circuit portion in a non-laser-welded portion.

Description

Semiconductor device

[0001] The present disclosure relates to a semiconductor device.

[0002] Conventionally, power semiconductor devices have been used, for example, in motor control of electric railway equipment or automotive equipment.

[0003] For example, in Patent Document 1, a technique is disclosed in which in a semiconductor device, by providing a Ni plating film having a higher laser absorption rate than copper on the entire surface of a copper lead frame (corresponding to a terminal) having a laser irradiation surface, welding with low power and low energy is made possible.

[0004] Japanese Patent Application Laid-Open No. 2008-212977

[0005] In the technique described in Patent Document 1, by providing a metal film having a high laser absorption rate and a high melting point on the laser irradiation surface of the lead frame, joining with less laser power is made possible. However, the assumed thickness of the lead frame is 0.5 mm, and the thickness of the metal plate (corresponding to the circuit portion) disposed on the opposite side of the laser irradiation surface is 1.0 mm. That is, the lead frame is assumed to have a thickness thinner than that of the metal plate to be joined to the lead frame.

[0006] In a general power semiconductor device, it is necessary to weld a copper terminal having a thickness of about 0.5 mm or more and 1.5 mm or less and a copper circuit portion having a thickness of about 0.3 mm constituting the upper surface of the insulating substrate with a laser. Since the thickness of the circuit portion is thinner than that of the terminal, if the laser power is not suppressed further, all of the terminal and the circuit portion will be completely melted during heat input by the laser, and the laser will reach the insulating layer located below the circuit portion, resulting in a problem that the insulating performance of the insulating substrate is impaired.

[0007] Therefore, an object of the present disclosure is to provide a technique capable of welding a terminal having a thickness thicker than that of the circuit portion of an insulating substrate to the circuit portion without impairing the insulating performance of the insulating substrate in a semiconductor device.

[0008] The semiconductor device according to this disclosure comprises an insulating substrate having an insulating layer and a circuit portion provided on the insulating layer; a terminal having a laser irradiation surface and being joined to the circuit portion by laser welding, and having a thickness greater than the thickness of the circuit portion; a semiconductor element being joined to the circuit portion via a bonding material and electrically connected to the terminal; a metal film having a higher melting point than the terminal, formed on the surface of the terminal opposite to the laser irradiation surface; and a void formed between the terminal and the circuit portion in a location that is not laser welded.

[0009] According to this disclosure, since a gap is formed between the terminal and the circuit portion in the area that is not laser-welded, the dissipation of heat applied to the terminal to the insulating substrate is suppressed. As a result, laser welding at a lower power density becomes possible.

[0010] Furthermore, because a metal film with a higher melting point than the terminal is formed on the side of the terminal opposite to the laser irradiation surface, the terminal and the metal film melt together, increasing the temperature at which they come into contact with the circuit, thereby enhancing the heat conduction type welding action.

[0011] Therefore, it is possible to weld terminals with a thickness greater than the thickness of the circuit portion of the insulating substrate to the circuit portion without impairing the insulating performance of the insulating substrate.

[0012] The purpose, features, aspects, and benefits of this disclosure will become clearer from the following detailed description and accompanying drawings.

[0013] This is a cross-sectional view of a semiconductor device according to an embodiment. This is a cross-sectional view of the laser-welded portion between the terminal and the circuit portion and its surrounding area in the semiconductor device according to an embodiment. This is a cross-sectional view of the laser-welded portion between the terminal and the circuit portion and its surrounding area in a semiconductor device according to a modified example of the embodiment.

[0014] <Embodiment> An embodiment will be described below with reference to the drawings. Figure 1 is a cross-sectional view of a semiconductor device according to the embodiment.

[0015] As shown in Figure 1, the semiconductor device is a power semiconductor device used, for example, for motor control in railway equipment or automobile equipment. The semiconductor device comprises a heat sink 9, an insulating substrate 3, a semiconductor element 5, a plurality (e.g., two) of terminals 1, a case 10, a sealing material 11, and a cover 12.

[0016] A plate with high thermal conductivity is used as the heat sink 9. Specifically, the heat sink 9 may be a plate made of a metal such as copper, aluminum, or molybdenum, or a plate made of a composite material such as AlSiC or MgSiC. The thickness of the heat sink 9 is approximately 3 mm to 10 mm.

[0017] The insulating substrate 3 is joined to the upper surface of the heat sink 9 via a bonding material 8. The bonding material 8 is Pb-based solder, Sn-based solder, brazing material, or sintered material. The insulating substrate 3 includes an insulating layer 31, a circuit section 32, and a heat dissipation section 33. The circuit section 32 is provided on the upper surface of the insulating layer 31. The heat dissipation section 33 is provided on the lower surface of the insulating layer 31. The insulating layer 31 is made of aluminum nitride, silicon nitride, or alumina, etc. When the insulating layer 31 is made of aluminum nitride or alumina, the thickness of the circuit section 32 is typically 0.3 mm. The circuit section 32 and the heat dissipation section 33 are made of copper or aluminum, etc.

[0018] The semiconductor element 5 is mounted on the upper surface of the insulating substrate 3 (more specifically, the upper surface of the circuit section 32) via a bonding material 6. The semiconductor element 5 is formed of a semiconductor such as Si. Preferably, the semiconductor element 5 is formed of a wide bandgap semiconductor such as SiC or GaN. By using a wide bandgap semiconductor, which has lower current loss than Si, the temperature rise of the entire semiconductor device is suppressed, making it possible to improve the current conducting ability of the semiconductor device.

[0019] The semiconductor element 5 includes, for example, an IGBT (Insulated Gate Bipolar Transistor), a MOSFET (Metal Oxide Semiconductor Field Effect Transistor), or a Schottky barrier diode. Alternatively, the semiconductor element 5 may include an RC-IGBT (Reverse-Conducting IGBT) in which an IGBT and a freewheeling diode are formed on a single semiconductor chip. The number of semiconductor elements 5 is not limited to one, but may be two or more. The bonding material 6 is a Pb-based solder, a Sn-based solder, a brazing material, or a sintered material.

[0020] The semiconductor element 5 is electrically connected to the circuit section 32 or terminal 1 via a wire 7. The wire 7 is typically an aluminum or copper wire with a diameter of approximately φ200 to φ500 mm. Alternatively, a copper plate, copper-molybdenum plate, or CIC plate may be used instead of the wire 7. In this case, the wire 7 is connected to the semiconductor element 5 or circuit section 32 using Pb-based solder, Sn-based solder, brazing material, or sintered material.

[0021] The case 10 is formed in a frame shape when viewed from above, and surrounds the sides of the insulating substrate 3 and the semiconductor element 5. The case 10 is fixed to the peripheral edge of the heat sink 9, for example, via an adhesive (not shown). The case 10 is made of a resin such as PPS (Poly Phenylene Sulfide), PET (Polyethylene Terephthalate), or PBT (Poly Butylene Terephthalate), but may also be made of a ceramic material.

[0022] The sealing material 11 is filled inside the case 10 and seals the insulating substrate 3, the semiconductor element 5, and a portion of the terminal 1. The sealing material 11 is, for example, a silicone gel or silicone resin, but if the dielectric strength can be ensured even in the atmosphere, the sealing material 11 does not need to be filled inside.

[0023] The cover 12 is attached to the upper end inside the case 10 and covers the upper surface of the sealing material 11. The cover 12 is made of a resin such as PPS, PET, or PBT, but may also be made of a ceramic material.

[0024] Terminal 1 is formed in a Z-shape in cross-section and has a lower end portion 21 extending laterally, a vertical portion 22 connected to one end of the lower end portion 21 and extending vertically, and an upper end portion 23 connected to the upper end of the vertical portion 22 and extending laterally. The lower end portion 21 of terminal 1 is joined to the upper surface of the circuit portion 32, and the upper end portion 23 of terminal 1 is fixed to the upper surface of the case 10, for example, by screws (not shown). Although not shown, the lower end portion 21 of terminal 1 is also electrically connected to the semiconductor element 5 via a wire 7. With the above configuration, terminal 1 is electrically connected to the circuit portion 32 by a welded portion 2 formed on the lower end portion 21 of terminal 1, and plays the role of supplying power from an external power source (not shown) to the semiconductor element 5. Since a large current flows through terminal 1, it is necessary to design terminal 1 so that its electrical resistance is as low as possible. For this reason, the thickness of terminal 1 must be at least 0.5 mm. Also, terminal 1 is made of copper, which has high electrical conductivity.

[0025] Here, the welded portion 2 is an irradiation mark formed on the lower end portion 21 and the circuit portion 32 by laser irradiation of the upper surface of the lower end portion 21 of the terminal 1. The welded portion 2, which is the irradiation mark, is formed by laser heat input, extending from the upper surface of the lower end portion 21 to the upper surface of the circuit portion 32. Figure 1 is used to illustrate that the welded portion 2, which is the irradiation mark, welds the lower end portion 21 and the circuit portion 32.

[0026] The temperature at the center of the laser irradiation surface of terminal 1 reaches a temperature high enough to generate metal fumes (the boiling point of copper, 2562°C). As described above, the insulating layer 31 is formed of aluminum nitride, silicon nitride, or alumina, but since silicon nitride melts at 1900°C, alumina at 2000°C, and aluminum nitride at 2200°C, the insulating performance of the insulating layer 31 is impaired at 2562°C, which is the boiling point of copper. In order to protect the insulating performance of the insulating layer 31, the temperature of the circuit part 32 must be below the boiling point of copper when welding terminal 1 and the circuit part 32. Here, the laser irradiation surface of terminal 1 is the upper surface of the lower end portion 21 of terminal 1.

[0027] Next, the structure of the terminal 1 and the circuit section 32, which is a characteristic of this embodiment, will be described. Figure 2 is a cross-sectional view of the laser-welded portion of the terminal 1 and the circuit section 32 and its surrounding area in the semiconductor device according to this embodiment.

[0028] In high-power-density lasers (infrared lasers) such as keyhole welding, when laser welding a terminal 1, which is thicker than the circuit portion 32 of the insulating substrate 3, to the circuit portion 32, thermal damage occurs to the insulating layer 31 located beneath the circuit portion 32. To avoid this, thermal conduction welding using a low-power-density laser (such as a blue laser) or welding using a combination of a low-power-density laser and a high-power-density laser is performed. However, in the latter case, the output of the high-power-density laser is used at a lower output than when using the high-power-density laser alone. However, with a low-power-density laser, it is difficult to sufficiently heat the terminal 1, which often leads to welding defects.

[0029] Therefore, the semiconductor device according to this embodiment has two features. The first feature is that, as shown in Figure 2, a gap 36 is provided to prevent the heat input to the terminal 1 from dissipating to the circuit section 32. The gap 36 is formed between the terminal 1 and the circuit section 32 in a place that has not been laser-welded, in other words, in a place where the welded section 2 has not been formed. The gap 36 is formed by inserting a spacer 35 between the terminal 1 and the circuit section 32 in a place that has not been laser-welded.

[0030] As the spacer 35, a metal plate, a projection provided in advance on the terminal 1 or circuit section 32, or a thin wire can be used. The spacer 35 may be removed after welding the terminal 1 and the circuit section 32, or it may be left attached. Alternatively, the gap 36 may be formed by adjusting the height of the terminal 1 with a jig without using the spacer 35, so the spacer 35 is not essential. However, if the width of the gap 36 is set wider than the width of the welded section 2, it will lead to welding defects, so using the spacer 35 makes it easier to ensure joining quality than adjusting the height of the terminal 1 with a jig. Here, the width of the welded section 2 is the width of the part of the welded section 2 that protrudes between the terminal 1 and the circuit section 32.

[0031] The second feature is that, in order to further increase the temperature at which the terminal 1, which has been melted by laser irradiation, contacts the circuit section 32, a metal film 24 having a higher melting point than the terminal 1 is plated onto the surface of the terminal 1 opposite to the laser irradiation surface. Since the base material of the terminal 1 is copper (melting point 1085°C), the metal film 24 is a plating film of nickel (melting point 1455°C), titanium (melting point 1688°C), or chromium (melting point 1907°C), and the melting point of the metal film 24 is 300K or more higher than the melting point of copper. The thickness of the metal film 24 is 0.1 μm or more. Here, the surface of the terminal 1 opposite to the laser irradiation surface is the lower surface of the lower end portion 21 of the terminal 1.

[0032] The semiconductor device is given two features, and then the laser output is adjusted so that the width of the welded area 2 is 100 μm or more during welding. This is because a wider welded area 2 increases the current-conducting capability of the semiconductor device, and welding conditions that result in a welded area 2 width of less than 100 μm have insufficient process margins, making it difficult to absorb variations in component dimensions, which can lead to welding defects. The welded area 2 is provided as a point or line on the laser irradiation surface of the terminal 1. The high-melting-point metal film 24 is melted into the terminal 1 and the circuit section 32 during welding.

[0033] As described above, in this embodiment, the semiconductor device comprises an insulating substrate 3 having an insulating layer 31 and a circuit portion 32 provided on the insulating layer 31; a terminal 1 having a laser irradiation surface and being joined to the circuit portion 32 by laser welding, and having a thickness greater than the thickness of the circuit portion 32; a semiconductor element 5 being joined to the circuit portion 32 via a bonding material 6 and electrically connected to the terminal 1; a metal film 24 having a higher melting point than the terminal 1, formed on the surface of the terminal 1 opposite to the laser irradiation surface; and a gap 36 formed between the terminal 1 and the circuit portion 32 in a location that has not been laser welded.

[0034] Therefore, since a gap 36 is formed between terminal 1 and circuit section 32 in the area that is not laser-welded, the dissipation of heat from terminal 1 to the insulating substrate 3 is suppressed. As a result, laser welding at a lower power density becomes possible.

[0035] Furthermore, since the metal film 24, which has a higher melting point than the terminal 1, is formed on the side of the terminal 1 opposite to the laser irradiation surface, the terminal 1 and the metal film 24 melt together, increasing the temperature at which they come into contact with the circuit section 32, thereby enhancing the heat conduction type welding action.

[0036] As described above, it is possible to weld terminals 1 having a thickness greater than the thickness of the circuit portion 32 of the insulating substrate 3 to the circuit portion 32 without impairing the insulating performance of the insulating substrate 3.

[0037] Furthermore, the melting point of the metal film 24 is at least 300 K higher than the melting point of the terminal 1. Also, the thickness of the metal film 24 is at least 0.1 μm.

[0038] Therefore, since the terminal 1 located on the upper side of the metal film 24 reaches a high temperature of 300K or higher, a welding effect due to heat conduction is obtained, making it possible to join a thicker terminal 1 to the circuit section 32.

[0039] Furthermore, since the width of the welded portion 2, which is an irradiation mark formed on the terminal 1, is 100 μm or more, it is possible to increase the current-conducting capacity of the welded portion 2.

[0040] In addition, since the welded portion 2, which is an irradiation mark, can be linearly formed by laser scanning, a wide welding range can be obtained, and it becomes possible to enhance the current-carrying capacity compared to spot welding.

[0041] In addition, since the thickness of the terminal 1 is 1.5 times or more the thickness of the circuit portion 32, it becomes possible to give the terminal 1 the current-carrying capacity.

[0042] Also, between the terminal 1 and the circuit portion 32, a spacer 35 for forming a gap 36 is arranged at a location where laser welding is not performed, so it is easier to ensure the joining quality than when adjusting the height of the terminal 1 with a jig.

[0043] In addition, since the semiconductor material of the semiconductor element 5 is a wide-bandgap semiconductor, it becomes possible to enhance the current-carrying capacity of the semiconductor device.

[0044] <Modification Example of the Embodiment> Next, a modification example of the embodiment will be described. FIG. 3 is a cross-sectional view of a laser-welded portion between the terminal 1 and the circuit portion 32 and its periphery in the semiconductor device according to the modification example of the embodiment.

[0045] When there is only one welded portion 2, the electrical resistance is large, and it is often difficult to give the semiconductor device sufficient current-carrying capacity. Therefore, as shown in FIG. 3, a plurality of welded portions 2 are formed. Specifically, the first welded portion 41, the second welded portion 42, and the third welded portion 43 are formed in this order. When forming the second welded portion 42, in the state where the previously formed first welded portion 41 and the second welded portion 42 are connected, heat dissipates to the circuit portion 32 through the first welded portion 41. Therefore, the second welded portion 42 is formed with a gap so that the first welded portion 41 and the second welded portion 42 are not connected. In order to suppress the heat input to the terminal 1 from dissipating to the circuit portion 32, it is preferable that the intervals between the plurality of welded portions 2 are wider.

[0046] As described above, in the modification example of the embodiment, in the semiconductor device, the welded portion 2, which is an irradiation mark, is formed at a plurality of locations on the terminal 1, and there is a region where laser welding is not performed between adjacent welded portions 2, which are irradiation marks. Therefore, it becomes possible to further suppress the dissipation of heat input to the terminal 1 to the insulating substrate 3.

[0047] Although this disclosure has been described in detail, the above description is illustrative and not restrictive in all aspects. An infinite number of variations not illustrated can be conceived.

[0048] Note that the embodiments can be appropriately modified and omitted.

[0049] 1 Terminal, 2 Welding portion, 3 Insulating substrate, 5 Semiconductor element, 6 Bonding material, 24 Metal film, 31 Insulating layer, 32 Circuit portion, 35 Spacer, 36 Gap, 41 First welding portion, 42 Second welding portion, 43 Third welding portion.

Claims

1. A semiconductor device comprising: an insulating substrate having an insulating layer and a circuit portion provided on the insulating layer; a terminal having a laser irradiation surface and being joined to the circuit portion by laser welding, and having a thickness greater than the thickness of the circuit portion; a semiconductor element being joined to the circuit portion via a bonding material and electrically connected to the terminal; a metal film having a higher melting point than the terminal, formed on the surface of the terminal opposite to the laser irradiation surface; and a void formed between the terminal and the circuit portion in a location that is not laser-welded.

2. The semiconductor device according to claim 1, wherein the melting point of the metal film is 300 K or more higher than the melting point of the terminal.

3. The semiconductor device according to claim 1 or claim 2, wherein the thickness of the metal film is 0.1 μm or more.

4. The semiconductor device according to any one of claims 1 to 3, wherein the width of the irradiation mark formed on the terminal is 100 μm or more.

5. The semiconductor device according to claim 4, wherein the irradiation marks are formed at multiple locations on the terminal, and there are areas that are not laser-welded between adjacent irradiation marks.

6. The semiconductor device according to claim 4, wherein the irradiation marks are formed in a linear shape.

7. The semiconductor device according to any one of claims 1 to 6, wherein the thickness of the terminal is 1.5 times or more the thickness of the circuit portion.

8. The semiconductor device according to any one of claims 1 to 7, wherein a spacer for forming the gap is placed between the terminal and the circuit portion where laser welding is not performed.

9. The semiconductor device according to any one of claims 1 to 8, wherein the semiconductor material of the semiconductor element is a wide-bandgap semiconductor.

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