JP2005259790A - Flexible printed wiring board and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a flexible printed wiring board having stabilized quality in which a film is not curled, even after a circuit is formed on the conductor side, and to provide its production method. <P>SOLUTION: In the flexible printed wiring board, a base layer composed of at least one kind of low thermal expansion polyimide based resin is interposed in between a bottom layer touching a conductor and a top layer on a side opposite to the conductor, wherein the bottom layer and the top layer are composed of thermoplastic polyimide based resin, exhibiting thermal expansion higher than that of the base layer, and a condition P<SB>1</SB><P<SB>2</SB>is satisfied between the thickness P<SB>1</SB>of the bottom layer and the thickness P<SB>2</SB>of the top layer. In the production process of a flexible printed wiring board, the bottom layer and the top layer are coated with polyimide precursor resin solution, convertible into thermoplasic polyimide based resin exhibiting thermal expansion higher than that of the base layer, such that the thickness P<SB>1</SB>of the bottom layer and the thickness P<SB>2</SB>of the top layer satisfy the condition P<SB>1</SB><P<SB>2</SB>and then they are dried and thermally set. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a flexible printed wiring board and a method for manufacturing the same in response to the demand for downsizing and weight reduction of electronic devices, and in particular, a high-quality single-sided conductor that does not warp in a polyimide film part after wiring processing. The present invention relates to a flexible printed wiring board and a method for manufacturing the same.

  In recent years, with the progress of miniaturization and weight reduction of highly functional mobile phones, digital cameras, navigators, and other various electronic devices, flexible printed circuit boards (wiring boards) used as electronic wiring materials are used. There is a growing demand for smaller, higher density, multilayer, finer, lower dielectric, etc. This flexible printed wiring board was previously manufactured by bonding polyimide film and metal foil together with an adhesive that can be cured at low temperature. However, the adhesive layer has deteriorated characteristics as a wiring board, especially polyimide base film. There is a problem that the excellent heat resistance and flame retardancy are impaired. Another problem with the adhesive layer is that the circuit processability of the wiring is deteriorated.

  Specifically, there are problems such as generation of resin smear due to drilling during through-hole processing and a large dimensional change rate during conductor through-hole processing. In particular, in the case of a double-sided through-hole structure, a base film, which is an insulator layer, is formed by bonding a copper foil, etc., of a conductor with an adhesive on both sides compared to a single-sided flexible printed circuit board. In general, there is a problem that its flexibility is low. On the other hand, heat generation increases with the increase in the density of ICs and the miniaturization and density of printed wiring, and it may be necessary to attach a good heat conductor. There is also a method of integrating the housing and the wiring in order to make it more compact. Furthermore, wiring with different electric capacities may be required, and wiring materials that can withstand higher temperatures may be required. Therefore, various methods for producing a flexible printed board have been proposed in which a polyamic acid solution before curing is directly applied to a conductor such as a copper foil without using an adhesive and is cured by heating.

For example, a polyamic acid synthesized with a diamine having a linear expansion coefficient of 3.0 × 10 ⁻⁵ or less and a tetracarboxylic acid anhydride applied to a metal foil and cured by heating (for example, see Patent Document 1), A resin solution containing a polyamidoimide precursor compound having a specific structural unit is applied onto a conductor and imidized (see, for example, Patent Document 2), a diamine containing diaminobenzanilide or a derivative thereof, an aromatic tetracarboxylic acid, Examples include those in which a precursor solution of an insulating material having a structural unit obtained by this reaction is directly applied onto a conductor and cured (see, for example, Patent Document 3). Further, a method for producing a flexible printed wiring board having a plurality of polyimide resin layers by applying and drying a plurality of times using a plurality of polyimide precursor resin solutions on a conductor in order to improve adhesion to a metal foil (For example, see Patent Document 4) has also been proposed.
JP-A-62-212140 JP-A-63-84188 JP-A 63-245988 Japanese Examined Patent Publication No. 6-49185

  According to the method for manufacturing a substrate for flexible printed wiring described in Patent Document 4, the curling of the substrate that occurs due to the difference in the adhesion between the metal foil and the resin as the conductor and the linear expansion coefficient between the metal foil and the resin. It is possible to obtain a high-quality flexible printed wiring board in which the above is suppressed. However, even with such a substrate, if the structure of the polyimide resin of the plurality of layers on one side of the metal foil does not satisfy specific conditions, circuit processing is actually performed on the conductor side, and unnecessary metal It has been found that there is a problem that the exposed polyimide film tends to curl when the foil is removed.

  As a cause of this phenomenon, there is a difference in the degree of orientation of polyimide molecules in the thickness direction of the formed polyimide resin layer in the process of applying and drying the polyimide precursor resin solution and imidizing by heat treatment. Although it was expected, the mechanism was not yet elucidated. SUMMARY OF THE INVENTION An object of the present invention is to provide a flexible printed wiring board having a stable quality that does not cause curling in a film even after circuit processing on a conductor side, and a method for manufacturing the same.

  As a result of intensive studies on the above problems, the present inventors have predicted the difference in the linear expansion coefficient in the thickness direction resulting from the difference in the degree of orientation of the polyimide molecules in the thickness direction of the multilayer polyimide layer, and contacted the conductor A polyimide resin layer having a higher thermal expansion coefficient than the base layer present between the layers and the top layer is disposed, and the thickness of the layer in contact with the conductor is made smaller than the thickness of the top layer. The present invention has been completed by finding out that the object can be achieved.

That is, the flexible printed wiring board of the present invention is a flexible printed wiring board having a multilayer polyimide layer having different linear expansion coefficients on one side of a conductor, and a bottom layer in contact with the conductor and a top layer opposite to the conductor. A base layer made of a low thermal expansion polyimide resin having a linear expansion coefficient of 30 × 10 ⁻⁶ (1 / ° C.) or less is disposed in the middle of the base layer, and the bottom layer and the top layer have higher thermal expansion than the base layer. The bottom layer thickness P ₁ and the top layer thickness P ₂ satisfy the condition of P ₁ <P ₂ .

Moreover, it is desirable that the ratio P _m / (P ₁ + P ₂ ) of the total thickness of the bottom layer and the top layer on both sides to the thickness P _m of the base layer in the present invention is in the range of 2-100.

Furthermore, the thickness P ₁ of the bottom layer in the present invention is in the range of 0.2 to ₁₀ μm, and the ratio P ₁ / P ₂ to the thickness P _{2 of the} top layer is in the range of 0.2 to 0.8. It is desirable that

The method for producing a substrate for flexible printed wiring of the present invention comprises a method of forming a multilayer polyimide layer having different linear expansion coefficients by directly applying and drying a plurality of layers of a polyimide precursor resin solution on one side of a conductor, followed by heat curing. In the method for manufacturing a flexible printed wiring board, a low thermal expansion having a linear expansion coefficient of at least 30 × 10 ⁻⁶ (1 / ° C.) or less between the bottom layer in contact with the conductor and the top layer opposite to the conductor. A base layer that can be converted into a functional polyimide resin is disposed, and a polyimide precursor resin solution that can be converted into a thermoplastic polyimide resin having a higher thermal expansion than the base layer is formed on the bottom layer and the top layer. P ₁ and the thickness P _{2 of the} top layer are characterized by being heat-cured after coating and drying so as to satisfy the condition of P ₁ <P ₂ .

In the present invention, the ratio P _m / (P ₁ + P ₂ ) of the total thickness of the bottom layer and the top layer on both sides to the thickness P _m of the base layer is preferably in the range of 2-100.

Further, in the present invention, the thickness P ₁ of the bottom layer is in the range of 0.2 to ₁₀ μm, and the ratio P ₁ / P ₂ to the thickness P _{2 of the} top layer is in the range of 0.2 to 0.8. It is desirable to apply the polyimide precursor resin solution so as to satisfy.

  According to the present invention, even if circuit processing is performed on a flexible printed wiring board and unnecessary metal foil is removed, the polyimide film does not curl or warp, and as a result, a printed wiring board having excellent dimensional stability is manufactured. can do.

  The best mode for carrying out the present invention will be described in detail below. First, examples of conductors used in the present invention include conductive metal foils such as copper, aluminum, iron, silver, palladium, nickel, chromium, molybdenum, tungsten, zinc, and alloys thereof having a thickness of 5 to 150 μm. Preferably, it is copper. In the case of copper, there are a rolled copper foil and an electrolytic copper foil, but both can be used. For the purpose of improving the adhesion, the surface may be subjected to chemical or mechanical surface treatment with siding, nickel plating, copper-zinc alloy plating, aluminum alcoholate, aluminum chelate, silane coupling agent or the like.

An insulator layer can be formed by forming a plurality of polyimide resin layers on one side of a conductive metal foil that is a conductor, but the polyimide resin used as the insulator layer has an imide ring structure. A general term for resins, such as polyimide, polyamideimide, polyesterimide, and the like. And as a polyimide-type resin layer, the thing of the low thermal expansion as described in the said patent documents 1-4, the thermoplastic polyimide etc. which fuse | melt or soften when heated can be utilized, and it is not specifically limited. However, a particularly preferred insulator layer is a low thermal expansion resin layer having a thermal expansion coefficient of 30 × 10 ⁻⁶ (1 / k) as a base layer, and a thermoplastic polyimide resin having a higher thermal expansion than the base layer above and below it. It is desirable to use at least three polyimide resin layers in which two layers (bottom layer and top layer) are arranged.

Here, the low thermal expansion polyimide resin forming the base layer preferably has a linear expansion coefficient of 30 × 10 ⁻⁶ (1 / ° C.) or less, and has excellent performance in heat resistance and flexibility of the film. Good. Here, the linear expansion coefficient is a sample having a sufficiently completed imidation reaction, heated to 250 ° C. using a thermomechanical analyzer (TMA), cooled at a rate of 10 ° C./min, and 240 to 100 ° C. The average linear expansion coefficient in the range is obtained. As a specific example of the low thermal expansion polyimide resin having such properties, a polyimide resin having a unit structure represented by the following general formula (I) is desirable.

（但し、式中Ｒ₁〜Ｒ₄は低級アルキル基、低級アルコキシ基、ハロゲン基又は水素を示す）

(Wherein R _{1 to} R ₄ represent a lower alkyl group, a lower alkoxy group, a halogen group or hydrogen)

  The thermoplastic polyimide resin forming the bottom layer and the top layer used above and below the base layer is higher in thermal expansion than the base layer and has a glass transition temperature of 350 ° C. or lower. Although it may have any structure, it is preferable that the adhesive strength at the interface is sufficient when pressure bonding is performed under heat and pressure. In this case, the thermoplastic polyimide resin species of the bottom layer and the top layer may be the same or have different unit structures as long as the above conditions are satisfied. Here, the thermoplastic polyimide resin does not necessarily have sufficient fluidity in a normal state above the glass transition point, and includes those that can be bonded by pressurization. Specific examples of the thermoplastic polyimide resin having such properties have a unit structure represented by the following general formula (II) or general formula (III).

（但し、式中Ａｒ₁は２価の芳香族基であってその炭素数が１２以上である。）

（但し、式中Ａｒ₂は２価の芳香族基であってその炭素数が１２以上である。）

(In the formula, Ar ₁ is a divalent aromatic group having 12 or more carbon atoms.)

(In the formula, Ar ₂ is a divalent aromatic group having 12 or more carbon atoms.)

等を挙げることができる。 Here, specific examples of the divalent aromatic group Ar ₁ or Ar ₂ include

Etc.

  Moreover, as a method for producing a flexible printed wiring board having a single-sided conductor, as described in Patent Document 4, a polyimide precursor solution or a polyimide solution is cured with a known acid anhydride type or amine type curing agent. Applying various additives and catalysts such as adhesives, silane coupling agents, titanate coupling agents, adhesion imparting agents such as epoxy compounds, flexibility imparting agents such as rubber, etc., and coating on one side of conductive metal foil Then, it can be thermoset by heat treatment to obtain a single-sided conductor laminate. The single-sided conductor laminate has a conductive metal foil as a bottom layer with a higher thermal expansion thermoplastic polyimide resin layer than the base layer, an intermediate base layer with at least one low thermal expansion polyimide resin layer, The outermost top layer is preferably laminated in the order of a thermoplastic polyimide resin layer having a higher thermal expansion than the base layer.

  Here, the intermediate base layer must be a polyimide resin layer having a lower thermal expansion than the thermoplastic polyimide resin layer of the bottom layer or the top layer. The base layer has the function of suppressing the occurrence of curling and warping of the substrate for the flexible printed wiring board to be manufactured, and the bottom layer in contact with the conductor has the function of ensuring the adhesiveness with the conductive metal foil. Is used in anticipation of curling the film itself. In some cases, another conductive metal foil is laminated on the top layer and thermocompression-bonded so that the adhesiveness when used as a substrate for a flexible printed wiring board with a double-sided conductor is also expected.

At that time, the ratio P _m / (P ₁ + P ₂ ) of the total thickness of the thermoplastic polyimide resin layers (bottom layer and top layer) on both sides of the thickness P _m of the low thermal expansion polyimide resin layer (base layer) Is in the range of 2-100, preferably in the range of 5-20. If the thickness ratio is smaller than 2, the thermal expansion coefficient of the entire polyimide resin layer is too high compared to that of the metal foil, and the warp and curl of the resulting flexible printed wiring board substrate becomes large, so that during circuit processing The workability is significantly reduced. In addition, if the total thickness (P ₁ + P ₂ ) of the thermoplastic polyimide resin layers on both sides is too small and the ratio of the thickness exceeds 100, the adhesive strength with the conductive metal foil is sufficiently exerted. In some cases, it will disappear.

It is important that the ratio of the thickness (P ₁ ) of the bottom layer in contact with the conductor and the thickness (P ₂ ) of the top layer opposite to the conductor is P ₁ <P ₂ . The ratio of the thickness varies depending on the thickness of the base layer having low thermal expansion, but P ₁ / P ₂ = 0.2 to 0.8, more preferably 0.3 to 0.7. If it is smaller than this range, the curl correcting effect is too strong, and conversely, curling occurs. On the other hand, if it is larger than this range, the film curl suppressing effect is not exhibited. The thickness (P ₁ ) of the bottom layer in contact with the conductor layer is preferably in the range of 0.2 to ₁₀ μm. If it is thinner than this range, the adhesive strength with the conductor layer cannot be secured, and if it is thick, heat resistance will be reduced.

  The application of the polyimide precursor resin that can be converted into the plurality of polyimide resins on the conductive metal foil can be performed in the form of the resin solution, but preferably as described in Patent Document 4 above. In the form of the precursor solution, it is preferable to perform batch conversion of a plurality of precursor solutions or a solvent removal treatment at a temperature equal to or lower than the imide ring-closing temperature and then perform heat conversion of the precursors to polyimide in a batch. . If another polyimide precursor solution is applied onto the layer that has been completely converted to polyimide and then heat-treated to cause imide ring closure, the adhesive strength between the polyimide resin layers may not be fully demonstrated. This causes the quality of the double-sided laminate to deteriorate.

As a method of coating a polyimide precursor resin solution (polyamic acid solution) or a resin solution containing the precursor compound on a conductive metal foil, for example, a knife coater, a die coater, a roll coater, a curtain coater or the like is used. In particular, a die coater or a knife coater is suitable for thick coating. The polymer concentration of the polyimide precursor resin solution used for coating is usually 5 to 30% by weight, preferably 10 to 20% by weight, although it depends on the degree of polymerization of the polymer. When the polymer concentration is lower than 5% by weight, a sufficient film thickness cannot be obtained by one coating, and when the polymer concentration is higher than 30% by weight, the solution viscosity becomes too high and coating becomes difficult.

  Next, the polyimide precursor resin solution (polyamic acid solution) applied to the conductive metal foil with a uniform thickness is subjected to a heat treatment to remove the solvent and further imide ring closure. In this case, if the heat treatment is suddenly performed at a high temperature, a skin layer is formed on the resin surface, and the solvent hardly evaporates or foams. Therefore, it is desirable to perform the heat treatment while gradually raising the temperature from a low temperature to a high temperature. The final heat treatment temperature at this time is usually preferably 300 to 400 ° C., and at 400 ° C. or higher, the thermal decomposition of the polyimide begins to occur gradually, and at 300 ° C. or lower, the polyimide film is sufficiently deposited on the conductive metal foil. A single-sided conductor laminate with good flatness cannot be obtained without orientation. Thus, the whole thickness of the polyimide-type resin layer as an insulator formed is normally 10-150 micrometers.

  Hereinafter, based on an Example and a comparative example, embodiment of this invention is described concretely. In the following examples, the coefficient of thermal expansion, the curl and adhesion of a single-sided copper-clad product, and the curl of the film were measured by the following methods.

  That is, the coefficient of thermal expansion is an average linear expansion between 240 ° C. and 100 ° C. using a thermomechanical analyzer (TMA100) manufactured by Seiko Electronics Industry Co., Ltd., after cooling to 250 ° C. and cooling at a rate of 10 ° C./min. The coefficient was calculated and obtained.

  As the curl of a single-sided copper-clad product, the radius of curvature of a copper-clad product having a size of 100 mm × 100 mm after heat treatment and imidization was measured.

  The adhesive strength of the single-sided copper-clad product is the value when the copper foil is peeled off at a speed of 50 mm / min in the direction of 180 ° using a pattern with a conductor width of 3 mm according to JIS C5016: 7.1 (kg / Cm).

  As the solder heat resistance, the solder bath temperature was gradually increased from 260 ° C. at intervals of 10 ° C. according to the method of JIS C5016, and measured up to 400 ° C.

In the examples and comparative examples, the following abbreviations were used.
PMDA: pyromellitic anhydride BTDA: 3,3 ′, 4,4′-benzophenone tetracarboxylic anhydride DDE: 4,4-diaminodiphenyl ether MABA: 2′-methoxy-4,4′-diaminobenzanilide

(Synthesis Example 1)
While nitrogen was passed through a glass reactor, 2532 g of N, N-dimethylacetamide was charged, followed by stirring with 0.5 mol of DDE and 0.5 mol of MABA, and then completely dissolved. The solution was cooled to 10 ° C., and 1 mol of PMDA was added little by little so that the reaction solution was kept at a temperature of 30 ° C. or lower. After the addition was completed, the mixture was stirred at room temperature for 2 hours to complete the polymerization reaction. I let you. The obtained polyimide precursor solution had a polymer concentration of 15% by weight and an apparent viscosity of 1000 mPa · s at 25 ° C. using a B-type viscometer.

(Synthesis Example 2)
A polyimide precursor solution was prepared in the same manner as in Synthesis Example 1 except that 1 mol of DDE was used as the diamine component and 1 mol of BTDA was used as the acid anhydride component. The resulting polyimide precursor solution had a polymer concentration of 15% by weight and an apparent viscosity of 300 mPa · s at 25 ° C. using a B-type viscometer.

Example 1
After the polyimide precursor solution 2 prepared in Synthesis Example 2 was uniformly coated with a thickness of 20 μm on a roughened surface of a 35 μm roll-shaped electrolytic copper foil (manufactured by Nikko Gould Co., Ltd.) using a die coater as a bottom layer, 120 μm was applied. The solvent was removed by continuous treatment in a hot air drying oven at 0 ° C. Next, the polyimide precursor solution 1 prepared in Synthesis Example 1 using a reverse roll coater was applied on the polyimide precursor layer uniformly as a base layer with a thickness of 200 μm, and continuously in a hot air drying oven at 120 ° C. After the treatment, the solvent was removed and the polyimide precursor solution 2 prepared in Synthesis Example 2 was applied uniformly as a top layer to a thickness of 40 μm, and then heated from 120 ° C. to 360 ° C. over 30 minutes in a hot air drying oven. A single-sided conductor laminate (single-sided copper-clad product a) having a good flatness with no warpage and no curling when the polyimide resin layer had a thickness of 25 μm was heated and heated for imidization. However, the base point of the bottom layer thickness was measured as 1/2 the surface roughness of the rough surface of the conductor. The 180 ° peel strength (JIS C-5016) measured between the copper foil layer and the polyimide resin layer of the single-sided conductor laminate “a” was 1.8 kg / cm. Next, when the circuit side is applied to the conductor side of the single-sided conductor laminate a and the unnecessary metal foil is removed, the exposed polyimide film is not curled, and the linear expansion coefficient of the film after etching is 23.5. × 10 ^-6 (1 / ° C).

Examples 2-3 and Comparative Examples 1-3
Various changes were made to the thicknesses of the polyimide resin layers of the bottom layer, base layer and top layer in Example 1, followed by drying in the same manner, followed by heating from 120 ° C. to 360 ° C. over 30 minutes in a hot air drying furnace. A laminate (single-sided copper-clad product) a was obtained. This single-sided conductor laminate a is warped and curled, 180 ° peel strength, and the conductor side is subjected to circuit processing to remove the undesired metal foil and curl of the exposed polyimide film. Tables 1 and 2 collectively show the linear expansion coefficient of the film after etching.

Claims (5)

A flexible printed wiring board having a multilayer polyimide layer having different linear expansion coefficients on one side of a conductor, wherein at least one kind of linear expansion coefficient is 30 × between the bottom layer in contact with the conductor and the top layer opposite to the conductor. A base layer made of a low thermal expansion polyimide resin having a temperature of 10 ⁻⁶ (1 / ° C.) or less is disposed, and a bottom layer and a top layer are made of a thermoplastic polyimide resin having a higher thermal expansion than the base layer, and further a bottom layer A flexible printed wiring board characterized in that the layer thickness P ₁ and the top layer thickness P ₂ satisfy a condition of P ₁ <P ₂ . _2. The flexible printed wiring board according to claim 1, wherein a ratio P _m / (P ₁ + P ₂ ) of a total thickness of the bottom layer and the top layer on both sides to the thickness P _{m of the} base layer satisfies a range of ₂ to 100. . The thickness P _{1 of the} bottom layer is in the range of 0.2 to ₁₀ µm, and the ratio P ₁ / P ₂ to the thickness P _{2 of the} top layer satisfies the range of 0.2 to 0.8. 2. The flexible printed wiring board according to 2. In a method for manufacturing a flexible printed wiring board having a multilayer polyimide layer having different linear expansion coefficients by directly applying and drying a plurality of layers of polyimide precursor resin solution on one side of the conductor, followed by heat curing, the conductor A base layer that can be converted into a low thermal expansion polyimide resin having a linear expansion coefficient of 30 × 10 ⁻⁶ (1 / ° C.) or less is disposed between the bottom layer in contact with the conductor and the top layer opposite to the conductor. In addition, a polyimide precursor resin solution that can be converted into a thermoplastic polyimide resin having a higher thermal expansion than the base layer is formed on the bottom layer and the top layer. The thickness P _{1 of the} bottom layer and the thickness P _{2 of the} top layer are P _1. <method of manufacturing a substrate for a flexible printed circuit, characterized in that the heat curing after coating and drying so as to satisfy the condition P _2. The polyimide precursor is such that the thickness P _{1 of the} bottom layer is in the range of 0.2 to ₁₀ μm and the ratio P ₁ / P ₂ to the thickness P _{2 of the} top layer is in the range of 0.2 to 0.8. The manufacturing method of the board | substrate for flexible printed wiring of Claim 4 which coats a body resin solution.