JP7476482B2 - Thermally conductive sheet - Google Patents

Description

The present invention relates to a thermally conductive sheet. More specifically, the present invention relates to a thermally conductive sheet used as a heat dissipation member for an electronic component structure.

Components that transfer heat from heat-generating components to heat-dissipating components in electrical and electronic devices are required to have excellent thermal conductivity and electrical insulation. To meet this requirement, thermally conductive sheets in which inorganic fillers with excellent thermal conductivity and electrical insulation are dispersed in a matrix resin are widely used. Alumina, boron nitride (BN), silica, aluminum nitride, and other inorganic fillers with excellent thermal conductivity and electrical insulation are known, but boron nitride is the most suitable for use in thermally conductive sheets because it has excellent chemical stability in addition to thermal conductivity and electrical insulation, is non-toxic, and is relatively inexpensive.

For example, Patent Document 1 discloses a thermally conductive sheet containing secondary particles composed of primary particles of boron nitride. In Patent Document 1, a varnish-like resin composition is applied to a substrate and dried to produce a thermally conductive sheet in a B-stage state, and the resulting sheet is heated and cured to obtain a thermally conductive member.

International Publication No. 2012/070289

The inventors have found that even in thermally conductive sheets with a high inorganic filler content, such as those described in Patent Document 1, there is still room for improvement in thermal conductivity and voltage resistance.

The present invention was made in consideration of the above problems, and provides a thermally conductive sheet that has good moldability and also has improved thermal conductivity and voltage resistance.

The present invention provides a thermally conductive sheet that has good moldability and improved thermal conductivity and voltage resistance.

The following describes an embodiment of the present invention.

The thermally conductive sheet according to this embodiment contains an epoxy resin and boron nitride particles. In the thermally conductive sheet according to this embodiment, the boron nitride particles are dispersed in the epoxy resin. The boron nitride particles used in the thermally conductive sheet according to this embodiment have two peaks in their particle size distribution. The particle size distribution of the boron nitride particles is measured using a laser diffraction particle size distribution measuring device or a dynamic light scattering particle size distribution measuring device known in the field.

In the thermally conductive sheet of this embodiment, the epoxy resin is highly filled with boron nitride particles. The inventors have found that the use of boron nitride particles having a particle size distribution with two peaks in the particle size distribution improves the thermal conductivity and voltage resistance of the resulting cured product of the thermally conductive sheet. The improvement in thermal conductivity is believed to be due to the fact that boron nitride particles having two peaks in the particle size distribution are a mixture of large and small particles, and the presence of small particles between the large particles improves the packing. In addition, the improved packing of the boron nitride particles makes the thermally conductive sheet in the B-stage state flat without undulations, which is believed to improve the voltage resistance of the cured product of the thermally conductive sheet. In addition, when boron nitride particles having such a particle size distribution are used, even when the resin is highly filled, the resulting sheet does not have an uneven surface, and a sheet with good formability having a smooth plane is obtained.

In this embodiment, the term "thermally conductive sheet" refers to a resin composition in a B-stage state obtained by semi-curing a thermally conductive resin composition. In addition, a thermally conductive sheet cured by heat treatment is referred to as a "cured product of the thermally conductive sheet."

In one embodiment, the boron nitride particles used in the thermally conductive sheet are preferably agglomerated particles of scaly boron nitride from the viewpoint of improving thermal conductivity. By using such boron nitride particles, it is possible to obtain a cured thermally conductive sheet with an excellent balance between thermal conductivity and electrical insulation.

The average particle size of the agglomerated particles of scaly boron nitride is, for example, preferably 5 μm or more and 180 μm or less, and more preferably 10 μm or more and 100 μm or less. This makes it possible to realize a cured thermally conductive sheet with an excellent balance between thermal conductivity and electrical insulation.

In one embodiment, the boron nitride particles are preferably contained in an amount of 50% by mass or more and 95% by mass or less, more preferably 55% by mass or more and 90% by mass or less, and even more preferably 60% by mass or more and 88% by mass or less, based on the entire thermal conductive sheet. By containing boron nitride particles in the above content range, the uniformity of the film thickness of the obtained thermal conductive sheet can be improved, and the cured product of the thermal conductive sheet has high thermal conductivity.

In one embodiment, of the two peaks in the particle size distribution of the boron nitride particles, the small particle side is the first peak and the large particle side is the second peak. The first peak is preferably in the range of 5 μm to 30 μm, and the second peak is preferably in the range of 30 μm to 200 μm. The first peak is more preferably in the range of 10 μm to 20 μm. The second peak is more preferably in the range of 40 μm to 100 μm. Furthermore, the difference between the first peak position (μm) and the second peak position (μm) ((second peak position (μm)) - (first peak position (μm)) is preferably in the range of 30 μm to 80 μm. By having the first peak and the second peak in the above range, the filling property of the boron nitride particles is further improved, and the thermal conductivity and voltage resistance of the cured product of the obtained thermal conductive sheet are improved.

In one embodiment, when the cumulative 50% particle diameter of the boron nitride particles measured by a laser diffraction particle size distribution analyzer is defined as _D50 and the cumulative 90% particle diameter is defined as _D90 , the ratio _D50 / _D90 is 0.25 to 0.65. When the value of _D50 / _D90 is within the above range, the packing property of the boron nitride particles is further improved, and the thermal conductivity and voltage resistance of the cured product of the obtained thermal conductive sheet are improved.

In one embodiment, the boron nitride particles include small particles having a particle size of 5 μm to 30 μm and large particles having a particle size of 30 μm to 200 μm, the small particles being present in an amount of 50% to 75% by volume relative to the total boron nitride particles, and the large particles being present in an amount of 5% to 20% by volume relative to the total boron nitride particles. The small particles are present in an amount of 0.2% to 30% by volume relative to the large particles. By using the small particles and the large particles in amounts within the above ranges, the packing property of the boron nitride particles is further improved, and the thermal conductivity and voltage resistance of the cured product of the obtained thermal conductive sheet are improved.

The thermally conductive sheet of this embodiment contains an epoxy resin. As the epoxy resin, a resin generally used in the relevant component can be used, for example, an epoxy resin having a dicyclopentadiene skeleton, an epoxy resin having a biphenyl skeleton, an epoxy resin having an adamantane skeleton, an epoxy resin having a phenol aralkyl skeleton, an epoxy resin having a biphenyl aralkyl skeleton, or an epoxy resin having a naphthalene aralkyl skeleton can be used. As the epoxy resin, an epoxy resin that is liquid at room temperature can be used. As such an epoxy resin, an epoxy resin having a biphenol skeleton that is liquid at room temperature is preferably used. These can be used alone or in combination of two or more.

In one embodiment, the epoxy resin is preferably contained in an amount of 1% by mass or more and 30% by mass or less, and more preferably 5% by mass or more and 28% by mass or less, based on the entire thermally conductive sheet. By containing the epoxy resin in an amount within the above range, the handleability of the resin composition, which is the material of the sheet, is improved, and it becomes easier to form the thermally conductive sheet. Furthermore, by containing the epoxy resin in an amount within the above range, the surface of the obtained thermally conductive sheet does not have any unevenness caused by the boron nitride particles, and a thermally conductive sheet having a smooth surface is obtained.

In one embodiment, the thermally conductive sheet may contain a cyanate resin in addition to the epoxy resin. By including a cyanate resin, the insulating properties of the cured product of the thermally conductive sheet obtained at high temperatures are improved. Examples of cyanate resins include, but are not limited to, novolac-type cyanate resins; bisphenol-type cyanate resins such as bisphenol A-type cyanate resin, bisphenol E-type cyanate resin, and tetramethylbisphenol F-type cyanate resin; naphthol aralkyl-type cyanate resins obtained by reacting naphthol aralkyl-type phenolic resins with cyanogen halides; dicyclopentadiene-type cyanate resins; and biphenyl alkyl-type cyanate resins. When a cyanate resin is used, the amount of the cyanate resin used is preferably 2% by mass or more and 25% by mass or less, and more preferably 5% by mass or more and 20% by mass or less, based on the entire thermally conductive sheet.

The thermally conductive sheet according to the present embodiment preferably contains a phenol-based curing agent or a curing catalyst. Examples of phenol-based curing agents include novolac-type phenolic resins such as phenol novolac resin, cresol novolac resin, naphthol novolac resin, aminotriazine novolac resin, novolac resin, and trisphenylmethane-type phenol novolac resin; modified phenolic resins such as terpene-modified phenolic resin and dicyclopentadiene-modified phenolic resin; aralkyl-type resins such as phenol aralkyl resins having a phenylene skeleton and/or biphenylene skeleton and naphthol aralkyl resins having a phenylene skeleton and/or biphenylene skeleton; bisphenol compounds such as bisphenol A and bisphenol F; and resol-type phenolic resins, which may be used alone or in combination of two or more. When a phenol-based curing agent is used, the amount of the curing agent used is preferably 0.1% by mass or more and 30% by mass or less, and more preferably 0.3% by mass or more and 15% by mass or less, based on the entire thermally conductive sheet.

Examples of curing catalysts include organic metal salts such as zinc naphthenate, cobalt naphthenate, tin octoate, cobalt octoate, cobalt bisacetylacetonate (II), and cobalt trisacetylacetonate (III); tertiary amines such as triethylamine, tributylamine, and 1,4-diazabicyclo[2.2.2]octane; 2-phenyl-4-methylimidazole, 2-ethyl-4-methylimidazole, 2,4-diethylimidazole, 2-phenyl-4-methyl-5-hydroxyimidazole, Examples of the curing catalyst include imidazoles such as 2-phenyl-4,5-dihydroxymethylimidazole; organic phosphorus compounds such as triphenylphosphine, tri-p-tolylphosphine, tetraphenylphosphonium tetraphenylborate, triphenylphosphine triphenylborane, and 1,2-bis-(diphenylphosphino)ethane; phenolic compounds such as phenol, bisphenol A, and nonylphenol; organic acids such as acetic acid, benzoic acid, salicylic acid, and p-toluenesulfonic acid; and mixtures thereof. These may be used alone or in combination of two or more. When a curing catalyst is used, the amount of the curing catalyst used is preferably 0.001% by mass or more and 1% by mass or more based on the entire thermal conductive sheet.

The thermally conductive sheet of this embodiment may further include a coupling agent. By blending a coupling agent, the interfacial wettability between the epoxy resin and the boron nitride particles can be improved. Examples of the coupling agent that can be used include, but are not limited to, epoxy silane coupling agents, cationic silane coupling agents, amino silane coupling agents, titanate-based coupling agents, and silicone oil-type coupling agents. When a coupling agent is used, it is preferably used in an amount of 0.1% by mass or more and 10% by mass or less, and more preferably 0.5% by mass or more and 7% by mass or less, based on the entire thermally conductive sheet.

The thermally conductive sheet of this embodiment may further contain a phenoxy resin. By using a phenoxy resin, the bending resistance of the thermally conductive sheet can be improved. As the phenoxy resin, a phenoxy resin having a bisphenol skeleton, a phenoxy resin having a naphthalene skeleton, a phenoxy resin having an anthracene skeleton, a phenoxy resin having a biphenyl skeleton, etc. can be used, but is not limited to these. When a phenoxy resin is used, it is preferably used in an amount of 2% by mass or more and 15% by mass or less with respect to the entire resin conductive sheet.

The thermally conductive sheet of this embodiment may contain additives such as antioxidants, leveling agents, defoamers, and dispersants, to the extent that the effects of the present invention are not impaired.

The thermally conductive sheet of this embodiment can be obtained by preparing a varnish-like resin composition containing the above materials, applying this resin composition to a substrate, and then heat-treating and drying it. More specifically, first, the above resin components are added to a solvent to prepare a resin varnish. Boron nitride particles are added to this resin varnish and kneaded to obtain a varnish-like resin composition. The obtained varnish-like resin composition is then applied to a substrate, dried to remove the solvent, and a sheet-like resin composition in a B-stage state can be obtained as a thermally conductive sheet. Examples of substrates include metal foils that form heat dissipation members, lead frames, peelable carrier materials, etc. The heat treatment for drying the resin composition is performed, for example, at 80 to 150°C for 5 minutes to 1 hour. The thickness of the resin sheet is, for example, 60 μm to 500 μm.

In one embodiment, the specific gravity of the sheet-shaped resin composition in the B-stage state obtained as described above is preferably 1.0 or more and 1.3 or less. If it is within the above range, sufficient thermal conductivity can be obtained when the sheet-shaped resin composition is cured to form a thermally conductive member.

The thermally conductive sheet of this embodiment is provided, for example, between a heat generating element such as a semiconductor chip and a substrate such as a lead frame or wiring board (interposer) on which the heat generating element is mounted, or between the substrate and a heat dissipation member such as a heat sink. This allows the heat generated by the heat generating element to be effectively dissipated to the outside of the semiconductor device while maintaining the insulation of the semiconductor device.

Although the embodiments of the present invention have been described above, these are merely examples of the present invention, and various configurations other than those described above can be adopted.
Examples of embodiments are given below.
1. A thermally conductive sheet comprising an epoxy resin and boron nitride particles,
A thermally conductive sheet, wherein the boron nitride particles have two peaks in their particle size distribution.
2. The thermally conductive sheet according to 1., wherein the boron nitride particles have a cumulative 50% particle diameter D50 and a cumulative 90% particle diameter D90, both measured with a laser diffraction particle size distribution analyzer, and the _ratio D50 _{/ D90} is _0.25 or more and 0.65 or less.
3. The thermally conductive sheet according to 1. or 2., wherein one of two peaks in the particle size distribution of the boron nitride particles is in the range of 5 μm or more and 30 μm or less, and the other is in the range of 30 μm or more and 200 μm or less.
4. The thermally conductive sheet according to any one of 1. to 3., wherein the boron nitride particles include small particles having a particle size of 10 μm or more and 30 μm or less and large particles having a particle size of 70 μm or more and 200 μm or less, and the small particles account for 0.2 vol % or more and 30 vol % or less of the large particles.
5. The thermally conductive sheet according to any one of 1. to 4., wherein the specific gravity of the thermally conductive sheet in a B-stage is 1.0 or more and 1.3 or less.

The present invention will be described below with reference to examples and comparative examples, but the present invention is not limited to these.

(Preparation of boron nitride particles 1 to 3)
Boron carbide is nitrided in a nitrogen atmosphere, for example, at 1200 to 2500° C. for 2 to 24 hours. Then, diboron trioxide is added to the obtained boron nitride, and the resultant is fired in a non-oxidizing atmosphere to form boron nitride particles. The firing temperature is, for example, 1200 to 2500° C. The firing time is, for example, 2 to 24 hours. The boron nitride particles obtained after firing are called boron nitride particles 1. The boron nitride particles 1 are passed through a sieve with a mesh size of 100 μm, and the particles remaining on the sieve are collected and called boron nitride particles 2. The particles that passed through the sieve are also collected and called boron nitride particles 3. The particle size distribution of the obtained boron nitride particles 1 to 3 was measured using a laser diffraction type particle size distribution measuring device (LA-950V2, manufactured by Horiba, Ltd.) using water as a medium. The results are shown in Table 1.

(Preparation of boron nitride particles 4)
The mixture obtained by mixing melamine borate and scaly boron nitride powder (average major axis: 15 μm) was added to an aqueous solution of ammonium polyacrylate and mixed for 2 hours to prepare a slurry for spraying. The slurry was then fed to a spray granulator and sprayed under the conditions of an atomizer rotation speed of 15,000 rpm, a temperature of 200° C., and a slurry feed rate of 5 ml/min to produce composite particles. The composite particles obtained were then fired under a nitrogen atmosphere at 2000° C. to form the boron nitride particles obtained after firing. The particle size distribution of the obtained boron nitride particles 4 was measured using a laser diffraction particle size distribution measuring device (LA-950V2, manufactured by Horiba, Ltd.) using water as a medium. The results are shown in Table 1.

(Preparation of thermally conductive sheet)
According to the formulation shown in Table 1, an epoxy resin and a curing agent were added to a solvent, methyl ethyl ketone, and the mixture was stirred to obtain a mixed solution. The boron nitride particles obtained by the above-mentioned method were then added to the mixed solution and premixed, and the mixture was kneaded with a three-roll mill to obtain a varnish-like thermosetting resin composition in which the boron nitride particles were uniformly dispersed. The obtained thermosetting resin composition was then aged at 60° C. for 15 hours.

The components shown in Table 1 are as follows:
(Epoxy resin)
Epoxy resin 1: Epoxy resin having a dicyclopentadiene skeleton (XD-1000, manufactured by Nippon Kayaku Co., Ltd.)
Epoxy resin 2: Bisphenol F type epoxy resin (830S, manufactured by Dainippon Ink Co., Ltd.)
(Hardening agent)
Phenol-based hardener 1: Trisphenylmethane type phenol novolac resin (MEH-7500, manufactured by Meiwa Kasei Co., Ltd.)
(Curing catalyst)
Curing catalyst 1: 2-phenyl-4,5-dihydroxymethylimidazole (2PHZ-PW, manufactured by Shikoku Kasei Co., Ltd.)
(Boron nitride particles)
Boron nitride particle 1: Boron nitride particle 1 prepared as described above
Boron nitride particles 2: Boron nitride particles 2 prepared as described above
Boron nitride particles 3: Boron nitride particles 3 prepared as described above
Boron nitride particles 4: Boron nitride particles 4 prepared as described above
・Boron nitride particles 5: "HP-40J2" made by Mizushima Ferroalloy
・Boron nitride particles 6: "PTX-60" manufactured by Momentive Performance Materials Japan, Inc.

(Thermal Conductivity Test)
The above-mentioned thermally conductive resin composition was heat-treated at 100° C. for 30 minutes to prepare a B-stage thermally conductive sheet having a thickness of 400 μm. The thermally conductive sheet was then heat-treated at 180° C. and 10 MPa for 40 minutes to obtain a thermally conductive sheet cured product. The thermal conductivity of the thermally conductive sheet cured product in the thickness direction was then measured using a laser flash method.
Specifically, the thermal conductivity was calculated using the following formula from the thermal diffusion coefficient (α) measured by the laser flash method (half-time method), the specific heat (Cp) measured by the DSC method, and the density (ρ) measured in accordance with JIS-K-6911. The unit of thermal conductivity is W/(m·K). The measurement temperature is 25°C. Thermal conductivity [W/(m·K)] = α [mm ² /s] × Cp [J/kg·K] × ρ [g/cm ³ ]. The evaluation criteria are as follows:
○: 12W/(m.K) or more △: 5W/(m.K) or more, less than 12W/(m.K) ×: Less than 5W/(m.K)

(Breakdown voltage)
The dielectric breakdown voltage of the cured thermally conductive sheet obtained above was measured in accordance with JIS K6911 as follows. First, the cured thermally conductive sheet obtained was cut into a 30 mm square to obtain a test piece. Further, the obtained test piece was sandwiched between circular electrodes and placed in insulating oil.
Next, an AC voltage was applied to both electrodes using a TOS9201 manufactured by Kikusui Electronics Co., Ltd., so that the voltage increased at a rate of 2.5 kV/sec. The voltage at which the test piece broke down was taken as the dielectric breakdown voltage. The evaluation criteria were as follows:
○: 8.0 kV or more △: 5.0 kV or more and less than 8.0 kV ×: Less than 5.0 kV

(Wavage; Contact type)
The surface unevenness (waviness) of the resin sheet surface was measured by a contact method at intervals of 10 μm.
The magnitude of unevenness is determined by the difference between the maximum and minimum values.
3: 100 μm or more 2: 5 μm or more and less than 50 μm 1: 5 μm or less

(Specific gravity of B-stage thermal conductive sheet)
The specific gravity of the thermally conductive sheet in the B stage obtained as described above was measured by the underwater substitution method (Archimedes method).
The results of the above evaluation are shown in Table 1.

Claims (5)

A thermally conductive sheet in a B-stage state , comprising an epoxy resin and boron nitride particles,
the boron nitride particles include agglomerated boron nitride particles;
The particle size distribution of the boron nitride particles was measured using a laser diffraction particle size distribution measuring device (LA-950V2, manufactured by Horiba, Ltd.) using water as a medium.
The boron nitride particles have two peaks in their particle size distribution,
one of two peaks in the particle size distribution of the boron nitride particles is in the range of 5 μm or more and 30 μm or less, and the other is in the range of 30 μm or more and 200 μm or less;
the boron nitride particles have a cumulative 50% particle size _D50 and a cumulative 90% particle size _D90 measured by the laser diffraction particle size distribution measuring device, and the ratio _D50 / _D90 is 0.25 or more and 0.65 or less;
the boron nitride particles include small particles having a particle size of 10 μm or more and 30 μm or less and large particles having a particle size of 70 μm or more and 200 μm or less, the small particles being present in an amount of 0.2 vol % or more and 30 vol % or less relative to the large particles;
The specific gravity of the thermal conductive sheet is 1.0 or more and 1.3 or less.
Thermally conductive sheet. The thermally conductive sheet according to claim 1,
A thermally conductive sheet, wherein the boron nitride particles account for 50 mass % or more and 95 mass % or less of the entire thermally conductive sheet. The thermally conductive sheet according to claim 1 or 2,
A thermally conductive sheet, wherein the epoxy resin is contained in an amount of 1 mass % or more and 30 mass % or less with respect to the entire thermally conductive sheet. The thermally conductive sheet according to any one of claims 1 to 3,
The thermally conductive sheet, wherein the epoxy resin includes an epoxy resin having a dicyclopentadiene skeleton. The thermally conductive sheet according to any one of claims 1 to 4,
The thermally conductive sheet further comprises a phenol-based curing agent.