JP2002009212A - Method for manufacturing heat dissipation structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing at low cost with high reproducibility a heat dissipation structure, with which semiconductor components can be bonded without impairing heat dissipation property even for fluxless case. SOLUTION: The method for manufacturing a heat dissipation structure comprises a first step, where a ceramic circuit-board having at least an aluminum circuit on the front and a metal plate on the back is brazed to a copper heatsink through a brazing filler metal, and a second step, where a nickel layer is coated on the area of the heatsink except the surface the brazing filler metal is bonded to and an aluminum circuit surface on the ceramic circuit-board.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ceramic circuit board and a heat sink made of copper, which are preferably used for modules, in particular, for power supplies, which are formed by joining a ceramic circuit board to a radiating fin or the like via a copper heat sink. The present invention relates to a method for manufacturing a heat dissipation structure.

[0002]

2. Description of the Related Art In power modules such as IGBT and IPM, aluminum oxide (Al
₂ O ₃ ), silicon nitride (Si ₃ N ₄ ), aluminum nitride (AlN), etc.
It is generally known that a power module is formed through a process of attaching a resin case or the like after soldering to a heat sink made of metal such as copper or aluminum.

[0003] Regarding the power module, higher reliability is required especially for electric vehicles including electric railway vehicles and hybrid cars. However, in conventional power modules, in actual use, cracks occur in the ceramic substrate used, and cracks occur in the solder used to join the ceramic substrate and the heat sink, thereby impairing the reliability of the power module. It is known that sometimes happens. Cracks that occur in the ceramic substrate cause insulation failure, and cracks that occur in the solder between the ceramic substrate and the heat sink deteriorate heat dissipation, resulting in inoperability of the semiconductor element.

Since the occurrence of the problem directly leads to shortening of the life of the power module, there is a need for a module which can minimize the occurrence of the problem and have long-term reliability.

[0005] Under the circumstances described above, in order to prevent cracks occurring in the ceramic substrate, aluminum having excellent stress relaxation properties is used as a circuit metal, and thermal stress generated in solder between the ceramic substrate and the heat sink is reduced. For this purpose, the use of a composite material such as an Al—SiC composite material having a thermal expansion coefficient close to that of a ceramic substrate as a heat sink has been studied.

A module obtained by combining a ceramic substrate and a heat sink made of a composite material has high reliability and is suitable for electric railway vehicles and hybrid cars, but the module body is expensive. This is a major drawback, and is a hindrance to expanding applications.

[0007] The reason is that a composite material such as an Al-SiC composite material must employ a special manufacturing method as compared with a conventional metal heat sink, and the cost of a processing step and a surface treatment step is high. This is because the heatsink is several times as expensive as the heatsink made. In addition, heat sinks occupy a large part of the module in terms of shape, which is one of the reasons that the ratio of the heat sink to the power module must be large. Therefore, development of a power module using an inexpensive metal heat sink and having higher reliability than conventional products has been desired.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and uses an inexpensive metal plate as a heat sink and uses a heat sink made of a composite material such as an Al-SiC composite material. It is an object of the present invention to provide a module that can achieve high reliability equal to or higher than that of the module.

As described above, in a power module using an inexpensive metal heat sink, the occurrence of solder cracks or the deterioration of solder joints due to the thermal stress received during the assembly process and actual use conditions. It has been known.

[0010] In view of the above problem, the use of easily degradable solder for the ceramic substrate itself is considered to cause a reduction in reliability. Improving reliability by adopting a structure in which a plated heat sink and a ceramic substrate having a nickel-plated circuit are joined via an aluminum foil having an aluminum melting point depressing layer on both surfaces (hereinafter, referred to as direct attachment). Expected studies are also being made (see JP-A-10-270596).

However, according to the study of the present inventors,
In the technique disclosed in Japanese Patent Application Laid-Open No. Hei 10-270596, the solder wettability of the nickel plating film on the element joint side is extremely deteriorated due to the heat history at the time of joining the ceramic substrate and the heat sink. There is a problem that becomes defective. When solder wetting becomes poor, the thermal resistance between the semiconductor element and the ceramic substrate increases,
A problem arises from the viewpoint of heat dissipation.

As a method of avoiding defective soldering, a certain improvement can be achieved by using a flux when reflowing the solder between the semiconductor element and the ceramic substrate. However, after the reflow, the flux is cleaned. Need to be added, which increases costs.

[0013]

Means for Solving the Problems As a result of intensive studies in view of the above circumstances, the present inventor has found that, in a method of manufacturing a heat dissipation structure for brazing a ceramic substrate and a heat sink, heat dissipation can be performed without flux at the time of joining semiconductor elements. The present inventors have found that a heat dissipation structure capable of obtaining solderability without impairing the reproducibility can be manufactured at low cost and with high reproducibility, and have arrived at the present invention.

That is, the present invention provides a first step of joining the metal plate of the ceramic circuit board having at least an aluminum circuit on the front surface and a metal plate on the back surface to a copper heat sink via a brazing material, A second step of sequentially providing a nickel layer on a portion of the heat sink except for a surface to be joined to the brazing material and a circuit surface made of aluminum on the ceramic circuit board. And preferably, the metal plate is an aluminum plate, and the brazing material is made of an aluminum alloy containing magnesium.

[0015] Further, the present invention provides a method for manufacturing a semiconductor device, comprising:
The method for manufacturing a heat dissipation structure according to the above, further comprising a step of subjecting the metal plate and the brazing material to a surface treatment.

Further, the present invention provides a method for manufacturing a semiconductor device, comprising:
A method for producing the heat dissipation structure, which comprises a step of providing a nickel layer on at least a bonding surface of the heat sink with a brazing material, preferably further comprising a first step and a second step.
And a step of removing a nickel layer in a portion other than a joint surface of the heat sink with the brazing material between the steps of the above.

Further, in the present invention, in the second step, the portion of the heat sink other than the joint surface with the brazing material and the circuit surface made of aluminum on the ceramic circuit board are treated with an aqueous solution containing fluoride. The method of manufacturing a heat dissipation structure according to claim 1, wherein the heat dissipation structure is treated with an aqueous solution containing palladium and tin, and then provided with a nickel layer by electroless plating.

[0018]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates to a first method for joining a metal plate of a ceramic circuit board having at least an aluminum circuit on a front surface and a metal plate on a back surface to a copper heat sink via a brazing material. A step of providing a nickel layer on a portion of the heat sink excluding the surface to be joined with the brazing material and a circuit surface made of aluminum on the ceramic circuit board, in order, in joining semiconductor elements. It has a feature that a heat dissipation structure having solderability without impairing heat dissipation even without flux can be manufactured at low cost and with high reproducibility.

In particular, when the metal plate is an aluminum plate and the brazing material is made of an aluminum alloy containing magnesium, the joining force between the ceramic substrate and the heat sink is increased, and the object of the present invention is more easily achieved, which is preferable. .

Regarding the ceramic circuit board used in the present invention, the ceramic circuit board may have any structure as long as it has the above-mentioned structure, and satisfies required properties such as insulation properties, thermal conductivity and mechanical strength. However, aluminum nitride (AlN) which is a high thermal conductivity ceramic or silicon nitride (Si ₃ N ₄ ) having both high strength and relatively high thermal conductivity is more preferably selected.

The ceramic circuit board used in the present invention may be provided with a circuit on which a semiconductor element or the like can be mounted as usual on the front surface of the ceramic circuit board. Is expected to have an effect of alleviating the thermal stress received during the assembling process and the actual use conditions. It is even more preferred for the above reasons when the circuit consists entirely of aluminum.

As the metal plate provided on the back surface of the ceramic substrate, copper, aluminum, or an alloy thereof can be used. Of these, aluminum or an aluminum alloy is subject to heat received during the assembling process or actual use conditions. It is preferable because the effect of relieving stress is expected, and in particular, plastic deformation ability is high with respect to stress generation,
High-purity aluminum having a purity of 99.5% or more is preferable.

Regarding the brazing material for joining the metal plate on the back surface of the ceramic circuit board to the heat sink made of copper, Al-Si, Al-Zn, Al-
It is used by appropriately selecting from various materials such as Cu. When an aluminum plate is selected as the metal plate as described above, an aluminum alloy containing magnesium is preferably selected for the reason described in detail below. Is done.

When an appropriate amount of magnesium is contained in the brazing material, an oxide film serving as a barrier to a chemical reaction is eliminated by a gettering effect of magnesium, and a strong bonding can be realized. In particular, in the case of aluminum having a strong oxide film, the effect is essential.

Elements other than magnesium include Cu,
Zn, Si, Ni, Ag, Ce, Ge and the like may be contained, and among these, Zn, Cu, Si, and Ge are preferred because the object of the present invention can be more easily achieved.

Further, in joining the aluminum plate and the copper heat sink, the aluminum-copper eutectic temperature of 54
When performed at 8 ° C. or higher, a mechanically brittle aluminum-copper intermetallic compound is formed at the bonding interface, and depending on the degree of its formation,
Cracks are likely to enter during the heat cycle, which may cause a problem in reliability. For this reason, it is preferable to use an aluminum alloy having a melting point lower than 548 ° C. as the brazing material.

In order to join the ceramic circuit board and the copper heat sink using the brazing material, the ceramic circuit board, the aluminum alloy brazing material, and the copper heat sink are sequentially stacked in a vacuum or in a non-oxidizing atmosphere, and a load is applied. The heating may be performed while applying the pressure.

When an aluminum alloy brazing material containing an appropriate amount of magnesium is used, the oxide film on the surface layer disappears due to a gettering effect of magnesium in the brazing material, and bonding is realized by a chemical reaction. In this case, when the bonding energy is insufficient (such as low-temperature bonding), even if magnesium is present, the oxide film on the aluminum surface cannot be broken, and perfect bonding may not be achieved.

Therefore, it is preferable to remove an oxide film existing on the surface of the aluminum and / or aluminum alloy brazing material on the joining side of the ceramic circuit board by a physical or chemical method before joining. As a physical method, a conventionally known method such as polishing using a wrapping film or the like, and as a chemical method, an acid treatment with a mixed solution of nitric acid and hydrofluoric acid may be used. By performing the above-described processing, bonding can be performed at a relatively low temperature, so that stress applied to the substrate after bonding can be reduced and reliability can be improved.

Further, in the present invention, when a heat sink provided with a nickel layer in advance by plating or the like is used, it is possible to join at a higher temperature than when a ceramic circuit board and a copper heat sink are directly joined. ,
The selection range of the brazing material is increased, and a strong joint can be obtained without performing a surface treatment before joining. Of course, introducing a surface treatment before joining is a more preferable method.

In the second step of the present invention, the joint between the ceramic circuit board and the copper heat sink obtained in the first step is formed by removing a portion of the heat sink except for a joint surface with the brazing material and the ceramic circuit. This is a step of providing a nickel layer on the circuit surface made of aluminum on the substrate and by adopting this step, in the subsequent step of soldering the semiconductor element, there is no loss of heat dissipation even with fluxless. It has a feature that a heat dissipation structure having solderability can be manufactured at low cost and with high reproducibility.

In the manufacture of a heat dissipation structure, when nickel plating is performed at a desired position after brazing, in a heat dissipation structure having both aluminum and copper on its surface, it is necessary to apply plating to copper and aluminum at the same time. Difficult with preprocessing. That is, for example, when electroless nickel plating is performed on copper, an activation treatment (containing palladium chloride) is performed.
On the other hand, in electroless nickel plating on aluminum, it is usually necessary to form a zinc-substituted layer on the aluminum surface layer by a process called a double zincate method, and it is difficult to perform both at the same time. .

In the present invention, as a pretreatment, an acid or an alkali (for example, nitric acid) and a fluoride (for example, hydrofluoric acid) are used.
After removing the oxide film on the aluminum surface, it is preferable to immerse (catalyst) in an aqueous solution containing palladium chloride and stannous chloride, and to perform electroless nickel plating. By this processing, the nickel layer can be provided in one operation on the portion of the copper heat sink except for the bonding surface with the brazing material and on the circuit surface made of aluminum on the ceramic circuit board. Further, it is more preferable to carry out a sulfuric acid treatment after the catallation, since the adsorbed palladium is more activated.

The above-mentioned processing method can be applied without any change even when the ceramic circuit board is joined to a heat sink made of nickel plated in advance with nickel. However, the nickel plating on the copper heat sink is peeled off in the fluoride (for example, hydrofluoric acid) immersion step in an acid or an alkali (for example, nitric acid).
This is because nickel plating can be formed on aluminum and copper by the same processing as nickel plating on a product obtained by bonding an aluminum circuit ceramics substrate and a copper heat sink.

[0035]

EXAMPLE 1 As shown in FIG. 1, a ceramic substrate 11 was a silicon nitride substrate of 35 × 35 × 0.635 mm and had a thermal conductivity of 75 W / mK by laser flash method and three-point bending. Average strength is 560MP
a was prepared. Also, metal plates 12 and 13 for circuits
A 1085 aluminum plate having a JIS designation of 32 × 32 × 0.4 mm was prepared.

On both front and back surfaces of the silicon nitride substrate 11, J
IS designation 2017 aluminum alloy foil (20μm thickness)
And the aluminum plates 12 and 13 were overlapped with each other and pressed vertically at 300 MPa. And, in vacuum, temperature 6
The aluminum plate and the silicon nitride plate were joined while heating at 20 ° C. × 20 min. After joining, an etching resist is screen-printed on a desired portion of the aluminum plate surface, and a circuit pattern is formed by etching with a ferric chloride solution.
0 was produced.

Next, as shown in FIG. 2, a 70 × 130 × 3 mm size tough pitch copper plate 1 was used as a heat sink.
5 were prepared. And the ceramic circuit board 10
A JIS 7075 aluminum alloy foil having a thickness of 20 μm is inserted between the heat sink 15 and the copper heat sink 15, and 540 ° C. × 4 with a graphite jig under a pressure of 300 MPa.
Bonding was performed under heating conditions for one minute to obtain an aluminum circuit board integrated with a copper heat sink.

Next, the copper heat sink-integrated aluminum circuit board is first degreased with alkali, and then subjected to a hydrofluoric / nitric acid solution (nitric acid; 900 ml / l, hydrofluoric acid; 50 ml / l).
For 1 minute, and then a catalyst solution (palladium chloride; 0.2 g / l, stannous chloride; 10 g / l, hydrochloric acid;
After immersion in 200 ml / l for 2 minutes, immersion in sulfuric acid (50 g / l) for 30 seconds, electroless Ni-P plating (Nimden SX)
(Uemura Industry). As a result, as shown in FIG. 3, the nickel plating 16 was uniformly formed on the copper heat sink except for the bonding surface and on the circuit-side aluminum of the ceramic circuit board.

Further, as shown in FIG. 4, a 13 × 13 mm silicon chip 18 was soldered on the nickel-plated aluminum circuit to obtain a heat dissipation structure. This heat dissipation structure was subjected to the following reliability test. Table 1 shows the results.

<Reliability test method>
A heat cycle test in which -40 ° C. × 30 minutes to 125 ° C. × 30 minutes is performed once is performed, and 500 times, 1000 times, and 300 times
After a lapse of 0 times, the occurrence of solder cracks under the Si chip was examined by SAT (ultrasonic image flaw detector).

[0041]

[Table 1]

Embodiment 2 The same ceramic circuit board 10 and copper heat sink 15 as in Embodiment 1 were prepared. Before joining them, the surface of the joining side aluminum 13 of the ceramic circuit board 10 and the JI
A polishing treatment was performed on the aluminum circuit bonding side of the 7075 aluminum alloy foil 14 with a wrapping film. Thereafter, bonding was performed under a heating condition of 520 ° C. × 4 minutes while applying a pressure of 300 MPa with a graphite jig to obtain an aluminum circuit board integrated with a copper heat sink. Subsequent steps were performed in the same manner as in Example 1. Finally, a heat dissipation structure as shown in FIG. 4 was obtained and subjected to a reliability test. The results are shown in Table 1.

[Embodiment 3] As shown in FIG. 5, the same ceramic circuit board 10 as that of the first embodiment is prepared, and the copper heat sink 15 is the same in size as the first embodiment. Was prepared. And
A 20 μm-thick JIS 2017 aluminum alloy foil having a thickness of 20 μm was inserted between the ceramic circuit board 10 and the copper heat sink 15, and a pressure of 300 MPa was applied with a graphite jig.
Under a load of 620 ° C. for 4 minutes,
A copper heat sink integrated aluminum circuit board was obtained.

Next, the copper heat sink-integrated aluminum circuit board is first degreased with an alkali, and then immersed in a hydrofluoric / nitric acid solution (nitric acid; 900 ml / l, hydrofluoric acid; 50 ml / l) for 2 minutes to obtain a portion other than the joint surface. The nickel plating on the copper heat sink was peeled off, and then immersed in a catalyst solution (palladium chloride; 0.2 g / l, stannous chloride; 10 g / l, hydrochloric acid; 200 ml / l) for 2 minutes, and then sulfuric acid (50 g / l).
l) for 30 seconds to perform electroless NiP plating (Nimden SX (Uemura Kogyo)). As a result, as shown in FIG. 6, the nickel plating 16 was formed uniformly on the copper heat sink and the circuit-side aluminum of the ceramic circuit board except for the joint surface. Further, through the same steps as in Example 1,
Finally, a heat dissipation structure as shown in FIG. 7 was obtained. Table 1 shows the results of the reliability test.

[0045]

According to the method for manufacturing a heat radiation structure of the present invention, the conventional A
There is a feature that a heat radiation structure having the same reliability as the case of using a heat sink made of a composite material such as an l-SiC composite material can be provided at low cost, easily, and thus stably.
Very useful in industry.

[Brief description of the drawings]

FIG. 1 is a sectional view of a ceramic circuit board according to Examples 1 to 3 of the present invention.

FIG. 2 is an explanatory view of a method of joining a ceramic circuit board and a heat sink in Examples 1 and 2 of the present invention.

FIG. 3 is a cross-sectional view of a heat dissipation structure according to the first and second embodiments of the present invention.

FIG. 4 is a cross-sectional view of a heat dissipation structure formed by soldering silicon chips according to the first and second embodiments of the present invention.

FIG. 5 is an explanatory view of a method of joining a ceramic circuit board and a heat sink according to a third embodiment of the present invention.

FIG. 6 is a sectional view showing a state after nickel plating is applied to a heat sink-integrated substrate according to a third embodiment of the present invention.

FIG. 7 is a sectional view of a heat dissipation structure formed by soldering a silicon chip according to a third embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 11 ceramic substrate 12, 13 metal plate 14 brazing material 15 copper heat sink 16, 17 nickel layer 18 silicon chip

Claims (6)

[Claims] A first step of joining the metal plate of a ceramic circuit board having at least an aluminum circuit on a front surface and a metal plate on a back surface to a copper heat sink via a brazing material; A second step of sequentially providing a nickel layer on a portion excluding a bonding surface with a brazing material and a circuit surface made of aluminum on the ceramic circuit board, the method further comprising sequentially performing a second step. 2. The method according to claim 1, wherein the metal plate is an aluminum plate, and the brazing material is made of an aluminum alloy containing magnesium. 3. The method according to claim 1, further comprising a step of subjecting the metal plate and the brazing material to a surface treatment before the first step. 4. The method according to claim 1, further comprising a step of providing a nickel layer on at least a bonding surface of the heat sink with the brazing material before the first step. 5. The heat radiation according to claim 4, further comprising a step of removing a nickel layer in a portion other than a joining surface of the heat sink with the brazing material between the first step and the second step. The method of manufacturing the structure. 6. In a second step, a portion of the heat sink except for a bonding surface with a brazing material and a circuit surface made of aluminum on the ceramic circuit board are treated with an aqueous solution containing fluoride to form palladium and tin. 3. The method for manufacturing a heat dissipation structure according to claim 1, wherein a nickel layer is provided by an electroless plating method after the treatment with an aqueous solution containing