JP2007288095A - Semiconductor device and spot friction agitation joining device used for manufacturing it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a spot friction agitation joining device in which thickness, shape and material of a joining member are optimized to acquire joining strength of high reliability, for a semiconductor having an assembly structure wherein a lead frame is joined to an insulating substrate or a semiconductor chip. <P>SOLUTION: The semiconductor device has the assembly structure wherein joining leg pieces 4a and 5a of lead frames 4 and 5 are piled on an insulating substrate 2 and the semiconductor chip 3 mounted on the insulating substrate 2, and a rotary tool 7 is pushed into here for spot friction agitation joining. Related to the joining leg piece of the lead frame, a thickness t1 of its spot joining part is made thin to 500 μm or less to promote plastic fluidization at friction agitation joining. Further, a thickness t2 of a conductor pattern 2b of the insulating substrate 2 is made thick to 0.5-3 mm to expand a range of plastic flow at friction agitation joining, and the rotary tool penetrating a region of the conductor pattern is prevented from hit against a ceramic substrate 2a of the insulating substrate. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention is directed to a power semiconductor device such as an IGBT module. A semiconductor device in which a lead frame is spot-bonded to a conductor pattern of an insulating substrate or an upper surface electrode of a semiconductor chip by a friction stir welding method, and spot friction applied to the bonding The present invention relates to a stir welding apparatus.

  First, an assembly structure of a conventional semiconductor device is shown in FIG. 7 taking an IGBT module as an example. In the figure, 1 is a heat dissipating metal base, 2 is an insulating substrate (for example, a direct conductive pattern 2b, 2c formed by directly bonding a copper foil having a thickness of about 0.2 to 0.25 mm on both front and back surfaces of a ceramic substrate 2a) (Copper Bonding substrate) 3 is a semiconductor chip (IGBT) mounted by soldering on the conductor pattern 2b of the insulating substrate 2, and 3a is a metal film (aluminum or solder) having a thickness of about 10 μm deposited on the electrode surface of the semiconductor chip 3. A metallized layer in which the surface of the aluminum film is plated with Ni—Au so as to enhance the wettability), 4 straddles between the upper surface electrode (IGBT emitter electrode) of the semiconductor chip 3 and the conductor pattern 2 b of the insulating substrate 2. Bonded and wired lead frames 5 are lead frames (external lead-out terminals) that are joined to the conductor pattern 2b of the insulating substrate 2 and drawn upward. Here, the lead frames 4 and 5 are L-shaped at both ends of a foil-like conductor piece made of a copper alloy having a width of 3 mm and a thickness of about 0.5 to 1.5 mm, depending on the model and current capacity. The joining leg pieces 4a and 5a are formed, and the joining leg pieces 4a and 5a are superposed on the conductor pattern 2b of the insulating substrate 2 and the upper surface side metal film 3a of the semiconductor chip 3 and are reflow soldered.

  By the way, in the assembly structure in which the lead frames 4 and 5 are solder-bonded to the insulating substrate 2 and the semiconductor chip 3 as described above, an intermetallic compound is generated at the bonding interface due to the thermal history at the time of solder bonding, and based on this, the bonding strength is increased. Deteriorated and reliability and life are reduced. In addition, due to the difference in coefficient of linear expansion between the ceramic and silicon chips and the lead frame (copper material), cracks occur in the solder layer due to the stress repeatedly applied by the temperature cycle at the joint interface of the solder joint, and the power cycle resistance There is a problem such as lowering.

Therefore, in recent years, as a bonding method that does not use solder, an ultrasonic bonding method has been widely used for wire bonding and lead frame bonding. Recently, a friction stir welding method in which solid-phase joining between metals is performed with a small amount of heat input has attracted attention, and research on applying this friction stir welding method to the joining of electronic circuit components (for example, see Patent Document 1) is also progressing. .
In this friction stir welding method, two metal plates to be joined are butted or overlapped, and then a pin-shaped rotating tool (made of cemented carbide similar to a tool) is pushed into the joint, This is a method in which metal materials are softened and plastically flowed by frictional heat generated by rotation, and the metal plates to be joined are solid-phase bonded. A micro-plate in which thin plates (thickness 0.2 mm) are overlapped and joined by a rotating tool. An experimental example of the spot friction stir welding method has also been reported (see Non-Patent Document 1).
JP 2003-25078 A (page 1-2) Yoshikawa Katsuyuki, “Microspot Friction Stir Welding”, Japan Welding Society Microjoining Research Committee, Proceedings of the 11th Symposium on Microjoining and Mounting Technology in Electronics, 2005, p421-424

  By the way, the above-mentioned microspot friction stir welding method is applied to the assembly of the semiconductor device of FIG. 7, and a joining experiment conducted by the inventors on the method of joining the lead frames 4 and 5 to the insulating substrate 2 and the semiconductor chip 3, According to the knowledge obtained through the examination, it has become clear that there are the following problems as problems to ensure high power cycle resistance in combination with the power supply and heat transfer required for the power semiconductor device.

  That is, the size of the joining leg pieces 4a and 5a formed on the lead frames 4 and 5 to be incorporated in the semiconductor device is about 3 mm, and the lead frame is pushed into the minute area of the leg pieces to place the lead frame on the conductive pattern of the insulating substrate 2. 2b, in order to spot friction stir weld the metal film 3a formed on the electrode surface of the semiconductor chip 3, it is necessary to use a thin rotary tool having a tip diameter of φ1 to 2 mm as the joining tool.

However, with a rotating tool with a small tip diameter, the amount of heat generated at the joint is very small, and when applied to a conventional lead frame with a plate thickness of 0.5 to 1.5 mm, plastic fluidization at the joint interface will occur. It is difficult to secure a reliable joint strength without sufficiently progressing.
Further, in order to expand the plastic flow range, when the rotary tool is pushed deeply so as to reach the area of the lower mating member that is overlapped with the lead frame, the rotary tool that rotates at a high speed causes the conductor pattern of the insulating substrate 2 to move. (Thickness of about 0.2 mm), penetrating through the metallized metal film (thickness of about 10 μm) on the surface of the semiconductor chip 3 and directly hitting the ceramic substrate 2a of the insulating substrate 2 and the silicon chip of the semiconductor chip 3 The ceramic substrate 2a and the semiconductor chip 3 may be damaged.

  In addition, in the lead frame friction stir welding process, it is necessary to use a fixing jig to fix the lead frame joining leg pieces superimposed on the insulating substrate and the semiconductor chip. Since other circuit parts are lined up around the part, it is actually difficult to fix the lead frame joint leg piece to the predetermined joint position with a fixing jig so as not to interfere with the rotating tool. is there.

  The present invention has been made in view of the above points, and is based on the above-mentioned consideration results for a semiconductor device having an assembly structure in which a lead frame is bonded to an insulating substrate or a semiconductor chip by a spot friction stir welding method. An object of the present invention is to provide a structure of a semiconductor device capable of ensuring a highly reliable joint strength by optimizing the thickness, shape and material conditions of a member, and to provide a spot friction stir welding apparatus applied to the lead frame joining. And

To achieve the above object, according to the present invention, an insulating substrate, a semiconductor chip mounted on the insulating substrate, a conductor pattern of the insulating substrate, an upper surface electrode of the semiconductor chip, and a heat spreader bonded to the upper surface electrode of the semiconductor chip A metal film having a lead frame bonded to any one of the upper surfaces of the metallization and metallized on the conductor pattern of the insulating substrate, which is a bonding partner, and the upper electrode surface of the semiconductor chip, the bonding leg piece formed on the end of the lead frame Or a semiconductor device in which a heat spreader is superimposed and a rotary tool is pushed in from above to spot bond between the conductor pattern of the lead frame / insulating substrate and between the metal film of the lead frame / semiconductor chip by a friction stir welding method.
(1) Regarding the joint leg piece of the lead frame, the thickness of the spot joint portion is reduced to 500 μm or less, and plastic fluidization is promoted when a rotary tool is pushed into the joint portion and friction stir welding is performed. (Claim 1).
(2) The thickness of the conductor pattern of the insulating substrate, which is the lead frame joining partner member, is increased to 0.5 to 3 mm to expand the plastic flow range during friction stir welding and penetrate into the conductor pattern region. The rotating tool is prevented from hitting the ceramic substrate of the insulating substrate (claim 2).
(3) The thickness of the metal film metallized on the electrode surface of the semiconductor chip to which the lead frame is bonded is increased to 30 μm or more, and the rotating tool hits the silicon chip in conjunction with the expansion of the plastic flow range as in the previous item (2). (Claim 3).
(4) For a lead frame made of a copper material, at least spot joint portions of the joining leg pieces are formed of an aluminum material having a higher plastic flowability than the copper material.
(5) For a lead frame having a plate thickness of 0.5 to 1.5 mm as a wiring component, leave a wall thickness of 30 to 200 μm on the top surface side of the spot joint portion of the joining leg piece formed on the tip side. A concave depression is formed to increase plastic fluidization of the joint during friction stir welding (Claim 5).
(6) A thin rotary tool with a tip diameter of 0.5 to 2 mm is used as a lead frame joining tool, and spot welding is performed on a plurality of joint leg pieces with the rotary tool to achieve the required electrical conductivity, heat transfer, and joining. Strength is ensured (Claim 6).
(7) With respect to the lead frame joint leg piece of (6) above, a stress relaxation hole is formed at a site deviated from the spot joint part, and due to the difference in the linear expansion coefficient of the material, Reduction of thermal stress applied to the joint interface is achieved (claim 7).
(8) Further, the friction stir welding apparatus applied to the spot welding of the lead frame is provided with a support mechanism that presses the joining leg piece of the lead frame into the joining position in combination with the rotary tool, and the leg of the lead frame is supported by this supporting mechanism. With the piece held in place, the rotary tool is pushed into the joint so that friction stir welding is performed (claim 8).

  According to the above configuration, the joining leg of the lead frame is superimposed on the conductive pattern of the insulating substrate or the metal film formed on the electrode surface of the semiconductor chip in the assembly process of the semiconductor device, and the rotary tool with a small tip diameter is placed thereon. When pushing and spot friction stir welding is performed, plastic fluidization of the joint is promoted, and the rotating tool is prevented from being damaged by hitting the ceramic substrate of the insulating substrate and the silicon chip of the semiconductor element, thereby ensuring a highly reliable joint strength. it can.

  In addition, the friction stir welding apparatus is provided with a support mechanism that presses the lead leg of the lead frame into the joint position in combination with the rotary tool, so that the joint leg of the lead frame is not restricted by the assembly structure of the semiconductor device. Friction stir welding can be performed efficiently while holding the piece in place, and application to semiconductor devices of different models is also possible.

Hereinafter, embodiments of the present invention will be described based on examples shown in FIGS. In addition, in the figure of an Example, the same code | symbol is attached | subjected to the member corresponding to FIG. 7, and the description is abbreviate | omitted.
First, an overall structure of a semiconductor device assembled by joining a lead frame by a spot friction stir welding method, and a schematic diagram of a lead frame joined portion spot spot stir welded using a rotary tool are shown in FIGS. In addition, FIG. 2 shows the spot joints of the lead frame. FIG. 1A is an assembly structure diagram corresponding to the semiconductor device shown in FIG. 7. In the assembly structure of FIG. 7, the joining leg pieces 4a. 5a is soldered to the upper surface of the insulating substrate 2 and the semiconductor chip 3, whereas in the structure shown in FIG. 5a is superimposed on the conductor pattern 2a of the insulating substrate 2 and the upper surface side metal film 3a of the semiconductor chip 3, and a rotary tool 7 rotating at a high speed is pushed in from above to perform spot friction stir welding.

  Here, the rotary tool 7 is configured by a pin 7a as shown in FIG. 1A or a structure in which a small diameter pin 7a and a large diameter shoulder 7b are combined as shown in FIG. 1B. The tip diameter of the pin 7a is φ0.5 to 2 mm. Then, when the rotating tool 7 rotating at high speed is pushed onto the joining leg piece 5a from above as shown in the drawing while the pressing force F is applied, the surrounding joining members are softened, plastically flowed and stirred by frictional heat. After that, when the rotary tool 7 is pulled up, it is well known that the joint leg piece 5a of the lead frame 5 and the conductor pattern 2b of the insulating substrate 2 are spot-bonded (solid phase bonding). The part is denoted by reference numeral 8. 1B and 1C show the bonding between the lead frame 5 and the conductor pattern 2b of the insulating substrate 2, the bonding between the lead frame 4 and the insulating substrate 2 and the semiconductor chip 3 is also possible. Spot friction stir welding is performed by pushing the rotary tool 7 in the same manner as described above.

On the other hand, with respect to the semiconductor device having the above structure, in the embodiment of FIG. 1, the thickness t1 of the joining leg pieces 4a and 5a of the lead frames 4 and 5, the thickness t2 of the conductor pattern 2a of the insulating substrate 2, and the semiconductor Spot friction stir welding is performed after the thickness t3 of the metal film 3a formed on the upper surface of the chip 3 is specified as follows.
First, with respect to the joining leg pieces 4a and 5a formed by bending at the ends of the lead frames 4 and 5 made of a copper alloy having a thickness of about 0.5 to 1.5 mm, the joining leg pieces are pushed by pushing the rotary tool 7. In order to promote the plastic fluidization of the joined portion in a short time, the thickness is reduced so that the thickness t1 of the joined leg pieces 4a and 5a is 500 μm or less (in the range of 10 to 500 μm) by pressing or the like.

  On the other hand, the conductor pattern 2b of the insulating substrate 2 to be joined to the lead frames 4 and 5 has a thickness of 0.5 to 3 mm (conventional product has a thickness of about 0.2 mm). As shown in FIG. 1C, the pin 7a of the rotary tool 7 penetrates the conductor pattern 2b beyond the bonding interface to expand the plastic flow range, and on the other hand, the tip of the pin of the rotary tool 7 is the insulating substrate. 2 is prevented from being damaged by hitting the ceramic substrate 2a (see FIG. 1).

Also, the metal film 3a metallized on the upper surface electrode of the semiconductor chip 3 (the thickness of the conventional product is about 10 μm) is increased to 30 μm or more, and the friction stir welding shown in FIG. Along with the expansion of the plastic flow range at the time, the pin 7a of the rotary tool 7 is prevented from being damaged by hitting the silicon chip of the semiconductor chip 3.
When spot friction stir welding is performed on the lead frame to the mating member under the above conditions, the joint frame 5a (outer dimension: about 3 mm) of the lead frame 5 shown in FIG. Separately, a rotary tool 7 having a tip diameter of 0.5 to 2 mm is pushed in and joined. In addition, the code | symbol 9 in a figure represents the spot joining part, By carrying out spot joining of the joining leg piece 5a of the lead frame 5 in multiple places in this way, the electroconductivity required as a power semiconductor device, heat dissipation, And bonding strength is obtained.

  Next, some application examples of the joining leg piece of the lead frame are shown in FIGS. First, the Example corresponding to Claim 4 of this invention is shown to Fig.3 (a)-(d). In this embodiment, at least a spot friction stir welding portion is formed of the aluminum material 10 with respect to the joining leg piece 5a of the lead frame 5 made of a copper material, and FIGS. The aluminum material 10 is filled in a hole drilled in the surface area of the leg piece to be the spot joint portion. 3C, the joining leg piece is formed of an aluminum material 10, and FIG. 3D shows that the aluminum material 10 is attached to the back surface (joining surface) of the joining leg piece 5a. In other words, aluminum has a slightly lower electrical conductivity and thermal conductivity than copper, but has a low softening temperature and high plastic fluidity, and has the property of being easy to friction stir weld compared to copper. Therefore, by forming the spot friction stir contact portion with an aluminum material as shown in the drawing, the lead frame can be reliably spot friction stir contact with a small amount of heat input.

  Next, the Example corresponding to Claim 5 of this invention is shown to Fig.4 (a), (b). That is, as described above, a lead frame having a thickness of 0.5 to 1.5 mm is thinned by increasing the thickness t1 of the joining leg piece to increase the plastic fluidity by friction stir welding. However, in this embodiment, the entire area of the joining leg piece 5a is not thinned, and instead the portion corresponding to the spot joining portion is left with a thickness of t4 = 30 to 200 μm and the rotary tool 7 is pushed into the upper surface side thereof. A concave depression 5b is formed. Thereby, the plastic fluidity | liquidity of the junction part by friction stir welding can be improved, ensuring the electrical conductivity, heat conductivity, and mechanical rigidity which are requested | required as components of a lead frame.

  5 (a) and 5 (b) show an embodiment corresponding to claim 7 of the present invention. In this embodiment, spot joining is performed on a plate surface along a portion that does not overlap with the arrangement of the spot joining portions 9 with respect to the joining leg pieces 5a (see FIG. 2) of the lead frame 5 that are divided into a plurality of places. A slit-shaped hole 5c is formed. According to this structure, the hole 5c absorbs and relaxes the shear stress in the surface direction applied to the spot joint 9 due to the thermal cycle of the semiconductor device, and the reliability of the joint strength can be improved. The stress relaxation hole 5c is not limited to the slit-shaped hole, and the same effect can be obtained even if round holes are dispersed and drilled between the arrangements of the spot joints 9.

  Next, FIG. 6 shows a schematic configuration of a spot friction stir welding apparatus corresponding to claim 8 of the present invention. This spot friction stir welding apparatus 11 is provided with a function of pressing and fixing the joining leg piece 5a of the lead frame 5 to a predetermined joining position in the assembling process of the semiconductor device. The equipped drive unit 11a includes a support mechanism for the lead frame 5 including a pair of linear guides 11b extending in the vertical direction with the rotary tool 7 sandwiched from the left and right and a pressing leg 11c.

  In the spot friction stir welding process of the lead frame, the spot friction stir welding apparatus 11 is lowered from above in a state where the joining leg pieces 5a of the lead frame 5 are superimposed on the conductor pattern 2b of the insulating substrate, and the pressing leg is pressed. 11c is abutted against the joining leg piece 5a of the lead frame and pressed into a predetermined position. Subsequently, the linear guide 11b is controlled to be lowered, and the rotating tool 7 rotating at high speed is pushed into the joining leg piece 5a to perform spot friction stir welding (see FIGS. 1B and 1C).

By using this spot friction stir welding apparatus 11, spot friction stir welding can be performed efficiently by pressing the lead frame to a predetermined joining position without interfering with circuit components arranged on the insulating substrate of the semiconductor device. .
In the above embodiment, the case where the lead frame 4 is friction stir welded to the upper surface side metal film 3a of the semiconductor chip 3 has been described as an example, but the present invention is not limited to this. For example, after the heat spreader is bonded to the upper surface side electrode of the semiconductor chip 3, the lead frame 4 may be friction stir bonded to the upper surface of the heat spreader.

By bonding a heat spreader to the upper surface of the semiconductor chip 3, heat can be radiated from the upper surface of the semiconductor chip. Further, since the lead frame 4 is friction stir welded to the heat spreader, damage to the semiconductor chip 3 can be avoided.
As the material of the heat spreader, molybdenum or tungsten having a thermal expansion coefficient close to that of the semiconductor chip as compared with copper or aluminum as the material of the lead frame is preferable. Although it is difficult to join molybdenum or tungsten to copper or aluminum due to differences in thermal expansion coefficient, melting point, etc., good joining can be obtained by the friction stir welding method.

BRIEF DESCRIPTION OF THE DRAWINGS It is a block diagram of the semiconductor device by the Example of this invention, (a) is the assembly structure figure of the whole semiconductor device, (b), (c) is a lead frame using a rotary tool of a different shape, Enlarged cross-sectional view of the joint that schematically represents the stir welded state FIG. 1 is a perspective view showing a spot bonding state of a lead frame in FIG. FIG. 4 is a structural diagram of an application example in which the spot joint portion of the lead frame is formed of an aluminum material, where (a) is a perspective view of the lead frame, (b) is a cross-sectional view of (a), and (c) and (d) are respectively Sectional view of an application example different from (a) FIG. 2 is a structural diagram of an application example in which a concave depression is formed in a spot joint portion of a lead frame, (a) is a perspective view of the lead frame (b) is a cross-sectional view of (a) FIG. 4 is a structural diagram of an application example in which stress relaxation holes are formed in the joint leg pieces of the lead frame, where (a) is a perspective view of the lead frame and (b) is a cross-sectional view of (a). Schematic configuration diagram of a spot friction stir welding apparatus according to an embodiment of the present invention Assembly structure diagram of conventional semiconductor device with lead frame soldered

Explanation of symbols

2 Insulating substrate 2a Ceramic substrate 2b Conductive pattern 3 Semiconductor chip 3a Metal film 4, 5 Lead frame 4a, 5a Joining leg piece 5b Concave depression 5c Stress relaxation hole 7 Rotating tool 8 Friction stir welding 9 Spot welding 10 Aluminum material 11 Spot Friction stir welding device 11c

Claims (8)

An insulating substrate, a semiconductor chip mounted on the insulating substrate, and a lead frame bonded to any one of the conductor pattern of the insulating substrate, the upper surface electrode of the semiconductor chip, and the upper surface of the heat spreader bonded to the upper surface electrode of the semiconductor chip; The joining leg piece formed at the end of the lead frame is superimposed on the conductor pattern of the insulating substrate that is the mating partner, the metal film metalized on the upper electrode surface of the semiconductor chip, or the heat spreader, and the rotary tool is pushed in from there. In the semiconductor device in which the lead frame / insulating substrate conductor pattern and the lead frame / semiconductor chip metal film are spot-bonded by the friction stir welding method,
The semiconductor device according to claim 1, wherein a thickness of the spot joint portion in the joint leg piece is set to 500 μm or less. 2. The semiconductor device according to claim 1, wherein the thickness of the conductor pattern of the insulating substrate to which the lead frame is bonded is set to 0.5 to 3 mm. 2. The semiconductor device according to claim 1, wherein the thickness of the metal film formed on the electrode surface of the semiconductor chip to which the lead frame is bonded is set to 30 [mu] m or more. 2. The semiconductor device according to claim 1, wherein at least spot joint portions of the joining leg pieces are formed of an aluminum material with respect to the lead frame made of copper. 2. The semiconductor device according to claim 1, wherein a concave depression is formed on the upper surface side of the spot joint portion of the lead frame having a thickness of 0.5 to 1.5 mm, leaving a thickness of 30 to 200 [mu] m. A semiconductor device formed. 2. The semiconductor device according to claim 1, wherein a rotating tool having a tip diameter of 0.5 to 2 mm is used as a lead frame joining tool, and a plurality of joint leg pieces are spot-joined. 6. The semiconductor device according to claim 5, wherein a stress relaxation hole is formed in a portion deviated from the spot joint portion with respect to the joint leg piece of the lead frame. A spot used for friction stir welding of the lead frame to a joining partner member, comprising a support mechanism that is combined with a rotary tool and presses a joining leg piece of the lead frame into a predetermined joining position. Friction stir welding device.

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