Electronic control device

Abstract

The purpose of the present invention is to provide an electronic control device that enhances cooling performance while suppressing an increase in size.　An electronic control device according to the present invention comprises: a first substrate 11 on which a high heat-generating chip 5 is mounted; a second substrate 12 on which a low heat-generating chip 6 and a cable connector 71 are mounted; a housing 9 that houses the first substrate 11 and the second substrate 12; and a cooling path 8. The first substrate 11 and the second substrate 12 are electrically connected and are arranged such that planar parts thereof overlap each other. On the first substrate 11, the high heat-generating chip 5 is mounted on the surface opposite to a facing surface that faces the second substrate 12. On the second substrate 12, the low heat-generating chip 6 is mounted on the surface opposite to a facing surface that faces the first substrate 11. The high heat-generating chip 5 is thermally connected to the cooling path 8, and the low heat-generating chip 6 is thermally connected to the housing 9.

Description

Electronic control unit

[0001] The present invention relates to an electronic control device.

[0002] In recent years, automotive electronic systems have become increasingly sophisticated, with cars equipped with autonomous driving systems (AD) and advanced driver assistance systems (ADAS) being commercialized. In such cars, numerous sensors such as cameras, radar, and lidar are connected to the ECU (Electrical Control Unit), and the ECU performs advanced processing such as recognition of the surrounding environment, action planning, and vehicle control.

[0003] Furthermore, as new functions such as connectivity and infotainment are being added to automobiles, the architecture is changing to one in which many of these functions are processed by a centralized ECU. Such centralized ECUs are called integrated ECUs (electronic control units) because they integrate various functions, and high-performance integrated ECUs are required to keep up with the trend toward more advanced autonomous driving systems and the addition of new functions.

[0004] To achieve a high-performance integrated ECU, the ECU is equipped with multiple printed circuit boards each equipped with a high-performance chip (SoC: System on Chip), and a multi-board structure is being considered to electrically connect these. In addition to a structure in which boards are stacked vertically, as in information and communications servers, there is also a multi-board structure in which boards are arranged horizontally, taking into account the reliability and cost specific to automotive applications.

[0005] Chips mounted on a substrate generate heat and therefore require cooling. For example, Patent Document 1 discloses a cooling structure for vertically stacked substrates.

[0006] In Patent Document 1, two boards are mounted upright on a motherboard via connectors, facing each other. A plurality of boards equipped with memory and other components are mounted horizontally on each of the two opposing boards, and these boards are stacked vertically. Furthermore, a heat sink is fixed to the motherboard, and the boards mounted horizontally are positioned on top of this heat sink. Heat from the chips mounted on the boards is dissipated into the air via the heat sink. In Patent Document 1, a board unit is configured as described above, and the chips are cooled using a natural cooling method.

[0007] International Publication No. 2003 / 022024

[0008] Along with high-performance integrated ECUs, SoCs are also consuming more power, which requires improved cooling performance. For this reason, the natural cooling method described in Patent Document 1 is unable to cool integrated ECUs. Therefore, water-cooling methods are becoming more common as a cooling method for integrated ECUs. Meanwhile, due to space constraints within vehicles, there is also a demand for miniaturization of integrated ECUs.

[0009] However, because of the multi-board structure mentioned above and the increasing adoption of water-cooling structures, the size of the integrated ECU is inevitably large. Given this background, the challenge is how to reduce the size of the ECU while improving its performance.

[0010] Furthermore, when applying a water-cooled structure to the technology described in Patent Document 1, the board unit needs to be covered with a housing, which poses the problem that the heat generated by chips that are not in contact with the cooling part of the water-cooled structure will cause the temperature inside the housing to rise, resulting in a decrease in cooling performance.

[0011] An object of the present invention is to solve the above-mentioned problems and to provide a technique for improving the cooling performance of an electronic control unit and suppressing an increase in the size of the electronic control unit.

[0012] In order to achieve the above object, the present invention provides an electronic control device comprising a first substrate on which a high heat generating chip is mounted, a second substrate on which a low heat generating chip and an external connector are mounted, a housing for accommodating the first substrate and the second substrate, and a cooling path, wherein the first substrate and the second substrate are electrically connected and arranged so that their planar portions overlap, the first substrate has a high heat generating chip mounted on the surface opposite to the opposing surface facing the second substrate, the second substrate has a low heat generating chip mounted on the surface opposite to the opposing surface facing the first substrate, the high heat generating chip is thermally connected to the cooling path, and the low heat generating chip is thermally connected to the housing.

[0013] According to the present invention, it is possible to provide a technique for improving the cooling performance of an electronic control unit and suppressing an increase in the size of the electronic control unit.

[0014] FIG. 1 is a schematic diagram showing the structure of an integrated ECU according to a first comparative example; FIG. 2 is a schematic diagram showing the structure of an integrated ECU according to a second comparative example; FIG. 3 is an external perspective view of an integrated ECU according to a first embodiment of the present invention; FIG. 4 is a cross-sectional view of an integrated ECU according to a first embodiment of the present invention; FIG. 5 is an external perspective view showing a partial configuration of an integrated ECU according to a second embodiment of the present invention; FIG. 6 is an exploded perspective view of an integrated ECU according to a second embodiment of the present invention; FIG. 7 is a cross-sectional view of an integrated ECU according to a third embodiment of the present invention; FIG. 8 is a side view showing a partial configuration of an integrated ECU according to a fourth embodiment of the present invention; FIG. 9 is a cross-sectional view of an integrated ECU according to a fifth embodiment of the present invention.

[0015] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In principle, identical elements are designated by the same reference numerals in all drawings. Furthermore, descriptions of parts having identical functions will be omitted. Note that the configurations described below are merely examples, and it is not intended that the embodiments of the present invention be limited to the specific embodiments below.

[0016] In the following embodiments, an integrated ECU will be used as an example of an electronic control device. Before describing the embodiments of the present invention, the structure of a comparative example will be described first. FIG. 1 is a schematic diagram showing the structure of an integrated ECU according to a first comparative example. FIG. 2 is a schematic diagram showing the structure of an integrated ECU according to a second comparative example.

[0017] In FIG. 1, a plurality of chips 3a are mounted on a substrate 2a constituting an integrated ECU 1. Similarly, a plurality of chips 3b are mounted on a substrate 2b. The substrates 2a and 2b are arranged so that the surfaces on which the chips 3a and 3b are mounted face each other. A cooling path 4a is arranged between the opposing substrates 2a and 2b. A liquid flows through the cooling path 4a. The plurality of chips 3a mounted on the substrate 2a and the plurality of chips 3b mounted on the substrate 2b are arranged so as to be in contact with the cooling path 4a, and are cooled by the fluid flowing through the cooling path 4a. The integrated ECU in FIG. 1 is configured by stacking two substrates and arranging a cooling path between the substrates.

[0018] In recent years, demand for improved performance in integrated ECUs has led to an increase in the number of chips mounted on a board. Accordingly, the number of boards on which chips are mounted has also increased. The integrated ECU 1 shown in FIG. 2 adds a board 2c, on which chip 3c is mounted, to the structure shown in FIG. 1 . The board 2c is positioned so that the surface on which chip 3c is mounted faces board 2b. A cooling path 4b through which a fluid flows is disposed between boards 2b and 2c, and multiple chips 3c are brought into contact with the cooling path 4b to cool the chips 3c. The integrated ECU shown in FIG. 2 is configured with three stacked boards, with cooling paths disposed between each board.

[0019] As shown in Figure 2, if the number of boards on which chips are mounted increases and the number of stacked layers increases, the number of cooling paths also increases, resulting in an increase in the size of the integrated ECU. Means for solving this problem will be described below.

[0020] Fig. 3 is a perspective view of the appearance of the integrated ECU according to the first embodiment of the present invention. Fig. 4 is a cross-sectional view of the integrated ECU according to the first embodiment of the present invention. In Fig. 3, the cooling path 8 and the housing 9 are omitted.

[0021] The integrated ECU has a two-tiered board structure in which the planar portion of the first board 11 and the planar portion of the second board 12 face each other so as to overlap, and the first board 11 and the second board 12 are electrically connected. The first board 11 and the second board 12 can be electrically connected, for example, by a board-to-board (B2B) connector such as the board-to-board connector 41. The board-to-board connector 41 of this embodiment is disposed so as to be sandwiched between the first board 11 and the second board 12.

[0022] A high heat-generating chip 5 is mounted on the surface of the first substrate 11 opposite to the surface facing the second substrate 12. A high heat-generating chip is a chip that consumes 10 W or more of power and is difficult to cool by natural air cooling, such as an SoC, memory, accelerator, or GPU (Graphics Processing Unit). If there are multiple high heat-generating chips 5, at least one of them is electrically connected to the inter-board connector 41.

[0023] On the other hand, a cable connector 71 (external connector) is mounted on the second substrate 12. In addition, a low heat generating chip 6 is mounted on the surface of the second substrate 12 opposite to the surface facing the first substrate 11.

[0024] The cable connector 71 of this embodiment is a connector for inserting a power cable or a high-speed communication cable. The low heat-generating chip 6 is a chip that consumes less than 10 W of power and is, for example, a chip that can be cooled by natural air cooling, such as a microcomputer, a communication IC, or a power supply IC. The low heat-generating chip 6 is electrically connected to the board-to-board connector 41, and if the low heat-generating chip 6 exchanges signals with an external device, such as a communication IC or a power supply IC, it is also electrically connected to the cable connector 71. The high heat-generating chip 5 is electrically connected to the cable connector 71 (external connector) via the low heat-generating chip 6.

[0025] Next, thermal connections within the electronic control unit will be described with reference to Fig. 4. The integrated ECU 10 of this embodiment includes a housing 9 that forms an outer shell and is made of metal.

[0026] The housing 9 accommodates the first substrate 11, the second substrate 12, and the cooling path 8. The cooling path 8 is provided with a connecting pipe 8a that penetrates the housing 9, and a refrigerant circulates through the cooling path 8 via the connecting pipe 8a. The connecting pipe 8a is connected to a heat exchanger (not shown).

[0027] With the first substrate 11, the second substrate 12, and the cooling path 8 housed in the housing 9, the high-heat-generating chip 5 mounted on the first substrate 11 is thermally connected to the cooling path 8. A typical form of the cooling path 8 is a water-cooled jacket, but it may also be in physical contact with the coolant itself, as in immersion cooling. Also, a thermally conductive substance such as a thermal interface material (TIM) may be present between the high-heat-generating chip 5 and the cooling path 8.

[0028] On the other hand, the low heat generating chip 6 mounted on the second substrate 12 is thermally connected to the housing 9. As with the high heat generating chip 5, a thermally conductive material such as a TIM may be present between the low heat generating chip 6 and the ECU housing 9.

[0029] When the high-heat-generating chip 5 generates heat, the heat is transferred to the cooling path 8, which is in thermal contact with the high-heat-generating chip 5, and the refrigerant in the cooling path 8 is heated. The high-heat-generating chip 5 is cooled by heat exchange with the refrigerant in the cooling path 8. The heated refrigerant in the cooling path 8 is sent to an external heat exchanger via a connecting pipe 8a, where it dissipates heat into the outside air. The refrigerant that has been cooled and dissipated heat in the heat exchanger is sent back to the cooling path 8, where it exchanges heat with the high-heat-generating chip 5. The refrigerant circulates between the cooling path 8 and the heat exchanger. To circulate the refrigerant, it is preferable to connect a pump (not shown) to the cooling path 8.

[0030] On the other hand, when the low heat generation chip 6 generates heat, the heat is transferred to the housing 9 that is in thermal contact with the low heat generation chip 6, heating the housing 9. The low heat generation chip 6 is cooled by heat exchange with the housing 9. Since the outer peripheral surface of the housing 9 is exposed to the atmosphere, the heat of the heated housing 9 is released into the atmosphere.

[0031] According to this embodiment, the high heat generating chip 5 is thermally connected to the cooling path 8, and the low heat generating chip 6 is in thermal contact with the housing 9. This allows cooling according to the amount of heat generated by the chip without enlarging the water cooling mechanism, thereby improving the cooling performance of the integrated ECU 10 and preventing the integrated ECU 10 from becoming larger.

[0032] Second Embodiment A second embodiment of the present invention will be described with reference to Figures 5 to 7. Components common to the first embodiment are given the same reference numerals, and detailed descriptions thereof will be omitted.

[0033] Fig. 5 is an external perspective view showing a partial configuration of an integrated ECU according to a second embodiment of the present invention. Fig. 6 is an exploded perspective view of the integrated ECU according to the second embodiment of the present invention. Fig. 7 is a cross-sectional view of the integrated ECU according to the second embodiment of the present invention. In Fig. 5, a cooling path 8 and a housing 9 are omitted.

[0034] The second embodiment differs from the first embodiment in that a plurality of first boards 11 and a plurality of second boards 12 are provided. In an integrated ECU, it is common to separate boards for each domain, and the configuration of the first embodiment alone does not allow multiple domains to be mounted on the ECU. In the second embodiment, an example of the structure of an integrated ECU that combines a plurality of the structures shown in the first embodiment to support multiple domains will be described.

[0035] The integrated ECU 10 of the second embodiment includes a first unit 10a made up of a first board 11a and a second board 12a, and a second unit 10b made up of a first board 11b and a second board 12b.

[0036] The first board 11a and the second board 12a that make up the first unit 10a form a two-tiered board structure in which the planar portions of the boards face each other and overlap, and the first board 11a and the second board 12a are electrically connected via an inter-board connector 41a.

[0037] High-heat-generating chips 5a are mounted on the surface of the first substrate 11a opposite to the surface facing the second substrate 12a. A substrate connector 42 is mounted on the first substrate 11a and is electrically connected to one of the high-heat-generating chips 5a. The high-heat-generating chip 5a is electrically connected to a high-heat-generating chip 5b (a chip mounted on another substrate) mounted on the first substrate 11b via this substrate connector 42.

[0038] On the other hand, a cable connector 71a (external connector) is mounted on the second substrate 12a, and a low heat generating chip 6a is mounted on the surface of the second substrate 12a opposite to the surface facing the first substrate 11a.

[0039] Furthermore, in this embodiment, a second unit 10b that constitutes the integrated ECU 10 is provided across the cooling path 8.

[0040] The first board 11b and the second board 12b that make up the second unit 10b form a two-tiered board structure in which the planar portions of the boards face each other and overlap, and the first board 11b and the second board 12b are electrically connected via an inter-board connector 41b.

[0041] The first substrate 11b has a high heat-generating chip 5b mounted on the surface opposite to the surface facing the second substrate 12b. Similar to the first substrate 11a, the first substrate 11b also has a board connector 42 mounted thereon, which is electrically connected to one of the high heat-generating chips 5b.

[0042] On the other hand, a cable connector 71b (external connector) is mounted on the second substrate 12b, and a low heat-generating chip 6b is mounted on the surface of the second substrate 12b opposite to the surface facing the first substrate 11b.

[0043] The cable connectors 71a and 71b of this embodiment are connectors into which a power cable or a high-speed communication cable is inserted.

[0044] Next, the thermal connections within the electronic control unit will be described with reference to Figure 4. The integrated ECU 10 of this embodiment includes a metal housing 9 that forms the outer shell. The housing 9 includes a box-shaped housing main body 9a and a lid 9b that covers the opening of the housing main body 9a. The housing main body 9a also has a notch 91 formed therein for passing the connecting pipe 8a of the cooling path 8. The housing main body 9a also has an open portion where the cable connectors 71a and 71b are located.

[0045] The housing 9 accommodates the first substrate 11a and the second substrate 12b that constitute the first unit 10a, as well as the cooling path 8. The cooling path 8 is provided with a connecting pipe 8a that protrudes from a cutout portion 91 in the housing 9, and a refrigerant circulates within the cooling path 8 via the connecting pipe 8a. The connecting pipe 8a is connected to a heat exchanger (not shown). The housing 9 also accommodates the first substrate 11b and the second substrate 12b that constitute the second unit 10b.

[0046] When the first unit 10a, the second unit 10b, and the cooling path 8 are housed in the housing 9, the high-heat-generating chip 5a mounted on the first substrate 11a and the high-heat-generating chip 5b mounted on the first substrate 11b are thermally connected to the cooling path 8.

[0047] On the other hand, the low heat generating chip 6a mounted on the second substrate 12a is thermally connected to the housing body 9a (housing 9), and the low heat generating chip 6b mounted on the second substrate 12b is thermally connected to the lid 9b (housing 9).

[0048] When the high-heat-generating chips 5a, 5b generate heat, the heat is transferred to the cooling paths 8, which are in thermal contact with the high-heat-generating chips 5a, 5b, and the refrigerant in the cooling paths 8 is heated. The high-heat-generating chips 5a, 5b are cooled by exchanging heat with the refrigerant in the cooling paths 8. The heated refrigerant in the cooling paths 8 is sent to an external heat exchanger via a connecting pipe 8a, where it dissipates heat into the outside air. The refrigerant that has dissipated heat and been cooled in the heat exchanger is sent back to the cooling paths 8, where it exchanges heat with the high-heat-generating chips 5a, 5b. The refrigerant circulates between the cooling paths 8 and the heat exchanger. To circulate the refrigerant, a pump (not shown) may be connected to the cooling paths 8.

[0049] On the other hand, when the low heat generation chips 6a, 6b generate heat, the heat is transferred to the housing 9 that is in thermal contact with the low heat generation chips 6a, 6b, heating the housing 9. The low heat generation chips 6a, 6b are cooled by heat exchange with the housing 9. Since the outer peripheral surface of the housing 9 is exposed to the atmosphere, the heat of the heated housing 9 is released into the atmosphere.

[0050] According to this embodiment, the high heat generating chips 5a, 5b are thermally connected to the cooling path 8, and the low heat generating chips 6a, 6b are thermally in contact with the housing 9. This allows cooling according to the amount of heat generated by the chips without enlarging the water cooling mechanism, thereby improving the cooling performance of the integrated ECU 10 and preventing the integrated ECU 10 from becoming larger.

[0051] 6 and 7, various other structures using the configuration of Example 2 are conceivable for the integrated ECU. For example, a plurality of configurations of Example 2 may be connected to a main board larger than the first board 11 and the second board 12 by the board connector 42. Alternatively, a plurality of configurations of Example 2 may be connected to a backplane board by the board connector 42. This configuration realizes an integrated ECU structure that employs a plurality of structures of the present invention, making it possible to further reduce the size of the integrated ECU.

[0052] Third Embodiment A third embodiment of the present invention will be described with reference to Fig. 8. Fig. 8 is an external perspective view showing a partial configuration of an integrated ECU according to the third embodiment of the present invention. Components common to those in the first embodiment are given the same reference numerals, and detailed descriptions thereof will be omitted.

[0053] In the third embodiment, in order to increase the packaging density of the interfaces, cable connectors 71 are mounted along multiple sides of the second substrate 12. As in the first embodiment, all cable connectors 71 are electrically connected to the low heat generating chips 6 and the high heat generating chips 5 via the inter-board connectors 41. The integrated ECU 10 needs to receive signals related to multiple functions, and therefore needs to be equipped with multiple connectors. However, the configuration of the third embodiment makes it possible to receive many signals with a small number of substrates.

[0054] A fourth embodiment of the present invention will be described with reference to Fig. 9. Fig. 9 is a side view showing a partial configuration of an integrated ECU according to the fourth embodiment of the present invention. Components common to the first embodiment are given the same reference numerals, and detailed descriptions thereof will be omitted.

[0055] In the fourth embodiment, the first substrate 11 and the second substrate 12 are electrically connected by a cable. A cable connector 72 is mounted on the first substrate 11 on the side opposite the second substrate 12, and a cable connector 72 is mounted on the second substrate 12 on the side opposite the first substrate 11. These are then electrically connected by a cable 80. This connection method makes it possible to reduce the space between the first substrate 11 and the second substrate 12, thereby enabling further miniaturization of the integrated ECU 10. Note that, depending on the amount of heat generated by the high-heat-generating chip 5, it may be necessary to thermally separate the first substrate 11 and the second substrate 12. Therefore, it is recommended to provide a spacer between the first substrate 11 and the second substrate 12 or to interpose a heat insulating member such as a heat insulating sheet between the first substrate 11 and the second substrate 12.

[0056] A fifth embodiment of the present invention will be described with reference to Fig. 10. Fig. 10 is a cross-sectional view of an integrated ECU according to the fifth embodiment of the present invention. Components common to the first embodiment are designated by the same reference numerals, and detailed descriptions thereof will be omitted.

[0057] The fifth embodiment aims to suppress electromagnetic noise generated between the boards and the housing in an integrated ECU. In an integrated ECU using a metal housing 9, a physical gap 92 exists between the cable connector 71 and the housing 9, and electromagnetic noise generated in the gap 92 propagates inside the housing 9. To suppress such electromagnetic noise, the boards (first board 11, second board 12) inside the housing 9 must be electrically connected to the housing 9 to equalize their potentials. That is, the second board 12 is electrically connected at least to the housing 9, and depending on the noise intensity, the second board 12 is also electrically connected directly or indirectly to the first board 11 and the cooling path 8. A specific electrical connection method for achieving this is, for example, a conductive gasket 13 made of a material containing metal powder. The gasket 13 is positioned to electrically connect the cooling path 8, the first board 11, the second board 12, and the housing 9. With this configuration, according to the fifth embodiment, electromagnetic noise can be efficiently suppressed even when the housing 9 is made of metal.

[0058] A sixth embodiment of the present invention will be described with reference to Figures 3 and 4. The same components as those in the first embodiment are designated by the same reference numerals, and detailed descriptions thereof will be omitted.

[0059] In the sixth embodiment, the purpose is to improve communication quality by the board layout of the integrated ECU.

[0060] The side on which the cable connectors 71 of the second substrate 12 are mounted becomes longer as the number of connectors increases, and accordingly, the low heat generating chip 6f may be mounted at a position far from the inter-board connector 41. In this case, the transmission distance between the low heat generating chip 6f and the high heat generating chip 5 becomes longer, which may cause problems in communication with the low heat generating chip 6f mounted at a position far from the inter-board connector 41.

[0061] In order to correct this difference in communication quality due to the mounting position of the connector, the low heat generating chip 6n with a high communication frequency is positioned near the inter-board connector 41 (electrical connection point), and the low heat generating chip 6f with a low communication frequency is positioned farther from the inter-board connector 41. In other words, the low heat generating chips are mounted on the second substrate 12 at positions closest to the inter-board connector 41 in descending order of the frequency of the signals they transmit and receive.

[0062] This makes it possible to improve the communication quality of interfaces that use high frequencies and are prone to deterioration in communication quality.

[0063] A seventh embodiment of the present invention will be described. The seventh embodiment aims to provide scalability in the processing performance of the integrated ECU by devising an SoC mounting configuration. While the first embodiment is based on the premise that multiple high-heat-generating chips 5 are mounted on the first substrate 11, the high-heat-generating chips 5 can be mounted in a removable configuration, thereby providing scalability in the processing performance of the device. Specifically, a socket may be mounted on the first substrate 11, and the high-heat-generating chip 5, such as an SoC, may be connected to the socket. Alternatively, a board-to-board connector may be mounted on the first substrate 11, and a module substrate on which the high-heat-generating chip 5 and the board-to-board connector are mounted may be connected.

[0064] The present invention is not limited to the above-described embodiments, but includes various modifications. For example, the above-described embodiments have been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to those including all of the described configurations. Furthermore, it is possible to replace part of the configuration of one embodiment with the configuration of another embodiment, or to add the configuration of another embodiment to the configuration of one embodiment. Furthermore, it is possible to add, delete, or replace part of the configuration of each embodiment with other configurations.

[0065] 5...high heat generating chip, 5a...high heat generating chip, 5b...high heat generating chip, 6...low heat generating chip, 6a...low heat generating chip, 6b...low heat generating chip, 6f...low heat generating chip, 6n...low heat generating chip, 8...cooling path, 8a...connecting pipe, 9...casing, 9a...casing main body, 9b...lid, 10...integrated ECU, 10a...first unit, 10b...second unit, 11...first board, 11a...first board, 11b...first board, 12...second board, 12a...second board, 12b...second board, 13...gasket, 41...board-to-board connector, 41a...board-to-board connector, 41b...board-to-board connector, 42...board connector, 71...cable connector (external connector), 71a...cable connector, 71b...cable connector, 72...cable connector, 80...cable, 91...notch, 92...gap

Claims

1. An electronic control device comprising: a first substrate on which a high heat generating chip is mounted; a second substrate on which a low heat generating chip and an external connector are mounted; a housing that houses the first substrate and the second substrate; and a cooling path, wherein the first substrate and the second substrate are electrically connected and arranged so that their planar surfaces overlap; the first substrate has a high heat generating chip mounted on the surface opposite to the surface facing the second substrate; the second substrate has a low heat generating chip mounted on the surface opposite to the surface facing the first substrate; the high heat generating chip is thermally connected to the cooling path; and the low heat generating chip is thermally connected to the housing.

2. The electronic control device according to claim 1, wherein the high heat generating chip mounted on the first substrate is electrically connected to the external connector via the low heat generating chip mounted on the second substrate.

3. The electronic control device according to claim 1, wherein the first board further comprises a board connector, and the high-heat-generating chip is electrically connected to a chip mounted on another board via the board connector.

4. The electronic control device according to claim 1, wherein the external connectors are mounted on multiple sides of the second substrate, and the high heat-generating chip mounted on the first substrate is electrically connected to the multiple external connectors via the low heat-generating chip mounted on the second substrate.

5. An electronic control device as described in claim 1, wherein cable connectors are mounted on the surface of the first board on which the high heat-generating chip is mounted and on the surface of the second board on which the low heat-generating chip is mounted, and the first board and the second board are electrically connected by a cable.

6. The electronic control device according to claim 5, wherein the first board and the second board overlap with a heat insulating member interposed therebetween.

7. The electronic control device according to claim 1, wherein the second board is electrically connected to the housing.

8. The electronic control device according to claim 7, wherein the housing is made of metal, and the second substrate and the housing are electrically connected by a conductive gasket.

9. An electronic control device as described in claim 1, wherein the low heat generation chips are mounted at positions closest to the electrical connection point between the first board and the second board in descending order of the frequency of the signals transmitted and received by the low heat generation chips.

10. The electronic control device according to claim 1, wherein the high heat generating chip is removably mounted on the first substrate.

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