JP3257869B2 - Circuit board - Google Patents

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 ceramic substrate having a metal circuit and a metal radiator plate, and is used for a power module of an electronic component.

[0002] In recent years, with the advancement of the performance of industrial equipment such as robots and motors, the transition of high-power modules such as high-power and high-efficiency inverters has been progressing, and the heat generated from semiconductor elements has been increasing steadily. . In order to efficiently dissipate this heat, various methods have conventionally been used for large power module substrates. Especially recently, ceramic substrates with good thermal conductivity have become available,
A structure in which a metal plate such as a copper plate is bonded on a substrate to form a circuit, and then a semiconductor element is mounted as it is or after performing a process such as plating has been adopted.

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

In particular, in the case of high power module substrates, attention has been paid to aluminum nitride substrates having high thermal conductivity in place of conventional alumina, and a method of bonding a copper plate will include an active metal between the copper plate and the aluminum nitride substrate. An active metal brazing method (hereinafter referred to simply as "brazing material"), which is subjected to a heat treatment to form a joined body (for example, Japanese Unexamined Patent Publication No. 60-177)
No. 634), and a DBC method in which an aluminum nitride substrate whose surface is oxidized and a copper plate are heated and joined at a temperature lower than the melting point of copper and higher than the eutectic temperature of Cu—O (for example, JP-A-56-163093). There is.

[0005] The active metal brazing method has the following advantages over the DBC method. (1) Since the processing temperature for obtaining the joined 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, the durability against heat shock and heat cycle is high.

[0006]

However, even if the active metal brazing method is used, it cannot be said that it has sufficient durability against damage caused by heat shock and heat history such as heat shock and heat cycle, and a new method is used. Technology proposals were awaited. Therefore, the volume of the metal heat sink (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). Was.

However, in these techniques, since the material of the metal circuit and the metal radiator plate are both copper, changing the volume of the two results in a different balance of stress due to thermal expansion. As a result, the thermal shock resistance of the joined body itself was improved, and the metal circuit or the metal radiator plate was less likely to peel.However, when the base copper plate was soldered to the metal radiator plate and the semiconductor element was soldered to the metal circuit, respectively. Due to the excessive temperature rise, the displacement of the warpage of the joined body becomes remarkable, and a gap is formed between the metal heat radiating plate and the base copper plate, and there is a risk that the portion becomes a void after soldering.

The present inventors have conducted intensive studies in order to solve the above-mentioned problems, and as a result, by applying a heat treatment while applying a load to the joined body or the circuit board, the temperature of the joined body or the circuit board is reduced.
It has been found that the amount of warpage when heated to 50 ° C. can be adjusted to 200 μm or less, and the present invention has been completed.

[0009]

That is, according to the present invention, a ceramic substrate is provided with a metal circuit on one side and a metal heat radiating plate on the other side. A circuit board characterized by being larger than 3 mm and having a warpage of 200 μm or less when heated to a temperature of 250 ° C.

Hereinafter, the present invention will be described in more detail. 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 the sintered density thereof is desirably 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 sink, but copper is generally used. In recent years, the thickness of the metal circuit is 0.3
mm, and the thickness of the metal heat sink is preferably 0.2 mm or less in order to reduce the thermal resistance.

A metal circuit is provided on one side 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 a metal plate and / or a pattern of a metal heat sink to a ceramic substrate For example, an active metal brazing method, a DBC method, or the like can be used as a bonding method between the metal plate or pattern and the ceramic substrate in these cases.

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 in order to ensure wettability with the ceramic substrate during melting. The 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. These ratios are 3 to 35 parts by weight of the active metal per 100 parts by weight of the total 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 is prepared by adding an organic solvent and, if necessary, an organic binder to the metal component of the brazing material, and preparing a roll, kneader, Banbury mixer, universal mixer, raika It can be adjusted by mixing with a machine or the like. Examples of the organic solvent include methylcellosolve, terpineol, isophorone, and toluene, and examples of the organic binder include ethylcellulose, methylcellulose, and polymethylmethacrylate. When the material of the metal circuit or the metal radiator plate is aluminum, it is not necessary to use the above brazing material. For example, a material containing aluminum and silicon as metal components is sufficient.

The present invention relates to such a circuit board,
200μ warpage when heated to 250 ° C
A major feature is that the distance is equal to or less than m. In the prior art, from the viewpoint of electrical conductivity and the like, the material of the metal circuit or the metal radiator plate is preferably oxygen-free copper or tough pitch copper mixed with a small amount of oxygen. The coefficient of thermal expansion of such copper is approximately equal to the literature value, 17 ×
Since it is 10 ⁻⁶ / ° C., which is larger than 4.5 × 10 ⁻⁶ / ° C. of a ceramic substrate, for example, an aluminum nitride substrate, thermal stress is generated when heat is applied to the circuit substrate.

In the method of changing the volume ratio between the metal circuit and the metal radiator plate, the method may be used when manufacturing a joined body of a metal plate and a ceramic substrate, or when attaching the obtained circuit board to a base copper plate. As described above, there were many opportunities to apply thermal stress due to thermal shock or the like resulting from a temperature difference during use.

However, the thickness of the metal circuit is going to become increasingly thicker in the future in order to maintain the current capacity, and it is desirable that the metal radiating plate is thinner from the viewpoint of lowering the thermal resistance. In order to prevent thermal stress from being transmitted to the ceramic substrate as much as possible even in the case of a circuit board having such a structure, the present invention provides a method for manufacturing a joined body or a circuit board, which is performed after 0.3 to 10 kg / cm ^2. Under an inert atmosphere such as nitrogen at a temperature of 100 to 200 ° C. while applying a load of
After holding for 4 hours, remove the load and cool down.
The warpage of the circuit board when heated to a temperature of 250 ° C. is 200 μm or less.

[0018]

The present invention will be specifically described below with reference to examples and comparative examples. Examples 1 to 3 Comparative Examples 1 to 7 75 parts by weight of silver powder, 25 parts by weight of copper powder, 5 parts by weight of zirconium powder, 15 parts by weight of terpineol 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 filler metal paste. This brazing material paste is 60 × 30 ×
The entire surface was coated on both sides of a 0.65 mm aluminum nitride substrate by screen printing. The coating amount (after drying) at that time was 6 to 8 mg / cm ² .

Next, a 60.times.30.times.0.5 mm copper plate is placed on the front surface of the aluminum nitride substrate on which the brazing material paste is applied, and a 60.times.30.times.0.2 mm copper plate is placed on the back surface of the aluminum nitride substrate. Then, the load shown in Table 1 was applied to the furnace, the mixture was heated in a high vacuum at a temperature of 900 ° C. for 30 minutes, and then cooled at a rate of 2 ° C./min to produce a joined body.

After a UV-curable etching resist is applied to the circuit pattern by screen printing on the copper plate of the obtained joined body, an etching process is performed using a cupric chloride solution to dissolve and remove unnecessary portions of the copper plate. The etching resist was stripped with a 5% sodium hydroxide solution. In the circuit board after the etching process, there is a residual unnecessary brazing material or a reaction product between the active metal component and the aluminum nitride substrate between the copper circuits and the like. Dipped in the solution for 10 minutes. After the obtained circuit board was subjected to a heat treatment under the conditions shown in Table 1, the load was removed and the circuit board was cooled to room temperature.

A heat cycle (thermal shock) test was performed on the circuit board manufactured as described above. The heat cycle test was carried out at -40 ° C for 30 minutes in air, then at 25 ° C.
X 10 minutes, and after further holding at 125 ° C x 30 minutes, 25
One cycle of standing at 10 ° C. × 10 minutes was performed, and the number of heat cycles at which copper peeling started was measured. In addition, temperature 25
The amount of warpage of the circuit board when heated in air at 0 ° C. for 5 hours was measured by a non-contact laser displacement meter. Furthermore, in a reflow furnace at a temperature of 250 ° C., the circuit board was soldered to the base copper plate having a thickness of 4 mm with no load, and the void was measured with an ultrasonic probe to determine the void generation rate (%). area/
It was calculated as (circuit board area) × 100. Table 1 shows the results.

[0022]

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

[0023]

According to the circuit board of the present invention, the displacement of the warpage due to the temperature change is extremely small, so that the occurrence of voids when soldering the circuit board to the base copper plate is reduced, and the durability against thermal shock and thermal history, that is, Heat cycle resistance is improved.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-U 4-32557 (JP, U) (58) Field surveyed (Int. Cl. ⁷ , DB name) H05K 1/02

Claims (1)

(57) [Claims] A metal circuit is provided on one surface of a ceramic substrate, and a metal radiator plate is provided on the other surface, wherein the thickness of the metal circuit is greater than 0.3 mm and the temperature is 250 ° C. A circuit board having a warpage of 200 μm or less when heated.