JP2010129964A - Sheet for flexible printed wiring board reinforcement, and flexible printed wiring board using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a sheet for flexible printed wiring board reinforcement which has a thermal expansion coefficient close to the coefficient of a thermosetting polyimide film used as a substrate film for a flexible printed wiring board and metal foil such as copper foil laminated on the thermosetting polyimide film, is sufficiently reduced in thickness variation, superior in reflow heat resistance, superior in blister resistance, and can be manufactured at a low cost, and to provide a flexible printed wiring board using the same. <P>SOLUTION: A sheet 2 for reinforcement for reinforcing a flexible printed wiring board 1 is used for flexible printed wiring board reinforcement which is obtained by extrusion-molding and biaxial stretching a composition containing a crystalline thermoplastic resin in a softening start temperature Tg of 120°C or higher and a fibrous inorganic filler, and has a thermal expansion rate of 5 to 30 ppm/K in either of MD and TD directions. The sheet for reinforcement is stuck on a predetermined place of the flexible printed wiring board to form a reinforcing layer of the flexible printed wiring board. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a flexible printed wiring board reinforcing sheet or film and a flexible printed wiring board using the same. Hereinafter, the term “reinforcing sheet” is used as a generic term for “reinforcing sheet” and “reinforcing film” in the present specification.

  With the recent increase in the density of electronic devices, the flexible printed wiring boards used therein are becoming smaller and thinner. In addition, with the increase in the number of functions of electronic devices, the number of components used inside the devices has increased, and the demand for flexible printed wiring boards that can be mounted by connecting these components has increased. In this case, a thin and flexible flexible printed wiring board is often provided with a reinforcing layer in order to maintain the connection strength of the connection portion when connecting and mounting with components. In addition, a backing layer is generally provided as a reinforcing layer in a tape carrier of a tape automated bonding (TAB) product which can be said to be a kind of flexible printed wiring board.

  The reinforcing layer is generally formed by adhering a reinforcing sheet with an adhesive at a predetermined position of a flexible printed wiring board. The reinforcing sheet is also referred to as a stiffener or a reinforcing plate, which gives the flexible printed wiring board the flatness of the part where components such as connectors are mounted and the rigidity of the connector insertion part, and the thickness can be adjusted. Used to do. In general, a thin film of about 10 to 50 μm is used as the base film of the flexible printed wiring board, whereas a thick film of about 100 μm or more, usually 125 μm or more is used as the reinforcing sheet. Further, the reinforcing sheet is required to have a small thickness variation.

  As a material for the reinforcing layer of the flexible printed wiring board and the backing layer of the TAB tape carrier, a thermosetting polyimide resin film (see Patent Documents 1 to 4) and other heat resistant resins are generally used. As described in Patent Documents 2 and 3, a thermosetting polyimide resin film is generally a polyimide resin obtained by casting and applying a precursor polyamic acid, followed by heating and imidization reaction (dehydration condensation reaction). It is produced by making it into a film. However, such a casting method has a manufacturing process complicated and inferior in productivity, and impurities are likely to be mixed therein. In addition, a monomer residue and a residual solvent are present, which causes a decrease in electrical characteristics. . Further, due to restrictions on the sheet manufacturing method, there has been a problem that the production cost becomes high particularly when the thickness is 175 μm or more.

  It is also known to add an inorganic filler to a resin composition for the purpose of mechanical properties such as dimensional stability and cost reduction. For example, it is composed of at least two kinds of resins such as a polyether aromatic ketone resin and a thermoplastic resin having a glass transition temperature of 100 ° C. or higher such as a polysulfone resin, and 5 parts by weight of a plate-like filler with respect to 100 parts by weight of the resin. A film or sheet comprising a resin composition containing 50 parts by weight (see Patent Document 5), an amorphous thermoplastic resin having a glass transition temperature of 230 ° C. or higher, such as a polysulfone resin, and a polyether aromatic ketone resin A sheet containing 100 parts by weight of a thermoplastic resin containing a crystalline thermoplastic resin having a glass transition temperature of 130 ° C. or higher, and 5 to 50 parts by weight of a plate-like filler having an average particle diameter of 1 to 20 μm ( It has also been proposed to use Patent Document 6) as a reinforcing sheet.

By the way, since the reinforcing sheet is used by being bonded to a flexible printed wiring board, the thermal expansion coefficient of a metal foil such as a thermosetting polyimide film used as a substrate film of the flexible printed wiring board or a copper foil laminated thereon ( It is desired to have a coefficient of thermal expansion close to (CTE). Moreover, the reflow heat resistance of 250 degreeC or more is requested | required. For example, in the solder reflow process, the reflow temperature is generally 250 ° C. or higher in many cases. However, in the case of the reinforcing sheet as described above, it has been difficult to control the thermal expansion coefficient to a value close to the thermal expansion coefficient of the thermosetting polyimide film used as the substrate film or the copper foil laminated thereon. . Further, since the glass transition point is 250 ° C. or lower, there is a problem that the reflow heat resistance is inferior and the flexible printed wiring board is likely to be warped or twisted in the solder reflow process. Since a film using a plate-like filler has a high gas barrier property, when heated, moisture adsorbed on the film stays inside, and there is a problem that blisters are easily generated. Blister is a so-called popcorn phenomenon that occurs due to the foaming of water vapor, and more specifically, is a phenomenon in which the water remaining inside the molded product is suddenly gasified and expanded by heating to cause blistering on the surface of the molded product. )
JP 2000-260823 A (Claims, Examples) JP-A-2004-43831 (Claims) JP 2004-149591 A (Claims) JP 2002-231769 A (Claims) JP 2004-266105 A (Claims) JP-A-2005-243757 (Claims)

  The present invention has a thermal expansion coefficient close to that of a metal foil such as a thermosetting polyimide film used as a substrate film of a flexible printed wiring board or a copper foil laminated thereon, and thickness variation is sufficiently reduced. An object of the present invention is to provide a flexible printed wiring board reinforcing sheet that is excellent in reflow heat resistance and blister resistance and can be produced at low cost with high productivity, and a flexible printed wiring board using the same.

  The present invention relates to a reinforcing sheet for reinforcing a flexible printed wiring board, in which a composition containing a crystalline thermoplastic resin having a softening start temperature Tg of 120 ° C. or higher and a fibrous inorganic filler is extruded and biaxial. Flexible printed wiring obtained by stretching and having a coefficient of thermal expansion in the MD direction (sheet extrusion direction) and TD direction (direction orthogonal to the sheet extrusion direction) in the range of 5 to 30 ppm / K The present invention relates to a sheet reinforcing sheet.

  The present invention also relates to a flexible printed wiring board, wherein the reinforcing sheet is bonded to a predetermined portion of the flexible printed wiring board to form a reinforcing layer.

  The flexible printed wiring board reinforcing sheet of the present invention is a sheet obtained by extruding a composition containing a predetermined crystalline thermoplastic resin and a fibrous inorganic filler, and further biaxially stretched. There is almost no difference or a sufficiently small difference in coefficient of thermal expansion between a thermosetting polyimide film generally used as a substrate film of a flexible printed wiring board and a metal foil such as a copper foil laminated thereon. The reinforcing sheet of the present invention is also excellent in reflow heat resistance, blister resistance and dimensional stability as a result of improving the softening start temperature Tg by biaxial stretching, and can be produced with low cost and high productivity. In addition, when a thermoplastic polyimide resin is used as the crystalline thermoplastic resin, excellent heat resistance, electrical characteristics, mechanical strength, and the like inherent to polyimide can be obtained.

  Specifically, since the reinforcing sheet of the present invention is produced by an extrusion method using a crystalline thermoplastic resin as a raw material, a sheet having an arbitrary thickness can be produced, and a sheet having a thickness necessary for the reinforcing plate can be reduced. It can be supplied stably at a low cost. Further, since the fibrous inorganic filler is contained, the gas barrier property is not changed even by heating, and the generation of blisters can be effectively suppressed.

Furthermore, the following effects are obtained by the synergistic effect of the addition of the fibrous inorganic filler to the crystalline thermoplastic resin and the biaxial stretching.
(1) The coefficient of thermal expansion (CTE) is sufficiently reduced. As a result, the reflow heat resistance is improved.
(2) The draw ratio can be reduced. As a result, the sheet thickness before stretching is reduced and the temperature controllability is improved, so that the thickness change in the stretching process is also reduced. Moreover, equipment cost can be suppressed.
(3) Since the crystalline thermoplastic polyimide resin is expensive, the cost of manufacturing the reinforcing sheet can be reduced by mixing an inexpensive fibrous inorganic filler.

[Usage]
The flexible printed wiring board reinforcing sheet of the present invention (hereinafter simply referred to as “reinforcing sheet”) is also referred to as a stiffener or a reinforcing plate, and reinforces the wiring board by bonding to the flexible printed wiring board. . In particular, a flexible wiring board in which a metal foil such as a copper foil is laminated on the wiring portion is effective. In the reinforcing sheet of the present invention, the thermal expansion coefficient in both the MD direction and the TD direction is almost the same as the thermal expansion coefficient of a metal foil such as a thermosetting polyimide film or copper foil generally used as a substrate film of a wiring board. It can be effectively reduced. Even if metal foil is laminated | stacked on a part of bonding area | region of the reinforcing sheet in a wiring board, it may be laminated | stacked on the whole surface.

  Specifically, for example, as shown in FIG. 1, the reinforcing sheet 1 is bonded to a predetermined portion of the flexible printed wiring board 1 to form a reinforcing layer. In FIG. 1, the reinforcing sheet is bonded to a part of the area on one side of the wiring board, but is not limited to this, for example, even if the reinforcing sheet is bonded to the entire surface of one side of the wiring board, It may be bonded to at least some areas on both sides of the plate.

  By sticking the reinforcing sheet to the flexible printed wiring board, it is possible to impart flatness and rigidity to a portion of the wiring board on which a component such as a connector is mounted. Also, the overall thickness of the wiring board can be adjusted.

  The reinforcing sheet may be bonded to the flexible printed wiring board using an adhesive such as an acrylic resin, an epoxy resin, or a phenol resin, or a hot-melt adhesive, or the softening start temperature Tg of the reinforcing sheet is higher than the melting point. It can also be carried out by a laminating method in which heat and pressure are applied at the following temperature, preferably 300 to 380 ° C. As a laminating method, in addition to a continuous laminating method such as roll laminating, a method of laminating reinforcing sheets one by one on a sheet-like flexible printed wiring board and pressing one by one or a plurality of sheets simultaneously by a press can be employed.

  Prior to bonding the reinforcing sheet to the flexible printed wiring board, it is possible to further increase the adhesive strength by performing a modification treatment on the surface of the reinforcing sheet. As surface modification treatment, general surface treatments such as corona discharge treatment, plasma treatment, ozone treatment, excimer laser treatment, and alkali treatment are possible. From the viewpoint of cost and treatment effect, corona discharge treatment, plasma Treatment is preferred.

[Reinforcing sheet]
The reinforcing sheet of the present invention is obtained by extrusion molding and biaxial stretching of a composition containing a crystalline thermoplastic resin and a fibrous inorganic filler.

  The coefficient of thermal expansion (CTE) can be sufficiently reduced by the synergistic effect of the addition of the fibrous inorganic filler to the crystalline thermoplastic resin and biaxial stretching. Specifically, by further biaxially stretching a sheet obtained by extruding a composition containing a crystalline thermoplastic resin and a fibrous inorganic filler, the crystalline thermoplastic resin is isotropic in the plane direction of the sheet. The thermal expansion coefficient is isotropically reduced effectively, and the softening start temperature Tg is improved. As a result, the reflow heat resistance is improved. In particular, by adjusting the addition amount of the fibrous inorganic filler, the draw ratio, and the composition of the composition, the thermal expansion coefficient in the MD direction and the TD direction is equal to that of the copper foil or thermosetting polyimide resin film substrate. The expansion rate can be reduced.

  Specifically, by biaxially stretching an unstretched sheet obtained by extruding a composition containing a crystalline thermoplastic resin and a fibrous inorganic filler, both the thermal expansion coefficients in the MD direction and the TD direction can be obtained. Reinforcing sheets in the range of 5 to 30 ppm / K, preferably 10 to 25 ppm / K can be obtained. Such a reinforcing sheet preferably has a difference (absolute value) in thermal expansion coefficient between the MD direction and the TD direction of 10 ppm / K or less, particularly 5 ppm / K or less, from the viewpoint of further improving the reflow heat resistance. If at least one of the thermal expansion coefficient in the MD direction or the thermal expansion coefficient in the TD direction is too large or the difference between them is too large, warping occurs due to the difference in expansion coefficient during heating in the reflow process. Unbearable for use as.

The thermal expansion coefficient in the MD direction and the thermal expansion coefficient in the TD direction can be controlled by adjusting the addition amount of the fibrous inorganic filler, the draw ratio, and the composition of the composition.
For example, when the addition amount of the fibrous inorganic filler is increased within the range described later, the thermal expansion coefficient in the MD direction is reduced. When the addition amount of the fibrous inorganic filler is reduced within the range described later, the thermal expansion coefficient in the MD direction is increased.
Moreover, for example, the greater the draw ratio in the MD direction is within a range described later, the greater the width in which the coefficient of thermal expansion in that direction is reduced. The smaller the draw ratio in the MD direction is within the range described later, the smaller the width in which the coefficient of thermal expansion in that direction is reduced.
On the other hand, the expansion coefficient in the TD direction is an extremely anisotropic film with almost no reduction effect of the expansion coefficient even when the fibrous inorganic filler is blended, and the thermal expansion coefficient is reduced only in the MD direction. . Therefore, by setting the stretching ratio in the TD direction high and setting the stretching ratio in the MD direction low, a film having the same difference in thermal expansion coefficient between the MD direction and the TD direction can be finally obtained.
In addition, for example, even if a part of the crystalline resin is replaced with an amorphous resin, the effect of reducing the thermal expansion coefficient by biaxial stretching is equally exhibited.

In this specification, the thermal expansion coefficient is the thermal expansion coefficient α _20-200 when the temperature is raised from 20 ° C. to 200 ° C., and in the measurement method of thermal expansion shown below, the length in a predetermined direction at 20 ° C. is can be calculated on the basis of the beta ₂₀ and a predetermined direction when the 200 ° C. length beta ₂₀₀ Metropolitan the following equation.
α ₂₀₋₂₀₀ = (β ₂₀₀ −β ₂₀ ) / β ₂₀

  The thermal expansion can be measured according to a method described in “5.17.1 TMA method” of JIS C 6481: 1996 by thermomechanical analysis (TMA). Specifically, for example, using a thermomechanical measuring device TMA-60 manufactured by Shimadzu Corporation, a test piece 2 × 23 mm according to the method described in “5.17.1 TMA method” of JIS C 6481: 1996, Under a tensile load of 5 gf, a change in thermal expansion is measured under the condition of a heating rate of 5 ° C./min.

  In the present invention, since there is an effect of reducing the coefficient of thermal expansion due to the blending of the fibrous inorganic filler, the draw ratio can be reduced, so that the thickness of the unstretched sheet before stretching can be reduced. As a result, the thickness variation after stretching is reduced, and the change in thickness due to stretching is also reduced, thereby reducing the difficulty of the stretching process. That is, for example, the crystalline thermoplastic polyimide resin sheet itself has a high stretching temperature, and as the thickness increases, it takes time to increase to an arbitrary heating temperature. And since the thickness of an unstretched sheet can be made thin, the difficulty of a extending process becomes low.

  Furthermore, since both the biaxial stretching and the inorganic filler mixing have an effect of improving the elastic modulus, the rigidity of the reinforcing sheet is increased. In addition, since the fibrous inorganic filler is contained, there is no change in gas barrier properties even by heating, and the generation of blisters can be effectively suppressed.

  In the present invention, it is possible to increase the softening start temperature Tg by biaxial stretching. For example, the softening start temperature Tg of the unstretched thermoplastic resin sheet whose softening start temperature Tg was 258 ° C. is biaxial. By stretching, the temperature rises to 305 ° C or higher. The same applies to a sheet obtained by extruding a composition containing a thermoplastic resin and a fibrous inorganic filler.

  Specifically, the reinforcing sheet of the present invention has a softening start temperature Tg that is 10 to 200 ° C. higher than the softening start temperature Tg of the sheet before stretching. Preferably, the softening start temperature Tg of the reinforcing sheet is 20 to 100 ° C. higher than the softening start temperature Tg of the sheet before stretching. Since the reflow heat resistance temperature of the flexible printed wiring board needs to be 250 ° C. or higher, it is desirable (necessary) that the softening start temperature Tg of the reinforcing sheet is 260 ° C. or higher. When such a reinforcing sheet is used for a flexible printed wiring board, reflow heat resistance, particularly solder heat resistance during solder reflow is improved.

  In this specification, the softening start temperature Tg is defined as the softening start temperature Tg at a temperature at which the thermal expansion rapidly increases in the same method as the method for measuring the coefficient of thermal expansion. For example, FIG. 2 is a schematic diagram showing TMA curves of an unstretched thermoplastic polyimide resin film and a stretched thermoplastic polyimide resin film. As is apparent from FIG. 2, the softening start temperature Tg is improved by biaxially stretching the thermoplastic polyimide resin film. The softening start temperature Tg is an intersection of a tangent line with a slowly rising thermal expansion and a tangent line with a rapidly rising line. The softening start temperature Tg is not limited to this, and may be a value measured using another similar device under the same conditions.

  The thickness of the reinforcing sheet is not particularly limited, but is usually 100 to 1000 μm, preferably 125 μm to 800 μm.

  In addition, by fixing (thermally fixing) the polymer crystal by heating with limited shrinkage after biaxial stretching, the original thermal expansion is achieved even in the temperature range exceeding the softening start temperature Tg of the crystalline thermoplastic resin used. The reduced thermal expansion coefficient can be maintained in a temperature range not lower than the softening start temperature Tg and not higher than the melting point. Further, the residual stress in the sheet generated during extrusion molding is also removed, and the sheet is excellent in dimensional stability without causing dimensional change even after being heated and cooled to a temperature capable of bonding. This makes it possible to form a reinforcing layer having excellent dimensional accuracy and dimensional stability without causing warpage or the like when bonded to a metal foil or a conductor circuit.

  If biaxial stretching is not performed, the coefficient of thermal expansion is not sufficiently reduced, and the anisotropy of the coefficient of thermal expansion increases, resulting in a decrease in reflow heat resistance. If a fibrous inorganic filler is not included, a thick sheet exceeding 1 mm needs to be stretched at a high magnification in order to obtain a predetermined coefficient of thermal expansion. Therefore, uniform heating becomes difficult in the stretching process, resulting in variations in thickness. appear. Furthermore, if the draw ratio is lowered, the coefficient of thermal expansion is not sufficiently reduced. When a spherical inorganic filler is used instead of the fibrous inorganic filler, since the reinforcing effect is small, the coefficient of thermal expansion is not sufficiently reduced, and in order to obtain a predetermined coefficient of thermal expansion, a thick sheet exceeding 1 mm is increased. Since it is necessary to stretch at a magnification, uniform heating becomes difficult and thickness variation occurs. When a plate-like inorganic filler is used instead of the fibrous inorganic filler, the gas barrier property is increased. Therefore, during reflow heating, moisture adsorbed on the sheet stays inside and blisters are generated.

[Method of manufacturing reinforcing sheet]
The manufacturing method of the reinforcing sheet will be described in detail.
The reinforcing sheet of the present invention is produced by a method including the following steps;
Preparing a composition containing a crystalline thermoplastic resin and a fibrous inorganic filler (compounding step);
A step of extruding the prepared composition to obtain an unstretched sheet (extrusion molding step); and a step of biaxially stretching the unstretched sheet (biaxial stretching step).

(Composition preparation process)
In this step, a crystalline thermoplastic resin and a fibrous inorganic filler are added and mixed to prepare a composition. For example, pellets or powder of a crystalline thermoplastic resin, a fibrous inorganic filler, and optionally other resins and additives are dry-mixed in advance and then melted, kneaded and extruded by a twin-screw kneading extruder. The extruded strand is cooled in water and cut to obtain composition pellets. In particular, when a thermoplastic polyimide resin is used as the crystalline thermoplastic resin, a reinforcing sheet that sufficiently exhibits the electrical properties and mechanical strength inherent to the polyimide resin material itself is obtained by subjecting the composition to a compounding process. It is done.

  The method for adding and mixing and kneading the resin component and the fibrous inorganic filler in the present invention is not particularly limited, and various mixing and kneading means are used. For example, when adding a filler to a thermoplastic resin, each may be separately fed to a melt extruder and mixed, or only the powder raw material may be preliminarily utilized using a mixer such as a Henschel mixer, a ball mixer, a blender, or a tumbler. It can be dry pre-kneaded and melt mixed in a melt extruder.

The crystalline thermoplastic resin used in the present invention is a polymer having thermoplasticity and crystallinity.
In this specification, having thermoplasticity means having thermoreversibility between curing and softening and capable of producing a sheet by extrusion.

  Having crystallinity means that a crystalline region can be formed when a molten thermoplastic resin is cooled and solidified. A crystalline thermoplastic resin has two transition points, a glass transition point and a melting point, while an amorphous thermoplastic resin does not have a melting point and has only a glass transition point.

  By using a thermoplastic resin having such crystallinity, it is possible to improve characteristics by biaxial stretching, for example, increase in softening start temperature Tg and decrease in thermal expansion coefficient. If an amorphous thermoplastic resin is used instead of the crystalline thermoplastic resin, these effects are not exhibited.

  A crystalline thermoplastic resin having a softening start temperature Tg of 120 ° C. or higher is used, and the softening start temperature Tg is 200 ° C. or higher, particularly 220 ° C. or higher, from the viewpoint of easily reaching the required reflow heat resistance temperature. Is more preferable. When the softening start temperature Tg of the crystalline thermoplastic resin is too low, even if the softening start temperature Tg is improved, the required reflow heat resistance temperature is not reached, and the performance required as a reinforcing plate for printed wiring boards is not achieved.

As crystalline thermoplastic resin, polyimide, polyethylene, polypropylene, polyamide, aromatic polyamide, polyacetal, polyethylene terephthalate, polybutylene terephthalate, polyphenylene sulfide, polyether aromatic ketone, polytetrafluoroethylene, perfluorovinylether-tetrafluoroethylene Polymers, hexafluoroethylene-tetrafluoroethylene copolymers, polymethylpentene, syndiotactic polystyrene, polyethylene naphthalate, polycyclohexane terephthalate, and ultrahigh molecular weight polyethylene can be used. The crystalline thermoplastic resin of the present invention includes a liquid crystal polymer that exhibits the same behavior as the crystalline thermoplastic resin.
Crystalline thermoplastic resins may be used alone or in admixture of two or more. From the viewpoint of having electrical insulation and heat resistance and strength suitable for use as a flexible printed wiring board reinforcing sheet, it is preferable to use a crystalline thermoplastic polyimide resin, a crystalline thermoplastic polyether aromatic ketone resin, A crystalline thermoplastic polyimide resin is particularly preferable.

  Crystalline thermoplastic polyether aromatic ketone resins are commercially available, and include PEEK (registered trademark, Tg: 146 ° C.) manufactured by Victrex.

  The crystalline thermoplastic aromatic polyamide resin is available as a commercial product, and examples thereof include ARLEN (registered trademark, Tg: 125 ° C.) manufactured by Mitsui Chemicals.

Examples of the crystalline thermoplastic polyimide resin include those having a repeating structural unit represented by the following general formula (1).

In the general formula (1), X is a direct bond, —SO ₂ —, —CO—, —C (CH ₃ ) ₂ —, —C (CF ₃ ) ₂ — or —S—, and R ¹ , R ² , R ³ and R ⁴ each independently represent a hydrogen atom, an alkyl group having 1 to 6 carbon atoms, an alkoxy group, a halogenated alkyl group, a halogenated alkoxy group, or a halogen atom, and Y represents the following formula (2) A group selected from the group consisting of:

  The thermoplastic polyimide resin having the repeating structural unit represented by the above general formula (1) is an organic material using an ether diamine of the following general formula (3) and a tetracarboxylic dianhydride of the following general formula (4) as raw materials. It can be produced by reacting in the presence or absence of a solvent and imidating the resulting polyamic acid chemically or thermally. These specific manufacturing methods can utilize the conditions of a known polyimide manufacturing method.

上記一般式（３）において、Ｒ^１、Ｒ^２、Ｒ^３及びＲ^４はそれぞれ前記式（１）における記号と同じ意味を示す。

In the general formula (3), R ¹ , R ² , R ³ and R ⁴ each have the same meaning as the symbol in the formula (1).

上記一般式（４）において、Ｙは前記一般式（１）における記号と同じ意味を示す。

In the said General formula (4), Y shows the same meaning as the symbol in the said General formula (1).

Specific examples of R ¹ , R ² , R ³ , and R ^{4 in} the general formula (1) and general formula (3) include alkyl groups such as hydrogen atom, methyl group, and ethyl group, methoxy group, ethoxy group, and the like. And a halogenated alkyl group such as a fluoromethyl group and a trifluoromethyl group, a halogenated alkoxy group such as a fluoromethoxy group, and a halogen atom such as a chlorine atom and a fluorine atom. Preferably, it is a hydrogen atom. X in the formula is a direct bond, —SO ₂ —, —CO—, —C (CH ₃ ) ₂ —, —C (CF ₃ ) ₂ — or —S—, preferably a direct bond, — SO ₂ —, —CO—, —C (CH ₃ ) ₂ —.

  In the general formulas (1) and (4), Y is represented by the above formula (2), and preferably pyromellitic dianhydride is used as the acid dianhydride.

尚、上記式（５）で表される繰り返し構造単位を有する熱可塑性ポリイミド樹脂は、三井化学株式会社製の「オーラム」（登録商標）として購入可能である。 More preferable as the crystalline thermoplastic polyimide resin is a thermoplastic polyimide resin having a repeating structural unit represented by the following formula (5).

In addition, the thermoplastic polyimide resin which has a repeating structural unit represented by the said Formula (5) can be purchased as "Aurum" (trademark) by Mitsui Chemicals.

  As a crystalline thermoplastic polyimide resin, a thermoplastic polyimide resin having repeating structural units represented by the following formulas (6) and (7) is also preferred.

  In the above formulas (6) and (7), m and n mean the molar ratio of each structural unit (not necessarily a block polymer), and m / n is 4-9, more preferably 5-9, More preferably, the number is in the range of 6-9.

  The thermoplastic polyimide resin having the repeating structural unit of the formula (6) and the formula (7) reacts in the presence or absence of an organic solvent using a corresponding ether diamine and tetracarboxylic dianhydride as raw materials. The resulting polyamic acid can be produced by imidizing chemically or thermally. These specific manufacturing methods can utilize the conditions of a known polyimide manufacturing method.

  In the present invention, the crystalline thermoplastic polyimide resin is represented by the following formula (8) instead of the thermoplastic polyimide resin having the repeating structural unit represented by the general formula (1) or in combination with the resin. It is also preferable to use a thermoplastic polyimide resin having a repeating structural unit. Further, it is also preferable to use a copolymer of a monomer having a structural unit represented by the formula (6) and a monomer having a structural unit represented by the following formula (8). In this case, the copolymer represented by the formula (6) is used. The molar ratio between the repeating structural unit and the repeating structural unit represented by the following formula (8) is suitably a ratio of 1: 0 to 0.75: 0.25.

  The thermoplastic polyimide resin having the repeating structural unit of the above formula (8) was obtained by reacting each of the corresponding ether diamine and tetracarboxylic dianhydride as raw materials in the presence or absence of an organic solvent. Polyamic acid can be produced by chemically or thermally imidizing. These specific manufacturing methods can utilize the conditions of a known polyimide manufacturing method.

  The amount of the crystalline thermoplastic resin added is usually 50 to 95% by weight, preferably 60 to 95% by weight, based on the total amount of the composition, in order for the crystalline resin to exhibit properties effectively by biaxial stretching. Is appropriate. As the crystalline thermoplastic resin, two or more different types of crystalline thermoplastic resins may be used. In that case, those total addition amount should just be in the said range.

  In the present invention, together with the crystalline thermoplastic resin, in a quantitative ratio that does not impair the effects of the present invention, for example, in the ratio of 40% by weight or less of the base thermoplastic resin component, other resins, for example, An amorphous thermoplastic resin may be contained.

  The amorphous thermoplastic resin is a polymer having a softening start temperature Tg of 120 ° C. or higher and thermoplasticity and amorphous properties.

  The softening start temperature Tg of the amorphous thermoplastic resin is preferably 180 ° C. or higher, particularly 210 ° C. or higher, more preferably 250 ° C. or higher, from the viewpoint that it easily reaches the required reflow heat resistance temperature.

  As the amorphous thermoplastic resin, for example, polycarbonate, polyphenylene ether, polyarylate, polysulfone, polyethersulfone, polyetherimide, amorphous polyimide, polyamideimide and the like can be used. Amorphous thermoplastic resins may be used alone or in admixture of two or more. Preferred amorphous thermoplastic resins are amorphous thermoplastic polyetherimide resin and amorphous thermoplastic polyethersulfone resin.

  Suitable amorphous thermoplastic polyetherimide resins include polyetherimide resins having a repeating structural unit represented by the following general formula (9).

上記一般式（９）において、Ｄは３価の芳香族基であり、ＥとＺは共に２価の残基である。

In the general formula (9), D is a trivalent aromatic group, and E and Z are both divalent residues.

  The polyetherimide resin having the repeating structural unit of the general formula (9) was obtained by reacting the corresponding ether diamine and tetracarboxylic dianhydride as raw materials in the presence or absence of an organic solvent. Polyamic acid can be produced by chemically or thermally imidizing. These specific manufacturing methods can utilize the conditions of a known polyimide manufacturing method.

  Specific examples of such a polyetherimide resin include, for example, polyetherimide resins having at least one repeating structural unit selected from repeating structural units represented by the following general formulas (10) to (12). It is done.

  In the general formulas (10) to (12), the symbol E is a divalent aromatic residue such as a group represented by the following formula.

The polyetherimide resin that is particularly preferably used is a polyetherimide resin having a repeating structural unit represented by the following formula (13).

  The polyetherimide resin having a repeating structural unit represented by the above formula (13) can be purchased as ULTEM (registered trademark) manufactured by Savic.

  The diamine or tetracarboxylic dianhydride that is a raw material of the crystalline thermoplastic polyimide resin or amorphous thermoplastic polyetherimide resin specifically shown can be used singly or in combination. Other copolymerization components can be included as long as the purpose is not impaired. Further, a plurality of polyimide resins obtained from different monomers may be arbitrarily polymer blended as long as the object of the present invention is not impaired.

With respect to each of the crystalline thermoplastic resin and the amorphous thermoplastic resin used in the reinforcing sheet-forming composition of the present invention, the melt viscosity that can be formed into a sheet by extrusion molding is 5 × 10 ¹ to 1 × 10 ⁴ [ Pa · S], preferably 4 × 10 ² to 3 × 10 ³ [Pa · S]. When the melt viscosity is less than 5 × 10 ¹ [Pa · S], the drawdown after discharging from the die is remarkable and sheet production becomes impossible. On the other hand, when the melt viscosity exceeds 1 × 10 ⁴ [Pa · S], the load applied to the extrusion screw at the time of melting is large, or the discharge from the die becomes difficult, and the sheet cannot be manufactured. Here, the melt viscosity [Pa · S] is a value measured using a Shimadzu flow tester CFT-500 in accordance with JIS K-7199, but is not limited to this, and under the same conditions. Any value that can be measured is acceptable.

  The fibrous inorganic filler used in the present invention is not particularly limited as long as the object of the present invention is achieved, and is an inorganic fiber having an average fiber length of 0.1 to 100 μm and an average fiber diameter of 0.01 to 20 μm. Fillers can be used.

  The average fiber length and average fiber diameter can be obtained by averaging 30 values measured with a scanning electron microscope (SEM).

  Specific examples of the fibrous inorganic filler include, for example, potassium titanate whisker, magnesium sulfate whisker, aluminum borate whisker, silicon nitride whisker, silicon carbide whisker, mullite whisker, graphite whisker, zinc oxide whisker, titanium boride whisker, and wallast. Examples thereof include knight, zonotlite, alumina silica fiber, quartz fiber, calcium carbonate whisker, carbon fiber, carbon nanotube, and glass fiber. From the viewpoint of obtaining a film having good film moldability, a smooth surface and a good appearance, preferred fibrous inorganic fillers are potassium titanate whisker and aluminum borate whisker.

The fibrous inorganic filler preferably has a generated gas (volatile content) amount of 1% by weight or less when heated at 350 ° C.
The amount of such generated gas can be measured by weight reduction when heated to 350 ° C. under a nitrogen gas stream in a measuring facility such as a thermogravimetric measuring device (TGA).

  The average fiber length of the fibrous inorganic filler is preferably 0.3 to 50 μm, more preferably 0.5 to 30 μm. The average fiber diameter is preferably 0.05 to 5 μm.

  The addition amount of the fibrous inorganic filler is usually 5 to 40% by weight of the total amount of the composition from the viewpoint that proper biaxial stretching can be performed at the time of heating and the effect of filler mixing can be expressed. 10 to 35% by weight is appropriate. As the fibrous inorganic filler, two or more kinds of fibrous inorganic fillers having different types, average fiber lengths, or average fiber diameters may be used. In that case, those total addition amount should just be in the said range.

  The reinforcing sheet-forming composition of the present invention may contain other additive components as necessary within a range in which the object of the present invention can be achieved. Other additive components include, for example, granular inorganic fillers (spherical silica, spherical alumina, glass beads, carbon beads, glass balloons, etc.), amorphous inorganic fillers (alumina, powdered silica, barium sulfate, calcium carbonate, zinc oxide, titanium oxide) Various other fillers such as antimony oxide, indium oxide, tin oxide, iron oxide, aluminum nitride, boron nitride, aluminum borate, carbon black), colorants such as dyes and pigments, mold release agents, heat stabilizers, etc. Various stabilizers, plasticizers, lubricants, antioxidants, antistatic agents, ultraviolet absorbers, oils and other additives, thermosetting resins (phenolic, epoxy, silicone, polyamideimide, etc.), etc. Is mentioned. The addition amount of other additive components is usually 20% by weight or less, preferably 10% by weight or less of the total amount of the composition. Two or more kinds of other additive components may be used in combination, and in that case, the total addition amount thereof may be within the above range.

  Other inorganic fillers to be added preferably have a median diameter of 0.1 to 10 μm or less, particularly 5 μm or less.

(Extrusion process)
In this step, a composition containing at least a crystalline thermoplastic resin and a fibrous inorganic filler is extruded to obtain an unstretched sheet. For example, the composition pellets obtained in the composition preparation step are optionally dried by heating to remove the adsorbed moisture, and then heated and melted by a single-screw or twin-screw extruder, and provided at the tip of the extruder. The sheet is discharged from the die into a flat film, and is cooled or solidified by contact or pressure contact with a cooling roll to obtain a sheet.

  The thickness of the unstretched sheet obtained by extrusion molding, that is, the sheet thickness immediately before biaxial stretching is set to 200 to 800 μm, particularly 200 to 500 μm, from the viewpoint of further effectively reducing the thermal expansion coefficient and thickness variation. preferable.

  The thickness of the unstretched sheet can be controlled by adjusting the gap size of the slit of the T die and the pulling speed after discharging from the T die.

(Biaxial stretching process)
In this step, the unstretched sheet obtained in the extrusion process is biaxially stretched in the TD direction and the MD direction, and is heat-set as desired to obtain a reinforcing sheet. In the MD direction, the thermal expansion coefficient is isotropically reduced mainly due to the orientation of the fibrous inorganic filler, and in the TD direction mainly due to the effect of crystallization by stretching. Therefore, the draw ratio can be set relatively small. As a result, since the thickness of the unstretched sheet can be reduced, variations in the thickness and characteristics of the finally obtained reinforcing sheet are very small, and a reinforcing sheet having improved characteristics can be obtained uniformly. In addition, the softening start temperature can be effectively increased.

  The stretching ratio in the TD direction achieved in this step is preferably larger than the stretching ratio in the MD direction. This is because an isotropic sheet having the same thermal expansion coefficient in the TD direction and the MD direction is obtained for the first time, and as a result, the reflow heat resistance is improved. When the draw ratio in the TD direction is equal to or less than the draw ratio in the MD direction, the thermal expansion coefficient is not isotropically reduced in the TD direction and the MD direction. Therefore, even if the MD direction has a low coefficient of thermal expansion, the TD direction becomes an anisotropic film whose coefficient of thermal expansion is not sufficiently reduced. In the case of simultaneous biaxial stretching, the stretching process is as described above. However, in the case of sequential biaxial stretching, the stretching ratio varies depending on the stretching conditions, and thus the stretching ratio in the TD direction is not necessarily increased. .

The draw ratio in the MD direction is usually 1.1 to 3.0.
The draw ratio in the TD direction is usually 1.3 to 3.0.
If the draw ratio in the TD direction and MD direction is too low, the thermal expansion coefficient may not be sufficiently reduced. If the draw ratio in the TD direction and the MD direction is too high, problems such as breakage of the sheet during stretching occur.

  From the viewpoint of improving the isotropic property in the TD direction and the MD direction, the difference between the draw ratio in the TD direction and the draw ratio in the MD direction is preferably 0.2 to 1.9, particularly preferably 0.3 to 1.0.

  The stretching temperature is preferably in the range of the softening start temperature Tg + 5 ° C. to Tg + 50 ° C. of the crystalline thermoplastic resin used. If the stretching temperature is too low, stretching stress is strong and stretching is impossible, or the sheet is torn or unevenly stretched during stretching. On the other hand, if the stretching temperature is too high, the effect of crystallization is small, and the effect of reducing the coefficient of thermal expansion due to stretching is not exhibited.

  The stretching speed is preferably in the range of 50 to 10,000% / min. If the stretching speed is low, the effect of crystallization is small and the coefficient of thermal expansion does not decrease. On the other hand, there is an upper limit to the stretching speed due to the limitations of the stretching equipment.

  As a biaxial stretching method, a conventionally known method such as a method of stretching using a plurality of roll groups, a method of stretching using a tenter, a stretching method by rolling using a roll, a tubular stretching method, or the like may be used. it can. Stretching methods using tenters that are often used in industry include sequential stretching in which the machine direction and the orthogonal direction are stretched in two stages, respectively, and simultaneous stretching in which the machine direction and the orthogonal direction are simultaneously stretched. Biaxial stretching may be performed by this method.

  The heat setting is a process of heating and cooling the sheet while maintaining the tension state of the biaxially stretched sheet. Thus, the dimensional change during reheating can be suppressed while maintaining the orientation of the polymer main chain and the fibrous inorganic filler achieved by biaxial stretching.

  As the heat setting conditions, the heating temperature is the softening start temperature Tg + 5 ° C. to the melting point −10 ° C. of the crystalline thermoplastic resin used, the limited shrinkage is 2 to 20%, preferably 4 to 10%, and the time is 1 to 5000. Can be set arbitrarily within minutes. When the heat setting temperature is too low, a large dimensional change occurs when the stretched sheet is reheated. On the other hand, when the heat setting temperature is higher than the melting point, the crystallization formed by stretching is eliminated.

The biaxial stretching process will be described with a specific example.
For example, in the case of simultaneous biaxial stretching, the unstretched sheet is preheated at a temperature range of the softening start temperature Tg + 5 ° C. to Tg + 20 ° C. of the used crystalline thermoplastic resin, and is uniformly heated to a predetermined temperature. The film is stretched at a predetermined magnification simultaneously in the TD direction and the MD direction. Next, the stretched sheet is heat-set under tension in the temperature range of the softening start temperature Tg + 5 ° C. to the melting point−10 ° C. of the used crystalline thermoplastic resin. In heat setting, the sheet is contracted after stretching, but the sheet is cooled while being gradually contracted to 2 to 20% while maintaining a tension state in which the contraction is restricted.
For example, in the case of sequential biaxial stretching, first, the unstretched sheet was preheated at a temperature range of the softening start temperature Tg + 5 ° C. to Tg + 50 ° C. of the used crystalline thermoplastic resin, and was uniformly heated to a predetermined temperature. In the state, the MD direction is stretched at a predetermined magnification. Next, the used crystalline thermoplastic resin is stretched at a predetermined magnification in the TD direction at a softening start temperature Tg + 5 ° C. to Tg + 50 ° C. Next, the stretched sheet is heat-set under tension in the temperature range of the softening start temperature Tg + 5 ° C. to the melting point−10 ° C. of the used crystalline thermoplastic resin. In heat setting, the sheet is contracted after stretching, but the sheet is cooled while being gradually contracted to 2 to 20% while maintaining a tension state in which the contraction is restricted.

  EXAMPLES The present invention will be specifically described below with reference to examples and the like, but the present invention is not limited to these examples. In addition, the present invention can be carried out in various modifications, changes, and modifications based on the knowledge of those skilled in the art without departing from the spirit of the present invention.

[Example 1]
(1) Compound process Thermoplastic polyimide whose chemical structural formula is the above formula (6) (Aurum (registered trademark) PD500A manufactured by Mitsui Chemicals, Inc .; Tg258 [° C], melting point 388 [° C], shear of 100 sec ⁻¹ A pellet or powder having a melt viscosity of 1000 [Pa · S] (hereinafter abbreviated as TPI) measured at a speed, an inorganic filler shown in Table 1, and other resins and additives as required, such as a Henschel mixer or a ribbon blender After dry-mixing, melting, kneading and extrusion were performed with a twin-screw kneading extruder. The extruded strand was cooled and cut to obtain a pellet of the mixture.

(2) Extrusion process After the obtained pellets are dried by heating to remove adsorbed moisture, they are heated and melted by a single-screw or twin-screw extruder, and a flat film is formed from a T die provided at the tip of the extruder. Then, it was pressure-bonded to a cooling roll and cooled and solidified to obtain various polyimide sheets (thickness: 660 μm).

(3) Biaxial stretching step The obtained sheet was preheated in a temperature range of 260 ° C, and was stretched simultaneously in two directions perpendicular to each other while being uniformly heated to a predetermined temperature. The draw ratios in the MD and TD directions were as shown in Table 1). Next, the obtained stretched sheet was heat-set at 300 ° C. under tension. In heat setting, the sheet was contracted after stretching, but the sheet was cooled while being gradually contracted to about 5% while maintaining a tension state in which the contraction was restricted, and a reinforcing sheet was obtained.

  The thermal expansion coefficient, softening start temperature (Tg by TMA measurement method), reflow heat resistance, thickness and variation of the unstretched sheet and the reinforcing sheet obtained as described above were measured by the following methods.

<Coefficient of thermal expansion (CTE)>
Using a thermomechanical measuring device TMA-60 manufactured by Shimadzu Corporation, the coefficient of thermal expansion up to 20 to 200 ° C. was measured at a heating rate of 5 ° C./min under a tensile load of 2 × 23 mm and a test piece of 5 gf. The unit is ppm / K.

<Softening start temperature Tg by TMA measurement method>
In accordance with the method described in “5.17.1 TMA method” of JIS C 6481: 1996, using a thermomechanical measuring device TMA-60 manufactured by Shimadzu Corporation, a tensile load of 2 × 23 mm and a test piece of 5 gf Below, the softening start temperature Tg at which the elongation increases rapidly under the condition of the temperature rising rate of 5 ° C./min was measured.

<Reflow heat resistance>
A reinforcing sheet is bonded to the polyimide side of a copper-clad laminate (total thickness 25 microns) composed of a thermosetting polyimide film and copper foil with an acrylic adhesive and heated to 150 ° C. to cure the adhesive. It was. Then, the reflow test which passes through the reflow furnace with the highest achieved temperature of 260 ° C. was performed. After cooling to room temperature, it was judged visually whether there was warping or deformation. In addition, the adhesive surface of the reinforcing sheet was subjected to corona discharge treatment in order to improve the adhesive strength. Using a corona treatment device manufactured by Sakai Kogyo Co., Ltd., the watt density per minute was 120 W / m ² .
○: No warpage.
Δ: Slightly warped (warp amount 10 mm or more) (practical problem).
X: There is curl.

<Thickness and thickness variation>
The sheet was sampled into a size of 100 mm in length and 100 mm in width excluding the gripping portion of the chuck at the time of stretching, and the thickness was measured at 4 places at intervals of 2 cm each in length and width, for a total of 16 places. The measuring instrument used was a dial gauge having a minimum scale of 1 μm and a flat tip terminal. With respect to the average thickness at 16 locations, the difference between the maximum value and the minimum value was less than 10% as ◯, 10% or more but less than 20% as Δ (practical problem), and 20% or more as x.

<Blister>
In the reflow heat resistance evaluation test, the surface of the copper foil surface was observed after passing through the reflow furnace. After cooling to room temperature, it was visually judged whether or not there was blistering at the interface between the copper foil and the adhesive due to gasification of the contained water.
○: No blister.
X: There is a blister.

[Examples 2-7 / Comparative Examples 1-8]
A biaxially stretched sheet was produced in the same manner as in Example 1 except that the resins, inorganic fillers, and other resins and additives as shown in Table 1 were used, and that biaxial stretching was performed at a predetermined magnification. Were manufactured and evaluated.

PEI: Amorphous thermoplastic polyetherimide resin having a repeating structural unit represented by the formula (13) (Ultem 1000 manufactured by Savic)
Whisker A: (Fibrous): Aluminum borate whisker (Shikoku Chemicals Co., Ltd .; Arborex-Y, average fiber length = 22 μm, average fiber diameter = 0.7 μm, amount of gas generated when heated at 350 ° C. <1 weight %)
Whisker B: (Fibrous): Potassium titanate whisker (Otsuka Chemical Co., Ltd .; Tismo-D, average fiber length = 16 μm, average fiber diameter = 0.4 μm, amount of gas generated when heated at 350 ° C. <1% by weight)
Alumina: (spherical): (manufactured by Tatsumori Co., Ltd.), median diameter = 1.0 μm, aspect ratio = 1
BaSO ₄ : (spherical): (B55; manufactured by Sakai Chemical Industry Co., Ltd.), median diameter = 0.7 μm, aspect ratio = 1)
Talc: (plate-like): (LMS200 manufactured by Fuji Talc Co., Ltd.), median diameter = 4.3 μm, aspect ratio = 16
Mica: (plate-like): (PDM5B manufactured by Topy Industries, Ltd.), median diameter = 5.0 μm, aspect ratio = 50
In the table, the “original” measured value is a measured value of the unstretched sheet immediately before biaxial stretching.
The “after” measurement value is a measurement value of the reinforcing sheet finally obtained.

It is a general | schematic fragmentary sectional perspective view which shows an example of the flexible printed wiring board by which the sheet | seat for reinforcement is bonded together and the reinforcement layer is formed. It is a schematic diagram which shows the TMA curve of a thermoplastic polyimide resin unstretched film and a biaxially stretched thermoplastic polyimide resin film.

Explanation of symbols

  1: Flexible printed wiring board, 2: Reinforcing sheet (stiffener).

Claims (11)

  A reinforcing sheet for reinforcing a flexible printed wiring board obtained by extrusion molding and biaxial stretching of a composition containing a crystalline thermoplastic resin having a softening start temperature Tg of 120 ° C. or more and a fibrous inorganic filler The flexible printed wiring board reinforcing sheet is characterized in that the thermal expansion coefficient in both the MD direction and the TD direction is in the range of 5 to 30 ppm / K.   The sheet for reinforcing a flexible printed wiring board according to claim 1, wherein the thermal expansion coefficient in both the MD direction and the TD direction is in the range of 10 to 28 ppm / K.   The sheet for reinforcing a flexible printed wiring board according to claim 1 or 2, wherein a difference in thermal expansion coefficient between the MD direction and the TD direction is 10 ppm / K or less.   The flexible printed wiring board reinforcing sheet according to any one of claims 1 to 3, wherein a softening start temperature Tg of the reinforcing sheet is 10 to 200 ° C higher than a softening starting temperature Tg of the sheet before stretching.   The flexible printed wiring board reinforcing sheet according to any one of claims 1 to 4, wherein a softening start temperature Tg of the reinforcing sheet is 260 ° C or higher.   The sheet for reinforcing a flexible printed wiring board according to any one of claims 1 to 5, wherein the reinforcing sheet has a thickness of 100 to 1,000 µm.   The flexible printed wiring board reinforcing sheet according to claim 1, wherein the crystalline thermoplastic resin is a crystalline thermoplastic polyimide resin having a softening start temperature Tg of 200 ° C. or higher.   The flexible printed wiring board reinforcing sheet according to any one of claims 1 to 7, wherein the composition further contains an amorphous thermoplastic resin having a softening start temperature Tg of 120 ° C or higher.   The flexible inorganic filler according to any one of claims 1 to 8, wherein the fibrous inorganic filler has an average fiber length of 0.1 to 100 µm and an average fiber diameter of 0.01 to 20 µm, and is contained in an amount of 5 to 40% by weight of the entire composition. Sheet for reinforcing printed wiring boards.   The flexible printed wiring board reinforcing sheet according to any one of claims 1 to 9, wherein the composition further contains a spherical inorganic filler having a median diameter of 0.1 to 10 µm.   A flexible printed wiring board, wherein the reinforcing sheet according to any one of claims 1 to 10 is bonded to a predetermined portion of the flexible printed wiring board to form a reinforcing layer.

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