JP2010199162A - Method of producing heat sink material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heat sink material consisting of a composite material of a high thermal conductive material whose thermal conductivity is improved and low thermal expansion hard material at low cost. <P>SOLUTION: In a method of producing a heat sink material, when production is carried out along a powder metallurgy process of powder mixing, molding, and sintering, a plurality of local compositions of material powder configuring a composite material is defined as X, an average value of a measured value for X is defined as Xave. When a standard deviation is defined as S, a mixing degree M of the mixed powder, which is represented by M=S/Xave×100, is defined to be ≤20. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for producing a heat sink material made of a composite material of a high thermal conductivity material such as Cu or Ag and a low thermal expansion hard material such as Mo or W.

  In recent years, with the increase in output of integrated circuits such as CPUs and GPUs, the amount of heat generated by electronic devices has increased, and on the other hand, the miniaturization of devices has also progressed. It has become. One of the countermeasures against heat generation of such a high heat generating element is the use of a heat sink material.

  However, the heat sink material does not have to have only high heat dissipation performance, but it may be used together with a semiconductor having a low coefficient of thermal expansion such as GaAs as in the case of a semiconductor device or a certain amount. Therefore, there is a demand for a material that can flexibly adapt characteristics depending on the environment in which it is used.

  Among them, the Cu-Mo composite material is a material that combines the characteristics of Mo with excellent thermal conductivity and low thermal expansion coefficient and high strength, and for heat sink by changing the composition of Cu and Mo. The characteristics as a material can be adjusted. For example, as described in the following patent document, a Cu—W based composite material is known as a material having the same characteristics as this Cu—Mo composite material. Cu-Mo systems are supported in fields where weight reduction is important, such as the space field.

  However, since the reaction of Cu—Mo system is slower than that of Cu—W system, densification is difficult, and the optimum processing conditions for densification have not yet been found.

  In addition, as described in the following patent documents and non-patent documents, a complicated process such as a Cu film treatment on the surface of the Mo particles is required for manufacturing the conventional Cu—Mo composite material.

  Under such circumstances, as represented by the Cu-Mo composite material, the production of the composite material as the heat sink material requires a large amount of money, and the material must be expensive. Therefore, the current situation is that the application must be special.

JP-A-8-78578 JP2003-309232A

T.W.Kirik, S.G.Galdwell and J.J.Oakes: Advances in Powder Metallurgy, Vol.9.pp.115-122 P. Yih and D.D.L.Chung: J. Electric Mater. 24 841-851

  A first object of the present invention is to improve characteristics as a heat sink material made of a composite material of a high thermal conductivity material and a low thermal expansion hard material.

  Another object is to provide a heat sink material made of the composite material with improved thermal conductivity at a low cost.

  The heat sink material made of the composite material according to the present invention is produced in accordance with a powder metallurgy process of powder mixing, molding and sintering.

  In general, it is known that a sintered body having good characteristics can be obtained from a powder in which particles are uniformly mixed. Further, it has been conceptually known that there is a correlation between the mixed state of the composite material and the thermal conductivity of the sintered body.

  This invention quantitatively manages the thermal conductivity characteristics of the obtained sintered material by defining the thermal conductivity of the heat sink material made of the composite material by the mixing degree M of the composite material powder.

  Specifically, when a plurality of local compositions of the raw material powder constituting the composite material is X, the average value of the measured values is Xave, and the standard deviation is S, the mixed powder represented by the following formula is mixed. The degree M is regulated to 20 or less, and the characteristics of the obtained sintered body are managed.

                      M = S / Xave × 100

  In obtaining a composite heat sink material by the powder metallurgy method, a heat sink material excellent in uniformity of heat conduction characteristics and mechanical characteristics is obtained by maintaining the mixing degree M of the mixed powder at a low value of 20 or less. be able to.

The Cu—Mo composite material having the highest thermal conductivity obtained by the present invention has a thermal conductivity of 166.4 W · m−1K−1 and a density of 9.18 g / cm ³ .

The outline of the evaluation method of a mixed state is shown. The microstructure of a sintered compact by the difference in mixing speed is shown. The difference of the heat conduction of the sintered compact by the difference of mixed atmosphere is shown. The heat conduction difference based on the difference in mixing speed is shown. It is the graph which compared the heat conductivity of the preparation sample with commercially available material. It is the graph which compared the density of the preparation sample with commercially available material.

  Embodiment of this invention is described based on the Example which produces Cu-Mo composite material.

  The physical property value of the sintered body sample made of the prepared Cu-Mo composite was measured, and the optimum mixing condition for obtaining high thermal conductivity was searched. Further, in order to examine the influence of the dispersed state (mixed state) of Cu and Mo particles on the thermal conductivity, the dispersed state of the particles was quantitatively evaluated.

(Mixing raw material powder)
Cu and Mo powders shown in Table 1 were mixed as starting materials using a planetary ball mill.

  The composition of the mixed powder was 28.75% by mass of Cu in order to obtain a volume ratio that maximizes densification by liquid phase sintering.

  The weighed raw material powders were mixed using a planetary ball mill. The ball mill pot used was 250 cc made of SUS304, and the ball used was φ5 mm made of SUS304. The rotation speed of the ball mill was three patterns of 250 rpm, 300 rpm, and 400 rpm, and the mixing time was 60 minutes and 600 minutes.

  Mixing was performed dry or wet. In the case of dry mixing, a method of mixing with the atmosphere without replacing the atmosphere in the container and a method of mixing with 99.9% Ar gas were performed.

  In the case of wet mixing, 2-propanol was used as a solvent, and the atmosphere was left as it was without being replaced. After wet mixing, drying was performed for about 1 hour using an evaporator.

  Furthermore, when mixing for 5 hours or more when the inside of the container was hot, milling was interrupted every 5 hours to prevent overheating of the container.

(Molding)
CIP (cold isostatic pressing) was used for forming the mixed powder. A molded amount of about 25 g was weighed from the powder after mixing and sealed in a polystyrene foam container (inner diameter 19 mm). The sealed container was covered with a professional cover (natural rubber latex), and sealed after removing air in the cover with an aspirator. The molding conditions were 1500 kgf / cm ² . After molding, the expanded polystyrene adhering to the sample was removed.

(Sintering)
The molded sample was sintered using a siliconit furnace under Ar flow. At that time, the titanium piece cut out by a drilling machine for the purpose of preventing oxidation of the sample was placed in a furnace together with the sample as a getter, and liquid phase sintering was performed. After the completion of sintering, it was cooled to room temperature in a furnace.

(Characteristic evaluation)
The manufactured sintered body was processed into a diameter of 10 mm and a thickness of about 2 mm with a lathe or a diamond cutter, and the surface was polished with water-resistant abrasive paper was used as a sample for measuring thermal conductivity. A laser flash constant heat constant measuring device TC-7000H type (manufactured by Vacuum Riko Co., Ltd.) was used for the thermal conductivity measurement. The sample after the thermal conductivity measurement was polished again, and the density measurement and the microstructure were observed. The density was measured by the Archimedes method. An optical microscope (manufactured by Nikon) was used for observation of the fine structure.

(Measurement of mixed state)
The degree of mixing was measured using a microstructural photograph. A method for measuring the degree of mixing is shown in FIG. As shown in the figure, a 100 μm line is drawn on the microstructure photograph, and the total lengths of Cu and Mo on this line are ΣLCu and ΣLMo, respectively. Here, the local composition of the sample is defined as X and is defined as follows.

        X = ΣLCu / (ΣLCu + ΣLMo) × 100

  36 local compositions of such samples were measured. Using the average value of these 36 measured values and the standard deviation S, the degree of mixing M is expressed by the following equation.

        M = S / Xave × 100

  When Cu and Mo particles are uniformly dispersed, the mixing degree M shows a small value.

(Influence of mixing speed)
Samples were prepared by changing only the mixing speed (ball mill rotation speed) under the mixing conditions.

  The atmosphere was an Ar atmosphere, and the mixing time was 60 min. The effect of the mixing speed on the sample characteristics of three patterns of 400 rpm high speed (a), 300 rpm intermediate speed (b), and 250 rpm low speed (c) was investigated.

  FIGS. 2A, 2B, and 2C show microstructural photographs of sintered samples prepared from mixed powders having different mixing speeds according to the pattern of the mixing speed.

  In the microstructural photographs in these figures, the black portions are pores, the gray portions are Cu, and the white portions are Mo. As is clear from the figure, only the mixing speed was different, and a large difference was observed in the appearance and microstructure of the sintered body.

  In the sample (c) having a small mixing speed in the fine structure, it was observed that many pores were distributed as a whole. In the sample (a) with a high mixing speed, slightly larger pores were observed although the number was smaller than that of the sample with a low mixing speed.

  Also from the density measurement result, the sample (b) having an intermediate mixing speed had the highest density and was densified, and the mixing degree M was 20 or less. On the other hand, in the case of the high speed (a) and the low speed (c), the degree of mixing M was 20 or more.

  From these results, it can be said that the difference in the mixing speed has a great influence on the density of the sintered body, that is, the degree of mixing M. The higher the mixing speed, the better the density is not affected and there is an optimum mixing speed. If the mixing speed is too high, it is considered that segregation of Cu is caused by the pressure bonding of Cu particles during mixing. If there is a large segregation of Cu, it is considered that the densification is hindered because the dissolution-precipitation reaction does not proceed efficiently during sintering.

  In addition, if there is segregation of Cu before sintering, formation of closed pores surrounded by Mo due to the rearrangement of particles during sintering occurs, and it is assumed that densification is inhibited. Optimization of the mixing speed reduces the pores in the sample and improves the density. Since the thermal conductivity is proportional to the density, it is considered that the increase in the density leads to the improvement of the thermal conductivity as a result.

  In addition, it is considered that the shape change of the particles can occur in the mixing by the ball mill. In particular, the change in shape of the Mo particles from a square shape to a spherical shape promotes the rearrangement of particles during sintering.

(Influence of mixing method and mixing time)
Samples were prepared by changing only the mixing speed and mixing time under mixing conditions. The mixed powder was sintered under the same conditions.

  From these results, it is known that when the atmosphere in mixing is not controlled, the mixed powder is oxidized as the mixing time increases, and the thermal conductivity of the sintered body is lowered. Therefore, for the purpose of suppressing oxidation, dry mixing in an Ar atmosphere and wet mixing using 2-propanol as a solvent were performed.

  The mixing time was 60 minutes and 600 minutes, and all the mixing speeds were 250 rpm. Sintered samples were prepared from the powders mixed under each condition, and the thermal conductivity was compared. The result is shown in FIG.

  As shown in the figure, in the case of mixing for a short time (60 min), the sample mixed in the Ar atmosphere showed higher thermal conductivity.

  In wet mixing with 2-propanol, it must be dried to remove the solvent after mixing.

  In the case of this example, the mixed powder is exposed to the atmosphere for about one hour during drying of the mixed powder, and the apparatus is heated above the boiling point of the solvent, so the mixed powder is significantly more than the sample mixed in the Ar atmosphere. It is thought that it was oxidized.

  On the other hand, in the case of mixing for a long time (600 min), the thermal conductivity of the sample mixed in the Ar atmosphere decreased by about 5%.

  On the other hand, the thermal conductivity of the sample mixed in 2-propanol increased by about 15%. The cause of the decrease in thermal conductivity during long-time mixing in an Ar atmosphere is considered to be the oxidation of the powder by oxygen contained in the Ar gas.

  Moreover, it is speculated that the increase in thermal conductivity accompanying long-time mixing in 2-propanol is caused by the refinement and shape change of Cu and Mo particles due to the increased grinding effect of the ball mill. In the case of mixing using a solvent, it can be said that mixing in an Ar atmosphere is more efficient in consideration of the need to completely remove the solvent before sintering.

(Measurement of mixing degree)
In order to investigate the influence of the dispersion state of Cu and Mo particles on the thermal conductivity, the degree of mixing was measured for five types of sintered samples having different mixing methods, atmospheres, and mixing times.

  All mixing speeds were 250 rpm. The relationship between the measured mixing degree and thermal conductivity is shown in FIG. From this figure, it can be seen that when the mixing speed is equal, the measured mixing degree is different when the mixing method and the mixing time are different. It can also be seen that there is a clear correlation between the thermal conductivity and the degree of mixing. That is, a sample having a small degree of mixing (M) of 20 or less in which Cu and Mo particles are uniformly distributed exhibits high thermal conductivity, whereas a sample having a large degree of mixing has low thermal conductivity. In FIG. 4, the sample mixed in the atmosphere shows a high thermal conductivity, which indicates that the thermal conductivity is governed more strongly by the degree of mixing than by the influence of oxidation. From this, it can be said that Cu and Mo particles need to be distributed as uniformly as possible in order to obtain high thermal conductivity. Therefore, in order to obtain a Cu—Mo composite material having a high thermal conductivity, it is indispensable to prepare a mixed powder in which Cu and Mo particles are uniformly dispersed.

FIG. 5 is a graph comparing the thermal conductivity of the fabricated sample with a commercially available material. FIG. 6 shows a comparison of density. In this comparison, data of commercially available Cu—Mo composite materials cited from catalogs of Allied Materials and AMETEK were used. What is shown as This study is that of this example, and shows three patterns of a high mixing speed, an intermediate speed, and a low speed. Of these three patterns, the middle speed showed the best results. In this example, a thermal conductivity of 166.4 W · m ⁻¹ K ⁻¹ and a density of 9.18 g / cm ³ were obtained. This is improved to a value of about 90% in the thermal conductivity ratio and about 94% in the density ratio as compared with the conventional commercially available material.

  As an example of the present invention, a Cu-Mo composite has been described as an example. However, the present invention is not limited to a Cu-Mo composite and can be applied to the production of heat sink materials of other composite systems.

Claims (4)

In a powder mixing process in a method of producing a heat sink material composed of a composite material by powder metallurgy along the processes of powder mixing, molding, and sintering,
When a plurality of local compositions of the raw material powder constituting the composite material are measured using a microstructural photograph, the average value of the measured values is Xave, and the standard deviation is S,
M = S / Xave × 100
A method for producing a heat sink material made of a composite material having a mixing degree M of the mixed powder represented by ≦ 20. The method for producing a heat sink material according to claim 1, wherein the mixing process is performed in a dry Ar atmosphere. The method for producing a heat sink material according to claim 1, wherein the mixing degree M of the mixed powder is adjusted by the rotational speed of the planetary ball mill. The method for producing a heat sink material according to claim 1, wherein the densified compression molding before sintering is wrapped in a foamed polystyrene and press-molded by CIP.