LOW-DIELECTRIC POROUS POLYMER FILM AND FILM FOR MILLIMETER-WAVE ANTENNA

Abstract

Provided is a low-dielectric porous polymer film having a low dielectric constant at high millimeter-wave frequencies and thereby useful as a sheet for a millimeter-wave antenna. The low-dielectric porous polymer film comprises: a base material layer made of a polymer material and formed with fine pores dispersed therein; and a substantially smooth skin layer made of the same polymer material as that of the base material layer and formed on at least one of the surfaces of the base material layer.

Claims

1. A low-dielectric porous polymer film comprising: a base material layer made of a polymer material and formed with fine pores dispersed therein; and a substantially smooth skin layer made of the same polymer material as that of the base material layer and formed on at least one of the surfaces of the base material layer.

2. The film as recited in claim 1, wherein the skin layer has a thickness which is less than 10% of a total thickness of the film.

3. The film as recited in claim 2, wherein the thickness of the skin layer is equal to or less than 5% of the total thickness of the film.

4. The film as recited in claim 1, wherein the skin layer has a thickness of 1 to 5 m.

5. The film as recited in claim 1, wherein the skin layer is formed on both of the surfaces of the base material layer.

6. The film as recited in claim 1, wherein the skin layer is liquid-impermeable.

7. The film as recited in claim 1, wherein the base material layer has a porosity of 60% or more.

8. The film as recited in claim 1, wherein the base material layer has a porosity of 95% or less.

9. The film as recited in claim 1, wherein the base material layer has an average pore diameter of 10 m or less.

10. The film as recited in claim 1, wherein the base material layer has a pore diameter distribution with a full width at half maximum of 10 m or less.

11. The film as recited in claim 1, which has a dielectric constant as measured at 60 GHz of 2.0 or less.

12. The film as recited in claim 1, wherein the polymer or a precursor of the polymer is soluble in an organic solvent.

13. The film as recited in claim 12, wherein the organic solvent is N-methylpyrrolidone.

14. The film as recited in claim 1, wherein the polymer is selected from the group consisting of polyimide, polyetherimide, fluorinated polyimide, and polycarbonate.

15. The film as recited in claim 1, which has a thickness of 50 m to 500 m.

16. The film as recited in claim 1, which is used in a board for a millimeter-wave antenna.

17. The film as recited in claim 1, which has a porous structure formed as an independent-cell structure.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0044] FIG. 1a is an SEM photograph of a cross-section of a film obtained in Inventive Example 1.

[0045] FIG. 1b is an SEM photograph of a surface of the film obtained in Inventive Example 1.

[0046] FIG. 2a is an SEM photograph of a cross-section of a film obtained in Inventive Example 2.

[0047] FIG. 2b is an SEM photograph of a surface of the film obtained in Inventive Example 2.

[0048] FIG. 3a is an SEM photograph of a cross-section of a film obtained in Inventive Example 3.

[0049] FIG. 3b is an SEM photograph of a surface of the film obtained in Inventive Example 3.

[0050] FIG. 4a is an SEM photograph of a cross-section of a film obtained in Comparative Example.

[0051] FIG. 4b is an SEM photograph of a surface of a film obtained in Comparative Example.

[0052] FIG. 5 is an SEM photograph of a cross-section of a film obtained in Inventive Example 4.

[0053] FIG. 6 is an SEM photograph of a cross-section of a film obtained in Inventive Example 5.

DESCRIPTION OF EMBODIMENTS

[0054] The film according to the present invention comprises a base material layer made of a polymer material in which fine pores are dispersed, and a substantially smooth skin layer made of the same polymer material as that of the base material layer and formed on the surface of the base material layer.

[0055] In the film according to the present invention, the skin layer is formed on one or both of the surfaces of the base material layer, and made of the same polymer material as that of the base material layer. The state of the film according to the present invention in which the base material layer has fine pores dispersedly formed therein, whereas the skin layer is almost or absolutely devoid of such pores and is substantially smooth, can be checked, for example, by microscope observation.

[0056] In the film according to the present invention, from a viewpoint of avoiding a rise in dielectric constant of the entire film, the thickness of the skin layer is preferably less than 10%, more preferably equal to or less than 5%, of the total thickness of the film. Further, from a viewpoint of forming a smooth skin layer while keeping the dielectric constant of the entire film low, the thickness of the skin layer is preferably in the range of 1 to 5 m. The thickness of the skin layer can be measured by scanning electron microscope (SEM) observation.

[0057] Further, from a viewpoint of easiness of plating in a process of forming a wiring pattern thereon, the skin layer is preferably liquid-impermeable.

[0058] In order to obtain a high antenna gain, it is desirable for the film according to the present invention to have a lowered dielectric constant. From this view point, the porosity of the film is preferably 60% or more. Further, the porosity of the film is preferably 95% or less. The porosity of the film can be obtained by calculation based on the specific gravity of a film without pores and the specific gravity of the film with pores, each measured by an electronic specific gravity meter.

[0059] From a viewpoint that an excessive enlargement of the pores leads to significant deterioration in mechanical strength during bending of the porous polymer film, the film according to the present invention is formed such that the pores have an average pore diameter of 10 m or less. Further, in the film according to the present invention, the skin layer is useful in forming antenna wires on the surface of the porous polymer film. In this case, if there are irregularities on a surface of the skin layer, irregularities will be undesirably formed in the wires formed thereon. For this reason, the skin layer needs to be smooth. On the other hand, as the skin layer is increased in thickness, the dielectric constant of the entire film undesirably rises. Thus, the skin layer needs to be thin. In the film according to the present invention, the average pore diameter of the pores is set to 10 m or less. This makes it possible to easily realize formation of a thin and smooth skin layer as a surface layer of the porous polymer film.

[0060] Further, from viewpoints of further improving the mechanical strength during bending of the porous polymer film, and further improving the smoothness of the skin layer, the pores preferably have a pore diameter distribution with a full width at half maximum of 10 m or less. The average pore diameter of the pores and the full width at half maximum in the pore diameter distribution of the pores can be measured by image analysis of an SEM photograph of a cross-section of the film.

[0061] For example, the film can be obtained by forming a polymer composition having a microphase-separated structure through the following drying-induced phase separation method, and then using a supercritical extraction method. Specifically, a porosifying agent is added to a solution of a polyimide precursor using an organic solvent (NMP or the like) at a given mixing ratio, and the resulting solution is formed in a desired shape (e.g., a sheet, a film or the like), for example, by applying it onto a substrate such as a PET film, a copper foil or the like. Then, the solvent is removed from the resulting shaped body by drying, and the porosifying agent is insolubilized within the polyimide precursor to obtain a polymer composition having a microphase structure in which non-continuous phases comprised of the porosifying agent are dispersed over a continuous phase of the polyimide precursor. Further, the porosifying agent is extracted using supercritical carbon dioxide or the like, and then the polyimide precursor is converted to polyimide (imidized). In the above process, the drying is performed at a relatively low temperature for a relatively short time period to intentionally allow part of the organic solvent such as NMP to remain, and, in this state, the porosifying agent is extracted using supercritical carbon dioxide or the like, whereby a film having a desired porosity and a desired average pore diameter can be obtained.

[0062] The polyimide precursor usable for obtaining the film according to the present invention may be any intermediate capable of being converted to polyimide, and can be obtained by a known or commonly-used method. For example, the polyimide precursor can be obtained from a reaction between an organic tetracarboxylic dianhydride and a diamino compound (diamine).

[0063] Examples of the organic tetracarboxylic dianhydride include pyromelletic dianhydride, 3,3,4,4-biphenyltetracarboxylic dianhydride, 2,2-bis(2,3-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride, 3,3 4,4-benzophenonetetracarboxylic dianhydride, bis(3,4-dicarboxyphenyl)-ether dianhydride, and bis(3,4-dicarboxyphenyl)-sulfonic dianhydride. These organic tetracarboxylic dianhydrides may be used independently or in the form of a mixture of two or more of them.

[0064] Examples of the diamino compound include m-phenylenediamine, p-phenylenediamine, 3,4-diaminodiphenyl ether, 4,4-diaminodiphenyl ether, 4,4-diaminodiphenyl sulfone, 3,3-diaminodiphenyl sulfone, 2,2-bis(4-aminophenoxyphenyl) propane, 2,2-bis(4-aminophenoxyphenyl) hexafluoropropane, 1,3-bis(4-aminophenoxy) benzene, 1,4-bis(4-aminophenoxy)benzene, 2,4-diaminotoluene, 2,6-diaminotoluene, diaminodiphenylmethane, 4,4-diamino-2,2-dimethylbiphenyl, and 2,2-bis(trifluoromethyl)-4,4-diaminobiphenyl. These diamino compounds may be used independently or in the form of a mixture of two or more of them.

[0065] The polyimide precursor can be obtained by causing a reaction between the organic tetracarboxylic dianhydride and the diamino compound (diamine), typically in an organic solvent at 0 to 90 C. for 1 to 24 hours. Examples of the organic solvent include polar solvents such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, and dimethylsulfoxide. Among them, from the viewpoint that a film having a relatively high porosity and a relatively small average pore diameter can be obtained by intentionally creating the state in which part of the organic solvent remains in the film, and, in this state, extracting the porosifying agent, it is preferable to use N-methyl-2-pyrrolidone whose remaining amount can be easily controlled through control of a drying condition, in a production process of the film, as described later,

[0066] The porosifying agent usable for obtaining the film according the present invention may be a component comprising the non-continuous phases in the microphase-separated structure (corresponding to pores in the porous polymer film) and capable of dispersing in the polyimide precursor upon being mixed with the polyimide precursor, more specifically a compound capable of being separated with respect to the polyimide precursor, in the form of fine particle-shaped microphases, so as to form a sea-island structure. More preferably, the porosifying agent is a component removable from the polyimide precursor by an extractive removal operation using supercritical carbon dioxide or the like.

[0067] More specifically, examples of the porosifying agent include: polyalkylene glycol such as polyethylene glycol or polypropylene glycol; a compound in which either or both of the terminals of the polyalkylene glycols is blocked by methyl or (meth)acrylate; a compound in which one of the terminals of polyalkylene glycol such as phenoxypolyethylene glycol (meth)acrylate is blocked by an alkyl or aryl group, and the other terminal is blocked by (meth)acrylate; urethane prepolymer; polyhydric alcohol poly(meth)acrylates such as trimethylolpropane tri(meth)acrylate and dipentaerythritol hexa(meth)acrylate; and a (meth)acrylate-based compound such as -caprolactone (meth)acrylate, urethane (meth)acrylates, epoxy (meth)acrylates, and oligoester (meth)acrylates. These porosifying agent may be used by selecting one of them or simultaneously selecting two or more of them.

[0068] In the film according to the present invention, when a porous structure of the film is formed as an independent-cell structure, it is preferable to use, as the porosifying agent, polyoxyethylene dimethyl ether. Further, in this case, it is preferable to use a nucleus agent such as a polytetrafluoroethylene (PTFE) powder, if necessary.

[0069] In a process of obtaining the film according to the present invention, first of all, the porosifying agent is added to a solution of the polyimide precursor using the organic solvent at a given mixing ratio, and, after forming the resulting solution into a sheet, a film or the like, the solvent is removed from the sheet or the like by drying, as mentioned above.

[0070] The temperature of the drying for removing the solvent may be set in the range of 60 to 180 C., preferably in the range of 60 to 120 C., although it varies depending on a type of solvent used. Further, the time period of the drying is preferably set in the range of 5 to 60 minutes, more preferably in the range of about 5 to 30 minutes.

[0071] Without being bound by any particular theory, a film having a high porosity and a small average pore diameter which could not be obtained by conventional techniques can be obtained by performing the drying at a lower temperature for a shorter time period than those in the conventional techniques to thereby intentionally create the state in which part of the organic solvent such as NMP remains in the film, and, in this state, extracting the porosifying agent using supercritical carbon dioxide.

[0072] A remaining amount of the solvent is preferably set in the range of 15 to 250 parts by weight, particularly in the range of 25 to 250 parts by weight, more preferably in the range of 50 to 150 parts by weight, with respect to the amount of the polyimide precursor.

[0073] From a viewpoint of setting the average pore diameter to a sufficiently small range, the porosifying agent is preferably added in an amount of 200 parts by weight or less with respect to 100 parts by weight of the polyimide precursor. Further, from a viewpoint of setting the dielectric constant of the film to a sufficiently small value, the porosifying agent is preferably added in an amount of 10 parts by weight or more with respect to 100 parts by weight of the polyimide precursor.

[0074] Then, a porous structure is formed by removing the porosifying agent from a polymer composition having a microphase-separated structure composed of the polyimide precursor and the porosifying agent. A method for removing the porosifying agent is not particularly limited, and the removal may be performed by any method like vaporization or decomposition, a removal method using an extraction operation is preferred. The removal using the extraction operation may involve decomposition or transformation of the porosifying agent, or may be performed after decomposition or transformation.

[0075] The solvent used for the extractive removal of the porosifying agent is not particularly limited, as long as it is capable of solving the porosifying agent. Carbon dioxide, particularly supercritical carbon dioxide, is preferred from a viewpoint of its removal performance and harmlessness. In a method for removing the porosifying agent from the polyimide composition by using supercritical carbon dioxide, a temperature for performing the method is equal to or greater than the critical point of supercritical carbon dioxide. The temperature is preferably set in a range where imidization of the polyimide precursor is not extremely progressed in the course of the removal. In addition, as the temperature goes higher, solubility of the porosifying agent to the supercritical carbon dioxide becomes lower. Therefore, a temperature during removal of the porosifying agent using supercritical carbon dioxide (extraction temperature) is preferably set in the range of 32 to 230 C., more preferably in the range of 40 to 200 C.

[0076] The pressure of the supercritical carbon dioxide during the removal (extraction pressure) is equal to or greater than the critical point of supercritical carbon dioxide. The extraction pressure is preferably set in the range of 7.3 to 100 MPa, more preferably in the range of 10 to 50 MPa.

[0077] The supercritical carbon dioxide may be pressurized and then continuously supplied to a pressure-resistant container containing the polymer composition having the microphase-separated structure by using a metering pump, or the supercritical carbon dioxide pre-pressurized at a given pressure may be injected into the pressure-resistant container. The time period of the extraction is set in the range of about 1 to 10 hours, and can be varied depending on the extraction temperature, the extraction pressure, and the amount of the porosifying agent added to the polyimide precursor.

[0078] The porosified polyimide precursor from which the porosifying agent has been removed in the above manner is subsequently converted to a porous polyimide through, e.g., dehydration ring-closure reaction. The dehydration ring-closure reaction of the polyimide precursor is performed by heating to the temperature of about 300 to 400 C., or by utilizing a cyclodehydrating agent such as a mixture of acetic anhydride and pyridine.

[0079] The film according to the present invention which can be produced by the above method preferably has a dielectric constant as measured at 60 GHz of 2.0 or less, from a viewpoint of lowering of dielectric constant. The dielectric constant of the film can be measured by an open resonator method or the like.

[0080] Although the process has been described above in detail based on an example in which the polymer is polyimide, the polymer is preferably selected from the group consisting of polyimide, polyetherimide, fluorinated polyimide, and polycarbonate, from a viewpoint that a drying-induced phase separation method and a supercritical extraction method can be applied to the film according to the present invention.

[0081] The film according to the present invention preferably has a thickness of 50 m to 500 m, from a viewpoint that the film is formed through the coating and drying process.

[0082] The film according to the present invention is suitably usable as a film for use in a board for a millimeter-wave antenna.

EXAMPLES

[0083] Although the present invention will be specifically described below based on examples, it should be understood that the present invention is not limited to these examples.

(Evaluation of Thickness of Skin Layer, Average Pore Diameter and Pore Diameter Distribution)

[0084] The thickness of the skin layer, the average pore diameter and the pore diameter distribution were evaluated by observing a porous configuration using an scanning electron microscope (JSM-6510LV manufactured by JEOL Ltd.). A sample was cut by a razor to expose a cross-section (cut surface). Further, the cross-section was subjected to platinum evaporation deposition, and then observed. The average pore diameter and the pore diameter distribution (full-width at half-maximum) were calculated by SEM image analysis. In the image analysis, an SEM image was subjected to binarization to identify pores, and then pore diameters were calculated to form a histogram. As for software for the analysis, ImageJ was used. The maximum pore diameter best representing an actual structure was used as a value of the pore diameter in the evaluation of the pore diameter.

(Evaluation of Porosity)

[0085] The specific gravity was measured using an electronic specific gravity meter (MD-3005 manufactured by Alfa Mirage). Further, the porosity was calculated using the following formula.

Porosity (%)=(1specific gravity of porous polyimide body/specific gravity of non-porous polyimide body)100

(Evaluation of Electric Properties)

[0086] The dielectric constant (relative permittivity) and the dielectric loss tangent at 10 GHz were measured using a PNA network analyzer (Agilent Technologies Inc.) and a split post dielectric resonator (SPDR). Further, the dielectric constant and the dielectric loss tangent at 60 GHz were measured using a vector network analyzer and an open resonator.

(Evaluation of Mechanical Strength by Bending)

[0087] The mechanical strength by bending was evaluated by bending a porous polyimide film by an angle of 90 and observing whether or not breaking occurs during the bending.

(Evaluation of Plating Property)

[0088] The obtained film was subjected to sputtering and electrolytic plating to form a 12 m-thick Cu foil layer, and, after undergoing an actual wiring pattern-forming process (subtractive process), subjected to a confirmation test for checking whether or not a plating solution intrudes into the film. Subsequently, the cross-section of the film was analyzed using an X-ray microanalyzer (XMA) to check the presence or absence of detection of an element originating from the plating (such as Cu or S). In case of no detection, the film was evaluated as having processability.

(Evaluation of Liquid Penetration)

[0089] A porous polyimide body was cut by a razor to expose a resulting cross-section. The cross-section was immersed in a red penetrant (NRC-ALII manufactured by Taiyo Bussan Co. Ltd.) for 5 minutes, and the penetrant adhering to the cross-section was cleaned off. The porous polyimide body was further cut perpendicularly to the exposed cross-section to evaluate the liquid penetration length by an optical microscope.

(Evaluation of Collapse)

[0090] A porous polyimide body was cut into a size of 50 mm50 mm, and a resulting sample was pressed by hot-press at 180 C. and 3 MPa for 60 minutes. Respective thicknesses of the sample before and after the press were measured, and, based on the measured values, a reduction in thickness of the sample after the press was calculated as change rate.

(Migration Test)

[0091] A plurality of through-holes each having a bore diameter of 0.3 mm were made in a porous polyimide body at a pitch of 1.52 mm, and a positive electrode and a negative electrode are formed in each of the through-holes. Then, a voltage of 60 V was applied between the positive and negative electrodes at 85 C. and 85% RH to measure an insulation resistance value.

Reference Example

(Synthesis of Polyimide Precursor [BPDA/PDA, DPE])

[0092] 43.2 g of p-phenylenediamine (PDA) and 20 g of diaminodiphenyl ether (DPE) were put into a 1000-ml flask equipped with a stirrer and a thermometer, and 768.8 g of N-methyl-2-pyrolidone (NMP) was added thereto and dissolved therein by stirring. Subsequently, 147 g of biphenyltetracarboxylic acid dianhydride (BPDA) was gradually added to the above solution, and the resulting solution was stirred at 40 C. for 2 hours to promote reaction. Further, the mixture was stirred at 75 C. for 12 hours to perform aging. As a result, a polyimide precursor solution having a solid content concentration of 20 wt % was obtained. This polyimide precursor has the following composition in terms of substance amount ratio: PDA:DPE:BPDA=0.8 mol:0.2 mol:1 mol.

Inventive Example 1

[0093] To 100 parts by weight of the polyimide precursor solution obtained in Reference Example, 200 parts by weight of polypropylene glycol having a weight-average molecular weight of 400 (grade: D400, manufactured by NOF Corporation) and 400 parts by weight of dimethylacetamide were added. Then, the resulting solution was stirred to obtain a transparent homogeneous solution. Subsequently, 4.2 parts by weight of 2-methylimidazole serving as an imidizing catalyst and 5.4 parts by weight of benzoic anhydride serving as a chemical imidizing agent were added to the above obtained solution to form a mixed solution. This mixed solution was applied onto a PET film or a copper foil by a die process, and dried by hot air at 85 C. for 15 minutes to produce a 100 m-thick polyimide precursor film having a phase-separated structure.

[0094] This film was immersed in carbon dioxide circulated at 40 C. while being pressurized to 30 MPa, for 8 hours, to promote extractive removal of the polypropylene glycol, phase separation of residual NMP, and pore formation. Subsequently, the carbon dioxide was depressurized to obtain a porous polyimide precursor film.

[0095] Further, the obtained porous polyimide precursor film was subjected to heat treatment under vacuum at 380 C. for 2 hours to promote removal of remaining components and imidization to thereby obtain a porous polyimide film.

Inventive Example 2

[0096] To 100 parts by weight of the polyimide precursor solution obtained in Reference Example, 200 parts by weight of polypropylene glycol having a weight-average molecular weight of 400 (grade: D400, manufactured by NOF Corporation) and 400 parts by weight of dimethylacetamide were added. Then, the resulting solution was stirred to obtain a transparent homogeneous solution. Subsequently, 4.2 parts by weight of 2-methylimidazole serving as an imidizing catalyst and 1.1 parts by weight of benzoic anhydride serving as a chemical imidizing agent were added to the above obtained solution to form a mixed solution. This mixed solution was applied onto a PET film or a copper foil by a die process, and dried by hot air at 85 C. for 15 minutes to produce a 100 m-thick polyimide precursor film having a phase-separated structure.

[0097] This film was immersed in carbon dioxide circulated at 40 C. while being pressurized to 30 MPa, for 8 hours, to promote extractive removal of the polypropylene glycol, phase separation of residual NMP, and pore formation. Subsequently, the carbon dioxide was depressurized to obtain a porous polyimide precursor film.

[0098] Further, the obtained porous polyimide precursor film was subjected to heat treatment under vacuum at 380 C. for 2 hours to promote removal of remaining components and imidization to thereby obtain a porous polyimide film.

Inventive Example 3

[0099] To 100 parts by weight of the polyimide precursor solution obtained in Reference Example, 200 parts by weight of polypropylene glycol having a weight-average molecular weight of 400 (grade: D400, manufactured by NOF Corporation) and 400 parts by weight of dimethylacetamide were added. Then, the resulting solution was stirred to obtain a transparent homogeneous solution. This solution was applied onto a PET film or a copper foil by a die process, and dried by hot air at 85 C. for 15 minutes to produce a 100 m-thick polyimide precursor film having a phase-separated structure.

[0100] This film was immersed in carbon dioxide circulated at 40 C. while being pressurized to 30 MPa, for 8 hours, to extractively remove the polypropylene glycol. Subsequently, the carbon dioxide was depressurized to obtain a porous polyimide precursor film.

[0101] Further, the obtained porous polyimide precursor film was subjected to heat treatment under vacuum at 380 C. for 2 hours to promote removal of remaining components and imidization to thereby obtain a porous polyimide film.

Inventive Example 4

[0102] To 100 parts by weight of the polyimide precursor solution obtained in Reference Example, 200 parts by weight of polyoxyethylene dimethyl ether having a weight-average molecular weight of 400 (grade: MM400, manufactured by NOF Corporation) and 150 parts by weight of NMP were added. Then, the resulting solution was stirred to obtain a transparent homogeneous solution. Subsequently, 4.2 parts by weight of 2-methylimidazole serving as an imidizing catalyst was added to the above obtained solution to form a mixed solution. This mixed solution was applied onto a PET film or a copper foil by a die process, and dried by hot air at 120 C. for 30 minutes to produce a 100 m-thick polyimide precursor film having a phase-separated structure.

[0103] This film was immersed in carbon dioxide circulated at 40 C. while being pressurized to 30 MPa, for 8 hours, to promote extractive removal of the polyoxyethylene dimethyl ether, phase separation of residual NMP, and pore formation. Subsequently, the carbon dioxide was depressurized to obtain a porous polyimide precursor film.

[0104] Further, the obtained porous polyimide precursor film was subjected to heat treatment under vacuum at 380 C. for 2 hours to promote removal of remaining components and imidization to thereby obtain a porous polyimide film.

Inventive Example 5

[0105] To 100 parts by weight of the polyimide precursor solution obtained in Reference Example, 200 parts by weight of polyoxyethylene dimethyl ether having a weight-average molecular weight of 400 (grade: MM400, manufactured by NOF Corporation), 10 parts by weight of PTFE powder having a particle size of about 2 m and 150 parts by weight of NMP were added. Then, the resulting solution was stirred to obtain a transparent homogeneous solution. Subsequently, 4.2 parts by weight of 2-methylimidazole serving as an imidizing catalyst was added to the above obtained solution to form a mixed solution. This mixed solution was applied onto a PET film or a copper foil by a die process, and dried by hot air at 120 C. for 30 minutes to produce a 100 m-thick polyimide precursor film having a phase-separated structure.

[0106] This film was immersed in carbon dioxide circulated at 40 C. while being pressurized to 30 MPa, for 8 hours, to promote extractive removal of the polyoxyethylene dimethyl ether, phase separation of residual NMP, and pore formation. Subsequently, the carbon dioxide was depressurized to obtain a porous polyimide precursor film.

[0107] Further, the obtained porous polyimide precursor film was subjected to heat treatment under vacuum at 380 C. for 2 hours to promote removal of remaining components and imidization to thereby obtain a porous polyimide film.

Comparative Example

[0108] To 100 parts by weight of the polyimide precursor solution obtained in Reference Example, 300 parts by weight of polypropylene glycol having a weight-average molecular weight of 400 (grade: D400, manufactured by NOF Corporation) and 400 parts by weight of dimethylacetamide were added. Then, the resulting solution was stirred to obtain a transparent homogeneous solution. This solution was applied onto a PET film or a copper foil by a die process, and dried by hot air at 140 C. for 20 minutes to produce a 100 m-thick polyimide precursor film having a phase-separated structure.

[0109] This film was immersed in carbon dioxide circulated at 40 C. while being pressurized to 30 MPa, for 8 hours, to extractively remove the polypropylene glycol. Subsequently, the carbon dioxide was depressurized to obtain a porous polyimide precursor film.

[0110] Further, the obtained porous polyimide precursor film was subjected to heat treatment under vacuum at 380 C. for 2 hours to promote removal of remaining components and imidization to thereby obtain a porous polyimide film.

[0111] Results obtained by observing the cross-sections and the surfaces of the films obtained in Inventive Examples 1 to 3 and Comparative Example by SEM are shown in FIGS. 1a and 1b (Inventive Example 1), FIGS. 2a and 2b (Inventive Example 2), FIGS. 3a and 3b (Inventive Example 3) and FIGS. 4a and 4b (Comparative Example), wherein FIG. Xa presents an SEM image of the cross-section, and FIG. Xb presents an SEM image of the surface, where X denotes an integer number of 1 to 4.

[0112] These figures show that each of the films obtained in Inventive Examples 1 to 3 has a surface layer formed as a smooth skin layer and a cross-section with no pore, whereas the film obtained in Comparative Example is devoid of such a skin layer, wherein there are pores in its cross-section uniformly, and the presence of irregularities is observed in its surface.

[0113] From the results of the SEM observation, the skin layer formed in each of the films obtained in Inventive Examples 1 to 3 has a thickness shown in Table 1.

TABLE-US-00001 TABLE 1 Thickness of Skin Total thickness of Rate of Thickness of Layer (m) Film (m) Skin Layer (%) Inventive 2.2 130 1.7 Example 1 Inventive 3.6 121 3.0 Example 2 Inventive 3.7 108 3.4 Example 3

[0114] Further, results of measurements performed for films obtained in Inventive Examples 1 to 3 and Comparative Example are shown in Table 2.

TABLE-US-00002 TABLES 2 Average Pore Full Width at Dielectric Dielectric Dielectric Dielectric Porosity Diameter Half Maximum Constant Loss Tangent Constant Loss Tangent Bending Process- (%) (m) (m) 10 GHz 10 GHz 60 GHz 60 GHz Strength ability Inventive Example 1 91 4.4 4 1.22 0.0017 1.20 0.0027 Good Good Inventive Example 2 84 7.5 6 1.40 0.0024 1.33 0.0046 Good Good Inventive Example 3 63 9.6 8 1.93 0.0063 1.80 0.014 Good Good Comparative Example 76 19.3 Unevaluable 1.54 0.0029 1.42 0.0056 Not Good Not Good

[0115] As is apparent from the above results, the film according to the present invention exhibits a relatively low dielectric constant and a relatively low dielectric loss tangent, i.e., has excellent electric properties, at high frequencies.

[0116] Further, the films obtained in Inventive Examples 1 to 3 and Comparative Example were evaluated in regard to the plating property. As a result, the film obtained in Comparative Example has a problem of liquid penetration, whereas the films obtained in Inventive Examples 1 to 3 could be adequately plated without such a problem.

[0117] The films according to the present invention are also excellent in terms of a mechanical or physical property during bending.

[0118] Next, results obtained by observing the cross-sections of the films obtained in Inventive Examples 4 and 5 by SEM are shown in FIG. 5 (Inventive Example 4) and FIG. 6 (Inventive Example 5).

[0119] From the results of the SEM observation, the skin layer formed in each of the films obtained in Inventive Examples 4 and 5 has a thickness shown in Table 3.

TABLE-US-00003 TABLE 3 Thickness of Skin Total thickness of Rate of Thickness of Layer (m) Film (m) Skin Layer (%) Inventive 4.8 120 4.0 Example 1 Inventive 6.0 120 5.0 Example 2

[0120] Further, results of measurements performed for films obtained in Inventive Examples 4 and 5 are shown in Table 4.

TABLE-US-00004 TABLE 4 Average Pore Full Width at Dielectric Dielectric Porosity Diameter Half Maximum Constant Loss Tangent Liquid Evaluation (%) (m) (m) 10 GHz 10 GHz Penetration of Collapse Migration Inventive Example 4 80 9.8 10 1.49 0.0040 200 m 6% 5.88E+10 () Inventive Example 5 81 6.5 8 1.48 0.0040 20 m 4% 6.93E+10 ()

[0121] As is apparent from the above results, the film according to the present invention whose porous structure is an independent-cell structure is excellent in terms of liquid penetration and pressing resistance, as well as excellent electric properties, and exhibits a high insulation resistance value even after processing, so that it is excellent in terms of processability of a circuit board.