JP2007189112A - Silicon nitride substrate and circuit board and module using the same. - Google Patents

Abstract

A silicon nitride substrate used for a module or the like has high strength and high insulation, but has a low thermal conductivity, and therefore a silicon nitride substrate having a high thermal conductivity is desired. For silicon nitride substrates, columnar silicon nitride particles grow on the substrate surface during the sintering of silicon nitride, and the substrate surface is amorphized with a sintering aid and columnar short axis particles with low thermal conductivity. As a result, the thermal conductivity decreases, and the bonding strength between the substrate and the metal circuit or the heat dissipation surface decreases due to the unevenness of the surface, and the reliability of the silicon nitride circuit substrate and the module using the same is impaired. there were.
A silicon nitride substrate having high thermal conductivity and high reliability by mechanically removing the short axis particles of silicon nitride having low thermal conductivity and the amorphous sintering aid covering the surface of the substrate. Is obtained.
[Selection figure] None

Description

The present invention relates to a silicon nitride substrate excellent in thermal conductivity and reliability, and a circuit board and a module using the same.

Conventionally, a module is used in which a metal circuit such as copper is formed on one surface of a silicon nitride substrate, a metal heat sink such as copper is formed on the opposite surface, and a semiconductor element is soldered on the metal circuit surface. In such a module, the silicon nitride substrate plays an important role as an insulating and heat radiating member. However, since the thermal conductivity is low, high thermal conductivity is desired. The reason for the low thermal conductivity of the silicon nitride substrate is that when the silicon nitride substrate is sintered, columnar silicon nitride particles grow on the surface of the substrate, and the amorphous sintering aid and the columnar shape with low thermal conductivity. It is mentioned that it is covered with short axis particles of crystal. However, if the surface covered with the short axis particles of silicon nitride and the amorphous sintering aid is mechanically removed, the substrate and the metal circuit or heat dissipation surface are joined by the unevenness caused by the roughening of the surface. There existed a subject that intensity | strength fell and the reliability of a board | substrate was impaired. Regarding the method for processing the surface of the silicon nitride substrate so far, Patent Document 1 has a surface from which the surface layer portion having a burnt surface is removed, and the surface roughness (Rmax) of the removed surface is 10 μm or less, In addition, a silicon nitride substrate or the like substantially free from grinding marks is shown. As a specific method, a method of spraying ceramic particles having an average particle diameter of 1 mm or less onto a silicon nitride substrate and removing a surface layer portion having a burnt surface is exemplified. However, such a spraying method certainly improves the mechanical properties, but even if there is virtually no grinding streak, the locally remaining unevenness causes the bonding between the substrate and the metal circuit or heat dissipation surface. There was still a problem that the strength was lowered and the reliability of the substrate was impaired.
Japanese Laid-Open Patent Publication No. 8-29569

An object of the present invention is to provide a silicon nitride substrate excellent in thermal conductivity and reliability, and a circuit board and a module using the same.

That is, according to the present invention, in the silicon nitride substrate from which the surface layer of the silicon nitride substrate is removed by grinding, the centerline average roughness (Ra) after grinding is 0.1 to 0.8 μm and the maximum height (Ry) is 5 μm or less. A silicon nitride substrate having a thermal conductivity of 80 W / m · K or more, a silicon nitride substrate having a surface layer of the silicon nitride substrate removed of 5 μm or more; This is a silicon nitride circuit board in which a metal circuit is formed on the main surface and a heat sink is bonded to the other main surface, and the module uses the silicon nitride circuit board.

According to the present invention, a silicon nitride substrate excellent in thermal conductivity and reliability, and a circuit board and a module using the same are provided.

The present invention relates to the surface treatment of a silicon nitride substrate, and there is no particular limitation on the method for producing the silicon nitride substrate, and the same method as before can be used for each step of raw material preparation, molding, degreasing, and sintering. is there.

Silicon nitride powder (hereinafter referred to as SN powder), which is the main raw material of the silicon nitride substrate (hereinafter referred to as SN substrate) according to the present invention, is produced by a known method such as direct nitridation method, silica reduction method, and imide pyrolysis method. Can be used. The amount of oxygen in the SN powder is preferably 3% by mass or less, and more preferably 2% by mass or less. As for the particle size of the SN powder, the average particle diameter (D50 value) measured by Microtrac SPA (Leeds & Northrup) is 0.6 to 0.8 μm, the D90 value is 1.5 to 3.0 μm, and the D100 value is 8. It is preferably 0 μm or less. Furthermore, it is preferable that the specific surface area of SN powder is 10-15 m <2> / g. When the range of the specific surface area is deviated, the SN substrate may not be densified or the mechanical strength of the SN substrate may decrease.

Examples of main impurities in the SN powder according to the present invention include Fe, Al, Ca, C and oxygen. The amount of inevitable impurities derived from industrial raw materials is about 180 μg of Fe, 730 μg of Al, 80 μg of Ca, and about 2800 μg of C per 1 g of SN powder. The oxygen contained in the SN powder is derived from SiO2 and inevitable impurities including oxygen, and is usually about 1.2% by mass.

The sintering aid according to the present invention is not particularly limited, but includes rare earth elements such as Y, La, Ce, Ho, Yb, Gd, Nb, Sm, and Dy, alkaline earth elements such as Mg, Ca, and Sr. The use of one kind or two or more kinds of oxide, fluoride, etc. is common.

As for the usage-amount of a sintering auxiliary agent, 5-15 mass parts is preferable with respect to 100 mass parts of SN powder. If the amount used is less than 5 parts by mass, a dense SN substrate may not be obtained. On the other hand, if it exceeds 15 parts by mass, the grain boundary phase in the substrate increases and the thermal conductivity of the SN substrate decreases. There is a case.

In order to manufacture the SN substrate according to the present invention, the SN powder and the sintering aid are mixed and molded. Moreover, an organic binder, a solvent, a plasticizer, etc. can be added suitably as needed in the range which does not affect the characteristic of SN board | substrate of this invention.
The mixing of the SN powder and the sintering aid, the organic binder, the plasticizer, the solvent, etc. is not particularly limited, and a known mixing method such as a universal mixer, a raking machine, a mixer, a vibrating sieve, a ball mill, or a rod mill can be used. All raw materials may be mixed at once, or some raw materials such as organic binder, plasticizer, solvent, etc. are mixed first, then SN powder and sintering aid are added later and mixed. It doesn't matter. Further, the mixed raw material powder may be molded as it is, or may be molded after granulation once to increase the molding density. In the present invention, the molding method is not particularly limited, and a known molding method such as a doctor blade method, an extrusion molding method, a dry press method, an injection molding method, or a slip casting method can be used.

In this invention, the organic binder used for shaping | molding is not specifically limited, A well-known thing can be used. Illustrating the types of organic binders, it is possible to use a single organic cellulose binder or an acrylic organic binder alone or in combination. As for the usage-amount of an organic binder, 0.1-15 mass parts is preferable with respect to 100 mass parts of SN powder. When using it as a liquid, the solid content of the organic binder is adjusted to fall within the above range. If the amount of the organic binder used is less than 0.1 parts by mass, sufficient strength of the molded body may not be obtained and cracking may occur. On the other hand, if it exceeds 15 parts by mass, the binder is removed during degreasing. Since the body density decreases, the shrinkage rate during sintering increases, which may cause dimensional defects and deformation.

The plasticizer which concerns on this invention is not specifically limited, A well-known plasticizer can be used. Examples of plasticizers include purified glycerin, glycerin trioleate and diethylene glycol. As for the usage-amount of a plasticizer, 0.1-3 mass parts is preferable with respect to 100 mass parts of SN powder. When the amount used is less than 0.1 parts by mass, the flexibility of the molded sheet becomes insufficient, the molded body becomes brittle during press molding, and the sheet tends to crack. On the other hand, if the amount exceeds 3 parts by mass, it becomes difficult to maintain the sheet shape, which may cause uneven thickness in the sheet width direction.

Further, when a solvent is used in the present invention, examples of the solvent include ethanol and toluene, but ion-exchanged water or pure water can be used in consideration of consideration for the global environment and handling of explosion-proof equipment. As for the usage-amount of a solvent, 15-20 mass parts is preferable with respect to 100 mass parts of SN powder. If the amount used is less than 15 parts by mass, sheet molding may be hindered. On the other hand, if it exceeds 20 parts by mass, it will be difficult to maintain the sheet shape, and thickness unevenness in the sheet width direction may occur.

For degreasing the molded body, it is preferable to remove the organic binder by heating at 350 to 450 ° C. for 3 to 15 hours in an air stream such as nitrogen gas or air. The residual carbon content after the heat degreasing treatment in the molded body is preferably 2.0% by mass or less. If the residual carbon content exceeds 2.0 mass%, sintering may be inhibited and a dense substrate may not be obtained.

The degreased molded body is preferably fired at a temperature of 1600 to 1900 ° C. under a pressure of 0.6 MPa or more in a non-oxidizing atmosphere such as nitrogen gas or argon gas. If the pressure is less than 0.6 MPa, SN may be decomposed during sintering, and the substrate may not be obtained. When the sintering temperature is less than 1600 ° C., the sintering is insufficient and the density of the substrate does not increase, so that the thermal conductivity and the bending strength may be insufficient. On the other hand, if the sintering temperature exceeds 1900 ° C., the sintering aid may be scattered in the firing furnace, which may make it difficult to achieve densification. The firing time is preferably as short as possible. That is, the firing time in the temperature region where the substrate density is 98% or more is within 10 hours, more preferably within 5 hours.

When the SN substrate according to the present invention is used as a substrate, the thickness varies depending on the purpose of use, but a thickness of about 0.2 to 1.0 mm is common.

In the present invention, the SN substrate is ground, and the centerline average roughness (Ra) after grinding is 0.1 to 0.8 μm and the maximum height (Ry) is 5 μm or less. Necessary for 80 W / m · K or more and for improving reliability. When the center line average roughness (Ra) and the maximum height (Ry) satisfy the above ranges at the same time, an SN substrate satisfying both thermal conductivity and reliability can be obtained. When the center line average roughness (Ra) after grinding is less than 0.1 μm, the productivity is significantly reduced. On the other hand, when it exceeds 0.8 μm, the reliability may not be sufficiently improved. Similarly, when the maximum height (Ry) exceeds 5 μm, the reliability may not be sufficiently improved.

The surface layer of the SN substrate is preferably removed by grinding 5 μm or more. If the thickness is less than 5 μm, the surface layer covered with the amorphous sintering aid and the columnar crystal short axis particles having low thermal conductivity may not be completely removed.

The SN substrate grinding means according to the present invention is not particularly limited, and a known method generally used for ceramic processing can be employed. For example, a surface grinder, a double-side grinder, a buff grinder, a lap Examples thereof include a surface polishing machine using a processing machine.

As the metal plate according to the present invention, copper, aluminum, tungsten, molybdenum, an alloy containing these, or the like is used, and copper, aluminum, or an alloy thereof is common. The thickness of a metal plate is not specifically limited, It determines suitably according to the electric current which flows. In general, those having a thickness of 0.1 to 1.0 mm are often used.

Either a DBC method or an active metal brazing method can be employed as a method for joining the SN substrate and the metal plate. The brazing material used in the active metal brazing method is mainly composed of silver and copper and an active metal as a minor component. Specific examples of the active metal include titanium, zirconium, hafnium, niobium, tantalum, vanadium, and compounds thereof. An example of the ratio of these metal components is 1 to 10 parts by mass of active metal per 100 parts by mass in total of 80 to 97 parts by mass of silver and 20 to 3 parts by mass of copper.

The brazing material may be used as a foil or powder, but is preferably used as a paste. The paste can be prepared by adding an organic solvent and, if necessary, an organic binder to the metal component of the brazing material, and mixing with a known mixer such as a roll, a kneader, a universal mixer, a raker, or the like. As the organic solvent, methyl cellosolve, terpineol, isophorone, toluene and the like can be used, and as the organic binder, ethyl cellulose, methyl cellulose, polymethacrylate and the like can be used.

The coating amount of the brazing material is preferably 5 to 20 mg / cm 2 on a dry basis. If the coating amount is less than 5 mg / cm 2, an unreacted part may occur. On the other hand, if it exceeds 20 mg / cm 2, the brazing material oozes out during bonding, and the metal plate may be eroded, which may adversely affect the quality. . The coating method is not particularly limited, and a known coating method such as a screen printing method or a roll coater method can be employed.

In the case of the active metal brazing method, a joined body is manufactured by interposing a brazing material between an SN substrate and a metal plate, and heating and cooling in a vacuum. The brazing material may be applied or disposed on either the SN substrate or the metal plate. When using alloy foil, it is also possible to clad a metal plate and alloy foil beforehand. The heating conditions are appropriately determined according to the brazing material to be used. For example, when the composition of the brazing material is 90 parts by mass of silver and 5 parts by mass of zirconium per 100 parts by mass of copper, the temperature is 830 to 860 ° C. Bonding is performed for 30 to 60 minutes under conditions of a degree of vacuum of 1 × 10 −10 to 5 × 10 −5 Pa.

In order to form a metal circuit on the joined body of the SN substrate and the metal plate, an etching resist is applied to the metal surface and etched. The method of etching the circuit pattern shape on the metal plate is not particularly limited, and a method of etching after drawing a circuit pattern on the metal plate with an etching resist is common. A known method can be used to remove the etching resist. As the etching resist, a known ultraviolet curing type or thermosetting type can be used. Further, as the etching solution, a suitable etching solution is selected according to the type of the metal plate to be used. For example, when the metal plate is copper, solutions of ferric chloride solution, cupric chloride solution, sulfuric acid, hydrogen peroxide solution, etc. are used, and preferably ferric chloride solution, cupric chloride solution. Is mentioned.

Between the metal circuits on the circuit board where unnecessary metal parts have been removed by etching, the applied brazing material, its alloy layer, nitride layer, and the brazing material unnecessary for forming the circuit that protrudes outside the metal circuit pattern remain. Therefore, it is preferable to remove unnecessary brazing filler metal using an aqueous solution of ammonium halide such as NH4F as the first treatment and a solution containing an inorganic acid such as sulfuric acid and nitric acid and hydrogen peroxide solution as the second treatment. . The concentration of the inorganic acid is generally 2 to 4% by mass, and the concentration of hydrogen peroxide is generally 0.5 to 1% by mass. Thereafter, all the etching resist is removed with an alkaline solution.

In the present invention, the circuit can be subjected to a plating treatment such as nickel plating as necessary. In that case, the plating resist is not particularly limited, and solvent dry type ink, UV curable type ink, and the like can be used. The coating method is not particularly limited, and a known coating method such as a screen printing method or a roll coater method can be employed. The coating thickness is preferably applied so as to have a thickness of 0.005 to 0.07 mm after drying. When the thickness is less than 0.005 mm, the metal may be partially exposed. On the other hand, when the thickness is more than 0.07 mm, it takes time to remove the plating resist, and the productivity may be lowered.

The plating treatment is not particularly limited, but electroless nickel plating, electroless nickel gold plating, and solder plating are preferable from the viewpoint of workability and cost. Although the thickness of a plating layer is not specifically limited, 2-8 micrometers is preferable. If the plating thickness is less than 2 μm, it may adversely affect mounting characteristics such as solder wettability and wire bonding characteristics. On the other hand, if the plating thickness exceeds 8 μm, the substrate characteristics may be adversely affected due to peeling of the plating film or the like.

The method for removing the plating resist is not particularly limited, and examples thereof include a method of removing using an organic solvent such as ethanol and toluene, and a method of immersing in an alkaline aqueous solution.

The circuit board manufactured as described above is joined to electronic components such as a base plate and a semiconductor element by soldering. The kind of the solder is not particularly limited, and lead-free solder can be used in addition to the commonly used tin-lead eutectic solder. The kind of lead-free solder is not particularly limited, and is Sn-Ag-Cu, Sn-Cu, Sn-Zn, Sn-Bi, Sn-Ag, Sn-Ag-Cu-Bi, Sn-Ag. Known compositions such as -In-Bi system and Sn-Sb system can be used. The soldering method is not particularly limited. For example, the solder paste is applied to a predetermined portion by a screen printing method or the like, the component is mounted, and soldered by being placed in a furnace having a predetermined temperature at which the solder melts. . In view of the reliability of the circuit board, it is preferable that the solder does not contact the side surface of the metal plate.

Example 1
In advance, 3 parts by mass of oleic acid was added to 100 parts by mass of SN powder, and the surface treatment of the SN powder was performed by mixing for 2 minutes by mixer mixing. After the surface treatment, 10 parts by weight of organic binder and 8.5 parts by weight of sintering aid (37 parts by weight of Y 2 O and 1.5 parts by weight of MgO) are added to 100 parts by weight of SN powder and mixed by a Bourton mixer. did.
The mixed powder is put into a mixer, and while stirring, a mixed solution composed of a plasticizer and ion-exchanged water is 3 parts by mass of plasticizer and 18 parts by mass of ion-exchanged water with respect to 100 parts by mass of the mixed powder. The mixture was sprayed with compressed air (0.2 MPa) to prepare a granular wet powder material.
<Raw materials>
Silicon nitride powder: manufactured by Denki Kagaku Kogyo Co., Ltd., trade name “DENKA SILICON NITRIDE” Grade name “NP-600” (alpha conversion 87%, D50 value 0.7 μm, D90 value 1.9 μm, D100 value 4.6 μm) Specific surface area 12 m 2 / g, oxygen content 1.2% by mass).
Y2O3: manufactured by Shin-Etsu Chemical Co., Ltd., trade name “Yttrium Oxide”, D50 powder particle size 1.0 μm
MgO: manufactured by Iwatani Chemical Co., Ltd., trade name “MTK-30”, D50 powder particle size 0.2 μm, specific surface area 160 m 2 / g
Organic binder: hydroxypropyl methylcellulose. Product name "Metrouse 65SH" manufactured by Shin-Etsu Chemical Co., Ltd.
・ Plasticizer: manufactured by Kao Corporation, trade name “EXCEPARL”, main component glycerin

Next, it was press-molded at a pressure of 30 MPa to produce a sheet having a size of 30 mm × 40 mm. After drying with a belt dryer held at 100 ° C. until the moisture content of the sheet reaches 2% by mass, it is filled in a boron nitride container and kept at 500 ° C. in the atmosphere under normal pressure for 3 hours. After degreasing, a carbon nitride electric furnace was used and maintained at 1800 ° C. for 3 hours in a nitrogen pressurized atmosphere of 0.8 MPa to produce a silicon nitride substrate.

The obtained silicon nitride substrate was surface ground using a surface grinding machine (manufactured by Okamoto Machine Tool Manufacturing Co., Ltd., model PSG64DX) to produce a silicon nitride substrate having a thickness of 0.6 mm, and surface roughness measurement (Ra, Ry), The thermal conductivity was measured. The results are shown in Table 1.
<Measuring method>
・ Surface roughness measurement: Conforms to JIS B0601-1994 (Mitutoyo ・ Model SJ-301 ・ Measurement length 0.8mm)
・ Thermal conductivity measurement: Uses a thermal conductivity measuring machine (vacuum Riko, model TC-7000, laser flash method)

<Production of SN circuit board>
In order to evaluate the performance as a circuit board, an oxygen-free copper plate was used for the metal circuit and the metal heat sink, and bonding and circuit pattern formation were performed by the following method.

30 parts by mass of terpineol is added to 100 parts by mass of mixed powder composed of 85% by mass of Ag, 10% by mass of Cu, 2% by mass of Zr, and 3% by mass of TiH, and a paste-like mixed liquid is prepared. It applied with the screen printer so that it might become a coating amount of 5 mg / cm <2> on a dry standard. Next, an oxygen-free copper plate having a thickness of 2.5 inches × 2 inches × 0.01 inches was attached to both sides. 14 layers of SN sintered body with oxygen-free copper plates pasted on each other were stacked, and installed on a carbon jig by tightening carbon screws and held at 850 ° C. for 45 minutes to produce a bonded body of silicon nitride substrate and copper plate did. In order to form a circuit pattern of a predetermined shape on one main surface of the joined body and a heat sink pattern on the other main surface, a UV curable resist ink is screen-printed and then irradiated with a UV lamp to form a resist film. Was cured. Next, the portion other than the portion where the resist was applied was etched with a cupric chloride solution, and then the resist was peeled off with an aqueous ammonium fluoride solution to produce a copper circuit silicon nitride circuit board. A heat cycle test was performed using the produced silicon nitride circuit board. The results are shown in Table 1.

<Materials used>
・ Oxygen-free copper plate: Oxygen-free copper plate (JIS H 3100) manufactured by Sumitomo Metal Mining Copper Co., Ltd.
UV curable resist ink: "PER-27B-6" manufactured by Kyoyo Chemical Co., Ltd.
<Measuring method>
Heat cycle test: A thermal history was performed for 3000 cycles on the circuit board, with a cycle of 10 minutes at -25 ° C, 10 minutes at 25 ° C, 10 minutes at 125 ° C and 10 minutes at 25 ° C. The presence or absence of the occurrence of a joint crack was confirmed by using a scanning ultrasonic flaw detector (Honda Electronics, Model HA701W). The case where a joint crack occurred in ˜3000 cycles was designated as symbol B, and the case where no joint crack occurred even in 3000 cycles was designated as symbol A.

(Examples 2 and 3, Comparative Examples 1 to 3)
The same procedure as in Example 1 was performed except that the surface grinding amount and grinding method of the produced silicon nitride substrate were changed. The results are shown in Table 1.

Claims (4)

In the silicon nitride substrate from which the surface layer of the silicon nitride substrate has been removed by grinding, the center line average roughness (Ra) after grinding is 0.1 to 0.8 μm and the maximum height (Ry) is 5 μm or less, and heat conduction A silicon nitride substrate having a rate of 80 W / m · K or more. 2. The silicon nitride substrate according to claim 1, wherein a surface layer of the silicon nitride substrate is removed by 5 μm or more. A silicon nitride circuit board comprising a metal circuit formed on one main surface of the silicon nitride substrate according to claim 1 and a heat sink attached to the other main surface. A module using the silicon nitride circuit board according to claim 3.