Porous film and method for producing circuit board with acoustic element

Abstract

This porous film is provided with a solid part containing a resin. The content of fluororesin contained in the porous film is less than 1 mass%. The product of the Young's modulus of the material constituting the solid part and the ratio of the volume of the solid part to the volume of the porous film is 1.0x104 Pa to 1.0x108 Pa. Maintain the ambient temperature of the porous film at 240°C for 5 minutes. Thereafter, measure the air resistance (Oken) of the porous film according to JIS P8117:2009. In this case, the air resistance (Oken) of the porous film is 100 seconds or less.

Description

Method for manufacturing porous membranes and circuit boards with acoustic elements

[0001] This invention relates to a method for manufacturing a porous membrane and a circuit board with an acoustic element.

[0002] Conventionally, porous polymer membranes that can prevent the entry of foreign matter such as dust and dirt in audio equipment while still being able to transmit sound are known.

[0003] For example, Patent Document 1 describes an aggregate comprising a polymer membrane containing stretched polytetrafluoroethylene (ePTFE) and at least one support structure. This aggregate has a predetermined acoustic impedance. The polymer membrane has a Young's modulus of 1 MPa to 1000 MPa.

[0004] Japanese Patent Publication No. 2023-109841

[0005] The polymer membrane described in Patent Document 1 has room for reconsideration from the viewpoint of exhibiting desired sound permeability and heat resistance while reducing the fluororesin content in a porous membrane containing resin. Therefore, the present invention provides a porous membrane that is advantageous from this viewpoint.

[0006] The present invention relates to a porous membrane comprising a solid portion containing a resin, wherein the content of fluororesin in the porous membrane is less than 1% by mass, and the product of the Young's modulus of the material constituting the solid portion and the ratio of the volume of the solid portion to the volume of the porous membrane is 1.0 × 10⁻⁶. 4 Pa ~ 1.0 × 10 8 The present invention provides a porous membrane in which the pressure is Pa, and the air permeability resistance (Wangyan) of the porous membrane, measured according to Japanese Industrial Standard (JIS) P8117:2009 after the ambient temperature of the porous membrane has been maintained at 240°C for 5 minutes, is 100 seconds or less.

[0007] Furthermore, the present invention provides a method for manufacturing a circuit board with an acoustic element, which includes arranging an acoustic element at a position corresponding to the opening of a circuit board having an opening covered with the porous film described above, and mounting the acoustic element to the circuit board by reflow soldering.

[0008] The porous membrane described above is advantageous in that it exhibits desired sound permeability and heat resistance while reducing the fluororesin content.

[0009] Figure 1 is a schematic plan view showing a porous membrane according to the present invention. Figure 2 is a schematic cross-sectional view of the porous membrane with the line II-II in Figure 1 as the cutting line. Figure 3 is a schematic diagram showing an example of a method for measuring the acoustic properties of a porous membrane. Figure 4 is a schematic diagram showing a method for manufacturing a circuit board with an acoustic element.

[0010] Embodiments of the present invention will be described with reference to the drawings. However, the present invention is not limited to the following embodiments.

[0011] Figure 1 is a schematic plan view showing a porous membrane according to the present invention. Figure 2 is a schematic cross-sectional view of the porous membrane with line II-II in Figure 1 as the cutting line. As shown in Figures 1 and 2, the porous membrane 1a comprises a solid portion 11 containing resin. The porous membrane 1a has pores 12. The porous membrane 1a has, for example, an open-cell porous structure in which the pores 12 are connected to each other. The fluororesin content in the porous membrane 1a is less than 1% by mass. Young's modulus E of the material making up the solid portion 11 11 The product E of the ratio τ of the volume of the solid portion 11 to the volume of the porous membrane 1a. 11 ×τ is 1.0 × 10 4 Pa ~ 1.0 × 10 8 It is Pa. The volume of the porous membrane 1a includes the volume of the pores 12. In other words, the volume of the porous membrane 1a is the apparent volume. The ambient temperature of the porous membrane 1a is maintained at 240°C for 5 minutes. After that, the air permeability resistance (Wangyan) of the porous membrane 1a is measured according to JIS P8117:2009. In this case, the air permeability resistance (Wangyan) of the porous membrane 1a is 100 seconds or less. Product E 11 Because the ×τ and the air permeability resistance (Wangyan) of the porous membrane 1a are adjusted to this range, the porous membrane 1a can easily exhibit the desired sound permeability even if the fluororesin content in the porous membrane is as low as less than 1% by mass. For this reason, the porous membrane 1a can be used as a sound permeable membrane. Furthermore, even when the porous membrane 1a is used at high temperatures, the air permeability of the porous membrane 1a remains within the desired range, and the porous membrane 1a can easily exhibit the desired heat resistance.

[0012] The content of the fluororesin contained in the porous membrane 1a may be 0.5% by mass or less, 0.1% by mass or less, 0.05% by mass or less, or 0.01% by mass or less.

[0013] Product E 11 ×τ is preferably 5.0×10 4 Pa to 1.0×10 8 Pa, more preferably 1.0×10 5 Pa to 1.0×10 8 Pa, still more preferably 2.0×10 5 Pa to 1.0×10 8 Pa, particularly preferably 5.0×10 5 Pa to 1.0×10 8 Pa, and most preferably 5.0×10 5 Pa to 5.0×10 7 Pa.

[0014] The air resistance (Oken method) of the porous membrane 1a described above is preferably 50 seconds or less, and more preferably 40 seconds or less. The air resistance (Oken method) may be 30 seconds or less, 20 seconds or less, 10 seconds or less, 5 seconds or less, 3 seconds or less, or 1 second or less. The air resistance (Oken method) is, for example, 0.1 seconds or more.

[0015] The shape of the porous membrane 1a in a plan view is not limited to a specific shape. As shown in FIG. 1, for example, the porous membrane 1a is circular in a plan view. The porous membrane 1a may be elliptical, triangular, quadrilateral, other polygonal, or irregular in a plan view.

[0016] FIG. 3 is a diagram schematically illustrating an example of a method for measuring acoustic characteristics of the porous membrane 1a. As shown in FIG. 3, for example, measurement of acoustic characteristics is performed in a state where the porous membrane 1a is arranged between a microphone M1 and a speaker MS serving as a sound source. At this time, the sound pressure level of the sound received by the microphone M1 is L P1 expressed in [dB]. In addition, measurement of acoustic characteristics is performed in a state (blank) where the porous membrane 1a is not arranged between the microphone M1 and the speaker MS serving as a sound source. At this time, the sound pressure level of the sound received by the microphone M1 is L P0It is expressed as [dB]. The insertion loss IL [dB] can be determined according to equation (1). Insertion loss IL [dB] = L P0 -L P1 Formula (1)

[0017] For example, the above acoustic measurements may be performed after the ambient temperature of the porous membrane 1a has been maintained at 240°C for 5 minutes. In this case, the insertion loss IL at 1000 Hz and 2000 Hz is, for example, 5 dB or less.

[0018] In the measurement system shown in Figure 3, the porous membrane 1a is circular in plan view. The porous membrane 1a is attached to the support plate S1 by a double-sided adhesive sheet T1 so as to cover the through hole H1 formed in the support plate S1. The double-sided adhesive sheet T1 is annular in plan view, and the outer diameter of the double-sided adhesive sheet T1 is the same as that of the porous membrane 1a. The microphone M1 has a sound-transmitting opening H2. The microphone M1 is positioned so that the axis of the through hole H1 passes through the sound-transmitting opening H2. The microphone M1 is electrically connected to the power supply 20. The microphone M1 is attached to the support plate S1 by a double-sided adhesive sheet T2. The double-sided adhesive sheet T1 is annular in plan view. The sound source, speaker MS, is electrically connected to amplifier 30. The power supply 20 and amplifier 30 are connected to a fast Fourier transform (FFT) analyzer 40. The FFT analyzer 40 is connected to a personal computer (PC) 50. The signal obtained by the Fast Fourier Transform (FFT) analyzer 40 is output to the PC 50.

[0019] The resin contained in the solid part 11 has a fluororesin content of less than 1% by mass in the porous membrane 1a, while the product E 11 As long as ×τ and the air permeability resistance (Wangyan) of the porous membrane 1a can be adjusted within the above range, the resin is not limited to a specific resin.

[0020] The solid portion 11 includes, for example, at least one selected from the group consisting of silicone resin and polyimide. In this case, the porous film 1a is more likely to exhibit the desired heat resistance.

[0021] The solid portion 11 preferably contains a silicone resin. In this case, the porous membrane 1a is more likely to exhibit desirable sound permeability even after the ambient temperature of the porous membrane 1a has been maintained at 240°C for 5 minutes.

[0022] The silicone resin is not limited to a specific silicone resin. The silicone resin may be derived, for example, from a condensation-type silicone resin composition or an addition-type silicone resin composition. An addition-type silicone resin composition is a type of silicone resin composition that cures by an addition reaction. A condensation-type silicone resin composition is a type of silicone resin composition that cures by a condensation reaction.

[0023] The thickness of the porous membrane 1a is not limited to a specific value. For example, the thickness of the porous membrane 1a is 10 μm or more. In this case, the porous membrane 1a is easier to handle. The thickness of the porous membrane 1a may be 20 μm or more, 30 μm or more, or 40 μm or more. For example, the thickness of the porous membrane 1a is 100 μm or less.

[0024] The porosity of the porous membrane 1a is not limited to a specific value. For example, the porosity of the porous membrane 1a is 20 to 95%. In this case, the porous membrane 1a is more likely to exhibit the desired sound permeability. The porosity of the porous membrane 1a can be determined, for example, according to the method described in the examples.

[0025] The porosity of the porous membrane 1a may be 20-90%, 20-80%, 20-70%, or 20-60%.

[0026] The applications of the porous membrane 1a are not limited to any particular application. For example, the porous membrane 1a can be used in a method for manufacturing a circuit board with acoustic elements.

[0027] Figure 4 is a schematic diagram illustrating a method for manufacturing a circuit board with an acoustic element. As shown in Figure 4, the opening 2h of the circuit board 2 is covered with a porous membrane 1a. This prevents foreign matter such as dust and dirt from entering through the opening 2h of the circuit board 2. The porous membrane 1a is attached to the circuit board 2, for example, by bringing double-sided adhesive tape 15 into contact with the area around the opening 2h.

[0028] Next, the acoustic element is arranged at a position corresponding to the opening 2h covered by the porous membrane 1a, and the acoustic element 3 is mounted on the circuit board 2 by reflow soldering. As described above, the porous membrane 1a can exhibit desired heat resistance, so even if reflow soldering is performed for mounting the acoustic element 3, desired air permeability can be maintained after the soldering.

[0029] Next, inspections such as operation check of the acoustic element 3 are performed. The product E of the porous membrane 1a 11 ×τ is adjusted within the above range, so the porous membrane 1a can exhibit desired sound permeability in the operation check of the acoustic element 3.

[0030] Next, if necessary, the porous membrane 1a and the double-sided adhesive tape 15 are peeled off from the circuit board 2. In this way, the circuit board with acoustic element 5 is obtained.

[0031] The acoustic element 3 is not limited to a specific acoustic element. The acoustic element 3 may be a sound emitting element such as a buzzer or a speaker, or may be a sound receiving element such as a microphone. The acoustic element 3 may be, for example, a MEMS microphone having a structure using micro-electro-mechanical system (MEMS) technology.

[0032] The method for producing the porous membrane 1a is not limited to a specific method. The porous membrane 1a can be produced, for example, by heating an emulsion membrane under predetermined conditions, where the precursor of the solid part 11 is a continuous phase and an aqueous solution or an aqueous dispersion is a dispersed phase. In heating the membrane, curing of the precursor of the solid part 11 and evaporation of water contained in the dispersed phase occur, and the porous membrane 1a including the solid part 11 and the voids 12 is obtained.

[0033] The porous membrane 1a may be produced, for example, by a method including bringing a coating film of a fluid containing a precursor of the solid part 11 and a porosifying agent into contact with water, and solidifying the precursor of the solid part 11. In this case, an example of the porosifying agent is a water-soluble inorganic salt. When the coating film comes into contact with water, the porosifying agent is eluted, and porosity of the coating film occurs. The coating film may be immersed in water.

[0034] For example, when the solid portion 11 contains polyimide, the solidification of the precursor of the solid portion 11 may include imidization. The imidization includes, for example, heating the precursor of the solid portion 11 at a temperature of 200°C to 300°C in a nitrogen atmosphere for about 1 hour.

[0035] Hereinafter, the present invention will be described in more detail by way of examples. However, the present invention is not limited to the following examples.

[0036] <Example 1> 100 parts by mass of Silpot184 elastomer manufactured by Dow Toray Industries, Inc., 10 parts by mass of Silpot184 curing agent manufactured by the same company, and 9 parts by mass of Leodol MO-60, a surfactant manufactured by Kao Corporation, were stirred for 2 minutes in Awatori Ken-taro manufactured by Thinky Corporation. Thereafter, 210 parts by mass of an aqueous solution in which bentonite W-100U manufactured by Hojun Co., Ltd. was dispersed at 0.9 mass% in distilled water was added, and the mixture was stirred for 18 minutes in Awatori Ken-taro to obtain an emulsion. Thereafter, the emulsion was stirred under reduced pressure in a vacuum defoaming Awatori Ken-taro manufactured by Thinky Corporation to perform defoaming. In this way, the emulsion according to Example 1 was obtained. The emulsion according to Example 1 was placed on a polyethylene terephthalate (PET) film A manufactured by Mitsubishi Chemical Corporation. Next, a PET film B manufactured by the same company was placed on the emulsion, and the emulsion and the pair of PET films were pressed using a desktop small heat roll press (HSRP-60150H) to obtain a film of the emulsion. Thereafter, this film was heated at 90°C for 6 minutes to cure the silicone resin. Next, the PET film B was peeled off from the cured silicone resin, and the cured silicone resin was further subjected to heat drying treatment at 150°C for 10 minutes, to obtain a porous membrane according to Example 1.

[0037] <Example 2> 100 parts by mass of Silpot 184 elastomer manufactured by Dow-Toray, 10 parts by mass of Silpot 184 curing agent manufactured by the same company, and 9 parts by mass of the surfactant Rheodol SP-O10V manufactured by Kao Corporation were stirred for 2 minutes in a foam remover manufactured by Thinky Co., Ltd. Then, 140 parts by mass of an aqueous solution in which bentonite W-100U manufactured by Hojun Co., Ltd. was dispersed at 0.9% by mass in distilled water was added, and the mixture was stirred for 30 minutes using a disperser under reduced pressure in a vacuum stirring kettle to obtain the emulsion according to Example 2. The emulsion according to Example 2 was placed on a PET film A manufactured by Mitsubishi Chemical Corporation. Next, a PET film S10 manufactured by Toray was placed on the emulsion, and the emulsion and the pair of PET films were pressed using a tabletop small-scale hot roll press (HSRP-60150H). Then, the film was heated at 90°C for 6 minutes to cure the silicone resin. Next, the PET film S10 was peeled off from the cured silicone resin, and the cured silicone resin was then subjected to a heat drying treatment at 150°C for 6 minutes to obtain the porous film according to Example 2.

[0038] <Example 3> 100 parts by mass of Silpot 184 elastomer manufactured by Dow-Toray, 10 parts by mass of Silpot 184 curing agent manufactured by the same company, and 9 parts by mass of the surfactant Rheodol SP-O10V manufactured by Kao Corporation were stirred for 2 minutes in a foam remover manufactured by Thinky Co., Ltd. Then, 140 parts by mass of an aqueous solution in which bentonite W-100U manufactured by Hojun Co., Ltd. was dispersed at 0.9% by mass in distilled water was added and stirred for 12 minutes in a disperser manufactured by Primix Co., Ltd. to obtain an emulsion. Then, the emulsion was stirred under reduced pressure in a vacuum foam remover manufactured by Thinky Co., Ltd. to remove foam. In this way, the emulsion according to Example 3 was obtained. The emulsion according to Example 1 was placed on a PET film A manufactured by Mitsubishi Chemical Corporation. Next, a PET film B manufactured by the same company was placed on top of the emulsion, and the emulsion and the pair of PET films were pressed using a tabletop small-scale hot roll press (HSRP-60150H) to obtain an emulsion film. Subsequently, the film was heated at 90°C for 6 minutes to cure the silicone resin. Next, the PET film B was peeled off from the cured silicone resin, and the cured silicone resin was further subjected to a heat drying treatment at 150°C for 10 minutes to obtain the porous film according to Example 3.

[0039] <Example 4> Under a nitrogen atmosphere at room temperature (20°C to 30°C), 1.59 g of 4,4'-diphenyl ether manufactured by Wakayama Seika Kogyo Co., Ltd. and 3.02 g of polyetheramine D-2000 manufactured by Mitsui Chemicals Fine Co., Ltd. were added to 10 g of N,N-dimethylacetamide solvent and stirred until the 4,4'-diphenyl ether was completely dissolved. Then, 2.06 g of pyromellitic anhydride manufactured by Puyang Sheng Huade Chemical Co., Ltd. was added little by little while stirring to obtain a low-elasticity polyimide (PI) varnish of the polyamic acid solution. To 5.00 g of this low-elasticity polyimide (PI) varnish, 2.00 g of N,N-dimethylacetamide (DMAc) manufactured by Tokyo Chemical Industries, Ltd., 3.00 g of 3-butoxy-N,N-dimethylpropanamide KJCBPA-100 manufactured by KJ Chemicals, 1.00 g of ultrapure water, 0.55 g of lithium chloride manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., and 0.01 g of the antioxidant Irganox 1010 manufactured by BASF Japan were added. The mixture was then stirred for 5 minutes using a foam remover and degassed for 3 minutes to prepare the solution according to Example 4. The solution according to Example 4 was applied to a substrate of PET film Diafoil MRF#38 manufactured by Mitsubishi Chemical Corporation, which had been corona-treated, using an applicator to obtain a coating film with a thickness of 50 μm. The obtained coating film was immersed in water for 1 hour. After that, the obtained film was peeled off the substrate and dried in an oven set to 80°C for 30 minutes. Next, the dried film was subjected to a heat treatment at 285°C under a nitrogen atmosphere for 1 hour to induce imidization, thereby obtaining the porous film according to Example 4.

[0040] <Comparative Example 1> 100 parts by mass of Silpot 184 elastomer manufactured by Dow-Toray, 10 parts by mass of Silpot 184 curing agent manufactured by the same company, and 9 parts by mass of the surfactant Rheodol SP-O10V manufactured by Kao Corporation were stirred for 2 minutes in a foam remover manufactured by Thinky Co., Ltd. Then, 140 parts by mass of an aqueous solution in which bentonite W-100U manufactured by Hojun Co., Ltd. was dispersed at 0.9% by mass in distilled water was added using a homomixer, and the mixture was stirred with a disperser manufactured by Primix Co., Ltd. to obtain an emulsion. Then, the emulsion was stirred under reduced pressure in a vacuum foam remover manufactured by Thinky Co., Ltd. to remove foam. In this way, the emulsion according to Comparative Example 1 was obtained. The emulsion according to Comparative Example 1 was placed on a PET film A manufactured by Mitsubishi Chemical Corporation. Next, a PET film B manufactured by the same company was placed on top of the emulsion, and the emulsion and the pair of PET films were pressed using a tabletop small-scale hot roll press (HSRP-60150H) to obtain an emulsion film. Subsequently, the film was heated at 85°C for 6 minutes to cure the silicone resin. Next, PET film B was peeled off from the cured silicone resin, and the cured silicone resin was further subjected to a heat drying treatment at 150°C for 10 minutes to obtain the porous film according to Comparative Example 1.

[0041] <Comparative Example 2> 5.00 g of porous PI varnish BP-BM manufactured by Unitika Corporation was mixed with 3.19 g of tetraethylene glycol dimethyl ether manufactured by Tokyo Chemical Industry Co., Ltd. The mixture was then stirred for 5 minutes in a foam remover and degassed for 3 minutes to prepare the liquid according to Comparative Example 2. The liquid according to Comparative Example 2 was applied to a substrate of PET film Diafoil MRF#38 manufactured by Mitsubishi Chemical Corporation, which had been corona treated, using an applicator to obtain a coating film with a thickness of 150 μm. The obtained coating film was dried in an oven set to 130°C for 20 minutes. Next, the dried film was heat-treated in a nitrogen atmosphere at 300°C for 1 hour to induce imidization and obtain the porous film according to Comparative Example 2.

[0042] (Heat Treatment) The porous membranes according to each example and comparative example were placed in a reflow oven RDT-250EC, and the internal temperature of the reflow oven was increased from 25°C to 240°C. The internal temperature of the reflow oven was then maintained at 240°C for 5 minutes. After that, the porous membranes according to each example and comparative example were removed from the reflow oven.

[0043] (Air permeability resistance) The air permeability resistance (Wang Ken) of the porous membranes in each example and comparative example after the above heat treatment was measured in accordance with JIS P8117:2009 using a Wang Ken type air permeability tester manufactured by Asahi Seiko Co., Ltd. The results are shown in Table 1.

[0044] (Young's modulus) Non-porous film α was prepared using the materials of the solid portion of the porous membranes according to Examples 1 to 3 and Comparative Example 1. Non-porous film α was prepared as follows: 100 parts by mass of Silpot 184 elastomer manufactured by Dow Toray and 10 parts by mass of Silpot 184 curing agent manufactured by the same company were stirred for 2 minutes in a foam remover to obtain a precursor for non-porous film α. This precursor was formed into a film of a predetermined thickness and heated at 90°C for 6 minutes to cure the silicone resin, and then subjected to a heat drying treatment at 150°C for 10 minutes. In this way, a non-porous film A with a thickness of 100 μm was obtained.

[0045] A non-porous film β was prepared using the material of the solid portion of the porous membrane according to Example 4. A precursor solution for non-porous film β was obtained in the same manner as in Example 4, except that lithium chloride was not added. The precursor solution for non-porous film β was applied to a substrate of PET film Diafoil MRF#38 manufactured by Mitsubishi Chemical Corporation, which had been corona-treated, using an applicator to adjust the thickness of the coating so that the non-porous film β would be 100 μm thick. The obtained film was then dried in an oven set to 80°C for 30 minutes. Next, the dried film was heat-treated at 285°C in a nitrogen atmosphere for 1 hour to induce imidization, and a non-porous film β with a thickness of 100 μm was obtained.

[0046] For the Young's modulus of the non-porous film γ formed from the solid material of the porous membrane according to Comparative Example 2, a reference value provided by the supplier of the porous PI varnish was used.

[0047] Each of the above-mentioned non-porous films α, β, and γ was cut into a rectangular shape with a short side length of 10 mm and a long side length of 60 mm in a plan view to obtain test specimens. The test specimens were fixed in a Shimadzu Autograph AGX-V2 with the chuck distance adjusted to 20 mm, and a tensile test was performed at a speed of 300 mm / min to obtain stress-strain curves (S-S curves). The tensile tests were performed at room temperature (20°C to 30°C). A tangent line was drawn at the origin of the S-S curve, and the Young's modulus of the material was calculated from the slope of the tangent line. The results are shown in Table 1.

[0048] (Porosity) Samples for porosity measurement were obtained by cutting out circular pieces with a diameter of 25 mm in a plan view from the porous membranes of each example and comparative example. The basis weight of the membrane was calculated from the mass and membrane area of ​​these samples. In other words, the basis weight of each sample was calculated using the relationship basis weight = mass / membrane area. Next, the bulk density of each porous membrane was calculated from the basis weight and thickness of the samples. In other words, the bulk density of each porous membrane was calculated using the relationship bulk density of porous membrane = basis weight / thickness. The porosity ρ of each porous membrane was calculated from the true density (density of the solid part) and bulk density of the porous membrane according to the following formula (2). The results are shown in Table 1. The density of non-porous film α was considered to be the true density of the porous membranes of Examples 1 to 3 and Comparative Example 1. The density of non-porous film β was considered to be the true density of the porous membrane of Example 4. The density of non-porous film γ was considered to be the true density of the porous membrane of Comparative Example 2. Table 1 shows the product of the Young's modulus of the material forming the solid part of each porous membrane and the ratio of the volume of the solid part to the volume of each porous membrane (1 - ρ). Porosity ρ [%] = (1 - true density of the porous membrane / bulk density of the porous membrane) × 100 Equation (2)

[0049] (Acoustic Characteristics) The acoustic characteristics of the porous membranes according to each example and comparative example after the heat treatment described above were evaluated using the measurement system shown in Figure 3. Each porous membrane was cut into a circle with a diameter of 3.1 mm in plan view to prepare a sample for acoustic characteristic evaluation. The double-sided adhesive sheet T1 in the measurement system had an outer diameter of 3.1 mm and an inner diameter of 1.22 mm in plan view, and Nitto Denko Corporation's double-sided adhesive tape No. 585 was used. The support plate S1 was a stainless steel plate with a thickness of 2 mm, and the diameter of the through hole H1 was 1 mm. As the double-sided adhesive sheet T2, Nitto Denko Corporation's double-sided adhesive tape No. 5610 was used. The outer diameter of the double-sided adhesive sheet T2 was 5.8 mm, its inner diameter was 1.6 mm, and its thickness was 0.1 mm. As the microphone M1, Knowles Corporation's MEMS microphone SPU0410LR5H was used. A Bruel & Kjaer Type 4227 speaker MS was used. A Kikusui Electronics PMC35-1A DC power supply was used as the power supply 20. A Bruel & Kjaer Type 2735 amplifier was used as the amplifier 30. A Bruel & Kjaer Type 3560-B-030 FFT analyzer 40 was used as the FFT analyzer 40. The sound pressure level of the sound emitted from the speaker MS was measured when the porous membrane was placed in the sound path and when the porous membrane was not placed in the sound path and the sound pressure level was measured. The insertion loss IL [dB] at 1000 Hz and 2000 Hz was calculated according to equation (1). The results are shown in Table 1.

[0050]

[0051] The insertion loss was low when using the porous membranes according to each example, and the air permeability resistance (Wang Ren) of the porous membranes after heat treatment was also low. Therefore, it can be understood that the sound permeability and heat resistance of the porous membranes according to each example are good. On the other hand, the insertion loss was not low when using the porous membranes according to each comparative example, and the air permeability resistance (Wang Ren) of the porous membrane according to Comparative Example 1 after heat treatment exceeded 1000 [sec]. Therefore, it is not possible to say that the sound permeability of the porous membranes according to Comparative Examples 1 and 2 is good, and it is not possible to say that the heat resistance of the porous membrane according to Comparative Example 1 is good. Comparing each example with each comparative example, the product of the Young's modulus of the material constituting the solid part and the ratio of the volume of the solid part to the volume of the porous membrane (1 - ρ) was 1.0 × 10⁻⁶. 4 Pa ~ 1.0 × 10 8 It is understood that being within the Pa range and having an air permeability resistance (Wangyan) of the porous membrane after heat treatment of 100 seconds or less are important from the viewpoint of achieving desired sound permeability and heat resistance in the porous membrane while reducing the fluororesin content.

[0052] A first aspect of the present invention is a porous membrane comprising a solid portion containing a resin, wherein the content of fluororesin in the porous membrane is less than 1% by mass, and the product of the Young's modulus of the material constituting the solid portion and the ratio of the volume of the solid portion to the volume of the porous membrane is 1.0 × 10⁻⁶. 4 Pa ~ 1.0 × 10 8 The present invention provides a porous membrane in which the pressure is Pa, and the air permeability resistance (Wangyan) of the porous membrane, measured according to Japanese Industrial Standard (JIS) P8117:2009 after the ambient temperature of the porous membrane has been maintained at 240°C for 5 minutes, is 100 seconds or less.

[0053] A second aspect of the present invention is to provide a porous membrane in which, in the first aspect, the solid portion comprises at least one selected from the group consisting of silicone resin and polyimide.

[0054] A third aspect of the present invention is to provide a porous membrane in which, in the first or second aspect, the solid portion contains a silicone resin.

[0055] A fourth aspect of the present invention is to provide a porous membrane having a thickness of 10 μm or more in any one of the first to third surfaces.

[0056] A fifth aspect of the present invention is to provide a porous membrane having a porosity of 20 to 95%, in any one of the first to fourth aspects.

[0057] A sixth aspect of the present invention provides a method for manufacturing a circuit board with an acoustic element, which includes arranging an acoustic element at a position corresponding to the opening of a circuit board having an opening covered with a porous film described in any one of the first to fifth aspects, and mounting the acoustic element to the circuit board by reflow soldering.

Claims

1. A porous membrane comprising a solid portion containing a resin, wherein the content of the fluororesin contained in the porous membrane is less than 1% by mass, and the product of the Young's modulus of the material constituting the solid portion and the ratio of the volume of the solid portion to the volume of the porous membrane is 1.0 × 10⁻⁶. 4 Pa ~ 1.0 × 10 8 A porous membrane having a pressure of Pa, wherein the air permeability resistance (Wangyan) of the porous membrane, measured according to Japanese Industrial Standard (JIS) P8117:2009 after the ambient temperature of the porous membrane has been maintained at 240°C for 5 minutes, is 100 seconds or less.

2. The porous membrane according to claim 1, wherein the solid portion comprises at least one selected from the group consisting of silicone resin and polyimide.

3. The porous membrane according to claim 1, wherein the solid portion comprises a silicone resin.

4. The porous membrane according to claim 1, having a thickness of 10 μm or more.

5. The porous membrane according to claim 1, wherein the porous membrane has a porosity of 20 to 95%.

6. A method for manufacturing a circuit board with an acoustic element, comprising arranging an acoustic element at a position corresponding to the opening of a circuit board having an opening covered with a porous film according to any one of claims 1 to 5, and mounting the acoustic element to the circuit board by reflow soldering.

Images