JP2001313355A - Module and method of manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a module, having a high reliability equal to or higher than that of a module employing a heat sink made of an expensive composite material which uses inexpensive metal plate as a heat sink. SOLUTION: In the module, where a ceramic circuit board is bonded onto a metallic heat sink, rear surface of the heat sink has a convex warpage of 20 μm-120 μm in a measuring span of 50 mm. Preferably, a metal principally comprising Mo is bonded to the circuit of the ceramic circuit board via a metal principally, comprising one or more of Ni, Cr and Ti.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a module in which a ceramic circuit board is joined to a radiating fin or the like via a heat sink made of metal such as copper, and more particularly to a power module for power supply and a method of manufacturing the same.

[0002]

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

On the other hand, for electric vehicles including electric railway vehicles and hybrid cars, recently, higher reliability has been demanded for power modules. However, it is known that as a factor that impairs the reliability of the power module, cracks occur in the ceramic substrate used, and cracks occur in the solder used for joining the ceramic substrate and the heat sink.

[0004] 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 is directly linked to shortening the life of the power module, there is a need for a module which can prevent the problem as much as possible and has high reliability for a long period of time.

[0005] In view of the above circumstances, in order to prevent cracks generated in a ceramic substrate, aluminum (Al) metal having excellent stress relaxation properties is used as a circuit metal, and solder generated between a ceramic substrate and a heat sink is generated. Use of a composite material such as an Al-SiC composite material having a coefficient of thermal expansion close to the coefficient of thermal expansion of a ceramic substrate as a heat sink has been studied in order to reduce thermal stress.

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, the shape of the heat sink occupies a large part of the module, which is one of the reasons why the heat sink shape has a large proportion in the price of the power module. Therefore, development of a highly reliable power module while using an inexpensive metal heat sink has been eagerly desired.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has clarified the relationship between the power module structure and its reliability which have not been omitted until now. Is used as a heat sink, and A
It is an object of the present invention to provide a module that achieves a high reliability equal to or higher than that when a heat sink made of a composite material such as an l-SiC composite material is used.

As described above, in a power module using an inexpensive metal heat sink, solder cracks may occur or solder joints may deteriorate due to the thermal stress received during the assembly process or actual use conditions. Are known.

In view of the above problem, it is considered that the use of easily deteriorated solder for the ceramic substrate itself causes a decrease in reliability. There is also a study to improve reliability by adopting a structure in which a ceramic substrate is directly bonded (hereinafter, referred to as a direct mounting) (Japanese Patent Application Laid-Open No. Hei 9-1997).
No. 97865, JP-A-10-270596).

However, the present inventor examined the power module having the above-mentioned direct mounting structure,
It has been found that there are two problems inherent to the direct mounting structure described later.

<Problem of Deterioration of Heat Dissipation Caused by Warpage of Module Bottom> When a metal heat sink, for example, copper (coefficient of thermal expansion is 17 ppm / ° C.) is used, the coefficient of thermal expansion is the coefficient of thermal expansion of the ceramic substrate (4 to 4). 5 ppm / ° C.), the surface of the heat sink (the surface to be connected to a heat radiating unit such as a radiating fin, hereinafter referred to as a module bottom surface) is greatly warped due to the generated thermal stress after bonding at 600 ° C. to 900 ° C. .

[0013] Since the warp is concave on the bottom surface of the module, a gap is generated between the heat dissipation unit and the bottom surface of the module when joining to the heat dissipation unit. Thermal conductivity deteriorates, and the temperature of electronic components such as semiconductor elements constituting the module rises.

<Problem of Solder Cracks Occurring in Semiconductor Device Mounting Portion During Use> Even when the ceramic substrate and the heat sink are directly attached, the semiconductor device on the substrate is still soldered to the circuit on the ceramic circuit substrate. However, in this case, the stress applied to the substrate is directly applied, so that it becomes larger than that of the conventional module, and the thermal stress applied to the solder under the semiconductor element on the ceramic substrate is also large. Therefore, solder cracks under the semiconductor element are likely to occur.

Although the reason is not clear, the present inventor has
In the case of the conventional module, the thermal stress between the ceramic substrate and the heat sink was considerably relaxed by the plastic deformation of the solder itself, so it was speculated that the thermal stress applied to the solder of the semiconductor element was reduced. I have.

When the above two problems occur, in any case, the heat dissipation of heat generated from electronic components and circuits such as semiconductor elements in the module is deteriorated, and the temperature of the semiconductor elements increases. Causes malfunctions such as malfunctioning of the semiconductor element and shortening of the service life.
Not practically preferable.

[0017]

Means for Solving the Problems The present inventor has studied diligently in view of the above circumstances. More specifically, for the combination of a ceramic circuit board and a metal heat sink, various modules have been trial-produced and the reliability thereof has been improved. Through the evaluation, it has been found that a module having a specific shape hardly generates solder cracks and substrate cracks over a long period of time, and is excellent in heat dissipation, and has led to the present invention.

That is, the present invention relates to a module in which a ceramic circuit board is joined to a metal heat sink, and the back surface of the heat sink is warped convexly, and the amount of the warp is 20 μm or more at a measurement span of 50 mm. A module having a size of less than 120 μm, preferably nickel (Ni), chromium (Cr) or titanium (T
i) via a metal mainly containing any one or more of
The above module is characterized in that a metal containing molybdenum (Mo) as a main component is joined.

Further, according to the present invention, a ceramic circuit board and molybdenum are sequentially arranged on one side of a metal heat sink to obtain a laminate, and a pair of a concave surface and a convex surface having a predetermined radius of curvature in any direction is obtained. Using a mold to be, under the state of sandwiching the laminate between the uneven surface, pressurized, a module manufacturing method characterized by being integrated,
Preferably, in the above-mentioned module manufacturing method, the laminate is characterized in that a metal mainly containing at least one of nickel, chromium and titanium is interposed between the ceramic circuit board and molybdenum. is there.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION As a result of various experiments by the present inventor, the present invention is directed to a module in which a ceramic circuit board is joined to a metal heat sink by brazing, and the back surface of the heat sink has a convex shape. Warp, and the amount of the warp is 20 μm in a measurement span of 50 mm.
When having a specific shape of less than 120 μm or more, it has been found that a solder crack and a board crack are unlikely to occur over a long period of time and a module excellent in heat dissipation can be obtained. Things. When the shape of the back surface of the heat sink does not have the specific shape,
The object of the present invention cannot be achieved.

Further, in the present invention, a metal mainly composed of molybdenum is bonded to a circuit of the ceramic circuit board via a metal mainly composed of at least one of nickel, chromium and titanium. Is preferred. By adopting the above configuration, the coefficient of thermal expansion of the ceramic circuit board can be suppressed, and when joined to a metal heat sink, a desired amount of convex warpage can be easily achieved, and the object of the present invention is further achieved. Easier to do.

Regarding the ceramic circuit board used in the present invention, any ceramic board may be used as long as it satisfies required properties such as insulation properties, thermal conductivity and mechanical strength. Aluminum nitride (AlN) or silicon nitride (Si ₃ N ₄ ) having both high strength and relatively high thermal conductivity is more preferable.

As the circuit provided on the ceramics substrate, any circuit may be used as long as it is a metal having good conductivity, but copper and aluminum which are inexpensive and have high thermal conductivity are preferably used. Further, as the copper or aluminum, the electrical conductivity is high, the plastic deformation ability is high with respect to stress generation,
High purity is preferred.

As the metal used for the metal heat sink, copper or aluminum, which is inexpensive and has high thermal conductivity, or an alloy containing these as a main component is preferably used.

As a method for obtaining the module of the present invention,
Although it can be obtained by applying a conventionally known method,
The method described later can obtain the module of the present invention with good reproducibility and high productivity.

That is, a ceramic substrate (ceramic circuit substrate) having a circuit provided on one principal surface between the concave and convex surfaces in a mold having a pair of a concave surface having a desired shape and a convex surface having the same shape, On the side of the board where the circuit is not provided, via a brazing material, a metal heat sink is arranged and heated so as to melt a part of the brazing material under pressure,
The ceramic circuit board and the metal heat sink are joined.

Thereafter, electronic components such as semiconductor elements are mounted on the circuit of the ceramic circuit board, and a circuit is formed by performing operations such as wire bonding. Then, after applying high thermal conductive grease to the back surface of the metal heat sink, it is attached to a heat radiation unit such as a heat radiation fin.

The brazing material used in the present invention may be appropriately selected according to the ceramic substrate and the metal constituting the circuit. When the metal for the circuit is copper, an Ag-Cu-Ti-based brazing material is used. And a metal heat sink, or a ceramic substrate and a circuit metal are preferred because of their high bonding strength.

When an aluminum nitride circuit board having a circuit made of aluminum is joined to a heat sink made of copper, it contains magnesium and two or more elements selected from the group consisting of copper, germanium and silicon. Aluminum alloys are preferred because they have excellent adhesion between the aluminum nitride substrate and the copper heat sink and can reliably achieve the object of the present invention.

In particular, when an aluminum nitride circuit board having a circuit made of aluminum is made of JIS 2017 aluminum alloy as a brazing material, the aluminum alloy is easily integrated with the aluminum of the circuit at the time of joining. Is more preferable.

In order to join a metal mainly composed of molybdenum to a circuit of a ceramic circuit board via a metal mainly composed of at least one of nickel, chromium and titanium, One of nickel, chromium or titanium on the outside of the circuit on the board
A metal mainly composed of at least one species may be arranged, and a metal mainly composed of molybdenum may be arranged outside.

The nickel, chromium, and titanium may be formed on a circuit by applying a known technique such as a plating method such as electroless plating, or may be formed on a metal containing molybdenum as a main component. The latter method has an advantage that the workability is excellent.

When joining molybdenum to the circuit of the ceramic circuit board, the thickness of the metal of nickel, chromium and titanium may be any thickness as long as the joining force between the circuit and the molybdenum plate can be exhibited. According to 5-20μ
m is preferred. Further, if the thickness of the molybdenum plate is 100 to 500 μm, the molybdenum plate can be used without any problem such as generation of solder cracks.

[0034]

EXAMPLES [Examples 1 to 3] As ceramic substrates,
A silicon nitride substrate of 35 × 35 × 0.635 mm having a thermal conductivity of 75 W / mK by a laser flash method and an average of three-point bending strengths of 560 MPa was prepared. Also, as a metal plate for a circuit, a J of 32 × 32 × 0.4 mm is used.
An IS name 1085 aluminum plate was prepared.

The silicon nitride substrate has JIS on both front and back sides.
The aluminum plates are stacked on each other via a 2017 aluminum alloy foil (20 μm thick),
Pressurized at a. Then, in a vacuum of 10 ^-2 Pa, a temperature of 63
The aluminum plate and the silicon nitride plate were joined while heating under the conditions of 0 ° C. × 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 performing an etching treatment with a ferric chloride solution, thereby producing a ceramic circuit board.

Next, as a heat sink, 70 × 130
A 10 mm thick electroless Ni-P plating was applied to the entire surface of an oxygen-free copper plate having a size of 3 mm. Then, between the ceramic circuit board and the copper heat sink, a 20 μm-thick JIS 2017 aluminum alloy foil was placed, and furthermore, a 13 × 13 × 0.2 mm-thick molybdenum plate plated with electroless Ni—P was placed on the ceramics substrate. JIS 2017 aluminum alloy foil was sandwiched between the circuits on the circuit board (see FIG. 1), and pressed with a graphite jig. Here, as the graphite jig, a plate having a concave surface and a plate having a convex surface are paired, and the uneven surface has a span of 50 mm.
At 300 μm (Example 1), 200 μm (Example 2),
100 μm (Example 3) was used. Further, when joining while pressurizing with a graphite jig, the joining was carried out under a heating condition of 610 ° C. × 4 min under a load of 300 MPa. The warped state of the bottom surface of the obtained module was measured by the following method.

<Method of Measuring Warpage of Module Bottom> At the bottom of the module, a laser type depth gauge was used to move the sample stage from the center of the ceramic circuit board to the three-dimensional shape at 10 mm intervals within a width of 50 mm span by moving the sample stage. Is quantified. Then, each point is corrected with respect to the obtained numerical value so that the end of the span becomes the origin, and the numerical value at the position where the absolute value is maximum is defined as the amount of warpage.

Next, the circuit of the module and the surface of molybdenum are subjected to electroless Ni-P plating so as to be in a solderable state, and further, a heater is soldered to the circuit instead of a semiconductor element, and a radiating unit is provided on the bottom of the module. Was placed via silicone grease for heat radiation, and the cooling performance of the module was examined by measuring the temperature of each part with a thermocouple. The heater output during the test was set to 100
W, the temperature of the heat radiation unit was fixed at 60 ° C. and measured.
Table 1 shows the results.

[0039]

[Table 1]

[Comparative Example 1] A module was manufactured in the same manner as in Example 1 except that bonding was performed using a graphite jig having a flat surface, and no molybdenum plate was laminated.
The same evaluation as in Example 1 was performed. Table 1 shows the results.

From the comparison of Examples 1 to 3 and Comparative Example 1,
As the warp warps in the + direction, the temperature of the semiconductor element becomes lower and the object of the present invention can be achieved at 50 μm or more. However, if the warpage is too large, the soldering will be hindered. It is clear that there is. In addition, the module of the present invention, in the cooling characteristic evaluation,
The temperature of the semiconductor element is lower than that of the comparative example, and has sufficient performance as a power module.

[Embodiment 4] In the embodiment 1, the electroless N
Changed to a 13 × 13 × 0.1 mm molybdenum plate plated with i-P, and furthermore, a 13 × 13 m
A m-type Si chip was soldered to produce a module having a laminated structure shown in FIG. Here, the bonding of the electroless Ni-P plated molybdenum plate to the circuit on the ceramic circuit board was performed simultaneously with the bonding of the electroless Ni-P plated copper heat sink. After evaluating the warpage of the bottom surface of the obtained module by the method shown in Example 1, the module was subjected to a reliability test shown below. Table 2 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 the lapse of 0 times, the occurrence of solder cracks under the Si chip is examined by SAT (ultrasonic image flaw detector).

[0044]

[Table 2]

Comparative Example 2 A module having the laminated structure of FIG. 3 was obtained by performing the same operation as in Example 4 except that solder was used for joining the circuit of the ceramic circuit board and the molybdenum plate. The same evaluation as in Example 4 was performed. Table 2 shows the results.

Comparative Example 3 A module having the laminated structure shown in FIG. 4 was obtained in the same manner as in Example 4 except that no molybdenum plate was interposed, and the same evaluation as in Example 4 was performed. Table 2 shows the results.

As shown in Table 2, Comparative Examples 2 and 3 have less than 500 heat cycles and less than 1000 heat cycles, respectively.
A solder crack has occurred under the Si chip. Since the number of heat cycles required for a module for a hybrid car or a railway car is 3000 or more, the reliability is low and not practical. On the other hand, in Example 4, 3
It is clear that no solder cracking occurs even after 2,000 heat cycles, and the reliability is high.

[0048]

According to the module of the present invention, the conventional Al-SiC is used while using an inexpensive metal as a heat sink material.
It has the same reliability as a module using a heat sink made of a composite material such as a composite material, and is very useful in industry. Further, the method for manufacturing a module according to the present invention includes:
By simply using a jig such as graphite in the conventional process, a highly reliable and highly reliable power module for a power supply can be provided, which is extremely industrially useful.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a laminated structure of modules according to Examples 1 to 3 of the present invention.

FIG. 2 is a sectional view showing a laminated structure of a module according to a fourth embodiment of the present invention.

FIG. 3 is a cross-sectional view showing a laminated structure of a module according to Comparative Example 2.

FIG. 4 is a sectional view showing a laminated structure of a module according to Comparative Example 3.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Heat sink made of metal 2 Metal plate for heat dissipation (aluminum plate) 3 Ceramic substrate 4 Circuit 5 Molybdenum plate 6 Solder 7 Semiconductor element

Claims (4)

[Claims] 1. A module in which a ceramic circuit board is joined to a metal heat sink, wherein the back surface of the heat sink is warped convexly, and the amount of the warp is 20 μm or more and less than 120 μm in a measurement span of 50 mm. A module characterized by the above-mentioned. 2. The method according to claim 1, wherein a metal mainly composed of molybdenum is bonded on the circuit of the ceramic circuit board via a metal mainly composed of at least one of nickel, chromium and titanium. The module according to claim 1, wherein 3. A laminate is obtained by sequentially disposing a ceramic circuit board and molybdenum on one side of a metal heat sink.
Further, by using a mold having a pair of concave and convex surfaces having a predetermined radius of curvature in an arbitrary direction, under a state where the laminate is sandwiched between the concave and convex surfaces, pressure is applied, and integrated. Module manufacturing method. 4. The laminate according to claim 3, wherein a metal mainly composed of at least one of nickel, chromium and titanium is interposed between the ceramic circuit board and molybdenum. Module manufacturing method.