JPH10145039A - Printed circuit board and evaluation and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve resistance for heat cycle by conducting analysis by specifying the electron irradiating condition using an electron micro-analyzer (EPMA) after polishing the cross section of a circuit board and then measuring the thickness of the layer indicating the particular value of the Ag strength and setting this thickness as the thickness of the Ag layer. SOLUTION: Thickness of Ag layer which is solid-dispersed to a copper circuit is set as the thickness of the layer in which Ag strength IA is in the range of 20<=IA<=40 40 by conducting the analysis, after polishing the cross section of the circuit substrate, under the condition that IPMA is used and electron irradiating condition per 1μm is 15.0kV, 1.06×10<-7> A, 30msec. The region of the Ag strength exceeding 40 includes a large amount of active metal element and the region including Ag strength under 20 does not allow solid- dispersion of Ag. Therefore, it is inadequate for this region to evaluate thickness of the Ag layer. Resistance for heat cycle of the circuit substrate can be improved depending on the value of IA. It is preferable that average thickness of Ag layer measured at the positions larger than the five positions of the circuit substrate is ranged from about. 5 to 40μm.

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 of an electronic component and the like, and to a method for evaluating heat cycle resistance and a manufacturing method thereof.

[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. Particularly recently, ceramic substrates with good thermal conductivity have become available, so a metal plate such as copper is bonded on a ceramic substrate, and after forming a metal circuit, the substrate is subjected to a treatment such as plating or plating. A structure for mounting an element is employed. In this case, there is also a structure in which a metal heat radiating plate for attaching a heat radiating fin is joined to the opposite surface of the metal circuit.

In order to manufacture such a circuit board, Ag is used.
After a brazing material containing Cu and Cu as main components and an active metal component such as Zr, Ti, Hf or the like as an auxiliary component is printed on a ceramic substrate in a circuit pattern shape, a metal plate having the same shape as the circuit pattern is placed and heated. A method of joining, a method of printing a brazing material in a circuit pattern, heating and joining a solid metal plate, etching unnecessary metal to form a metal circuit, and applying a brazing material to the entire surface and then heating and joining the solid metal plate. A method of forming a metal circuit by removing unnecessary metal and brazing material by etching or the like (for example, WO91 / 1680).
No. 5). Since such a manufacturing method is a technique using a brazing material containing an active metal, it is also called an active metal brazing method.

In the active metal brazing method, (1) since the temperature at which the metal plate and the ceramic substrate are joined is lower than that of the DBC method, the residual thermal stress caused by the difference in thermal expansion between the ceramic substrate and the metal plate is small. (2) Since the brazing material is a ductile metal, there is an advantage that heat cycle resistance due to heat shock or heat cycle is large. Here, the DBC method is a method of directly joining a ceramics substrate and a copper plate using a eutectic without using a brazing material.

[0005]

However, a circuit board manufactured by such an active metal brazing method is required to have a further improved heat cycle resistance.

An object of the present invention is to provide a circuit board having improved heat cycle resistance more than before.

[0007]

That is, the present invention relates to a method for evaluating a circuit board in which a copper circuit and a ceramic substrate are joined using a brazing material containing an Ag component, a Cu component and an active metal component. The heat cycle resistance of the circuit board is evaluated based on the thickness of the Ag layer solid-diffused in the copper circuit measured by the following method.

[Measurement Method] After polishing a cross section of a circuit board, the cross section of the circuit board is 1 μm by an electron beam microanalyzer (EPMA).
The electron beam irradiation conditions per unit were 15.0 kV, 1.06 × 1
The analysis was performed at 0 ^-7 A, 30 msec, and the Ag intensity I _A
There is the thickness of the Ag layer by measuring the thickness of the layer shows a 20 ≦ I _A ≦ 40.

Further, according to the present invention, when evaluated by the above-described evaluation method, the thickness of the Ag layer solid-diffused in the copper circuit is 5%.
It is a circuit board characterized by being about 40 μm.

Further, the present invention provides, among the metal components, an Ag component and a Cu component as main components and an active metal component as an auxiliary component, respectively, and the weight ratio of the Ag component to the Cu component is 80.
95: 20-5, after the brazing material is interposed between the copper plate and the ceramic substrate, the degree of vacuum is 10 ⁻⁵ to 1 × 10 ^−6.
Manufacturing a circuit board characterized by starting heating in a high vacuum of Torr, increasing the temperature from 700 ° C. or higher at a rate of 10 ° C./min to 800 to 840 ° C., holding at that temperature, and then cooling. Is the way.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail below. In order to improve the heat cycle resistance (reliability) of a circuit board in which a copper plate and a ceramic substrate are joined, the strength of the ceramic substrate itself must be improved. In addition to this, it is important not to reduce the fatigue resistance of the copper plate, that is, to bond the copper plate to the ceramic substrate without lowering the mechanical characteristics.

Since high-purity oxygen-free copper is softened and easily plastically deformed by heat treatment, it does not easily damage the ceramic substrate even when joined to a ceramic substrate having a different coefficient of thermal expansion, but contains a small amount of impurities. In this way, it becomes harder and loses its inherent property of being easily plastically deformed, which is the original property of copper, and causes residual stress to damage the ceramic substrate (Takami Maruta: Journal of Copper and Copper Technology 2 (196
3), 89.).

Accordingly, oxygen-free copper is preferable as the copper plate of the circuit board, but it is particularly important to prevent impurities other than oxygen from being mixed into the copper plate. In the active metal brazing method, Ag is a substance that easily diffuses into the copper plate.

In the present invention, when the temperature is increased in a state where the ceramic substrate and the copper plate are interposed with the brazing material paste, first, the brazing material is melted from a portion in contact with the copper plate,
Subsequently, the composition shifts from the Ag-rich composition to the eutectic composition, and it is considered that Ag diffusion starts when the temperature is further increased. Therefore, in this case, the energy E received by the work is E = E when the reaction between the copper plate and Ag alone is considered.
1 + E2. Here, E1 is Ag (S) + Cu (S) →
Energy consumed by AgCu (eutectic),
E2 is the energy consumed when Ag diffuses into the copper plate. E1 is determined by the amount of Ag in the brazing filler metal, and is considered to increase as the Ag-rich composition increases.
Therefore, when the amount of Ag in the brazing material is extremely small or close to the eutectic composition, E1 becomes small, and Ag diffusion easily occurs.

Since the solid diffusion of Ag occurs even by a small amount of energy, the energy consumption when transitioning from an Ag-rich composition to a eutectic composition is increased as much as possible to reduce the Ag consumption.
It is important to reduce the energy given to the diffusion of methane. That is, E ≒ E1 (E2 ≒ 0). To this end, it is conceivable to lower the bonding temperature as much as possible. The means adopted in the present invention is to optimize the metal component of the brazing material and the joining conditions. That is, the weight ratio of the Ag component and the Cu component, which are the main components of the metal component of the brazing filler metal, is set to A.
g component: Cu component = 80 to 95: 20 to ⁵ and bonding is performed in a high vacuum of a degree of vacuum of 10 ⁻⁵ to 1 × 10 ⁻⁶ Torr at a temperature of 800 to 840 ° C. and a rise from 700 ° C. This is to increase the temperature rate to 10 ° C./min or more.

In the present invention, solid-state diffusion in a copper circuit
The thickness of the Ag layer is determined by polishing the cross section of the
Microanalyzer (EPMA), eg JEOL
Per 1 μm using "JXA-8600M"
Line irradiation conditions were 15.0 kV, 1.06 × 10^-7A, 30
The analysis was performed as msec and the Ag intensity I_AIs 20 ≦ I _A
It is defined as the thickness of the layer showing ≦ 40.

The circuit board of the present invention comprises a ceramic substrate,
When it is considered to be composed of a bonding layer and a copper circuit, A
region g the intensity I _A exceeds 40, and active metal component, I
Since the bonding layer contains a large amount of an intermetallic compound such as n, Sn, or Zn, an alloy, or the like, it is not appropriate as a region for evaluating the thickness of the Ag layer that is solid-diffused in the copper circuit. Further, Ag intensity I _A is not in Ag solid diffusion in the region of the surface portion or the ceramic substrate of the copper circuit is a region of less than 20, not suitable this also as an area to evaluate the thickness of the Ag layer. In the present invention, if defined as the thickness of the Ag layer Ag intensity I _A is solid diffusion layer thicknesses in the copper circuit showing a 20 ≦ I _A ≦ 40, resistance of the circuit board by the magnitude of the value The heat cycle property can be evaluated as good or bad.

In the circuit board of the present invention, the thickness of the Ag layer solid-diffused in the copper circuit is 5 to 40 μm.
When the thickness of the Ag layer is less than 5 μm, the bonding between the ceramic substrate and the copper plate becomes insufficient, and when the thickness exceeds 40 μm,
The plastic deformation characteristics of the copper plate are lost and the ceramic substrate is damaged, and in any case, the heat cycle resistance of the circuit board cannot be further improved. In the present invention, it is preferable that the average value of the thickness of the Ag layer measured at any five or more locations on the circuit board is 5 to 40 μm.

Next, a method of manufacturing a circuit board according to the present invention will be described.

Examples of the ceramic substrate used in the present invention include alumina, silicon nitride, and aluminum nitride. Aluminum nitride having high thermal conductivity is preferable. As a sintering aid for producing an aluminum nitride substrate, rare earth oxides (for example, yttria), alkaline earth metal oxides (for example, calcia) and the like are preferable, and yttria is particularly preferable. The ratio of the sintering aid is preferably 2 to 5% by weight based on the total of the aluminum nitride powder and the sintering aid. Further, the thickness of the ceramic substrate is preferably about 0.5 to 0.8 mm, because if it is too thick, the thermal resistance will be large, and if it is too thin, the durability will be lost.

The surface properties of the ceramic substrate are important, and it is desirable that the surface of the ceramic substrate be smooth because minute defects and dents have a large effect on the contact area at the time of bonding with the copper plate. Therefore, it is desirable to perform polishing by honing, machining, or the like.

On the other hand, the thickness of the copper circuit is preferably larger than 0.3 mm in view of the tendency of current density to increase in recent years, and when a heat-dissipating copper plate is provided on the back surface, the thickness is preferably 0.2 mm or less. Is preferred.

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 The pattern of the heat-dissipating copper plate can be bonded to the ceramic substrate by a method or the like. In such a case, as a bonding method of the copper plate or the copper pattern and the ceramic substrate, an active metal brazing method is adopted.

The metal component of the brazing material in the active metal brazing method contains Ag component and Cu component as main components, and an 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 Ti, Zr, and H.
f, Nb, Ta, V, and these compounds. These ratios are 1 to 7 parts by weight of the active metal per 100 parts by weight of the total of 80 to 95 parts by weight of Ag and 20 to 5 parts by weight of Cu.

If the joining temperature between the copper plate and the ceramic substrate is too high, the diffusion of Ag into the copper circuit proceeds and the residual stress of the copper circuit tends to remain, and if the temperature is too low, the copper plate and the ceramic substrate become uncomfortable. The temperature is set to 800 to 840 ° C. because bonding is not sufficiently performed. Further, it is preferable to increase the heating rate up to the joining temperature as much as possible,
The rate of temperature rise from 00 ° C. is 10 ° C./min or more. The atmosphere at the time of joining is a high vacuum with a degree of vacuum of 1 × 10 ⁻⁵ to 1 × 10 ⁻⁶ Torr. If the degree of vacuum is less than 1 × 10 ⁻⁵ , the bonding becomes insufficient, and if the degree of vacuum exceeds 1 × 10 ⁻⁶ Torr, the activity of the brazing filler metal becomes too high, and the heat cycle resistance is not improved in any case. .

[0026]

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

Examples 1-5 Comparative Examples 1-6 2 parts by weight of oleic acid as a surface treatment agent was added to 96 parts by weight of aluminum nitride powder and 4 parts by weight of a sintering aid (yttria) in total of 100 parts by weight. Then, premixing was performed with a vibration mill. Further, 8 parts by weight of ethyl cellulose as an organic binder, 3 parts by weight of glycerin triolate as a plasticizer and 12 parts by weight of water are mixed by a mixer, and the kneaded product is molded at a molding speed of 1.0 m / min.
Extrusion was performed at kg / cm ² .

The extruded product was heated at 120 ° C., 5
After drying for 1 minute, the material was degreased in air at 480 ° C. for 10 hours and fired at 1850 ° C. The surface of the obtained aluminum nitride sintered body was sufficiently cleaned by a honing treatment and processed into a size of 60 mm × 36 mm × 0.65 mm to obtain an aluminum nitride substrate.

The ratio of Ag powder to Cu powder was varied as shown in Table 1. 100 parts by weight, 3 parts by weight of Zr powder, 3 parts by weight of Ti powder, 15 parts by weight of terpineol, and polyisobutyl as an organic binder were used. Add 5 parts by weight of a methacrylate toluene solution as a solid content and knead well.
The brazing material paste was adjusted. This brazing material paste was applied on the front surface (circuit side) of the aluminum nitride substrate by screen printing to form an L-shaped pattern having a pattern ratio of 0.20, and on the entire back surface (radiation side). The coating amount (after drying) at that time was 9 mg / cm ² .

Next, on the circuit side, 60 mm × 36 mm ×
0.3mm copper plate and 60mm × 36m on the heat dissipation side
After placing the mx 0.15mm copper plates in contact with each other,
Table 1 shows the degree of vacuum in the atmosphere and the rate of temperature rise above 700 ° C.
Then, the temperature was raised to the bonding temperature shown in the same table, maintained at that temperature for 30 minutes, and then cooled at a cooling rate of 2 ° C./min to produce a bonded body.

After applying a UV curing type etching resist to the obtained joined body by screen printing in accordance with the brazing material paste pattern, etching 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 joined body subjected to the etching treatment, there is an unnecessary brazing material or a reactant between the active metal component and the aluminum nitride substrate remaining between the copper circuit patterns. It was immersed in an ammonium chloride solution for 10 minutes.

With respect to the circuit board manufactured through these series of processes, the three-point bending strength when pressed from the heat radiation side was measured under the conditions of a span of 30 mm and a crosshead speed of 0.5 mm / min. Further, in order to evaluate the heat cycling resistance, after holding at -40 ° C for 30 minutes in the air, 25 ° C x 1
Leave for 0 minutes, hold at 125 ° C x 30 minutes, then 25 ° C x 1
A durability test was performed in which one cycle was left for 0 minutes, and the number of cycles at which the copper circuit or the heat-dissipating copper plate started peeling was measured.
In addition, the thickness of the Ag layer in which Ag was solid-diffused in the copper circuit was measured at five arbitrary positions as described above, and the values were averaged. Table 1 shows the results.

[0033]

[Table 1]

[0034]

According to the present invention, a circuit board with further improved heat cycle resistance can be provided.

Claims (3)

[Claims] 1. A method for evaluating a circuit board in which a copper circuit and a ceramic substrate are joined using a brazing material containing an Ag component, a Cu component, and an active metal component, wherein the copper circuit is measured by the following method. A method for evaluating a circuit board, comprising evaluating the heat cycle resistance of the circuit board based on the thickness of the Ag layer that is solid-diffused in the circuit. [Measurement Method] After polishing the cross section of the circuit board, the electron beam irradiation conditions per 1 μm were set to 15.0 kV, 1.06 × 10 ⁻⁷ A, 30 μm by an electron beam microanalyzer (EPMA).
analyzed as msec, Ag intensity I _A is 20 ≦ I _A
The thickness of the layer that satisfies ≦ 40 is measured and defined as the thickness of the Ag layer. 2. The method according to claim 1, wherein the Ag layer solid-diffused in the copper circuit has a thickness of 5 to 4%.
A circuit board having a thickness of 0 μm. 3. The metal component contains an Ag component and a Cu component as main components and an active metal component as an auxiliary component, respectively, and has a weight ratio of Ag component: Cu component of 80-95: 20-.
After the brazing material No. ⁵ is interposed between the copper plate and the ceramic substrate, heating is started in a high vacuum at a degree of vacuum of 1 × 10 ⁻⁵ to 1 × 10 ⁻⁶ Torr, and the temperature is raised from a temperature of 700 ° C. or more. A method for manufacturing a circuit board, comprising increasing the speed to 800 to 840 ° C. at a rate of 10 ° C./min, holding at that temperature, and then cooling.