JP2002110875A - Power module - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive power module exhibiting excellent heat dissipation properties in which high reliability is sustained even if thermal stress is applied repeatedly during actual use. SOLUTION: Onto a heat sink having a surface of Cu or Cu alloy or Al or Al alloy, a ceramic circuit board having an Al circuit firmed on the surface of a ceramic substrate and a metal plate formed on the backside thereof is bonded through a soldering material such that the soldering material touches the metal plate and an electric component is bonded onto the Al circuit through solder. In such a power module, the solder is an Sn-Pb based solder containing 45-85 mass% of Pb.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power module using a circuit board composite for a semiconductor device which has excellent heat dissipation and high reliability.

[0002]

2. Description of the Related Art Conventionally, various types of semiconductor devices called power modules have a structure in which one surface of a ceramic substrate is coated with Cu.
A circuit in which a metal plate made of Cu for heat dissipation is bonded to the other surface is used as a circuit board. In particular, when high heat dissipation is required due to high integration and high power, silicon nitride or aluminum nitride is used as a ceramic substrate because of high thermal conductivity.

In a circuit board in which electric components are soldered to a Cu circuit, usually, a metal plate for heat radiation is soldered to a heat sink made of Cu, and the heat sink is connected to a heat radiating member such as a water cooling plate or a heat radiation fin via grease. It is used by tightening with screws, etc., but problems such as cracks in the solder layer between the electric components and the circuit board due to repeated thermal cycles and temperature changes in the operating environment due to the operation of electric components such as transistors When high reliability is required, attempts have been made to join Al or an Al alloy having a low yield strength as a circuit material (hereinafter, a circuit board using Al or an Al alloy as a circuit material has been proposed. Simply called Al circuit board).

In order to avoid solder cracks in the solder layer between the Al circuit board and the heat sink, a heat sink made of a material having a smaller coefficient of thermal expansion than Cu has also been used. Aluminum-silicon carbide composites (hereinafter referred to as Al / SiC composites) obtained by impregnating the voids of the silicon carbide porous body with a metal mainly composed of
The composite material is inferior to the Cu plate or the Al plate in heat dissipation and has a problem that it is expensive.

Recently, attempts have been made to use a circuit board composite in which the solder between the metal plate of the Al circuit board and the heat sink is omitted and the metal plate is directly joined to a heat sink made of a Cu plate or an Al plate with a brazing material. I have. By joining the metal plate of the Al circuit board directly to the heat sink with brazing material,
The problem of solder cracks between the metal plate and the heat sink in repeated thermal cycles is eliminated.

[0006]

In the above-mentioned circuit board composite, a thinner heat sink is more advantageous for heat dissipation. However, the rigidity of the heat sink is weakened, and the change in the swell of the heat sink during repeated thermal cycles is increased due to the difference in the coefficient of thermal expansion between the Al circuit board and the heat sink in the circuit board composite. In a state where the heat sink is screwed to a heat dissipating member made of aluminum having a high coefficient of thermal expansion, the change in the undulation of the heat sink in a repeated thermal cycle is further increased. As a result, for example, stress tends to concentrate on the solder end under the Si chip of a transistor as an electric component, and a crack (hereinafter referred to as a solder crack) occurs at the solder end under the Si chip in a heat cycle test as a reliability evaluation test. Is generated.

The present invention has been made in view of the situation of the prior art described above, and an object of the present invention is to use a solder having an appropriate composition range so as to have excellent heat radiation properties and to repeat thermal stress repeatedly applied in actual use. It is an object of the present invention to provide an inexpensive power module that can maintain high reliability in spite of this.

[0008]

That is, the present invention has a circuit made of Al on the surface of a ceramic substrate via a brazing material on a heat sink whose surface is made of Cu or Cu alloy or Al or Al alloy. A power module comprising a ceramic circuit board having a metal plate on a back surface joined so that the metal plate is in contact with a brazing material, and furthermore, an electric component is joined to the circuit made of Al via solder. The power module is characterized in that the solder is a Sn—Pb-based solder containing 45% by mass or more and 85% by mass or less of Pb. The brazing material is C
at least one selected from the group consisting of u, Zn and Si, and M
g is preferably made of an Al alloy, and the metal plate is preferably made of Al or an Al alloy. Further, it is preferable that the ceramic substrate is made of silicon nitride or aluminum nitride.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail.

As the ceramic substrate used in the present invention, a conventionally known ceramic substrate can be used, but it is preferable to use silicon nitride or aluminum nitride which is excellent in thermal conductivity and mechanical strength.

As the circuit material formed on the ceramic substrate, a circuit made of Al having high conductivity and high thermal conductivity (hereinafter simply referred to as an Al circuit) is used. Al
The purity of Al is high and the plastic deformability against stress generation is high, for example, the purity of Al is 98.5%.
The above are preferred. As for the metal plate formed on the other surface of the ceramic substrate, Al having high thermal conductivity is used.
Although Al and Cu are used, when Al is used, it is possible to join simultaneously with the circuit material, which is preferable for improving productivity. In order to form a circuit and a metal plate, each plate is bonded to a ceramic substrate via a brazing material and then etched, and a pattern punched from the plate is bonded to a ceramic substrate via a brazing material and the like. be able to.

The heat sink used in the present invention is Cu or Cu which can be fastened to the heat sink via grease.
Alloy or Al or Al alloy,
In particular, as Cu or Cu alloy, oxygen-free Cu, Cu
-W alloy and Cu-Mo alloy are preferable. In addition, since a thinner heat sink is more advantageous for heat dissipation, it is preferable that the thickness be 2 mm or more and 4 mm or less.

When the heat sink is made of Cu or a Cu alloy, it is preferable that at least the surface of the ceramic substrate to be joined to the metal plate is covered with a film containing Ni as a main component. If the film containing Ni as the main component is not covered, the brazing material made of an Al alloy diffuses into the heat sink after joining with the Al-based brazing material, and Al and C
and a hard alloy layer containing u is formed to lower the bonding strength,
This is because the reliability may be reduced. By coating the heat sink with a film containing Ni as a main component, diffusion of the brazing material into the heat sink is eliminated, and highly reliable bonding is enabled by forming an alloy layer containing Al and Ni. As a method of coating the film containing Ni as a main component, a conventionally known method such as Ni plating or cladding by rolling can be used.

For joining a ceramic substrate (hereinafter simply referred to as an Al circuit board) to which an Al circuit and a metal plate are joined and a heat sink, one type selected from the group consisting of Cu, Ge and Si, as in the formation of the Al circuit. It is preferable to use an Al alloy brazing material containing the above and Mg, for example, an Al-Mg-Cu alloy, an Al-Mg-Ge alloy, an Al-Mg-Si alloy, or the like. The reason is as follows.

For joining a ceramic substrate and a metal plate, A
Although l-Si alloys and Al-Ge alloys are known,
Mg and 1 selected from the group consisting of Cu, Ge and Si
The brazing material made of an Al alloy containing at least one of the above-described alloys has a wider allowable range of bonding conditions with a ceramic substrate than the above-mentioned alloys, and has a feature that it can be bonded even in a vacuum. Joining is possible.

That is, a conventionally known Al—Si alloy or Al
In the case of a -Ge alloy, the melting point does not decrease unless Si or Ge is added in a relatively large amount. However, if a large amount is added, the alloy becomes hard and brittle. In order not to cause such a problem, for example, in an Al-Si based alloy, if the proportion of Si is reduced to 5%, the melting point becomes 615 ° C., and it is difficult to join at a temperature of 620 ° C. or less even if pressure is applied. Become.

On the other hand, in the case of an Al alloy containing Mg, particularly an Al-Mg-Cu alloy, an Al-Mg-Ge alloy or an Al-Mg-Si alloy, Cu, Ge, Si
Even if the ratio is reduced to about 4%, bonding at about 600 ° C. becomes possible by taking appropriate measures such as pressurization, and the allowable range of bonding conditions is widened. Furthermore, Al-Mg-C
u-based alloy, Al-Mg-Ge-based alloy and Al-Mg-
In the Si system, Cu, Ge, Si, and Mg are easily diffused into Al more easily than in the case where Si or Ge is solely added to Al. The protrusion phenomenon that the material is extruded hardly occurs, and stable bonding can be performed in a relatively short time.

The addition of a small amount of Mg in an Al alloy containing Mg improves the bonding state. This is presumed to be due to the effect of removing the oxide layer on the Al surface and the effect of improving the wettability between the ceramic substrate surface and the brazing material. The ratio of Mg is preferably 0.05 to 3% by mass.
If the content is less than 0.05% by mass, the effect of addition becomes insignificant,
If the content exceeds mass%, the hardness of Al or an Al alloy is adversely affected, and a large amount may be volatilized at the time of joining to hinder furnace operation. Particularly preferably, 0.1 to 1.0
% By mass.

If necessary, the surface of the circuit board composite may be A
The entire surface or the portion where the l or Al alloy is exposed is plated with Ni. In particular, the soldering portion is plated with Ni in order to enhance the solder bonding property, but the portion to be bonded to the Al wire is more reliable if there is no Ni plating.
Partial plating in which only desired portions are selectively plated is preferred.

In the soldering of electric components onto an Al circuit of a circuit board composite in which an Al circuit board is joined to a heat sink, the length of one side of an electric component such as a transistor Si chip is preferably 20 mm or less. .
The larger the Si chip, the larger the heat transfer area and the better the heat dissipation. However, the larger the Si chip, the higher the unit cost of the chip, and the greater the stress applied to the solder end under the Si chip, and the solder due to repeated thermal cycling. This is because cracks and Si chip damage are likely to occur. As for the thickness of the Si chip, a thinner one is more advantageous for heat dissipation,
It is preferably 0.5 mm or less.

When soldering an electric component on the Al circuit of the circuit board composite, a thinner solder layer is more advantageous for heat dissipation, but if it is too thin, the ability to relieve stress due to repeated thermal cycling. And solder cracks tend to occur. The thickness of the solder layer is 50μm or more and 150μ
m or less.

In the present invention, when an electric component is to be soldered on the Al circuit of the circuit board composite, 45% by mass or more is used.
Sn-Pb based solder containing 5% by mass or less of Pb is used. The content of impurity elements other than Pb and Sn is preferably 1% by mass or less. The reason for using the Sn-Pb-based solder having the above composition is as follows.

When the electric component is soldered to the circuit board composite, especially when the heat sink is screwed to a heat dissipating member made of aluminum having a high coefficient of thermal expansion, Al
The change of the swell of the heat sink in the repetitive thermal cycle due to the difference in the coefficient of thermal expansion between the circuit board and the heat sink is large, and the stress tends to concentrate on the solder end under the electric component.

Sn—P having a Pb content of 85% by mass or more
When the b-based solder is used, the solder is relatively soft and has a high effect of relaxing stress, but stress tends to concentrate on the solder layer as compared with other portions. Therefore, the content of Pb is 85% by mass.
In a power module in which electrical components such as a transistor Si chip are joined using the above Sn-Pb solder, in a heat cycle test in which the above-described heat sink is screwed to a heat dissipation member made of aluminum, a solder layer is formed. Exceeding the deformation limit, solder cracks occur at the solder end under the Si chip.

On the other hand, when the content of Pb is 45% by mass or less,
When n-Pb-based solder is used, the solder is relatively hard, and the stress is distributed to the Al circuit under the solder layer, so that solder cracks are less likely to occur, but the effect of relaxation is low. For this reason, in a power module in which an electric component such as a transistor Si chip is joined using a Sn—Pb-based solder having a Pb content of 45% by mass or less, a screw is fastened to a heat dissipation member made of aluminum as described above. In the heat cycle test, the Si chip, which can completely reduce the change in the undulation of the heat sink, is destroyed.

According to the present invention, the circuit board composite A
(1) When a power module is manufactured by using an Sn-Pb-based solder containing 45% by mass or more and 85% by mass or less of Pb for soldering electric components on a circuit, a solder layer having an appropriate hardness is formed. It is possible to moderately change the undulation of the heat sink while appropriately distributing the stress applied to the layer to the Al circuit under the solder layer. As a result, even in a heat cycle test in a state where the heat sink as described above is screwed to a heat radiating member made of aluminum, a solder module is unlikely to occur in a heat cycle test and an electric component is not broken. Becomes possible.

[0027]

The present invention will be described below in more detail with reference to examples and comparative examples.

Examples 1 to 4 and Comparative Examples 1 to 4 (1) Ceramic substrate Silicon nitride and aluminum nitride of 35 mm × 35 mm × 0.635 mm having a thermal conductivity of 95 W / mK and 175 W by a laser flash method, respectively. / MK, the average values of the three-point bending strengths are 700 MPa and 420 MPa, respectively.
Prepared what is a

(2) Al Circuit As an aluminum plate serving as a circuit and a metal plate, a commercially available Al material having a thickness of 0.4 mm (purity ≧ 99.99%) was cut into a ceramic substrate.

(3) Preparation of Circuit Board A composition having a composition of 95% by mass A
JIS designation 2017 of 1-1 mass% Mg-4 mass% Cu
The aluminum plates were overlaid with an aluminum alloy foil brazing material (thickness: 20 μm) and pressed vertically at 300 MPa. Then, at a temperature of 630 ° C. in a vacuum of 10 ⁻² Pa ×
The aluminum plate and the ceramics substrate were joined while heating under the condition of 20 min. After joining, an etching resist was screen-printed on a desired portion of the aluminum plate surface, and a circuit pattern was formed by etching with a ferric chloride solution, thereby producing a circuit board.

(4) Heatsink The heatsink is a 10 μm-thick electroless Ni-P (phosphorus) over the entire surface of an oxygen-free copper material of 70 mm × 130 mm × 3 mm.
A plated one was prepared. Holes were provided at the four corners of the heat sink for fastening screws to a heat dissipating member made of aluminum.

(5) Preparation of a Circuit Board Composite A composition having a composition of 9 between the circuit board and the heat sink.
5% by mass Al-1% by mass Mg-4% by mass Cu thickness 20
The aluminum alloy foil brazing material having a JIS designation of 2017 μm was put therein, and pressed with a graphite jig at a pressure of 300 MPa. 610 ° C x 4mi in a vacuum of 10 ^-2 Pa under pressure
The bonding was performed under heating conditions of n. Furthermore, Ni plating was applied to the Al circuit of each manufactured circuit board composite to obtain a circuit board composite.

(6) Electronic parts 15 mm × 15 with the back plated with Au as electronic parts
A mm × 0.4 mm Si chip was prepared.

(7) Soldering of electronic parts Sn-Pb-based solders of each composition shown in Table 1 having a weight of 0.1 mm in thickness of the solder layer after soldering were used, and the Al of the circuit board composite was used. A power module was obtained by soldering to the circuit surface.

(8) Evaluation of Power Module 100 mm × 150 mm × 20 mm pure Al material (A10
After applying silicon grease to a thickness of 200 μm on the mounting portion of the power module of the heat radiating member comprising 50),
Through the screw holes provided in the heat sink of the obtained power module, the power module was tightened to the heat radiating member with bolts to obtain an evaluation sample. For each sample, −40 ° C., 30 minutes → room temperature,
A heat cycle test was performed with 10 minutes → 125 ° C., 30 minutes → room temperature, 10 minutes as one cycle, and the appearance after 3000 cycles was visually observed to confirm the presence or absence of abnormality, and the Si chip when the sample was cut Observations were made on solder cracks in the solder layer between the aluminum plate and the Al circuit, and on the joint surface between the metal plate and the heat sink. Table 1 shows the results.

[0036]

[Table 1]

As is clear from the examples and comparative examples, in the power module of the present invention, in each of the samples,
The solder layer under the Si chip shows a good bonding condition,
No damage to the i-chip was observed. On the other hand, in Comparative Examples 1 and 3, Sn-P having a Pb content of 45% by mass or less was used.
In the case of using the b-based solder, some of the heat sinks may be damaged by the change in the undulation of the heat sink, and may be damaged only by the Si chip. In addition, the Pb content of Comparative Examples 2 and 4 is 85% by mass.
When the Sn-Pb-based solder described above was used, the limit of deformation of the solder layer was exceeded, and the rate of occurrence of solder cracks under the Si chip increased.

[0038]

According to the power module of the present invention, an electric component is formed on a circuit of a circuit board composite in which a ceramic circuit board having an Al circuit and a heat sink are directly joined by using a Sn-Pb-based solder having a specific composition range. Has a feature that it has excellent heat dissipation properties and can maintain high reliability even if it is subjected to repeated thermal stress in actual use. It is suitable for applications such as power modules.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Masahiro Ibukiyama 3-5-1 Asahimachi, Machida-shi, Tokyo Denki Kagaku Kogyo Co., Ltd. Central Research Laboratory F-term (reference) 5E322 AA01 AB02 AB09 EA11 5E338 AA01 AA18 BB71 BB75 CC01 CD11 EE02 5F036 AA01 BB08 BC06 BD03 BD14

Claims (4)

[Claims] 1. A ceramic circuit board having a circuit made of Al on the surface of a ceramic substrate and a metal plate on the back surface on a heat sink made of Cu or Cu alloy or Al or Al alloy on a heat sink made of brazing material. A power module, wherein the metal plate is joined so as to be in contact with the brazing material, and an electric component is joined to the circuit made of Al via solder, wherein the solder is 45% by mass or more and 85% by mass or more. A power module, which is a Sn-Pb-based solder containing Pb by mass% or less. 2. The power module according to claim 1, wherein the brazing material is made of an Al alloy containing Mg and at least one selected from the group consisting of Cu, Zn and Si. 3. The power module according to claim 1, wherein the metal plate is made of Al or an Al alloy. 4. The ceramic substrate according to claim 1, wherein said ceramic substrate is made of silicon nitride or aluminum nitride.
Or the power module according to claim 3.