JPH10247763A - Circuit board and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable a circuit board to be protected against cracking and improved in reliability by a method wherein a metal circuit or a circuit metal plate and a metal heat sink are bonded to both the sides of a ceramic board keeping the ceramic board which is intrinsically free from warpage deflected in one direction, and moreover the metal circuit or the circuit metal plate is so warped as to be concave. SOLUTION: A metal circuit or a circuit metal plate 2 and a metal heat sink 3 are bonded to both the sides of a ceramic board keeping the ceramic board which is intrinsically free from warpage deflected in one direction, whereby a circuit board warped in a prescribed direction can be obtained. When a module is formed by the use of the warped circuit board, the module is generally joined flat to a heat sink or used as fixed to a stationary part, so that cracks occur less in the circuit board if a compressive stress is previously induced in the required surface of the circuit board, and the circuit board can be improved in reliability.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit board having a metal circuit provided on a ceramic substrate used for a power module of an electronic component requiring high reliability and heat radiation, and a method of manufacturing the circuit board.

[0002]

2. Description of the Related Art Conventionally, alumina (Al ₂ O ₃ ), aluminum nitride (Al
N), beryllium oxide (BeO), and other ceramic sintered body substrates have been widely used as circuit boards in which a circuit metal plate such as copper (Cu) is integrally joined as a conductive layer.

[0003] In these circuit boards, the circuit board is formed of a metal such as copper which is excellent in heat conductivity and electric conductivity. Therefore, there is an advantage that a delay in circuit operation is small and a life of circuit wiring is long. . In addition, the semiconductor element (IC chip) and the electrode plate can be bonded to the surface of the ceramic sintered body with high bonding strength because of good wettability to a bonding material such as solder. As a result, heat generated from the semiconductor element can be sufficiently dissipated, and the operation reliability of the element can be kept good. Further, by joining a heat-dissipating metal plate such as copper to the ceramic substrate on the side opposite to the circuit surface, not only promotes heat dissipation to a heat sink joined through the heat-dissipating metal plate, but also This has the advantage that the objects of stress relaxation of the substrate and prevention of thermal deformation can also be achieved.

[0004]

With respect to a circuit board using a ceramic substrate, thermal stress during mounting to a module, such as bonding the circuit board to a heat sink or mounting and bonding electrodes, etc., and mounting the module to various devices. The micro-cracks may be generated on the ceramics substrate due to the load applied at the same time. In particular, the microcracks tend to occur between metal circuits or near the ends of the metal circuits. The minute cracks are enlarged by stress due to repeated heating and cooling applied to the module during use. If the cracks grow into relatively large cracks, insulation failure occurs at the cracks, and the power module becomes unusable.

Conventionally, several methods have been known as a method for manufacturing a circuit board. In order to obtain good productivity, a full-etch method is often used. In the full-etch method, a ceramic substrate and a metal plate are joined by a method such as using a brazing material paste or a eutectic of the substrate and the metal, and a circuit pattern is formed on the metal plate to be a metal circuit by an etching resist. This is a method of removing unnecessary portions by etching. Although the full-etch method has good productivity, it undergoes an unnecessary metal circuit and a step of removing a brazing material and a eutectic. Cracks are easy to occur.

A mounting method is known as another method of manufacturing a circuit board. In the mounting method, a brazing material paste is printed in a circuit pattern on a ceramic substrate, and a metal plate previously formed on the circuit pattern is soldered. The method of arranging and joining on the material paste does not go through an etching process, so that the residual stress is relatively small. However, there are problems such as that the amount of brazing material applied by the screen printing method is limited, that advanced printing technology is required, and that productivity is reduced due to complicated processes.

[0007] Regarding the circuit boards manufactured by any of the above manufacturing methods, in the step of mounting on the power module, the semiconductor chips and electrodes are joined to the circuit board, and the circuit board is joined to the heat sink copper plate. Further, since the tensile stress in the ceramic substrate remaining between the metal circuits after etching and in the vicinity of the end of the metal circuit is further increased, there is a problem that cracks are easily generated. Furthermore, even when the power module is mounted on the device, the fixing bolt is passed through the hole previously drilled in the heat sink, and the load when screwing the device is used.
In order to further increase the large tensile stress remaining on the ceramic substrate, there is a problem that cracks are generated even at this point.

[0008] In order to solve the above problems, a circuit board using an aluminum nitride sintered body is made of copper in order to have sufficient durability against damage caused by heat shock and heat history such as heat shock and heat cycle. A method has been proposed in which the thickness of a bonding layer interposed between a circuit and an aluminum nitride substrate is increased to, for example, 20 μm or more (Japanese Patent Laid-Open No. 6-162828). However, when the thickness of the joining layer is increased, there is still a problem to be solved such that it becomes difficult to remove unnecessary brazing material.

The present invention has been made in view of the above circumstances, and it is possible to easily obtain a highly reliable module by reducing the occurrence of cracks without impairing the heat dissipation and insulation withstand voltage of a circuit board. It is an object to provide a circuit board and to provide the circuit board with high productivity.

[0010]

According to the present invention, a metal substrate or a metal plate for a circuit and a metal radiating plate are joined while bending a ceramic substrate having essentially no warp in at least one direction. A circuit board characterized in that a circuit or a metal plate side for a circuit is warped to a concave surface, wherein the amount of warpage is 1/4000 or more and 3/100 or less with respect to the length in the direction of warping. The circuit board described above.

Further, the present invention relates to a circuit board characterized in that the coefficient of thermal expansion of the ceramic substrate is 5 × 10 ⁻⁶ ° C.- ¹ or less, and preferably, the ceramic substrate is made of silicon nitride. Yes, or the ceramic substrate is made of aluminum nitride, and the amount of warpage after joining a metal circuit or a metal plate for a circuit and a metal radiator plate is 1/250 or more to 3 / L of the length in the warped direction. The circuit board as described above, wherein the number is 100 or less.

Further, the present invention provides a ceramic substrate having essentially no warp in at least one direction having a length of one direction.
A method for manufacturing a circuit board, comprising joining a metal circuit or a metal plate for circuit and a metal plate for heat dissipation while bending the metal plate between / 4000 and 3/100 inclusive, preferably in the manufacturing method, A metal circuit or a metal plate for a circuit on the concave side of the ceramic substrate, and a metal plate for heat radiation on the convex side. Further, the present invention is the above-mentioned method for manufacturing a circuit board, wherein a metal circuit is formed from a circuit metal plate bonded to a ceramic substrate.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 1 is an explanatory diagram of a circuit board of the present invention and a method of manufacturing the same. (A) is a cross-sectional view of a ceramic substrate to be used, (b) is a cross-sectional view of a bent ceramic substrate immediately before joining a metal circuit or a metal plate for a circuit and a metal plate for heat radiation, and (c) is a concave side. FIG. 4 is a cross-sectional view of a warped circuit board having a metal circuit and having a metal plate for heat dissipation on a convex surface side.

In the present invention, an essentially warp-free ceramic substrate is bent while being bent in at least one direction.
It is essential to obtain a circuit board that is warped in a desired direction by joining metal plates having different thicknesses and shapes to both surfaces of the ceramic substrate. When a module is formed using a circuit board having such a warp,
Since a module is generally used by bonding a heat sink so as to be planar and fixing it to a fixed component, if a compressive stress is generated on a desired surface of the circuit board, it is possible to assemble the module at the time of assembling or actualizing the module. This is based on the finding that the generation and growth of cracks during use can be reduced, and as a result, a highly reliable module can be obtained.

As described above, the circuit board of the present invention comprises:
When the circuit board is used in a flat state, it is formed so that a compressive stress remains on a desired surface of the circuit board.
For the purpose of suppressing generation and growth of micro cracks, warping so that the metal circuit or circuit metal plate is arranged on a concave surface so that compressive stress remains on a certain surface of the metal circuit or circuit metal plate. Is essential. Therefore, it has a structure in which the metal plate for heat radiation is arranged on the convex surface.

With respect to the amount of warpage, the magnitude of thermal fluctuations and impacts received during assembly into a module or under the use conditions of the module, or the size of a ceramic substrate, a metal circuit, a metal plate for heat dissipation, and a bonding material, It may be appropriately selected according to the shape, material characteristics, etc. According to the study of the present inventors, the warpage amount is 1/400 of the length in the warped direction.
It is preferably 0 or more and 3/100 or less. 1/40
If it is less than 00, the effect of the present invention cannot be sufficiently obtained in many cases, and if it exceeds 3/100, the ceramic substrate may be broken at the time of module production. In the case where the pattern cut portion of the metal circuit is continuous, when the circuit board is warped in a direction perpendicular thereto, the effect of the present invention is greatly exerted.

The ceramic substrate used in the present invention has a coefficient of thermal expansion smaller than the coefficient of thermal expansion of a metal plate for a circuit or a metal plate for heat radiation, which will be described later. Any of them can be used. Examples of such a ceramic substrate include alumina, mullite, silicon nitride, aluminum nitride, and beryllium oxide. Of these,
When a ceramic circuit board having a coefficient of thermal expansion of 5 × 10 ⁻⁶ ° C. ^-1 or less is selected, a large difference in coefficient of thermal expansion from the metal plate is large, and a large compressive stress can be expected even if the circuit board is slightly warped. As such a ceramic substrate, a non-oxidized ceramic substrate such as silicon nitride and aluminum nitride can be used.

The ceramic substrate further includes:
In order to improve the heat dissipation of the obtained circuit board, those having a thermal conductivity of 60 W / mK or more are suitable. Further, the bending strength of the ceramic substrate is preferably 350 MPa or more because it affects the strength of the circuit board.

As described above, the preferred coefficient of thermal expansion, thermal conductivity,
Ceramic substrates that fall into the range of bending strength include:
Silicon nitride and aluminum nitride are given. In particular, in the case of aluminum nitride, the reason is unknown, but the amount of warpage is 1/250 or more and 1/3 of the length in the warped direction.
When it is less than 00, a module having high thermal conductivity and high reliability can be obtained, which is more preferable.

Regarding the thickness of the ceramic substrate,
Although it depends on the required heat radiation characteristics, mechanical properties, electrical characteristics, etc. of the circuit board, it is usually 0.3 mm or more and 1.5 mm or less. In the present invention, those having a normal thickness are used, but the present invention is not limited thereto.

The metal circuit, the metal plate for the circuit and the metal plate for heat radiation used in the present invention include metals such as copper, nickel, aluminum, molybdenum and tungsten, alloys containing the above metals as main components, or metals or alloys containing the above metals. Are used, and the thickness thereof is 0.1 to 2.0 mm.
Is common. In the present invention, since the metal circuit and the heat-dissipating metal plate do not need to be the same, the material, thickness,
They may differ in shape and the like.

The metal circuit, the circuit metal plate and the heat dissipation metal plate are joined to the ceramic substrate via a joining layer. In the case of the present invention, the joining layer is formed by using a brazing material paste. It may be formed of a eutectic layer of ceramic and metal. The brazing material paste is, for example, a brazing material containing copper or copper and silver when the material of the metal circuit or the heat-dissipating metal plate is copper, and an active material such as titanium when the material is a nitride ceramic substrate. The brazing material containing a metal is preferably used. On the other hand, when forming a eutectic layer,
For example, when alumina is used for the ceramic substrate and copper is used for the metal plate, a copper oxide is selected as the eutectic layer.

The method of manufacturing the circuit board of the present invention will be described in detail by way of example of a full etching method, but the present invention is not limited thereto.

First, a brazing material paste containing, for example, copper and silver is applied to both surfaces of an essentially non-warped ceramic substrate whose warpage is less than 1/4000 of the length in the warped direction. . Next, a pair of uneven jigs having the same curved surface as the warp according to the claims of the present invention is prepared, and the heat dissipating metal having a width sufficient to cover the brazing material paste on the jig having the concave surface on the upper surface is provided. A plate is placed, and the ceramic substrate is placed thereon. Further, a circuit metal plate is placed on the ceramic substrate, and a jig having a convex surface on the lower surface is placed thereon. Further, a load for bending the ceramic substrate is applied, and heat treatment is performed while the load is applied to join the metal plate and the ceramic substrate.

Regarding how to warp the ceramic substrate, it is preferable that the ceramic substrate bend from 1/4000 to 3/100 in length in the same direction as the warping direction of the circuit board having the target warpage. This is because the direction in which the circuit board is deflected and the amount thereof substantially match the direction in which the circuit board is warped and the amount thereof.

The method of applying the load may be such that the joining of the metal and the ceramic substrate and the deformation of the ceramic substrate may be performed in the above-mentioned operation. Any method such as a method of applying hydraulic pressure or mechanical pressure such as hot pressing may be used. However, in the former method, the method in which the jig is made of a high-melting-point metal such as tungsten or molybdenum, which can serve as a high-density high-density metal, does not require a special device for load application, so the circuit is inexpensive. This is a preferable method because a substrate can be manufactured.

With respect to the ceramic substrate (hereinafter referred to as a “joined body”) having a metal plate bonded to both surfaces obtained above, a circuit pattern is printed on the metal plate using an etching resist to form a resist circuit pattern.

Next, after performing an etching process to remove unnecessary metal and brazing material outside the pattern, the etching resist is removed to obtain a circuit board having a metal circuit. Further, for the purpose of preventing oxidation and corrosion of the metal circuit, nickel plating or the like is performed as necessary to form a protective film on the metal circuit.

Now, the present invention will be described in further detail with reference to Examples.

[0031]

【Example】

[Examples 1 to 6] Size: 40 mm x 40 mm, thickness: 0.1 mm
A silver-copper-based brazing material paste containing titanium as an active metal is applied on both sides of a 635-mm aluminum nitride sintered body by a screen printing method and dried, and then a copper plate for a metal circuit having a thickness of 0.3 mm and a copper plate having a thickness of 0.15 mm are formed. The heat dissipating copper plate is placed so as to be in contact with it. A silicon nitride jig was machined so that it had a concave surface on the upper surface, and a copper plate for metal circuits was placed on the upper surface of the jig. Place the jig, and 10
The aluminum nitride substrate is bent by placing tungsten of 0 × 100 × 50 mm. These were heat-treated in a vacuum at 830 ° C. for 30 minutes to obtain a joined body of an aluminum nitride substrate and a copper plate.

Next, an ultraviolet-curable etching resist was printed on the circuit board by a screen printing method and cured on the copper plate of the joined body, and unnecessary copper outside the pattern was removed with a ferric chloride solution. Next, the substrate was placed in an aqueous solution containing ammonium hydrogen fluoride and hydrogen peroxide to remove unnecessary brazing material between copper circuit patterns, and then the resist was removed. Further, a nickel protective film was selectively formed on the copper circuit by electroless nickel plating.

Example 7 In Example 7, a sample was obtained by the method shown in Examples 1 to 6, except that a silicon nitride sintered body was used as a ceramic substrate.

Example 8 In Example 8, samples were obtained by the methods shown in Examples 1 to 6, except that an alumina sintered body was used as a ceramic substrate.

[Comparative Examples 1 and 2] With respect to Comparative Examples 1 and 2, samples were obtained in the same manner as in Examples 1 to 6, except that the metal circuit copper plate was placed downward.

Comparative Example 3 Comparative Example 3 was performed in the same manner as in Examples 1 to 6, except that a silicon nitride sintered body having no warped plate was used instead of the above silicon nitride jig. A sample was obtained.

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 minutes and the temperature was held at 125 ° C. for 30 minutes was defined as one cycle, and the test was performed in accordance with JIS C 0025 temperature change test method. After 30, 50, and 100 heat cycle tests, the presence or absence of cracks was evaluated.

The presence or absence of cracks was determined by the following method. That is, after the test, the copper circuit board and the brazing material are dissolved and removed by the above-described method, and cracks generated on the ceramic substrate along the edge of the copper circuit board below the copper circuit board (appearing between the circuits). (A crack which cannot be confirmed without removing the copper circuit board because there is no copper circuit board) was evaluated with a stereoscopic microscope. The length of the edge where cracks occur at the edge of the circuit board is expressed as a percentage of the total length of the edges of the circuit board (total length), and this is calculated as the crack occurrence rate%. The results are shown in Table 1.

[0039]

[Table 1]

[0040]

As is clear from the examples and the comparative examples, the circuit board according to the comparative example has cracks caused by heat sink bonding in the heat cycle test 30 times, and as the number of repetitions of the heat cycle increases, Since the number of cracks generated increased, sufficient reliability could not be obtained, and the amount of deflection was large and many cracks were observed before the heat cycle was performed, whereas the circuit board according to the present invention was subjected to a heat cycle test. No cracks are generated in the ceramic substrate even after the heat sink bonding after 30 times, and the ceramic substrate has high reliability and is practical.

In particular, the coefficient of thermal expansion at room temperature is 5 × 10 ^-6 ° C ^-1
When the following ceramic substrate is used, the effect of the present invention is remarkable, and no crack is generated even after 100 heat cycle tests, indicating that the reliability is extremely high.

That is, according to the present invention, a circuit board having high reliability and heat dissipation is provided, and the circuit board can be stably provided by the method of the present invention, which is industrially useful.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a circuit board according to the present invention and a method of manufacturing the same.

[Description of Signs] 1 Ceramic substrate 2 Metal plate for circuit 3 Metal plate for heat radiation 4 Bonding layer w Deflection amount of ceramic substrate

Claims (8)

[Claims] A metal circuit or a metal plate for a circuit and a metal heat radiating plate are joined while bending an essentially warped ceramic substrate in at least one direction, and the metal circuit or the metal plate for a circuit has a concave surface. A circuit board characterized by being warped. 2. The circuit board according to claim 1, wherein the amount of warp is not less than 1/4000 and not more than 3/100 with respect to the length in the direction in which the warp is present. 3. The ceramic substrate has a coefficient of thermal expansion of 5 × 1.
The circuit board according to claim 2, wherein the temperature is 0 ^-6 ° C ^-1 or less. 4. The circuit board according to claim 3, wherein the ceramic substrate is made of silicon nitride. 5. The ceramic substrate is made of aluminum nitride, and the amount of warpage after joining a metal circuit or a metal plate for a circuit and a metal radiator plate is 1/250 or more with respect to the length in the warped direction. 4. The circuit board according to claim 3, wherein the ratio is not more than / 100. 6. A ceramic substrate having essentially no warpage,
1/4000 or more of the length in at least one direction 3
A method for manufacturing a circuit board, comprising joining a metal circuit or a metal plate for a circuit and a metal plate for heat radiation while bending the metal plate to be less than / 100. 7. A method of manufacturing a circuit board according to claim 6, wherein a metal circuit or a metal plate for a circuit is bonded to a concave side of the bent ceramic substrate, and a metal plate for heat radiation is bonded to the convex side. A method for manufacturing a circuit board. 8. The method for manufacturing a circuit board according to claim 6, wherein a metal circuit is formed from a circuit metal plate bonded to a ceramic substrate.