Patent Application
20210024804
The present invention relates to a thermally conductive silicone rubber composition, a sheet thereof, and a method for producing the sheet.
Semiconductors used in computers (CPUs), transistors, light-emitting diodes (LEDs), etc. generate heat during operation, and the performance of electronic components may be reduced by the heat. To cope with this, heat dissipaters are attached to the electronic components that generate heat. The heat dissipaters are often made of metal. Therefore, the adhesion of a heat dissipating part to a CPU is enhanced by inserting a sheet-like or gel-like thermally conductive composition between them. In order to improve the thermal conductivity of a heat-dissipating material, which is the final purpose, the thermally conductive composition needs to contain a large amount of thermally conductive inorganic powder. However, merely increasing the amount of thermally conductive inorganic powder results in various problems. For example, when the heat-dissipating material is in the form of an elastomer, the hardness is too high, so that an electronic component and a heat dissipater cannot be spaced at a predetermined small distance from each other. Moreover, the space between the electronic component and the heat dissipater cannot be filled with the heat-dissipating material as desired. In the case of an elastomeric or gel-like heat-dissipating material, the compression set is increased and long-term reliability is likely to be deteriorated. Further, there is also a problem that the hardness may be increased by a high temperature thermal history.
To solve these problems, various approaches have been proposed conventionally. The present applicant proposes in Patent Document 1 that small particles of alumina are surface treated with an alkylsilane compound. It is proposed to use 0.1 to 5 μm of amorphous alumina and 5 to 50 μm of spherical alumina (Patent Document 2). Patent Document 3 proposes a thermally conductive sheet using a glass cloth.
Patent Document 1: JPWO 2009/136542 A1
Patent Document 2: JP 1102-041362 A
Patent Document 3: JP 2015-233104 A
However, thermally conductive silicone rubbers in the conventional technologies have problems of high thermal resistance values.
To solve the above conventional problems, the present invention provides a thermally conductive silicone rubber composition having a low thermal resistance value, a sheet thereof, and a method for producing the sheet.
A thermally conductive silicone composition of the present invention includes silicone as a matrix component and a thermally conductive filler. The matrix component includes a silicone base polymer having a vinyl group and a silicone oil having no vinyl group. The thermally conductive filler includes aluminum nitride particles. The thermally conductive silicone composition includes a peroxide as a curing component.
A thermally conductive silicone sheet of the present invention includes the thermally conductive silicone composition that is applied to at least one surface of a sizing sheet of a glass cloth. A thickness of the thermally conductive silicone sheet is 0.1 to 1 mm.
A method for producing the thermally conductive silicone sheet of the present invention includes adding a diluent to the thermally conductive silicone composition to prepare a coating liquid; impregnating a glass cloth with the coating liquid, drying the glass cloth, and then heating and curing the glass cloth to prepare a sizing sheet; and coating at least one surface of the sizing sheet of the glass cloth with the coating liquid, drying the sizing sheet, and then heating and curing the sizing sheet.
In the present invention, the matrix component includes a silicone base polymer having a vinyl group and a silicone oil having no vinyl group. The thermally conductive filler includes aluminum nitride particles. The thermally conductive silicone composition includes a peroxide as a curing component. Thus, the present invention can provide a thermally conductive silicone rubber composition having a low thermal resistance value, a sheet thereof, and a method for producing the sheet.
In the present invention, it is preferable not to use a platinum-based catalyst for the following reasons.
(1) The composition of the present invention is dissolved in a solvent before coating, and the remaining material will be used for the next production in terms of cost. However, if the composition includes a platinum-based catalyst (addition reaction system), the life becomes shorter and the curing is more likely to proceed than if the composition includes a peroxide. Thus, it is difficult to use the material including the platinum-based catalyst for the next production.
(2) When the composition is cured using the platinum-based catalyst (addition reaction system), only a part having a vinyl group reacts, resulting in insufficient curing. When the composition is cured using the peroxide, a part having a vinyl group and a part having a methyl group react, and thus the curing sufficiently proceeds.
The present inventor studied that whether the addition of a silicone base polymer having a vinyl group and a silicone oil having no vinyl group could improve the thermal resistance value problem. Here, silicone gum exhibits properties intermediate between the silicone oil (fluid) and the silicone rubber (solid). In the present invention, the silicone base polymer having a vinyl group refers to silicone gum and oil that have a vinyl group.
The silicone base polymer having a vinyl group of the matrix component of the present invention is highly reactive, and has a high strength in comparison with that having no vinyl group. The silicone oil having no vinyl group is less reactive, but has flexibility. Therefore, the silicone gum and oil that have a vinyl group and the silicone oil having no vinyl group can balance strength and flexibility.
To enhance the conduction of heat, alumina has been highly packed conventionally. However, if alumina is highly packed, the strength and the flexibility tend to decrease. Thus, aluminum nitride particles will be packed, which can enhance the conduction of heat and maintain good strength and flexibility.
In the present invention, curing proceeds due to the curing action by the radical reaction using a peroxide curing agent. The peroxide curing agent is preferably 0.01 to 10 parts by mass, and more preferably 0.1 to 8 parts by mass with respect to 100 parts by mass of the matrix component. Preferably, examples of the peroxide curing agent include the following: acyl peroxides such as benzoyl peroxide and bis(p-methylbenzoyl) peroxide; alkyl peroxides such as di-tert-butyl peroxide, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, tert-butyl cumyl peroxide, and dicumyl peroxide; and ester-based organic peroxides such as tert-butyl perbenzoate.
If the matrix component is 100 parts by mass, the silicone base polymer having a vinyl group (silicone gum) is 44 to 71.
If the matrix component is 100 parts by mass, the silicone oil having no vinyl group is 11 to 45. Moreover, the composition of the present invention may include a both-terminal vinyl silicone oil. If the matrix component is 100 parts by mass, the both-terminal vinyl silicone oil is 11 to 45.
The oil having no vinyl group may be basically any dimethylsilicone oil, and includes e.g., phenylmethylsilicone oil and fluorosilicone oil.
It is preferable that the matrix component is a polysiloxane having at least two alkenyl groups bonded to silicon atoms per molecule. Examples of the alkenyl groups include vinyl, allyl, and propenyl groups. Examples of organic groups other than the alkenyl groups include the following: alkyl groups such as methyl, ethyl, propyl, butyl, pentyl, hexyl, octyl, decyl, and dodecyl groups; aryl groups such as phenyl and tolyl groups; aralkyl groups such as a β-phenylethyl group; and halogen-substituted alkyl groups such as 3,3,3-trifluoropropyl and 3-chloropropyl groups. Moreover, small amounts of hydroxyl groups may be present at the ends of a molecular chain or the like. The molecular structure of the polysiloxane may be a linear, a linear with branches, a ring, or a mesh-like structure. Two or more types of diorganopolysiloxanes may be used together. The molecular weight of the polysiloxane is not particularly limited. The polysiloxane may include liquid polysiloxanes having a low viscosity and gum-like polysiloxanes having a high viscosity. To produce a rubber-like elastic body as a result of curing, a viscosity of 100 mPa·s or more at 25° C. is preferred. Gum-like polysiloxanes having a polystyrene-equivalent number-average molecular weight of 200,000 to 700,000 measured by gel permeation chromatograph (GPC) are more preferred.
The amount of the thermally conductive filler added is preferably 600 to 2000 parts by mass, more preferably 700 to 1900 parts by mass, and further preferably 800 to 1800 parts by mass with respect to 100 parts by mass of the matrix component. Moreover, if the thermally conductive filler is 100 parts by mass, the aluminum nitride particles are preferably 10 to 100 parts by mass, more preferably 15 to 90 parts by mass, and further preferably 20 to 80 parts by mass.
It is preferable that the thermally conductive filler further includes alumina particles. If the thermally conductive filler is 100 parts by mass, the alumina particles are preferably 20 to 100 parts by mass, more preferably 25 to 90 parts by mass, and further preferably 30 to 80 parts by mass. It is preferable that the alumina particles include a mixture of particles (A) with an average particle size of 10 μm or more and 20 μm or less and particles (B) with an average particle size of 0.01 μm or more and less than 10 μm. It is preferable that the mixing ratio of A to B is 90:10 to 10:90 in mass ratio.
The average particle size of the thermally conductive filler is preferably 0.01 to 20 μm, and more preferably 0.1 to 15 μm. Thus, the thermally conductive filler can be favorably mixed with the matrix resin and have good processability. The average particle size means D50 (median diameter) in a volume-based cumulative particle size distribution, which is determined by a particle size distribution measurement with a laser diffraction scattering method. The measuring device may be, e.g., a laser diffraction/scattering particle size distribution analyzer LA-950 S2 manufactured by HORIBA, Ltd.
It is preferable that the thermally conductive silicone composition does not include reinforcing silica. If reinforcing silica is included, it is disadvantageous in that the hardness increases, and thus contact thermal resistance increases.
The composition of the present invention may include components other than the above as needed. For example, the composition may include an inorganic pigment such as colcothar, and alkyltrialkoxysilane used, e.g., for the surface treatment of a filler, a flow control agent, a tackifier, and a flame retardant. Moreover, alkoxy group-containing silicone may be added, e.g., for the surface treatment of a filler.
A thermally conductive silicone sheet of the present invention includes the thermally conductive silicone composition that is applied to at least one surface of a sizing sheet of a glass cloth. It is preferable that both surfaces of the sizing sheet are coated. The thickness of the thermally conductive silicone sheet is 0.1 to 1 mm. If the thickness is less than 0.1 mm, the production is difficult. If the thickness is more than 1 mm, the coating is difficult. It is preferable that the glass cloth has a mass of 25 to 54 g/m2 and is a woven fabric having a plain weave texture in which a warp density and a weft density are each 56 to 60 threads/25 mm.
The thermal conductivity of a bulk in one example of the thermally conductive silicone sheet of the present invention is preferably 1 W/m·k or more, and more preferably 3.3 W/m·k or more. Here, the term “bulk” refers to the state of the composition that includes, e.g., the silicone base polymer, the filler, and other additive agents before it is dissolved in the solvent.
In a method for producing the thermally conductive silicone sheet of the present invention, first, a diluent is added to the thermally conductive silicone composition to prepare a coating liquid. To prepare the coating liquid, the matrix resin component and the thermally conductive filler, and optionally a flame retardant and a pigment, are added and uniformly mixed to prepare a composition. Subsequently, a peroxide curing component and a diluent solvent are added to the composition. The diluent solvent may be added in an appropriate amount so that the coating can be performed. The viscosity for coating is preferably of 3,000 to 10,000 cps.
Next, the glass cloth is impregnated with the coating liquid, dried, heated and cured to prepare a sizing sheet. The sizing sheet is a sealed glass cloth sheet.
Next, at least one surface of the sizing sheet of the glass cloth is coated with the coating liquid, dried, and then heated and cured to obtain the thermally conductive silicone sheet. The coating is preferably knife coating because it can provide a thin coating. It is preferable that the curing is performed at a temperature of 150 to 180° C. for 3 to 10 minutes.
Hereinafter, the present invention will be described with reference to the drawings.
Hereinafter, the present invention will be described by way of examples. However, the present invention is not limited to the following examples.
<Method for Measuring Thermal Resistance Value>
The measurement was performed using the apparatus illustrated in
The thermal resistance value was calculated by the following formula:
Rt=(Tc−Tf)/P0,
where Rt represents a thermal resistance value (K·cm2/W);
Tc represents a transistor temperature (° C.);
Tf represents a heat sink temperature (° C.); and
P0 represents a constant power (W).
The measurement apparatuses included the following:
a transistor 2SC2245 (TO-3 type); and
a heat sink 40CH104L-90-K.
(1) Raw Materials
(A) Matrix component
(A-1) Silicone gum having a vinyl group: gum having a vinyl group at both ends and a side chain, 80 g (manufactured by Elkem Japan K.K.)
(A-2) Both-terminal vinyl silicone oil having a viscosity of 350 mm2/s at a temperature of 25° C. (manufactured by Elkem Japan K.K.)
(A-3) Silicone oil having no vinyl group having a viscosity of 300 cs at a temperature of 25° C. (manufactured by Dow Corning Toray Co., Ltd.)
(B) Thermally conductive filler
(B-1) Aluminum nitride having an average particle size of 10 μm (manufactured by TOYO ALUMINIUM K.K).
(B-2) Alumina having an average particle size of 12 μm (manufactured by Nippon Light Metal Co., Ltd.)
(B-3) Alumina having an average particle size of 2 μm (manufactured by Showa Denko K.K.)
(B-4) Alumina having an average particle size of 0.3 μm (manufactured by Sumitomo Chemical Co., Ltd.)
(C) Pigment: “Brown 105A” manufactured by Wacker Asahikasei Silicone Co., Ltd.
(D) Peroxide curing component: Bis(4-methyl)benzoyl peroxide
The above raw materials were uniformly kneaded using a kneader to prepare a thermally conductive silicone composition.
(2) Coating Liquid 1
A coating liquid 1 was prepared by adding 3 g of a 50% paste liquid of bis(4-methyl)benzoyl peroxide as a peroxide curing component and an appropriate amount of solvent xylene as a diluent to 100 g of the above raw material composition.
(3) Coating Liquid 2
A coating liquid 2 was prepared by adding 0.8 g of a 50% paste liquid of bis(4-methyl)benzoyl peroxide as a peroxide curing component and an appropriate amount of solvent xylene as a diluent to 100 g of the above raw material composition.
(4) Coating
First, a glass cloth having a thickness of about 35 μm (a mass of 25 g/m2, a woven fabric having a plain weave texture in which a warp density and a weft density are each 56 threads/25 mm) was impregnated with the coating liquid 1, dried, heated and cured to prepare a sizing sheet.
Next, one surface of the sizing sheet was coated with the coating liquid 2 using a knife coater, dried, placed in a heater, and heated and cured at 180° C. for three minutes. Next, the other surface of the sizing sheet was coated with the coating liquid 2 using the knife coater, and dried. Then, the sizing sheet was placed in the heater, and heated and cured at 180° C. for three minutes. Thus, a thermally conductive silicone sheet having a total thickness of 0.2 mm and a thermally conductive silicone sheet having a total thickness of 0.33 mm were produced.
Comparative examples were performed as in Example 1 except that the above silicone oil (A-3) having no vinyl group was not added. Table 1 shows the addition amount of each of the components. Table 2 shows the thermal resistance values.
As apparent from Tables 1 and 2, the products of Examples of the present invention were able to obtain lower thermal resistance values than Comparative Examples 1-3.
The thermally conductive silicone composition and the sheet of the present invention can be applied to heat dissipating members or the like interposed between a heat generating part and a heat sink of an electronic component.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2018-239685 | Dec 2018 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2019/036279 | Sep 2019 | US |
| Child | 17070535 | US |
