JP2019079958A - Power module - Google Patents

Abstract

To provide a power module capable of achieving improvement in reliability.SOLUTION: A power module of the present invention comprises: a transistor (1) as a heat generation source; a first spreader (31) joined to a first electrode (collector electrode) of a transistor via a first joining layer (41); a spacer (2) for ensuring a space for a bonding wire which is joined to a second electrode (emitter electrode) of the transistor via a second joining layer (42) and joined to a third electrode (gate electrode) of the transistor; and a second spreader (32) joined to another surface side of the spacer via a third joining layer (43). In this case, the spacer is composed of a conductive material having a thermal expansion coefficient difference within a range of 0.5-7 ppm/K with a semiconductor material which composes the transistor. According to the present invention, by utilizing the spacer required in wire bonding, reliability of a double-sided cooling structure power module can be further improved.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a power module capable of improving reliability.

  As a motor driving inverter or the like, a power module mounted with a power device (semiconductor element) such as an IGBT (Insulated Gate Bipolar Transistor) is used.

  In order to ensure the reliability of the power module, it is important to efficiently dissipate the heat generated during operation of the device. Under such circumstances, a double-sided cooling type power module for cooling a power device from both sides has been proposed, and the related description is given in the following patent documents.

JP 2008-103623 A

  In Patent Document 1, first, a collector electrode (first electrode) of a transistor (IGBT) is joined to one surface of a first lead frame (spreader) and a first ceramic tube (cooler) is attached to the other surface of the first lead frame. It is joined. Next, the emitter electrode (second electrode) of the transistor (IGBT) is joined to one surface of the copper block (spacer), and the other surface of the copper block is joined to one surface of the second lead frame (spreader). A second ceramic tube is bonded to the other side of the frame. Incidentally, the copper block is provided to secure a space for avoiding the interference between the bonding wire bonded to the gate electrode (third electrode) of the transistor and the second lead frame or the like. In addition, joining between each member is made of Ag sintered body.

  In the case of Patent Document 1, a transistor having a small coefficient of thermal expansion (CTE) is joined to a copper block or lead frame having a large CTE via a low toughness Ag sintered body. For this reason, large thermal stress may be concentrated on the transistor and the junction due to CTE mismatch between the junction members.

  Therefore, when a temperature cycle test (a test in which a low temperature state and a high temperature state are repeated at a constant interval) is applied to a laminated structure imitating such a power module, as shown in FIG. ), Bonding layers (IMC: intermetallic compound / intermetallic compound), etc. may be cracked. Therefore, in the semiconductor device (power module) as disclosed in Patent Document 1, the durability against heat stress (heat fatigue resistance) is not sufficient.

In particular, when using a next-generation device made of SiC, GaN, Ga ₂ O ₃ or the like, it is expected to be used in a higher temperature range (for example, 150 ° C. or more) than conventional. In addition, the power modules are being further miniaturized to improve the degree of freedom in mounting and to reduce the weight, and the operating temperature of the devices tends to further increase. Therefore, there is a need for a power module that can ensure high reliability even when used at high temperatures.

  The present invention has been made in view of such circumstances, and it is an object of the present invention to provide a power module which can improve the reliability.

  As a result of earnest studies to solve this problem, the inventor of the present invention conceived to increase the reliability of a double-sided cooling type power module by using a spacer required for wire bonding. While realizing this and developing it, the present invention described below has been completed.

Power Module
(1) In the present invention, a transistor which is a heat source, a first spreader joined to a first electrode of the transistor through a first bonding layer, and a second bonding layer to a second electrode of the transistor The spacer includes: a spacer which is joined and which secures a space of a bonding wire joined to the third electrode of the transistor; and a second spreader joined to the other surface side of the spacer via the third joining layer, Is a power module made of a conductive material having a thermal expansion coefficient difference of 0.5 to 7 ppm / K with the semiconductor material constituting the transistor.

(2) The conductive material constituting the spacer according to the present invention has a thermal expansion coefficient (CTE) close to that of the semiconductor material constituting the transistor, and their CTE difference is small. Therefore, the thermal stress caused by the CTE mismatch generated between the spacer and the transistor is also small. As a result, in the power module of the present invention, generation of cracks and the like due to thermal stress is suppressed in the junction between the spacer and the transistor (second junction layer) and the transistor.

  Next, while the CTE difference between the spacer and the transistor is reduced, the CTE difference between the spacer and the second spreader is larger than in the prior art. For this reason, thermal stress due to CTE mismatch may occur between the spacer and the second spreader. However, this portion is at a lower temperature than between the spacer and the transistor. For this reason, the thermal stress generated between the spacer and the second spreader becomes relatively small, and the occurrence of cracks is also suppressed.

  Even if a crack is generated, the crack is preferentially generated in the spacer and the third bonding layer, and the generation of the crack in the transistor and the second bonding layer is suppressed. In other words, in the power module of the present invention, the origin of the crack is shifted (shifted) from the vicinity of the second junction layer where the transistor is located to the vicinity of the third junction layer where the second spreader is located.

  These actions act synergistically to prevent transistor damage and maintain the function of the power module. Thus, the power module of the present invention has a long life and exhibits high reliability.

<< Others >>
Unless otherwise stated, “x to y” as used herein includes the lower limit x and the upper limit y. Ranges such as “a to b” may be newly established as new lower limit values or upper limit values for arbitrary numerical values included in various numerical values or numerical ranges described in the present specification.

It is a schematic cross section which shows the principal part of the power module which is an Example. It is a microscope picture of the crack which arose in the laminated structure which imitated the conventional power module.

  One or more components arbitrarily selected from the present specification may be added to the components of the present invention. The contents described herein may apply not only to the power module of the present invention, but also to a method of manufacturing the same. The component relating to the “method” can also be a component relating to the “article”.

<< transistor >>
At least one of the devices included in the power module of the present invention is a transistor provided with a third electrode to be wire-bonded. Such transistors include power devices such as thyristors, bipolar transistors, MOSFETs and IGBTs. In the representative IGBT, the third electrode in the present invention corresponds to the gate electrode (G), the first electrode to the collector electrode (C), and the second electrode to the emitter electrode. The power module of the present invention may include various devices (such as diodes) and control circuits in addition to the above-described transistors.

  Incidentally, the semiconductor material used for the transistor has a CTE of about 2 to 5 and further about 3 to 6 ppm / K. For example, Si (CTE: 3 ppm / K), SiC (CTE: 3.7 ppm / K), GaN (CTE: 5.5 ppm / K) or the like is used as the semiconductor material.

"Spacer"
The spacer according to the present invention is preferably made of a conductive material having a CTE difference of 0.5 to 7 ppm / K, 1 to 6 ppm / K, and more preferably 2 to 5 ppm / K with the semiconductor material constituting the transistor described above. If the CTE difference is too large, suppression of thermal stress in the vicinity of the second bonding layer will be insufficient. The CTE of the spacer is preferably an intermediate value between the CTE of the transistor and the CTE of the second spreader because thermal stress in the vicinity of the second bonding layer can be suppressed. Therefore, the CTE of the conductive material is preferably, for example, 4.4 to 10 ppm / K, and more preferably 5 to 8 ppm / K.

  The electrical conductivity (conductivity) of the conductive material is at least higher than that of the semiconductor material, and Cu-based materials (pure Cu, Cu alloys, Cu compounds, etc.), Al-based materials (pure Al, Al alloys, Al compounds, etc.) The closer to the electrode material, the better.

  There are various conductive materials having a relatively small CTE, but it is preferable to use, for example, a composite material having a Cu alloy, Cu or a Cu alloy (simply referred to as "Cu-based metal") as a matrix. Examples of Cu alloys include Cu-W alloys and Cu-Mo alloys. Further, as a composite material, there are Cu-diamond, Cu-Si, Cu-C and the like.

  The thickness of the spacer is set according to the transistor (spreader) and the specifications (material, thickness, etc.) of each bonding layer, and for example, 0.1 mm to 2 mm, and further 0.5 mm to 1.5 mm or so preferable. If the thickness is too large, thinning of the power module can not be achieved, and if the thickness is too small, reduction of thermal stress acting in the vicinity of the second bonding layer becomes insufficient.

<< First bonding layer / Second bonding layer >>
The first bonding layer and the second bonding layer in contact with the heat generating source transistor (also referred to simply as “device”) are required to be made of a high melting point material which does not melt even at the highest temperature that the device can reach during operation. . On the other hand, the temperature at which such a bonding layer is formed (bonding temperature) is required to be at least smaller than the heat resistance temperature of the device. As such a bonding layer, there is, for example, an intermetallic compound layer or a metal sintered layer.

(1) The intermetallic compound layer is prepared, for example, by reacting a low melting point metal and a high melting point metal metallized on at least one of the surfaces to be joined, and an intermetallic compound (IMC) having a high melting point than the low melting point metal. Is obtained by generating. Such IMC bonding is referred to as solid-liquid interdiffusion bonding or simply "SLID (Solid Liquid Interdiffusion) bonding".

  The combination of the low melting point metal and the high melting point metal (and the composition of the intermetallic compound) involved in the SLID bonding is selected in consideration of the heat resistance temperature of the power module, the heating temperature during the bonding process, the thermal expansion coefficient and the like. The low melting point metal includes, for example, Sn, In, Ga, Pb, Bi, Zn, etc., and their alloys. As the high melting point metal, there are Ni, Cu, Ti, Mo, W, Si, Cr, Mn, Co, Zr, Nb, Ta, Ag, Au, Pt, etc., and their alloys.

  As an example, Sn (melting point: about 230 ° C.) and Ni (melting point: about 1450 ° C.) or Cu (melting point: about 1085 ° C.) may be combined. For example, when the Sn layer and the Ni layer are brought into contact and heated at about 350 ° C. for about 5 minutes, an intermetallic compound layer made of nickel tin (NiSn / melting point: about 795 ° C.) is obtained. Thus, a high melting point bonding layer can be obtained while suppressing the bonding temperature. Of course, the combination of high melting point metal / low melting point metal may be Cu / Sn, Ag / Sn, Pt / Sn /, Au / Sn or the like.

(2) The sintered metal layer can be obtained, for example, by sintering metal nanoparticles interposed between bonding surfaces. The metal nanoparticles are sintered at low temperature due to their very high surface activity, but the sintered body itself exhibits the high melting point inherent to the metal. Therefore, also when metal nanoparticles are used, a high melting point bonding layer can be formed while suppressing the bonding temperature. The metal nanoparticles are made of, for example, Ag and Cu. The metal nanoparticles are usually coated with a protective layer made of an organic substance, an oxide or the like that decomposes and disappears at the heating temperature at the time of bonding in order to prevent its aggregation.

(3) By the way, although intermetallic compounds and metal sintered bodies are excellent in high temperature resistance, ductility and toughness are not necessarily sufficient. Therefore, in order to ensure the reliability of the power module, it is preferable that the bonding layer made of an intermetallic compound or a metal sintered body be in contact with a layer or member excellent in stress relaxation property and thermal fatigue resistance.

  Therefore, the first bonding layer is made of, for example, an intermetallic compound or a sintered metal, and a high temperature resistant layer bonded to the first electrode surface, and an Al-based metal (pure Al or And a relaxation layer made of an Al alloy). The other surface side (first spreader side) of the relaxation layer may be a solder layer or the like in consideration of the thickness of the relaxation layer, the heat dissipation from the first spreader, etc. It is preferable that it is a high temperature resistant layer formed of an intermetallic compound or a sintered metal, as in the one side (transistor side).

  Pure Al has a total content of impurity elements other than Al of less than 1% by mass (purity 99% (2N) Al), less than 0.1% by mass (purity 99.9% (3N) Al) and further 0.01 Less than% by mass (purity 99.99% (4N) Al) or the like.

  In the present specification, a material in which the total amount of elements other than Al (including the impurity element and “alloy element”) is 1 mass% or more is referred to as “Al alloy”. The relaxation layer need not have high strength as long as sufficient ductility and toughness are ensured. Therefore, the total amount of the alloying elements is preferably 5% by mass or less, and more preferably 3% by mass or less. For this reason, it is also possible to use, for example, a 3000-series Al alloy or the like having a relatively small amount of alloy elements for the relaxation layer, as long as it is other than pure Al.

  The thickness of the relaxation layer is set in accordance with the specifications (material, thickness, etc.) of the transistor, the spreader, etc., but is preferably about 5 to 200 μm, more preferably about 10 to 500 μm. If the thickness is too large, thinning of the power module can not be achieved, and if the thickness is too small, securing of the stress relaxation property and thermal fatigue resistance by the relaxation layer becomes insufficient.

  The second bonding layer is preferably made of an intermetallic compound layer or a metal sintered layer, as in the high temperature resistant layer of the first bonding layer. The second bonding layer may be only a high-temperature resistant layer (single layer) without a relaxation layer because the second bonding layer bonds a transistor and a spacer with a small CTE difference.

<< Third bonding layer >>
The third bonding layer is located between the relatively thick spacer and the second spreader on the heat dissipation side, and is usually on the lower temperature side than the first bonding layer (and the second bonding layer). For this reason, the third bonding layer may be a solder layer or the like in addition to a high temperature resistant layer (for example, an intermetallic compound layer, a metal sintered layer) similar to the second bonding layer, a composite bonding layer similar to the first bonding layer. .

  However, in the power module of the present invention, the starting point of the crack is moved from the vicinity of the second bonding layer to the vicinity of the third bonding layer. Therefore, it is preferable that the third bonding layer be composed of a composite bonding layer excellent in stress relaxation property and thermal fatigue resistance, or a solder layer softer than an intermetallic compound layer or the like.

Spreader
The spreader is a member that receives heat from the transistor and conducts heat to the outside. The spreader in the present invention does not have to be a dedicated member (heat sink) for diffusing heat. In other words, a lead frame having another function, an electrode, a cooler, and the like are also included in the spreader in the present invention.

  Such a spreader (at least one of the first spreader and the second spreader) is often made of pure Cu (oxygen-free copper) or a Cu alloy (also simply referred to as "Cu-based metal"). Since the spreader made of a Cu-based metal has a CTE considerably larger than that of a device made of a semiconductor, a large thermal stress can be exerted between the two depending on the CTE difference. However, in the power module of the present invention, the thermal stress is greatly relieved by the spacer made of Al-based metal, the relaxation layer in the first bonding layer, etc., and the fatigue due to the thermal stress is also greatly suppressed.

"Constitution"
A cross-sectional view schematically showing a double-sided cooling structure type power module M according to an embodiment of the present invention is shown in FIG. The power module M includes an IGBT transistor 1, a spacer 2, a spreader 31 (first spreader) and a spreader 32 (first spreader) as a lead frame, a collector electrode (first electrode) of the transistor 1, and a spreader 31. Bonding layer 41 (first bonding layer) for bonding, a bonding layer 42 (second bonding layer) for bonding the emitter electrode (second electrode) of the transistor 1 and the spacer 2, and bonding for bonding the spacer 2 and the spreader 32. And a layer 43 (third bonding layer). The gate electrode of the transistor 1 is bonded to the signal terminal 52 by the bonding wire 51.

  The transistor 1 is made of SiC (CTE: 3.7 ppm / K), the spacer 2 is made of a Cu-W alloy (CTE: 6 to 7.1 ppm / K), the spreaders 31 and 32 are oxygen free copper (high in purity 3N) Made of pure copper).

  The bonding layer 41 is a three-layer (composite) having a relaxation layer 413 made of pure Al (purity: 2 N) and intermetallic compound layers 411 and 412 (high temperature resistant layers) made of NiSn bonded to each surface side thereof. Bonding layer). The bonding layers 42 and 43 are made of an intermetallic compound (NiSn). The bonding layer 43 can also be a solder layer made of Sn-Ag-Cu system or the like.

  Incidentally, in this embodiment, the thickness of the transistor 1 is 100 μm, the thickness of the spacer 2 is 1.15 μm, the thickness of the spreaders 31 and 32 is 2 mm, the thickness of the bonding layer 41 is 110 μm (of which the thickness of the relaxation layer 413 is The thickness of the bonding layer 42 is 5 μm, and the thickness of the bonding layer 43 is 100 μm.

Bonding
The bonding layer 41 is formed as follows. First, a bonding sheet in which Ni and Sn are sequentially metallized is prepared on both sides of a pure Al thin plate (foil) to be the relaxation layer 413. This bonding sheet is interposed between the collector electrode surface of the transistor 1 in which Ni is metallized on each surface and the bonding surface of the spreader 31. By heating and holding in this state, the Ni and Sn between the surfaces to be joined cause a SLID reaction, and the intermetallic compound layers 411 and 412 are formed. Thus, the bonding layer 41 formed of the composite bonding layer including the intermetallic compound layers 411 and 412 is formed on both surface sides of the relaxation layer 413 made of pure Al.

  The bonding layer 42 is formed as follows. First, Ni and Sn are sequentially metallized on one side of the spacer 2 (the lower surface in FIG. 1). Also on the emitter electrode surface of the transistor 1, Ni is metallized. Next, one surface of the spacer 2 is laminated on the emitter electrode surface of the transistor 1. By heating and holding in this state, the Ni and Sn between the surfaces to be joined cause a SLID reaction, and the joining layer 42 made of an intermetallic compound is formed. In addition, it is preferable that the bonding layers 41 and 42 heat the laminated body which laminated | stacked each member at once, and are simultaneously formed.

  The bonding layer 43 forms the bonding layers 41 and 42 by SLID bonding, and then uses Sn—Ag—Cu solder paste between the other surface (upper surface in FIG. 1) of the spacer 2 and the bonding surface of the spreader 32. It forms by heating with a reflow furnace etc. in the state which made it intervene. The bonding layer 43 may be a composite bonding layer similar to the bonding layer 41.

M power module 1 transistor 2 spacer 31 (first) spreader 32 (second) spreader 41 (first) bonding layer 42 (second) bonding layer 43 (third) bonding layer

Claims (6)

A transistor that is a heat source,
A first spreader joined to a first electrode of the transistor via a first bonding layer;
A spacer which is joined to the second electrode of the transistor through the second joining layer and which secures a space of a bonding wire joined to the third electrode of the transistor;
A second spreader joined to the other surface of the spacer via a third bonding layer,
The said spacer is a power module which consists of an electrically-conductive material whose thermal expansion coefficient difference with the semiconductor material which comprises the said transistor is 0.5-7 ppm / K.   The power module according to claim 1, wherein the conductive material is any one of a Cu alloy, a Cu compound, a Cu or a composite material having a Cu alloy as a matrix.   The power module according to claim 1, wherein the second bonding layer comprises an intermetallic compound layer or a metal sintered layer. The first bonding layer or the third bonding layer is
A high temperature resistant layer formed of an intermetallic compound or a sintered metal and joined to the first electrode surface;
A relaxation layer made of pure Al or an Al alloy and joined to the other surface of the high temperature resistant layer;
The power module according to any one of claims 1 to 3, comprising a composite bonding layer having:   The power module according to any one of claims 1 to 3, wherein the third bonding layer comprises a solder layer.   The power module according to any one of claims 1 to 5, wherein at least one of the first spreader and the second spreader is made of pure Cu or a Cu alloy.