JP2000058723A - Circuit board - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a highly reliable circuit board which does not warp much when the board is subject to heat history by joining a copper circuit to one surface of a ceramic substrate and a heat radiating copper plate to the other surface of the substrate with a brazing material containing an active metal component and making the joining layer formed on the heat radiating plate side thicker in thickness than the joining layer formed on the copper circuit side by a specific value or larger. SOLUTION: The thickness of the joining layer formed on a copper circuit side is made different from that of the joining layer formed on a heat radiating copper plate side by changing the applied amount of brazing material paste applied to a ceramic substrate and/or the copper plate. Since the joining layer formed at the time of joining the heat radiating copper plate to one surface of a ceramic substrate is made thicker in thickness than the joining layer formed at the time of joining the copper plate to the other end face of the substrate by >=10 μm, the generated thermal stress difference becomes smaller. Therefore, a highly reliable circuit board which does not warp much when the board is subject to heat history and has a good heat cycle property can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of a circuit board comprising a ceramic substrate provided with a copper circuit and a heat radiating copper plate, and an object thereof is to improve the reliability of the circuit board. is there. The circuit board of the present invention is suitable for assembling a power module or the like of electronic components.

[0002]

2. Description of the Related Art In recent years, power modules such as high-power and high-efficiency inverters have been changing with the advancement of the performance of industrial equipment such as robots and motors, and the heat generated from semiconductor devices has been increasing steadily. I have. In order to efficiently dissipate this heat, various methods have conventionally been used for power module substrates. Especially recently, ceramic substrates with good thermal conductivity have become available,
After joining a metal plate such as a copper plate on the board and forming a circuit,
A structure in which a semiconductor element is mounted as it is or after performing a process such as Ni plating has been adopted.

[0003] Such modules have initially been used in simple machine tools, but in recent years have been used in welding machines, train drives, electric vehicles and in more severe environmental conditions. Durability and further miniaturization have been required. Therefore, the ceramic substrate is required to have an increased metal circuit thickness for increasing the current density, and to have improved durability against thermal shock and the like.

[0004]

Conventionally, a generally used circuit board has a structure in which a copper circuit is formed on one surface of an alumina substrate or an aluminum nitride substrate and a heat-dissipating copper plate is formed on the other surface. In such a circuit board, since the thermal expansion coefficients of copper and ceramics are different, thermal stress is repeatedly applied to the joining end face of the ceramic and copper, so that the copper circuit or the heat-dissipating copper plate is peeled off, There are problems such as cracking.

One of the means for solving this problem is to change the thickness of the copper circuit and the heat-dissipating copper plate and to warp the substrate in a direction in which thermal stress is applied during a thermal cycle, thereby causing cracks in the ceramic substrate, copper circuit or heat dissipation. There is a method for improving the reliability with respect to peeling of a copper plate.

However, when the thickness of the copper circuit and the thickness of the heat-dissipating copper plate are changed, when the circuit board and the base copper plate are soldered in module assembly, the molten solder due to the warpage of the circuit board is not uniformly applied and spread. There is a problem that solder voids are generated and solder cracks are easily generated during a heat cycle.

[0007] The present invention has been made in view of the above, and an object of the present invention is to provide a highly reliable circuit board which is less warped when subjected to a thermal history.

[0008]

That is, the present invention relates to a ceramic substrate having a copper circuit on one side and a heat-dissipating copper plate on the other side, each of which is joined by using a brazing material containing an active metal component. A circuit board characterized in that the thickness of the bonding layer on the heat radiating copper plate side is 10 μm or more thicker than that of the copper circuit side. Also, the present invention provides the circuit board, wherein the volume ratio of the heat radiating copper plate to the copper circuit is 30 to 9;
JIS B after conducting a furnace test five times in one cycle of 350 ° C. × 5 minutes and 25 ° C. × 5 minutes in air.
A circuit board characterized in that the flatness according to No. 0621 is 100 μm or less.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail below. A circuit board is assembled four to five times by assembling a module by soldering a silicon chip, a base copper plate, curing silicone gel, epoxy resin, and the like. Receive a degree of heat history. The temperature of the heat treatment is adjusted so that the type of solder to be used and the solder used in the previous process are not melted, and is about 350 ° C. at the maximum. Here, when the volume of the copper circuit is different from that of the heat-dissipating copper plate, the circuit board is warped due to the different thermal stresses generated by the heat treatment, and the molten solder is not spread evenly, causing solder voids.

In order to prevent such solder voids, the volume of the copper circuit and that of the heat-dissipating copper plate may be made equal. In such a case, cracks may occur in the ceramic substrate due to the inability to withstand the generated thermal stress, Is peeled off, and the reliability during a thermal cycle is greatly reduced.

Therefore, in a circuit board having a different volume between the copper circuit and the heat-dissipating copper plate, in order to reduce the warpage during the heat cycle, the difference in generated thermal stress may be reduced. The present inventors have found that by making the thickness of the bonding layer formed at the time of bonding the ceramic substrate and the copper plate 10 μm or more thicker on the heat-dissipating copper plate than on the copper circuit, it is possible to reduce the generated thermal stress difference. Was.

[0012] High-purity oxygen-free copper becomes soft and easily plastically deformed by heat treatment, whereas the bonding layer is relatively hard and is hardly plastically deformed. Therefore, the rigidity of the entire circuit board greatly changes depending on the thickness of the bonding layer.

In the present invention, the thickness of the bonding layer formed on the copper circuit side and the heat radiating copper plate side can be changed by changing the amount of the brazing material paste applied to the ceramic substrate and / or the copper plate. Can be. And
By increasing the thickness of the bonding layer on the heat dissipating copper plate side more than the copper circuit side by 10 μm or more with respect to the circuit board in which the volume ratio of the copper heat dissipating plate to the copper circuit is 30 to 90%,
The flatness according to JIS B 0621 after conducting the furnace test five times with 50 ° C. × 5 minutes and 25 ° C. × 5 minutes as one cycle can be suppressed to 100 μm or less. Also,
The transverse rupture strength is 25 kgf / mm ² or more, which maintains the original high reliability. Note that the thickness of the bonding layer formed on the copper circuit side is preferably about 10 to 20 μm.

The material of the ceramic substrate used in the present invention is silicon nitride, aluminum nitride, alumina or the like, but aluminum nitride is suitable for the power module. As the thickness of the aluminum nitride substrate, if it is too thick, the thermal resistance increases, and if it is too thin, the durability is lost.

[0015] The surface properties of the ceramic substrate are important, and minute defects and dents have an adverse effect when joining a copper circuit, a heat-radiating copper plate, or a copper plate that is a precursor thereof to the ceramic substrate. Is desirable.
Therefore, it is preferable that the ceramic substrate has been subjected to a honing process or a polishing process such as machining.

The purity of copper forming the copper circuit and the heat dissipation copper plate is preferably 99.5% or more, and the thickness is 100 to 500 μm.
Is preferred.

As a method of forming a copper circuit and a heat radiating copper plate on a ceramic substrate, there are a method of etching a joined body of the ceramic substrate and the copper plate, a method of bonding a circuit punched from the copper plate and a pattern of the heat radiating plate to the ceramic substrate, and the like. An active metal brazing method is used as a joining method in these cases.

In the active metal brazing method, the metal component of the brazing filler metal is silver and copper as main components, and the active metal is a subcomponent to ensure wettability with the ceramic substrate during melting. The active metal component reacts with the ceramic substrate to generate a compound such as a composite oxide or a composite nitride, thereby strengthening the bond between the brazing material and the ceramic substrate. Specific examples of the active metal include titanium, zirconium, hafnium, niobium, tantalum, vanadium, and compounds thereof. In the present invention, these ratios are, as weight ratios, 60 to 100 parts of silver and 0 to 40 parts of copper in a total amount of 1
The amount of the active metal is 1 to 30 parts per 00 parts.

The bonding temperature is preferably 780-830 ° C., and the holding time is preferably 20-60 minutes. When the temperature is low and the holding time is too short, the bonding is insufficient.On the other hand, when the holding time is too high and the holding time is too long, the diffusion of the brazing material component into the metal plate is too much and the metal plate is hard. And the heat cycle resistance decreases.

[0020]

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

Examples 1 to 2 90 parts by weight of Ag powder, 10 parts of Cu powder, TiH
₍₂₎ 3 parts of powder and 3 parts of Zr powder were mixed with 15 parts of terpineol, and a terpineol solution of polyisobutyl methacrylate was added and kneaded to prepare a brazing filler metal paste. An aluminum nitride substrate (size: 60 mm × 36)
mm × 0.65 mm Flexural strength: 40 kg / mm ² Thermal conductivity: 135 W / mK) The coating amount (after drying) is 9 mg / cm ^{2 on} the copper circuit forming surface, and the coating amount is on the heat radiation copper plate forming surface. (After drying) was applied at 13 mg / cm ^{2 in} Example 1 and 17 mg / cm ^{2 in} Example 2.

Next, a copper circuit pattern of 56 mm × 32 mm × 0.3 mm was formed on the copper circuit forming surface of the aluminum nitride substrate, and a 56 mm × 32 mm × 0.
A pattern of a heat-dissipating copper plate of 15 mm was placed in contact with the pattern, heated under the conditions shown in Table 1 under a vacuum of 1 × 10 ⁻⁵ Torr or less, rapidly cooled to 600 ° C., and then cooled to 2 ° C. /
The circuit board was manufactured by cooling at a temperature lowering rate of 1 minute.

Comparative Example 1 The amount of paste applied on both sides of an aluminum nitride substrate was 9 mg.
A circuit board was produced in the same manner as in Example 1 except that the ratio was set to / cm ² .

Comparative Example 2 A circuit board was manufactured in the same manner as in Comparative Example 1 except that the thickness of the heat-dissipating copper plate was 0.30 mm.

The coating amount of the brazing material paste is a copper circuit forming surface of Comparative Example 3 aluminum nitride substrate (after drying) 9 mg / cm ^2, the coating amount is in heat dissipating copper plate forming surface (after drying) to 11 mg / cm ² A circuit board was prepared in the same manner as in Example 1 except that the substrate was applied.

The circuit board manufactured through these series of processes was subjected to a furnace test five times in air at 350 ° C. × 5 minutes and 25 ° C. × 5 minutes as one cycle.
The flatness and bending strength according to SB 0621 were measured.
In addition, in order to evaluate heat cycle resistance, in air,-
After holding at 40 ° C. × 30 minutes, leave at 25 ° C. × 10 minutes, and further 12
After holding at 5 ° C. × 30 minutes, a durability test was performed with one cycle of standing at 25 ° C. × 10 minutes, and the number of cycles at which the copper circuit or the heat-dissipating copper plate started peeling was measured. Further, the thickness of the joining layer of the copper circuit and the heat-dissipating copper plate was measured at five arbitrary points with a scanning electron microscope after the circuit board was cut and the cross section was polished, and the values were averaged. Table 1 shows the results.

[0027]

[Table 1]

[0028]

According to the present invention, there is provided a highly reliable circuit board which is less warped after heat history and has excellent heat cycle properties.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Yoshihiko Tsujimura, Inventor 1 Shinkaicho, Omuta-shi, Fukuoka Denki Kagaku Kogyo Co., Ltd. Inside the Omuta Plant F-term in Omuta factory (reference) 5E338 AA02 AA18 BB80 CC01 CC08 EE02 EE28 5F036 AA01 BB01 BC06 BC33 BD01 BD13 BD14

Claims (2)

[Claims] A copper circuit is provided on one side of a ceramic substrate,
The heat-dissipating copper plate is bonded to the other surface using a brazing material containing an active metal component, and the thickness of the bonding layer on the heat-dissipating copper plate side is at least 10 μm thicker than the copper circuit side. Circuit board. 2. A copper circuit on one side of a ceramic substrate,
A heat radiation copper plate formed on the other surface, wherein the volume ratio of the heat radiation copper plate to the copper circuit is 30 to 90%;
2. The flatness according to JIS B 0621 after performing a furnace test five times in air at 350 ° C. × 5 minutes and 25 ° C. × 5 minutes as one cycle is 100 μm or less. Circuit board.