JP3529055B2 - Insulating heat sink - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an insulated heat radiating plate excellent in heat radiation and reliability, and more particularly to a device mounted with elements such as a power transistor, a power diode and a power IC. Is what is used for 2. Description of the Related Art As the size of semiconductor devices is reduced, the power consumption per unit area is rapidly increasing, and circuit designers are struggling with measures for heat dissipation. In particular, heat generated from elements such as power transistors, power diodes, and power ICs used in igniters and welding power sources is large, and the heat is dissipated. This is used as an insulating heatsink and soldered to a heat sink made of metal such as copper or aluminum. If a copper heat spreader or a copper heat spreader is used on the element mounting side as necessary, Mo The element was soldered through the foil. [0003] However, in the case of an insulating radiator in which an alumina substrate is provided with a thick film metallization, the thermal conductivity of alumina is not sufficiently high, so that it is limited to use in a region where there is a margin in element performance. was there. Therefore, it is conceivable to use an aluminum nitride substrate instead of the alumina substrate. Aluminum nitride has the same electrical properties as alumina, but has a thermal conductivity of 5 to 13 times that of alumina.
There is a feature that the thermal expansion coefficient is twice that of Si. The characteristic of the thermal expansion coefficient close to that of Si is that it enables direct mounting of a large-sized Si element, but has a problem that cracks occur in the aluminum nitride substrate and the solder layer due to thermal stress caused by a thermal cycle. This is due to the difference in thermal expansion between aluminum nitride and the heat spreader material when using aluminum nitride and a heat sink material, or when using a heat spreader, and because a larger stress is generated than when using alumina. [0005] From the above, when the alumina substrate is replaced with an aluminum nitride substrate in an insulating radiator in which a thick film metallization is applied to an alumina substrate, cracks occur in the aluminum nitride substrate and insulation failure occurs. Also,
When an Si element is directly mounted on an aluminum nitride substrate with a thick film metallized on an insulating radiator without a heat spreader, there is no difference in expansion between the Si element and the insulating radiator, so the thermal stress generated is extremely small. Destruction of the board is unlikely to occur, but large stress is generated on the heat sink side, and a horizontal crack is generated in the solder layer joining the heat sink and the insulating heat sink with respect to the joint surface, and the heat transfer area is reduced due to the reduction of the heat transfer area Progresses to peeling. SUMMARY OF THE INVENTION An object of the present invention is to provide an insulating and heat radiating plate which does not cause cracks in a ceramic substrate or a solder layer due to thermal stress caused by a thermal cycle and has excellent heat radiating properties and reliability. Aim. That is, the present invention provides a table
E of the grain boundary layer existing in a portion within 100 μm from the surface
s is 8 to 20%, ± 100 μm from the center in the thickness direction
The ratio Ei of the grain boundary layer existing in the portion within m is 16% or less
And a copper foil having a thickness of 0.075 to 0.2 mm is provided on each of the element mounting side and the heat sink side of the aluminum nitride substrate having Es / Ei of 1.25 or more. It is an insulating heat sink. Hereinafter, the present invention will be described in more detail. The metal foil types formed on the surface of the ceramic substrate in the present invention include copper, aluminum, a copper-aluminum alloy, an iron-nickel alloy, and the like. General. The thickness of the metal foil is not particularly limited, but is preferably 0.05 to 0.2 mm so that the metal foil functions as a buffer layer for thermal stress. [0009] In particular, when the metal foil type is copper, the thickness is set to 0.075 mm or more, particularly 0.1 mm or more, which is generated when the copper foil is bonded to the aluminum nitride substrate by the active metal method. Wrinkles of the copper foil can be prevented. As the ceramic substrate used in the present invention, an aluminum nitride substrate, a beryllia substrate, an alumina substrate, a silicon nitride substrate and the like can be mentioned. An aluminum nitride substrate is preferable, and the sintering density is mechanical. From the viewpoint of strength and electrical characteristics, it is desirable that the relative density is 95% or more. Among them, the ratio Es of the grain boundary layer existing in a portion within 100 μm from the surface is 8 to 20%, and the ratio Ei of the grain boundary layer existing in a portion within ± 100 μm from the center in the thickness direction is 16%. It is preferable that the aluminum nitride substrate having Es / Ei of 1.25 or more has a high durability against a thermal cycle. Such an aluminum nitride substrate contains 3 to 5% by weight, based on the aluminum nitride powder, of a sintering aid, for example, a rare earth oxide such as yttria or ceria, or an alkaline earth oxide such as calcia or magnesia. After firing the resulting green sheet in a non-oxidizing atmosphere such as nitrogen or argon, the green sheet can be manufactured by cooling to a temperature of 1700 ° C. at a cooling rate of 8 ° C./min or less. The ratio Es or Ei of the grain boundary layer can be measured by taking a COMPO image of a section of the aluminum nitride substrate with a scanning electron microscope (SEM) and processing the image. For example, when the molecular weight of the grain boundary component (sintering aid component) is larger than that of aluminum nitride, the grain boundary component looks white. Image processing is performed so that the grain boundary layer is white and the aluminum nitride particles are black in order to unify the contrast of the photograph. The chromaticity (L value) of the image-processed photo is measured with a color difference meter, and the value is corrected so that the value is 100 when the image is completely white and 0 when the image is completely black, and the L value of the photograph is obtained. From the L value, the ratio Es or Ei of the grain boundary layer is determined according to a calibration curve created in advance.
If it is difficult to discriminate based on the difference in molecular weight between the grain boundary component and aluminum nitride, a surface analysis is performed using an EPMA device,
Similarly, it can be performed by obtaining the L value. The magnification of the scanning electron microscope or the EPMA apparatus is suitably about 500 times, and it is desirable to adjust the size of the photograph so that a circle having a diameter of about 100 μm is contained as a measurement range of the color difference meter. For details of the ratio Es or Ei of the grain boundary layer and the measuring method thereof, see Japanese Patent Application No. 6-17 / 1990.
640. [0014] The thickness of the ceramic substrate is a typical thickness of 0. 0, which is used for a circuit board for a power module.
It can be thinner than 635mm, 0.4m
There is no problem if it is more than m. As a method of bonding a metal foil to a ceramic substrate, an active metal method using an active metal brazing and a DBC method are generally used. According to the active metal method, a paste of active metal brazing is applied to the surface of a ceramic substrate and then a metal foil is placed in contact therewith, or a metal foil is placed in contact with an active metal brazing foil sandwiched therebetween, and 1 × 10 ^{− The} joining process is performed in a vacuum of ⁴ torr or less, at a temperature equal to or higher than the melting point of the active metal brazing, generally at a temperature about 50 ° C. higher than the melting point of the active metal brazing. The active metal wax used in the active metal method includes:
There is no particular limitation as long as it has a melting point equal to or higher than the melting point of the solder used when assembling the semiconductor component. However, a eutectic silver-copper is generally used. As the active metal species, Ti, Zr, and Hf have been used, but are not limited thereto. As long as they become alloys with those metals or other metals and act as active metals by the joining temperature, Ti, Zr, and Hr are used. , Hf hydrides, carbonates, oxides and other compounds can also be used. In order to form a pattern for mounting an element on a joined body of a ceramic substrate and a metal foil, unnecessary metal foil and unnecessary brazing are removed by a method such as chemical etching, and patterning is performed. It can be performed by performing a process such as plating in response. There are various methods for patterning, such as placing the active metal brazing only on the part to be joined and joining the metal foil that has been patterned in advance, or joining the solid metal foil after placing the active metal brazing only on the part to be joined Then, there is a method of removing unnecessary metal foil by a chemical etching method. The DBC method is used when the metal foil type is copper and the ceramic substrate is an alumina substrate or an aluminum nitride substrate. When the ceramic substrate is an aluminum nitride substrate, the surface thereof is oxidized before use. As the copper foil, one containing a little oxygen, such as tough pitch copper, is preferably used.
The bonding process is performed in a nitrogen stream containing a small amount of oxygen at a temperature of ℃. D
In the BC method, no load is applied at the time of joining.
Even with a copper foil of m, it can be joined without generating wrinkles. The patterning of the copper foil can be performed by the method described above. A ceramic substrate such as an aluminum nitride substrate bonded with a metal foil such as a copper foil has already been put into practical use as a circuit board for a power module such as an inverter. This has a circuit having at least three or four or more islands, and a current flows through the metal foil portion. Regarding the thickness of the metal foil, the circuit side is in the direction of becoming even thicker than the conventional 0.3 mm in order to respond to the recent demand for large current, and the heat sink side is considering the warpage of the circuit board. Make it the same thickness so that it balances with the metal foil on the circuit side,
It is thinner by about 0.05 to 0.1 mm than the metal foil on the circuit side. Such a structure has been avoided as an insulating heat radiating plate in terms of the risk of metal foil peeling and the cost, and a ceramic substrate provided with a thick film metallization has been the mainstream. On the other hand, the insulating radiator plate of the present invention is almost always composed of one island for mounting an element or a heat spreader (sometimes a large current is supplied to the spreader) and is composed of a metal foil. No current flows through the part. When an element or a heat spreader is mounted on the insulating heat radiating plate of the present invention, the metal foil acts as a buffer layer for thermal stress, and the life is extended as compared with a conventional aluminum nitride substrate having a thick metallized insulating heat radiating plate. However, the service life is shorter than that of an insulating radiator in which a thick film metallization is applied to an alumina substrate, and peeling of a metal foil and cracking of a ceramic substrate may occur due to thermal stress of a cooling / heating cycle. To eliminate such a problem, an aluminum nitride substrate is used as the ceramic substrate, and a metal foil having a thickness of 0.07 is used.
This is to use a copper foil of 5 to 0.2 mm, which can withstand a minimum of 2000 cycles of cooling and heating. The present invention will be described more specifically with reference to examples and comparative examples. Example 1 97 parts of aluminum nitride powder ("parts" are "parts by weight", the same applies hereinafter), 3 parts of yttria as a sintering aid, 8 parts of polyvinyl butyral as an organic binder, and 4 parts of butyl phthalate as a plasticizer A slurry comprising 1 part of glycerin triolate as a dispersant and 50 parts of xylene as a solvent was defoamed to adjust the viscosity, and then formed into a sheet by a doctor blade method. After drying, 29 x 29 mm by press punching
The resultant was subjected to a degreasing treatment in nitrogen at 700 ° C. and then baked at 1850 ° C. for 2 hours to produce an aluminum nitride substrate. This substrate had a thermal conductivity of 150 W / mK, a relative density of 99.9%, and dimensions of 25 × 25 × 0.635 mm. In addition, Es value 6.5%, Ei value 6.3%, Es /
Ei was 1.03. An active metal brazing paste was prepared by kneading 75 parts of silver powder, 25 parts of copper powder, 10 parts of zirconium powder, 15 parts of terpineol and 15 parts of a toluene solution of polyisobutyl methacrylate at a solid content of 1.5 parts. did. This was applied on the entire surface of each of the front and back surfaces of the aluminum nitride substrate by screen printing at a rate of 10 mg / cm ² (application amount after drying). Next, a copper foil having a thickness of 0.1 mm is placed in contact therewith, and then put into a furnace, heated at 900 ° C. for 30 minutes under a vacuum of 1 × 10 ^-4 torr, and then cooled at a rate of 2 ° C./min. The assembly was manufactured by cooling at a speed. On the copper foil of the obtained joined body, a UV-curable etching resist was applied by a screen printing method.
After applying in a shape of × 20 mm, the unnecessary copper foil portion is dissolved and removed using a cupric chloride solution, and the unnecessary brazing material and reaction products remaining outside the pattern are dissolved in a 10% ammonium fluoride solution at 60 ° C. Removed. Thereafter, the etching resist was peeled off with a 5% caustic soda solution to obtain an insulating heat sink having a desired shape. Five samples were subjected to Ni-P plating for surface protection to produce five test pieces. As shown in FIG. 1, 30
× 30 × 4mm heat sink copper plate, and 2 on the surface
When a heat spreader made of a copper plate having a size of 0 × 20 × 1 mm was soldered and subjected to a thermal cycle test, no abnormality was observed at all for the five specimens up to 2500 cycles. The thermal cycle test was performed at -40 ° C. in the atmosphere.
After holding for 30 minutes at 25 ° C., leave for 10 minutes at 25 ° C. and further 125
After holding for 30 minutes at 25 ° C. for 10 minutes, one cycle of standing at 25 ° C. for 10 minutes was performed.
Alternatively, when the test piece peeled from the solder layer (solder peeling), the number of thermal cycles at that time was evaluated as the thermal cycle resistance. Examples 2 to 9 Specimens were prepared and evaluated in the same manner as in Example 1 except that the thickness of the copper foil bonded to the aluminum nitride substrate was changed variously. In Examples 2, 3, 7 and 9, 25, 15, 17 and 1 out of the 50 bonded bodies, respectively.
Each of the five sheets had copper foil wrinkles.
The specimens were selected and used as test specimens. Comparative Example 1 100 parts of a tungsten powder (average particle size: 1.5 μm) and a toluene solution of polyisobutyl methacrylate were solidified on each of the front and back surfaces of a 29 × 29 mm compact obtained by press punching in Example 1. 2 parts by volume was diluted with terpineol to form a metallized paste, applied to a 23 × 23 mm shape by screen printing, degreased at 700 ° C. in nitrogen, and baked at 1850 ° C. for 2 hours to form aluminum nitride. An insulating heat sink having a thick metallized substrate was manufactured. The thermal conductivity of this insulating heat sink is 150 W / mK, the relative density is 99.9%, and the dimensions are 25 ×
25 x 0.635 mm, thick metallized area is 20 x 20
mm. Five samples were subjected to Ni-P plating for surface protection to produce five test pieces. A heat sink copper plate and a heat spreader made of the same copper plate as in Example 1 were soldered to this test piece, and a cooling / heating cycle test was carried out.
In all of the sheets, through cracks occurred in the aluminum nitride substrate and insulation failure occurred. Comparative Example 2 Dimensions 25 × 25 × 0.635 mm, Thermal conductivity 20 W / m
Mo-Mn paste is screen-printed on each of the front and back surfaces of a commercially available aluminum oxide substrate of
× 20 mm, and subjected to thick film metallizing treatment at 1450 ° C. in a wet forming gas (nitrogen-hydrogen). This was subjected to Ni-P plating to produce five test pieces. When a heat sink copper plate and a heat spreader made of the same copper plate as in Example 1 were soldered to this test body and a cooling / heating cycle test was carried out, cracks were observed in the solder layer on the heat sink side after 300 cycles, but the solder layer was peeled off. Instead, it had sufficient strength. However, 1
At the time when 500 cycles had elapsed, all five sheets peeled off from the heat sink copper plate. The results of Examples 1 to 9 and Comparative Examples 1 and 2 are summarized in Table 1. [Table 1] Example 10 Except that an aluminum plate of 30 × 30 × 2 mm was used as a heat sink material, the same procedure as in Example 1 was performed up to the cooling / heating cycle test. No abnormality was found at 2500 cycles. Was. Examples 11 to 18 Specimens were prepared in the same manner as in Example 10 except that the thickness of the copper foil bonded to the aluminum nitride substrate was changed variously, and the same evaluation as in Example 1 was performed. In Example 11,
In 12 and 16, there were some wrinkles in the copper foil, and five pieces each having no wrinkles were selected as test specimens. Comparative Example 3 A test specimen was prepared and evaluated in the same manner as in Comparative Example 1 except that a 30 × 30 × 2 mm aluminum plate was used as a heat sink material. Cracks occurred and insulation failure occurred. Comparative Example 4 A test piece was prepared and evaluated in the same manner as in Comparative Example 2 except that a 30 × 30 × 2 mm aluminum plate was used as a heat sink material. All five stripped from the heat sink aluminum plate. The above Examples 10 to 18 and Comparative Examples 3 and 4
Table 2 summarizes the results. [Table 2] Example 19 A cooling and heating cycle test was performed in the same manner as in Example 1 except that a heat sink copper plate of 30 × 30 × 4 mm was soldered only to the back surface of the test piece. No abnormality was found up to 2500 cycles. I couldn't. Examples 20 to 28 Specimens were prepared and evaluated in the same manner as in Example 19 except that the thickness of the copper foil bonded to the aluminum nitride substrate was changed variously. Examples 20, 21 and 25
Since some of the copper foils had wrinkles, five without wrinkles were selected as test specimens. Comparative Example 5 A cooling and heating cycle test was performed in the same manner as in Comparative Example 1 except that a 30 × 30 × 4 mm heat sink copper plate was soldered only to the back surface of the test piece. All five stripped from the heat sink copper plate. Comparative Example 6 A cooling and heating cycle test was performed in the same manner as in Comparative Example 2 except that a heat sink copper plate of 30 × 30 × 4 mm was soldered only to the back surface of the test piece. All five stripped from the heat sink copper plate. The above Examples 19 to 28 and Comparative Examples 5 to 6
Table 3 summarizes the results. [Table 3] Example 29 As an aluminum nitride substrate, an Es value of 8.2%, an Ei value of 5.5%, and an Es / Ei of 1.4 produced by the following method.
A test specimen was prepared in the same manner as in Example 1 except that the test piece No. 9 was used, and the conditions were more severe than in Example 1.
After holding in air at −60 ° C. × 30 minutes, and then standing at 25 ° C. × 15 minutes, and further holding at 150 ° C. × 30 minutes, and then standing at 25 ° C. × 15 minutes as one cycle, the thermal cycling resistance was evaluated. No abnormalities were observed. Incidentally, in the test piece of Example 1, 2000
A substrate crack occurred during the cycle. For a total of 100 parts by weight of 97 parts by weight of aluminum nitride powder and 3 parts by weight of a sintering aid (yttria),
8 parts by weight of polyvinyl butyral as an organic binder, 4 parts by weight of butyl phthalate as a plasticizer, 1 part by weight of glycerin triolate as a dispersant, and xylene 5 as a solvent
0 parts by weight were mixed in a nylon pot for 24 hours. After adjusting the viscosity of the obtained slurry, it was spread into a PET film by doctor blading, and was forcedly dried to form a green sheet having a predetermined thickness. This green sheet was punched out to a size of 60 × 35 mm, ten of them were piled, a tungsten weight was placed thereon, and the organic binder was removed by heating at 500 ° C. for 1 hour in the air. After firing at 1950 ° C, the temperature up to 1700 ° C is increased by 1 ° C.
Per minute. According to the present invention, it is possible to provide an insulating and heat radiating plate which is excellent in heat radiating property and reliability without causing cracks in a ceramic substrate or a solder layer due to thermal stress caused by a thermal cycle.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram for explaining that a heat sink copper plate is soldered to the back surface of a test body manufactured in Example 1 and a copper plate heat spreader is soldered to the front surface. [Description of Signs] 1 heat sink copper plate 2 solder layer 3 copper foil on heat sink side 4 aluminum nitride substrate 5 copper foil on element mounting side 6 solder layer 7 copper plate heat spreader

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshihiko Tsujimura 1 Shinkaicho, Omuta-shi, Fukuoka Prefecture Inside the Omuta Plant of Denki Kagaku Kogyo Co., Ltd. (56) References JP-A-7-69750 (JP, A) JP-A-59- 150453 (JP, A) JP-A-3-114289 (JP, A) JP-A-3-141169 (JP, A) JP-A-59-40404 (JP, A) JP-A-7-257973 (JP, A) JP-B 3-51119 (JP, B2) (58) Field surveyed (Int. Cl. ⁷ , DB name) C04B 41/88 H01L 23/373

Claims (1)

(57) [Claims] [Claim 1] Exists in a portion within 100 μm from the surface
The ratio Es of the grain boundary layer is 8 to 20%, and the central portion in the thickness direction
Of the grain boundary layer existing in a portion within ± 100 μm from
i is 16% or less, and Es / Ei is 1.25 or less
An insulating radiator plate comprising: a copper foil having a thickness of 0.075 to 0.2 mm provided on each of an element mounting side and a heat sink side of an upper aluminum nitride substrate.