US20140295189A1

US20140295189A1 - Coverlay for high-frequency circuit substrate

Info

Publication number: US20140295189A1
Application number: US14/230,298
Authority: US
Inventors: Shotaro Hidaka; Takeshi Tanaka; Hideaki Machida; Takeshi Inaba; Shinya Murakami
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 2013-03-29
Filing date: 2014-03-31
Publication date: 2014-10-02
Also published as: US20140291926A1; JP5803969B2; US9382084B2; EP2783875A1; EP2783875B1; JP2014197319A

Abstract

To provide a coverlay for a high-frequency circuit substrate, that uses polyimide film and fluororesin, has excellent mechanical properties and heat resistance, and can increase workability during the manufacture of high-frequency circuit substrates. Resolution Means: The coverlay for a high-frequency circuit substrate including a polyimide film and a fluororesin bonded together, and an adhesive strength between the polyimide film layer and the fluororesin layer being greater than 3.0 N/cm.

Description

BACKGROUND OF THE DISCLOSURE

The present invention pertains to a coverlay for a high-frequency circuit substrate.
Printed wiring boards are used widely in electronic and electrical equipment. Among them, flexible printed wiring boards that can be bent are widely used in the bending parts of personal computers, portable telephones, and the like, and in parts that require bending, such as hard disks.
As substrates for such flexible printed wiring boards and substrates for coverlays for protecting printed wiring boards, normally various types of polyimide film are used in consideration of such properties as heat resistance, dimensional stability, flexibility, high bendability, and ease of making into a thin film, and most coverlays are made of a combination of polyimide film and adhesive.
With the growth in the volume of information being transmitted in recent years, there is an increasing demand for circuit substrates for high frequency applications. As the frequency used for transmission increases, the increase in frequency is also accompanied by more transmission loss. Because circuits with high transmission loss are not practical, in order to transmit information efficiently at high frequencies it is necessary to reduce the transmission loss.
Transmission loss can be reduced by lowering the dielectric constant of the substrate or coverlay, and because of this, low-dielectric-constant substrates and coverlays are in demand. Normally the dielectric constant of the polyimide film that is used in the substrates and coverlays of flexible printed circuit boards is 3.0 to 3.5, which is insufficient for a low-dielectric-constant material.
So as a way to reduce the dielectric constant, methods have been developed in which a fluororesin of low dielectric constant is laminated between the copper and polyimide layer in the circuit wires that are used in the substrate (the copper foil is etched on the side of a copper-clad laminate (CCL)) (see Japanese Patent No. 2890747 and Japanese Patent No. 4917745 (Patent Documents 1 and 2)).
But in the process of manufacturing a circuit substrate using the coverlay, if for example, kiss lamination is done by aligning the positions of a copper-clad laminate and a coverlay, in some cases, some time may be needed for the aligning in order to prevent attachment misalignment, and further improvements in manufacturing efficiency have been requested for industrial implementation.
From such considerations, it has been desired that a coverlay be developed that uses fluororesin, and has as a base material a new polyimide film that is easy to process and is a low dielectric constant material for use in high-frequency circuit substrates.

Patent Document 1: Japanese Patent No. 2890747
Patent Document 2: Japanese Patent No. 4917745

SUMMARY

An object of the present invention is to provide a coverlay for a high-frequency circuit substrate that employs polyimide film and fluororesin, has excellent mechanical properties and heat resistance, and can improve workability during manufacture of a high-frequency circuit substrate.
That is, the present invention pertains to the following invention.
1. A coverlay for a high-frequency circuit substrate, the coverlay comprising a polyimide film and a fluororesin bonded together, and an adhesive strength between the polyimide film layer and the fluororesin layer being greater than 3.0 N.
2. The coverlay according to 1 above, wherein a thermal shrinkage thereof at 260° C. for 30 minutes is less than ±0.1%.
3. The coverlay according to 1 or 2 above, wherein the fluororesin has a melting point of 200° C. or less.
4. The coverlay according to any of 1 to 3 above, wherein the fluororesin is a fluorine-containing ethylenic polymer, and the fluorine-containing ethylenic polymer contains a carbonyl group.
5. The coverlay according to 4 above, wherein a quantity of carbonyl groups contained in the fluorine-containing ethylenic polymer totals 3 to 1000 groups per 1×10⁶main-chain carbon atoms.
6. The coverlay according to any of 1 to 3 above, wherein the fluororesin is made up of fluorine-containing ethylenic polymer that has at least one type selected from a group made up of carbonate groups, carboxylic acid halide groups, and carboxylic acid groups totaling 3 to 1000 groups per 1×10⁶main-chain carbon atoms.
7. The coverlay according to any of 1 to 3 above, wherein the fluororesin is one or more types of fluorine-containing ethylenic monomer selected from a group made up of tetrafluoroethylene, vinylidene fluoride, chlorotrifluoroethylene, vinyl fluoride, hexafluoropropylene, hexafluoroisobutene, monomers represented by the following formula (X):
CH₂═CR¹(CF₂)_nR² (X)
(wherein R¹represents H or F, R²represents H, F, or Cl, and n is a positive integer in the range of 1 to 10), and perfluoro(alkyl vinyl ethers) having 2 to 10 carbon atoms, or a fluorine-containing ethylenic polymer made by polymerizing the fluorine-containing ethylenic monomer and an ethylenic monomer having 5 or fewer carbon atoms.
8. The coverlay according to any one of 1 to 3 above, wherein the fluororesin is a copolymer made by polymerizing at least the following (a), (b), and (c).
(a) 20 to 90 mol % of tetrafluoroethylene,
(b) 10 to 80 mol % of ethylene, and
(c) 1 to 70 mol % of a compound represented by the formula:
CF₂═CFR³ (Y)
(wherein R³represents CF₃or OR⁴, and R⁴represents a perfluoroalkyl group having 1 to 5 carbon atoms).
9. The coverlay according to any of 1 to 8 above, wherein the polyimide film is made up mainly of one or more aromatic diamine components selected from a group made up of paraphenylene diamine, 3,4′-diaminodiphenyl ether, and 4,4′-diaminodiphenyl ether, and one or more acid anhydride components selected from a group made up of pyromellitic acid dianhydride and 3,3′,4,4′-biphenyl tetracarboxylic acid dianhydride.
The high-frequency circuit substrate coverlay of the present invention is industrially useful because, with its excellent mechanical properties and heat resistance, it enhances workability during the manufacture of high-frequency circuit substrates. Also, with a high-frequency circuit substrate that employs the coverlay of the present invention, the low-dielectric-constant fluororesin can be made so that the dielectric constant is low and the transmission loss is kept in check.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the results of measuring the transmission properties of circuits employing the coverlays of the working examples and of the comparison examples.

DETAILED DESCRIPTION

The coverlay of the present invention for a high-frequency circuit substrate is a coverlay that is made up of a polyimide film and a fluorosein bonded together, and the adhesive strength between the polyimide film layer and the fluororesin layer (initial adhesive force) exceeds 3.0 N/cm.
In manufacturing the polyimide film that is used in this coverlay, first, a polyamic acid solution is prepared by polymerizing an aromatic diamine component and an acid anhydride component in an organic solvent.
Specific examples of the aromatic diamine component include paraphenylene diamine, metaphenylene diamine, benzidine, paraxylilene diamine, 4,4′-diaminodiphenyl ether, 3,4′-diaminodiphenyl ether, 4,4′-diaminodiphenyl methane, 4,4′-diaminodiphenyl sulfone, 3,3′-dimethyl-4,4′-diaminodiphenyl methane, 1,5-diaminonaphthalene, 3,3′-dimethoxybentidine, 1,4-bis (3 methyl-5 aminophenyl)benzene, and amidic derivatives thereof. Among these, it is preferable for circuit substrate applications to adjust the amounts of the diamines such as paraphenylene diamine, 3,4′-diaminodiphenyl ether, and the like, which are effective in increasing the tensile strength of the film, so that the tensile strength of the polyimide film that is ultimately obtained is 3.0 GPa or more. Of these aromatic diamines, paraphenylene diamine, 4,4′-diaminodiphenyl ether, and 3,4′-diaminodiphenyl ether are preferable. These may be used either singly as one type, or with two or more type thereof mixed together. If paraphenylene diamine and 4,4′-diaminodiphenyl ether and/or 3,4′-diaminodiphenyl ether are used together, there are no particular restrictions on their blending ratio (mole ratio), but it is preferable that the ratio of 4,4′-diaminodiphenyl ether and/or 3,4′-diaminodiphenyl ether:paraphenylene diamine=69:31 to 100:0 (except 0), and more preferable that it be 70:30 to 90:10.
Specific examples of the acid anhydride component include pyromellitic acid, 3,3′,4,4′-biphenyl tetracarboxylic acid, 2,3′,3,4′-biphenyl tetracarboxylic acid, 3,3′,4,4′-benzophenone tetracarboxylic acid, 2,3,6,7-naphthalene dicarboxylic acid, 2,2-bis(3,4-dicarboxylphenyl)ether, pyridine-2,3,5,6-tetracarboxylic acid, and acid anhydrides of amidic derivatives thereof and the like. Among these acid anhydrides, pyromellitic acid, 3,3′,4,4′-biphenyl tetracarboxylic acid, 2,3′,3,4′-biphenyl tetracarboxylic acid are preferable. These may be used either singly as one type, or with two or more types mixed together. As the mole ratio of the acid anhydride components, if pyromellitic acid dianhydride and 3,3′,4,4′-biphenyl tetracarboxylic acid dianhydride are used together, there are no particular restrictions on their blending ratio (mole ratio), but it is preferable that the ratio of pyromellitic acid dianhydride:3,3′,4,4′-biphenyl tetracarboxylic acid dianhydride=0 (except 0):100 to 97:3, and more preferable that it be 30:70 to 95:5.
Examples of the polyimide film to be used in the coverlay of the present invention preferably include mainly ones that are made up of one or more aromatic diamine components selected from a group made up of paraphenylene diamine, 3,4′-diaminodiphenyl ether, and 4,4′-diaminodiphenyl ether, and one or more acid anhydride components selected from a group made up of pyromellitic acid dianhydride and 3,3′,4,4′-biphenyl tetracarboxylic acid dianhydride.
As organic solvents that can be used for forming the polyamic acid solution in the present invention, examples include dimethyl sulfoxide, diethyl sulfoxide, and other sulfoxide solvents; N,N-dimethyl form[amide], N,N-diethyl formamide, and other formamide solvents; N,N-dimethyl acetoamide, N,N-diethyl acetoamide, and other acetoamide solvents; N-methyl-2-pyrrolidone, N-vinyl-2-pyrrolidone, and other pyrrolidone solvents; phenol, o-, m-, or p-cresol, xylenol, phenol halide, catechol, and other phenol solvents; or hexamethyl phosphoramide, γ-butyrolactone, and other aprotic polar solvents; it is desirable to use these either singly or as a mixture of two or more types, and in addition, aromatic hydrocarbons such as xylene or toluene may be used.
There are no particular restrictions on the polymerization method; any well known method may be used, such as (1) a method of polymerizing in which first the entire amount of the aromatic diamine component is inserted into the solvent, and the acid anhydride component is then added so that it becomes an amount equivalent to the entire amount of the aromatic diamine component.
(2) A method of polymerizing in which first the entire amount of the acid anhydride component is inserted into the solvent, then the aromatic diamine component is added so that it becomes an amount equivalent to the acid anhydride component.
(3) A method of polymerizing in which a first aromatic diamine component (a1) is inserted into the solvent, then a first acid anhydride component (b1) is mixed at a ratio that becomes 95 to 105 mol % with respect to the reaction components for the time needed for a reaction to occur, after which a second aromatic diamine component (a2) is added, following which a second acid anhydride component (b2) is added so that the entire aromatic diamine component and the entire acid anhydride component become roughly equivalent amounts.
(4) A method of polymerizing in which a first acid anhydride component (b1) is inserted into the solvent, then a first aromatic diamine component (a1) is mixed at a ratio that becomes 95 to 105 mol % with respect to the reaction components for the time needed for a reaction to occur, after which a second acid anhydride component (b2) is added, following which a second aromatic diamine component (a2) is added so that the entire aromatic diamine component and the entire acid anhydride component become roughly equivalent amounts.
(5) A method wherein, in a solvent, a polyamic acid solution (A) is prepared by causing reactions such that one or the other of the first aromatic diamide component and an acid anhydride component is in excess, and in a separate solvent, a polyamic acid solution (B) is prepared by causing reactions so that one or the other of the second aromatic diamine component and an acid anhydride component is in excess, and then the polyamic acid solutions (A) and (B) thereby obtained are mixed together and the polymerization is completed. If when preparing the polyamic acid solution (A), the aromatic diamine component is in excess, then with the polyamic acid solution (B) the acid anhydride component is made in excess, or when with the polyamic acid solution (A), the acid anhydride component is in excess, then the aromatic diamine component is made in excess with the polyamic acid solution (B). The polyamic acid solutions (A) and (B) are mixed together, and an adjustment is made so that the total aromatic diamine component and acid anhydride component to be used in these reactions are of roughly equivalent amounts.
Also, the polymerization methods are not limited thereto, and one may also use any other well known method.
The polyamic acid solution that is thus obtained normally contains a solid portion of 5 to 40 wt %, and preferably 10 to 30 wt %. Moreover, its viscosity is normally 10 to 10,000 Pa·s as measured by a Brookfield viscometer, and is preferably 300 to 5,000 Pa·s for sake of stable liquid feeding. It is acceptable for the polyamic acid in an organic solvent solution to be partially imidized.
Next, the method for manufacturing the polyimide film of the present invention, which employs the above polyamic acid solution, is described.
Examples as methods for making the polyimide film include a method in which the polyimide film is obtained by casting the polyamic acid solution in film form and then thermally removing rings and the solvent, and a method in which the polyimide film is obtained by mixing a ring removal catalyst and dehydrating agent into the polyamic acid solution to chemically remove rings and make a gel film, which is then heated to remove the solvent.
The polyamic acid solution may contain a ring removal catalyst (imidization catalyst), dehydrating agent, gelling delaying agent, and the like.
Specific examples of the ring removal catalysts to be used in the present invention include trimethyl amine, triethylene diamine, and other aliphatic tertiary amines; dimethyl aniline and other aliphatic tertiary amines; and isoquinoline, pyridine, beta picoline, and other heterocyclic tertiary amines, and the like, but heterocyclic tertiary amines are preferable. These may be used either singly as one type or as a mixture of two or more types.
Specific examples of the dehydrating agents to be used in the present invention include acetic anhydride, propionic anhydride, butyric anhydride, and other aliphatic carboxylic anhydrides, and benzoic anhydride and other aromatic carboxylic anhydrides, and the like, but acetic anhydride and/or benzoic anhydride are preferable.
As a method for manufacturing polyimide film from a polyamic acid solvent, examples include a method in which a polyamic acid solution that is made to contain the ring removal catalyst and the dehydrating agent is made to flow from a slitted nozzle onto a support body and is molded into a film form, imidization on the support body is allowed to proceed partway, a gel film that can support itself is made, then it is peeled off from the support body, is heated, dried, and imidized, and is heat-treated.
The “support body” is a metal rotating drum or endless belt, and its temperature is controlled by a liquid or gas heat transfer medium and/or an electric heater or other radiant heat.
The gel film is heated normally to 30 to 200° C., and preferably to 40 to 150° C. by being heated by the support body and/or by being heated by hot air, an electric heater, or another heat source, thereby causing ring closure reactions, and by drying the volatile component, such as the organic solvent, that is set free, the film acquires the ability to support itself, and is peeled away from the support body.
The gel film that is peeled from the support body may as necessary undergo a stretching extension treatment in the running direction while regulating the running speed by a rotating roll. The extension magnification (MDX) in the mechanical conveyance direction and the extension magnification (TDX) in the direction perpendicular to the mechanical conveyance direction are implemented at 1.01 to 1.9-times, and preferably at 1.05 to 1.6-times.
The film dried in the drying zone is heated from 15 seconds to 10 minutes by hot air, an infrared heater, or the like. Next, it is heat-treated for 15 seconds to 20 minutes at a temperature of 250 to 500° C. by hot air and/or an electric heater, or the like.
Also, the running speed is adjusted to adjust the thickness of the polyimide film, and the thickness of the polyimide film is normally about 2 to 250 μm, and preferably about 2 to 100 μm. Thicknesses thinner or thicker are undesirable because that would significantly degrade the manufacturability of the film.
As the polyimide film to be used in the present invention, commercially available products may be used. There are no particular restrictions on the commercially available products, and examples include Capton EN types (for example, 50EN-S (brand name, made by Dupont-Toray Co., Ltd.), 100EN (brand name, made by Dupont-Toray Co., Ltd.), and the like), Capton H types (for example, Capton 100H (brand name, made by Dupont-Toray Co., Ltd.), etc.), and the like.
The polyimide film in the present invention may include a plasticizer or other resin, or the like to the extent that doing so does not detract from the purpose of the present invention.
For the plasticizers, there are no particular restrictions, and examples include hexylene glycol, glycerin, β-naphthol, dibenzyl phenol, octyl cresol, bisphenol A, and other bisphenol compounds; p-hydroxyoctyl benzoate, p-hydroxy benzoic acid-2-ethyl hexyl, p-hydroxy benzoic acid peptyl, p-hydroxy benzoic acid ethylene oxide and/or propylene oxide adducts, ε-caprolactone, phosphoric acid ester compounds of phenols, N-methyl benzene sulfonamide, N-ethyl benzene sulfonamide, N-butyl benzene sulfoneamide, toluene sulfonamide, N-ethyl toluene sulfonamide, N-cyclohexyl toluene sulfonamide, and the like.
As the other resins to be blended into the polyimide, those having superior compatibility are preferable, and examples include ester and/or carboxylic acid modified olefin resin, acrylic resin (in particular, acrylic resin that has a glutarimide group), ionomer resin, polyester resin, phenoxy resin, ethylene-propolyene-diene copolymer, polyphenylene oxide, and the like.
The polyimide film in the present invention may include colorants and various types of additives, insofar as this would not detract from the purpose of the present invention. As the additives, examples include antistatic agents, flame retardants, heat stabilizers, ultraviolet ray absorbents, lubricants, mold release agents, crystal nucleus agents, reinforcing agents (fillers), and the like. Also, the surface of the polyimide film may be coated with ink, and the like.
There are no particular restrictions on the fluororesin that is used in the coverlay of the present invention, but a fluorine-containing ethylenic polymer is preferable. In the fluorine-containing ethylenic polymer in the present invention, a carbonyl group or a functional group that contains a carbonyl group is joined to the fluorine-containing ethylenic polymer chain.
The “carbonyl group” means a functional group having —C(═O)— that can basically react with the imide groups or amino groups in the polyimide film. Specifically examples include carbonates, carboxylic acid halides, aldehydes, ketones, carboxylic acid, esters, acid anhydrides, isocyanate groups, and the like. There are no particular restrictions on the carbonyl group, but preferable for ease of introduction and high reactivity with the polyamide resin are carbonate groups, carboxylic acid halide groups, carboxylic acid groups, ester groups, and acid anhydride groups, and more preferable are carbonate groups and carboxylic acid halide groups.
The number of carbonyl groups in the fluorine-containing ethylenic polymer in the present invention can be suitably selected according to differences in the types of materials that are laminated together, the shape, the purpose of the adhesion, the application, the adhesive force that is required, the mode of polymerization, and the method of adhesion, and the like, but it is preferable that the number of carbonyl groups total 3 to 1000 groups per 1×10⁶carbon atoms in the main chain. If the number of the carbonyl groups per 1×10⁶carbon atoms in the main chain is less than 3, sometimes there will not be sufficient adhesive force. Moreover, if it is greater than 1000, sometimes the adhesive force will be reduced due to chemical changes of the carbonyl groups during the adhesion operation. More preferable is 3 to 500, even more preferable is 3 to 300, and particularly preferable is 5 to 150. Also, the amount of carbonyl groups in the fluorine-containing ethylenic polymer can be measured by infrared absorption spectrum analysis.
Therefore if the fluorine-containing ethylenic polymer of the present invention for example is one that has carbonate groups and/or carboxylic acid halide groups, if it has carbonate groups, it is preferable that the number of carbonate groups be 3 to 1000 per 1×10⁶carbon atoms in the main chain, and if the fluorine-containing ethylenic polymer of the present invention has carboxylic acid halide groups, it is preferable that the number of carboxylic acid halide groups be 3 to 1000 per 1×10⁶carbon atoms in the main chain. If the fluorine-containing ethylenic polymer of the present invention has both carbonate groups and carboxylic acid halide groups, it is preferable that the total number of carbonate groups and carboxylic acid halide groups be 3 to 1000 per 1×10⁶carbon atoms in the main chain. If the number of the carbonate groups and/or carboxylic acid halide groups is less than 3 per 1×10⁶carbon atoms in the main chain, sometimes there will not be sufficient adhesive force. Moreover, if it is greater than 1000, sometimes, due to chemical changes of the carbonate groups or carboxylic acid halide groups during the adhesion operation, the production of gas coming from the adhesive interface will have an adverse effect, and the adhesive force will be reduced. From the standpoint of heat resistance and resistance to chemicals, it is more preferable that it be 3 to 500, even more preferable that it be 3 to 300, and particularly preferable that it be 5 to 150. Also, if carboxylic acid halide groups, which have excellent reactivity with polyamide resin, are present in the fluorine-containing ethylenic polymer in a quantity of 10 or more, and more preferably 20 or more per 1×10⁶carbon atoms in the main chain, then even if the quantity contained in the total of carbonyl groups is less than 150 per 1×10⁶carbon atoms in the main chain, excellent adhesion can be obtained with a layer (A) made up of polyamide resin.
The carbonate group in the fluorine-containing ethylenic polymer in the present invention is generally a group that has a —OC(═O)O— bond; specifically, it is one with a structure of an —OC(═O)O—R group (where R is an organic group (for example, a C₁to C₂₀alkyl group (and preferably a C₁to C₁₀alkyl group), a C₂to C₂₀alkyl group having an ether bond, or the like) or a group VII element). As carbonate groups, preferable examples include —OC(═O)OCH₃, —OC(═O)OC₃H₇, —OC(═O)OC₈H₁₇, —OC(═O)OCH₂CH₂CH₂OCH₂CH₃, and the like.
The carboxylic acid halide group in the fluorine-containing ethylenic polymer in the present invention is specifically one of a —COY structure (where Y is a halogen element), and examples include —COF and —COCl.
A fluorine-containing ethylenic polymer that has these carbonyl groups can itself retain the excellent properties of a fluorine-containing resin, and can confer them to the laminate after formation, with no diminution in such excellent properties that a fluorine-containing resin has.
The fluorine-containing ethylenic polymer in the present invention includes carbonyl groups in its polymer chain, but there are no particular restrictions on how the carbonyl groups are contained in the polymer chain; for example a carbonyl group or a functional group that contains a carbonyl group may be joined to the end of the polymer chain or to a side chain. Among them, those that have a carbonyl group on the end of the polymer chain are preferable, because they do not significantly reduce heat resistance, mechanical properties, or resistance to chemicals, or because they are beneficial from a productivity and cost perspective. Here, a mode that is preferable, because it is very easy to introduce and because it is also easy to control the quantity introduced, is the method in which either carbonyl groups are included such as peroxy carbonate or peroxy ester, or the carbonyl groups are introduced to the ends of the polymer chain using a polymerization initiator that has a functional group that can be changed into a carbonyl group. Also, in the present invention, a carbonyl group that originates in a peroxide means a carbonyl group that is introduced directly or indirectly from a functional group that is included in the peroxide.
Also, in the fluorine-containing ethylenic polymer in the present invention, even if a fluorine-containing ethylenic polymer that does not contain carbonyl groups is present, it suffices that as an overall polymer, it have a number of carbonyl groups in the above range as a total per 1×10⁶carbon atoms in the main chain.
In the present invention, the type and structure of the fluorine-containing ethylenic polymer can be suitably selected according to the purpose, the application, and the method of use, but here it is preferable that its melting point be 160 to 270° C. With such a polymer, this is advantageous especially if the lamination is done by a heating, melting, sticking-together process, because, in particular, sufficient adhesiveness can be obtained between the carbonyl groups and the other material, and a strong adhesive force with the other material can be obtained. From the perspective of enabling lamination to an organic material of relatively low heat resistance, the melting point is more preferably 250° C. or less, even more preferably 230° C. or less, and particularly preferably 200° C. or less. For the melting point, using a Seiko model DSC device (made by Seiko Electronics Co.), the melting peak was recorded when the temperature was increased at a rate of 10° C./min, and the temperature corresponding to the maximum value was taken as the melting point (Tm).
With regard to the molecular weight of the fluorine-containing ethylenic polymer in the present invention, it is preferable that it be in a range in which the polymer can be formed below the thermal breakdown temperature and that the resulting formed body be able to exhibit the excellent mechanical properties that are characteristic of the fluorine-containing ethylenic polymer. Specifically, taking the melt flow rate (MFR) as an index of the molecular weight, it is preferable that the MFR be 0.5 to 100 g/10 min at any temperature in the range of about 230 to 350° C., which is the general molding temperature range for fluororesin. For the MFR, using a melt indexer (made by Toyo Seiki Seisaku-Sho, Ltd.), measurements were taken at unit intervals (10-minute intervals) of the weight (g) of the polymer flowing out of a 2 mm diameter nozzle having a length of 8 mm under a load of 5 kg at various temperatures.
The structure of the fluorine-containing ethylenic polymer chain is in general a homopolymer chain or copolymer chain that has repeated units that are derived from at least one type of fluorine-containing ethylenic monomer, and it may be a polymer chain made up of either a fluorine-containing ethylenic monomer only, or made by polymerizing a fluorine-containing ethylenic monomer with an ethylenic monomer that does not have any fluorine atoms.
The fluorine-containing ethylenic monomer is an olefinic unsaturated monomer that has fluorine atoms, and specific examples include tetrafluoroethylene, vinylidene fluoride, chlorotrifluoroethylene, vinyl fluoride, hexafluoropropylene, hexafluoroisobutene, monomers represented by the formula (X):
CH₂═CR¹(CF₂)_nR² (X)
(wherein R¹represents H or F, R²represents H, F, or Cl, and n is a positive integer in the range 1 to 10), and perfluoro(alkyl vinyl ethers) having 2 to 10 carbon atoms, and the like.
The above ethylenic monomer that does not have any fluorine atoms is preferably selected from ethylenic monomers having 5 or fewer carbon atoms, in order not to reduce the heat resistance, and the like. Specific examples include ethylene, propylene, 1-butene, 2-butene, vinyl chloride, vinylidene chloride, and the like.
If a fluorine-containing ethylenic monomer and an ethylenic monomer that does not have any fluorine atoms are used, the composition of the monomers may have a weight ratio such that the fluorine-containing ethylenic monomer is 10 mol % or more to less than 100 mol % (for example, 30 mol % or more to less than 100 mol %), and the ethylenic monomer that does not have any fluorine atoms is greater than 0 mol % to no greater than 90 mol % (for example, greater than 0 mol % to no greater than 70 mol %).
In the fluorine-containing ethylenic polymer in the present invention, the melting point or glass transition point of the polymer can be adjusted by selecting the type, combination, composition ratio, and other properties of the fluorine-containing ethylenic monomer and the ethylenic monomer that does not have any fluorine atoms.
As the fluorine-containing ethylenic polymer in the present invention, preferable for heat resistance and resistance to chemicals is a fluorine-containing ethylenic polymer that contains a carbonyl group in which a tetrafluoroethylene unit is an essential component, and preferable for formability and workability is a fluorine-containing ethylenic copolymer that contains a carbonyl group in which a vinylidene fluoride unit is an essential component.
Preferable specific examples of the fluorine-containing ethylenic polymer in the present invention include fluorine-containing ethylenic copolymers (I) to (V) that contain a carbonyl group in which the fluorine-containing ethylenic polymer is essentially made by polymerizing the following monomers:
(I) a copolymer made by polymerizing at least tetrafluoroethylene and ethylene,
(II) a copolymer made by polymerizing at least tetrafluoroethylene and a compound represented by the formula (Y):
CF₂═CFR³ (Y)
(wherein R³represents CF₃or OR⁴, and R⁴represents a perfluoroalkyl group having 1 to 5 carbon atoms),
(III) a copolymer made by polymerizing at least vinylidene fluoride,
(IV) a copolymer made by polymerizing at least (a), (b), and (c) below,
(a) tetrafluoroethylene 20 to 90 mol %
(b) ethylene 10 to 80 mol %
(c) 1 to 70% of a compound expressed by
CF₂═CFR³ (Y)
(where R³has the same meaning as above), and
(v) a copolymer made by polymerizing at least (d), (e), and (f) below.
(d) vinylidene fluoride 15 to 60 mol %
(e) tetrafluoroethylene 35 to 80 mol %
(f) hexafluoropropylene 5 to 30 mol %
Other known monomers may be added to these specific examples, including the above monomers, insofar as they do not interfere with the effects of the present invention.
Each of these examples of a fluorine-containing ethylenic polymer that contains a carbonyl group is preferable especially in that it has excellent heat resistance.
Examples of the copolymer (I) include polymer-chain carbonyl group-containing copolymers that, with respect to the monomers as a whole excluding monomers that have a carbonyl group (if it has a functional group that has a carbonyl group on a side chain), are made up of 20 to 90 mol % tetrafluoroethylene units (for example, 20 to 60 mol %), 10 to 80 mol % ethylene units (for example, 20 to 60 mol %), and 0 to 70 mol % of other monomers that can be copolymerized therewith.
As other monomers that can be copolymerized, examples include hexafluoropropylene, chlorotrifluoroethylene, and monomers represented by the formula (X):
CH₂═CR¹(CF₂)_nR² (X)
(wherein R¹represents H or F, R²represents H, F, or Cl, and n is a positive integer in the range 1 to 10), and perfluoro(alkyl vinyl ethers) having 2 to 10 carbon atoms, and normally these are used as a single types or as two or more types together.
As the copolymer (I), the following can be suitably listed in that they maintain excellent performance in a tetrafluoroethylene/ethylene copolymer, they can be made to have a relatively low melting point as well, and they can exhibit maximum adhesion with other materials.
(I-1) Polymer-chain carbonyl group-containing copolymers made up of 62 to 82 mol % tetrafluoroethylene units, 20 to 38 mol % ethylene units, and 0 to 10 mol % of other monomer units,
(I-2) Polymer-chain carbonyl group-containing copolymers made up of 20 to 80 mol % tetrafluoroethylene units, 10 to 80 mol % ethylene units, 0 to 30 mol % hexafluoropropylene units, and 0 to 10 mol % of other monomer units.
Suitable examples of the copolymer (II) include the following.
(II-1) Polymer-chain carbonyl group-containing copolymers made up of 65 to 95 mol % (preferably, 75 to 95 mol %) tetrafluoroethylene units and 5 to 35 mol % (preferably, 5 to 25 mol %) hexafluoropropylene units,
(II-2) polymer-chain carbonyl group-containing copolymers made up of 70 to 97 mol % tetrafluoroethylene units and 3 to 30 mol % CF₂═CFOR⁴(where R⁴is a perfluoroalkyl group having 1 to 5 carbon atoms) units,
(II-3) and a polymer-chain copolymer having carbonyl groups that is made up of tetrafluoroethylene units, hexafluoropropylene units, and CF₂═CFOR⁴(where R⁴is as above) units, wherein the total of the hexafluoropropylene units and the CF₂═CFOR⁴units is 5 to 30 mol %.
The (II-1) to (II-3) above are also perfluoro copolymers, and even among fluorine-containing polymers, they are the most excellent in heat resistance, electrical insulation performance, and other properties.
As the copolymer (III), examples include polymer-chain carbonyl group-containing copolymers that, with respect to the monomer total excluding the monomers that have a carbonyl group (if they have a carbonyl group-containing functional group in a side chain), are made up of 15 to 99 mol % vinylidene fluoride units, 0 to 80 mol % tetrafluoroethylene units, and 0 to 30 mol % of either one or more types of hexafluoropropylene or chlorotrifluoroethylene units. As specific examples of the copolymer (III), the following can be suitably listed.
(III-1) Polymer-chain carbonyl group-containing copolymers made up of 30 to 99 mol % vinylidene fluoride units and 1 to 70 mol % tetrafluoroethylene units,
(III-2) polymer-chain carbonyl group-containing copolymers made up of 60 to 90 mol % vinylidene fluoride units, 0 to 30 mol % tetrafluoroethylene units, and 1 to 20 mol % chlorotrifluoroethylene units,
(III-3) polymer-chain carbonyl group-containing copolymers made up of 60 to 99 mol % vinylidene fluoride units, 0 to 30 mol % tetrafluoroethylene units, and 5 to 30 mol % hexafluoropropylene units,
(III-4) polymer-chain carbonyl group-containing copolymers made up of 15 to 60 mol % vinylidene fluoride units, 35 to 80 mol % tetrafluoroethylene units, and 5 to 30 mol % hexafluoropropylene units.
There are no particular restrictions on how to manufacture the fluorine-containing ethylenic polymer in the present invention. The fluorine-containing ethylenic polymer of the present invention can be manufactured by taking an ethylenic monomer that has a carbonyl group and copolymerizing it with a fluorine-containing and/or ethylenic monomer of a type and blend that fits the desired fluorine-containing polymer. As the ethylenic monomer that has a carbonyl group, preferable examples include fluorine-containing monomers, such as perfluoroacrylic acid (fluoride), 1-fluoroacrylic acid (fluoride), acrylic acid fluoride, 1-trifluoromethacrylic acid (fluoride), perfluorobutenic acid, and the like; and monomers that do not include fluorine, such as acrylic acid, methacrylic acid, acrylic acid chloride, vinylene carbonate, itaconic acid, citraconic acid, and the like.
On the other hand, various methods can be adopted for obtaining fluorine-containing ethylenic polymer that has a carbonyl group at the end of the polymer molecule, but the method of using peroxide, in particular peroxycarbonate or peroxy ester, as a polymerization initiator can be preferably adopted for its economy and for quality considerations such as heat resistance and resistance to chemicals. With this method, a carbonyl group originating in a peroxide (for example, a carbonate group that originates in a peroxide carbonate; an ester group that originates in a peroxy ester; or a carboxylic acid halide group or carboxylic acid group that is obtained by modifying these functional groups) can be introduced at the end of a polymer chain. Among these polymerization initiators, it is more preferable if peroxide carbonate is used, because the polymerization temperature can be made low and because the initiation reactions are not accompanied by side reactions.
As the peroxycarbonates, we can suitably list, for example, compounds that are represented by the following formulas (1) to (4):
(wherein R and R^arepresent a straight-chain or branched monovalent saturated hydrocarbon group having 1 to 15 carbon atoms, or a straight-chain or branched monovalent hydrocarbon group having 1 to 15 carbon atoms that contains an alkoxy group on the end, and R^brepresents a straight-chain or branched divalent saturated hydrocarbon group having 1 to 15 carbon atoms, or a straight-chain or branched divalent hydrocarbon group having 1 to 15 atoms that contains an alkoxy group on the end). Particularly preferable are diisopropyl peroxycarbonate, di-n-propyl peroxydicarbonate, t-butyl peroxyisopropyl carbonate, bis(4-t-butyl cyclohexyl)peroxydicarbonate, di-2-ethyl hexylperoxydicarbonate, and the like.
The amount of peroxycarbonate, peroxy ester, or other initiators that is used varies depending on the type (composition, and the like), molecular weight, and polymerization conditions of the desired polymer and the type of initiator that will be used, but normally it is preferable that it be 0.05 to 20 parts by weight, and in particular 0.1 to 10 parts by weight, per 100 parts by weight of the polymer that is obtained by the polymerization.
As the polymerization method, for industrial purposes, suspension polymerization using a fluorine solvent, in an aqueous medium using peroxycarbonate, or the like as the polymerization initiator, is preferable, but other polymerization methods may also be adopted, such as, for example, solution polymerization, emulsion polymerization, bulk polymerization, and the like. In suspension polymerization, a fluorine solvent may be used in addition to water. As the fluorine solvent used in suspension polymerization, one can use, for example, hydrofluorochloroalkanes (for example, CH₃CClF₂, CH₃CCl₂F, CF₃CF₂CCl₂H, CF₂CICF₂CFHCl), chlorofluoroalkanes (for example, CF₂CICFClCF₂CF₃, CF₃CFClCFClCF₃), and perfluoroalkanes (for example, perfluorocyclobutane, CF₃CF₂CF₂CF₃, CF₃CF₂CF₂CF₂CF₃, CF₃CF₂CF₂CF₂CF₂CF₃), and perfluoroalkanes are preferable. There are no particular restrictions on the amount of fluorine solvent that is used, but in the case of suspension polymerization, for suspendability and economy it is preferable to use 10 to 100 wt % with respect to the aqueous medium.
There are no particular restrictions on the polymerization temperature; 0 to 100° C. is acceptable. The polymerization pressure is appropriately set according to the type, amount, and vapor pressure of the solvent that is used, and the polymerization temperature and other polymerization conditions, but 0 to 9.8 MPaG is acceptable.
Also, to adjust the molecular weight, any well known chain transfer agent may be used. As chain transfer agents, one may use, for example, a hydrocarbon such as isopentane, n-pentane, n-hexane, cyclohexane, and the like; an alcohol such as methanol, ethanol, and the like; or a hydrocarbon halide such as carbon tetrachloride, chloroform, methylene chloride, methyl chloride, and the like. Also, the quantity of end carbonate groups or ester groups it contains can be controlled by adjusting the polymerization conditions, and can be controlled by the amount of peroxycarbonate or peroxy ester that is used, the amount of chain transfer that is used, the polymerization temperature, and the like.
Various methods can be adopted to obtain a fluorine-containing ethylenic polymer that has a carboxylic acid halide group or a carboxylic acid group at the end of the polymer molecule; for example, one can obtain it by heating, and causing thermal breakdown (decarboxylation) of a fluorine-containing ethylenic polymer that has at its end the aforementioned carbonate group or ester group. The heating temperature varies depending on the type of carbonate group or ester group and on the type of fluorine-containing ethylenic polymer, but normally it is 270° C. or more, preferably 280° C. or more, and particularly preferably 300° C. or more. Also, it is preferable that the heating temperature be below the thermal breakdown temperature of the parts of the fluorine-containing ethylenic polymer other than the carbonate groups or ester groups; specifically, 400° C. or less is preferable, and 350° C. or less is more preferable.
A white powder of the resulting fluorine-containing ethylenic polymer or cut pieces of its molten extruded pellets were compression-formed at room temperature and made into a uniform film of thickness 0.05 to 0.2 mm. By infrared spectrum absorption analysis of this film, the peak originating in the carbonyl group of the carbonate group (—OC(═O)O—) appeared at an absorption wavelength of 1809 cm⁻¹(v_c-o), and its absorbance at the v_c-opeak was measured. The number (N) of carbonate groups per 1×10⁶main-chain carbon atoms was computed by the following formula (5).
N=500 AW/∈df (5)
A: absorbance at the v_c-opeak of the carbonate group (—OC(═O)O—)
∈s: mole absorbance coefficient (l·cm⁻¹·mol⁻¹) of the v_c-opeak of the carbonate group (—OC(═O)O—). From model compounds, epsilon was set to ∈=170.
W: average molecular weight of the monomer as computed from the monomer composition
d: density of the film (g/cm³)
f: thickness of the film (mm)
Also, in the infrared absorption spectrum analysis, scanning was done 40 times using a Perkin-Elmer FTIR spectrometer 1760×(made by Perkin-Elmer Co.). The resulting IR spectrum was used to automatically determine the baseline using Perkin-Elmer Spectrum for Windows (registered trademark) Ver. 1.4, and the absorbance of the peak at 1809 cm⁻¹was measured. Also, the film thickness was measured with a micrometer.

Measurement Method for the Number of Carboxylic Acid Fluoride Groups

In the same way as with the above measurement method for the number of carbonate groups, by infrared spectrum analysis of the resulting film, the peak originating in the carbonyl group of the carboxylic acid fluoride group (—C(═O)F) appeared at an absorption wavelength of 1880 cm⁻¹(v_c-o), and the absorbance thereof at the v_c-opeak was measured. The number of carboxylic acid fluoride groups was measured in the same way as in the above measurement method for the number of carbonate groups using the above formula (5), except that from model compounds. the mole absorbance coefficient (l·cm⁻¹·mol⁻¹) of the v_c-opeak of the carboxylic acid fluoride group was set to ∈=600.

Measurement Method for the Number of Other Carbonyl Groups

In the same way as with the above measurement method for the number of carbonate groups, infrared spectrum analysis of the resulting film can be used to measure the number of other carbonyl groups that can basically react with amide groups, amino groups, and other functional groups in a polyamide resin such as carboxylic acid groups, ester groups, acid anhydride groups, and the like. Here, except for setting the mole absorbance coefficient (l·cm⁻¹·mol⁻¹) of the v_c-opeak originating in these carbonyl groups to ∈=530, the number of other carbonyl groups was measured in the same way as in the above measurement method for the number of carbonate groups using the above formula (5).

Measurement Method for the Composition of the Fluorine-Containing Ethylenic Polymer

Measurements were made by ¹⁹F-NMR analysis.
It is preferable that the fluorine-containing ethylenic polymer in the present invention be used singly, so as not to detract from the adhesiveness, heat resistance, resistance to chemicals, and other properties that it has itself, but insofar as its performance is not degraded, it may according to purpose and application be blended with various well known fillers such as inorganic powder, glass fiber, carbon fiber, metal compounds, carbon, or the like. Moreover, besides fillers, one may also mix in other additives as desired, such as pigments and ultraviolet ray absorbents. Besides additives, one may also blend in other fluororesins or thermoplastic resins, thermoplastic and other resins, synthetic rubber, and the like, thereby making it possible to improve the mechanical properties, improve the weather resistance, add decorative designs, prevent static electricity, improve the moldability, and the like.
As a result of combining the above polyimide film with the above fluororesins, the coverlay of the present invention has excellent thermal shrinkage and plenty of adhesive strength. The coverlay of the present invention at the least is formed by laminating the fluororesin to the polyimide film in an adhesive state. Various manufacturing methods can be applied to the manufacture of the coverlay, including a manufacturing method of forming, one after another or by extrusion together, the constituent layers that include the polyimide film and the fluororesin, a manufacturing method of thermocompression bonding of a molded body; and a manufacturing method of taking either the polyimide film or the fluororesin as a molded body that is coated with a precursor or molten version of the other resin, and is allowed to flow along to make a resin composition; and a good adhesive state is formed between the constituent layers, which include the polyimide film and the fluororesin. In this manufacturing, one may use any well known molding machine that is normally used for thermoplastic resin, such as an injection molding machine, a compression molding, a flow molding machine, or an extrusion molding machine.
The forming conditions vary with the carbonyl group, especially the type of carbonate group, and with the type of fluorine-containing ethylenic polymer, but with extrusion or flow molding it is appropriate to heat the cylinder to a temperature of 200° C. or more. It is preferable that the heating temperature be set to no greater than a temperature that will suppress foaming or other bad effects caused by the thermal breakdown of the fluorine-containing ethylenic polymer itself; specifically, 400° C. or less is preferable, and 350° C. or less is more preferable.
There are no particular restrictions on the manufacturing method through thermocompression bonding, and examples include the methods of vacuum pressing, lamination (the hot laminate method, and the like), and coating. Also, the fluorine-containing resin layer may include lamination and coating on either one side or both sides of the polyimide film.
With a vacuum press, a coverlay is obtained by, for example, thermocompressing together the polyimide resin and the fluororesin at the prescribed temperature and pressure using a well known vacuum press machine. To make the processing simple, it is preferable when doing this that the press temperature be in the range of 100 to 250° C. Annealing may be done following the pressing, and it is preferable that the annealing temperature be in the range of 100 to 250° C.
With the hot laminate method, on which there are no particular restrictions, a coverlay is obtained by using two heatable rollers whose distance between them can be adjusted as desired, layering film of two or more types between the rollers, and pressing them together while applying heat and pressure. If necessary, heat treatment can be done continuously immediately after the laminating is done. In the heat treatment, it is preferable that the temperature be no less than the glass transition point (Tg) of the fluororesin and no greater than its melting point +50° C., because this allows the pressing-together force to be increased. A temperature below Tg is undesirable because then the desired bonding force cannot be obtained, and a temperature above the melting point+50° C. is undesirable because then the fluororesin begins to break down, lowering the bonding force. There are no particular restrictions on the heating time; it can be set suitably as necessary. There are no particular restrictions on the equipment, as long as it does not hamper the effects of the present invention.
The adhesive strength between the polyimide layer and the fluororesin layer of the coverlay before the copper-clad laminate is attached is preferably more than 3.0 N/cm, more preferably at least 5.0 N/cm, and even more preferably at least 8.0 N/cm; this is for sake of improving the precision in the positioning of the copper-clad laminate and coverlay and raising the operational efficiency. There are no particular restrictions on the upper limit for the adhesive strength.
Although there are no particular restrictions on the thickness of the polyimide layer in the coverlay of the present invention, because this thickness affects the bonding force of the coverlay's polyimide layer and fluororesin layer, it is preferably about 0.01 to 2.0 times the thickness of the fluororesin layer, more preferably about 0.05 to 1.0 times, and even more preferably about 0.1 to 0.9 times. A thickness of the polyimide layer that exceeds a factor of 2.0 is undesirable because, although it improves the rigidity and dimensional stability for the substrate, it increases the dielectric constant. Moreover, if it is less than a factor of 0.01, the rigidity of the polyimide layer will decrease, the coefficient of linear expansion will tend to increase, and the rigidity and dimensional stability as a substrate will decrease.
The thermal shrinkage of the coverlay of the present invention as measured at 260° C. at 30 minutes is normally less than ±0.1%, preferably less than ±0.08%, and more preferably less than ±0.06%.
A high-frequency circuit substrate can be manufactured by affixing the coverlay of the present invention to a copper-clad laminate. There are no particular restrictions on how to make the copper-clad laminate; it may be manufactured by any well known method. Also, the copper-clad laminate may have either a one-sided structure or a two-sided structure.
As manufacturing methods for the copper-clad laminate to be affixed to the coverlay of the present invention, examples include a three-layer CCL in which a base-material film and copper foil are laminated together with an intervening adhesive, a method in which a copper layer is formed by making use of vapor deposition onto the base-material film along with sputtering and electroplating, a so-called cast-type two-layer CCL (COC) in which a polyimide layer is cast and formed on copper foil, and the copper layer is formed using electroless plating on the base-material film.
As the base-material film, examples include polyimide film and LCP film for use with high-frequency circuits. In addition, as the adhesive layer, examples include epoxy, acrylic, or polyimide adhesives, as well as fluororesins, and the like. Commercially available products may be used for the adhesive. As commercially available products, there are no particular restrictions, and examples include the LF Series (of acrylic adhesives) of Pyralux (made by Dupont Co., Ltd.), and the like. Preferable modes among these are copper-clad laminates that make use of LCP film, and copper-clad laminates in which the laminate is made with the fluororesin between the base-material film and the copper foil.
As the polyimide film to be used for the copper-clad laminate, ones similar to the above polyimide film for the coverlay can be cited, and their composition may be either the same as or different from the polyimide film for the coverlay.
There are no particular restrictions on the fluororesin to be used in the copper-clad laminate; any well known fluororesin may be used, including commercially available products. As such commercially available products, examples include Toyoflon F, FE, FL, FR, and FV (brand names; made by Toray Advanced Film Co., Ltd.), and the like.
There are no particular restrictions on the thickness of the polyimide layer in the copper-clad laminate (the layer including the adhesive, if a polyimide adhesive is used), but a thickness 0.01 to 2.0 times the thickness of the fluororesin layer is preferable, about 0.05 to 1.0 times is more preferable, and about 0.1 to 0.9 times is even more preferable. A thickness of the polyimide layer that exceeds 2.0 times the thickness of the fluororesin layer is undesirable because although the rigidity and dimensional stability as a copper-clad laminate will improve, the dielectric constant will increase. Moreover, if it is less than a factor of 0.01, the rigidity of the polyimide layer will decrease, the coefficient of linear expansion will tend to increase, and the rigidity and dimensional stability as a copper-clad laminate will decrease.
A wiring-processed copper-clad laminate is obtained by etching the copper-clad laminate. There are no particular restrictions on how the etching is to be done, and any well known method may be used.
In manufacturing a high-frequency circuit substrate, laminating is done in such a way that the fluororesin side of the coverlay comes into contact with the circuit of the copper-clad laminate, and the coverlay and the copper-clad laminate are held temporarily in place.
There are no particular restrictions on the step by which the copper-clad laminate and the coverlay are held temporarily in place, and any well known method may be used; examples include a method in which the copper-clad laminate and the coverlay are aligned in position, kiss lamination is done, then as necessary, quick-pressing is done to produce lamination at about 150 to 200° C., a method in which the copper-clad laminate and the coverlay are aligned in position, and multi-stage pressing is done, and the like. There are no particular restrictions on the maximum temperature in the step for temporarily holding in place, as long as it is lower than the melting point of the fluororesin that is to be used in the coverlay, but to obtain sufficient adhesive strength by subsequent annealing, it is appropriate that it be in the range of 100 to 250° C. There are no particular restrictions on the treatment time for the step of temporarily holding in place.
There are no particular restrictions on the manufacture of a high-frequency circuit substrate, but following the step of temporarily holding in place, it is preferable to execute a step in which the copper-clad laminate to which the coverlay is attached is annealed.
There are no particular restrictions on the maximum heating temperature in the annealing step, but for good adhesive strength of the resulting high-frequency circuit substrate, it is preferable to set it to a temperature within the range from 150° C. to 350° C. at a temperature that is higher than the maximum temperature in the step for temporarily holding in place, and it is preferable that the temperature difference be at least 20° C. Moreover, in the annealing step, it is preferable that it be done by free tension between 150° C. and 350° C. Free tension has the advantage of simplifying the treatment steps, and with regard to the annealing temperature, from 200° C. to 280° C. is more preferable, and from 205° C. to 275° C. is even more preferable. There are no particular restrictions on the annealing time.
The annealing increases the bonding force based on the adhesive force of the fluororesin layer, and results in a high-frequency circuit substrate that has practical adhesive strength (peel strength). To ensure performance as a coverlay, the adhesive strength after annealing of the resulting high-frequency circuit substrate preferably is a value that exceeds 8 N/cm, and more preferably is at least 10 N/cm. It is even more preferable that it be at least 14 N/cm. The adhesive strength in the present invention is the value that was measured by the method described below in the working examples.
A high-frequency circuit substrate can be manufactured by laminating together the coverlay of the present invention and a copper-clad laminate that has been wiring processed. Making the thickness of the polyimide film and the thickness of the fluororesin so that they have the above-specified ratio not only further improves the electrical and mechanical properties but also ensures excellent dimensional stability, further repressing the occurrence of curling, twisting, warping, and the like even if one carries out etching for circuit formation of the copper layer, and various heating steps in the steps following circuit formation.

WORKING EXAMPLES

Next, the present invention is described in greater specificity, citing working examples, but the present invention is not restricted by the working examples thereof, and many modifications can be made by a person who has the usual knowledge in this field, within the technical concept of the present invention.
The following is a description of the methods for measuring the various properties in the present invention.

(1) Peel Strength

A sample was cut into a strip 10 mm wide, and the peel strength (unit: N/cm) was measured in a 90° C. [sic; 90 degrees] pulling test (pulling speed: 50 mm/min, measurement length: 20 mm, measurement range: 5.0 to 20.0 mm) using the Autograph AG-IS universal tensile testing device made by Shimadzu Ltd.

(2) Thermal Shrinkage

A sample of size 190 mm×200 mm was cut, and its dimensions before heat treatment were measured with the CNC image processing device system NEXIVVM-250 made by Nikon Co., Ltd. Then the sample was put into an oven set to 260° C. and heat treated for 30 minutes. The sample after heat treatment was moisture-adjusted for 12 hours or more at a constant temperature and high humidity. The sample after moisture adjustment was measured for its dimensions in the same way as before the heat treatment, and the rate of change in the dimensions before and after the heat treatment was shown as a percentage.

(3) Transmission Loss

The high-frequency transmission characteristics from 1 to 40 GHz were measured using a prober device for substrate measurement made by Cascade Microtech.

Synthesis Example 1

Pyromellitic acid dianhydride (molecular weight 218.12)/3,3′,4,4′-biphenyl tetracarboxylic acid dianhydride (molecular weight 294.22)/4,4′-diaminodiphenyl ether (molecular weight 200.24)/paraphenylene diamine (molecular weight 108.14) was prepared at mole ratio of 95/5/85/15, and was made into a 20 wt % solution in DMAc (N,N-dimethyl acetoamide) and polymerized, and a 3500-poise polyamic acid solution was obtained.

Synthesis Example 2

Pyromellitic acid dianhydride (molecular weight 218.12)/4,4′-diaminodiphenyl ether (molecular weight 200.24) was prepared at a mole ratio of 100/100, and was made into a 20 wt % solution in DMAc (N,N-dimethyl acetoamide) and polymerized, and a 3500-poise polyamic acid solution was obtained.

Synthesis Example 3

A 380-liter quantity of distilled water was put into an autoclave, and after carrying out nitrogen replacement sufficiently, it was charged with 75 kg 1-fluoro-1,1-dichloroethane, 155 kg hexafluoropropylene, and 0.5 kg perfluoro(1,1,5-trihydro-1-pentene), and the interior of the system was kept at 35° C. at a stirring speed of 200 rpm. Next, tetrafluoroethylene was pressurized to 0.7 MPa, and subsequently ethylene was pressured to 1.0 MPa, then 2.4 kg di-n-propyl peroxydicarbonate was inserted, and polymerization was initiated. Because the pressure within the system decreases as the polymerization proceeds, the pressure within the system was kept to 1.0 Mpa by continuously supplying a mixed gas made of tetrafluoroethylene/ethylene/hexafluoropropylene=40.5/44.5/15.0 mol %. Then, for the perfluoro(1,1,5-trihydro-1-pentene) as well, a total quantity of 1.5 kg was charged, and stirring was continued for 20 hours. Then, after the pressure was released and the system returned to atmospheric pressure, the reaction products were washed with water and dried, producing 200 kg of powder (fluorine-containing ethylenic polymer F-A). The analysis results thereof are presented in Table 1.

Synthesis Example 4

In the same way as in Synthesis Example 3, fluorine-containing ethylenic polymer F—B was obtained in the blends shown in Table 1. The analysis results thereof are presented in Table 1.

Synthesis Example 5

A 9.5 kg quantity of powder of the fluorine-containing ethylenic polymer F—B obtained in Synthesis Example 4, 700 g of 28% aqueous ammonia, and 10 liter of distilled water were charged into an autoclave, the system was heated while stirring, and kept at 80° C., and the stirring was continued for 7 hours. Then the content was water-washed and dried, yielding 9.2 kg of powder (fluorine-containing ethylenic polymer F—C). By carrying out such treatment, the active functional groups contained in the resin (carbonate groups and carboxylic acid fluoride groups) were transformed into amide groups that are stable both chemically and thermally. Also, it was confirmed by infrared spectrum analysis that this transformation proceeded quantitatively. The analysis results of the resin after treatment are presented in Table 2. Also, no carbonyl groups except carbonate groups and carboxylic acid fluoride groups were found in the fluorine-containing ethylenic polymer (F-A) shown in Synthesis Example 3. In Table 1, TFE represents tetrafluoroethylene, Et represents ethylene, HFP represents hexafluoropropylene, and HF-Pa represent perfluoro(1,1,5-trihydro-1-pentene).

	TABLE 1

			Quantity (number)
			per 10⁶carbon
	Fluoro-		atoms in main
	resin		chain

	(fluorine-	Monomer composition		carboxylic		MFR
	containing	(mol %)		acid	Melting	(g/10 min)

ethylenic				HF-	carbonate	fluoride	point	(measured
polymer	TFE	Et	HFP	Pa	groups	groups	(° C.)	temperature)

SE 3	F-A	40.8	44.8	13.9	0.5	300	3	162.5	2.6
									(230° C.)
SE 4	F-B	46.2	43.8	9.5	0.5	255	5	194.3	8.9
									(230° C.)
SE 5	F-C	46.1	43.8	9.5	0.5	not detected	not detected	193.5	9.8
									(230° C.)

SE = Synthesis example

Working Example 1

(1) Preparation of the Polyimide Film

Acetic anhydride (molecular weight 102.09) and β-picoline were mixed in the polyamic acid solution obtained in Synthesis Example 1 at respective ratios of 17 wt % and 17 wt % with respect to the polyamic acid solution, and stirred. The resulting mixture was cast by a T-shaped slit die onto a rotating stainless steel drum at 75° C., and after the mixture was allowed to flow and extend for 30 seconds, the resulting gel film was stretched 1.2-fold in the running direction while being heated for 5 minutes at 100° C. Next, both ends in the width direction were gripped, and the film was stretched 1.3-fold in the width direction while heating for 2 minutes at 270° C., after which it was heated for 5 minutes at 380° C., producing a polyimide film with a thickness of 12.5 μm.

(2) Preparation of the Fluororesin Film

The fluorine-containing ethylenic polymer polymerized in Synthesis Example 3 was pelletized using a 65-mm-diameter short-axis extruding machine with a temperature setting of 230° C. to 280° C., then it was made into a film using a 50-mm-diameter short-axis extruding machine equipped with a T-die and having a temperature setting of 230° C. to 280° C., thereby producing a fluororesin film with a thickness of 25 μm.

(3) Preparation of the Coverlay

Using the polyimide film obtained in (1) above and the fluororesin film obtained in (2) above, a coverlay was made by the vacuum press method. Specifically, the polyimide film and the fluororesin film were laid atop each other and then pressed for 90 seconds at 120° C. and 30 kN with a vacuum press machine, after which an electric furnace set to 180° C. was used to heat the material for 20 minutes with free tension, thereby producing a coverlay film. The above-described properties were then measured for the resulting coverlay, and the results are shown in Table 2.

Working Example 2

A coverlay was made in the same way as in Working Example 1, with the exception that instead of the polyimide film obtained in (1) of Working Example 1, a polyimide film made in the same way as in (1) of Working Example 1 using the polyamic acid solution obtained in Synthesis Example 2 was used, and instead of the fluorine-containing ethylenic polymer (F-A) obtained in Synthesis Example 3, the fluorine-containing ethylenic polymer (F—B) obtained in Synthesis Example 4 was used. Each of the above-described properties was measured, and the results are presented in Table 2.

Comparison Example 1

A coverlay was made using the same method as the preparation method of (3) in Working Example 1 and using the polyimide film obtained in (1) of Working Example 1 and the fluorine-containing ethylenic polymer (F—C) obtained in Synthesis Example 5.
TABLE 2

Working Working Comparison

Item Unit Example 1 Example 2 Example 1

Peel strength [N/cm] 3.1 3.6 0.1 or less

Thermal [%] 0.05 0.09 0.18

shrinkage

(In the table, each coverlay peel strength denotes the adhesive strength between the polyimide film layer and the fluororesin layer.)
In Comparison Example 1, it took time to align the positions of the copper-clad laminate and the coverlay, which is not suitable for industrial implementation. Moreover, it was learned that, as shown in Table 2, in Comparison Example 1, sufficient bonding force is not obtained, which makes it unsuitable industrially. On the other hand, with the coverlay of the present invention, the workability was excellent, and the thermal shrinkage was less than ±0.1%.

Preparation 1

A two-sided CCL was fabricated by using an epoxy adhesive to attach an 18 μm copper foil to the polyimide film obtained in (1) of Working Example 1.

Working Example 3

A coverlay was fabricated by the method shown in (3) of Working Example 1, using polyimide film (thickness: 12.5 μm) obtained in the same way as in (1) of Working Example 1 and the fluorine-containing ethylenic polymer (F—B) obtained in Synthesis Example 4.

Comparison Example 2

A coverlay was fabricated by using a bar coater to coat one side of a polyimide film (thickness: 12.5 μm) obtained in the same way as in (1) of Working Example 1 with epoxy adhesive to a thickness of 25 μm, heat-drying it for 5 minutes at 150 degrees [sic; degrees Celsius], performing B staging, and then bonding the separate film to the resin composition surface with a laminator.

Test Example

Transmission Properties

Using the CCL fabricated in Working Example 1, etching was done to produce the desired wiring, and using the etched CCL and the coverlay of Working Example 3 or the coverlay of Comparison Example 2, a circuit was fabricated, and the transmission properties thereof were measured. The measurement results are shown in FIG. 1.
As described above, when the coverlay of the present invention was used, a high-frequency circuit substrate was obtained that exhibits superior workability when manufacturing high-frequency circuit substrates, and that has excellent mechanical properties and heat resistance. Moreover, as shown in FIG. 1, the coverlay of the present invention also has better properties than a conventional coverlay, including transmission properties.

INDUSTRIAL POTENTIAL

The coverlay of the present invention requires no high-temperature pressing beforehand, and by low-temperature pressing and subsequent heating with free tension, one can obtain a FPC [flexible printed circuit] that has excellent high-frequency properties, dimensional stability, and wiring precision. Also, because it has a low dielectric constant, the high-frequency circuit substrate of the present invention can keep transmission loss in check.

Claims

What is claimed is:

1. A coverlay for a high-frequency circuit substrate, the coverlay comprising a polyimide film and a fluororesin bonded together, and an adhesive strength between the polyimide film layer and the fluororesin layer being greater than 3.0 N/cm.

2. The coverlay as described in claim 1, wherein a thermal shrinkage thereof at 260° C. for 30 minutes is less than ±0.1%.

3. The coverlay as described in claim 1 or 2, wherein the fluororesin has a melting point of 200° C. or less.

4. The coverlay as described in any one of claims 1 through 3, wherein the fluororesin is a fluorine-containing ethylenic polymer, and the fluorine-containing ethylenic polymer contains a carbonyl group.

5. The coverlay as described in claim 4, wherein a quantity of carbonyl groups contained in the fluorine-containing ethylenic polymer totals 3 to 1000 groups per 1×10⁶main-chain carbon atoms.

6. The coverlay as described in any one of claims 1 through 3, wherein the fluororesin is made up of fluorine-containing ethylenic polymer that has at least one type selected from a group made up of carbonate groups, carboxylic acid halide groups, and carboxylic acid groups totaling 3 to 1000 groups per 1×10⁶main-chain carbon atoms.

7. The coverlay as described in any one of claims 1 through 3, wherein the fluororesin is one or more types of fluorine-containing ethylenic monomer selected from a group made up of tetrafluoroethylene, vinylidene fluoride, chlorotrifluoroethylene, vinyl fluoride, hexafluoropropylene, hexafluoroisobutene, monomers represented by the following formula (X):

CH₂═CR¹(CF₂)_nR² (X)

(wherein R¹represents H or F, R²represents H, F, or Cl, and n is a positive integer in a range 1 to 10), and perfluoro(alkyl vinyl ethers) having 2 to 10 carbon atoms, or a fluorine-containing ethylenic polymer made by polymerizing the fluorine-containing ethylenic monomer and an ethylenic monomer having 5 or fewer carbon atoms.

8. The coverlay as described in any one of claims 1 through 3, wherein the fluororesin is a copolymer made by polymerizing at least the following (a), (b), and (c);

(a) 20 to 90 mol % of tetrafluoroethylene,

(b) 10 to 80 mol % of ethylene, and

(c) 1 to 70 mol % of a compound represented by the formula:

CF₂═CFR³ (Y)

(wherein R³represents CF₃or OR⁴, and R⁴represents a perfluoroalkyl group having 1 to 5 carbon atoms).

9. The coverlay as described in any one of claims 1 through 8, wherein the polyimide film is made up mainly of one or more aromatic diamine components selected from a group made up of paraphenylene diamine, 3,4′-diaminodiphenyl ether, and 4,4′-diaminodiphenyl ether, and one or more acid anhydride components selected from a group made up of pyromellitic acid dianhydride and 3,3′,4,4′-biphenyl tetracarboxylic acid dianhydride.