Thermally conductive composition and thermal grease

Abstract

[Problem] To provide a heat-conductive composition that has excellent heat-dissipating performance and that does not loosen bolts. [Solution] Provided is a heat-conductive composition comprising a first inorganic powder filler, a second inorganic powder filler, and a third inorganic powder filler having an average particle size of 10-100 μm, 1-50 μm, and 0.1-5 μm, respectively. The average particle size ratio of the second filler with respect to the first filler is not more than 0.8 times; the average particle size ratio of the third filler with respect to the second filler is not more than 0.6 times; with respect to all inorganic powder fillers, which is 100 vol%, the first filler is 40-80 vol%, the second filler is 10-50 vol%, and the third filler is 10-40 vol%. The heat-conductive composition further comprises a base oil, a wax-type resin demonstrating a penetration of at least 5 and having a melting point of 40-150°C, a rosin-based resin having a melting point of 40-150°C. With respect to the entire resin content, which is 100 parts by volume, the base oil is 100-1000 parts by volume, and the content ratio of the wax-based resin is 10-60 vol%.

Description

[Technical Field]

[0001] The present invention relates to a thermally conductive composition used between a heat generating element and a heat dissipating element, and to a thermally conductive grease containing the thermally conductive composition and a solvent. [Background technology]

[0002] In electronic devices, a heat sink or other heat sink made of a metal material with high thermal conductivity, such as aluminum or copper, is attached to dissipate heat generated by heat-generating elements such as semiconductor elements or mechanical components. To improve the efficiency of heat transfer to the heat sink, a thermally conductive member is sandwiched between the heat-generating element and the heat sink. These thermally conductive members come in various types, such as thermally conductive sheets (solid) and thermally conductive grease (liquid), and are used depending on the application. Technology related to thermally conductive grease is described, for example, in the following Patent Document 1.

[0003] However, although heat-dissipating sheets are easy to handle and have excellent workability, it is difficult to make them thin because the shapes of fillers that can be used are limited. Furthermore, since it cannot conform to the minute irregularities on the surfaces of electronic components and heat sinks, gaps are created, increasing contact thermal resistance and making it difficult to dissipate heat.

[0004] On the other hand, there are fewer restrictions on the selection of fillers for thermal grease, so it is possible to improve the filling amount and fluidity, making it possible to create a grease with excellent spreadability, which can fill the gap between electronic components and heat sinks and efficiently dissipate heat. However, while thermal grease is prone to thinning, it is paste-like at room temperature and flows when pressurized, making it easy to apply in a thin film and providing excellent adhesion. However, it has the drawback of easily flowing out from the applied area and pumping out due to repeated heat generation and heat dissipation.

[0005] To overcome these drawbacks, development is underway on phase-change heat dissipation components, which are thermally conductive materials that combine the easy handling of heat-dissipating resin sheets with the low thermal resistance of heat-dissipating grease, and have better pump-out resistance than heat-dissipating grease.These components are solid at room temperature, but after being incorporated into a heat-generating body, they absorb heat and soften, adhering closely to the adherend and lowering thermal resistance.Technology related to phase-change heat dissipation components is described, for example, in Patent Documents 2 and 3 below. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 4713161 [Patent Document 2] Patent No. 3794996 [Patent Document 3] International Publication No. 2016 / 185936 Summary of the Invention [Problem to be solved by the invention]

[0007] Phase change heat dissipation materials (hereinafter referred to as "PCM") are thermally conductive compositions that are solid at room temperature, but after being incorporated into a heat generating element, they absorb heat and soften, thereby providing excellent adhesion and low thermal resistance, and are able to exhibit high thermal conductivity and phase change properties. However, while PCMs are easy to handle because they are solid at room temperature, they cannot be applied by printing to fine structures, etc. Therefore, methods have been considered to soften the thermally conductive composition with a volatile solvent to create a paste or liquid thermal grease that can be printed. However, if the thermally conductive composition that forms the base of the thermal grease has poor viscosity, fluidity, smoothness, etc., even if a volatile solvent is used to make it printable, the printed state may be poor, with issues such as blurring and bleeding, and the applied product may not maintain its normal shape, resulting in problems such as insufficient heat dissipation properties.

[0008] Furthermore, thermal greases made into a paste or liquid form by adding a volatile solvent must be dried after printing to fully function as a PCM. Residual volatile solvent components in the printed thermal grease are undesirable because they can worsen adhesion or cause problems due to the release of the solvent components after assembly. Drying the volatile solvent in the thermal grease is typically performed by a drying process at 60–80°C after printing. For this reason, it is preferable to use a volatile solvent that can be reliably removed within this temperature range. After drying, the thermally conductive composition is interposed between the heating element and the heat sink, and the heating element and heat sink are secured with bolts or similar. After tightening and securing the components under a certain load, further heating is required to wet and spread the applied thermally conductive composition throughout the module. However, thermally conductive compositions converted into PCMs are solid at room temperature but soften when heated, increasing their fluidity. This can cause the bolts to loosen.

[0009] This is thought to be due to the difference in thickness between the thermally conductive composition when fastened and the thickness after softening due to heating, which causes the bolts to loosen. When the bolts loosen, the components loosen, causing a deterioration in adhesion and heat dissipation. Furthermore, if a PCM thermally conductive composition fixed between a heating element and a heat sink continues to be used with the bolts loosened, it not only deteriorates heat dissipation but can also lead to damage to the parts. The above-mentioned conventional technologies are insufficient in terms of printability and measures to prevent bolt loosening, so there is a need for the development of a PCM that is easy to work with and has excellent heat dissipation properties, and that is less likely to cause bolt loosening due to the difference in deformation between when tightened and after softening by heating.

[0010] The present invention has been made in view of the above-mentioned conventional circumstances, and aims to provide a thermally conductive composition that is used as a thermally conductive member sandwiched at the thermal boundary surface between a heat generating body and a heat dissipating body such as a heat sink for cooling electronic components, and that has excellent heat dissipation properties and does not loosen bolts. The thermally conductive composition can be used as a heat dissipating sheet.

[0011] Another object of the present invention is to provide a thermally conductive grease with improved workability, which can be applied by screen printing or the like by making the thermally conductive composition, which undergoes a phase change when a volatile solvent is used, into a paste or liquid form in its initial state before use.

[0012] Another object of the present invention is to provide a thermally conductive grease that has excellent printability through the selection of appropriate constituent materials, and that, by volatilizing the solvent after application, can be made into a thermally conductive composition that suppresses the outflow of the thermally conductive composition due to significant viscosity reduction, softening, or melting at temperatures within the operating range of electronic components, which has been a problem with conventional grease products. [Means for solving the problem]

[0013] As a result of extensive research into solving the above problems, the inventors have found that the above problems can be solved by optimizing the type, amount, and particle size ratio of the inorganic powder filler that constitutes the thermally conductive composition, as well as the types and amounts of the base oil, wax-based resin, and rosin-based resin, and that by selecting an appropriate volatile solvent as needed, the thermally conductive composition can be provided in a grease form that is easier to handle.

[0014] That is, a thermally conductive composition according to one embodiment of the present invention comprises (A) an inorganic powder filler, (B) a base oil, (C) a wax-based resin, and (D) a rosin-based resin, wherein the (A) inorganic powder filler is composed of three or more types of powders having different average particle sizes, the first inorganic powder filler having the largest average particle size has an average particle size of 10 μm or more and 100 μm or less, the second inorganic powder filler having the next largest average particle size has an average particle size of 1 μm or more and 50 μm or less, and the third inorganic powder filler having the next largest average particle size has an average particle size of 0.1 μm or more and 5 μm or less, the average particle size ratio of the second inorganic powder filler to the first inorganic powder filler is 0.8 times or less and the average particle size ratio of the third inorganic powder filler to the second inorganic powder filler is 0.6 times or less, and 100 volumes of the (A) inorganic powder filler is % of the total amount of the (C) wax-based resin and the (D) rosin-based resin, the wax-based resin (C) has a penetration of 5 or more and a melting point of 40°C or more and 150°C or less, and the rosin-based resin (D) has a melting point of 40°C or more and 150°C or less, the wax-based resin (C) has a content of 100 parts by volume or more and 1000 parts by volume or less, and the wax-based resin (C) has a content ratio of 10% by volume or more and 60% by volume or less, based on 100 parts by volume of the total amount of the (C) wax-based resin and the (D) rosin-based resin.

[0015] In the thermally conductive composition according to one aspect of the present invention, the base oil (B) is preferably at least one selected from mineral oils, synthetic hydrocarbon oils, diesters, polyol esters, and phenyl ethers.

[0016] In the thermally conductive composition according to one aspect of the present invention, the wax-based resin (C) is preferably at least one selected from the group consisting of paraffin-based wax, ester-based wax, polyolefin-based wax, and acrylic resin.

[0017] In the thermally conductive composition according to one embodiment of the present invention, the inorganic powder filler (A) is preferably at least one selected from copper, aluminum, zinc oxide, magnesium oxide, aluminum oxide, aluminum nitride, and silicon carbide.

[0018] Furthermore, the thermally conductive composition according to one aspect of the present invention preferably further contains at least one of (E) a thixotropy regulator, (F) a surfactant, and (G) a diffusion inhibitor.

[0019] A thermal grease according to one aspect of the present invention is characterized by containing the thermally conductive composition according to any one of the above aspects of the present invention and a volatile solvent. [Effects of the Invention]

[0020] According to the present invention, a thermally conductive composition is obtained that is useful as a heat dissipation material for CPUs, power semiconductors, etc., and that is disposed between a heat generating element and a heat dissipation element such as a heat sink. When placed between a heat generating element and a heat dissipation element such as a heat sink, the composition reduces in viscosity, softens, or melts to adhere to the adherend at temperatures within the operating range of the electronic component, thereby enabling efficient heat dissipation. The composition is also siloxane-free, does not cause contact failure, and provides excellent long-term reliability for electronic devices. Furthermore, by adding a volatile solvent to the composition to form a thermally conductive grease, the composition can be printed and applied to shapes that are difficult to install in sheet form. DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention will be described in more detail below.

[0022] (A) Inorganic powder filler The inorganic powder filler used in the thermally conductive composition of the present invention is not particularly limited as long as it has a higher thermal conductivity than the base oil, but suitable examples include powders of metal oxides, inorganic nitrides, metals, silicon compounds, etc. The thermally conductive composition of the present invention may use one type of inorganic powder filler, or two or more types of inorganic powder fillers in combination.

[0023] When electrical insulation is required, the inorganic powder filler is preferably a powder of a non-conductive material such as a semiconductor or ceramic, such as zinc oxide, magnesium oxide, aluminum oxide, aluminum nitride, boron nitride, silicon carbide, silica, or diamond. Powders of zinc oxide, magnesium oxide, aluminum oxide, aluminum nitride, boron nitride, silicon carbide, or silica are more preferred, and powders of zinc oxide, magnesium oxide, aluminum oxide, aluminum nitride, or silicon carbide are particularly preferred. These inorganic powder fillers may be used alone or in combination of two or more. When electrical insulation is not required but higher thermal conductivity is required, metal powders such as metallic aluminum, metallic silver, or metallic copper are preferably used, with aluminum or copper powder being more preferred. Metal powders may also be used in combination with the powders of the non-conductive materials.

[0024] The inorganic powder filler contains at least three types of powders with different average particle sizes: a first inorganic powder filler with the largest average particle size in the range of 10 μm to 100 μm, a second inorganic powder filler with the next largest average particle size in the range of 1 μm to 50 μm, and a third inorganic powder filler with the next largest average particle size in the range of 0.1 μm to 5 μm. Note that multiple inorganic powder fillers of different types may be contained as long as they have the same average particle size.

[0025] That is, the inorganic powder filler used in the thermally conductive composition of the present invention is composed of a combination of inorganic powder fillers with different average particle sizes of 100 μm or less that satisfy the following relational expressions 1 and 2. The reason why the average particle size of the inorganic powder filler is set to 100 μm or less is because the use of inorganic powder fillers with an average particle size exceeding 100 μm deteriorates fluidity. This is thought to be because coarse inorganic powder fillers exceeding 100 μm tend to cause turbulence in the fluid, and fine particles are not evenly packed between the inorganic powder fillers, thereby impairing fluidity.

[0026] The ratio of the average particle size of each inorganic powder filler is such that the value of R1, represented by the following relational expressions 1 and 2, is 0.8 or less, and the value of R2 is 0.6 or less. If the value of R1 is greater than 0.8 or the value of R2 is greater than 0.6, high frictional forces are generated between the inorganic powder fillers, making it difficult to form a thin film and achieve low thermal resistance, and also worsening printability, which is undesirable. (Relationship 1) R1 = D2 / D1 [In the formula, R1 represents the average particle size ratio of the second inorganic powder filler to the first inorganic powder filler, D1 represents the average particle size of the first inorganic powder filler, and D2 represents the average particle size of the second inorganic powder filler.] (Equation 2) R2 = D3 / D2 [In the formula, R2 represents the average particle size ratio of the third inorganic powder filler to the second inorganic powder filler, D2 represents the average particle size of the second inorganic powder filler, and D3 represents the average particle size of the third inorganic powder filler.]

[0027] In addition, an inorganic powder filler having an average particle size smaller than that of the third inorganic powder filler may be further added, as long as it satisfies the above relational expressions 1 and 2 and does not adversely affect various properties such as fluidity during heating. Furthermore, the inorganic powder filler used in the thermally conductive composition of the present invention contains 40% by volume or more and 80% by volume or less of a first inorganic powder filler, 10% by volume or more and 50% by volume or less of a second inorganic powder filler, and 10% by volume or more and 40% by volume or less of a third inorganic powder filler, relative to 100% by volume of the total inorganic powder filler, which is the sum of the individual inorganic powder fillers.

[0028] (B) Base oil The base oil is not particularly limited, but is preferably at least one selected from mineral oil, synthetic hydrocarbon oil, diester, polyol ester, and phenyl ether, and more preferably mineral oil, polyalphaolefin, ester oil, and phenyl ether. Furthermore, an oil that can dissolve polysulfide, which is an anti-corrosion additive, is desirable.

[0029] (C) Wax-based resin The wax-based resin is a material that solidifies at room temperature and is added to maintain the shape of the thermally conductive composition. It has a melting point of 40°C to 150°C and a penetration of 5 or greater. Having a melting point within the above range allows the resin to remain solid at room temperature but easily deform when heated, making it possible to turn it into a low-viscosity liquid. Furthermore, a material with a penetration within the above range can maintain appropriate softness in its solid state. Using a wax with a penetration of less than 5 results in a wide joint gap between the module and heat sink because the grease does not stretch when joining the module and heat sink. This can lead to significant grease flow upon heating, resulting in loosening and bolt loosening. Using a wax with a high penetration of 5 or greater allows the thermally conductive composition to deform easily during fastening at room temperature, allowing for a certain degree of thinning of the film thickness. This can suppress fluctuations in the joint gap that occur when the grease becomes flowable upon heating, making it less likely to loosen the bolts. Furthermore, adding a volatile solvent can make the grease into a thermally conductive grease at the time of application, making it easier to thin the film.

[0030] The melting point of the wax-based resin is set to 40°C or higher and 150°C or lower because if the melting point is lower than 40°C, the wax-based resin may soften due to the heat generated during operation and flow out of its designated position, while if the melting point is higher than 150°C, the wax-based resin may not soften at all even with the heat generated by the heat-generating component and may not provide a stress relaxation effect. Note that the heat generation temperature of heating elements such as semiconductor elements and machine parts varies depending on the field of use, so a wax-based resin with an appropriate melting point can be selected according to the field of use of the heating element.

[0031] (D) Rosin-based resin The rosin-based resin used has a melting point of 40°C or higher and 150°C or lower. Selecting a resin with a melting point close to that of the wax-based resin is preferable, as it simplifies the softening behavior. By incorporating a rosin-based resin, the thermal conductive composition can be given heat resistance and its viscosity can be adjusted. Furthermore, when a volatile solvent is added to create a grease-like consistency, the smoothness of the resulting film can be improved. This results in good printability and easy plate release during printing. By using a combination of wax-based resin and rosin-based resin, even when applied to thermal grease, the grease's shape retention after printing is improved, and adhesion to heat-generating components such as modules can be improved. Furthermore, because wax-based resin alone has poor deformation performance after heating, adding rosin-based resin can improve this performance. The melting point of the rosin-based resin, like that of wax-based resin, can be selected according to the application field of the heating element.

[0032] The content of (B) base oil is 100 to 1,000 parts by volume per 100 parts by volume of the total of (C) wax-based resin and (D) rosin-based resin, and the content of (C) wax-based resin is 10 to 60% by volume per 100 parts by volume of the total of (C) wax-based resin and (D) rosin-based resin. If the content of base oil is less than 100 parts by volume, the thermally conductive composition will be hard and have low fluidity, which may result in a printed shape that is not smooth and prone to smearing. On the other hand, if the content of base oil exceeds 1,000 parts by volume, the thermally conductive composition will have excessive fluidity, which may lead to sagging and the printed shape may not be maintained at the time of ejection. Alternatively, even if the printed shape is maintained at the time of ejection, the thermally conductive composition may overflow from its intended position during subsequent use due to thermal deformation of the module when heated, which may result in a so-called pump-out phenomenon. The total content of the (B) base oil, (C) wax-based resin, and (D) rosin-based resin is preferably 10% by volume or more and 40% by volume or less relative to 100% by volume of the thermally conductive composition. If it is less than 10% by volume, the thermally conductive composition may not be integrated, and if it is more than 40% by volume, there is a risk that liquid components may flow out due to pumping out or the like when heated, which is not preferable.

[0033] (E) Thixotropy adjuster The thixotropy modifier is a material that may be added as needed to develop appropriate thixotropy when the thermally conductive composition becomes liquid upon heat generation. The thixotropy modifier is not particularly limited, and materials used in general greases can be used, but it is preferable to use at least one selected from bentonite, mica, kaolin, sepiolite, saponite, hectorite, lithium complex soap, aluminum complex soap, carbon nanotubes, and cellulose nanofibers. The inclusion of a thixotropy modifier can also further improve the shape retention of the thermally conductive grease. The content of the thixotropy modifier is preferably 1 part by volume or more and 10 parts by volume or less per 100 parts by volume of the base oil.

[0034] (F) Surfactant The surfactant is a component that may be added as needed to disperse the components in the thermally conductive composition more uniformly during melt stirring during production, and is not particularly limited and may be selected appropriately depending on the compatibility with other components. When added, it is preferable to use at least one selected from the group consisting of acid-based hydrocarbon polymers, higher fatty acid esters, and phosphate esters.

[0035] (G) Diffusion inhibitor The diffusion inhibitor is a component that may be added as needed to suppress bleed-out, and is not particularly limited, but it is preferable to use a diffusion inhibitor of a perfluoroalkyl compound type. Although there is no particular specification for the amount to be added, it is preferable to add 0.01% by mass to 1.5% by mass relative to 100% by mass of the thermally conductive composition. If it is less than 0.01% by mass, no effect can be expected, and if it is added in excess of 1.5% by mass, no further effect can be expected.

[0036] (H) Volatile solvent The volatile solvent is added when the thermally conductive composition is made into a printable thermal grease. The thermally conductive composition of the present invention is a solid at room temperature, so it needs to be diluted with a solvent to make it suitable for printing application. The volatile solvent used in the present invention is not particularly limited, but it is preferable that it can dissolve wax-based resins or rosin-based resins, and more preferably has a solubility parameter of 9.0 cal(1 / 2) / cm(3 / 2) or more and 13.0 cal(1 / 2) / cm(3 / 2) or less. If the solubility parameter is outside the above range, the wax-based resin or rosin-based resin used in the present invention becomes difficult to dissolve in the volatile solvent, which may lead to aggregation of the inorganic powder filler, which is undesirable. If the inorganic powder filler aggregates, problems such as an inability to form a sufficiently thin film may occur, and the desired heat dissipation characteristics may not be achieved, which is undesirable.

[0037] Such volatile solvents include ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-ethyl-1-butanol, 3-methyl-1-butanol, 1-pentanol, 2-pentanol, 1-hexanol, 1-heptanol, 1-octanol, 2,4-dimethyl-3-pentanol, 3-pentanol, 2-ethyl-1-hexanol, 1-nonanol, 3,5,5-trimethyl-1-hexanol, 2-methyl-3-butyn-2-ol, α-terpineol, β-terpineol, γ-terpineol, texanol, diethylene glycol, ethylene glycol dibutyrate, diethylene Examples of the alkyl ether include glycol monobutyl ether, hexyl glycol, methylpropyl triglycol, diethylene glycol monobutyl ether, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, propylene glycol monomethyl ether, phenylpropylene glycol, dipropylene glycol monomethyl ether, cyclopentanone, cyclohexanone, 4-hydroxy-4-methyl-2-pentanone, limonene, methyl ethyl ketone, acetone, acetonitrile, 2,4,4-trimethyl-1,3-pentanediol-1-monoisobutyrate, butyl butyrate, and butyl propionate. As the volatile solvent, one or more of those exemplified above are used. The amount of volatile solvent added should be adjusted so that the shear viscosity of the thermal grease at 6 / s is between 25 Pa·s and 250 Pa·s. This viscosity range is generally considered to provide good printability; if the viscosity is less than 25 Pa·s, it becomes too liquid and may have difficulty maintaining its shape after application, resulting in insufficient printability. If the viscosity is more than 250 Pa·s, the grease may become too hard and difficult to print, which is undesirable. The viscosity of the thermal grease within this range can be selected appropriately based on the printing conditions of the usage environment.

[0038] Furthermore, the volatile solvent used in the present invention is assumed to be completely evaporated during heat drying after printing. If the volatility is poor and solvent components remain in the grease, curing will be insufficient, making it difficult to control the thickness when joining the module and heat sink, and there is also the possibility of adversely affecting surrounding components when mounted. Therefore, it is preferable to confirm in advance whether the amount of solvent added to achieve the above viscosity range will be sufficiently evaporated during heat drying after printing. [Example]

[0039] EXAMPLES The present invention will be described in detail below based on examples and comparative examples of the present invention, but the present invention is not limited to the following examples in any way.

[0040] [evaluation] The formability of the thermally conductive composition, the printability of the prepared thermal grease, and whether or not the thermally conductive composition obtained by applying the thermal grease to a module and then heating it to cause bolt loosening were evaluated using the following methods.

[0041] (Formability of thermally conductive composition (grease formation)) The prepared materials were heated and mixed at 150°C for 30 minutes, and then checked to see if they would combine into a grease-like substance at 150°C. If only a portion of the materials had combined into a grease-like substance, the mixing time was extended to 60 minutes to check whether they would combine. If the resulting thermally conductive composition had combined into a fluid grease-like substance, it was marked "Good." If the composition was clearly not combined after 30 minutes of heating and mixing, or if the composition was not fully combined and did not become grease-like even after extending the heating and mixing time to 60 minutes, it was marked "Poor," indicating that the thermally conductive composition could not be formed, and further evaluation was discontinued.

[0042] (Printability of thermal grease) To print on the module, a module and a 130 μm thick metal mask were prepared. Thermal grease was then applied to the module using the metal mask for printing. If the thermal grease did not sag at the edges and was formed to the exact shape of the metal mask, it was rated as "Good" for good printability. If the shape of the metal mask could not be maintained, causing sagging at the edges, spreading too much, causing fading or chipping in the printed shape, or the viscosity of the grease was too high to print properly, it was rated as "Poor" for bad printability.

[0043] (Loose bolts) The module with thermal grease printed on it and the heat sink, with the thermal grease between them, were tightened at a pressure of 3.5 N·m at room temperature, and then heated and dried in a dryer heated to 70°C for 90 minutes. After the heat drying process, the module was removed from the dryer and allowed to cool sufficiently, after which the bolts were checked for looseness at a pressure of 2.5 N·m. If the bolts could not be turned, the pressure was increased in 0.1 N·m increments, until they were tightened to 3.5 N·m and looseness was checked. If the bolts could not be turned and there was no looseness up to 3.5 N·m, the result was marked "Good", and if the bolts turned while checking the tightening after increasing the pressure from 2.5 N·m to 3.5 N·m, the result was marked "Poor".

[0044] Example 1 The thermally conductive composition to be produced was prepared in a ratio of 100% by volume, with 0.5% by volume of thixotropy modifier, 1.6% by volume of surfactant, and a total of 22.9% by volume of base oil, wax-based resin, and rosin-based resin, as shown in Table 1, i.e., 19.08% by volume of base oil, 1.34% by volume of wax-based resin, and 2.48% by volume of rosin-based resin. The materials were then placed in a universal mixer and heated to 150°C, followed by stirring until the wax-based resin and rosin-based resin were melted. In addition, 75 volume% of inorganic powder fillers were prepared in the proportions shown in Table 1, i.e., 45 volume% of alumina powder (first inorganic powder filler) having an average particle size of 20 μm, 15 volume% of alumina powder (second inorganic powder filler) having an average particle size of 10 μm, and 15 volume% of zinc oxide powder (third inorganic powder filler) having an average particle size of 3 μm. In addition, the average particle size ratio between the first inorganic powder filler and the second inorganic powder filler was 0.5, and the average particle size ratio between the second inorganic powder filler and the third inorganic powder filler was 0.3. The three inorganic powder fillers prepared were added to the base oil in which the thixotropy modifier, wax-based resin, rosin-based resin, and surfactant had been dissolved, and the mixture was stirred for 30 minutes while heating at 150°C to obtain the thermally conductive composition of Example 1. Table 1 shows some of the data, such as the characteristics and blending ratios of the materials used.

[0045] (Grease confirmation) Immediately after the stirring was completed, the resulting thermally conductive composition was checked to see if it had been unified into a grease-like substance at a temperature of 150°C. As a result, it was confirmed that the composition had been unified into a grease-like substance and had fluidity. The evaluation results are shown in Table 2. After confirming that the mixture had been integrated into a grease-like state while heated, the temperature of the stirrer was lowered to 70°C and vacuum degassing was carried out to remove the air layer present inside the thermally conductive composition. A thermally conductive composition having a total volume of 100 parts by volume was mixed with 12 parts by volume of terpineol, a volatile solvent, and stirred for 5 minutes to obtain a thermally conductive grease.

[0046] (Printability check) When the thermal grease printed using the metal mask was checked, it was confirmed to be printed well without any smearing or bleeding. The evaluation results are shown in Table 2.

[0047] (Check for loose bolts) A sample consisting of a module with thermal grease printed on it and a heat sink fastened together at a pressure of 3.5 N m was heated and dried, and the bolts were checked for loosening. The bolts did not turn at a pressure of 2.5 N m, and no loosening was observed even when tightened to 3.5 N m. The evaluation results are shown in Table 2.

[0048] (Examples 2 to 6, Comparative Examples 1 to 10) Thermally conductive compositions and thermally conductive greases were prepared and evaluated in the same manner as in Example 1, except that the particle size and blending ratio of the inorganic powder filler, the type and blending ratio of the wax and rosin, and the blending ratio of the base oil to the total content of the wax and rosin were changed as shown in Table 1. The evaluation results for each of the Examples and Comparative Examples are shown in Table 2. The first and second inorganic powder fillers were alumina powder, the third inorganic powder filler was zinc oxide powder, and the fourth finer inorganic powder filler added only in Example 6 was also alumina powder.

[0049] [Table 1] R1=D2 / D1 R2=D3 / D2

[0050] [Table 2]

[0051] It was confirmed that the samples of Examples 1 to 6 of the present invention were able to be integrated into a grease-like form by heating and mixing the materials, the printability of the heat-dissipating grease was good, and no loosening of the bolts occurred after heat treatment.

[0052] The sample of Comparative Example 1, which was made using an alumina powder with an average particle size of 110 μm (greater than 100 μm) as the first inorganic powder filler, and the sample of Comparative Example 2, which was made using an alumina powder with an average particle size of 52 μm (greater than 50 μm) as the second inorganic powder filler and a zinc oxide powder with an average particle size of 6 μm (greater than 5 μm) as the third inorganic powder filler, were able to be integrated into a grease-like state and no loosening of the bolts occurred, but the fluidity of the thermal grease state was poor, causing smearing during printing and preventing the desired shape from being obtained. Therefore, there is concern that the samples of Comparative Examples 1 and 2 have poor thermal conductivity.

[0053] In the sample of Comparative Example 3, in which the difference in particle size between the first inorganic powder filler and the second inorganic powder filler was too small, and in the sample of Comparative Example 4, in which the difference in particle size between the second inorganic powder filler and the third inorganic powder filler was too small, the inorganic powder filler remained in a powdery state to the extent that it aggregated to some extent even after continued stirring, so it was determined that it could not be made into a grease, and the evaluation was discontinued. This is thought to be because the small particle size inorganic powder filler could not be effectively inserted into the gaps by utilizing the difference in particle size, and the filling rate of the inorganic powder filler could not be increased, so the gaps became wider than necessary, and the amount of liquid component required to fill the gaps was insufficient.

[0054] The sample of Comparative Example 5, in which the average particle sizes of the first inorganic powder filler and the second inorganic powder filler were too small and the blending ratio of the first inorganic powder filler was too high, and the sample of Comparative Example 6, in which the blending ratio of the first inorganic powder filler was too low and the blending ratio of the second inorganic powder filler was too high, also failed to turn into grease even after continued stirring, and evaluation was discontinued. This is thought to be because the sample of Comparative Example 5 had too large a surface area, and the amount of liquid component was insufficient to cover all of the powder surfaces, and the sample of Comparative Example 6 had too much second inorganic powder filler and could not fit into the gaps in the first inorganic powder filler, making it impossible to increase the filling rate of the inorganic powder filler, resulting in an insufficient amount of liquid component, and also because the ratio of the amount of base oil to the total amount of resin was too low.

[0055] The sample in Comparative Example 7, which was prepared using a wax with a high melting point of 160°C and a low penetration of 3, and a rosin with a low melting point of 37°C, and in which the base oil content was increased beyond the upper limit of the range of the present invention, was successfully greased. However, the excessive base oil resulted in excessive fluidity, resulting in a lack of edge formation and drooping after printing, making it impossible to obtain the desired shape. Furthermore, when checking for bolt loosening, the bolts did not loosen at 2.5 N·m, but at 3.0 N·m, they began to turn, confirming looseness. This is thought to be due to the inappropriate melting points of the wax-based resin and rosin-based resin, which resulted in insufficient dissolution in the base oil during heating and mixing. Under pressure above a certain level, the resins deformed but did not return to their original shape. Furthermore, the high amount of base oil, resulting in excessive fluidity, caused some of the thermally conductive composition to leak out when heated under pressure.

[0056] The sample of Comparative Example 8, which was prepared by increasing the ratio of rosin in the resin and decreasing the ratio of the amount of base oil to the total amount of resin, turned into a grease by heating and mixing, but it became a hard thermally conductive composition with low fluidity. As a result, the printed shape after application was not smooth, and some smearing was observed, resulting in an unsatisfactory shape. Furthermore, although no bolt loosening occurred, there are concerns about poor thermal conductivity.

[0057] The sample in Comparative Example 9, which was made using only wax-based resin with no rosin-based resin components, was able to be made into a grease, and thermal grease with added volatile solvent could be printed without any problems, but the bolts loosened at 3.1 N m. This is thought to be because wax-based resin alone has poor deformability, and although it does not deform up to a certain pressure, it will collapse above that value.

[0058] The sample of Comparative Example 10, which was made using only rosin-based resin without any wax-based resin components, was able to be made into a grease, just like the sample of Comparative Example 8, but the printed shape was inferior. Rosin-based resin has high deformability and no loosening of the bolts was observed, but there are concerns about poor thermal conductivity due to poor application conditions during printing.

Claims

1. A thermally conductive composition comprising (A) an inorganic powder filler, (B) a base oil, (C) a wax-based resin, and (D) a rosin-based resin, the (A) inorganic powder filler is composed of three or more types of powders having different average particle sizes, the first inorganic powder filler having the largest average particle size has an average particle size of 10 μm or more and 100 μm or less, the second inorganic powder filler having the second largest average particle size has an average particle size of 1 μm or more and 50 μm or less, and the third inorganic powder filler having the next largest average particle size has an average particle size of 0.1 μm or more and 5 μm or less, the average particle size ratio of the second inorganic powder filler to the first inorganic powder filler is 0.8 times or less, and the average particle size ratio of the third inorganic powder filler to the second inorganic powder filler is 0.6 times or less, and the (A) inorganic powder filler contains 40 volume % or more and 80 volume % or less of the first inorganic powder filler, 10 volume % or more and 50 volume % or less of the second inorganic powder filler, and 10 volume % or more and 40 volume % or less of the third inorganic powder filler, relative to 100 volume % of the inorganic powder filler, The wax-based resin (C) has a penetration of 5 or more and a melting point of 40°C or more and 150°C or less, The melting point of the (D) rosin-based resin is 40°C or higher and 150°C or lower, the content of the (B) base oil is 100 parts by volume or more and 1,000 parts by volume or less relative to 100 parts by volume of the total content of the (C) wax-based resin and the (D) rosin-based resin, A thermally conductive composition characterized in that the content ratio of the wax-based resin (C) is 10% by volume or more and 60% by volume or less relative to 100% by volume of the total content of the wax-based resin (C) and the rosin-based resin (D).

2. 2. The thermally conductive composition according to claim 1, wherein the base oil (B) is at least one selected from the group consisting of mineral oil, synthetic hydrocarbon oil, diester, polyol ester, and phenyl ether.

3. 2. The thermally conductive composition according to claim 1, wherein the wax-based resin (C) is at least one selected from the group consisting of paraffin-based wax, ester-based wax, polyolefin-based wax, and acrylic resin.

4. 2. The thermally conductive composition according to claim 1, wherein the inorganic powder filler (A) is at least one selected from the group consisting of copper, aluminum, zinc oxide, magnesium oxide, aluminum oxide, aluminum nitride, and silicon carbide.

5. 2. The thermally conductive composition according to claim 1, further comprising at least one of (E) a thixotropy adjuster, (F) a surfactant, and (G) a diffusion inhibitor.

6. A thermally conductive grease comprising the thermally conductive composition according to any one of claims 1 to 5 and (H) a volatile solvent.

Images