Semiconductor unit with cooler

A semiconductor unit includes a cooler having a fluid flow space, an insulating substrate bonded to the cooler through a metal, a semiconductor device soldered to the insulating substrate, an intermediate member interposed between the insulating substrate and the fluid flow space and having a first surface where the insulating substrate is mounted, and a mold resin having a lower coefficient of liner expansion than the intermediate member. The insulating substrate, the semiconductor device and the cooler are molded by the mold resin. The intermediate member has a second surface that extends upward or downward relative to the first surface. The first surface is covered by the mold resin. The second surface is covered by a resin cover.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor unit.

There is known a semiconductor unit in which a power semiconductor device mounted on one side of a die pad, an insulating plate mounted on the other side of the die pad and a hollow heat exchange member mounted on the side of the insulating plate opposite from the die pad are molded with a mold resin into a module, as disclosed for example in Japanese Unexamined Patent Application Publication No. 2007-329163. Such molded power semiconductor device has high reliability.

There is also known a semiconductor unit in which the hollow heat exchange member is brazed or soldered to the insulating plate.

In the semiconductor unit of such structure, however, the mold resin may be detached from the hollow heat exchange member due to the difference in the coefficient of linear expansion between the mold resin and the hollow heat exchange member.

The present invention is directed to providing a semiconductor unit which prevents such detachment of the mold resin.

SUMMARY OF THE INVENTION

In accordance with an aspect of the present invention, a semiconductor unit includes a cooler having a fluid flow space, an insulating substrate bonded to the cooler through a metal, a semiconductor device soldered to the insulating substrate, an intermediate member interposed between the insulating substrate and the fluid flow space and having a first surface where the insulating substrate is mounted, and a mold resin having a lower coefficient of liner expansion than the intermediate member. The insulating substrate, the semiconductor device and the cooler are molded by the mold resin. The intermediate member has a second surface that extends upward or downward relative to the first surface. The first surface is covered by the mold resin. The second surface is covered by a resin cover.

In accordance with another aspect of the present invention, a semiconductor unit includes a cooler having a fluid flow space, an insulating substrate bonded to the cooler through a metal, a semiconductor device soldered to the insulating substrate, and a mold resin having a lower coefficient of liner expansion than the cooler. The insulating substrate, the semiconductor device and the cooler are molded by the mold resin. The cooler includes a first plate having a first surface where the insulating substrate is mounted; and a second plate having a base, a vertical portion extending upward from the periphery of the base and bonded to the first plate, and an extension extending upward from the vertical portion beyond the first surface. The mold resin covers the first surface and the entire of the extension.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following will describe the embodiments of the semiconductor unit according to the present invention with reference to the accompanying drawings. It is noted that, in the drawings, some components of the semiconductor unit are shown with exaggerated dimensions for simplicity.

Referring toFIG. 1, the semiconductor unit of the first embodiment which is designated generally by1includes two semiconductor devices20mounted on a circuit board10and a heat sink30thermally coupled to the circuit board10. The circuit board10, the semiconductor devices20and the heat sink30are molded by a mold resin70. The semiconductor unit1is applicable to an inverter for use in a vehicle which converts DC power of a battery into AC power to drive a travel motor of the vehicle.

The circuit board10is composed of an insulating substrate11, a metal plate12bonded to the upper surface of the insulating substrate11, and a metal plate13bonded to the lower surface of the insulating substrate11. The insulating substrate11is a rectangular thin plate and provided, for example, by a ceramic substrate that is made of aluminum nitride, alumina or silicon nitride.

The metal plate12serves as a wiring layer or an electrode and also serves to release the heat generated by the semiconductor devices20. The metal plate12is made of, for example, an aluminum-based metal or copper. The term “aluminum-based metal” includes pure aluminum and aluminum alloys.

Each of the semiconductor devices20is mounted to the metal plate12through a solder layer21. That is, the semiconductor device20is soldered to the metal plate12of the circuit board10. The semiconductor device20is thermally coupled to the insulating substrate11through the metal plate12. The semiconductor device20may be provided, for example, by an insulated gate bipolar transistor (IGBT), a metal oxide semiconductor field effect transistor (MOSFET) or a diode.

The metal plate13bonded to the lower surface of the insulating substrate11serves to connect the insulating substrate11to the heat sink30and also to release the heat generated by the semiconductor devices20. The metal plate13is made of, for example, an aluminum-based metal or copper.

A stress relief member14in the form of a rectangular plate is provided between the metal plate13of the circuit board10and the heat sink30. The stress relief member14is made of a material with high thermal conductivity such as an aluminum-based metal. The stress relief member14is brazed at its upper surface to the metal plate13and at its lower surface to the heat sink30. That is, metal bonding layers made of brazing metal (not shown) are formed between the stress relief member14and the metal plate13and between the stress relief member14and the heat sink30. The heat sink30, the stress relief member14and the circuit board10are bonded together through a metal. The circuit board10and the heat sink30are thermally coupled through the stress relief member14, so that the heat generated by the semiconductor device20is transferred through the circuit board10and the stress relief member14to the heat sink30.

The stress relief member14has plural holes14X formed therethrough and extending in the direction of its thickness. In other words, the holes14X of the stress relief member14forms a region where the stress relief member14is not in contact with the metal plate13and the heat sink30. Such region or the hole14X allows deformation of the stress relief member14and hence reduces the thermal stress occurring in the stress relief member14.

The heat sink30(cooler) is made of a material with high thermal conductivity such as aluminum-based metal. The heat sink30has an upper plate40(first plate), a lower plate50(second plate) and a fluid flow space60.

The upper plate40and the lower plate50are brazed together at their outer marginal portions to form therebetween a space that serves as the fluid flow space60. The fluid flow space60thus formed inside the heat sink30has plural fins or partition walls61extending between the lower plate50and the upper plate40. Each of the partition walls61is brazed at its upper and lower ends to the upper and lower plates40,50, respectively. The partition walls61are spaced at a regular interval and extend parallel to each other. In the fluid flow space60, each partition wall61cooperates with its adjacent partition wall61or its adjacent portion of the lower plate50to form therebetween a channel62through which coolant such as water flows. The fluid flow space60with a plurality of channels62is disposed at a position immediately below the semiconductor devices20that are to be cooled.

Although not shown in the drawing, the heat sink30is connected to a supply pipe through which coolant is supplied to the channels62and also connected to a discharge pipe through which the coolant having passed through the channels62is discharged.

The following will describe in detail the structure of the lower plate50and the upper plate40. The upper plate40is brazed at its upper surface40A to the stress relief member14on which the metal plate13and the insulating substrate11are mounted.

The lower plate50has a base51, a vertical portion52and a horizontal bonding portion53. The base51is of a rectangular planar shape and extends horizontally to form the bottom for the fluid flow space60. The vertical portion52extends vertically upward from the periphery of the base51to form a side wall for the fluid flow space60. The bonding portion53extends horizontally outwardly from the upper end of the vertical portion52. The bonding portion53extends away from the base51or the fluid flow space60. The bonding portion53has a side surface53A that extends vertically downward relative to the upper surface40A of the upper plate40, or extends in the direction that is different from the direction of the upper surface40A of the upper plate40.

The upper plate40(intermediate member) is of a rectangular planar shape and includes a base41and a bonding portion42. The base41extends horizontally to form the top for the fluid flow space60. The base41is brazed at its upper surface to the stress relief member14.

The bonding portion42extends horizontally outwardly from the base41. The bonding portion42extends away from the base41or the fluid flow space60. The bonding portion42is brazed at its lower surface to the upper surface of the bonding portion53of the lower plate50. The interior space formed between the upper and lower plates40,50thus bonded together, that is defined by the base41of the upper plate40, the base51and the vertical portion52of the lower plate50corresponds to the fluid flow space60.

The bonding portion42has an extension43that extends outwardly beyond the bonding portion53of the lower plate50. The extension43has a lower surface43A that is not in contact with the bonding portion53of the lower plate50. The bonding portion42or the extension43has a side surface42A that extends vertically downward from the upper surface40A of the upper plate40.

In the heat sink30, the bonding portions42,53of the upper and lower plates40,50are formed extending outwardly away from the fluid flow space60having the partition walls61. In other words, the heat sink30has an outer profile that is slightly larger than the profile of the fluid flow space60.

In the semiconductor unit1of the present embodiment, part of the heat sink30and the components mounted thereon are molded by the mold resin70in such a way that the mold resin70covers the upper surface40A (first surface) of the upper plate40, the side surface42A (second surface) of the bonding portion42of the upper plate40, the lower surface43A of the extension43of the upper plate40, the side surface53A of the bonding portion53of the lower plate50, the stress relief member14, the circuit board10and the semiconductor devices20. The lower surface of the bonding portion53, the outer surface of the vertical portion52and the lower surface of the base51of the lower plate50are exposed out of the mold resin70. Thus, the lower surface of the heat sink30is exposed out of the mold resin70. The mold resin70is provided by an insulating resin that has a lower coefficient of linear expansion than the heat sink30. Such molding with the mold resin70is performed under a temperature that is lower than the temperature under which the semiconductor device20are soldered to the metal plate12and also under which the metal plate13is brazed to the heat sink30and the stress relief member14, after the stress relief member14and the circuit board10are brazed to the heat sink30and also the semiconductor devices20are soldered to the circuit board10. In the present embodiment, the mold resin70serves as a resin cover for covering the side surface42A of the bonding portion42of the upper plate40.

The following will describe the operation of the semiconductor unit1of the present embodiment. The heat generated by the semiconductor devices20while the semiconductor unit1is energized is transferred through the metal plate12, the insulating substrate11, the metal plate13and the stress relief member14and finally to the heat sink30. The heat sink30, the stress relief member14and the metal plate13bonded together through the metal provides good heat transfer between the metal plate13or the circuit board10and the heat sink30, thereby allowing the heat generated by the semiconductor devices20to be transferred efficiently to the heat sink30.

The heat transferred to the heat sink30is released therefrom to the coolant which is supplied from a source (not shown) through a supply pipe (not shown) to the channels62in the heat sink30and flows therethrough in the same direction. Thus, the heat of the semiconductor devices20transferred through the stress relief member14to the heat sink30is released to the coolant flowing in the channels62. The coolant having passed through the channels62is discharged out of the heat sink30through a discharge pipe.

Receiving the heat generated by the semiconductor devices20, the heat sink30and its adjacent mold resin70are heated and thermally expanded. The difference in the coefficient of linear expansion between the heat sink30and the mold resin70causes thermal stress in the semiconductor unit1. In the semiconductor unit1of the present embodiment wherein the mold resin70covers the side surfaces42A,53A of the bonding portions42,53of the upper and lower plates40,50, when the heat sink30receives a force tending to cause the heat sink30to expand horizontally in the extending direction of the bonding surface between the upper plate40and the mold resin70, the side surfaces42A,53A of the bonding portions42,53of the upper and lower plates40,50press the mold resin70, while the bonding portions42,53receive reaction force from the mold resin70. Such reaction force serves to restrict the expansion of the heat sink30. If there is a large difference in the coefficient of linear expansion between the heat sink30and the mold resin70, the difference in the amount of expansion between the heat sink30and the mold resin70is small, which prevents the mold resin70from being detached from the heat sink30.

The semiconductor unit1of the first embodiment offers the following advantages.

(1) The circuit board10, the semiconductor device20and the stress relief member14are molded integrally with the heat sink30by the mold resin70. There is no need to provide an additional member such as bracket for fixing such components to the heat sink30, resulting in a reduced size of the semiconductor unit.
(2) The mold resin70covers the side surfaces42A,53A of the bonding portions42,53which extend downward from the upper surface40A of the upper plate40, thereby restricting the horizontal expansion of the heat sink30. If there is a large difference in the coefficient of linear expansion between the heat sink30and the mold resin70, the difference in the amount of expansion between the heat sink30and the mold resin70is small, which prevents the mold resin70from being detached from the heat sink30and hence increases the reliability of the connection between the heat sink30and the mold resin70.
(3) The bonding portion42has the extension43that extends beyond the bonding portion53of the lower plate50, and the mold resin70covers the lower surface43A of the extension43. When the heat sink30and the mold resin70are thermally expanded, the mold resin70covering the lower surface43A of the extension43and the upper surface40A of the upper plate40serves to restrict the expansion of the upper plate40in the direction of its thickness, which prevents the mold resin70from being detached from the side surface42A of the bonding portion42. In other words, the provision of the bonding portion42having the extension43allows the sealing of the whole of the upper plate40by the mold resin70with the vertical portion52of the lower plate50exposed out of the mold resin70. This helps to decrease the amount of the mold resin70to be used for sealing and also to prevent thermal deformation of the upper plate40, resulting in increased reliability of the connection between the upper plate40and the mold resin70.
(4) The bonding portions42,53of the upper and lower plates40,50of the heat sink30extend outward of the fluid flow space60, and the bonding portion42is brazed at its lower surface to the upper surface of the bonding portion53so that the fluid flow space60is formed between the upper plate40and the lower plate50. This leads to an increased bonding area, which makes it easy to braze the upper plate40to the lower plate50.
(5) The circuit board10, the stress relief member14and the heat sink30are bonded together through the metal. Such structure provides good heat transfer between the circuit board10and the heat sink30as compared to the case that the circuit board10is bonded to the heat sink30through silicone grease, thereby allowing the heat generated by the semiconductor devices20to be efficiently transferred to the heat sink30.
(6) Molding the circuit board10, the semiconductor devices20and part of the heat sink30by the resin increases the reliability of the connection of such components and prevents deterioration of the semiconductor unit1, thus allowing the performance of the semiconductor unit1to be maintained for a long period of time.
(7) The lower surface of the heat sink30is exposed out of the mold resin70. This allows a heating element bonded to the lower surface of the heat sink30to be cooled.
(8) The stress relief member14having the plural holes14X is interposed between the heat sink30and the circuit board10. The holes14X serves to disperse and reduce the thermal stress caused by the difference in the coefficient of linear expansion between the heat sink30and the insulating substrate11of the circuit board10. This prevents cracks from occurring at the connections between the insulating substrate11and the metal plate12and also between the insulating substrate11and the metal plate13, thereby preventing the mold resin70from being detached.
(9) The mold resin70that covers the side surface42A of the bonding portion42doubles as the resin cover. Therefore, there is no need to provide an additional member to cover the side surface42A of the bonding portion42, which leads to a reduced number of components of the semiconductor unit.

FIGS. 2A and 2Bshow the second embodiment of the semiconductor unit according to the present invention. In the drawings, same reference numerals are used for the common elements or components in the first and second embodiments, and the description of such elements or components of the second embodiment will be omitted or simplified.

As shown inFIGS. 2A and 2B, the semiconductor unit which is designated generally by80has the circuit board10in which a first metal plate81and a second metal plate82are bonded to the upper surface of the insulating substrate11and the metal plate13is bonded to the lower surface of the insulating substrate11.

Semiconductor devices83,84are mounted to the first and second metal plates81,82, respectively, through the solder layers21. Each of the first and second metal plates81,82serve as a wiring layer. A bus bar85is provided to electrically connect the upper surface of the first metal plate81to the upper surface of the semiconductor device84. An electrode86is connected to the upper surface of the semiconductor device83, and an electrode87is connected to the upper surface of the second metal plate82. The electrodes86,87are connected to a power source (not shown).

A resin case90(resin cover) is screwed to the extension43of the upper plate40. The case90has side walls92forming a box shaped body91of the case90and a fastening portion93extending horizontally inward of the body91from the lower end of each side wall92. The distance between the opposite side walls92of each pair is substantially equal to the distance between their corresponding paired opposite side surfaces42A of the upper plate40.

With the upper surface of the fastening portion93of the case90set in contact with the lower surface43A of the extension43of the upper plate40, a screw94is screwed through the extension43into a threaded hole in the fastening portion93to fix the case90. In the case90thus fixed to the upper plate40, the inner peripheral surfaces of the fastening portion93are in contact with the side surfaces53A of the bonding portion53. The distance between the opposite side walls92of each pair is substantially equal to the distance between their corresponding paired opposite side surfaces42A of the upper plate40, and the inner surfaces of the side walls92are in contact with the side surfaces42A. The side walls92of the case90covers the side surfaces42A (second surface) of the upper plate40.

The case90has a rectangular hole92A formed through the upper part of each of one pair of opposite side walls92. The electrodes86,87are inserted through the respective holes92A in such a way that the electrodes86,87are supported by the side walls92. Part of the respective electrodes86,87exposed out of the case90serves as the terminals which are to be connected to the power source. The side walls92of the case90also serve as the support for such terminals.

The opening of the case90adjacent to the fastening portion93is closed by the upper plate40that is screwed at the extension43thereof to the case90. The upper plate40serves as the bottom of the case90. The case90is filled with the mold resin70in such a way that the mold resin70covers the upper surface40A of the upper plate40. In the semiconductor unit80of the second embodiment, the mold resin70and the case90cooperate to cover the entire of the extension43. Specifically, the mold resin70covers the upper surface of the extension43or the upper surface40A of the upper plate40, and the case90covers the side surface of the extension43or the side surface42A of the bonding portion42and the lower surface43A of the extension43. The fastening portion93of the case90covers the side surface53A of the bonding portion53.

When thermal stress occurs in the semiconductor unit80and the heat sink30receives a force tending to cause the heat sink30to expand horizontally in the extending direction of the bonding surface between the upper plate40and the mold resin70, the side surfaces42A,53A of the bonding portions42,53of the upper and lower plates40,50press the case90, while the bonding portions42,53receive reaction force from the case90. Such reaction force serves to restrict the expansion of the heat sink30. The case90also serves to restrict the mold resin70from expanding horizontally in the extending direction of the bonding surface between the upper plate40and the mold resin70. If there is a large difference in the coefficient of linear expansion between the heat sink30and the mold resin70, the difference in the amount of expansion between the heat sink30and the mold resin70is small, which prevents the mold resin70from being detached from the heat sink30.

The second embodiment offers the following advantages, as well as the advantages (1), and (4) to (8) of the first embodiment.

(10) The case90covers the side surfaces42A,53A of the bonding portions42,53and the mold resin70, thereby restricting the horizontal expansion of the heat sink30and the mold resin70. If there is a large difference in the coefficient of linear expansion between the heat sink30and the mold resin70, the difference in the amount of expansion between the heat sink30and the mold resin70is small, which prevents the mold resin70from being detached from the heat sink30and hence increases the reliability of the connection between the heat sink30and the mold resin70.
(11) The case90is used not only as a mold in which the mold resin70is poured, but also as the resin cover, which results in a reduced number of components.
(12) The case90is fixed with its fastening portion93screwed to the extension43of the upper plate40. The extension43can be used as a flange for fixing the case90to the upper plate40of the heat sink30.
(13) The fastening portion93of the case90covers the lower surface43A of the extension43of the upper plate40. The mold resin70and the case90serve to restrict the expansion of the upper plate40in the direction of its thickness, which prevents the mold resin70from being detached from the side surface42A of the bonding portion42.
(14) The case90is used as a support for the terminal. When the mold resin70is poured in the case90, the electrodes86,87are supported by the case90. There is no need to provide an additional member for supporting the electrodes86,87when the mold resin70is poured in the case90.

The above embodiments may be modified in various ways as exemplified below.

The mold resin70does not necessarily need to cover the side surface53A of the bonding portion53of the lower plate50as in the case of the first embodiment. It may be so modified that the mold resin70covers the side surface42A of the bonding portion42of the upper plate40, but the lower surface43A of the extension43is exposed out of the mold resin70. This may reduce the amount of the mold resin70and hence the manufacturing cost of the semiconductor unit1. Such structure offers the advantages similar to the advantages (1), (2), (4) to (6), and (8) of the first embodiment.

The first embodiment may be modified in such a way that the mold resin70covers the lower surface of the bonding portion53and also part of the outer surface of the vertical portion52, as shown inFIG. 3. Such configuration increases the force of the mold resin70that serves to restrict the horizontal expansion of the heat sink30and hence increases the reliability of the connection between the heat sink30and the mold resin70. The heat sink30is sealed tightly, resulting in improved heat radiation.

The first embodiment may be modified in such a way that the mold resin70covers the lower surface of the bonding portion53and also the entire of the outer surface of the vertical portion52, as shown inFIG. 4. Such configuration increases the force of the mold resin70that serves to restrict the horizontal expansion of the heat sink30as compared to the case ofFIG. 3, thereby further increasing the reliability of the connection between the heat sink30and the mold resin70. The heat sink30is sealed tightly, resulting in improved heat radiation.

The first embodiment may be modified in such a way that the mold resin70covers the lower surface of the bonding portion53, the entire of the outer surface of the vertical portion52and also the entire of the lower surface of the base51thereby to cover the entire of the heat sink30.

The first embodiment may be modified in such a way that the bonding portion42of the upper plate40has no extension such as43, as shown inFIG. 5. Such structure offers the advantages similar to the advantages (1), (2), (4) to (6), and (8) of the first embodiment. This modification may be further modified in such a way that the mold resin70covers the lower surface of the bonding portion53, covers the lower surface of the bonding portion53and the entire of the outer surface of the vertical portion52, or covers the entire of the heat sink30. In a similar manner, the second embodiment may be modified in such a way that the bonding portion42of the upper plate40has no extension such as43. In this case, the fastening portion93of the case90may be disposed covering the lower surface of the bonding portion53of the lower plate50and screwed to the bonding portion53.

In the first embodiment, the heat sink30may be replaced by a heat sink30A (cooler) including the upper and lower plates40,50as shown inFIG. 6. Specifically, the base41A of the upper plate40is of a rectangular planar shape and extends horizontally to form the top for the fluid flow space60. The base41A is bonded at its side surface to the lower plate50. The base51of the lower plate50is of a rectangular planar shape and extends horizontally to form the bottom for the fluid flow space60. The vertical portion52of the lower plate50extends vertically upward from the periphery of the base51to form a side wall for the fluid flow space60. The vertical portion52is bonded at its inner surface to the upper plate40. The side surface of the base41A of the upper plate40is brazed to the inner surface of the vertical portion52of the lower plate50. The interior space formed by the base41of the upper plate40, the base51and the vertical portion52of the lower plate50corresponds to the fluid flow space60. The lower plate50has an extension54that extends upward from the vertical portion52beyond the upper surface40A of the upper plate40. The extension54has a side surface54A that extends vertically upward from the upper surface40A of the upper plate40.

In the semiconductor unit1with such structure, the upper surface40A of the upper plate40, the side surface54A of the extension54and part of the outer surface of the vertical portion52in the lower plate50, the stress relief member14, the circuit board10and the semiconductor devices20are covered by the mold resin70.

Such structure also prevents the mold resin70from being detached from the heat sink30A. Specifically, when the heat sink30A receives a force tending to cause the heat sink30A to expand horizontally in the extending direction of the bonding surface between the upper plate40and the mold resin70, the side surface of the vertical portion52and the side surface54A of the extension54press the mold resin70, while the vertical portion52and the extension54of the lower plate50receive reaction force from the mold resin70. Such reaction force serves to restrict the expansion of the lower plate50and hence the horizontal expansion of the upper plate40bonded to the lower plate50. If there is a large difference in the coefficient of linear expansion between the heat sink30A and the mold resin70, the difference in the amount of expansion between the heat sink30A and the mold resin70is small, which prevents the mold resin70from being detached from the heat sink30A.

The heat sink30A has no bonding portion such as42,53ofFIG. 1extending outwardly of the fluid flow space60, resulting in the heat sink30A of a smaller profile.

It may be so modified that the upper plate40has a projection44on its upper surface40A and the entire of the projection44is molded by the mold resin70, as shown inFIG. 7. The projection44is formed integrally with the upper plate40and has a side surface44A (second surface) that extends upward from the upper surface40A of the upper plate40. The projection44may be provided, for example, by a screw or boss. The projection44is made of a material having a coefficient of linear expansion that is close to that of the heat sink30, such as aluminum-based metal.

Such structure also prevents the mold resin70from being detached from the heat sink30. Specifically, when the heat sink30receives a force tending to cause the heat sink30to expand horizontally, the side surface44A of the projection44presses the mold resin70, while the projection44receives reaction force from the mold resin70. Such reaction force serves to restrict the expansion of the projection44and hence the horizontal expansion of the upper plate40where the projection44is formed. If there is a large difference in the coefficient of linear expansion between the heat sink30and the mold resin70, the difference in the amount of expansion between the heat sink30and the mold resin70is small, which prevents the mold resin70from being detached from the heat sink30.

In addition, the side surface of the heat sink30is exposed out of the mold resin70, which may reduce the amount of the mold resin70and hence the manufacturing cost of the semiconductor unit1.

In the embodiments shown inFIGS. 1,3,4and5, the mold resin70covers the side surfaces42A,53A of the bonding portions42,53both extending outward of the fluid flow space60. Alternatively, it may be so modified that the mold resin70covers the entire of a stress relief member15interposed between the metal plate13of the circuit board10and the heat sink30and extending outward of the heat sink30or the fluid flow space60, as shown inFIG. 8. Specifically, the stress relief member15(intermediate member) includes a base15A brazed to both the metal plate13and the upper plate40, a bonding portion15B brazed only to the upper plate40, and an extension15C extending outward of the heat sink30or the fluid flow space60. The extension15C has a side surface15D (second surface) that extends downward from the upper surface (first surface) of the stress relief member15. The mold resin70covers the upper surface of the stress relief member15and the side surface15D and the lower surface of the extension15C. The mold resin70covering the side surface15D and the lower surface of the extension15C serves to restrict the expansion of the stress relief member15. If there is a large difference in the coefficient of linear expansion between the stress relief member15and the mold resin70, the difference in the amount of expansion between the stress relief member15and the mold resin70is small, which prevents the mold resin70from being detached from the stress relief member15. In addition, the structure ofFIG. 8leads to an increased bonding area between the stress relief member15and the heat sink30, as compared to the case that the stress relief member is disposed only at a position immediately below the semiconductor device20. The stress relief member15may be used in the second embodiment so that the extension15C is screwed to the fastening portion93of the case90.

The semiconductor unit1shown inFIG. 8does not necessarily require the metal plate13. In this case, the base15A of the stress relief member15is brazed to the insulating substrate11and the upper plate40. Similarly, the semiconductor unit80of the second embodiment does not necessarily require the metal plate13.

Alternatively, it may be so modified that the mold resin70covers the entire of a metal plate16replacing the metal plate13and the stress relief member15ofFIG. 8and extending outward of the heat sink30or the fluid flow space60, as shown inFIG. 9. Specifically, the metal plate16(intermediate member) includes a base16A bonded to both the insulating substrate11and the upper plate40, a bonding portion16B bonded to the upper plate40, and an extension16C extending outward of the heat sink30or the fluid flow space60. The extension16C has a side surface16D (second surface) that extends downward from the upper surface (first surface) of the metal plate16. The mold resin70covers the upper surface of the metal plate16and the side surface16D and the lower surface of the extension16C. The mold resin70covering the side surface16D and the lower surface of the extension16C serves to restrict the expansion of the metal plate16. If there is a large difference in the coefficient of linear expansion between the metal plate16and the mold resin70, the difference in the amount of expansion between the metal plate16and the mold resin70is small, which prevents the mold resin70from being detached from the metal plate16. In addition, the structure ofFIG. 9leads to an increased bonding area between the metal plate16and the heat sink30, as compared to the case that the metal is disposed only at a position immediately below the semiconductor device20.

In the embodiments shown inFIGS. 1,3,4and5, the side surface42A of the bonding portion42covered by the mold resin70does not necessarily need to extend perpendicularly to the upper surface40A of the upper plate40.

In the second embodiment, the case90does not necessarily require the fastening portion93. For example, the case90may be fixed to the upper plate40in such a way that the inner surface of the side walls92of the case90is bonded to the side surface42A of the bonding portion42by adhesive. Alternatively, the screw94may be screwed through the side wall92into a threaded hole in the side surface42A of the bonding portion42to fix the case90. The case90may be fixed by any other suitable method.

In the second embodiment, the screw94is screwed through the fastening portion93into a threaded hole in the extension43.

Not only water but also other liquid such as alcohol or a gas such as air may be used as the coolant flowing through the fluid flow space60of the heat sink30.

The partition wall61of the heat sink30may be of any suitable shape. For example, corrugated fins may be provided between the upper and lower plates40,50.

The heat sink30does not necessarily require the partition walls61.

The cross section of the holes14X of the stress relief member14may be of any suitable shape such as circular, elliptical or square as long as the holes14X function to reduce the thermal stress occurring in the stress relief member14.

The number of components mounted on the heat sink30may be changed. For example, two or more metal plates such as12may be mounted on the insulating substrate11and one or three or more semiconductor devices20may be mounted on each metal plate12.

The semiconductor units1,80do not necessarily require the stress relief member14.

The semiconductor units1,80do not necessarily require the metal plate13.

The semiconductor units1,80are not limited to in-vehicle use.

The second embodiment may be modified in such a way that the case90is fixed to the upper surface of the extension43or the upper surface40A of the upper plate40and also that the inner surfaces of the respective side walls92of the case90are located inward of the side surfaces42A of the bonding portion42of the upper plate40, as shown inFIG. 10. The structure ofFIG. 10may reduce the capacity of the case90, thereby reducing the amount of the mold resin70poured in the case90and hence the manufacturing cost of the semiconductor unit80.