JP6338524B2 - Thermally conductive adhesive sheet - Google Patents

Description

  The present invention relates to a heat conductive adhesive sheet interposed between a heat generating element such as a semiconductor element and a heat radiating component such as a heat sink in an electronic device.

A pressure-sensitive adhesive using acrylic acid is generally known, and a heat conductive pressure-sensitive adhesive sheet obtained by adding a heat conductive filler to such a pressure-sensitive adhesive is disclosed in, for example, JP-A-2001-279196 (Patent Document 1). Have been described.
On the other hand, it is known to increase the adhesive force by adding a monomer having a polar group such as hydroxyl group or carboxyl group to the adhesive. Japanese Patent Application Laid-Open No. 2011-173983 (Patent Document 2) describes a technique related to a pressure-sensitive adhesive in which a monomer having a group and acrylic acid are used in combination.

JP 2001-279196 A JP 2011-173983 A

However, the heat-conductive adhesive sheet containing acrylic acid and the heat-conductive filler exemplified in JP-A No. 2001-279196 (Patent Document 1) has insufficient initial adhesive force, There was a problem that the initial adhesive strength was likely to vary due to the influence. This is a phenomenon that has not been observed with pressure-sensitive adhesives using general acrylic acid that does not contain a thermally conductive filler.
In addition, the present inventors applied the technique of Japanese Patent Application Laid-Open No. 2011-173983 (Patent Document 2), and studied the improvement of adhesive force and the reduction of variation by using a monomer having a hydroxyl group and acrylic acid in combination. did. As a result, the initial adhesive force was successfully increased and variation was reduced, but the heat resistance was insufficient as a heat conductive adhesive sheet to be attached to a heat-generating component.

  Then, this invention was made | formed in order to solve the said subject, Comprising: It aims at provision of the heat conductive adhesive sheet with high initial adhesive force and excellent in heat resistance.

In the study by the inventors, a heat conductive adhesive sheet containing acrylic acid and a heat conductive filler has a problem that the initial adhesive force is low and the initial adhesive force varies greatly in a plurality of experiments. Therefore, referring to the description of JP 2011-173983 A (Patent Document 2), heat conduction that can effectively prevent a decrease in adhesive strength and durability using an acrylic adhesive component containing a hydroxyl group and a carboxyl group. Development of adhesive sheet. However, in the development of the heat conductive pressure-sensitive adhesive sheet, the desired effect did not occur and the heat resistance deteriorated. From this result, it was estimated that the presence of a thermally conductive filler inhibits the reaction of hydroxyl groups and carboxyl groups or accelerates the decomposition reaction.
Therefore, the inventors consider that the reason why the combination of the heat conductive filler and the acrylic adhesive component containing hydroxyl group and carboxyl group cannot exhibit sufficient adhesiveness and heat resistance is the presence of the carboxyl group. I presumed that.

And about the heat conductive adhesive sheet containing a heat conductive filler and an acrylic adhesive component, an acrylic adhesive component contains the (meth) acrylic oligomer and the (meth) acrylate monomer which has a hydroxyl group, and a carboxyl group The heat conductive adhesive sheet characterized by being the hardened | cured material of the composition which does not contain.
By adding a (meth) acrylate monomer having a hydroxyl group and no carboxyl group to a (meth) acrylic oligomer, a heat conductive adhesive sheet having excellent initial adhesive strength and excellent heat resistance can be obtained. .

  Content of the (meth) acrylate monomer which has a hydroxyl group and does not have a carboxyl group can be made into 16%-35% (weight%) in the said acrylic adhesive component. Since it was set to 16% to 35% (% by weight), the initial adhesive force can be sufficiently increased, and the temperature can be suppressed to a desired temperature range without increasing the glass transition point of the cured product.

  The thermally conductive filler can be aluminum hydroxide. Since aluminum hydroxide is used as the heat conductive filler, there is no possibility of curing inhibition and there is no possibility that the adhesive strength becomes weak. Furthermore, flame retardance can be increased.

  According to the heat conductive adhesive sheet of this invention, the initial adhesive force is high and it is excellent in heat resistance.

This will be described in more detail based on the embodiment.
The heat conductive adhesive sheet includes a heat conductive filler and an acrylic adhesive component, and the acrylic adhesive component includes a (meth) acrylic oligomer and a (meth) acrylate monomer having a hydroxyl group, and a carboxyl group. It is a cured product of the composition not containing.

(Thermal conductive filler)
The heat conductive filler is a conductive component (heat conductive component) for facilitating heat transfer in the heat conductive adhesive sheet. Examples of the thermally conductive filler include metal powders such as copper powder, aluminum powder, nickel powder, silver powder, and iron powder, and alloy powders thereof, and various conductive substances such as graphite, zinc oxide, carbon fiber, and aluminum hydroxide ( Heat conductive substance). However, when graphite, zinc oxide, or carbon fiber is used, it is necessary to increase the cumulative amount of ultraviolet light to be irradiated due to deterioration of curability, or there is a risk of causing inhibition of curing. When used, the adhesive strength may be reduced. On the other hand, it is not necessary to irradiate excessive ultraviolet rays at the time of curing, and there is no curing inhibition, and aluminum hydroxide that can further improve the adhesion and improve the flame retardancy is preferable.

(Acrylic adhesive component)
The acrylic pressure-sensitive adhesive component is a component for imparting pressure-sensitive adhesiveness to the heat-conductive pressure-sensitive adhesive sheet, and causing the heat-conductive pressure-sensitive adhesive sheet to adhere to the heating element and the heat dissipation component in the electronic device.
This acrylic adhesive component is a cured product of a composition containing a (meth) acrylic oligomer and a (meth) acrylate monomer having a hydroxyl group, and a (meth) acrylic oligomer and a (meth) acrylate monomer having a hydroxyl group are also included. It does not contain a carboxyl group.

  Content of a heat conductive filler is 10 weight part-300 weight part with respect to 100 weight part of acrylic adhesive components. When the amount is less than 10 parts by weight, there is a possibility that sufficient thermal conductivity is not exhibited as a thermally conductive adhesive sheet. On the other hand, if the amount exceeds 300 parts by weight, the sheet becomes hard and the tack and adhesive strength may be reduced, and flexibility and adhesive strength suitable for the thermally conductive adhesive sheet may not be obtained.

((Meth) acrylic oligomer)
The (meth) acrylic oligomer is an acrylic copolymer (oligomer) having a (meth) acryloyl group and a liquid at room temperature having no carboxyl group and having a molecular weight of about 100 to 10,000. Such (meth) acrylic oligomers include polyester acrylate, epoxy acrylate, urethane acrylate, and the like. The viscosity is preferably 500 mPa · s or more and 100,000 mPa · s or less, and more preferably 1000 mPa · s or more and 15000 mPa · s or less. If it is less than 500 mPa · s, the molecular weight is small and the cohesive force of the thermally conductive adhesive sheet after photocuring may be small. On the other hand, when it exceeds 100,000 mPa · s, it is difficult to increase the filling amount of the heat conductive filler. If the viscosity is 1000 mPa · s or more, the molecular weight of the acrylic pressure-sensitive adhesive component after curing can be sufficiently increased because it has a certain molecular weight. Therefore, the strength of the acrylic pressure-sensitive adhesive component as the binder of the heat conductive filler can be increased, and a heat conductive pressure-sensitive adhesive sheet that is difficult to cohesive failure can be obtained. In addition, if the viscosity is 15000 mPa · s or less, the blending amount of the heat conductive filler can be increased within a desired viscosity range suitable for production, and a heat conductive pressure-sensitive adhesive sheet excellent in heat conductivity is obtained. be able to.
In addition, (meth) acryl means acryl or methacryl, and (meth) acrylate means acrylate or methacrylate.

((Meth) acrylate monomer having hydroxyl group)
The (meth) acrylate monomer having a hydroxyl group is a component that forms a polymer by polymerizing with the (meth) acrylic oligomer.
Specific examples of the (meth) acrylate monomer having a hydroxyl group include 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 4-hydroxybutyl acrylate, 2-hydroxybutyl acrylate, 1,4-cyclohexanedimethanol monoacrylate, Examples include 2-hydroxy-3-phenoxypropyl acrylate, 2-hydroxyethyl methacrylate, and 2-hydroxypropyl methacrylate.

The glass transition point (Tg) of such a (meth) acrylate monomer having a hydroxyl group is preferably in the range of -70 ° C to 20 ° C. This is because using a (meth) acrylate monomer in this temperature range makes it easy to increase the adhesive strength. More specifically, by setting the glass transition point within this temperature range, it is possible to suppress an increase in the glass transition point of the acrylic adhesive component after polymerization, and to give tack to the surface of the heat conductive adhesive sheet. it can. When the acrylic pressure-sensitive adhesive component has a tack, the heat-conductive pressure-sensitive adhesive sheet is easily wetted with respect to the adherend, and can increase the adhesive force with respect to the adherend. Furthermore, since it is easy to wet with respect to a to-be-adhered body, a heat conductive adhesive sheet can be closely_contact | adhered without gap and the thermal resistance of the interface can be made low.
Specifically, 2-hydroxyethyl acrylate (Tg = −15 ° C.), 2-hydroxypropyl acrylate (Tg = −7 ° C.), 1,4-cyclohexanedimethanol monoacrylate (Tg = 18 ° C.), 2-hydroxy -3-phenoxypropyl acrylate (Tg = 17 ° C.) and the like.

  The content of the (meth) acrylate monomer is preferably 16% to 35% (% by weight) in the acrylic adhesive component. When the content of the (meth) acrylate monomer is less than 16%, the initial adhesive force cannot be sufficiently increased. On the other hand, if it exceeds 35%, the monomer component increases and the glass transition point of the cured product may increase.

  The monomer to be reacted with the (meth) acrylic oligomer can include a (meth) acrylate monomer having no polar group other than the (meth) acrylate monomer having a hydroxyl group. Such a monomer preferably has a glass transition point of -70 ° C to 0 ° C. By adding a monomer having a glass transition point in such a temperature range, the tackiness can be increased.

  More specifically, by setting the glass transition point within this temperature range, it is possible to suppress an increase in the glass transition point of the acrylic adhesive component after polymerization, and to give tack to the surface of the heat conductive adhesive sheet. it can. When the acrylic pressure-sensitive adhesive component has a tack, the heat-conductive pressure-sensitive adhesive sheet is easily wetted with respect to the adherend, and can increase the adhesive force with respect to the adherend. Furthermore, since it is easy to wet with respect to a to-be-adhered body, a heat conductive adhesive sheet can be closely_contact | adhered without gap and the thermal resistance of the interface can be made low.

  The upper limit of the glass transition point of the (meth) acrylate monomer having a hydroxyl group is 20 ° C., whereas the upper limit of the glass transition point of the (meth) acrylate monomer having no polar group is 0 ° C. This is because a (meth) acrylate monomer having no polar group having the same boiling point as the (meth) acrylate monomer having a group has a low glass transition point. That is, the vapor pressure of the monomer in the composition can be lowered by using a (meth) acrylate monomer having no polar group having a low glass transition point. Thus, even if there is unreacted residual monomer in the heat conductive adhesive sheet, the vapor pressure can be lowered, and problems due to volatilization of the unreacted monomer can be prevented.

  Examples of the monomer having a low glass transition point in the temperature range of −70 ° C. to 0 ° C. include alkyl (meth) acrylates having 4 to 20 carbon atoms. Specifically, n-butyl acrylate ( Tg = −55 ° C.), 2-ethylhexyl acrylate (Tg = −70 ° C.), isooctyl acrylate (Tg = −56 ° C.), n-lauryl acrylate (Tg = −65 ° C.), etc. Can be mentioned.

  In addition, if it is a desired range in which the adhesive strength does not extremely decrease, a small amount of a (meth) acrylate monomer having a glass transition point exceeding 0 ° C. can be added. Moreover, you may add a small amount of (meth) acrylate monomers which have polar groups other than a carboxyl group in the range which does not affect adhesiveness.

A solvent or a diluent can be added to the composition for improving dispersibility and adjusting the viscosity. Examples of such a solvent include various solvents such as toluene, xylene, ethyl acetate, butyl acetate, acetone, methyl ethyl ketone, and isopropyl alcohol, and examples of the diluent include an acrylic polymer having no functional group.
Furthermore, various additives such as a photocuring initiator, an antioxidant, an ultraviolet absorber, a flame retardant, a light stabilizer, and an antistatic agent are added to the composition as long as the adhesiveness and durability are not affected. Can be added.

Preparation of sample As a (meth) acrylic oligomer, 28 parts by weight of an ester copolymer acrylate having a viscosity of 5000 mPa · s, 16 parts by weight of hydroxypropyl acrylate as a (meth) acrylate monomer having a hydroxyl group, and having a polar group (Meth) acrylate monomer 48 parts by weight (ethylhexyl acrylate 9 parts by weight, n-butyl acrylate 7 parts by weight, acrylic ester monomer 32 parts by weight), acrylic polymer having no functional group as a diluent (molecular weight 3000, viscosity 1000 mPa · 5 parts by weight of s, Tg: -77 ° C, 5 parts by weight of aromatic condensed phosphate ester flame retardant (1,3 phenylene bis (diphenyl phosphate)), photocuring initiator Irgacure 184 (BASF Japan Ltd.) Company) and average particle size 3μ In yield and pulverized aluminum hydroxide powder 110 parts by weight, a uniformly mixed to mixed composition. Next, this mixed composition is sandwiched between a pair of release-treated polyethylene terephthalate films (release films), stretched with a roll so as to have a thickness of 200 μm in that state, and then irradiated with ultraviolet rays having an integrated light quantity of 500 mJ / cm ^2. Thus, a heat conductive adhesive sheet was obtained. This heat conductive adhesive sheet was designated as Sample 1.

Samples 2 to 19 were prepared by changing the raw materials used in Sample 1 to the compositions (components) shown in Table 1 or Table 2. However, acrylic acid is used for the carboxyl group-containing monomers shown in Tables 1 and 2.
Tables 1 and 2 also show the ratio (content ratio: wt%) of the predetermined monomer to the resin component (total amount of oligomer and monomer) and various test results shown below.

<Initial adhesive strength>
About each sample, adhesive force was measured by the method of JISZ0237 regulation. Specifically, each sample was cut into a width of 24 mm, one release film was peeled off and attached to a polyethylene terephthalate film having a thickness of 25 μm, and the other release film was peeled off and attached to a stainless steel plate. Then, a 180 ° peeling test was performed at a peeling speed of 5 mm / s, and the value of the adhesive strength (N / 10 mm) at that time was defined as the initial adhesive strength.
The initial adhesive strength is shown in Table 1 or Table 2 as “◯” when 7.5 N / 10 mm or more, “Δ” when 4.0 N / 10 mm or more and less than 7.5 N / 10 mm, and “x” when less than 4.0 N / 10 mm. Indicated.

<Adhesive strength after applying thermal history>
Each sample in the state of being sandwiched between a polyethylene terephthalate film and a stainless steel plate (same as the above test piece for initial adhesive strength evaluation) was allowed to stand in a high-temperature bath at 120 ° C. for 72 hours (given thermal history) (N / 10 mm) was measured by the same method as the measurement of the initial adhesive strength, and was defined as the adhesive strength after applying the thermal history.
The adhesive strength after applying the heat history is also shown in Table 1 as “◯” when 7.5 N / 10 mm or more, “Δ” when 4.0 N / 10 mm or more but less than 7.5 N / 10 mm, and “x” when less than 4.0 N / 10 mm. It is shown in Table 2.

  Table 1 or Table 2 shows the rate of change of the adhesive strength after application of the heat history from the initial adhesive strength as the rate of change of the adhesive strength. Evaluated as “◯” when the adhesive strength did not decrease below the initial adhesive strength, “△” when the adhesive strength decrease was 30% or less, and “X” when the adhesive strength decrease exceeded 30%. did.

<Thermal conductivity>
In each of the above samples, the thickness was changed to 100 μm instead of the thickness of 200 μm, and a disk shape with a diameter of 10 mm was formed. Then, a sample for thermal conductivity measurement was prepared by removing the release films on both sides. The sample numbers of these samples were the same as those of each sample having a thickness of 200 μm. And about these samples, the thermal conductivity (W / m * K) was measured by the laser flash method.

Evaluation results The heat conductive adhesive sheets of Sample 1 to Sample 6 containing a monomer having a hydroxyl group and no monomer having a carboxy group are both 4.0 N / 10 mm in initial adhesive force and adhesive force after imparting thermal history. As described above, the adhesive strength did not decrease even after the thermal history was imparted. In particular, with respect to Sample 2 to Sample 5 in which the amount of the monomer having a hydroxyl group is 16.8% to 31.6% (weight%), both the initial adhesive strength and the adhesive strength after applying the thermal history are 7.5 N. / 10 mm or more and excellent adhesive strength.

On the other hand, Sample 7 which does not contain a monomer having a hydroxyl group and a monomer having a carboxy group did not decrease the adhesive strength after imparting thermal history, but the initial adhesive strength and the adhesive strength after imparting thermal history were 2.4 N / The value was as extremely low as 10 mm.
Samples 8 and 9 containing no monomer having a hydroxyl group and containing a monomer having a carboxy group have a low or moderate initial adhesive strength, and a very low adhesive strength after applying a thermal history. Value.

  In addition, regarding samples 10 to 15 including monomers having a hydroxyl group and monomers having a carboxy group, although the initial adhesive strength was improved in some samples, the adhesive strength after applying the thermal history was remarkably increased in all samples. Declined.

  For comparison, the samples 16 to 18 which do not contain the heat conductive filler were evaluated. The remarkable decrease in the adhesive strength after the thermal history was imparted regardless of the presence or absence of the monomer having a hydroxyl group and the monomer having a carboxy group. I could not confirm.

  Regarding the thermal conductivity, the values of Samples 1 to 15 in which the thermally conductive filler was blended were 0.8 W / m · K, and the values of Samples 16 to 18 in which the thermally conductive filler was not blended were 0. It was confirmed that the thermal conductivity was superior to that of 1 W / m · K.

  The above description is merely an example of the present invention, and the production method, curing method, usage method, raw material and the like of the composition are appropriately changed, for example, the use of known raw materials other than the above does not depart from the spirit of the present invention. Therefore, such changes are also included in the scope of the technical idea of the present invention.

Claims (3)

In the heat conductive adhesive sheet containing the heat conductive filler and the acrylic adhesive component,
The acrylic adhesive component includes a (meth) acrylic oligomer having a (meth) acryloyl group and a (meth) acrylate monomer having a hydroxyl group occupying 16% to 35% (wt%) in the acrylic adhesive component; A thermally conductive pressure-sensitive adhesive sheet, which is a cured product of a composition containing no carboxyl group. The heat conductive adhesive sheet of Claim 1 whose molecular weight of the said (meth) acrylic-type oligomer is 100-10000. The heat conductive adhesive sheet according to claim 1, wherein the (meth) acrylic oligomer is any one of polyester acrylate, epoxy acrylate, and urethane acrylate.