JP5195314B2 - Manufacturing method of semiconductor device - Google Patents

Description

The present invention relates to the production how a semiconductor device, in particular an insulating substrate such as ceramic or the like on the heat radiating base about the manufacture how the semiconductor device is joined.

  In general, an inverter device applied to a variable speed device of a motor includes a power element that performs power conversion, a drive circuit that controls and drives the power element, a protection circuit, and a plurality of semiconductor integrated circuits such as a control circuit that performs overall control thereof It is constituted by. Recently, high power control semiconductor devices such as inverter devices are intelligent power modules (hereinafter referred to as IPMs) that incorporate a power element that converts direct current into alternating current, a drive circuit, and a protection circuit in one package. It has been commercialized as.

FIG. 7 is a schematic cross-sectional view of a main part of a conventional power semiconductor module.
The insulating substrate 101 of the power semiconductor module 100 shown in FIG. 7 is a substrate in which conductor layers 101b and 101c such as copper (Cu) and aluminum (Al) are formed on both surfaces of a ceramic substrate 101a such as aluminum nitride (AlN). A semiconductor chip 103 such as a power semiconductor is bonded to the insulating substrate 101 via a solder layer 102. Then, the insulating substrate 101 to which the semiconductor chip 103 is bonded in this way is copper, aluminum, aluminum silicon by the solder layer 104 for the purpose of radiating heat generated in the semiconductor chip 103 on the side opposite to the bonding surface side. It is joined to a plate-like heat dissipation base 105 made of a metal such as carbide (AlSiC) or copper molybdenum alloy (Cu · Mo).

  When the power semiconductor module 100 having such a structure is formed, two members having different thermal expansion coefficients, that is, the insulating substrate 101 including the ceramic substrate 101a and the metal heat dissipation base 105 must be joined together by the solder layer 104. . Therefore, the heat dissipation base 105 that was originally flat may be warped after soldering.

FIG. 8 is a schematic cross-sectional view of an essential part showing the heat dissipation base in a warped state. In FIG. 8, the same elements as those shown in FIG. 7 are denoted by the same reference numerals.
For example, when aluminum nitride is used for the ceramic substrate 101a of the insulating substrate 101 and copper is used for the heat dissipation base 105, the thermal expansion coefficient of aluminum nitride is about 4.5 ppm / K, and the thermal expansion coefficient of copper is about 16.5 ppm / K, which produces a relatively large difference. Therefore, in the cooling stage after soldering, the shrinkage of copper is larger than that of aluminum nitride, and the heat dissipation base 105 is warped in a convex shape toward the joint surface with the insulating substrate 101. When such a warp occurs in the heat dissipation base 105, the device assembly process after soldering is hindered, and depending on the degree of the warp, the heat dissipation performance of the power semiconductor module 100 may be reduced. .

  By the way, many solders used for manufacturing semiconductor devices contain lead (Pb) as a component. Electronic devices and electronic parts that use such lead-containing solder, if discarded and left outdoors and exposed to acid rain, may lead to environmental pollution due to the elution of lead from the solder. . For this reason, it is desirable to use so-called lead-free solder mainly composed of tin (Sn) containing no lead for various electronic devices and electronic parts. Based on the European Union (EU) RoHS directive on the restriction of the use of specific hazardous substances in electronic and electrical equipment, solder containing lead in particular is restricted. As a substitute, silver (Ag), copper, zinc (Zn), A solder joint including an alternative material such as bismuth (Bi) or indium (In) has been proposed.

  Such lead-free solder has a property that its hardness is higher than that of solder containing lead. When the insulating substrate 101 of the power semiconductor module 100 shown in FIGS. 7 and 8 is joined to the originally flat heat dissipation base 105 using solder containing lead, the insulating substrate of the heat dissipation base 105 is soldered at the time of soldering. Even if a convex warp toward the 101 side occurs, the solder itself is soft, so that the solder layer 104 undergoes creep deformation immediately after soldering, and the stress therebetween is relaxed. As a result, the warp of the heat radiating base 105 is eliminated, and the heat radiating base 105 returns to the original flat state or a state close to flat.

  On the other hand, when lead-free solder is used for joining them, since the solder is hard and the creep deformation of the solder layer 104 does not occur, the convex warpage generated in the flat heat radiation base 105 is restored. It will remain without. Further, the amount of warpage is as large as about 100 μm to 900 μm. As a result, as described above, the assembly process after soldering may be hindered or the performance of the power semiconductor module 100 may be degraded.

  FIG. 9 is a schematic cross-sectional view of the relevant part showing the assembly process of the power semiconductor module. In FIG. 9, the same components as those shown in FIGS. 7 and 8 are denoted by the same reference numerals.

  As shown in FIG. 9, the power semiconductor module 100 is usually fixed to the cooling fin 200 by screws or the like after the insulating substrate 101 and the heat dissipation base 105 are soldered. Yes.

  When solder containing lead is used for bonding between the insulating substrate 101 and the heat dissipation base 105, the convex warpage generated in the heat dissipation base 105 during the soldering tends to be eliminated thereafter. Therefore, the contact thermal resistance between the heat dissipation base 105 and the cooling fin 200 is kept relatively small, and the heat generated in the semiconductor chip 103 is efficiently dissipated from the heat dissipation base 105.

  On the other hand, when lead-free solder is used for bonding with the insulating substrate 101 and the heat dissipation base 105 is greatly bent in a convex shape toward the insulating substrate 101, the cooling fin 200 is flat as shown in FIG. A large gap 201 is generated between the surface. When such a gap 201 is generated, the contact thermal resistance is increased, so that the efficiency of dissipation of the heat generated in the semiconductor chip 103 is lowered, and the temperature at the junction of the semiconductor chip 103 is abnormally increased, which may cause thermal destruction. In addition, when the heat dissipation base 105 is greatly warped toward the insulating substrate 101, there is a problem that the ceramic substrate 101a is cracked when the heat dissipation base 105 is screwed to the cooling fin 200. There is also a case.

  Therefore, as a method for manufacturing a semiconductor device using lead-free solder that does not contain lead for joining the insulating substrate and the heat dissipation base and joining the insulating substrate and the semiconductor chip, for example, a technique as described in Patent Document 1 is available. Proposed. In this proposal, when soldering the heat dissipation base and the insulating substrate, the heat dissipation base is preliminarily given a predetermined amount of warpage (recessed) and then insulated using lead-free solder. As a result, the heat dissipation base is soldered to the insulating substrate in a substantially flat state. For this reason, when the heat radiation base is attached to a flat surface such as a cooling fin, a large gap is not formed between the member and the member, a necessary contact area is ensured, and damage to the ceramic substrate during attachment is prevented.

On the other hand, unbonded portions (voids) may occur when soldering the insulating substrate and the heat dissipation base. In general, voids are caused by the dissolved gas contained in the solder material, the formed sheet solder (pellet), the surface of the heat dissipation base, or the gas generated after reduction on the electrode oxide film of the insulating substrate (for example, water vapor) Etc.) remain in the solder, and it is assumed that the oxide film on the heat dissipation base, the insulating substrate, and the solder surface cannot be reduced and cannot be joined to the solder. In particular, if there is a gap between the sheet solder and the heat dissipation base due to the warp given in advance to the heat dissipation base, the outside air that is involved when the solder melts tends to become voids. Thus, it is known to prevent such voids from being generated by a vacuum soldering method in which solder is melted in an inert gas atmosphere or under reduced pressure.
JP 2006-202884 A (paragraph numbers [0019] to [0026], FIG. 1)

  However, in the decompression process for preventing the generation of such voids, when the voids escape from the molten solder, there is a risk that the solder will be scattered around. In particular, if the scattered solder splashes reach the outer peripheral end of the heat dissipation base, it interferes with the mating surface of the protective case in the subsequent process, causing assembly failure. In addition, voids generated due to warping applied to the heat dissipation base have a larger volume than voids such as dissolved gas, and therefore, both the number and amount of solder scattering tend to increase. Therefore, conventionally, in the manufacturing process of a semiconductor device, efficient suppression of solder splash has been demanded.

  The present invention has been made in view of these points, and an object thereof is to provide a method for manufacturing a semiconductor device in which solder scattering is prevented by a positioning jig used in an assembly process before soldering. To do.

  In the present invention, in order to solve the above problem, an insulating substrate having a wiring layer formed on one main surface is bonded onto a heat dissipation base, and a semiconductor chip is bonded to the wiring layer side of the insulating substrate by soldering. A method for manufacturing a semiconductor device is provided. In this method of manufacturing a semiconductor device, the heat dissipation base includes a step of processing a convex warp on a side opposite to the bonding surface with the insulating substrate with a predetermined amount of warpage, and a positioning jig having a curved bottom surface. A step of fixing the insulating substrate and a bonding material on a heat dissipation base, and a step of bonding the insulating substrate to the heat dissipation base by heating and melting the bonding material, the bottom surface of the positioning jig being It is characterized by having a curved surface shape so that the distance from the heat dissipation base is 100 μm or less.

  According to the present invention, it is possible to prevent solder from being scattered during the manufacture of a semiconductor device, to suppress the occurrence of product defects, and to improve the bonding quality.

Hereinafter, a conventional manufacturing method as a comparison object will be described with reference to the drawings, and then an embodiment of the present invention will be described.
(Conventional manufacturing method)
FIG. 10 is one of the manufacturing processes of a power semiconductor module using a conventional positioning jig, in which (A) is a member setting process, (B) is a heating process, and (C) is a pressure reducing process. It is a schematic diagram.

In the manufacture of the power semiconductor module, the insulating substrate 1 is set via the sheet solder 3 on the heat-radiating base 2 warped as shown in FIG. The insulating substrate 1 is configured as a substrate in which a Cu layer 1a, a ceramic plate 1b, and a Cu layer 1c are stacked. As the ceramic plate 1b, a substrate mainly composed of alumina (Al ₂ O ₃ ), silicon nitride (Si ₃ N ₄ ), aluminum nitride (AlN), or the like is used.

  An insulating substrate 1 having conductor layers on both sides is formed with a Cu layer 1 a as a wiring layer on one main surface, and the other Cu layer 1 c is soldered by a heat dissipation base 2 and a plate solder 3. When the plate solder 3 is melted in the subsequent heating process, the insulating substrate 1 is fixed on the heat radiation base 2 by the positioning jig 4 so that it does not move.

FIGS. 11A and 11B are positioning jigs installed on a heat dissipation base at the time of joining conventional insulating substrates, in which FIG. 11A is a plan view and FIG. 11B is a cross-sectional view taken along line BB.
The conventional positioning jig 4 is a jig for positioning the insulating substrate 1 on the heat radiating base 2 by fixing a frame body 40 whose upper and lower surfaces are parallel to a predetermined position on the heat radiating base 2. That is, a rectangular opening 41 having a long side X and a short side Y (≦ X) as shown in FIG. The insulating substrate 1 having substantially the same shape can be fitted into the opening 41, and the insulating substrate 1 together with the sheet solder 3 can be disposed on the heat dissipation base 2. The screw holes 42 formed on the left and right of the positioning jig 4 are for alignment on the heat dissipation base 2.

  The heat radiating base 2 is a heat radiating base that is fixed to a heat radiating fin (not shown) or the like in order to release heat generated from a power semiconductor chip (not shown) arranged on the insulating substrate 1. The heat dissipation base 2 is warped in advance so as to have a convex shape opposite to the main surface to which the insulating substrate 1 is soldered. Here, the amount of warping of the heat dissipation base 2 is, for example, when the convex warpage toward the insulating substrate 1 that occurs when the heat dissipation base 2 is flat during soldering is in the range of about 200 μm to about 1000 μm. The range is from about 100 μm to about 900 μm.

  A solder material (lead-free solder) that does not contain lead is used for the plate solder 3. The component of the plate solder 3 is tin-based solder containing silver, bismuth, copper, indium, antimony (Sb), zinc, aluminum and the like. Also, the lower the melting point of lead-free solder, the lower the heat applied to the heat dissipation base 2 during soldering, so the copper expansion / contraction is small and the warp generated in the heat dissipation base 2 is kept small. It becomes possible. In general, the melting point of the sheet solder 3 is preferably 250 ° C. or less.

  FIG. 10B shows a molten state of the solder 3 a by the reflow process in the heating furnace 6. When the sheet solder 3 is heated and melted to 280 ° C. to 400 ° C. by the heating furnace 6, the solder 3 a melted in the depression on the heat radiation base 2 entrains the gas, so that voids are formed in the solder 3 a melted under the insulating substrate 1. 5 is likely to occur.

  FIG. 10C shows a void-removed state in which the void 5 generated by the decompression process in a hydrogen or inert gas atmosphere is removed by suction. In the positioning jig 4 as shown in FIG. 11, since the entire structure is constituted by the parallel frame body 40, if the heat dissipation base 2 is warped in the opposite direction in advance, there is a gap between the heat dissipation base 2. Occurs. Therefore, in the decompression step, when the void 5 in the solder 3 a comes out from this gap, the molten solder droplet 3 b is likely to be scattered on the heat dissipation base 2.

Next, two positioning jigs according to the embodiment of the present invention will be described.
(First embodiment)
1A and 1B are schematic cross-sectional views showing a solder melting state of the power semiconductor module according to the first embodiment, in which FIG. 1A is a cross-sectional view along the longitudinal direction of the insulating substrate, and FIG. It is sectional drawing which follows a hand direction.

  As shown in FIGS. 1A and 1B, the positioning jig has, for example, a curved surface shape (which may be a stepped shape or a linear inclined surface) whose bottom surface follows the warped shape of the two-dimensional plane of the heat radiating base 2. The carbon jig 4a is used. If the bottom surface of the carbon jig 4a is not only matched with the warped shape of the outer peripheral portion of the heat radiating base 2, but also curved so as to match the warped shape at the position in contact with the opening of the heat radiating base 2, There is almost no gap between the heat dissipating base 2. Therefore, in the assembly process before soldering, the insulating substrate 1 is positioned on the heat dissipation base 2 using the carbon jig 4a, so that solder scattering is suppressed, and the solder reaches the outer peripheral end of the heat dissipation base 2. It will not scatter.

Next, a method for forming the carbon jig 4a will be described.
The carbon jig 4a can be formed by performing a cutting process after being pressed by a press die. In order to give warpage corresponding to the amount of warpage of the heat dissipation base 2 to the contact surface with the heat dissipation base 2, a press die having a shape corresponding to the amount of warpage to be applied is prepared, and the press die is filled with carbon powder. By doing so, a molded body is formed. Thereby, the carbon jig 4a having a shape corresponding to the press die is formed. According to such a method, by changing the press die, the carbon jig 4a having various warpage amounts and plane sizes can be formed without performing mechanical processing. Further, the bottom surface of the carbon jig 4 a can be stepped to give a warp corresponding to the warp amount of the heat dissipation base 2.

(Second Embodiment)
2A and 2B are schematic cross-sectional views showing a solder melting state of the power semiconductor module according to the second embodiment, where FIG. 2A is a cross-sectional view along the longitudinal direction of the insulating substrate, and FIG. 2B is a short view of the insulating substrate. It is sectional drawing which follows a hand direction.

  The carbon jig 4b is curved so as to coincide only with the warp shape in the longitudinal direction of the outer peripheral portion of the heat dissipation base 2. That is, the carbon jig 4b is the same as the carbon jig 4a of FIG. 1 in the cross-sectional shape along the longitudinal direction of the insulating substrate 1, but the short side direction of the insulating substrate 1 is as shown in FIG. As shown, the bottom surface of the carbon jig 4b is a plane parallel to the top surface.

  Such a carbon jig 4b is easier to process than the first embodiment in which the carbon jig 4b matches the warped shape of the two-dimensional plane of the heat dissipation base 2. On the other hand, a gap is formed between the bottom surface of the opening of the carbon jig 4b and the heat dissipation base 2. However, if the size of the gap at this time is small to some extent, as in the case of the first embodiment, the solder scattering is suppressed, and the solder does not splash up to the outer peripheral end of the heat dissipation base 2. It has been confirmed by the following experiment.

  FIGS. 3A and 3B are diagrams for explaining the relationship between the shape of the carbon jig and solder scattering, where FIG. 3A is a plan view of the carbon jig, FIG. 3B is a cross-sectional view taken along the line B-B, and FIG. CC sectional drawing, (D) is a figure which shows the relationship between the clearance gap between the carbon jig in the outer peripheral edge part of a thermal radiation base, and the solder scattering rate to the horizontal direction exterior.

  The solder scattering rate shown in FIG. 4D changes according to the maximum value (distance Da) of the gap with the carbon jig 4b at the outer peripheral end of the heat dissipation base 2. If the distance Da at which the distance gap is widest is 100 μm or less on the entire circumference, all the solder splashes 3b from the joint portion of the heat dissipation base 2 in the horizontal direction are blocked at the outer periphery joint portion of the heat dissipation base 2. . Therefore, if the bottom surface of the carbon jig 4b is processed into a curved surface shape so that the distance from the heat radiating base 2 is 200 μm or less at the maximum, the solder scattering rate becomes 40% or less, and the protective case in the subsequent process. Therefore, it is effective in suppressing the occurrence of defects in the semiconductor device.

  4A and 4B are diagrams for explaining the relationship between the shape of the carbon jig and the solder scattering, where FIG. 4A is a plan view of the carbon jig, FIG. 4B is a cross-sectional view taken along the line BB, and FIG. CC sectional drawing, (D) is a figure which shows the relationship with the solder scattering rate which goes to a perpendicular | vertical upward direction from the joint surface of a thermal radiation base. Here, a gap is formed between the insulating substrate 1 and the carbon jig 4b in order to release the void 5 generated in the molten solder 3a in a state where the gap between the heat dissipation base 2 and the carbon jig 4b is eliminated. Has been.

  As shown in FIG. 4D, since the momentum of the void 5 in the decompression process is determined according to the minimum value (distance Db) of the gap between the insulating substrate 1 and the carbon jig 4b, the solder scattering rate is the distance Db. Inversely proportional to Here, if the distance Db is approximately 300 μm or more around the entire circumference of the insulating substrate, all the melted solder 3a is solidified under the insulating substrate 1, and the solder crawls up vertically from the joint surface of the heat dissipation base 2. Such a phenomenon can be surely suppressed. Therefore, if the carbon jig 4b is configured to hold the insulating substrate 1 in the opening at an interval of at least 300 μm or more, solder droplets do not adhere to the Cu layer 1a, and the occurrence of defects in the semiconductor device can be suppressed. it can.

  Thus, the conventional positioning jig 4 that does not have a curved surface along the heat dissipation base on the bottom surface and a curved surface on the bottom surface as in the present invention, the distance Da is 100 μm or less, and the distance Db is 300 to 500 μm. When an experiment was performed using the carbon jig 4b, the former had a defect rate of 88% due to solder scattering, and the latter was 0%, and a great effect was obtained.

  When the distance Da described above is 100 μm or less, when the distance Db is 300 μm or less, the molten solder droplet 3b may reach and adhere to the circuit pattern of the insulating substrate 1 by capillary action. Therefore, in order to avoid such a phenomenon, it is preferable to set the distance Da to 100 μm or less and the distance Db to 300 μm or more. Furthermore, the upper limit of the distance Db is preferably set to about 500 μm where the solder scattering rate is 0% from the viewpoint of downsizing the jig.

FIG. 5 is a plan view showing the opening shape of the carbon jig.
The opening 41 of the carbon jig 4b needs to be large enough to allow the insulating substrate 1 to be inserted. This is because the carbon jig 4 b has an original role in positioning the insulating substrate 1 on the heat dissipation base 2. The screw hole 42 of the positioning jig 4 is for alignment on the heat dissipation base 2.

  In the carbon jig 4b, the opening 41 is formed to be larger than the insulating substrate 1 by one turn, and stepped protrusions 43 are provided at the four corners of the opening 41 so as to hold the four corners of the heat dissipation base 2, respectively. I have to. When the insulating substrate 1 is positioned on the heat dissipation base 2 by the protrusion 43 of the carbon jig 4b in this way, the distance Db between the insulating substrate 1 and the carbon jig 4b shown in FIG. It becomes possible to set to.

FIG. 6 is a schematic cross-sectional view of the relevant part showing a power semiconductor module that is brought to normal pressure and room temperature by a cooling process.
The resin case 7 protects the circuit pattern on the insulating substrate 1 and the semiconductor chip 9 joined thereto with solder 8. By suppressing solder scattering using the carbon jigs 4a and 4b described above, when the heat dissipation base 2 returns to a substantially flat state, the periphery of the upper surface of the heat dissipation base 2 to which the insulating substrate 1 is soldered is No solder splashes remain. Therefore, the resin case 7 and the like can be reliably adhered from above the heat radiating base 2 without performing subsequent correction processing or the like, and the product size can be finished to the standard. Moreover, if there is no gap between the heat radiating base 2 and the resin case 7, there is no possibility that the gel flows out even if no special treatment is performed when the semiconductor chip 9 is sealed with gel in a later process. .

  Further, since the heat radiating base 2 is warped in a concave shape in advance, the heat radiating base 2 becomes almost flat after being cooled and soldered to the insulating substrate 1. Therefore, the heat radiating base 2 can be securely attached to the cooling fin.

  Although the case where the plate solder 3 is used as the bonding material has been described above, a metal paste or a conductive resin can be used in addition to the solder. In any case, the material for constituting the positioning jig is heat resistant up to a heating temperature higher than the melting point of the joining material used, such as carbon or ceramic, and is not resistant to the joining material. It must be adhesive.

It is a cross-sectional schematic diagram which shows the solder melting state of the power semiconductor module which concerns on 1st Embodiment, (A) is sectional drawing which follows the longitudinal direction of an insulated substrate, (B) is along the transversal direction of an insulated substrate. It is sectional drawing. It is a cross-sectional schematic diagram which shows the solder melting state of the power semiconductor module which concerns on 2nd Embodiment, (A) is sectional drawing which follows the longitudinal direction of an insulated substrate, (B) is along the transversal direction of an insulated substrate. It is sectional drawing. It is a figure explaining the relationship between the shape of a carbon jig and solder scattering, (A) is a top view of a carbon jig, (B) is the BB sectional view, (C) is the CC cross section. FIG. 4D is a diagram showing the relationship between the gap with the carbon jig at the outer peripheral end of the heat dissipation base and the solder scattering rate to the outside in the horizontal direction. It is a figure explaining the relationship between the shape of a carbon jig and solder scattering, (A) is a top view of a carbon jig, (B) is the BB sectional view, (C) is the CC cross section. FIG. 4D is a diagram showing the relationship with the solder scattering rate from the joining surface of the heat dissipation base toward the vertically upward direction. It is a top view which shows the opening part shape of a carbon jig. It is a principal part cross-section schematic diagram which shows the power semiconductor module made into normal-pressure normal temperature by the cooling process. It is a principal part cross-sectional schematic diagram of the conventional power semiconductor module. It is a principal part cross-sectional schematic diagram which shows the thermal radiation base of the curved state. It is a principal part cross-sectional schematic diagram which shows the assembly process of a power semiconductor module. It is one of the manufacturing processes of the power semiconductor module using the conventional positioning jig, (A) is a member setting process, (B) is a heating process, (C) is a cross-sectional schematic diagram which shows a pressure reduction process. . It is a positioning jig installed on a heat dissipation base at the time of joining of conventional insulation substrates, and (A) is the top view and (B) is a sectional view which met a BB line.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Insulation board | substrate 2 Heat radiation base 3 Sheet solder 3a Solder 4 Positioning jig 4a, 4b Carbon jig 5 Void 6 Heating furnace 7 Resin case 8 Solder 9 Semiconductor chip 40 Frame 41 Opening part

Claims (5)

In a method of manufacturing a semiconductor device, in which an insulating substrate having a wiring layer formed on one main surface is bonded onto a heat dissipation base and a semiconductor chip is soldered to the wiring layer side of the insulating substrate.
A process of processing a convex warp with a predetermined warp amount on the side opposite to the joint surface with the insulating substrate on the heat dissipation base;
Fixing the insulating substrate and the bonding material on the heat dissipation base by a positioning jig having a curved bottom surface;
Bonding the insulating substrate to the heat dissipation base by heating and melting the bonding material;
Including
A method of manufacturing a semiconductor device, wherein the bottom surface of the positioning jig has a curved shape so that a distance from the heat dissipation base is 100 μm or less.   The method of manufacturing a semiconductor device according to claim 1, wherein the heat dissipation base is formed with a warpage amount of 100 μm to 900 μm.   The method of manufacturing a semiconductor device according to claim 1, wherein the bonding material is solder containing no lead.   4. The method of manufacturing a semiconductor device according to claim 3, wherein the solder containing no lead has a melting point of 250 [deg.] C. or lower.   4. The method of manufacturing a semiconductor device according to claim 3, wherein the lead-free solder contains at least one of silver, bismuth, indium, antimony, zinc, aluminum, and copper and tin.

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