JP2002305274A - Circuit board and module - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a reliable circuit board that can reduce soldering cracks and cracks to a ceramics substrate, and to provide a module. SOLUTION: In the circuit board, a circuit 2 is joined to one surface of an aluminum nitride substrate or a silicon nitride substrate, a heat sink 3 is jointed to the other, the circuit and heat sink are made of Al or an Al alloy, hardness in the circuit is set to 250 MPa or lower, the hardness of the heat sink is set to 300 to 460 MPa, and the amount of warpage in the circuit board at 260 deg.C is set to ±100 μm or lower. The circuit board is soldered to a base copper plate 4 for composing a circuit board, having the base copper plate. The circuit boards are used for assembling the module.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit board used for a power module or the like and a module assembled using the circuit board.

[0002]

2. Description of the Related Art Conventionally, in assembling a power module or the like, a circuit board having a circuit formed on a surface of a ceramic substrate made of alumina, beryllia, silicon nitride, aluminum nitride or the like and a radiator plate formed on a back surface is used. Such a circuit board is characterized by having higher insulating properties than a resin board or a composite board of a resin board and a metal board.

The advantage that the material of the circuit and the heat sink is made of Al or Al alloy rather than Cu or Cu alloy is that Cu or Cu alloy generates thermal stress due to a difference in thermal expansion between the ceramic substrate and solder. Unavoidable, the long-term reliability is insufficient, whereas Al or Al alloy is
Thermal conductivity and electrical conductivity are slightly inferior to those of Cu or Cu alloy, but are easily plastically deformed even when subjected to thermal stress, so that stress is relieved and reliability is dramatically improved.

[0004]

However, the above-mentioned plastic deformation of Al or an Al alloy (hereinafter, also referred to as "Al etc.") is caused by the magnitude of thermal stress and the amount of Al subjected to thermal stress.
And so on. If the plastic deformation is concentrated on a part of the circuit or the heat radiating plate, the amount of plastic deformation of the solder is exceeded, and a solder crack occurs. In order to avoid this, it is conceivable to use high hardness Al or the like, but the stress relaxation effect is reduced.

[0005] Therefore, the demand today is the emergence of an extremely advanced technology that achieves the trade-off of suppressing solder cracks while ensuring a sufficient stress relaxation effect on the ceramic substrate. As one example, it has been proposed to use a base plate having a similar thermal expansion coefficient to that of a ceramic substrate, as represented by Al / SiC. However, since this base plate is more expensive than a general base copper plate, it is often used only for special applications, and another technical development has been awaited.

SUMMARY OF THE INVENTION An object of the present invention is to provide a highly reliable circuit board and a highly reliable circuit board which exhibit both a stress relaxing effect on a ceramic substrate and a solder crack suppressing effect without using an expensive base plate such as Al / SiC. Is to provide a module that uses.

An object of the present invention is to pursue Al characteristics suitable for a circuit and a radiator plate from among many Al-based materials, and to manufacture a circuit board having an appropriate hardness difference and a thickness difference between the circuit and the radiator plate. Preferably, this can be achieved by soldering the base copper plate with solder raised to the side surface of the heat sink.

[0008]

That is, the present invention is as follows. (Claim 1) An aluminum nitride substrate or a silicon nitride substrate having a circuit bonded to one surface and a radiator plate bonded to the other surface, wherein the circuit and the radiator plate are made of Al or an Al alloy. Hardness 250MPa or less, heat sink hardness 3
A circuit board wherein the amount of warpage of the circuit board when heated at 00 to 460 MPa and 260 ° C. is ± 100 μm or less. (Claim 2) The circuit board according to claim 1, wherein the heat radiating plate and the base copper plate are brought into contact with each other and soldered to the base copper plate, and the solder is raised to the side surface of the heat radiating plate. Characterized circuit board with copper base plate. (Claim 3) A module assembled using the circuit board according to claim 1 or the circuit board with a base copper plate according to claim 2.

[0009]

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

A feature of the present invention is to provide an appropriate hardness difference and thickness difference between a circuit made of Al or the like and a radiator plate to adjust plastic deformation of Al or the like when subjected to a heat history, and to provide a radiator plate and a base. That is, when soldering the copper plate in contact with the copper plate, the solder is raised to the side surface of the heat radiating plate, thereby suppressing both solder cracks and cracks on the ceramic substrate.

Conventionally, in order to improve the reliability of a circuit board in which the material of the circuit and the heat sink is made of Al or the like, a plating composition (Japanese Patent Laid-Open No.
-260187), surface modification of Al and the like (Japanese Unexamined Patent Publication No.
260186), particle size regulation of Al etc.
56330), but it was not a sufficient solution.

The first method implemented by the present invention is the definition of Vickers hardness of Al or the like. The heat sink, which has a large soldering area and the occurrence of solder cracks is a dominant factor for failure, has a slightly higher hardness, and has a Vickers hardness of 300 to 460 MPa.
For circuits where the occurrence of cracks in the ceramic substrate is dominant under the influence of the circuit pattern,
This means that Al or the like having a Vickers hardness of 260 MPa or less was selected.

[0013] Among Al and the like, those that are soft after joining are easily plastically deformed, so that the thermal stress generated when the circuit board receives a thermal history is relaxed, and the ceramic substrate is less likely to crack. However, if the amount of plastic deformation exceeds that of the solder, the solder is broken, and solder cracks are likely to occur. Conversely, hard Al or the like easily causes cracks on the ceramic substrate, but suppresses solder cracks.

The heat radiating plate surface of the circuit board for the power module is a soldering surface, and is soldered to the entire surface to be fixed to the base plate. Dominates sex. On the other hand, since the circuit surface is only for attaching an IC chip or a diode, the soldering area is relatively small.
/ 3. Since the thermal stress on the ceramic substrate is concentrated at the end of the circuit pattern, the pattern shape cannot be ignored. So-called "solid pattern" without pattern
Although there is not much problem with the heat radiation surface, on the circuit surface, "horizontal cracks" which occur at the ends along the pattern and propagate into the pattern often dominate the heat radiation.

In the circuit board of the present invention, from these points,
The Vickers hardness of the circuit is set to 250 MPa or less to suppress cracks generated on the ceramic substrate, and the Vickers hardness of the heat sink is set to 300 to 460 MPa to suppress the generation of solder cracks.

If the Vickers hardness of the circuit exceeds 250 MPa, plastic deformation due to thermal stress becomes non-uniform, partial deformation increases, and there is a possibility that plating or peeling of bonding wires may occur. The hardness is preferably as small as possible, but is preferably 1 because it is easily damaged when it is too soft.
80 to 220 MPa. On the other hand, if the Vickers hardness of the heat sink is more than 460 MPa, plastic deformation becomes difficult, and cracks easily occur in both the ceramic substrate and the solder. Further, if the pressure is less than 300 MPa, plastic deformation becomes easy, so that when subjected to repeated heat history, large deformation occurs, and solder cracks are liable to occur. Particularly preferred Vickers hardness of the heat sink is 350 to 430 MPa.
It is.

The Vickers hardness is a method of reading the hardness by driving a fine indenter under a load, and is widely used as a method of measuring the hardness of metals and ceramics. In the present invention, the measurement is performed under the conditions of a weight of 9.8 N and a holding time of 15 seconds.

The hardness in the present invention is the hardness of the circuit and the heat sink, and is different from the hardness of Al or the like before joining. Since Al or the like is usually bonded to a ceramic substrate by heating at 500 to 640 ° C. using a bonding material, the microstructure changes due to heat treatment, and the bonding material diffuses to lower the purity of the Al and the like. . Further, heat treatment is also performed after the bonding, whereby the characteristics such as Al change. For these reasons, it is meaningless to strictly define the hardness of Al or the like before joining.

As a method for adjusting the hardness of Al or the like, there are a method of selecting a material and a method of adjusting the hardness under heat treatment conditions. As for the material, Al and the like after bonding become softer as the purity becomes higher.
What is necessary is just to select the purity. Usually, those commercially available as pure Al have a considerable range of 99.0 to 99.999%, and there is a great difference in hardness. That is, 99.5%
For the following Al, the Vickers hardness after joining is 250 MPa
It is difficult to reduce the content below, and it is difficult to set the content to 300 MPa or more after joining with Al of 99.9% or more. Therefore, in the present invention, it is possible to use an Al alloy for the circuit, but it is preferable to use a high-purity Al material and a relatively low-purity Al or the like for the heat sink. As an example of an Al alloy, Al-Mn starting with 3003 in AA symbol
Alloy or an Al-Mg alloy such as 5052.

A second technique implemented by the present invention is to provide a thickness difference between the circuit and the heat sink, and to reduce the warpage of the circuit board when heated to 260 ° C. to ± 100 μm or less. The “+” of the warp is a warp in which the circuit surface is concave, and the “−” of the warp is a warp in which the circuit surface is convex. Preferably, the warpage is 0 to +50 μm.

In the present invention, the thickness of the circuit is not particularly limited, but the thickness generally used is 0.3.
０．５0.5 mm. In the present invention, the amount of warpage is usually adjusted by adjusting the thickness of the heat sink in accordance with the thickness and the pattern ratio of the circuit board.

If there is no thickness difference between the circuit and the heat radiating plate, or if there is an extremely large thickness difference, the generated thermal stress will be biased, causing warpage and undulation, causing solder voids, damage such as solder cracks and the like. In addition, peeling of the bonding wire and plating cannot be sufficiently prevented. When the hardness of the heat sink is greater than the hardness of the circuit board, the thickness of the heat sink is reduced. When the hardness of the heat sink is smaller than the hardness of the circuit board, the thickness of the heat sink is increased.

As for the material of the ceramic substrate used in the present invention, a silicon nitride substrate or an aluminum nitride substrate having a thermal conductivity of 70 W / mK or more is selected in consideration of use as a circuit board for a power module. In particular, an aluminum nitride substrate is desirable.

A method for manufacturing a circuit board according to the present invention will be described.

In order to form a circuit and a heat radiating plate on a ceramic substrate, there is a method of joining those patterns, etching after joining a plate of Al or the like, or using both of them. In any case, it is necessary to join the ceramic substrate with the circuit and the heat sink. There is a joining method that does not use a joining material like the molten metal method,
In the present invention, the joining is preferably performed using an Al-Cu-Mg-based alloy foil. The joining material is not limited to this,
An Al-Si system, an Al-Ge system, or a system in which Mg is added thereto can also be used.

The thickness of the joining material depends on the type, but is 10 to 5
0 μm is common. If the thickness is less than 10 μm, joining becomes difficult. If the thickness is more than 50 μm, the alloy component diffuses into Al or the like and hard portions increase, which causes a decrease in reliability when subjected to thermal history. The preferred thickness of the bonding material is 15 to 35 μm. The bonding material may be disposed on either the ceramic substrate side or the Al side.

In the present invention, a plate, a pattern or both of Al or the like is disposed on both sides of the ceramic substrate via the above-mentioned alloy foil, and is placed in a direction perpendicular to the ceramic substrate by one to one.
It is preferable to apply a pressure of 10 MPa, especially 4 to 8 MPa. Pressing can be performed by placing a weight on the laminate, mechanically sandwiching it with a jig or the like, or the like.

The bonding temperature between the ceramic substrate, the circuit and the heat sink is 580 to 645 ° C., and the bonding is performed in a nitrogen atmosphere or in a vacuum.

Next, the joined body is etched as needed. Etching is not particularly necessary when the circuit or heat sink pattern is joined. The etching can be performed by a usual resist and etching process. Further, surface treatment such as plating is also performed as needed.

The circuit board of the present invention is preferably used by being soldered 5 to a base copper plate 4 as shown in FIG. In this case, it is important to suppress solder cracks between the base copper plate 4 and the heat sink 3. In order to suppress solder cracks, it is necessary to reduce the stress (strain) applied to the solder. The stress applied to the solder is large at the interface between the heat sink and the solder, and is particularly maximized at the corners of the heat sink.

In the present invention, in order to reduce the stress applied to the solder, the hardness and thickness of the heat sink are optimized as described above, and the thickness of the solder at the corner of the heat sink at which the maximum stress occurs is increased. . That is, when the heat radiating plate of the circuit board and the base copper plate are brought into contact with each other and soldered to the base copper plate, the solder is raised to the side surface of the heat radiating plate.

In order to raise the solder to the side of the heat sink, the solder may be slightly wetted on the side of the heat sink before soldering the base copper plate and the circuit board, or the solder may be placed on the circuit board at the time of soldering. It is necessary to devise measures such as placing weights.
Moreover, in order to suppress solder cracks, the thickness of the solder, the height at which it is raised, and the position are important.

The thickness (T) of the solder is 100 to 300 μm
Preferably, there is. If the thickness of the solder is less than 100 μm, when stress is applied to the solder, the plastic deformation cannot be sufficiently performed, so that the stress cannot be reduced and solder cracks are easily generated. Conversely, if the thickness of the solder is more than 300 μm, plastic deformation can be sufficiently performed. However, since the thermal conductivity of the solder is small, when the thickness is large, the thermal resistance increases and the function as a module is impaired. More preferably, it is 150 to 250 μm.

The swelling state of the solder on the side of the heat sink is as follows.
In the explanatory view shown in FIG. 2, it can be defined by the skirt length (W) of the solder and the height (H) of the swelling of the solder.

The skirt length (W) of the solder is 200 μm to 2 m
m is preferable. If it is less than 200 μm, a sufficient effect cannot be obtained to reduce the stress generated in the solder at the corners of the heat sink. Conversely, if the distance exceeds 2 mm, adjacent circuit boards may interfere with each other, so that a problem may occur during module assembly.

It is preferable that the height (H) of the raised solder is 50 μm or more. If it is less than 50 μm, a sufficient effect cannot be obtained to reduce the stress generated in the solder at the corners of the heat sink. The thickness is more preferably 100 μm or more, and still more preferably the thickness of the heat sink (until it comes into contact with the ceramic substrate).

[0037]

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

Examples 1 to 8 Comparative Examples 1 to 5 The aluminum nitride substrate and silicon nitride substrate used were commercially available products, each having a size of 2 inches square and having a thermal conductivity of 175 W / m2 of aluminum nitride by a laser flash method.
K, silicon nitride 72 W / mK, and three-point bending strength are 420 MPa for aluminum nitride and 780 MPa for silicon nitride. As the Al plates, those having respective thicknesses shown in Table 1 were used for the circuit formation and the heat radiation plate.

An Al plate and a bonding material shown in Table 1 were superimposed on the front and back surfaces of the ceramic substrate, and a carbon plate was screwed and pressed against the substrate using a jig, and pressure was uniformly applied vertically to the ceramic substrate. The bonding was performed while applying pressure at a temperature of 550 to 635 ° C. in a vacuum or nitrogen atmosphere.

After joining, the etching resist was screen printed and etched with a FeCl ₃ solution. The pattern of the circuit surface and the heat radiating surface was square (corner R was 2 mm) and formed at the center of the ceramic substrate (creeping distance 1 mm). Next, after the resist was peeled off, electroless Ni-P plating was performed at 5 μm to form a circuit board, and the following physical properties were measured.

(1) The amount of warpage of the circuit board: 26 of the circuit board
The amount of warpage during heating at 0 ° C. was measured on a hot plate heated to 260 ° C. with the circuit surface of the circuit board facing up, left for 5 minutes, and measured with a laser displacement meter. In the measurement of the amount of warpage, the case where the circuit surface is concave is defined as (+ side), and the case where the circuit surface is convex is defined as (− side).

(2) Vickers hardness of circuit and heat sink: A cross section of the circuit board was cut out, and Vickers hardness was measured at the center of each of the circuit and the heat sink in the thickness direction.

Thereafter, a 13 mm square Si chip was soldered to the center of the circuit board. The solder used was a Sn-Pb (Sn / Pb = 10/90) 100 μm thick solder.

Further, the circuit board on which the chip was soldered was heated on a hot plate at 260 ° C., and solder (Sn / Pb = 50/50) was pressed against the side surface of the heat sink to confirm that the solder was wet. Thereafter, the solder was once wiped off. When soldering the circuit board with the chip to the base copper plate, the base copper plate was previously masked to the same size as the ceramic substrate with a heat-resistant tape in order to set the foot length (W) of the solder to 1 mm. Was used. Solder is Sn-P
b (Sn / Pb = 50/50), and the thickness (T) of the solder is set to 100 to 300 μm.
μm was used. Further, a carbon jig for positioning the circuit board was used in order to make the skirt length (W) of the solder uniform. Furthermore, the thickness (T) of the solder is made uniform,
In order to make the solder height (H) 50 μm or more,
A weight was placed on the center of the circuit board.

In the measurement of the thickness (T) of the solder on the base copper plate, the hem length (W) of the solder, and the height (H) of the solder swelling, the four corners of the circuit board soldered to the base copper plate were used. The cross section was observed (SEM observation) and measured.

A heat cycle test was performed on the circuit board soldered to the base copper plate. Heat cycle test is -4
1000 cycles were performed with one cycle of 0 ° C. × 30 minutes → room temperature × 10 minutes → 125 ° C. × 30 minutes → room temperature × 10 minutes. After the heat cycle test, the appearance of the substrate was checked for damage such as solder cracks and pattern peeling, and the progress of the solder cracks (L) shown in FIG. 3 was examined by an ultrasonic inspection image (SAT). Regarding the solder crack under the chip, a solder crack exceeding 1 mm could be detected as "defective". Further, as for the solder cracks on the base copper plate, those in which a solder crack exceeding 3 mm was detected were regarded as "defective". Thereafter, the circuit and the heat sink were dissolved with hydrochloric acid, and the presence or absence of cracks generated on the ceramic substrate was observed. Table 1 shows the results.

[0047]

[Table 1]

As is clear from Table 1, in each of the examples of the present invention, even after 1000 cycles of the heat cycle test, the occurrence of solder cracks and cracks in the ceramic substrate was extremely small. On the other hand, in the comparative example,
Since the Vickers hardness was out of the range of the present invention, many cracks occurred after the heat cycle, which was insufficient for a highly reliable circuit board.

[0049]

According to the present invention, it is possible to provide a highly reliable circuit board and a module using the same, which can significantly reduce the occurrence of solder cracks and cracks in the ceramic substrate.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view of a circuit board with a base copper plate of the present invention.

FIG. 2 is an explanatory diagram for measuring a thickness (T) of a solder, a skirt length (W) of a solder, and a height (H) of a swelling of the solder.

FIG. 3 is an explanatory diagram for measuring a solder crack length (L).

[Description of Signs] 1 Ceramic substrate 2 Circuit 3 Heat sink 4 Base copper plate 5 Solder

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5E338 AA01 AA18 BB71 CC01 CD11 EE02 EE28 5F036 AA01 BA23 BB21 BC06 BD03

Claims (3)

[Claims] 1. A circuit in which a circuit is bonded to one surface of an aluminum nitride substrate or a silicon nitride substrate and a radiator plate is bonded to the other surface, wherein the circuit and the radiator plate are made of Al or an Al alloy. A circuit board having a hardness of 250 MPa or less, a heat sink hardness of 300 to 460 MPa, and an amount of warpage of the circuit board of ± 100 μm or less when heated at 260 ° C. 2. The circuit board according to claim 1, wherein the heat radiating plate and the base copper plate are brought into contact with each other and soldered to the base copper plate, and the solder is raised up to the side surface of the heat radiating plate. Characterized circuit board with copper base plate. 3. The circuit board according to claim 1, or claim 2.
A module assembled using the circuit board with a base copper plate as described.