JPH1050928A - Semiconductor device and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To avoid causing soldering troubles such as forming of solder balls, poor solder strength and positional deviation. SOLUTION: The semiconductor device comprises a metal base 2, insulation board 3 having upper and lower electrodes 7 and 4 on the upper and lower surfaces, solder film 5 inserted between the upper electrodes and metal base, semiconductor elements 6 which are disposed on the upper surface of the board 3 and electrically connected to the upper electrodes 7. The base 2 has bumps 22 contacted to the lower electrodes 4 at positions to hold a fixed gap between the board and metal base.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a semiconductor device such as a giant transistor and a method for manufacturing the same.

[0002]

2. Description of the Related Art As shown in FIGS. 25 and 26, a semiconductor device C such as a giant transistor for connecting a large insulating substrate A to a metal base B by soldering has a plurality of semiconductor devices on the insulating substrate A. Arrange element D,
An electrode of the semiconductor element D and an electrode provided on the insulating substrate A are connected by an aluminum wire F. The insulating substrate A is connected to a metal base B by a solder G. The material of the metal base B used at this time is generally copper, and its surface is plated with a nickel film. The metal base B is often used for heat radiation of the semiconductor device C, and its surface is generally flat to increase the contact area with the insulating substrate A.

[0003] Solder G used for soldering is used.
Is formed in a sheet shape. In the soldering method, a flux H is applied to the surface of the metal base B, and a sheet-like solder G is placed thereon. Further, a flux is applied to the sheet-like solder G, and after placing the insulating substrate A, the solder is heated to melt the solder, thereby making connection. As a heating method, there are (1) a method of directly heating from the metal base B side with a hot plate or the like, and (2) a method of heating the entire semiconductor device C itself to be soldered with a reflow furnace or the like.

[0004]

However, only the gap between the metal base B and the insulating substrate A of the conventional semiconductor device C is maintained by the solder G, and the amount of the solder G and the inclination of the insulating substrate A are different. The gap changes due to the like. At this time, the solder G in the narrowed portion protrudes from the gap between the metal base B and the insulating substrate A, and the solder ball J
Etc., or the amount of solder is insufficient,
Insufficient contact area occurs.

[0005] Further, a flux is applied before soldering. When the flux is applied to the outside of the insulating substrate A at the time of application, the flux is guided by the flux at the time of soldering, the solder G protrudes, and a solder ball or the like is formed. In addition to the failure, a flux may remain between the insulating substrate A and the metal base B, and outgas may be generated by heating, which may lead to generation of voids. The present invention has been made in view of the above circumstances, and has as its object to provide a semiconductor device and a method of manufacturing the same that solve the above-mentioned problems.

[0006]

According to a first aspect of the present invention, there is provided a semiconductor device, comprising: a metal base; an insulating substrate having an upper electrode provided on one surface and a lower electrode provided on the other surface; A solder film interposed between the metal base and
A semiconductor element electrically connected to the upper electrode, wherein at least one of the metal base and the lower electrode is provided with a projection for maintaining a constant distance between the metal base and the insulating substrate. ing.

According to a second aspect of the present invention, in the semiconductor device according to the first aspect, the projection is formed integrally with the metal base or the lower electrode. According to a third aspect of the present invention, in the semiconductor device according to the first aspect, a solder groove in which a part of the solder film is immersed is formed in the metal base at a position close to the protrusion.

According to a fourth aspect of the present invention, in the semiconductor device according to the first aspect, the projection is formed in a point-like body or a linear body. Thus, the semiconductor device according to claims 1 to 4 can prevent the formation of solder balls, insufficient solder strength, misalignment, and occurrence of soldering defects such as solder voids.

According to a fifth aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising: a metal base; an insulating substrate having an upper electrode provided on one surface and a lower electrode provided on the other surface; and electrically connected to the upper electrode. In manufacturing a semiconductor device having a semiconductor element and a projection provided on at least one of the metal base and the lower electrode to keep a distance between the metal base and the insulating substrate constant, a solder sheet is used. A superposing step of interposing between the lower electrode side of the insulating substrate and the projection forming side of the metal base, and the insulating substrate is formed by a solder film obtained by melting the soldering sheet after the superposing step. A reflow soldering step of performing reflow soldering on the metal base.

According to a sixth aspect of the present invention, in the method of the fifth aspect, the protrusion on the metal base is formed by pressing the metal base. According to a seventh aspect of the present invention, in the method of the fifth aspect, the protrusion on the metal base is formed by cutting the metal base itself.

According to an eighth aspect of the present invention, in the method of the fifth aspect, the projection on the lower electrode is formed by etching. According to a ninth aspect of the present invention, in the manufacturing method of the fifth aspect, the flux is applied to at least one of the lower electrode side of the insulating substrate and the protrusion forming side of the metal base before the overlapping step.

According to a fifth aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising: preventing formation of solder balls, insufficient soldering strength, misalignment, and occurrence of soldering defects such as solder voids. Is possible.

[0013]

An embodiment of the present invention will be described below in detail with reference to the drawings. 1 and 2 show a semiconductor device 1 of this embodiment. The semiconductor device 1 has a rectangular shape, for example, a metal base 2 having a length of 100 mm and a width of 30 mm, and a rectangular shape having a smaller area than the metal base 2 provided on the metal base 2 and having a length of, for example, 70 mm.
an insulating substrate 3 having a thickness of, for example, copper and a thickness of, for example, 200 μm, which is adhered to a lower surface of the insulating substrate 3 via a brazing material and has a thickness of, for example, 200 μm.
A solder film 5 for bonding the lower electrode 4 to the metal base 2; a plurality of semiconductor elements 6 such as transistors provided on the upper surface of the insulating substrate 3;
The material provided in the shape of an island on the upper surface of the insulating substrate 3 so as to surround the upper electrode 7 is, for example, copper and the thickness is, for example, 200 μm. It is made of, for example, an aluminum wire 9 for electrical connection. The lower electrode 4 is provided for the purpose of heat radiation and grounding, and is provided on substantially the entire lower surface of the rectangular insulating substrate 3. That is, the lower electrode 4 is provided in a rectangular shape, for example, 1.5 mm inward from the edge of the insulating substrate 3.
On the other hand, the upper electrode 7 is for electrically connecting the semiconductor element 6 and is formed in a predetermined pattern.

Thus, the metal base 2 has a thickness of, for example, 3
The main body 10 is formed to a thickness of about 5 mm and made of a metal having a small electric resistance such as copper, for example, and a coating 11 made of, for example, nickel or the like is applied on the surface of the main body to a thickness of about 5 μm. I have. Further, the material of the insulating substrate 3 is, for example, ceramic or the like. Thus, as shown in FIGS. 3 and 4, the metal base 2 has
For example, a projection 1 formed in a concave shape in cross section (vertical direction) at a position facing the four corners of the lower electrode 4 having a rectangular shape.
2 is engraved. The protrusion 12 has an outer diameter of, for example, 1
mm and height are, for example, 60 μm (5 μm to 100 μm
It is better within the range. Within this range, a solder film 5 having an appropriate thickness can be obtained. ) Is formed. The groove angle θ of the projection 12 is preferably 90 degrees to 120 degrees. Further, the solder film 5 has a thickness of, for example, 10
It is provided at 0 μm.

Next, a method of manufacturing the semiconductor device 1 will be described. The method for manufacturing the semiconductor device 1 includes a projection forming step of forming a plurality of projections 12 on a metal plate having a small electric resistance such as copper by, for example, press working (see symbol S1 in FIG. 5), and a projection after the projection forming step. Forming a coating portion 11 of, for example, nickel on the surface of the metal plate on which is formed by plating, for example, to a thickness of about 5 μm (see symbol S2 in FIG. 5), and after this coating portion forming step, A metal base forming step of cutting the plated metal plate to obtain a plurality of metal bases 2 (see symbol S3 in FIG. 5);
(See symbol S4 in FIG. 5), and a gap between the electrode 8 and the upper electrode 7 provided on the semiconductor element 6 after the semiconductor element fixing step is made of, for example, aluminum. Wire connection step (see symbol S5 in FIG. 5) for electrically connecting with wires 9 and the like, and a flux application step in which flux 13 is applied on metal base 2 and / or lower electrode 4 of insulating substrate 3 (see symbol 5 in FIG. 5). S6 / see FIG. 6), and a solder sheet placing step of placing the solder sheet 14 on the metal base 2 after the flux applying step (see S7 in FIG. 5 / see FIG. 6).
And a laminating step of laminating the insulating substrate 3 via the solder sheet 14 so that the lower electrode 4 comes into contact with the solder sheet 14 after the solder sheet mounting step (S in FIG. 5).
8 / see FIG. 6) and the metal base 2 and the insulating substrate 3 superposed after the superposing step are charged into a reflow furnace or placed on a hot plate, for example, 2
And a reflow soldering step of performing reflow soldering after heating at 75 ° C. for 130 seconds (see symbol S9 in FIG. 5).

In the reflow soldering step, the solder sheet 14 is melted. However, when the insulating substrate 3 is inclined, when the amount of molten solder is large, or when a gap is formed only with solder, the gap is small. A part of the melted solder 5a flows to the outside in the peripheral portion of the insulating substrate 3 that has become damaged. However, in this embodiment, the gap between the metal base 2 and the insulating substrate 3 is kept horizontal by the projections 12 and the inclination of the insulating substrate 3 with respect to the metal base 2 is eliminated, so that the molten solder 5a flows to the peripheral portion. This is effective for preventing the formation of solder balls.

As described above, the semiconductor device 1 of this embodiment
In the method of manufacturing the semiconductor device 1, the insulating substrate 3 and the metal base 2 by the protrusion 12 are provided at positions facing the edge of the lower electrode 4 provided on the insulating substrate 3 of the metal base 2.
To keep the distance between the flux 13
Can be reliably limited to the bonding region of the metal base 2 and the insulating substrate 3 by soldering. In addition, by forming a gap between the metal base 2 and the insulating substrate 3, the solder is filled between the metal base 2 and the insulating substrate 3 due to the surface tension of the solder, and the amount of solder is insufficient due to the inclination of the insulating substrate 3. Accordingly, it is possible to prevent the occurrence of soldering failure such as a shortage of the contact area due to soldering and a failure to obtain a sufficient adhesive strength. Furthermore, since the projections 12 are formed integrally with the metal base 2, the projections 12 also have a good effect in terms of heat dissipation for releasing heat generated in the semiconductor element 6 to the metal base 2 side.

In the above embodiment, the protrusion 12
The cross-sectional shape in the transverse direction is a shape in which the bottom is an edge (a sharp triangle) and the periphery is raised as shown in FIG. 4, but the metal base 2 is cut so that the projection 12 becomes a solid cylinder, and Even when the shape is rectangular as shown in FIG. 7, the same operation and effect as in the above embodiment can be obtained.

In the above embodiment, the number of projections is four. However, if the number is three or more, the number and size of the projections, the shape of the projections, etc. are determined according to the shape of the insulating substrate 3. It can be freely changed.

Further, as a modified example, as shown in FIG. 8, a groove 14 having a square frame shape in plan view for storing molten solder is provided in parallel with the projection 12 so that the solder 5a which has flowed out can be removed. Since it can be stored in the groove 14, the effect of keeping the distance between the insulating substrate and the metal base constant by the protrusions 12 has a remarkable effect to enhance the effect of preventing the formation of solder balls. The shape of the cross section of the groove 14 is not limited to a rectangle, and may be arbitrarily selected, such as a semicircle or a triangle, as long as it can store molten solder.

Further, the shape of the projection is not limited to a point-like body, and the insulating substrate 3 side may be held by at least a pair of linear bodies 30 as shown in FIG.

Further, as a modified example, as shown in FIG. 10, the same function and effect can be obtained even if the linear member 30a is formed in a "-" shape. Further, as shown in FIG. 11, the same function and effect can be obtained even if the linear bodies 30b are constituted by the point bodies 31 arranged at substantially equal intervals.

Further, as a modified example, as shown in FIG. 12, the same operation and effect can be obtained by forming the linear member 30c in the shape of a "mouth". Next, FIGS. 13 and 14 show a semiconductor device 1 according to a second embodiment of the present invention.
a will be described.

This semiconductor device 1a has a metal base 2 having a rectangular shape and dimensions of, for example, 100 mm in length and 30 mm in width.
It is provided on the metal base 2 and has a rectangular shape having an area smaller than that of the metal base 2.
An insulating substrate 3 having a width of 25 mm and a thickness of 1.5 mm, a lower electrode 4 made of, for example, copper and having a thickness of, for example, 200 μm, provided on the lower surface of the insulating substrate 3, and the lower electrode 4 and the metal base 2. A solder film 5 to be bonded, a plurality of semiconductor elements 6 such as transistors provided on the upper surface of the insulating substrate 3, and a material provided in an island shape on the upper surface of the insulating substrate 3 so as to surround these semiconductor elements 6. The upper electrode 7 is made of, for example, copper and has a thickness of, for example, 200 μm, and is made of, for example, an aluminum wire 9 that electrically connects the electrode 8 provided on the semiconductor element 6 and the upper electrode 7. In addition,
The lower electrode 4 is provided for the purpose of radiating heat and grounding, and is provided on substantially the entire lower surface of the rectangular insulating substrate 3. That is, the lower electrode 4 is provided in a rectangular shape, for example, 1.5 mm inward from the edge of the insulating substrate 3. As shown in FIGS. 15 and 16, protrusions 12 a formed in a conical shape (a rounded end is formed) are protruded from the four corner portions of the lower electrode 4. . The protrusion 12a has a bottom diameter of, for example, 1 mm and a height of, for example, 60 μm (5 μm
It is good to be within the range of μ100 μm. Within this range, a solder film 5 having an appropriate thickness can be obtained. ) Is formed. The tip angle θa of the projection 12a is 90
Degrees to 120 degrees are preferred. Further, the thickness of the solder film 5 is, for example, 100 μm. On the other hand, the upper electrode 7 is for electrically connecting the semiconductor element 6 and
It is formed in a predetermined pattern.

Thus, the metal base 2 has a thickness of, for example, 3
The main body 10 is formed to a thickness of about 5 mm and made of a metal having a small electric resistance such as copper, for example, and a coating 11 made of, for example, nickel or the like is applied on the surface of the main body to a thickness of about 5 μm. I have. Further, the material of the insulating substrate 3 is, for example, ceramic or the like.

Next, a method for manufacturing the semiconductor device 1a will be described. The method for manufacturing the semiconductor device 1a includes a coating forming step of applying a coating 11 of, for example, nickel to a surface of a metal plate having a small electric resistance, such as copper, to a thickness of, for example, about 5 μm by plating, for example (see FIG. 17). S1
1), a metal base forming step of cutting a plated metal plate after the covering part forming step to obtain a plurality of metal bases 2 (see symbol S12 in FIG. 17), and an electric resistance of, for example, copper or the like on the insulating substrate 3. Is small and the thickness is, for example, 200 μm
An electrode forming step of bonding a metal plate serving as the lower electrode 4 by, for example, brazing (see symbol S13 in FIG. 17), and a protrusion for forming the protrusion 12a on the lower electrode 4 by, for example, etching after the electrode forming step. A forming step (see S14 in FIG. 17) and a semiconductor element fixing step of fixing the semiconductor element 6 to the insulating substrate 3 via the upper electrode 7 with a high melting point solder or a conductive resin after the projection forming step (S15 in FIG. 17) And the electrode 8 and the upper electrode 7 provided on the semiconductor element 6 after the semiconductor element fixing step.
And a flux for applying a flux 13 on the metal base 2 and / or the lower electrode 4 of the insulating substrate 3 (see symbol S16 in FIG. 17). An application step (see S17 in FIG. 17 / see FIG. 18), a solder sheet mounting step of placing the solder sheet 14 on the metal base 2 after the flux applying step (see S18 in FIG. 17 / see FIG. 18), After the sheet mounting step, the insulating substrate 3 is overlapped via the solder sheet 14 such that the protrusions 12a formed on the lower electrode 4 are in contact with the solder sheet 14 (refer to symbol S19 in FIG. 17 / FIG. 18).
) And the metal base 2 and the insulating substrate 3 superimposed after this superimposing step are charged into a reflow furnace or placed on a hot plate, for example, at 275 ° C. and 13 ° C.
A reflow soldering step of performing reflow soldering after heating for 0 seconds (see symbol S20 in FIG. 17 / see FIG. 19)
And

In the reflow soldering process, the solder sheet 14 is melted. However, when the insulating substrate 3 is inclined, when the amount of molten solder is large, or when a gap is formed only with solder, the gap is small. A part of the melted solder 5a flows to the outside in the peripheral portion of the insulating substrate 3 that has become damaged. However, in this embodiment, the protrusion 12a keeps the gap between the metal base 2 and the insulating substrate 3 horizontal, and the inclination of the insulating substrate 3 with respect to the metal base 2 is eliminated. This is effective in preventing the flow and the formation of solder balls.

As described above, the semiconductor device 1 of this embodiment
In the method for manufacturing the semiconductor device 1a and the semiconductor device 1a, the distance between the insulating substrate 3 and the metal base 2 can be kept constant by the protrusions 12a provided on the lower electrode 4,
The application region of the flux 13 can be reliably limited to the adhesion region of the metal base 2 and the insulating substrate 3 by soldering. In addition, by forming a gap between the metal base 2 and the insulating substrate 3, the solder is filled between the metal base 2 and the insulating substrate 3 due to the surface tension of the solder, and the amount of solder is insufficient due to the inclination of the insulating substrate 3. Accordingly, it is possible to prevent the occurrence of soldering failure such as a shortage of the contact area due to soldering and a failure to obtain a sufficient adhesive strength. In addition, a good effect is exhibited in terms of heat dissipation for releasing the heat generated in the projections 12a and the semiconductor element 6 to the metal base 2 side.

As a modified example, as shown in FIG. 20, a groove 14a having a rectangular frame shape in plan view for storing molten solder is provided in parallel with the projection 12a.
Since the solder 5a that has flowed out can be stored in the groove 14a, this is notable for enhancing the effect of preventing the formation of solder balls, together with the function of keeping the distance between the insulating substrate and the metal base constant by the projections 12a. It works. In addition,
The cross-sectional shape of the groove 14a is not limited to a rectangle,
Any material such as a semicircle or a triangle may be selected as long as it can store the molten solder.

Further, the shape of the projection is not limited to a point-like body, and the insulating substrate 3 side may be held by at least a pair of linear bodies 30a as shown in FIG.

Further, as a modified example, as shown in FIG. 22, the same function and effect can be obtained by forming it in a "-"-shaped linear body 30p. Further, as shown in FIG. 23, the same function and effect can be obtained even if the linear bodies 30a are constituted by the dot-like bodies 30n arranged at substantially equal intervals.

Further, as a modified example, as shown in FIG. 24, the same operation and effect can be obtained by forming the linear member 30q in the shape of a "mouth". Further, it goes without saying that the present invention includes a mixture of the protrusions 12 of the first embodiment and the protrusions 12a of the second embodiment.

[0033]

According to the first to fourth aspects of the present invention,
On at least one of the metal base and the lower electrode,
With the projection provided to keep the distance between the metal base and the insulating substrate constant, it is possible to reliably limit the application area of the flux to the bonding area by soldering between the metal base and the insulating substrate. . In addition, even if the insulating substrate is inclined and the gap between the metal base and the insulating substrate is narrowed, the provision of the protrusion allows the insulating substrate not to approach the metal base more than the height of the protrusion, so that the solder flows between the metal base and the insulating substrate. It does not protrude. As a result, the above two effects are combined, and the protruding solder becomes a solder ball due to the effect of surface tension, or the amount of solder for soldering the metal base and the insulating substrate is insufficient, so that the It is possible to prevent the occurrence of defective soldering such as insufficient contact area and insufficient adhesive strength. Also, before the solder is melted, even if outgas is generated due to the gap, the gas can be discharged and the generation of voids can be prevented.

According to a fifth aspect of the present invention, in the method of manufacturing a semiconductor device, at least one of the metal base and the lower electrode is provided with a projection for maintaining a constant distance between the metal base and the insulating substrate. Therefore, the application region of the flux can be reliably limited to the adhesion region of the metal base and the insulating substrate by soldering. Also,
Even if the insulating substrate tilts and the gap between the metal base and the insulating substrate is narrowed, the protrusion protrudes so that the solder does not protrude from the metal base and the insulating substrate without approaching the metal base more than the height of the protrusion. Nothing. As a result, the above two effects are combined, and the protruding solder becomes a solder ball due to the effect of surface tension, or the amount of solder for soldering the metal base and the insulating substrate is insufficient, so that the It is possible to prevent the occurrence of defective soldering such as insufficient contact area and insufficient adhesive strength. Also, before the solder is melted, even if outgas is generated due to the gap, the gas can be discharged and the generation of voids can be prevented.

[Brief description of the drawings]

FIG. 1 is a plan view of a semiconductor device according to an embodiment of the present invention.

FIG. 2 is a sectional view taken along line II-II in FIG.

FIG. 3 is an enlarged sectional view of a main part of the semiconductor device according to one embodiment of the present invention;

FIG. 4 is an enlarged sectional view of a main part of a protrusion in FIG. 3;

FIG. 5 is a flowchart illustrating a method for manufacturing a semiconductor device according to an embodiment of the present invention.

FIG. 6 is a sectional view illustrating the method for manufacturing the semiconductor device according to the embodiment of the present invention;

FIG. 7 is an enlarged sectional view of a main part of a semiconductor device showing a modification of one embodiment of the present invention.

FIG. 8 is an enlarged sectional view of a main part of a semiconductor device showing a modification of one embodiment of the present invention.

FIG. 9 is an explanatory diagram showing a modified example of one embodiment of the present invention.

FIG. 10 is an explanatory diagram showing a modification of one embodiment of the present invention.

FIG. 11 is an explanatory diagram showing a modification of one embodiment of the present invention.

FIG. 12 is an explanatory diagram showing a modification of one embodiment of the present invention.

FIG. 13 is a plan view of a semiconductor device according to another embodiment of the present invention.

FIG. 14 is a sectional view taken along line XX of FIG. 1;

FIG. 15 is an enlarged sectional view of a main part of a semiconductor device according to another embodiment of the present invention.

FIG. 16 is an enlarged sectional view of a main part of a protrusion in FIG.

FIG. 17 is a flowchart illustrating a method for manufacturing a semiconductor device according to another embodiment of the present invention.

FIG. 18 is a sectional view illustrating a method for manufacturing a semiconductor device according to another embodiment of the present invention.

FIG. 19 is an enlarged sectional view of a main part of a semiconductor device showing a modification of another embodiment of the present invention.

FIG. 20 is an enlarged cross-sectional view of a main part of a semiconductor device showing a modification of another embodiment of the present invention.

FIG. 21 is an explanatory view showing a modified example of another embodiment of the present invention.

FIG. 22 is an explanatory diagram showing a modified example of another embodiment of the present invention.

FIG. 23 is an explanatory view showing a modified example of another embodiment of the present invention.

FIG. 24 is an explanatory view showing a modified example of another embodiment of the present invention.

FIG. 25 is an explanatory diagram for explaining a conventional technique.

FIG. 26 is an explanatory diagram for explaining a conventional technique.

[Explanation of symbols]

1, 1a: semiconductor device, 2: metal base, 3: insulating substrate, 4: lower electrode, 5: solder film, 6: semiconductor element, 1
2, 12a: protrusion, 13: flux, 14, 14a:
Groove.

Claims (9)

[Claims] A metal base; an insulating substrate provided with an upper electrode on one surface and a lower electrode on the other surface;
A solder film interposed between the lower electrode and the metal base, comprising a semiconductor element electrically connected to the upper electrode, at least one of the metal base and the lower electrode, A semiconductor device comprising a projection for maintaining a constant distance between the metal base and the insulating substrate. 2. The semiconductor device according to claim 1, wherein the projection is formed integrally with the metal base or the lower electrode. 3. The metal base is provided at a position close to the protrusion.
2. The semiconductor device according to claim 1, wherein a solder groove into which a part of the solder film is immersed is provided. 4. The semiconductor device according to claim 1, wherein the protrusion is formed in a point-like body or a linear body. 5. A metal base, an insulating substrate provided with an upper electrode on one surface and a lower electrode on the other surface,
A semiconductor comprising: a semiconductor element electrically connected to the upper electrode; and a projection provided on at least one of the metal base and the lower electrode for maintaining a constant distance between the metal base and the insulating substrate. In manufacturing the device, a superposition step of interposing a solder sheet between the lower electrode side of the insulating substrate and the protrusion forming side of the metal base is obtained by melting the soldering sheet after the superposition step. A reflow soldering step of reflow soldering the insulating substrate to the metal base using the solder film. 6. The method according to claim 5, wherein the projections on the metal base are formed by pressing the metal base. 7. The method according to claim 5, wherein the projections on the metal base are formed by cutting the metal base itself. 8. The method according to claim 5, wherein the projection on the lower electrode is formed by etching. 9. The method according to claim 5, wherein a flux is applied to at least one of the lower electrode side of the insulating substrate and the projection forming side of the metal base before the overlapping step.