JPH11121889A - Circuit board - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a circuit board durable against damage due to heat shock or heat record by specifying the mean grain size of copper on the cross-section of a copper circuit and/or a copper heat plate. SOLUTION: A copper plate is bonded to a ceramic board while avoiding adverse effect of As diffusion and the grain size of copper in the copper plate is increased in the vicinity of the ceramic board in order to enhance the reliability of a circuit board. More specifically, a copper plate subjected to high heat- treatment in nonoxidative atmosphere is employed. Alternatively, a plurality of laminates of a copper plate and a ceramic plate sandwiching a paste of brazing material are contained in a carbon heating member comprising a frame body, a pushing plate and a pushing means therefor and then they are heated in an infrared heating furnace and bonded in a short time thus accelerating growth of copper crystal particles. According to the arrangement, the mean grain size of copper on the cross-section of a copper circuit and a copper heat plate can be set at 300 μm or above.

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 or the like of an electronic component.

[0002] In recent years, power modules such as high-power and high-efficiency inverters have been transitioning with the advancement of the performance of industrial equipment such as robots and motors, 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 power module substrates. In particular, since ceramic substrates with good heat conduction have become available recently, a copper plate such as copper is bonded on the substrate, and after forming a circuit, the substrate is subjected to a treatment such as plating or plating, and then a semiconductor element is mounted on the circuit surface. Is installed. In this case, there is a structure in which a heat dissipating copper plate for attaching a heat dissipating fin is joined to the opposite side of the circuit.

[0003] Such modules were initially used in simple machine tools, but have been used in welding machines, train drives and electric vehicles in recent years, and have been used in more severe environments. There is a demand for durability and further miniaturization. Therefore, the ceramic substrate is also required to have an increased circuit copper thickness for increasing the current density and an improvement in durability against thermal shock and the like.

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 metallization method and the like. Of these, for joining copper and a ceramic substrate, an active metal brazing method is used to form a joined body by interposing a brazing material containing an active metal between the two and heating the joint (for example, JP-A-60-177634). ), And a DBC method (for example, JP-A-56-163093) in which a ceramic substrate and a copper plate whose surfaces are oxidized are heated and joined at a temperature lower than the melting point of copper and higher than the eutectic temperature of Cu-O.

In the active metal brazing method, (a) the processing temperature for obtaining the joined body is lower than that of the DBC method, so that the residual thermal stress caused by the difference in thermal expansion between the ceramic substrate and copper is small. Since the brazing filler metal containing the active metal is a ductile metal, there are advantages such as high durability against heat shock and heat cycle.

[0006]

However, even if the active metal brazing method is used, since the temperature is raised to a temperature close to the melting point of the brazing, a residual stress is generated at the time of bonding on the circuit board. Proposals for new technologies have been awaited, although they are not sufficiently durable against damage caused by heat shock and heat history such as heat shock and heat cycle.

[0007] Japanese Patent Application Laid-Open No. 8-139420 discloses a copper circuit measured by an intercept method (code method) using a SEM photograph on a circuit board in which a copper circuit or a copper circuit and a heat dissipation copper plate are formed on a ceramic substrate. Alternatively, by setting the average crystal grain size of the heat-dissipating copper plate to 400 μm or more and the average sub-boundary density to 20 mm / mm ² or less, the warpage, strength, and the like are improved, and the circuit board has high reliability against thermal shock and thermal history. It is described. However, in the present invention, the state of the copper crystal particles on the surface of the copper circuit or the heat-dissipating copper plate and the vicinity of the ceramic substrate may be significantly different, which is an obstacle in further improving the reliability.

An object of the present invention is to provide a circuit board with further improved reliability, and the average crystal grain size of copper in a copper circuit or a heat-dissipating copper plate is increased not only in the surface but also in the cross section. The feature is that they are joined under such conditions.

[0009]

That is, the present invention provides a ceramic substrate and a copper circuit or a copper circuit and a heat-dissipating copper plate which are joined by a brazing material containing an Ag component and an active metal component. A circuit board characterized in that the average crystal particle diameter of copper in a cross section of the copper circuit and / or the heat-dissipating copper plate is 300 μm or more.

Hereinafter, the present invention will be described in more detail. As to the reliability of a circuit board formed by forming a copper circuit or a copper circuit and a heat-radiating copper plate on a ceramic substrate, the strength of the ceramic substrate itself is improved as described above. Besides that, it is important to improve the fatigue resistance of the copper side. That is, it is necessary to join the copper circuit and the heat-dissipating copper plate to the ceramic substrate without deteriorating the mechanical properties of copper.

[0011] The deformation of copper to external stresses is transmitted by the movement of transitions in the copper crystal. At this time, if the defect density in the copper crystal is low, the transition of the transition due to the stress becomes smooth and the copper crystal is easily deformed. Conversely, if the defect density is high, the movement of the dislocation is hindered by the defect, making it difficult to deform.

When the copper plate and the ceramic substrate are heated and cooled after cooling, the ceramic substrate tends to shrink more because the shrinkage ratio of the ceramic substrate is smaller. In this case, if the yield strength of the copper is small, the copper tends to be deformed. Therefore, the circuit board tends to deform following the deformation of the ceramic substrate, and the warpage of the circuit board becomes small. On the other hand, if the yield strength of copper is large, the deformation stress does not try to follow, but the contraction stress is applied to the ceramic substrate as it is, and the warpage of the circuit substrate increases. From the above,
If the yield strength of copper is small and easily deformed, stress due to temperature change can be dispersed in the deformation of copper, so it shows good durability against thermal shock such as in heat cycle tests (Japanese Unexamined Patent Publication No.
139420).

[0013] Copper, particularly oxygen-free copper which is close to pure copper, is soft and easily plastically deformed by heat treatment, and therefore is a material which is hardly damaged even when joined to ceramics having a different coefficient of thermal expansion. However, if copper contains a small amount of impurities, it hardens easily and loses its inherent property of being easily plastically deformed, resulting in residual stress and damaging ceramics (Takumi Maruta: Journal of Copper and Copper Research, 2 (1963), 89.). This is because copper contains impurities to hinder the grain growth of copper crystals.

In the circuit board manufactured by the active metal brazing method as in the present invention, the substance which is most likely to diffuse and penetrate into the copper circuit or the heat-dissipating copper plate is the Ag of the brazing material in view of its properties and quantity. Component. The mechanism of the diffusion and intrusion is as follows: first, the brazing material melts from the part in contact with the copper plate, then shifts from the Ag-rich composition to the eutectic composition,
It is believed that the temperature is further increased and energy is added to initiate the diffusion of Ag.

Therefore, when the copper plate and the ceramic substrate are joined by the active metal brazing method, if excessive energy is applied, the diffusion of Ag into the copper plate easily occurs, and the copper plate near the ceramic substrate has more than the surface. Ag diffuses. As a result, the growth of copper crystal grains in the copper plate portion is hindered, and the copper plate is hardened, which adversely affects the improvement of the reliability of the circuit board.

In the present invention, the copper plate and the ceramics substrate are joined while minimizing the adverse effects of such Ag diffusion.
This is to increase the crystal grain size of copper in the copper plate near the ceramic substrate to further improve the reliability of the circuit board. Specifically, a rolled copper sheet is heat-treated in a non-oxidizing atmosphere at a high heat treatment, preferably at a temperature of 800 ° C. or more for several hours. Alternatively, a plurality of laminates of a copper plate and a ceramics substrate with a brazing material paste interposed are housed in a carbon heating member composed of a frame, a push plate, and a pressing means for the push plate (Japanese Patent Application No. 2002-214,878). Hei 9-21
No. 6219), which is heated in an infrared heating furnace to perform bonding in a short time to promote the growth of copper crystal particles.

According to the above method, the average crystal particle diameter of copper in the cross section of the copper circuit and the heat-dissipating copper plate can be made 300 μm or more. In the present invention, when the average crystal particle diameter of copper is less than 300 μm, the grain boundary density of the cross section of the copper plate increases, so that plastic deformation of copper is hindered, and no further improvement in reliability is observed.

The average crystal grain size of copper on the surface of the copper circuit and the heat-dissipating copper plate is described in JP-A-8-13942.
As described in Japanese Patent Publication No. 0, it is preferable that a value measured by an intercept method (code method) using an SEM photograph is 400 μm or more and an average sub-grain density is 20 mm / mm ² or less.

Examples of the ceramic substrate used in the present invention include alumina, silicon nitride, and aluminum nitride. Aluminum nitride is preferable. If the thickness of the ceramic substrate is too large, the thermal resistance will increase,
If it is too thin, the durability will be lost, so it is preferably about 0.5 to 2 mm.

As a method of forming a copper circuit on one surface of the ceramic substrate and a heat-dissipating copper plate on the other surface, a method of etching a joined body of the ceramic substrate and the copper plate, a copper circuit punched from the copper plate and / or a heat-dissipating copper plate A method of joining a pattern of a copper plate to a ceramic substrate or the like can be used, and an active metal brazing method is employed for joining the copper plate or pattern and the ceramic substrate in these cases.

The metal component of the brazing material in the active metal brazing method contains silver or silver and copper as main components, and uses the active metal as a sub-component 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 as follows: 80 to 100 parts by weight of silver, preferably 85 to 95 parts by weight, and 20 to 0 parts by weight of copper, preferably 15 to 5 parts by weight; Department.

If the bonding temperature is too high, the diffusion of Ag into the copper circuit or the heat-dissipating copper plate proceeds, and if the bonding temperature is too low, the copper plate and the ceramics substrate are not sufficiently bonded.
C. is desirable.

The effect of the present invention is to improve the reliability of the circuit board. The evaluation is based on the three-point bending strength after the heat cycle test and the crack (horizontal level) generated from the joint interface between the copper circuit or the heat-radiating copper plate and the ceramic substrate. When the cracks are developed, the strength of the product of the present invention after 30 heat cycles is improved by about 20 kg / mm ² compared to the conventional product, and the rate of development of the horizontal crack is about 0.65 times the speed.

[0024]

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

Examples 1 to 5 89 parts by weight of silver powder, 5 parts by weight of copper powder, 3 parts by weight of zirconium powder, 3 parts by weight of titanium powder, 15 parts by weight of terpineol, and a toluene solution of polyisobutyl methacrylate at a solid content of 5 parts A weight part was added and kneaded well to prepare a brazing filler metal paste.

An aluminum nitride substrate (60 mm × 36 mm × 0.65 mm: heat conductivity: 1)
An L-shaped pattern having a pattern ratio of 0.20 was applied to the circuit surface of 70 W / mK) by screen printing, and the entire surface was applied to the heat radiation surface (back surface). Application amount at that time (after drying)
Was 9 mg / cm ² .

Next, a 60 mm × 36 mm ×
0.3-0.5mm copper plate, and 60mm ×
A copper plate having a size of 36 mm × 0.15 mm is placed in contact therewith, and 30 of the laminates are made of a carbon heating member (Japanese Patent Application No. 9-21) composed of a frame, a pressing plate, and pressing means for the pressing plate.
No. 6219) under pressure of 5 kg / cm ² and stored in an infrared heating furnace at a degree of vacuum of 1 × 10 ⁻⁵ To.
After heating at the bonding temperature shown in Table 1 for 30 minutes or less,
The joined body was manufactured by cooling at a rate of ° C./min.

Next, after applying an UV-curable etching resist by screen printing on the copper plate of the joined body,
Unnecessary portions of the copper plate were dissolved and removed by performing an etching treatment using a cupric chloride solution, and the etching resist was peeled off with a 5% sodium hydroxide solution. In the joined body after the etching process, there is a residual unnecessary brazing material or a reactant between the active metal component and the aluminum nitride substrate between the copper circuit patterns. Dipped in the solution for 10 minutes.

Comparative Examples 1 and 2 In the example, instead of storing 30 pieces of the laminate of the copper plate and the aluminum nitride substrate in a heating member made of carbon and heating it by vacuum in an infrared heating furnace, 30 pieces of the laminate were used. Was placed on a carbon plate of a heating furnace using a carbon heater, and a 500 g tungsten weight was placed on the upper surface thereof, and a vacuum heating was carried out according to the example, except that a circuit board was manufactured.

The average crystal grain size of copper in the cross section of the circuit board manufactured through these series of processes was measured. The measurement was performed using a microscope after cutting the circuit board, embedding the cut piece with a resin, polishing the cut piece, immersing the cross section of the exposed copper plate in 5% diluted nitric acid for 5 minutes to remove the oxide film, and then using a microscope.

The three-point bending strength of the circuit board at the initial stage and after 30 cycles of the heat cycle test was measured. Measurement is 30mm span, 0.5mm crosshead speed
/ Min, and pushed from the heat radiation side.

Further, a heat cycle resistance test was performed, and the number of heat cycles at which the copper plate of the copper circuit or the heat radiating copper plate started to peel was measured. The heat cycle test was performed in the air at -40.
After maintaining the temperature for 30 minutes at 25 ° C for 10 minutes,
After holding at 30 ° C. for 30 minutes, one cycle of standing at 25 ° C. for 10 minutes was performed.

The above results are shown in Table 1.

[0034]

[Table 1]

[0035]

According to the present invention, a highly reliable circuit board having improved heat cycle resistance is provided.

Claims (1)

[Claims] A ceramic substrate and a copper circuit or a copper circuit and a heat-dissipating copper plate are joined by a brazing material containing an Ag component and an active metal component, and are formed in a cross section of the copper circuit and / or the heat-dissipating copper plate. Average crystal grain size of copper is 300μm
A circuit board characterized by the above.