Substrate for semiconductor devices

Provided is a substrate for semiconductor devices comprising: an insulating substrate; and a first metal board having a plurality of sides and formed on a first surface of the insulating substrate; wherein the first metal board includes: a corner portion positioned closer to a corner of a first side of the first metal board, for which a creepage distance between an edge of the first metal board and an edge of the insulating substrate reaches a smallest value for the first side; and a center portion positioned closer to a center of the first side than the corner portion, for which a creepage distance between the edge of the first metal board and the edge of the insulating substrate exceeds the smallest value; wherein a range of the center portion is larger than a range of the corner portion.

BACKGROUND

1. Technical Field

The present invention relates to a substrate for semiconductor devices.

2. Related Art

Conventionally, in the field of a substrate for semiconductor devices on which a semiconductor device is placed, it has been known to form a metal board below the substrate for semiconductor devices and solder it to a heat releasing board (refer to WO 2012/157584 and Japanese Patent Application Publication No. 2001-177053, for example).

However, for a conventional substrate for semiconductor devices, a heat stress generated in the substrate for semiconductor devices and the heat releasing board may cause a solder to overflow from an insulating substrate. The solder overflow from the insulating substrate reduces insulation properties.

SUMMARY

In a first aspect of the present invention, a substrate for semiconductor devices comprises: an insulating substrate; and a first metal board having a plurality of sides and formed on a first surface of the insulating substrate. The first metal board may include: a corner portion positioned closer to a corner of a first side of the first metal board, for which a creepage distance between an edge of the first metal board and an edge of the insulating substrate reaches a smallest value for the first side; and a center portion positioned closer to a center of the first side than the corner portion, for which a creepage distance between the edge of the first metal board and the edge of the insulating substrate exceeds the smallest value. Also, a range of the center portion may be larger than a range of the corner portion.

The substrate for semiconductor devices may further comprise a second metal board formed on a second surface opposite to the first surface of the insulating substrate. Also, the second metal board may have a smaller volume than the first metal board.

The second metal board may have the same film thickness as that of the first metal board.

The range of the center portion may be 1.3 times or more the range of the corner portion.

The range of the corner portion may be 30% or less of a shortest side of the first metal board in length.

The range of the corner portion may be 10% or more of a shortest side of the first metal board in length.

The first metal board may have a rectangular shape. The center portion of a longer side of the first metal board may be larger than the center portion of a shorter side of the first metal board.

The range of the corner portion on the longer side of the first metal board may be equal to the range of the corner portion on the shorter side of the first metal board.

The creepage distance for the center portion may be 1.5 times or more and 2.5 times or less the creepage distance for the corner portion.

A substrate100for semiconductor devices may comprise: a plurality of the insulating substrates; a plurality of the first metal boards arranged to correspond to the plurality of insulating substrates; and a heat releasing board of a rectangular shape on which the plurality of insulating substrates are placed via the plurality of first metal boards.

The plurality of first metal boards may include: an inner side around which the plurality of first metal boards are opposing to one another; and an outer side around which the plurality of first metal boards are not opposing to one another. The outer side may be formed to be parallel to a longer side of the heat releasing board.

The first metal board may include: the corner portion corresponding to the inner side; and the center portion corresponding to the inner side.

The substrate for semiconductor devices may comprise a soldered portion formed on a surface of the heat releasing board in a surrounding area of the first metal board. At least a portion of the soldered portion may be formed below the insulating substrate in a planar view.

The soldered portion corresponding to the center portion may be formed more inside of the first metal board than the soldered portion corresponding to the corner portion.

The soldered portion corresponding to the center portion may be formed not sequentially with the soldered portion corresponding to the edge.

The soldered portion may be a carbon scribe or an organic resist formed on the surface of the heat releasing board.

The soldered portion corresponding to the center portion may be a carbon scribe formed on the surface of the heat releasing board, and the soldered portion corresponding to the corner portion is an organic resist formed on the surface of the heat releasing board.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, the present invention will be described through the embodiments of the invention. However, the embodiments described below are not to limit the claimed inventions. Also, all of the combinations of the features described in the embodiments are not necessary for means for solving the problem of the invention.

FIG. 1illustrates an exemplary configuration of a semiconductor device200. The semiconductor device200comprises a substrate100for semiconductor devices and a semiconductor chip110.

As used herein, the X direction and the Y direction are directions vertical to each other while the Z direction is a direction vertical to the X-Y plane. The X direction, the Y direction and the Z direction form a so called right handed system. The substrate for semiconductor devices of the present example has a front surface in the +Z direction and a back surface in the −Z direction. Note that the terms “on” and “above” mean the +Z direction. In contrast, the terms “under” and “below” mean the −Z direction.

In one example, the semiconductor chip110is an insulated gate bipolar transistor (IGBT). Also, the semiconductor chip110may be a reverse-conducting IGBT (RC-IGBT). For example, the semiconductor chip110includes three units of two RC-IGBTs to configure a three phase inverter with the six RC-IGBTs.

The substrate100for semiconductor devices comprises an insulating substrate10, a first metal board20, a second metal board30, a solder40and a heat releasing board50. The semiconductor chip110is arranged above the substrate100for semiconductor devices using the solder45.

The insulating substrate10is an insulative substrate having surfaces parallel to the X-Y plane. For example, the insulating substrate10is a ceramic substrate containing at least one of alumina, aluminum nitride and silicon nitride. Preferably, the insulating substrate10is formed of a material of high heat conductivity. In one example, the insulating substrate10has a film thickness of 0.25 mm to 0.38 mm.

The first metal board20is formed on either one of the surfaces of the insulating substrate10. The first metal board20includes a plurality of sides. That is, the first metal board20has a predetermined polygon shape. The first metal board20of the present example is formed on a surface of the insulating substrate10positioned at a negative side of the Z axis. The first metal board20receives a heat generated by the semiconductor chip110through the insulating substrate10and emits the heat through its surface. For example, the first metal board20is formed of copper. The first metal board20may have a film thickness of 0.20 mm to 0.4 mm.

The second metal board30is formed on a surface of the insulating substrate10opposite to the first metal board20. The second metal board30of the present example is formed on a surface of the insulating substrate10positioned at a positive side of the Z axis. The second metal board30includes a predetermined circuit formed thereon depending on a structure of the semiconductor chip110. The second metal board30may be connected to an external terminal electrically connected to outside of the substrate100for semiconductor devices. The substrate100for semiconductor devices of the present example has a structure in which the first metal board20and the second metal board30are affixed to both surfaces of the insulating substrate10, respectively. For example, the second metal board30is formed of copper. The insulating substrate10, the first metal board20and the second metal board30may be affixed by way of direct joint or braze joint. The second metal board30may have a film thickness of 0.20 mm to 0.4 mm. In one example, the second metal board30is formed to have the same film thickness as the film thickness of the first metal board20. However, preferably, the second metal board30has a smaller volume, than the volume of the first metal board20.

The solder40is provided between the first metal board20and the heat releasing board50. The solder40is joined such that the heat resistance between the insulating substrate10and the heat releasing board50is reduced.

The heat releasing board50receives a heat generated by the semiconductor chip110and emits the heat towards the opposite side. The heat releasing board50is formed of, for example, a metal of high heat conductivity. In one example, the material of the heat releasing board50includes copper and copper alloy.

FIG. 2Aillustrates an exemplary plan view of an insulative substrate in accordance with Example 1.FIG. 2Billustrates an exemplary cross-sectional view taken along A-A′ of the insulative substrate in accordance with Example 1.FIG. 2Cillustrates an exemplary cross-sectional view taken along B-B′ of a substrate100for semiconductor devices in accordance with Example 1. Note thatFIG. 2Ais viewed from a negative side of the Z axis to illustrate the shape of the first metal board20.

The insulating substrate10has a rectangular shape in a planar view. The insulating substrate10of the present example includes a first side m1, a second side m2, a third side m3and a fourth side m4. The first side m1and the third side m3correspond to longer sides. The second side m2and the fourth side m4correspond to shorter sides of the rectangular shape.

The first metal board20has a polygon shape in a planar view. Although the first metal board20of the present example has a rectangular shape, it may have a square shape or other shapes. The first metal board20includes a corner portion22and a center portion24. The corner portion22and the center portion24are differentiated by a creepage distance. As used herein, the creepage distance refers to a distance between an edge of the first metal board20and an edge of the insulating substrate10.

The corner portion22is positioned closer to a corner of the first side m1of the first metal board20. The corner portion22is a portion for which the creepage distance between the edge of the first metal board20and the edge of the insulating substrate10reaches the smallest value for the first side m1. If the insulating substrate10has a rectangular shape, the first metal board20includes four corner portions22a,22a′,22b, and22b′. The corner portion22ais formed closer to a corner at which the first side m1and the fourth side m4intersect. The corner portion22a′is formed closer to a corner at which the first side m1and the second side m2intersect. The corner portion22bis formed closer to a corner at which the third side m3and the fourth side m4intersect. The corner portion22b′is formed closer to a corner at which the second side m2and the third side m3intersect.

A creepage distance D1ais a creepage distance for the corner portion22aon the first side m1. In the present example, the corner portion22aand the corner portion22a′have the same creepage distance. The creepage distance D1anot only refers to a distance when the distance between the edge of the first metal board20and the edge of the insulating substrate10becomes exactly the smallest, but also includes a region up to 10% greater in length than the smallest distance between the edge of the first metal board20and the edge of the insulating substrate10. That is, the corner portion22includes, in addition to portions for which the creepage distance is the smallest, portions for which the creepage distance is 10% greater than the smallest distance in length.

The center portion24is, on at least one side of the first metal board20, positioned closer to a center of the first metal board20than the corner portion22is. The center portion24is a portion for which the creepage distance is greater than the creepage distance for the corner portion22. That is, the center portion24represents a region other than the corner portion22.

A creepage distance D1brefers to a distance between the edge of the center portion24and the edge of the insulating substrate10. The creepage distance D1bis greater than the creepage distance D1a. In the substrate100for semiconductor devices, the creepage distance D1bfor the center portion24is greater, which suppresses overflow of the solder40from the insulating substrate10. For example, the creepage distance D1bfor the center portion24is 1.5 times or more and 2.5 times or less the creepage distance D1afor the corner portion22.

A range La of the corner portion22refers to a range of a region which corresponds to the corner portion22, on a side of the insulating substrate10corresponding to the corner portion22. For example, as an exemplary range La of the corner portion22, ranges L1a, L1a′, L2a, L2a′are illustrated.

The range L1ais a range of the corner portion22aon the first side m1. The range L1a′is a range of the corner portion22a′on the first side m1. Also, the range L2ais a range of the corner portion22a′on the second side m2. The range L2a′is a range of the corner portion22b′on the second side m2. In the first metal board20of the present example, all of the ranges of the corner portions22are equal. That is, all of the range L1a, the range L1a′, the range L2aand the range L2a′are equal. In other words, the first metal board20of the present example has a symmetric structure.

A range Lb of the center portion24refers to a range of a region corresponding to the center portion24, on a side of the insulating substrate10corresponding to the center portion24. For example, as an exemplary range Lb of the center portion24, ranges L1band L2bare illustrated.

The range L1brefers to a range of the center portion24on the first side m1. The range L2brefers to a range of the center portion24on the second side m2. The range L1bis larger than the range L2b. In one example, the ratio of the range L1bto the range L2bis equal to the ratio of the first side m1to the second side m2. The range L1bof the present example is larger than the range L1a. The range L1bmay be larger than any ranges of the corner portions22. For example, the range L1bis 1.3 times or more the range L1a.

Preferably, the range La of the corner portion22is, on at least one side of the first metal board20, 30% or less of the shortest side of the first metal board20in length. This can suppress the solder overflow from the center portion24. Also, preferably, the range La of the corner portion22is, on the shortest side of the first metal board20, 10% or more of the shortest side of the first metal board20in length. This results in the margin against crack growth from the corner portion22of the first metal board20, which can suppress decrease in the heat cycle reliability.

FIG. 3illustrates an exemplary configuration of the substrate100for semiconductor devices. In the substrate100for semiconductor devices of the present example, the second metal board30has a smaller volume than the first metal board20. The substrate100for semiconductor devices deforms to have a downwardly convex shape due to a heat stress after joint with the solder40. The substrate100for semiconductor devices can suppress voids to be generated between the first metal board20and the heat releasing board50, if it is designed to have a downwardly convex shape. If a heat stress is generated to deform the substrate100for semiconductor devices to be downwardly convex, the solder40around the center of the substrate100for semiconductor devices is easily pushed out.

Comparative Example 1

FIG. 4illustrates an exemplary configuration of a semiconductor device600in accordance with Comparative Example 1. The semiconductor device600of the present example comprises a substrate500for semiconductor devices and a semiconductor chip610. The substrate500for semiconductor devices includes an insulating substrate510, a first metal board520, a second metal board530, a solder540and a heat releasing board550. The semiconductor chip610is joined above the substrate500for semiconductor devices via the solder545.

The first metal board520is formed to have an approximately constant creepage distance on each side. The creepage distance of the first metal board520is shorter than the creepage distance of the second metal board530. Therefore, the substrate500for semiconductor devices of the present example allows the solder540to easily overflow. In particular, the solder540includes a region X with the solder540raised at the edge of the semiconductor device600.

In the semiconductor device600, the raised solder540causes the insulation distance to be reduced. This results in the semiconductor device600having reduced insulation properties, which causes the reduced reliability and yield rate.

FIG. 5Aillustrates an exemplary plan view of a substrate500for semiconductor devices in accordance with Comparative Example 1.FIG. 5Billustrates an exemplary cross-sectional view of the substrate500for semiconductor devices in accordance with Comparative Example 1. The substrate500for semiconductor devices of the present example comprises an insulating substrate510and a first metal board520which have a rectangular shape in a planar view.

A creepage distance D520refers to a distance between an edge of the first metal board520and an edge of the insulating substrate510. The creepage distance D520is constant on each side.

A creepage distance D530refers to a distance between an edge of the second metal board530and an edge of the insulating substrate510. The creepage distance D520is shorter than the creepage distance D530. That is, when the first metal board520and the heat releasing board55are joined by means of a solder, the substrate500for semiconductor devices is curved due to a stress to cause the solder to be wetting and spreading, which results in the solder540easily overflowing from the insulating substrate510.

FIG. 6illustrates an exemplary configuration of a substrate100for semiconductor devices in accordance with Example 2. For the substrate100for semiconductor devices of the present example, insulating substrates10a,10b, first metal boards20a,20band a heat releasing board50are shown. Other configurations are omitted for the sake of simplicity of the description.

The insulating substrate10ahas an approximately square shape in a planar view. The insulating substrate10bhas an approximately rectangular shape in a planar view. In one example, the insulating substrate10aand the insulating substrate10binclude a semiconductor chip110formed thereon, respectively.

The heat releasing board50has a rectangular shape in a planar view. The heat releasing board50is sized to allow at least the insulating substrate10aand the insulating substrate10bto be placed thereon. The heat releasing board50of the present example includes soldered portions60,65.

The first metal board20ais provided to correspond to the insulating substrate10a.

The first metal board20ahas an approximately square shape if the insulating substrate10ahas a square shape. The first metal board20aof the present example includes a corner portion22and a center portion24on each side.

The first metal board20bis provided to correspond to the insulating substrate10b. The first metal board20bhas an approximately rectangular shape if the insulating substrate10bhas a rectangular shape. The first metal board20bof the present example includes a corner portion22and a center portion24on each side.

The soldered portions60,65are formed in a surrounding area of the insulating substrate10to prevent the solder40from flowing out when melted. At least portions of the soldered portions60,65are formed below the insulating substrate10in a planar view. That is, at least portions of the soldered portions60,65may be formed inside of the insulating substrate10in a planar view. Also, the soldered portion60of the present example is formed not sequentially with the soldered portion65.

Here, if the soldered portions60,65are formed inside of the insulating substrate10, the solder40is prevented from overflowing from below the insulating substrate10. For example, the soldered portions60,65are an organic resist or a carbon scribe formed on the heat releasing board50. The organic resist is formed in a predetermined pattern on the heat releasing board50, which prevents the solder40from diffusing beyond the organic resist. Also, the carbon scribe is a concave portion formed on a surface of the heat releasing board50. This results in the carbon scribe suppressing the solder40diffusing beyond the carbon scribe. In one example, the carbon scribe is formed by scribing the surface of the heat releasing board50with a pencil or the like.

The soldered portion60is provided to correspond to the corner portion22. Being provided to correspond to the corner portion22refers to being provided opposing to a side of the edge of the corner portion22. The soldered portion60of the present example is an organic resist. Although the soldered portion60of the present example is formed outside of the insulating substrate10, it may also be formed inside of the insulating substrate10.

The soldered portion65is provided to correspond to the center portion24. Being provided to correspond to the center portion24refers to being provided opposing to a side of the edge of the center portion24. As the soldered portion65corresponds to the center portion24formed inside of the insulating substrate10in a planar view, it can be formed inside of the insulating substrate10. The soldered portion65of the present example is formed more inside of the insulating substrate10than the soldered portion60in a planar view. The soldered portion65of the present example is a carbon scribe.

Comparative Example 2

FIG. 7illustrates an exemplary substrate500for semiconductor devices in accordance with Comparative Example 2. For the substrate500for semiconductor devices of the present example, insulating substrates510a,510b, first metal boards520a,520band a heat releasing board550are shown. Other configurations are omitted for the sake of simplicity of the description. The heat releasing board550includes a soldered portion560and a soldered portion565formed thereon.

The first metal board520ais provided to correspond to the insulating substrate510a. The first metal board520ahas a square shape if the insulating substrate510ahas a square shape. The first metal board520ais formed inside of the insulating substrate510ain a planar view. The first metal board520ais formed to have an approximately constant creepage distance on each side.

The first metal board520bis provided to correspond to the insulating substrate510b. The first metal board520bhas a rectangular shape if the insulating substrate510bhas a rectangular shape. The first metal board520bis formed inside of the insulating substrate510bin a planar view. The first metal board520bis formed to have an approximately constant creepage distance on each side.

The soldered portion560and the soldered portion565are formed to cover an outer peripheral portion of the insulating substrate510aand an outer peripheral portion of the insulating substrate510b. The soldered portion560is formed sequentially with the soldered portion565.

If a heat stress is applied to the substrate500for semiconductor devices of the present example, the solder540may overflow from below the insulating substrate510. Also, particularly in a region in which the insulating substrate510aand the insulating substrate510bare opposing to each other, the overflow volume of the solder540may increase and reduce the reliability.

Also, in the semiconductor device600, the soldered portion560and the soldered portion565need to be provided in a region outside of the insulating substrate510in a planar view to form solder fillets. The solder fillet refers to one having a shape spreading toward the bottom after soldered. Also, misalignment may occur when assembled.

On the other hand, if the area of the semiconductor device600is simply reduced to increase a creepage distance in length, the joint area of the solder540is reduced, thereby reducing the margin against the solder crack growth due to a heat stress. This reduces the heat cycle resistance of the semiconductor device600.

FIG. 8illustrates an exemplary configuration of a substrate100for semiconductor devices in accordance with Example 3. The substrate100for semiconductor devices of the present example comprises a first metal board20bof a shape different from that of the substrate100for semiconductor devices in accordance with Example 2.

The first metal board20ais provided to correspond to the insulating substrate10a. The first metal board20ahas an approximately square shape if the insulating substrate10ahas a square shape. The first metal board20aof the present example includes a corner portion22and a center portion24on each side.

The first metal board20bis provided to correspond to the insulating substrate10b. The first metal board20bhas an approximately rectangular shape if the insulating substrate10bhas a rectangular shape. The first metal board20bof the present example includes a side having the corner portion22and the center portion24formed thereon and a side not having the corner portion22or the center portion24formed thereon.

The first metal boards20a,20binclude an inner side around which the first metal board20aand the first metal board20bare opposing to each other and an outer side around which the first metal board20aand the first metal board20bare not opposing to each other.

On the inner side, the first metal board20aincludes the corner portion22and the center portion24. Also, on the inner side, the first metal board20bincludes the corner portion22and the center portion24.

On the outer side, the first metal board20aincludes the corner portion22and the center portion24. The first metal board20aof the present example includes, on all of three outer sides, the corner portion22and the center portion24. Also, on the outer side, the first metal board20bat least includes a side parallel to the insulating substrate10. In one example, the first metal board20bincludes, on three outer sides, a side parallel to the insulating substrate10, respectively. Also, the first metal board20bmay be formed to include the outer side parallel to a longer side of the heat releasing board50.

Here, in a longitudinal direction of the first metal board20b, the solder40is not easily affected by the heat stress. This allows the first metal board20bof the present example to omit the center portion24in the longer side direction of the first metal board20b. This results in the substrate100for semiconductor devices of the present example preventing crack growth while suppressing overflow of the solder40from the inner side.

FIG. 9illustrates an exemplary expanded view of an edge of the substrate100for semiconductor devices at a positive side of the X axis. In the present example, described is an exemplary design method of the substrate100for semiconductor devices.

A creepage distance A refers to a distance between the edge of the insulating substrate10and the edge of the first metal board20. That is, the creepage distance A is a creepage distance for the first metal board20. The creepage distance A may be designed to be greater than the creepage distance for the second metal board30. The creepage distance A may be, as illustrated inFIG. 2A,FIG. 2B, andFIG. 2C, greater than a distance (D30) between the edge of the insulating substrate10and the edge of the second metal board30at the center portion24, and equal to or smaller than D30at the corner portion22.

A thickness B refers to a thickness of the first metal board20. The thickness B of the present example is equal to a thickness of the second metal board30. In one example, the creepage distance A and the thickness B are designed such that the volume of the first metal board20is greater than the volume of the second metal board30.

A thickness C refers to a thickness of the solder40. The thickness C is defined as a distance between a lower end of the first metal board20and an upper end of the heat releasing board50. The thickness C is determined by a material of the solder40, a joint temperature of the solder40and others.

The substrate100for semiconductor devices of the present example is designed such that the creepage distance A is equal to or greater than the sum of the thickness B and the thickness C. That is, the creepage distance A is determined to fulfill the following condition. (Creepage distance A)≥(thickness B of the first metal board20)+(thickness C of the solder40) In other words, the creepage distance A is equal to or greater than the distance between the lower end of the insulating substrate10and the upper end of the heat releasing board50. This results in the substrate100for semiconductor devices ensuring sufficient insulation properties even if the solder40overflows and is raised.

As described above, the substrate100for semiconductor devices described herein has a sufficient creepage distance for the center portion24of the first metal board20, which can suppress insulation breakdown along the insulating substrate10. Also, the substrate100for semiconductor devices described herein has a shorter creepage distance for the corner portion22of the first metal board20than the center portion24, which provides the margin against the crack growth and high heat cycle reliability. Thus, the substrate100for semiconductor devices described herein is of high quality and a good yield rate.

FIG. 10illustrates an exemplary configuration of a circuit300including the semiconductor device200. The circuit300of the present example is a three phase inverter circuit provided between a power source210and a load220. The load220is, for example, a third phase motor. The circuit300converts power supplied from the power source210into three phase signals (AC voltage) and supplies the converted signals to the load220.

The circuit300comprises three bridges corresponding to three phase signals. Each bridge includes an upper arm152and a lower arm154provided in series between a positive side wire and a negative side wire. Each arm is provided with a transistor202such as an IGBT and a diode204such as an FWD. The arm may be provided with an individual semiconductor chip as the transistor202and the diode204, or may be provided with a semiconductor chip on which an RC-IGBT having both features is formed. Signals of each phase are output from a connection point of the upper arm152and the lower arm154.

Also, the circuit300comprises two sensing units208. One sensing unit208detects an electric current at the connection point of the upper arm152and the lower arm154. The other sensing unit208detects an electric current at the connection point of the lower arm154and the reference potential.

In the present example, the semiconductor device200includes a pair of the upper arm152and the lower arm154. That is, the circuit300is configured by comprising three semiconductor devices200each including a pair of the upper arm152and the lower arm154. Also, the semiconductor device200may include the sensing unit208. In one example, the semiconductor device200may include all of the upper arms152and the lower arms154of the circuit300.

EXPLANATION OF REFERENCES