Patent Application
20090289344
The present invention relates to a semiconductor device. Particularly, it relates to a semiconductor device configured so that an insulating substrate having a semiconductor element mounted thereon is joined onto a heat radiator.
Power semiconductor modules operable under a large-current high-voltage environment have been used in various fields, for example, for general industrial purposes and in-vehicle purposes in recent years. The power semiconductor modules employ semiconductor devices such as IGBTs (Insulated Gate Bipolar Transistors), power MOSs (Metal Oxide Semiconductors) and FWDs (Free Wheel Diodes).
For example, a semiconductor device has a semiconductor element mounted on an insulating substrate of ceramics. When the semiconductor device is operated, the semiconductor element generates heat. The insulating substrate of the semiconductor device is joined to a metal heat radiator such as a heat radiating fin by a solder member. The heat generated by the semiconductor element is radiated to the outside through the heat radiator to thereby cool the semiconductor device (e.g. see Patent Document 1).
Because the semiconductor device in which the insulating substrate and the heat radiator having a large difference in heat expansion coefficient from the insulating substrate are joined by a solder member is used in various environments, for example, for general industrial purposes and in-vehicle purposes as described above, high reliability is required in the semiconductor device. Therefore, a member, such as an aluminum-silicon carbide (Al—SiC) composite material or a copper-molybdenum (Cu—Mo) composite material, having a heat expansion coefficient close to that of the insulating substrate is used as the heat radiator. A new structure for joining the insulating substrate and the heat radiator to each other without use of any solder member has been further proposed.
The semiconductor device improved in reliability by the aforementioned method, however, has the following problem. First, the Al—SiC composite material or the Cu—Mo composite material used as the heat radiator is expensive and low in recycling efficiency. In the structure for joining the insulating substrate and the heat radiator to each other without use of any solder member, the cost for reducing contact thermal resistance increases and the work for attaching the structure to a power semiconductor module is complicated.
Therefore, to obtain a low-cost semiconductor device with high reliability, a solder member containing tin (Sn) as a main component and about 5% by weight of antimony (Sb) has been used for joining the insulating substrate and the heat radiator to each other. Such a solder member can be used according to a conventional assembling method and a manufacturing apparatus. A solder member that is obtained as described above can have a lifetime of 3000 cooling-and-heating cycles. Both high reliability and low cost can be satisfied by the solder member. At present, use of the solder member, an aluminum oxide (Al2O3) type insulating substrate and a metal type heat radiator is chiefly the most suitable combination.
Higher reliability will be required as the power semiconductor module will be used for various purposes in the future. With respect to the aforementioned structure of the most suitable combination, it is necessary to attain higher reliability while the cost is kept low. It is therefore necessary to provide an insulating substrate using high heat-conductive ceramics such as aluminum nitride (AlN) and silicon nitride (Si3N4), which is high in heat conductivity, because of increase of heating density caused by size reduction and power increase.
[Patent Document 1] JP-A-2006-202884
The insulating substrate using high heat-conductive ceramics such as AlN and Si3N4 is higher in heat conductivity but lower in heat expansion coefficient than an Al2O3 type insulating substrate. For this reason, if the ceramic type insulating substrate is used in combination with a heat radiator of Cu, the heat expansion coefficient difference between the insulating substrate and the heat radiator becomes larger than that in the case where the Al2O3 type insulating substrate is used.
For this reason, if the insulating substrate of AlN or Si3N4 is used in combination with the heat radiator of Cu, stress imposed on the solder member becomes larger than that in the case where the Al2O3 type insulating substrate is used in combination with the heat radiator of Cu. Accordingly, there is a problem that the lifetime indicated by the number of cooling-and-heating cycles decreases and reliability decreases even when the solder member contains about 5% by weight of Sb which is relatively resistant to thermal deterioration.
The invention has been developed in consideration of such circumstances. An object of the invention is to provide a semiconductor device improved in reliability.
Further objects and advantages of the invention will be apparent from the following description of the invention.
To achieve the foregoing object, there is provided a semiconductor device configured so that an insulating substrate having a semiconductor element mounted thereon is joined onto a heat radiator.
The semiconductor device includes an insulating substrate; at least one semiconductor element mounted on a first principal surface of the insulating substrate; and a heat radiator joined through a solder member to a second principal surface of the insulating substrate opposite to the first principal surface on which the semiconductor element is mounted, wherein the solder member contains at least tin and antimony; and the antimony content of the solder member is in a range of from 7% by weight to 15% by weight, both inclusively.
According to the configuration, reliability of the semiconductor device can be improved.
Embodiments of the invention will be described below with reference to the drawings. Incidentally, the technical scope of the invention is not limited to the embodiments. In the drawings, the same or like numerals refer to the same or like parts.
First, a first embodiment of the invention will be described.
As shown in
A front electrode and a rear electrode (both not shown), each being made of a metal film, are provided on opposite surfaces of the semiconductor element 11, respectively. The rear electrode of the semiconductor element 11 is joined to the insulating substrate 12 by a solder member 14a. Any type lead-free (Pb-free) solder alloy such as Sn—Ag alloy, Sn—Cu alloy, Sn—In alloy, Sn—Bi alloy or Sn—Sb alloy (alloy containing Sn as a main component, and one or more elements as additional components selected from Ag, Cu, In, Bi, Sb, etc.) can be used for the solder member 14a. Preferably, the same alloy as used for a solder member 14 which will be described later may be used for the solder member 14a.
For example, the insulating substrate 12 has a ceramic substrate 12b containing any one of Al2O3, AlN and Si3N4 as a main component. Conducting layers 12a and 12c are joined to opposite surfaces of the ceramic substrate 12b, respectively. The conducting layer 12a is a conducting pattern of metal serving as an electric circuit. The conducting layer 12a is joined through the solder member 14a to the rear electrode of the semiconductor element 11. Similarly, the conducting layer 12c is a conducting pattern of metal serving as an electric circuit. Although the conducting layers 12a and 12c may be made of Al, it is preferable that the conducting layers 12a and 12c are made of Cu which is inexpensive and excellent in heat conduction.
The heat radiator 13 is joined through a solder member 14 to the conducting layer 12c of the insulating substrate 12. For example, the heat radiator 13 serves as a heat conductor for conducting heat to an external cooler of a semiconductor package (not shown). Although the heat radiator 13 may be made of a composite material such as Al—SiC or Cu—Mo, it is preferable that the heat radiator 13 is made of Cu which is inexpensive and excellent in heat conduction.
In the semiconductor device 10 configured as described above, heat distortion caused by the heat expansion coefficient difference between the ceramic substrate 12b and the heat radiator 13 is generated in a junction portion between the conducting layer 12c of the insulating substrate 12 and the heat radiator 13. Because the heat expansion coefficient difference between the ceramic substrate 12b and the heat radiator 13 of Cu is particularly large compared with any other combination, heat distortion generated in the junction portion in this case is relatively remarkable. Although it may be conceived that a material, such as an Al—SiC composite material or a Cu—Mo composite material, having a smaller heat expansion coefficient than that of Cu is used for the heat radiator 13, these composite materials are more expensive than Cu and the heat radiating characteristic of the semiconductor device 10 is lowered because these materials are lower in heat conductivity than Cu.
Therefore, while Cu is used for the conducting layer 12c and the heat radiator 13, an Sn—Sb solder alloy containing Sn as a main component, and 7% by weight to 15% by weight (both inclusively) of Sb, preferably 8% by weight to 10% by weight (both inclusively) of Sb is used as an optimum composition of the solder member 14 used for joining the conducting layer 12c and the heat radiator 13 to each other.
Determination of the optimum composition of the solder member will be described below.
Incidentally, the optimum composition of the solder member is determined in such a manner that the thermal fatigue lifetimes of solder members prepared in advance to have various compositions are evaluated. The compositions of the solder members prepared in advance are Sn—Sb solder alloys containing Sn as a main component and containing 5% by weight of Sb, 6% by weight of Sb, 8% by weight of Sb, 10% by weight of Sb, 13% by weight of Sb and 15% by weight of Sb, respectively.
The evaluation of the thermal fatigue lifetime is performed on samples using these solder members. Incidentally, these solder members are alloys adjusted by dissolving raw materials Sn and Sb in an electric furnace. The purity of each raw material is 99.99% by weight or higher, and each raw material contains impurities inevitably. Accordingly, the respective solder members contain inevitable impurities.
As shown in
A cooling-and-heating cycle test was applied to each sample 20. In the cooling-and-heating cycle test, a cooling-and-heating cycle for changing the atmospheric temperature of the sample 20 in a range of about −40° C. to about 125° C., both inclusively, was repeated in a range of 2000 cycles to 5000 cycles at intervals of a predetermined time. Each sample 20 was evaluated while the length of a crack X which occurred in a junction portion between the heat radiator 23 and the solder member 24 after such cycles was used as an index. Incidentally, the insulating substrate 22 suffered stress from its outer edge portion toward its central portion. Therefore, in the cooling-and-heating cycle test, the length of a crack X caused in this instance was used as an index of the thermal fatigue lifetime of the sample. The area ratio occupied by the crack may be used in place of the length of the crack as an index of the thermal fatigue lifetime. This is a ratio of the area of the crack produced in the junction portion to the area of contact between the solder member and the conducting layer.
A result of this test will be described below.
First, the case where the ceramic substrate 22b is made of Al2O3 will be described.
As shown in
The case where the ceramic substrate 22b is made of Si3N4 will be described next.
Similarly to
Incidentally, the crack length in the case where Si3N4 is used as the insulating substrate is larger than the crack length in the case where Al2O3 is used as the insulating substrate even when the two cases are equal in the number of cycles. For example, when the Sb content is 5% by weight and the number of cycles is 3000, the crack length in use of Al2O3 is a little smaller than 3 mm but the crack length in use of Si3N4 reaches about 11 mm. According to the results shown in
Although the result obtained in the case where, for example, AlN not thinner than about 0.5 mm but thinner than about 0.8 mm was used as the ceramic substrate 22b is not shown, it was confirmed that the average crack length versus the number of cooling-and-heating cycles decreased remarkably when the Sb content increased to 8% by weight, and then the average crack length decreased as the Sb content further increased, similarly to
The reason why the thermal fatigue lifetime was improved is conceivable as follows. That is, both heat resistance and thermal fatigue strength of the solder member 24 are improved by addition of Sb to Sn. Moreover, the thermal fatigue lifetime is improved because the melting temperature increases to improve heat resistance so that thermal stress prevents Sn crystal particles from coarse-graining. Although the thermal fatigue lifetime is improved as the Sb content increases, there is a possibility that the Sb content higher than 15% by weight may be an obstacle to the assembling process because the liquidus temperature exceeds 300° C.
Accordingly, it is found from the results of the cooling-and-heating cycle test shown in
For the aforementioned reason, in the semiconductor device 10 shown in
In the semiconductor device having the insulating substrate and the heat radiator joined to each other by the solder member made of the aforementioned composition, the thermal fatigue lifetime can be kept long even when a ceramic substrate with a high heat conductivity and a low heat expansion coefficient such as Si3N4 or AlN is used in combination with a heat radiating plate of Cu with a low cost and a high heat conductivity. Because it is therefore unnecessary to use an expensive composite material as the heat radiating plate, it is possible to provide a semiconductor device with a high reliability ensured at a low cost.
A second embodiment of the invention will be described below with reference to the drawings.
The second embodiment is an exemplary configuration of a power semiconductor module based on the first embodiment.
As shown in
The semiconductor device 30 has a semiconductor element 31, and an insulating substrate 32. The semiconductor element 31 is mounted on a principal surface of the insulating substrate 32.
A front electrode and a rear electrode (both not shown) made of metal films respectively are provided on opposite surfaces of the semiconductor element 31, respectively. The rear electrode of the semiconductor element 31 is joined to the insulating substrate 32 by a solder member 34a. The same constituent component as a solder member 34 which will be described later is used as the solder member 34a.
For example, similarly to the first embodiment, the insulating substrate 32 has a ceramic substrate 32b containing any one of Al2O3, Si3N4 and AlN as a main component. Incidentally, when, for example, Al2O3 is used, the thickness of the ceramic substrate 32b can be set to be not smaller than about 0.2 mm but smaller than about 0.4 mm. When, for example, Si3N4 is used, the thickness of the ceramic substrate 32b can be set to be not smaller than about 0.2 mm but smaller than about 0.7 mm. When, for example, AlN is used, the thickness of the ceramic substrate 32b can be set to be not smaller than about 0.5 mm but smaller than about 0.8 mm.
Conducting layers 32a1, 32a2, 32a3 and 32c are joined to opposite surfaces of the ceramic substrate 32b, respectively. Incidentally, the thickness of each of the conducting layers 32a1, 32a2, 32a3 and 32c can be set to be not smaller than about 0.2 mm but smaller than about 1.0 mm. The conducting layers 32a1, 32a2 and 32a3 are provided as a conductive pattern of a metal which serves as an electric circuit. Particularly, the conducting layer 32a2 is joined to the rear electrode of the semiconductor element 31 through the solder member 34a. Further, the conducting layers 32a1 and 32a3 are connected from the semiconductor element 31 to the lead-out terminals 42 through the bonding wires 42a respectively. The conducting layer 32c is also a conductive pattern of a metal which serves as an electric circuit. Although the conducting layers 32a1, 32a2, 32a3 and 32c may be made of Al, it is preferable that the conducting layers 32a1, 32a2, 32a3 and 32c are made of Cu which is inexpensive and excellent in heat conduction. The conducting layer 32c is joined to the heat radiating fin 33 through the solder member 34.
Each of the solder members 34 and 34a contains Sn as a main component, and 7% by weight to 15% by weight (both inclusively) of Sb, preferably 8% by weight to 10% by weight (both inclusively) of Sb, as described in the first embodiment. Because each of the solder members 34 and 34a does not contain Pb, environmental pollution can be prevented. Incidentally, when the same material as used for the solder member 34 is used for the solder member 34a, reliability on joining the insulating substrate 32 and the semiconductor element 31 is improved more greatly. Moreover, when the same material is used for the solder members 34 and 34a, production can be made easily to reduce the production cost compared with the case where different solder members are used. In addition, it is preferable that germanium (Ge) is added to the solder members 34 and 34a in order to improve joining characteristic between the conducting layer 32c and the heat radiating fin 33 and between the semiconductor element 31 and the conducting layer 32a2.
The lead-out terminals 42 can supply an external voltage to the semiconductor device 30 through the bonding wires 42a.
The enclosure resin casing 41 can contain the semiconductor device 30 in its inside. For example, the enclosure resin casing 41 is made of a PPS (poly phenylene sulfide) resin or a PBT (poly butylene terephthalate) resin. Incidentally, the semiconductor device 30 contained in the inside of the enclosure resin casing 41 is covered and fixed with a gel-state filler 43.
The upper cover 44 serves as a cap for the semiconductor device 30 which is contained in the inside of the enclosure resin casing 41 and fixed with the gel-state filler 43. The upper cover 44 is embedded and fixed by a sealing adhesive agent 45. For example, the upper cover 44 is made of a PPS resin or a PBT resin.
Comb-like grooves are formed in a surface of the heat radiating fin 33 opposite to a surface of contact between the heat radiating fin 33 and the conducting layer 32c of the insulating substrate 32. Although the heat radiating fin 33 may be made of a composite material such as Al—SiC or Cu—Mo, it is preferable that the heat radiating fin 33 is made of Cu which is inexpensive and excellent in heat conduction. The heat radiating fin 33 may be replaced by a heat radiating plate as provided in the first embodiment. In this case, for example, the thickness of the heat radiating plate can be set to be not smaller than about 2 mm but smaller than about 5 mm.
The cooler 46 is attached to the heat radiating fin 33. The inside of the cooler 46 is filled with the cooling medium 47 made of a material such as water or a mixture solution (antifreezing solution) of water and ethylene glycol. The cooling medium is brought into contact with the grooves of the heat radiating fin 33. The combination of the heat radiating fin 33 and the cooler 46 may be replaced by a heat radiating plate which has a flow channel in its inside so that a cooling medium such as water flows in the flow channel and which is brought into contact with the semiconductor device 30.
In the power semiconductor module 40 configured as described above, the thermal fatigue lifetime can be kept long even when a ceramic substrate with high heat conductivity and a low heat expansion coefficient such as Si3N4 or AlN is used in combination with a heat radiating plate of Cu with a low cost and a high heat conductivity. Because it is therefore unnecessary to use an expensive composite material as the heat radiating plate, it is possible to provide a semiconductor device with a high reliability ensured at a low cost.
The above description is provided only for explaining principles of the invention. Many changes and modifications can be made by those skilled in the art. The invention is not limited to the exact configuration and applied examples shown and described above. All corresponding modified examples and their equivalences can be regarded as being included in the scope of the invention based on accompanying claims and their equivalences.
The disclosure of Japanese Patent Application No. 2008-135086 filed on May 23, 2008 is incorporated herein by reference in its entirely.
While the invention has been explained with reference to the specific embodiments of the invention, the explanation is illustrative and the invention is limited only by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-135086 | May 2008 | JP | national |
