JPH07115252A - Circuit board - Google Patents

Abstract

PURPOSE:To reduce the generation of void when soldering a copper plate base and to improve durability to thermal shock and heat history. CONSTITUTION:A circuit board comprises a metal circuit on one surface of a ceramic substrate and a metal heat sink on the other surface and meets the following requirements (1) to (3): (1) the thickness of the metal circuit is larger than 0.3mm; (2) the minimum of the total length of a part where the metal circuit is joined to the ceramic substrate is less than 20% of the ceramic substrate length; and (3) warp when cooled and heated at temperatures -40 deg.C to 300 deg.C is less than 100mum.

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 composed of a ceramics substrate having a metal circuit and a metal heat dissipation plate, and is used for a power module of electronic parts.

In recent years, along with the high performance of industrial equipment such as robots and motors, the transition of high power modules such as high power and high efficiency inverters is progressing, and the heat generated from semiconductor elements is also increasing. . In order to efficiently dissipate this heat, various methods have been conventionally used for high power module substrates. Especially recently, since ceramic substrates having good thermal conductivity have become available,
A structure is also being adopted in which a metal plate such as a copper plate is bonded onto a substrate, a circuit is formed, and then a semiconductor element is mounted as it is or after a treatment such as plating is performed.

There are various methods for joining metal and ceramics, but from the viewpoint of manufacturing a circuit board, Mo-Mn is used.
Method, active metal brazing method, copper sulfide method, DBC method, copper metallizing method and the like.

Particularly in high-power module substrates, attention has been paid to aluminum nitride substrates having high thermal conductivity in place of conventional alumina, and as a method for joining copper plates, an active metal may be contained between the copper plates and the aluminum nitride substrate. An active metal brazing method in which a material (hereinafter simply referred to as "brazing material") is interposed and heat-treated to form a joined body (for example, JP-A-60-177).
No. 634), or a DBC method (for example, Japanese Patent Laid-Open No. 56-163093) in which an aluminum nitride substrate whose surface is oxidized and a copper plate are heat-bonded at a temperature below the melting point of copper and above the eutectic temperature of Cu—O There is.

The active metal brazing method has the following advantages over the DBC method. (1) Since the processing temperature for obtaining the bonded body is low, the residual stress caused by the difference in thermal expansion between the aluminum nitride substrate and the copper plate is small. (2) Since the brazing material is a ductile metal, it is highly durable against heat shock and heat cycles.

[0006]

However, even when the active metal brazing method is used, it cannot be said that it has sufficient durability against damage caused by heat shock such as heat shock or heat cycle, or heat history. A technology proposal was awaited. Therefore, the volume of the metal heat dissipation plate (usually provided on the lower surface of the ceramic substrate) is 50 to 90% of the volume of the metal circuit (usually provided on the upper surface of the ceramic substrate).
It is improved to some extent by adjusting the thickness of the metal heat sink to 50% or less of that of the metal circuit (Japanese Patent Laid-Open No. 5-170564). It was

However, in these techniques, since the metal circuit and the metal radiator plate are both made of copper, changing the volumes of both makes the balance of stress due to thermal expansion different. As a result, the thermal shock resistance of the joint itself became good, and the metal circuit or metal heat sink was less likely to peel off.However, when soldering the base copper plate to the metal heat sink and the semiconductor element to the metal circuit respectively, The warp displacement of the bonded body is remarkably increased due to such a temperature rise, and a gap is formed between the metal radiator plate and the base copper plate, and there is a risk that the portion becomes a void after soldering.

As a result of intensive studies to solve the above problems, the present inventors have found that such thermal stress is greatly influenced by the shape of the circuit pattern, and the metal circuit and the ceramic substrate are bonded together. The present invention has been completed by finding that there is a correlation between the ratio of the part that is joined to the part that is not joined and the change in the amount of warpage with respect to the temperature change of the circuit board.

[0009]

That is, according to the present invention, a metal circuit is provided on one surface of a ceramic substrate, and a metal heat dissipation plate is provided on the other surface.
It is a circuit board provided with the conditions (3) to (3). (1) The thickness of the metal circuit is larger than 0.3 mm. (2) The minimum total length of the portions where the metal circuit and the ceramics substrate are joined is 20% or less of the length of the ceramics substrate. (3) The amount of warpage when cooled / heated to a temperature of −40 ° C. to 300 ° C. is 100 μm or less.

The present invention will be described in more detail below. As the ceramic substrate used in the present invention,
Examples thereof include an aluminum nitride substrate, a beryllia substrate, and an alumina substrate. Among them, an aluminum nitride substrate is preferable, and its sintered density is preferably 95% or more in terms of mechanical strength and electrical characteristics. The thickness of the ceramic substrate is preferably about 0.4 to 0.7 mm.

On the other hand, copper, aluminum, tungsten, molybdenum or the like is used as the metal circuit and / or the metal heat dissipation plate, but copper is generally used. The thickness of the metal circuit is 0.3 because the current density tends to increase in recent years.
The thickness of the metal radiator plate is preferably 0.2 mm or less in order to reduce the thermal resistance.

A metal circuit is formed on one surface of the ceramic substrate,
As a method of providing a metal heat sink on the other surface, a method of etching a joined body of a ceramic substrate and a metal plate, a method of joining a metal circuit punched from the metal plate and / or a pattern of the metal heat sink to the ceramic substrate And the like. As a method for joining the metal plate or pattern to the ceramic substrate in these cases, an active metal brazing method, a DBC method, or the like can be adopted.

The metal component of the brazing filler metal in the active metal brazing method contains silver and copper as main components, and the active metal as a subcomponent for ensuring wettability with the ceramic substrate during melting. This active metal component reacts with the ceramic substrate to generate oxides and nitrides, and these products strengthen 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. The ratio of these is 3 to 35 parts by weight of the active metal per 100 parts by weight of the total amount of 69 to 75 parts by weight of silver and 25 to 31 parts by weight of copper.

The brazing material paste used in the active metal brazing method comprises a roll, a kneader, a Banbury mixer, a universal mixer, a raider, and the like, in which an organic solvent and, if necessary, an organic binder are added to the metal components of the brazing material. It can be adjusted by mixing with a machine or the like. Methylcellosolve, terpineol, isophorone, toluene and the like are used as the organic solvent, and ethylcellulose, methylcellulose, polymethylmethacrylate and the like are used as the organic binder. When the material of the metal circuit or the metal radiating plate is aluminum, it is not necessary to use the brazing material described above, and for example, a material containing aluminum and silicon as metal components is sufficient.

According to the present invention, in such a circuit board,
It satisfies the following conditions (1) to (3). (1) The thickness of the metal circuit is larger than 0.3 mm. (2) The minimum total length of the portions where the metal circuit and the ceramics substrate are joined is 20% or less of the length of the ceramics substrate. (3) The amount of warpage when cooled / heated to a temperature of −40 ° C. to 300 ° C. is 100 μm or less.

First, the first condition is the thickness of the metal circuit. In the present invention, the thickness is larger than 0.3 mm.
This is to cope with the recent increase in current density.

The second condition is that in the cross section of the circuit board,
Proportion of the minimum value of the total length of the joined portion of the metal circuit and the ceramics substrate to the length of the ceramics substrate (hereinafter,
This ratio is called the pattern ratio. ) Is less than 20%. If the pattern rate exceeds 20%, the thermal stress due to the difference in the thermal expansion coefficient between the metal circuit and the ceramics substrate increases, and the warp of the circuit substrate increases. As a result, the third
In order to satisfy the condition (1), it is necessary to increase the thickness of the metal heat dissipation plate on the back surface, so that the durability against damage caused by thermal shock or the like is significantly reduced.

In the calculation of the pattern ratio, when forming a metal circuit on a ceramic substrate, usually, the ceramic substrate and the metal circuit are arranged so that thermal stress caused by thermal expansion of the metal circuit is not directly propagated to the ceramic substrate. Although a non-bonded portion may be provided between the two, such a non-bonded portion is also regarded as a bonded portion in the present invention.

Further, the position of the circuit board for calculating the pattern ratio is optimal near the central portion in the longitudinal direction of the circuit board as shown by X in FIG. 1, but from the central portion in the longitudinal direction. It may be within a range of ± 20 mm.

Next, the third condition is a temperature of -40 ° C to 30 ° C.
The amount of warpage when cooled and heated to 0 ° C is 100 μm
It means that In the prior art, from the viewpoint of electrical conductivity, the material of the metal circuit or the metal heat dissipation plate is preferably oxygen-free copper or tough pitch copper mixed with a slight amount of oxygen, but such a copper The coefficient of thermal expansion is 17 × 10 ⁻⁶ / ° C., which is almost equal to the literature value, and is larger than 4.5 × 10 ⁻⁶ / ° C. of a ceramic substrate such as an aluminum nitride substrate. Thermal stress had occurred.

In addition, in the method of changing the volume ratio of the metal circuit and the metal heat sink, when manufacturing the joined body of the metal plate and the ceramics board, when mounting the obtained circuit board on the base copper plate, At the time of its use, there were many occasions where thermal stress was applied by thermal shock or the like caused by a temperature difference.

However, the thickness of the metal circuit tends to become thicker in the future in order to maintain the current capacity, and it is desirable that the metal radiator plate be thin from the viewpoint of reducing the thermal resistance. However, in the conventional technology, in the circuit board having such a structure, the thermal stress was large and the durability was lowered, but in the present invention, in order to disperse and transmit the thermal stress to the ceramic substrate, The third condition is necessary.

[0023]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples. Examples 1-2 Comparative Examples 1-3 Silver powder 75 parts by weight, copper powder 25 parts by weight, zirconium powder 5 parts by weight, terpineol 15 parts by weight and a toluene solution of polyisobutyl methacrylate as an organic binder in a solid content of 1. 5 parts by weight were added and kneaded well to prepare a brazing material paste. 51 × 36 × this brazing paste
Both sides of a 0.65 mm aluminum nitride substrate were applied to the entire surface by screen printing. The coating amount (after drying) at that time was 6 to 8 mg / cm ² .

Next, a 51 × 36 × 0.5 mm copper plate for forming a metal circuit is formed on one surface of the aluminum nitride substrate coated with the brazing material paste, and 51 × 36 is formed on the other surface.
X 0.2 mm copper plates for metal heat sinks are placed in contact with each other, then heated at 900 ° C for 30 minutes in a vacuum with a vacuum degree of 1 x 10 ^-5 Torr or less, and then cooled at a rate of 2 ° C / minute. To produce a joined body.

UV-curing type etching resist was screen-printed on the copper plate of the obtained bonded body by FIG. 1 and Table 1.
After applying to the circuit pattern shown in, the copper plate solution is etched to remove unnecessary portions of the copper plate,
Further, the etching resist was stripped with a 5% caustic soda solution. On the circuit board after this etching treatment, there is a residual unnecessary brazing material or a reaction product of the active metal component and the aluminum nitride substrate between the copper circuits. Immerse in the solution for 10 minutes.

A heat cycle (thermal shock) test was conducted on the circuit board manufactured as described above. The heat cycle test is carried out in the air at −40 ° C. for 30 minutes, then at 25 ° C.
× 10 minutes, 125 ° C × 30 minutes, then 25
The cycle of 10 minutes at ℃ × 10 was performed as one cycle, and the number of heat cycles at which copper started to peel was measured. Also, the temperature is 30
The amount of warpage of the circuit board when heated in 0 ° C. air for 10 minutes was measured by a non-contact laser displacement meter. Furthermore, in a reflow oven at a temperature of 250 ° C., after soldering the circuit board to a base copper plate having a thickness of 4 mm with no load, the voids are measured with an ultrasonic probe, and the void occurrence rate (%) is The area was calculated as (area / circuit board area) × 100. The results are shown in Table 1.

[0027]

[Table 1] (Note) The direction of warpage of the circuit board is indicated as + when the circuit surface is concave.

[0028]

Since the circuit board of the present invention has a significantly small warp displacement due to temperature change, the occurrence of voids during soldering to the base copper plate is reduced, and the circuit board has durability against heat shock and heat history, that is, heat resistance. Cycleability is improved.

[Brief description of drawings]

FIG. 1 is a top view showing an example of a circuit board of the present invention (unit of dimension is mm).

FIG. 2 is a bottom view of FIG. 1 (unit of dimension is mm).

[Explanation of symbols]

 1 ceramics substrate 2 metal circuit 3 metal heat sink a pattern width b pattern width X position where the minimum pattern ratio (a / 36) is calculated

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsunori Terano 1 Shinkai-cho, Omuta-shi, Fukuoka Electric Chemical Industry Co., Ltd. Omuta Factory

Claims (1)

[Claims] 1. A ceramic substrate having a metal circuit on one surface thereof and a metal radiator plate on the other surface thereof, which is provided with the following conditions (1) to (3). Characteristic circuit board. (1) The thickness of the metal circuit is larger than 0.3 mm. (2) The minimum total length of the portions where the metal circuit and the ceramics substrate are joined is 20% or less of the length of the ceramics substrate. (3) The amount of warpage when cooled / heated to a temperature of −40 ° C. to 300 ° C. is 100 μm or less.