JP7388145B2 - semiconductor cooling equipment - Google Patents

Description

The present invention relates to a semiconductor cooling device that cools a semiconductor element mounting board using a liquid cooling type cooler.

In recent years, semiconductor devices often handle large amounts of electric power, and the amount of heat generated has increased accordingly. For this reason, a cooler is bonded to the substrate on which the semiconductor element is mounted to radiate heat. Forced air cooling is possible for fixed equipment that can secure a large space for heat radiation, but liquid-cooled coolers are useful when equipment is placed in a limited space.

Conventional liquid-cooled coolers generally have a structure in which thin plate-shaped inner fins are installed in a cooling liquid circulation space, and heat is transferred to the cooling liquid via the fins. In a liquid-cooled cooler having such a structure, foreign matter mixed into the circulating cooling liquid may get stuck between the fins and obstruct the circulation of the cooling liquid, causing a decrease in cooling performance. Furthermore, if there is a large amount of foreign matter, it will also lead to a pressure loss in the coolant.

To deal with the blockage of foreign matter, methods have been proposed in which the foreign matter is allowed to flow between the fins together with the coolant, or the foreign matter is prevented from flowing into the coolant circulation space (see Patent Documents 1 and 2).

The cooler described in Patent Document 1 has two inner fins, upper and lower, including upper fins with a small fin pitch and lower fins with a large fin pitch, and a means for transferring foreign matter is provided between the upper fin and the lower fin, and Foreign matter that is too large to flow between the fins is guided to the lower fins and allowed to flow between the lower fins.

In the cooler described in Patent Document 2, a bag-shaped foreign matter removal member is installed in an inlet pipe leading to a coolant circulation space, and foreign matter is collected in the foreign matter removal member to prevent foreign matter from flowing into the coolant circulation space. is being prevented.

Japanese Patent Application Publication No. 2014-86641 Japanese Patent Application Publication No. 2008-275190

Liquid-cooled coolers have a small fin pitch and a large number of fins to improve the cooling performance of the inner fins. However, as in the cooler described in Patent Document 1, increasing the fin pitch of the lower fins in order to circulate foreign matter is not preferable from the viewpoint of cooling performance. Furthermore, the cooler described in Patent Document 2 has a problem in that the internal structure of the inlet piping is complicated.

SUMMARY OF THE INVENTION In view of the above-mentioned background art, an object of the present invention is to provide a semiconductor cooling device that can suppress a decrease in cooling performance and an increase in pressure loss due to foreign substances without increasing the fin pitch.

That is, the present invention has the configurations described in [1] to [7] below.

[1] An insulating substrate on which a semiconductor element is mounted on one surface of the insulating substrate via a wiring layer,
A heat dissipation board having a plurality of plate-like fins erected at predetermined intervals on one surface, and a recess for accommodating the fins, and is attached to one surface of the heat dissipation board to form a coolant circulation space. Equipped with a liquid-cooled cooler with a jacket,
a semiconductor cooling device in which the other side of the insulating substrate is joined or adhered to the other side of the heat dissipation substrate of the liquid-cooled cooler;
In the cooling liquid circulation space of the liquid-cooled cooler, a gap serving as a main flow path is formed between the tip of the fin and the bottom surface of the recess of the jacket, and the height H of the main flow path and the gap between the fins are A semiconductor cooling device characterized in that a dimension W of a fin gap satisfies the relationship H>W.

[2] The semiconductor cooling device according to item 1, wherein the fin gap has a dimension W of 0.1 mm to 0.6 mm.

[3] The semiconductor cooling device according to the above item 1 or 2, wherein the height H of the main flow path is 0.9 mm to 3 mm.

[4] The semiconductor cooling device according to any one of items 1 to 3 above, wherein the fin has at least one slit extending from the tip toward the heat dissipation substrate.

[5] The semiconductor cooling device according to item 4, wherein the width S of the slit and the dimension W of the fin gap satisfy the relationship S≧W.

[6] The semiconductor cooling device according to the above item 4 or 5, wherein the slit is provided at a position avoiding directly under the semiconductor element.

[7] The semiconductor cooling device according to any one of items 4 to 6 above, which has a plurality of semiconductor elements in the flow direction of the coolant, and the slit is provided between the semiconductor elements in the flow direction of the coolant.

In the semiconductor cooling device described in [1] above, there is a gap between the tip of the fin and the jacket in the cooling liquid circulation space of the liquid-cooled cooler. The height H of the gap on the fin tip side and the dimension W of the fin gap between the fins are in the relationship H>W, so cooling water flows more in the gap on the fin tip side where there is less flow resistance than in the fin gap. The flow and flow speed become faster, and the gap on the fin tip side becomes the main flow path for the coolant. Therefore, foreign matter mixed in the coolant either flows into the main flow path with the coolant and is discharged from the coolant circulation space, or even if it remains in the main flow path, the pressure loss is extremely small because the main flow path has a large cross-sectional area. . In addition, cooling water flows into the gaps between the fins, although the flow velocity is slower than that in the main channel, and it also flows from the main channel, where the flow velocity is faster, so the coolant in contact with the sides of the fins is constantly replaced, so there is no flow from the sides of the fins into the coolant. Heat transfer also occurs without any problems. Further, even if foreign matter does not flow into the main flow path and becomes clogged in the fin gap upstream, the cooling liquid will flow into the fin gap from the main flow path except for the clogged portion.

As described above, the liquid cooling type cooler has a structure that prevents foreign matter from clogging the fin gaps, and even if foreign matter becomes clogged, the cooling liquid flows avoiding the clogged area. Therefore, stable and excellent cooling performance is obtained, and an increase in pressure loss due to foreign matter is also suppressed. Furthermore, since there is no need to widen the dimension W of the fin gap to allow foreign matter to pass through, high cooling performance can be maintained by reducing the fin pitch.

In the semiconductor cooling device described in [2] above, since the dimension W of the fin gap is 0.1 mm to 0.6 mm, high cooling performance can be obtained.

In the semiconductor cooling device described in [3] above, since the height H of the main channel is 0.9 mm to 3 mm, the flow velocity of the coolant between the main channel and the fin gap is well balanced, and stable and excellent cooling can be achieved. Performance can be obtained.

In the semiconductor cooling device described in [4] above, the cooling liquid flows from the main channel into the fin gaps through the slits formed in the fins, and the flow is stirred in both the main channel and the fin gaps. Therefore, even if the upstream side is clogged with foreign matter, the cooling liquid flows downstream of the foreign matter without being stagnated due to this agitation, making it less susceptible to the influence of foreign matter. Furthermore, since the cooling liquid flows from the slits into the slit gaps, the flow velocity downstream of the slits becomes faster, so that variations in cooling performance are reduced.

In the semiconductor cooling device described in [5] above, the width S of the slit formed in the fin and the dimension W of the fin gap satisfy the relationship S≧W, so that a sufficient stirring effect can be obtained.

In the semiconductor cooling device described in [6] above, the slits of the fins are provided so as to avoid being directly under the semiconductor element, so that it is possible to suppress a decrease in heat transfer efficiency to the fins due to the slits.

The semiconductor cooling device according to [7] above has a plurality of semiconductors in the direction of flow of the coolant, and slits of the fins are provided between the semiconductor elements in the direction of flow of the coolant. Therefore, the cooling liquid flows from the main channel into the fin gaps through the slits and is stirred, so that variations in cooling performance for a number of semiconductor elements can be reduced.

FIG. 1 is an exploded perspective view of a first semiconductor cooling device that is an embodiment of the present invention. FIG. 2 is a sectional view taken along line 2A-2A in FIG. 1; FIG. 2 is a sectional view taken along line 2B-2B in FIG. 1; FIG. 3 is a longitudinal cross-sectional view of a second semiconductor cooling device that is an embodiment of the present invention. FIG. 6 is a top perspective view of the second semiconductor cooling device. It is a longitudinal cross-sectional view showing a modification of the second semiconductor cooling device. FIG. 7 is a longitudinal cross-sectional view showing another modification of the second semiconductor cooling device. FIG. 3 is a longitudinal cross-sectional view of a third semiconductor cooling device according to an embodiment of the present invention. FIG. 7 is a top perspective view of the third semiconductor cooling device.

FIGS. 1 to 6B show three embodiments of a liquid-cooled cooler and a semiconductor cooling device of the present invention, and variations thereof. Further, in the following description, common reference numerals will be used to indicate the same thing, and the description will be omitted.
[First semiconductor cooling device]
The semiconductor cooling device 1 shown in FIGS. 1 to 2B includes an insulating substrate 10 and a liquid-cooled cooler 20.

The insulating substrate 10 has a rectangular shape, and a wiring layer 12 for mounting a semiconductor element 11 is bonded to one surface, and a layer 12 for relieving stress generated between the insulating substrate 10 and the liquid cooler 20 is bonded to the other surface. At the same time, a buffer layer 13 that promotes heat transfer is bonded. 14 is a solder layer that joins the semiconductor elements 11 together.

The liquid cooling type cooler 20 includes a rectangular heat dissipation board 21 and a jacket 30. The heat dissipation board 21 has a plurality of plate-shaped fins 22 erected at the center of one surface with a predetermined gap therebetween, and a flange 23 is formed around the group of fins 22 . The jacket 30 has a box shape having a recess 31 that accommodates the group of fins 22, and the depth D of the recess 31 is set to be deeper than the height Hf of the fins 22. In the liquid-cooled cooler 20, fins 22 having a height Hf of 2 mm to 12 mm are preferably used. An inlet hole 32a and an outlet hole 33a for the cooling liquid C are formed in two opposing side walls 32 and 33 of the jacket 30, and a joint 34 is attached to each of them.

When the heat dissipation board 21 is placed over the jacket 30 so that both longitudinal ends of the fins 22 face the inlet holes 32a and the outlet holes 33, the fins 22 are accommodated in the recesses 31, and the flange 23 of the heat dissipation board 21 comes into contact with the upper surface of the jacket 30, and a coolant circulation space 35 is formed surrounded by one surface of the heat dissipation board 21 and the inner surface of the recess 31. The contact surface between the flange 23 of the heat dissipation board 21 and the jacket 30 is kept watertight by brazing or a sealing structure using an O-ring. The coolant C enters the coolant circulation space from the inlet hole 32a and is discharged from the outlet hole 33a.

In the liquid-cooled cooler 20, the depth D of the recess 31 and the height Hf of the fin 22 have a relationship of D>Hf, so the height H between the tip of the fin 22 and the bottom of the recess 31 is D. A flat gap 36 of −Hf is formed. The depth D of the recess 31 and the height Hf of the fin 22 are set such that the height H of the gap 36 is larger than the dimension W of the fin gap 24 between the fins 22, that is, the relationship H>W. is set to meet. Therefore, the coolant C that has entered the coolant circulation space 35 from the inlet hole 32a flows more into the gap 36 on the fin tip side where the flow resistance is lower than that in the fin gap 24, and the flow rate becomes faster. This becomes the main flow path 36 for the coolant C. Even if foreign matter Q is mixed in the coolant C, the foreign matter Q flows into the main channel 36 together with the coolant C, passes through the main channel 36, and is discharged from the outlet hole 33a. Even if the foreign matter Q remains in the main channel 36 without being discharged from the outlet hole 33a, the pressure loss due to the foreign matter Q is extremely small because the main channel 36 has a large cross-sectional area.

Note that the dimension W of the fin gap 24 is the dimension at the point where the gap is the narrowest in the height direction of the fins 22. For example, when the thickness of the fin becomes thinner from the base end (the heat dissipation board side) toward the tip, the gap at the base end becomes the narrowest.

The coolant C also flows into the fin gaps 24. Although the flow rate of the coolant C flowing through the fin gaps 24 is slower than that in the main flow path 36, it does not stagnate and flows from the upstream side (inlet hole 32a side) to the downstream side (outlet hole 33a side). Further, since the fin gap 24 and the main flow path 36 communicate with each other at the tip end side of the fin 22, the coolant C also flows into the fin gap 24 from the main flow path 36, which has a high flow rate. In this way, the coolant C in contact with the side surfaces of the fins 22 is also constantly replaced, so that heat is transferred from the side surfaces of the fins 22 to the coolant C without any problem. Further, even if the foreign matter Q does not flow into the main flow path 36 and becomes clogged in the fin gap 24 upstream, the coolant C flows into the fin gap 24 from the main flow path 36 except for the clogged portion.

As described above, the liquid cooling type cooler 20 has a structure in which the foreign matter Q does not easily clog the fin gaps 24, and even if the foreign matter Q becomes clogged, the cooling liquid C flows avoiding the clogged part. Therefore, stable and excellent cooling performance is obtained, and an increase in pressure loss due to foreign matter is also suppressed. Furthermore, since there is no need to widen the dimension W of the fin gap 24 to allow the foreign matter Q to pass through, high cooling performance can be maintained by reducing the fin pitch.

The dimension W of the fin gap 24 is preferably 0.1 mm to 0.6 mm. If the dimension W is less than 0.1 mm, the difficulty of assembly increases, leading to an increase in cost. On the other hand, as the dimension W increases, the number of fins decreases, and when it exceeds 0.6 mm, it becomes difficult to obtain high cooling performance. A particularly preferred dimension W is 0.2 mm to 0.5 mm.

The height H of the main flow path 36 is preferably in the range of 0.9 mm to 3 mm. If the height H of the main flow path 36 is less than 0.9 mm, the effect of preferentially causing the coolant C to flow through the main flow path 36 will be reduced. On the other hand, if the height H exceeds 3 mm, the coolant C flows too much into the main channel 36 without resistance, resulting in a decrease in the flow velocity in the fin gaps 24 and a decrease in heat transfer performance from the side surfaces of the fins 24. No improvement in performance is expected. When the height H is set in the range of 0.9 mm to 3 mm, the flow velocity between the main flow path 36 and the fin gap 24 is well balanced, and stable and excellent cooling performance can be obtained. A particularly preferable height H of the main flow path 36 is 0.9 mm to 1.5 mm.
[Second semiconductor cooling device]
The semiconductor cooling device 2 shown in FIGS. 3A and 3B differs from the first semiconductor cooling device 1 in the shape of the fins 42 of the liquid-cooled cooler 40. The configuration other than the fins 42 is the same as the first semiconductor cooling device 1. FIG. 3B is a diagram illustrating the positional relationship between the fins 42 and the semiconductor element 11 when the semiconductor cooling device 2 is viewed from above and the interior of the liquid cooling type cooler 40 is seen through.

The fin 42 has slits 43a and 43b extending from the tip of the fin 42 toward the heat dissipation board 21 at two locations closer to the center from both longitudinal ends of the plate-like fin having the same shape as the fin 22. ing. All the fins 42 have slits 43a, 43b at the same position, and each slit 43a, 43b is continuous in a direction transverse to the flow of the coolant C in plan view.

The cooling liquid C flowing through the main flow path 36 flows into the fin gap 44 through the slits 43a and 43b, and the flow is stirred in both the main flow path 36 and the fin gap 44. Even if the foreign matter Q becomes clogged on the upstream side, the coolant C flows downstream of the clogged foreign matter Q without being stagnated due to this agitation, making it less susceptible to the influence of the foreign matter Q. In addition, since the cooling liquid C flows into the slit gap 44 from the slits 43a and 43b, the flow velocity downstream of the slits 43a and 43b becomes faster, so that variations in cooling performance are smaller than in a cooler without slits.

It is preferable that the width S of the slits 43a, 43b and the dimension W of the fin gap 44 satisfy the relationship S≧W, so that a sufficient stirring effect can be obtained. When the width S of the slits 43a, 43b becomes smaller than the dimension W of the slit gap 44, the above-mentioned stirring effect becomes smaller. As the width S of the slits 43a and 43b increases, the stirring effect increases, but on the other hand, the surface area of the fins 42 decreases, resulting in a decrease in heat transfer performance. From this point of view, the particularly preferable width S of the slits 43a and 43b is 1.5 to 2 times the slit gap W.

Furthermore, since the above effect can be obtained with just one slit, the number of slits is not limited. The position of the slit is also not limited. However, if the slit is provided close to the semiconductor element, which is a heating element, the efficiency of heat transfer to the fins will decrease, so it is preferable to provide the slit so as not to be directly under the semiconductor element. By providing the slit so as not to be directly under the semiconductor element, it is possible to suppress a decrease in heat transfer efficiency. The liquid cooling type cooler 2 shown in FIGS. 3A and 3B has slits 43a and 43b provided at two locations, one on the upstream side and one on the downstream side of the semiconductor device 11, avoiding directly below the semiconductor device 11. Furthermore, since the foreign matter Q often clogs the upstream end of the fin, a plurality of slits may be provided on the upstream side to promote agitation in the region near the foreign matter Q. The liquid cooling type cooler 50 in FIG. 4 has slits 52a at two locations on the upstream side of the semiconductor element 11 of the fin 51, and a slit 52b at one location on the downstream side to promote agitation in the upstream region. It is a liquid-cooled cooler. Further, the liquid-cooled cooler 55 in FIG. 5 is a liquid-cooled cooler in which a slit 57 is provided in the fin 56 directly below the semiconductor element 11. These liquid-cooled coolers are also included in the present invention.

In addition, in the illustrated liquid type cooler 40, the plurality of fins 42 have slits 43a and 43b at the same position, but there are cases where only some of the fins are provided with slits, or where the plurality of fins are different. A case where a slit is provided at a certain position is also included in the present invention.
[Third semiconductor cooling device]
In the semiconductor cooling device 3 of FIGS. 6A and 6B, two semiconductor elements 11a are arranged on the upstream side in the flow direction of the cooling liquid C, and two semiconductor elements 11b are arranged on the downstream side, for a total of four semiconductor elements 11a, 11b is arranged. The four semiconductor elements 11a and 11b are mounted on separate insulating substrates 10, respectively. FIG. 6B is a diagram illustrating the positional relationship between the fins 62 and the semiconductor elements 11a and 11b, when the semiconductor cooling device 3 is viewed from above and the inside of the liquid-cooled cooler 60 is seen through.

In the liquid-cooled cooler 60, all the fins 62 have a slit 63 at the center in the flow direction of the cooling liquid C. In plan view, each slit 63 is continuous in a direction across the flow of the coolant C between the upstream semiconductor element 11a and the downstream semiconductor element 11b. The slit 63 is provided at a position that avoids being directly under the semiconductor elements 11a and 11b. Then, it flows from the main flow path 36 into the fin gap 64 in the slit 33, and the flow is stirred in both the main flow path 36 and the fin gap 64. Even if the foreign matter Q becomes clogged upstream, the cooling liquid C flows downstream of the clogged foreign matter Q due to this agitation without being stagnated, making it less susceptible to the influence of the foreign matter. Furthermore, as the cooling liquid C flows from the slit 63 into the slit gap 64, the flow velocity downstream of the slit 63 becomes faster, so the variation in cooling performance for the plurality of semiconductor elements 11a and 11b is smaller than in a cooler without slits. Become.

Further, even in a cooling device having a plurality of semiconductor elements, the preferable relationship between the width S of the slit 63 and the dimension W of the fin gap 64 is the same as in the cooling device having one semiconductor element (see FIGS. 3A and 3B).

[Materials of components of semiconductor cooling device]
Preferred materials for the constituent members of the semiconductor cooling device of the present invention are as follows.

It is preferable that the material constituting the insulating substrate 10 has not only excellent electrical insulation but also good thermal conductivity and excellent heat dissipation. In this respect, ceramics such as aluminum nitride (AlN), aluminum oxide (Al ₂ O _{3 )} , silicon nitride (Si ₃ N ₄ ), zirconium oxide (ZrO ₂ ), and silicon carbide (SiC) can be exemplified. Moreover, a composite material in which the ceramics are mixed as a filler into a silicone resin or an epoxy resin can also be used. Further, the thickness of the insulating substrate 10 is preferably in the range of 0.2 mm to 3 mm.

The material constituting the wiring layer 12 is preferably one having excellent electrical conductivity and excellent thermal conductivity, and preferably aluminum or an aluminum alloy, copper or a copper alloy. Among these, pure aluminum is particularly preferred. Further, the thickness of the wiring layer 12 is preferably in the range of 0.2 mm to 1 mm. Furthermore, the semiconductor element 11 is joined to the wiring layer 12 by brazing, soldering, welding, adhesive, or the like.

The material and thickness of the buffer layer 13 are based on the material of the wiring layer 12. The buffer layer 13 has the effect of promoting heat radiation generated by the semiconductor element 11 to the liquid-cooled coolers 20 , 40 , 50 , 55 , and 60 . Furthermore, the buffer layer 13 is not an essential layer, and a semiconductor cooling device in which the insulating substrate 10 is directly bonded to the coolers 20, 40, 50, 55, and 60 is also included in the present invention.

The material constituting the liquid-cooled coolers 20, 40, 50, 55, and 60 is preferably a highly thermally conductive material such as aluminum or aluminum alloy, copper, or copper alloy.

The present invention can be suitably used as a semiconductor cooling device mounting a semiconductor element that generates a large amount of heat.

1, 2, 3...Semiconductor cooling device 10...Insulating substrate 11, 11a, 11b...Semiconductor element 12...Wiring layer 13...Buffer layer 20, 40, 50, 55...Liquid cooling type cooler 21...Radiation substrate 22, 42, 51, 56, 62...Fins 24, 44, 64...Fin gap 30...Jacket 31...Recess 35...Cooling liquid circulation space 36...Main flow path 43a, 43b, 52a, 52b, 57, 63...Slit H...Height of main flow path S...Slit width Q...Foreign object C...Cooling liquid

Claims (8)

an insulating substrate on which a semiconductor element is mounted on one side of the insulating substrate via a wiring layer;
A heat dissipation board having a plurality of plate-like fins erected at predetermined intervals on one surface, and a recess for accommodating the fins, and is attached to one surface of the heat dissipation board to form a coolant circulation space. Equipped with a liquid-cooled cooler with a jacket,
a semiconductor cooling device in which the other side of the insulating substrate is joined or adhered to the other side of the heat dissipation substrate of the liquid-cooled cooler;
In the cooling liquid circulation space of the liquid-cooled cooler, a gap serving as a main flow path is formed between the tip of the fin and the bottom surface of the recess of the jacket, and the height H of the main flow path and the gap between the fins are The dimension W of the fin gap satisfies the relationship H>W ,
A semiconductor cooling device characterized in that the dimension W of the fin gap is 0.1 mm to 0.6 mm . The semiconductor cooling device according to claim 1, wherein the height H of the main flow path is 0.9 mm to 3 mm. The fin has at least one slit extending from the tip toward the heat dissipation board,
3. The semiconductor cooling device according to claim 1, wherein the width S of the slit and the dimension W of the fin gap satisfy the relationship S≧W. an insulating substrate on which a semiconductor element is mounted on one side of the insulating substrate via a wiring layer;
A heat dissipation board having a plurality of plate-like fins erected at predetermined intervals on one surface, and a recess for accommodating the fins, and is attached to one surface of the heat dissipation board to form a coolant circulation space. Equipped with a liquid-cooled cooler with a jacket,
a semiconductor cooling device in which the other side of the insulating substrate is joined or adhered to the other side of the heat dissipation substrate of the liquid-cooled cooler;
In the cooling liquid circulation space of the liquid-cooled cooler, a gap serving as a main flow path is formed between the tip of the fin and the bottom surface of the recess of the jacket, and the height H of the main flow path and the gap between the fins are The dimension W of the fin gap satisfies the relationship H>W ,
A semiconductor cooling device characterized in that the height H of the main flow path is 0.9 mm to 3 mm . The fin has at least one slit extending from the tip toward the heat dissipation board,
5. The semiconductor cooling device according to claim 4, wherein the width S of the slit and the dimension W of the fin gap satisfy the relationship S≧W . an insulating substrate on which a semiconductor element is mounted on one side of the insulating substrate via a wiring layer;
A heat dissipation board having a plurality of plate-like fins erected at predetermined intervals on one surface, and a recess for accommodating the fins, and is attached to one surface of the heat dissipation board to form a coolant circulation space. Equipped with a liquid-cooled cooler with a jacket,
a semiconductor cooling device in which the other side of the insulating substrate is joined or adhered to the other side of the heat dissipation substrate of the liquid-cooled cooler;
In the cooling liquid circulation space of the liquid-cooled cooler, a gap serving as a main flow path is formed between the tip of the fin and the bottom surface of the recess of the jacket, and the height H of the main flow path and the gap between the fins are The dimension W of the fin gap satisfies the relationship H>W ,
The fin has at least one slit extending from the tip toward the heat dissipation board,
A semiconductor cooling device characterized in that the width S of the slit and the dimension W of the fin gap satisfy the relationship S≧W . 7. The semiconductor cooling device according to claim 3, wherein the slit is provided at a position away from directly below the semiconductor element. Semiconductor cooling according to any one of claims 3, 5, 6, and 7, which has a plurality of semiconductor elements in the flow direction of the coolant, and the slit is provided between the semiconductor elements in the flow direction of the coolant. Device.

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