WO2016199410A1 - Thermally conductive sheet, flexible wiring board in which same is used, and method for manufacturing thermally conductive sheet - Google Patents

Abstract

A thermally conductive sheet provided with a graphite sheet (21), a first adhesive layer (22) bonded to the upper surface of the graphite sheet (21), and a first protective film (23) bonded to the upper surface of the first adhesive layer (22). The first adhesive layer (22) contains at least a polyolefin-based elastomer and a thermosetting resin. The glass transition point of the first protective film (23) is higher than that of the first adhesive layer (22).

Description

HEAT CONDUCTIVE SHEET, FLEXIBLE WIRING BOARD USING THE SAME, AND METHOD FOR PRODUCING HEAT CONDUCTIVE SHEET

The present disclosure relates to a heat conductive sheet used for heat dissipation of various electronic devices, a flexible wiring board using the heat conductive sheet, and a method for manufacturing the heat conductive sheet.

In recent years, the amount of heat generated from electronic components such as semiconductor elements has been increasing as various electronic devices have advanced functions and processing capabilities have been rapidly improved. And in order to transmit the heat which generate | occur | produced from these electronic components to a heat sink etc., a heat conductive sheet is used.

A sheet material used as a heat conductive sheet is a graphite sheet. The graphite sheet has a higher thermal conductivity in the plane direction than in the thickness direction. A heat conductive sheet using a graphite sheet is provided with an insulating layer for ensuring electrical insulation on the surface of the heat conductive sheet because the graphite sheet itself has conductivity.

For example, Patent Document 1 is known as prior art document information related to such a heat conductive sheet.

JP 2007-207800 A

A heat conductive sheet of the present disclosure includes a graphite sheet, a first adhesive layer bonded to the upper surface of the graphite sheet, and a first protective film bonded to the upper surface of the first adhesive layer, the first adhesive layer Includes at least a polyolefin-based elastomer and a thermosetting resin, and the glass transition point of the first protective film is higher than the glass transition point of the first adhesive layer.

With the above configuration, the heat conductive sheet of the present disclosure is useful as a heat conductive sheet used at high temperatures.

FIG. 1 is a cross-sectional view of a heat conductive sheet in the first embodiment. FIG. 2 is a cross-sectional view of a flexible wiring board using the heat conductive sheet in the first embodiment. FIG. 3 is a cross-sectional view of the heat conductive sheet in the second embodiment. FIG. 4 is a cross-sectional view of a flexible wiring board using the heat conductive sheet in the second embodiment. FIG. 5 is a cross-sectional view of a flexible wiring board of a comparative example in the second embodiment. FIG. 6 is a view for explaining the temperature distribution of the flexible wiring board using the heat conductive sheet in the second embodiment.

Each of the embodiments described below shows a specific example. Numerical values, shapes, materials, constituent elements, arrangement positions and connection forms of the constituent elements, and the like shown in the following embodiments are merely examples, and are not intended to limit the present invention. In addition, among the constituent elements in the following embodiments, constituent elements that are not described in the independent claims indicating the highest concept are described as optional constituent elements.

The problems with the conventional heat conductive sheet will be described.

With the increase in heat generation in electronic parts, heat conductive sheets are required to be used at high temperatures. In the conventional heat conductive sheet, there is a possibility that peeling occurs between the graphite sheet and the adhesive layer due to thermal strain caused by the difference in thermal expansion coefficient between the graphite sheet and the adhesive layer. That is, the conventional heat conductive sheet has low heat resistance.

Hereinafter, the heat conductive sheet of the present disclosure having higher heat resistance than the conventional heat conductive sheet will be described.

(Embodiment 1)
[1-1. Thermal conductive sheet]
FIG. 1 is a cross-sectional view of a heat conductive sheet in the first embodiment.

As shown in FIG. 1, the heat conductive sheet 10 includes a graphite sheet 11, an adhesive layer 12, and a protective film 13.

The thickness of the graphite sheet 11 is preferably thick in order to ensure thermal conductivity, and is desirably 10 μm or more in order to ensure sufficient thermal conductivity. On the other hand, the thickness of the graphite sheet 11 is desirably 70 μm or less in order to ensure the flexibility of the heat conductive sheet 10. In the present embodiment, the thickness of the graphite sheet 11 is 25 μm.

The graphite sheet 11 is a pyrolytic graphite sheet produced by pyrolyzing a polymer.

The adhesive layer 12 includes a first adhesive layer 12a bonded to the upper surface side of the graphite sheet 11, a second adhesive layer 12b bonded to the lower surface side of the graphite sheet 11, and both end portions of the graphite sheet 11 (FIG. 1). The adhesive layer 12c and the adhesive layer 12d are present on the left and right ends of FIG. Accordingly, the entire graphite sheet 11 is covered with the first adhesive layer 12a, the second adhesive layer 12b, the adhesive layer 12c, and the adhesive layer 12d.

The thickness of the first adhesive layer 12a and the second adhesive layer 12b is desirably 5 μm or more in order to ensure adhesion. On the other hand, the thickness of the first adhesive layer 12a and the second adhesive layer 12b is preferably 30 μm or less in order to transfer heat from the surface of the protective film 13 to the graphite sheet 11 via the adhesive layer 12. In the present embodiment, the thickness of the first adhesive layer 12a and the second adhesive layer 12b is 25 μm.

The adhesive layer 12 is made of a polymer alloy containing a polyolefin-based elastomer and a thermosetting resin. The adhesive layer 12 has two glass transition points. Of the two glass transition points, the first glass transition point is related to the temperature characteristics of the polyolefin-based elastomer, and the second glass transition point is related to the temperature characteristics of the thermosetting resin. The second glass transition point is higher than the first glass transition point. Here, details of the glass transition point of the polymer alloy according to the present embodiment will be described later in Examples.

Note that when the adhesive layer 12 according to the present disclosure is simply referred to as a glass transition point, the glass transition point refers to a second glass transition point.

In addition, the glass transition point measurement method according to the present disclosure is performed by DMA (Dynamic Mechanical Analysis).

Examples of the polyolefin-based elastomer contained in the adhesive layer 12 include styrene elastomers, butadiene elastomers, acrylic elastomers, and elastomers made of polyolefin block copolymers. The polyolefin-based elastomer imparts elastic deformation performance to the adhesive layer 12 and has higher compatibility with the thermosetting resin than elastomers other than the polyolefin-based elastomer.

Examples of the thermosetting resin contained in the adhesive layer 12 include copolymers such as bismaleimide resin and epoxy resin. The adhesive layer 12 has high mechanical strength and high moisture resistance by including these thermosetting resins.

The protective film 13 includes a first protective film 13a bonded to the upper surface side of the first adhesive layer 12a and a second protective film 13b bonded to the lower surface side of the second adhesive layer 12b. That is, the first protective film 13a and the second protective film 13b are bonded to the graphite sheet 11 via the first adhesive layer 12a and the second adhesive layer 12b.

The protective film 13 also has a glass transition point. The glass transition point of the protective film 13 is higher than both the first glass transition point and the second glass transition point in the adhesive layer 12.

The thickness of the first protective film 13a and the second protective film 13b is desirably 3 μm or more in order to ensure insulation. On the other hand, the thickness of the first protective film 13a and the second protective film 13b is preferably 30 μm or less in order to transfer heat from the surface of the protective film 13 to the graphite sheet 11 via the adhesive layer 12. In the present embodiment, the thickness of the first protective film 13a and the second protective film 13b is 15 μm.

In the present embodiment, the protective film 13 is made of polyimide and has a glass transition point of 285 ° C. The protective film 13 only needs to have a glass transition point higher than the first glass transition point and the second glass transition point of the adhesive layer 12. Examples of the constituent material of the protective film 13 include liquid crystal polymer and polyamideimide in addition to polyimide.

The protective film 13 may have a glass transition point higher than the melting point. In this case, the protective film 13 may have a melting point higher than the glass transition point of the adhesive layer 12.

With the above configuration, the heat conductive sheet 10 of the present disclosure has higher heat resistance than the conventional heat conductive sheet. That is, the adhesive layer 12 has high elastic deformation performance due to the polyolefin-based elastomer. Therefore, the adhesive layer 12 can relieve the thermal stress generated by the difference in coefficient of thermal expansion between the graphite sheet 11 and the protective film 13.

Further, the adhesive layer 12 includes a polyolefin-based elastomer, and thus has high flexibility near the temperature at which various electronic devices are used.

Furthermore, since the adhesive layer 12 is thermoset by the crosslinking reaction of the thermosetting resin, it has high mechanical strength and high moisture resistance.

Further, the glass transition point of the protective film 13 is higher than both the first glass transition point and the second glass transition point in the adhesive layer 12. Thereby, in the temperature environment which exceeds the 2nd glass transition temperature of the contact bonding layer 12, and is below the glass transition temperature of the protective film 13, the protective film 13 maintains the shape, without softening. On the other hand, the adhesive layer 12 is softened, and the distortion due to the difference in linear expansion coefficient between the graphite sheet 11 and the protective film 13 is reduced. As a result, peeling or breakage of the protective film 13 can be suppressed, and the shape of the heat conductive sheet 10 can be maintained.

The heat conductive sheet 10 has a shape that is symmetrical in the thickness direction with respect to a plane that passes through the center of the graphite sheet 11 in the thickness direction and is parallel to the main surface of the graphite sheet 11. Thereby, the thermal expansion difference between the different materials generated on the upper surface side and the lower surface side of the graphite sheet 11 is offset between the upper surface side and the lower surface side of the graphite sheet 11, and the warpage of the heat conductive sheet 10 is suppressed.

The end of graphite sheet 11 is covered with adhesive layer 12c and adhesive layer 12d. Thereby, detachment | desorption of the graphite piece in the edge part of the graphite sheet 11 can be suppressed. The same effect can be obtained even when the end portion of the graphite sheet 11 is covered with the protective film 13.

Further, the entire graphite sheet 11 is covered with the first adhesive layer 12a, the second adhesive layer 12b, the adhesive layer 12c, and the adhesive layer 12d, so that the graphite sheet 11 is protected from external stress.

[1-2. Flexible wiring board]
Next, a flexible substrate using the heat conductive sheet according to the present embodiment will be described.

FIG. 2 is a cross-sectional view of a flexible wiring board using the heat conductive sheet in the first embodiment.

As shown in FIG. 2, the flexible wiring board 100 includes a graphite sheet 11, an adhesive layer 12, a protective film 13, and a wiring pattern 14 that constitute the heat conductive sheet 10. The wiring pattern 14 includes a wiring pattern 14a provided on the upper surface of the first protective film 13a and a wiring pattern 14b provided on the lower surface of the second protective film 13b. The wiring pattern 14a and the wiring pattern 14b are both made of copper and have a thickness of 12 μm. Electronic components are appropriately mounted on the wiring pattern 14a and the wiring pattern 14b.

With the above configuration, the flexible wiring board 100 of the present disclosure diffuses the heat generated by the electronic components mounted on the flexible wiring board 100 through the graphite sheet 11 having a high thermal conductivity, thereby reducing the temperature of the heat generating components. Can be suppressed.

Moreover, the flexible wiring board 100 of the present disclosure can form two wiring patterns 14 a and 14 b on both the upper and lower surfaces of one heat conductive sheet 10. Therefore, it is useful for downsizing electronic devices and reducing the number of parts.

[1-3. Example of adhesive layer material]
Examples and variations of the material of the adhesive layer 12 according to the present embodiment will be described.

<Material composition of adhesive layer>
(Example 1)
The heat conductive sheet 10 and the flexible wiring board 100 were produced using the polymer alloy which mix | blended polyolefin-type elastomer and the thermosetting resin as the contact bonding layer 12. FIG. The polymer alloy has a storage modulus of 25 ° C. of 3.0 × 10 ⁸ Pa, a storage modulus of 150 ° C. of 1.3 × 10 ⁸ Pa, a first glass transition point of −46 ° C., and a second glass transition point of 240 ° C. is there.

(Example 2)
The heat conductive sheet 10 and the flexible wiring board 100 were produced using the polymer alloy which mix | blended polyolefin-type elastomer and the thermosetting resin as the contact bonding layer 12. FIG. The polymer alloy has a storage modulus of 25 ° C. of 2.0 × 10 ⁸ Pa, a storage modulus of 150 ° C. of 1.0 × 10 ⁸ Pa, a first glass transition point of 3 ° C., and a second glass transition point of 220 ° C. .

(Example 3)
The heat conductive sheet 10 and the flexible wiring board 100 were produced using the polymer alloy which mix | blended polyolefin-type elastomer and the thermosetting resin as the contact bonding layer 12. FIG. The polymer alloy has a storage modulus of 25 ° C. of 9.0 × 10 ⁸ Pa, a storage modulus of 150 ° C. of 1.2 × 10 ⁷ Pa, a first glass transition point of 32 ° C., and a second glass transition point of 188 ° C. .

Example 4
The heat conductive sheet 10 and the flexible wiring board 100 were produced using the polymer alloy which mix | blended polyolefin-type elastomer and the thermosetting resin as the contact bonding layer 12. FIG. The polymer alloy has a storage modulus of 25 ° C. of 5.8 × 10 ⁸ Pa, a storage modulus of 150 ° C. of 4.0 × 10 ⁶ Pa, a first glass transition point of 30 ° C., and a second glass transition point of 130 ° C. .

(Example 5)
The heat conductive sheet 10 and the flexible wiring board 100 were produced using the polymer alloy which mix | blended polyolefin-type elastomer and the thermosetting resin as the contact bonding layer 12. FIG. The polymer alloy has a storage modulus of 25 ° C. of 7.0 × 10 ⁸ Pa, a storage modulus of 150 ° C. of 1.1 × 10 ⁶ Pa, a first glass transition point of 42 ° C., and a second glass transition point of 120 ° C. .

(Comparative Example 1)
The heat conductive sheet 10 and the flexible wiring board 100 were produced using the thermoplastic polyimide which does not contain polyolefin-type elastomer and a thermosetting resin as the contact bonding layer 12. FIG. The thermoplastic polyimide has a 25 ° C. storage elastic modulus of 5.4 × 10 ⁸ Pa, a 150 ° C. storage elastic modulus of 8.0 × 10 ⁴ Pa, and a first glass transition point of 46 ° C.

(Comparative Example 2)
The heat conductive sheet 10 and the flexible wiring board 100 were produced using the polymer alloy which mix | blended the thermosetting resin which has as a main component the epoxy resin which does not contain polyolefin-type elastomer as the contact bonding layer 12. FIG.

The polymer alloy has a 25 ° C. storage elastic modulus of 3.2 × 10 ⁹ Pa, a 150 ° C. storage elastic modulus of 8.5 × 10 ⁸ Pa, and a second glass transition point of 165 ° C.

<Test items>
<< Folding resistance test >>
In order to confirm the adhesion of the graphite sheet 11, the adhesive layer 12, and the protective film 13, a fold resistance test (MIT (Masschus Institutes of Technology) test) was performed.

The evaluation sample used a heat conductive sheet 10 having a width of 15 mm and a length of 110 mm in order to make it easy to confirm the state of delamination. The load during the test was 500 g, the bending angle was 135 °, the bending clamp R was 0.38 mm, and the bending test was performed 1000 times.

<< Heat cycle test >>
In order to confirm the influence of the difference in thermal expansion coefficient between the constituent materials of the graphite sheet 11, the adhesive layer 12 and the protective film 13 on the adhesion between the respective layers, the holding time is set in the range of −40 ° C. to 150 ° C. A heat cycle test for 30 minutes each was performed 500 cycles.

Evaluation sample used flexible wiring board 100 having a width of 20 mm and a length of 60 mm. After the 500 cycle test, the presence or absence of swelling of the sample appearance was confirmed. Moreover, in order to confirm the presence or absence of interlayer swelling, the copper of the wiring pattern 14a and the wiring pattern 14b was removed by an etching process, and the presence or absence of swelling inside the heat conductive sheet 10 was confirmed.

<< Solder tank float heat resistance test >>
A solder bath float heat resistance test, which is generally used for reliability evaluation of flexible substrates, was performed. Test conditions were 260 ° C. for 1 minute and 288 ° C. for 1 minute, respectively, under two conditions.

Evaluation sample used flexible wiring board 100 having a width of 20 mm and a length of 60 mm. And the presence or absence of the swelling of the sample external appearance after a solder tank float heat test was confirmed. Moreover, in order to confirm the presence or absence of interlayer swelling, the copper of the wiring pattern 14a and the wiring pattern 14b was removed by an etching process, and the presence or absence of swelling inside the heat conductive sheet 10 was confirmed.

<Test results>
<< Folding resistance test >>
Those in which peeling did not occur between the layers in the 1000-fold bending test were normal (indicated by “◯” in Table 1), and those in which peeling occurred were abnormal (indicated by “x” in Table 1). The results are shown in Table 1.

No abnormality occurred in the 1000-fold bending test because the heat-conductive sheet 10 according to Examples 1 to 5 in which the adhesive layer 12 contains a polyolefin-based elastomer and a thermosetting resin, and the adhesive layer 12 is thermoplastic. It is the heat conductive sheet 10 which concerns on the comparative example 1 using a polyimide. On the other hand, in the heat conductive sheet 10 according to Comparative Example 2 using a thermosetting resin whose main component is an epoxy resin for the adhesive layer 12, swelling occurred at the interface between the graphite sheet 11 and the adhesive layer 12. This is because the adhesive layer 12 of Comparative Example 2 is made of a thermosetting material and is hard at room temperature and does not have flexibility.

From the above results, it is desirable that the storage elastic modulus at 25 ° C. is 9.0 × 10 ⁸ Pa or less from the viewpoint of ensuring folding resistance at room temperature.

<< Heat cycle test >>
In the confirmation of the swelling of the external appearance of the sample and the swelling of the inside, those that are not swollen are normal (indicated by “◯” in Table 1), and those that are swollen are abnormal (in Table 1) (Indicated by “x”). The results are shown in Table 1.

No abnormality occurred in the 500 heat cycle tests in the flexible wiring substrate 100 according to Examples 1 to 5 in which the adhesive layer 12 contains a polyolefin-based elastomer and a thermosetting resin. On the other hand, in the flexible wiring substrate 100 according to Comparative Example 1 using thermoplastic polyimide for the adhesive layer 12, a small undulation occurred at the interface between the wiring pattern 14 and the protective film 13. In addition, the flexible wiring board 100 according to Comparative Example 2 was not subjected to the heat cycle test because an abnormality occurred as a result of the folding resistance test (indicated by “-” in Table 1).

The adhesive layer 12 according to Comparative Example 1 has a glass transition point of 46 ° C. and, unlike the adhesive layers 12 according to Examples 1 to 5, does not have a glass transition point on the high temperature side, and is a single component thermoplastic polyimide It is. And since the storage elastic modulus at 150 ° C. is very low and it is in a state close to a melt at this temperature, the force to hold the wiring pattern 14 and the protective film 13 and the graphite sheet 11 is lost, and swell is generated Conceivable.

From the above results, in order to absorb the difference in the thermal expansion coefficient between the graphite sheet 11 having a very low thermal expansion coefficient and the protective film 13 having a thermal expansion coefficient of 14 ppm / ° C. to 20 ppm / ° C. The adhesive layer 12 according to the present disclosure including a polyolefin elastomer that is soft and sticky from a low temperature region to a high temperature region and a thermosetting resin that does not flow even in a high temperature region is desirable.

<< Solder tank float heat resistance test >>
In the confirmation of the swelling of the external appearance of the sample and the swelling inside, it is normal that no swelling occurs (indicated by a circle in Table 1), and any one that has swelling is abnormal (× in Table 1) ). The results are shown in Table 1.

No abnormality occurred in the solder bath float heat resistance test at 260 ° C. for 1 minute in the flexible wiring board 100 according to Examples 1 to 5 in which the adhesive layer 12 contains a polyolefin-based elastomer and a thermosetting resin. . On the other hand, in the flexible wiring board 100 according to the comparative example 1 using the thermoplastic polyimide for the adhesive layer 12, the adhesive layer 12 flows and an appearance abnormality occurs. This is because the adhesive layer 12 according to Comparative Example 1 has a low glass transition point of 46 ° C. and is a thermoplastic material.

No abnormality occurred in the solder bath float heat resistance test at 288 ° C. for 1 minute in the flexible wiring board 100 according to Examples 2 to 4 having a first glass transition temperature. On the other hand, in the flexible wiring substrate 100 according to Example 1 and Example 5, swelling was generated at the interface between the graphite sheet 11 and the adhesive layer 12 due to the etching confirmation of the wiring pattern 14.

Note that the flexible wiring board 100 according to Comparative Example 1 was not subjected to a solder bath float heat test at 288 ° C. for 1 minute because an abnormality occurred as a result of the solder bath float heat test at 260 ° C. for 1 minute. Further, the flexible wiring board 100 according to Comparative Example 2 was not subjected to the solder bath float heat resistance test because it was abnormal as a result of the folding resistance test (indicated by “-” in Table 1).

From the above results, in the heat cycle test in the range of −40 ° C. to 150 ° C. and the 260 ° C. solder bath heat resistant float test, no abnormality occurred in the adhesive layers 12 of Examples 1 to 5. Therefore, when the temperature range where the reliability of the heat conductive sheet 10 and the flexible wiring board 100 should be ensured is −40 ° C. or more and 260 ° C. or less, the material of the adhesive layer 12 has the physical property values according to Examples 1 to 5. It is desirable to have. Specifically, the material of the adhesive layer 12 desirably has a first glass transition point of −46 ° C. or higher and 42 ° C. or lower, and a second glass transition point of 120 ° C. or higher and 240 ° C. or lower. The material of the adhesive layer 12 desirably has a storage elastic modulus at 150 ° C. in the range of 1.1 × 10 ⁶ Pa to 1.3 × 10 ⁸ Pa.

By setting the first glass transition point of the adhesive layer 12 to −46 ° C. or higher and 42 ° C. or lower and the second glass transition point to 120 ° C. or higher and 240 ° C. or lower, the adhesive layer 12, the graphite sheet 11, and the protective film 13 Adhesiveness to each becomes high.

Moreover, the adhesiveness of the contact bonding layer 12 and the graphite sheet 11 becomes high because the viscoelasticity in 150 degreeC shall be 1.1 * 10 < ⁶ > or more and 1.3 * 10 < ⁸ > Pa or less. That is, due to the above-described viscoelasticity of the adhesive layer 12, the softened adhesive layer 12 enters the irregularities on the surface of the graphite sheet 11, and the adhesion between the adhesive layer 12 and the graphite sheet 11 is increased.

Furthermore, in order to ensure the reliability in the temperature range of −40 ° C. to 288 ° C., no abnormality occurred in the 288 ° C. solder bath heat-resistant float test, and the adhesion having the physical property values according to Examples 2 to 4 Layer 12 is desirable. Specifically, it is desirable that the first glass transition temperature is 3 ° C. or higher and 32 ° C. or lower, and the second glass transition temperature is 130 ° C. or higher and 220 ° C. or lower. The storage elastic modulus at 150 ° C. is preferably in the range of 4.0 × 10 ⁶ Pa to 1.0 × 10 ⁸ Pa.

(Embodiment 2)
[2-1. Thermal conductive sheet]
FIG. 3 is a cross-sectional view of the heat conductive sheet in the second embodiment.

As shown in FIG. 3, in the heat conductive sheet 20 according to the present embodiment, only one first adhesive layer 22 is adhered to the upper surface of the graphite sheet 21, and the first protective film 23 is adhered to the upper surface of the first adhesive layer 22. Is different from the heat conductive sheet 10 according to the first embodiment in that only one sheet is bonded. Other configurations are basically the same as those of the heat conductive sheet 10 according to the first embodiment. Therefore, only the difference will be described in detail, and the description of other configurations will be simplified or omitted.

The heat conductive sheet 20 includes a graphite sheet 21, a first adhesive layer 22, and a first protective film 23.

The first adhesive layer 22 is bonded to the upper surface of the graphite sheet 21. The first adhesive layer 22 corresponds to the adhesive layer according to the present disclosure.

The first protective film 23 is bonded to the upper surface of the first adhesive layer 22. In addition, the 1st protective film 23 corresponds to the protective film which concerns on this indication.

According to such a configuration, the configuration of the heat conductive sheet 20 can be simplified and the thickness can be reduced. Therefore, the heat conductive sheet 20 is useful when the wiring pattern is mounted only on one side.

[2-2. Flexible wiring board]
Next, a flexible wiring board using the heat conductive sheet according to the present embodiment will be described.

FIG. 4 is a cross-sectional view of a flexible wiring board using the heat conductive sheet in the second embodiment.

As shown in FIG. 4, the flexible wiring board 200 includes a graphite sheet 21, a first adhesive layer 22, a first protective film 23, and a wiring pattern 24 that constitute the heat conductive sheet 20. The first protective film 23 is bonded to the lower surface of the graphite sheet 21 via the first adhesive layer 22, and the wiring pattern 24 is formed on the lower surface of the first protective film 23.

The wiring pattern 24 is made of copper and has a thickness of 12 μm. As a method of forming the wiring pattern 24, a method of forming a pattern by etching or the like on the first protective film 23 bonded to the graphite sheet 21 via the first adhesive layer 22, or a wiring on the first protective film 23 There is a method of bonding the graphite sheet 21 through the first adhesive layer 22 to the pattern 24 formed.

It is desirable to provide a substrate protective layer 25a for protecting the upper surface of the flexible wiring board 200 on the upper surface of the graphite sheet 21. Further, it is desirable to provide a substrate protective layer 25 b that protects the lower surface of the flexible wiring substrate 200 on the lower surface of the wiring pattern 24. The substrate protective layer 25a and the substrate protective layer 25b can be configured such that a liquid thermoplastic polyimide is applied and dried, or another film is bonded through the thermoplastic polyimide. In addition, when providing the board | substrate protective layer 25a on the upper surface of the graphite sheet 21, in order to ensure the adhesive force of the graphite sheet 21 and the board | substrate protective layer 25a in a high temperature environment, the 1st contact bonding layer 22 which concerns on the heat conductive sheet 20 and It is desirable that the substrate protective layer 25a and the graphite sheet 21 are bonded together through an adhesive layer 28 made of the same material and configuration.

The flexible wiring board 200 is provided with a heat generating component 26 serving as a heat source such as a camera unit and a connector component 27.

Usually, polyimide having low thermal conductivity is used for the flexible wiring board. For this reason, when a component that easily generates heat, such as a camera module, is connected by a flexible wiring board, a countermeasure against heat becomes a problem. On the other hand, according to the flexible wiring board 200 according to the present embodiment, the flexible wiring board 200 itself can have good thermal conductivity, and heat dissipation can be improved. As a result, a highly reliable flexible wiring board 200 can be obtained.

[2-3. Example of flexible wiring board]
Next, examples of the flexible wiring board 200 using the heat conductive sheet 20 according to the present embodiment will be described.

In FIG. 4, the graphite sheet 21 has two thicknesses of 25 μm (hereinafter referred to as Example 1) and 70 μm (hereinafter referred to as Example 2). A ceramic heating element was attached instead of the heat generating component 26 of the flexible wiring board, and the temperature distribution on the surface of the flexible wiring board 200 when 0.7 W of power was supplied to the ceramic heating element was measured with a thermoviewer. The thermoviewer was disposed on the upper side of the drawing (substrate protective layer 25a side).

FIG. 5 is a cross-sectional view of a flexible wiring board of a comparative example in the second embodiment.

As shown in FIG. 5, in the flexible wiring substrate 250 of the comparative example, the wiring pattern 24 is laminated with a first protective film 23 and a substrate protective layer 25 made of polyimide and having a thickness of 30 μm. The flexible wiring board 250 is attached with a heat generating component 26 serving as a heat source such as a camera unit and a connector component 27.

FIG. 6 is a diagram for explaining the temperature distribution of the flexible wiring board using the heat conductive sheet in the second embodiment.

6, the diagrams showing the temperature distributions of Example 1, Example 2, and Comparative Example are denoted by L1, L2, and LR, respectively. In FIG. 6, the vertical axis indicates the surface temperature of the flexible wiring board 200. In FIG. 6, the horizontal axis indicates the horizontal position of the flexible wiring board 200, the position a corresponds to the back surface of the heating element, and the position b corresponds to the connector connecting portion (see FIGS. 4 and 5).

As shown in FIG. 6, the temperature at position a when 0.7 W of electric power was supplied to the ceramic heating element was 81.6 ° C. in the comparative example, while 64.2 ° C. in Example 1. Example 2 was 61.3 ° C. That is, in Example 1 and Example 2 in which the flexible wiring board 200 includes the graphite sheet 21, the maximum temperature on the surface of the flexible wiring board 200 is lower by 20.3 ° C. to 17.4 ° C. than the comparative example. This is because the graphite sheet 21 diffuses heat toward the connector part 27.

In comparison between Example 1 (25 μm) and Example 2 (70 μm) in which the thickness of the graphite sheet 21 is different, Example 2 (70 μm) in which the thickness of the graphite sheet 21 is larger is more likely to diffuse heat. However, the difference in the effect due to the thickness of the graphite sheet 21 is smaller than the difference in the effect of the presence (Example 1 and Example 2) and the absence (Comparative Example) of the graphite sheet 21.

As described above, the graphite sheet 21 included in the flexible wiring board 200 has an effect of diffusing the heat generated from the heat generating component 26 and keeping the temperature of the heat generating component 26 low.

[2-4. Production method]
An example of the manufacturing method of the heat conductive sheet 20 will be described.

First, an adhesive made of a liquid polymer alloy in which a polyolefin elastomer, a thermosetting resin and a solvent are mixed is generated. This liquid adhesive is applied to the upper surface of the first protective film 23 and heated at 150 ° C. for 5 minutes to volatilize the solvent and dry the liquid adhesive. By doing so, the first adhesive layer 22 is provided on the upper surface of the first protective film 23. Thus, the thickness of the 1st contact bonding layer 22 can be made thin by using a liquid adhesive agent.

Next, the graphite sheet 21 is disposed on the upper surface of the first adhesive layer 22, and this is laminated by heating to 150 ° C. while applying a load of 0.5 MPa in a vacuum state.

Next, the thermosetting resin contained in the polymer alloy is cured by a crosslinking reaction of the thermosetting resin by heating to 200 ° C. while applying a load of 2.0 MPa in a vacuum state and holding for 60 minutes, thereby causing the first The adhesive layer 22 is cured.

The heat conductive sheet 20 is manufactured through the above steps. That is, the manufacturing method of the heat conductive sheet 20 includes a step of applying a liquid adhesive containing a polyolefin-based elastomer, a thermosetting resin, and a solvent to the upper surface of the first protective film 23, and a solvent contained in the adhesive. And a step of providing the first adhesive layer 22 on the upper surface of the first protective film 23, a step of providing the graphite sheet 21 on the upper surface of the first adhesive layer 22, and a step of curing the first adhesive layer 22. .

Here, notes on manufacturing the heat conductive sheet 20 and the flexible wiring board 200 are described.

When the solvent contained in the polymer alloy of the adhesive is volatilized, the temperature for heating the adhesive is preferably 100 ° C. or higher and 150 ° C. or lower. This is because if it is less than 100 ° C., it takes too much time to dry, and if it exceeds 150 ° C., the thermosetting resin proceeds. That is, in the step of applying the liquid polymer alloy, it is preferable to dry in the above temperature range where the thermosetting resin is not cured.

The temperature at which the graphite sheet 21 and the first protective film 23 to which the wiring pattern 24 is pasted is primarily laminated is preferably the temperature range in which the viscosity of the polymer alloy containing the polyolefin-based elastomer and the thermosetting resin is lowest. This is because by lowering the viscosity of the polymer alloy, the polymer alloy enters the concavo-convex portion of the graphite sheet 21 and the anchor effect works to improve the adhesion between the first adhesive layer 22 and the graphite sheet 21. This temperature range is, for example, 110 ° C. or more and 160 ° C. or less.

In the above, in order to reduce the thickness of the first adhesive layer 22, the first adhesive layer 22 was applied with a polymer alloy containing a liquid polyolefin elastomer and a thermosetting resin and dried. However, a polymer alloy containing a sheet-like polyolefin-based elastomer and a thermosetting resin may be used as the first adhesive layer 22.

The first protective film 23 to which the graphite sheet 21 and the wiring pattern 24 are attached is subjected to a primary laminating process so that air is not caught, and then the polymer alloy of the first adhesive layer 22 is cured by pressing. The temperature ranges from 160 ° C to 220 ° C. In other words, in the method of manufacturing the heat conductive sheet 20, in the step of curing the first adhesive layer 22, it is preferable to heat the first adhesive layer 22 at a temperature of 160 ° C or higher and 220 ° C or lower.

At this time, the press curing temperature is preferably higher than the second glass transition point, but if it exceeds 220 ° C., the elastomer component contained in the first adhesive layer 22 is damaged, which is not preferable. That is, it is better to promote curing by working pressing at 220 ° C. for a long time than to thermally cure at a temperature exceeding 220 ° C.

The heat conductive sheet according to the present disclosure and a flexible wiring board using the heat conductive sheet are industrially useful because a heat conductive sheet with high heat resistance can be obtained and a flexible wiring board with good heat dissipation can be obtained.

10, 20 Thermal conductive sheet 11, 21 Graphite sheet 12 Adhesive layer 12a, 22 First adhesive layer 12b Second adhesive layer 13 Protective film 13a, 23 First protective film 13b Second protective film 100, 200 Flexible wiring board 14, 14a , 14b, 24 Wiring pattern

Claims (12)

Graphite sheet,
A first adhesive layer adhered to the upper surface of the graphite sheet;
A first protective film adhered to the upper surface of the first adhesive layer;
With
The first adhesive layer includes at least a polyolefin-based elastomer and a thermosetting resin, and the glass transition point of the first protective film is higher than the glass transition point of the first adhesive layer.
Thermal conductive sheet. The first adhesive layer is obtained by applying a liquid adhesive composed of the polyolefin-based elastomer, the thermosetting resin, and a solvent to the first protective film and drying it.
The heat conductive sheet according to claim 1. The first adhesive layer has a first glass transition point and a second glass transition point,
The first glass transition point is −46 ° C. or higher and 42 ° C. or lower,
The second glass transition point is 120 ° C. or higher and 240 ° C. or lower.
The heat conductive sheet according to claim 1. The graphite sheet has a thickness of 10 μm or more and 70 μm or less,
The thickness of the first protective film is 3 μm or more and 30 μm or less.
The heat conductive sheet according to claim 1. The thickness of the first adhesive layer is 5 μm or more and 30 μm or less.
The heat conductive sheet according to claim 1. The storage elastic modulus at 25 ° C. of the first adhesive layer is 9.0 × 10 ⁸ Pa or less,
The storage elastic modulus at 150 ° C. of the first adhesive layer is 1.1 × 10 ⁶ or more and 1.3 × 10 ⁸ Pa or less.
The heat conductive sheet according to claim 1. A second adhesive layer adhered to the lower surface of the graphite sheet;
A second protective film adhered to the lower surface of the second adhesive layer;
Further comprising
The second adhesive layer is made of the same material as the first adhesive layer, and the second protective film is made of the same material as the first protective film.
The heat conductive sheet according to claim 1. The wiring pattern is provided in the upper surface of the said 1st protective film in the heat conductive sheet of Claim 1,
Flexible wiring board. The wiring pattern is provided in at least any one of the upper surface of the said 1st protective film in the heat conductive sheet of Claim 7, or the lower surface of the said 2nd protective film,
Flexible wiring board. Applying a liquid adhesive containing a polyolefin-based elastomer, a thermosetting resin, and a solvent to the upper surface of the protective film;
Volatilizing the solvent contained in the adhesive to provide an adhesive layer on the upper surface of the protective film;
Providing a graphite sheet on the upper surface of the adhesive layer;
Curing the adhesive layer;
With
Manufacturing method of heat conductive sheet. In the step of volatilizing the solvent, the adhesive is heated at a temperature of 100 ° C. or higher and 150 ° C. or lower.
The manufacturing method of the heat conductive sheet of Claim 10. In the step of curing the adhesive layer, the adhesive layer is heated at a temperature of 160 ° C. or higher and 220 ° C. or lower.
The manufacturing method of the heat conductive sheet of Claim 10.

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