JPH10190176A - Circuit board - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a circuit board high in reliability by a method in which crack generated readily in a ceramic substrate due to residue stress attributed to a heat cycle, etc., are reduced when the circuit board is used for applications generating large heat for a power module, etc. SOLUTION: In a circuit board in which a metal circuit a is formed on one face of a ceramic board and a metal heat-radiating plate e is formed on the reverse face via junction layers, b, d, control is performed by projecting the junction layer 0.1 to 1.0 times the thickness of a metal member from an end part of the metal member, thereby a circuit board excellent in heat-cycle- resistance is obtained. Further, if a contact angle made by the junction layer and the board is set 90 deg. or less, preferably 45 deg. or less, further, is effective for reducing a residual stress and this affects cracks prevention.

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 of an electronic component requiring high reliability and heat radiation.

[0002]

2. Description of the Related Art Conventionally, alumina (Al2O3), aluminum nitride (Al
N), a circuit board in which a copper (Cu) circuit board or the like is integrally bonded as a conductive layer to the surface of a ceramic sintered body substrate such as beryllium oxide (BeO) is widely used.

[0003] In these circuit boards, since the circuit board is formed of a metal such as Cu having excellent thermal conductivity and electric conductivity, the delay of circuit operation is reduced and the life of circuit wiring is improved. Improves wettability to bonding materials such as solder and enables semiconductor elements (IC chips) and electrode plates to be bonded to the surface of the ceramic sintered body with high bonding strength.
As a result, the heat dissipation from the semiconductor element and the operation reliability of the element can be kept good. Further, by joining a metal plate such as Cu to the heat radiation surface of the ceramic substrate, the stress relaxation of the ceramic substrate can be reduced. It has the advantage that the object of preventing thermal deformation can also be achieved.

[0004] In the process of mounting on a power module, a crack occurs in the ceramic substrate between the patterns of the metal circuit and the ceramic substrate at the end of the metal circuit during the mounting process on the power module. There is a problem that the power module becomes inoperable. It is considered that the cause of cracks in the ceramic substrate is largely due to the residual stress caused by the difference in thermal expansion between the ceramic and the brazing material and between the circuit board and the metal plate for heat dissipation.

The following method is known as a method for forming a circuit on a circuit board. (1) A mounting method in which a brazing material paste is printed on a ceramic substrate in a circuit pattern, and a metal circuit board prepared in advance so as to cover at least the circuit pattern is bonded.

(2) A brazing material paste is applied to the entire surface of the ceramic substrate, a metal plate is bonded to the entire surface so as to cover the paste, and a circuit pattern is formed on the metal plate serving as a circuit surface by etching resist. A full-etch method in which unnecessary circuits and bonding materials are partially removed by etching.

(3) A pattern is printed on a ceramic substrate with a brazing material paste in a desired circuit shape, a metal plate is joined so as to cover the entire surface thereof, and a circuit pattern is formed on the joined metal plate with an etching resist. After that, a pattern printing method in which a metal circuit is formed by etching.

However, the above method has advantages and disadvantages. That is, in the method (1), although the residual stress applied to the ceramic substrate is small, it is difficult to control the positional relationship between the brazing material and the metal plate with high accuracy because the metal circuit board prepared in advance is joined. In addition, there is a problem that productivity is low and it is not suitable for mass production. Although the method (2) has good productivity, a large stress remains on the ceramic substrate after the etching because of an unnecessary circuit and a brazing material removing step. The end of the brazing material is inside or just below the end of the metal plate. The pattern printing method of (3) does not give residual stress to the ceramic substrate as much as the full etching method of (2). However, there is still a problem that the ceramics are liable to crack when the substrate is large.

In the case where aluminum nitride is used for the ceramic substrate, the aluminum nitride circuit board has sufficient durability against damage caused by heat shock and heat history such as heat shock and heat cycle.
There has been reported an example in which the thickness of a bonding layer interposed between a copper circuit and an aluminum nitride substrate is increased to, for example, 20 μm or more. However, when the thickness of the bonding layer is increased, there is still a problem to be solved such that it becomes difficult to remove unnecessary brazing material.

[0010]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and reduces the occurrence of cracks in a ceramic substrate without deteriorating the heat dissipation and dielectric strength of a circuit board, and achieves high reliability. An object of the present invention is to provide a power module circuit board.

[0011]

Means for Solving the Problems In order to achieve the above object, the present inventors reviewed the positional relationship between the brazing material of a circuit board using a ceramic substrate and a copper plate, and found that the brazing material was at least as large as the edge of the copper plate. The inventors have found that cracks are greatly reduced by joining them so as to protrude outside, and have reached the present invention.

That is, the present invention relates to a circuit board comprising a ceramic substrate having a metal circuit formed on one surface thereof and a metal radiating plate formed on the other surface thereof via a bonding layer, wherein the bonding layer is formed of metal from an end of the metal member. The circuit board protrudes over a length of 0.1 to 1.0 times the thickness of the member.

Further, the present invention is the above circuit board, wherein the angle between the bonding layer and the ceramic substrate is 90 degrees or less.

Further, the present invention is the above-mentioned circuit substrate using a ceramic substrate with an aluminum nitride sintered body or a silicon nitride sintered body.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The material of the ceramic substrate used in the present invention is not particularly limited. However, in the case of aluminum nitride or silicon nitride having a large thermal expansion difference from a metal, the effect of reducing cracks is particularly large. A ceramic substrate having a thermal conductivity of 80 W / mK or more is suitable for exhibiting good heat dissipation. Also, regarding the bending strength of the ceramic substrate, 350 MPa
a or more is preferable.

The thickness of the ceramic substrate varies depending on the required strength of the circuit board.
m to 1.5 mm are used.

In the present invention, a metal circuit or a metal circuit formed on a ceramic substrate and a metal heat radiating plate will be described. The material used is a pure metal or alloy such as copper, nickel, aluminum, molybdenum, and tungsten. The thickness of the metal circuit or metal radiator plate is preferably 0.1 to 2.0 mm.

What is important in the present invention is that when the thickness of the metal circuit is T and the width of the protrusion of the bonding layer is w in the schematic cross section of the circuit board shown in FIG.
1.0. Further, even if w / T is larger than 1.0, the effect of reducing the residual stress cannot be obtained. As shown in FIG. 2, when the contact angle between the bonding layer and the substrate is set to 90 degrees or less, preferably 45 degrees or less, the residual stress is further reduced, which is effective in preventing cracks.

The protrusion of the brazing material of the present invention does not need to be completely achieved around the entire periphery of the metal member, but it is sufficient if a part of the protrusion protrudes. That is, residual stress is likely to remain at the corners of the circuit, particularly at the corners of the convex shape, around the electrode mounting portion of the circuit, and at the circuit forming portion, and it is sufficient if the residual stress is achieved at that portion. If the length of the protruding portion is too small, the effect cannot of course be expected. It is preferable that 20% or more of the entire circumference of the metal circuit forms the protruding portion of the bonding layer. More preferably 40
% Or more. Further, if the distance between the patterns is small, there is a risk of causing interference between circuits. Therefore, the pattern interval is preferably at least twice the thickness of the metal circuit.

The bonding layer according to the present invention is formed using a brazing material paste. The brazing material paste used here is, for example, the material of the metal circuit or the metal radiator plate is Cu.
Is Ag or Cu or a brazing material containing Ag and Cu. When the substrate is made of aluminum nitride or silicon nitride, an active metal component made of Ti, Zr, or the like can be appropriately added to the brazing material.

The method of manufacturing a circuit board according to the present invention is the preferred method because the pattern printing method (3) is easy to control the amount of protrusion of the brazing material. However, the present invention relates to a full etching method, a mounting method, or a method. It is not limited to other manufacturing methods.

A method for manufacturing a case where the material of the metal circuit and the metal radiator plate is Cu will be described with reference to an example. First,
The solder paste is printed on the ceramic substrate larger than the intended copper circuit shape by the margin. The protruding margin is performed according to the claims.

In order to achieve the present invention, it is necessary to print with high accuracy. In order to improve the positioning accuracy of the reference ceramic substrate, it is preferable that the edge of the substrate is ground and finished with diamond, and the straightness and the squareness are processed to a high precision of 1/100 or less before use. Thereafter, a solid Cu plate is placed in contact so as to cover the entire surface. The aluminum nitride substrate on which the solid Cu plate is disposed is heat-treated to obtain a joined body of the two. A target circuit pattern is screen-printed on the Cu plate of the joined body manufactured in this manner with high precision using an etching resist to form a resist circuit pattern.

Next, after etching to remove unnecessary Cu outside the pattern, the etching resist film is removed to obtain an aluminum nitride circuit board having a Cu circuit. Thereafter, in order to prevent oxidation and corrosion of the Cu circuit, a protective film is selectively formed on the Cu circuit by Ni plating or the like as necessary.

[0025]

[Example] Length 60 mm x width 50 mm x thickness 0.635
mm of aluminum nitride sintered body, a brazing material is printed on one side by a screen printing method in a circuit pattern + protruding amount, a brazing material is similarly applied on the other side, and C is applied on both sides.
A u plate (thickness: Cu plate for metal circuit: 0.3 mm, Cu plate for metal heat dissipation: 0.15 mm) was placed in contact with the plate, and heat-treated at 830 ° C. for 30 minutes in vacuum to obtain a joined body of the aluminum nitride substrate and the Cu plate. .

An ultraviolet-curing etching resist is printed on the Cu plate of the joined body by printing a circuit pattern by a screen printing method so that the amount of protrusion of the joining layer becomes a predetermined size, and is cured. Unnecessary Cu outside the pattern was removed with a solution. Next, the resist was removed. Further, a Ni protective film was selectively formed on the Cu circuit by electroless Ni plating. Thus, Examples 1 to 1 of Table 1
4. The aluminum nitride circuit boards shown in Comparative Examples 1 and 2 were completed. Moreover, about Example 5, it carried out similarly to the above except having used the silicon nitride sintered compact for the ceramics board | substrate,
A circuit board was manufactured.

A heat cycle test was performed on these circuit boards. The heat cycle test was performed at -40 ° C for 3 hours.
The heating / cooling operation in which the temperature was held for 0 minute and the temperature was held at + 125 ° C. for 30 minutes was defined as one cycle, and the test was performed according to the temperature change test method shown in JIS-C-0025. 10 times, 3
Table 1 shows the results of evaluating the occurrence of cracks after 0, 50, and 100 heat cycles. Cracks are indicated by incidence. The evaluation of cracks was performed by observing the presence or absence of cracks between circuits by a fluorescence inspection after completion of the heat cycle.

As is clear from the results shown in Table 1, the circuit boards of Comparative Examples 1 and 2 show cracks in the heat cycle test 30 times. It is clear that no cracks occurred 30 times and high reliability was obtained.

The circuit board of Example 1 had cracks after 50 heat cycle tests, whereas the circuit board of Example 4 had no cracks after 50 heat cycles, indicating that the bonding layer and the ceramic substrate were not cracked. It is clear that the effect is higher when the contact angle with the substrate is 45 degrees or less. Further, in Example 5, no crack was generated even after 100 heat cycle tests, and the superiority of the present invention is apparent.

[0030]

[Table 1]

[0031]

According to the present invention, a circuit board used for a power module or the like having higher reliability and heat dissipation can be provided by extending a bonding layer from a metal member without changing a circuit pattern. be able to.

[Brief description of the drawings]

FIG. 1 schematically shows a cross section of a circuit board according to the present invention.

FIG. 2 Contact angle between bonding layer and ceramic substrate

FIG. 3 is a circuit pattern used in Examples and Comparative Examples.

[Explanation of symbols]

 a: metal circuit b: metal circuit bonding layer c: ceramic substrate d: metal heat sink bonding layer e: metal heat sink T: thickness of metal circuit w: protrusion width of metal circuit bonding layer θ1, θ2, θ3: bonding layer Contact angle with ceramic substrate

Claims (3)

[Claims] 1. A circuit board comprising a ceramic substrate having a metal circuit on one side and a metal radiating plate on the opposite side via a bonding layer, wherein the bonding layer has a thickness of 0.1 mm from the end of the metal member to the metal member. A circuit board having an extension of 1 to 1.0 times. 2. The angle between the bonding layer and the ceramic substrate is 90.
The circuit board according to claim 1, wherein the temperature is equal to or less than a degree. 3. The circuit board according to claim 1, wherein a sintered body of aluminum nitride or sintered body of silicon nitride is used as the ceramic substrate.