JP2006310359A - Substrate for flexible printed circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enhance adhesiveness between a sheet-like substrate of aromatic polyamide film and a thin film layer of copper. <P>SOLUTION: The substrate for flexible printed circuit is produced by laminating a thin film layer of copper and a substrate of aromatic polyamide film through an alloy layer of nickel and chromium wherein the ratio of chromium in the alloy layer is 15-25 wt.%. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a flexible printed circuit board having a copper thin film layer, and more particularly to a flexible printed circuit board having an aromatic polyamide film as a base material and a copper thin film layer.

  Conventionally, as a substrate for a flexible printed circuit, a substrate having a three-layer structure in which a copper foil as a conductive metal layer is bonded to a plastic film base via an organic adhesive layer is known. This type of flexible printed circuit board has a problem that the dimensional accuracy after processing is lowered because the heat resistance of a commonly used organic adhesive is lower than that of a plastic film substrate. In addition, since the thickness of the copper foil to be used is usually 10 μm or more, it is difficult to perform etching for patterning for high-density wiring with a narrow pitch, and because it has an adhesive layer, it has a two-layer structure. In comparison, it has been pointed out that the substrate becomes thick and has poor storage properties.

  On the other hand, a conductive metal layer is formed on a plastic film substrate by a wet plating method or a dry plating method (for example, a vacuum deposition method, a sputtering film forming method, an ion plating method, etc.) without using an organic adhesive. A two-layer structure type flexible printed circuit board substrate has been proposed.

  For example, Patent Document 1 describes that a polyimide film is used as a sheet-like substrate, and an adhesion improving layer made of a nickel-chromium alloy is used between the sheet-like substrate and a copper thin film layer. Patent Document 2 describes the use of chromium-based ceramics as such an adhesion improving layer.

  However, in the case of such a two-layer structure type, since the conductive metal layer can be made thinner than 10 μm, high-density wiring is possible, but a severe thermal load test is performed, tin, nickel, solder, or gold When the electroless plating process such as the above is performed, there is a problem that the adhesion between the film base material and the conductive metal layer is reduced. In addition, when a nickel-chromium alloy is used as the adhesion improving layer for the polyimide film base material, it is known that when the nickel content is low, the adhesion strength after the heating test is markedly reduced. Need to increase. However, when the chromium content is increased, chromium is difficult to be etched with iron chloride, so it is necessary to change the etching conditions and the etching solution for each layer to be etched, resulting in a decrease in printed circuit productivity. There is. In addition, when a nickel-chromium layer is laminated on a polyimide substrate, it is laminated after performing chemical treatments such as hydrazine treatment and permanganic acid treatment as well as physical treatment such as plasma treatment, corona treatment and ozone treatment. It is necessary, and when a nickel-chromium layer is laminated without combining these treatments, it is known that the adhesion strength between the polyimide substrate and nickel-chromium does not increase.

  On the other hand, Patent Document 3 proposes the use of an aromatic polyamide as a sheet base material. However, it is generally difficult for a two-layer copper-clad laminate using an aromatic polyamide as a sheet base material to maintain adhesion with a copper thin film layer.

JP-A-9-83134 Japanese Patent Publication No.8-8400 Japanese Patent No. 2626049

  An object of the present invention is to improve adhesion between an aromatic polyamide film and a copper thin film layer.

  The inventors of the present invention have intensively studied to achieve the above object, and particularly focused on materials that can be formed by a sputtering film forming method. As a result, a nickel-chromium alloy layer containing a specific proportion of chromium, for example, has a specific thickness. The present invention is completed by finding that it is excellent in adhesion to the substrate of the aromatic polyamide film and the copper thin film layer, and does not impair the productivity of the printed circuit board, by using it as an adhesion improving layer that is laminated only. It came to.

That is, the present invention is as follows.
[1] For flexible printed circuits in which a copper thin film layer and a base material made of aromatic polyamide are laminated via an alloy layer made of nickel and chromium, and the proportion of chromium in the alloy layer is 15 to 25% by weight substrate.
[2] The flexible printed circuit board as described above, wherein the alloy layer has a thickness of 2 to 10 nm.

  According to the present invention, by providing a thin film layer mainly composed of nickel and chromium containing chromium at a specific ratio between the aromatic polyamide film (sheet) substrate and the copper thin film layer, Since the base material and the copper thin film layer are firmly adhered to each other and high reliability is obtained, a flexible printed circuit board capable of producing a high-definition circuit can be provided. In particular, using a nickel-chromium alloy containing 15 to 25% by weight of chromium as the copper adhesion-improving layer and setting the thickness of the adhesion-improving layer to 2 to 10 nm improves the heat resistance.

  The flexible printed circuit board of the present invention is formed by laminating a sheet-like base material made of an aromatic polyamide film, a thin film layer mainly composed of nickel and chromium, and a thin film layer made of copper in this order.

The base material used for this invention consists of an aromatic polyamide film. As this aromatic polyamide, it is comprised by the unit selected from the group which consists of the following structural unit, for example.
—NH—Ar ¹ —NH— (1)
—CO—Ar ² —CO— (2)
—NH—Ar ³ —CO— (3)

Here, Ar ¹ , Ar ² and Ar ³ contain at least one aromatic ring and may be the same or different. Such an aromatic ring includes a divalent hydrocarbon group having 6 to 12 carbon atoms and optionally having a sulfonyl bond or an ether bond, such as a p-phenylene group, an m-phenylene group, an o-phenylene group, 2, A 6-naphthylene group, a biphenylene group, 4,4'-diphenylene ether, 3,4'-diphenylene ether and the like can be mentioned. The aromatic ring of these groups may have a substituent such as a halogen group, a nitro group, an alkyl group, or an alkoxy group. Typical examples of these are shown below. That is, substantially the following formula (4)
-(-CO-Ar < ¹¹ > -CONH-Ar < ²¹ > -NH-)-(4)
A para-oriented aromatic polyamide (polyparaphenylene terephthalamide, hereinafter may be referred to as PPTA) is preferable. Here, Ar ¹¹ and Ar ²¹ are both p-phenylene groups. PPTA is a sheet (film) with excellent dimensional stability against heat, humidity, and external force. Since this film has a very small coefficient of thermal expansion, its dimensional change is small and all layers are laminated. It has an excellent characteristic that it is difficult to cause deformation such as curling when heated. In particular, the thermal expansion coefficient from 25 ° C. to 250 ° C. is (0-15) ppm / ° C., the thermal shrinkage at 250 ° C. is 0.1% or less, and the hygroscopic expansion coefficient at 25 ° C. is 30 ppm / ° C. Those having a moisture absorption of 2.5% by weight or less at 25 ° C. and 50% RH are extremely useful as base materials for flexible printed circuit boards. It goes without saying that a plurality of these characteristics may be provided.

  In the aromatic polyamide used in the present invention, as long as the purpose and effect of the present invention are not impaired, colorants such as lubricants, dyes and pigments, flame retardants, antistatic agents, antioxidants, other additives, etc. Modifiers as well as other polymers may be included.

  If the degree of polymerization of the aromatic polyamide used in the present invention is too low, a film having good mechanical properties cannot be obtained. Therefore, a logarithmic viscosity ηinh of 3.5 or more, preferably 4.5 or more (polymer 0.1. A polymer having a degree of polymerization that gives a value obtained by dissolving 5 g and measuring at 30 ° C. is selected.

  As a method for producing a sheet-like base material comprising an aromatic polyamide film, if the aromatic polyamide is soluble in an organic solvent, the solution is obtained by polymerizing directly in a solvent, or once isolating the polymer and then re-dissolving it. Then, the film is formed by a dry method or a wet method. In addition, those which are hardly soluble in an organic solvent such as PPTA are dissolved in concentrated sulfuric acid to form a solution, and then formed into a film by a dry-wet method or a wet method. In the wet method, the solution is extruded directly from a die into a coagulating liquid, or cast onto a metal drum or an endless belt in the same manner as in the dry process, and then guided into the coagulating liquid and coagulated. Subsequently, these films are washed with a solvent, an inorganic salt, or the like in the film and subjected to treatment such as stretching, drying, and heat treatment.

  For example, a method described in Japanese Patent No. 2664959 can be used as a method for producing a sheet-like substrate made of PPTA.

  The thickness of the sheet-like substrate made of the aromatic polyamide film in the present invention is usually 3 to 25 μm, preferably 4 to 12 μm. In the case of a PPTA sheet substrate, a particularly thin one can be produced, so that the flexible print It is suitable for use as a circuit board, and preferably 4 to 12 μm.

  In the flexible printed circuit board of the present invention, a thin film layer mainly composed of a nickel-chromium alloy is formed on a sheet-like base material composed of the above aromatic polyamide film. The material surface may be subjected to a primary pretreatment for degreasing, removing dust and the like. If necessary, secondary treatment such as corona discharge treatment, plasma treatment, ozone treatment, chemical treatment, dehumidification treatment, and heat treatment may be performed.

  In the present invention, a thin film layer mainly composed of a nickel-chromium alloy containing a specific amount of chromium as a main component is preferably interposed between a copper thin film layer and a substrate composed of an aromatic polyamide film, preferably with a specific thickness. Thus, the adhesion between the substrate and the copper thin film layer is good. Moreover, the adhesive force between the copper thin film layer and the sheet-like substrate after the heat treatment is favorably maintained. Here, the main component means that the total amount of nickel and chromium is at least 90% by weight, preferably 95% by weight or more of the whole. As a material for forming the thin film layer mainly composed of nickel and chromium, it is appropriate to use an alloy mixed in a desired ratio. The thin film layer of the alloy mainly composed of nickel and chromium may contain a small amount (other than 10% by weight of other metals) of other metals as long as the object and effect of the present invention are not impaired. The nickel-chromium alloy containing chromium in the above proportion can keep the adhesion between the copper thin film layer and the sheet-like base material strong. Moreover, it is excellent in the adhesion holding | maintenance characteristic after a heat test like the board | substrate for flexible printed circuits which was until now. The chromium content in the nickel-chromium alloy is 15% by weight or more and 25% by weight or less. If it exceeds 25% by weight, the proportion of chromium in the nickel-chromium alloy layer becomes too high, and etching with iron chloride or iron chloride containing hydrochloric acid becomes difficult. On the other hand, if the weight is less than 15 weights, it is necessary to increase the thickness of the adhesion improving layer in order to maintain the adhesion after the heating test. Bring. The nickel-chromium thin film containing chromium within the range of 15% by weight or more and 25% by weight within the range of the present invention is excellent in the ability to maintain adhesion even if the thickness is as thin as 10 nm or less. It is possible to achieve both adhesion and does not hinder printed circuit productivity. The minimum thickness of the adhesion-improving layer of the present invention is much thinner than the adhesion-improving layer of a flexible printed circuit board made of polyimide film that is used in flexible printed circuits that have been used so far. I can do it. This is considered to be because the polyamide film base material used in the present invention has a remarkably high elastic modulus as compared with polyimide or the like, so that it is difficult to cause so-called migration in thermal contact or the like. The thickness of the thin film layer containing nickel-chromium as a main component is usually 2 to 10 nm, preferably 2 to 5 nm.

  Next, a method for forming a thin film layer of an alloy containing nickel-chromium as a main component will be described below. For the thin film layer mainly composed of nickel-chromium in the present invention, for example, a vapor deposition method, a DC magnetron sputtering method, an RF magnetron sputtering method, an ion plating method, an ion beam sputtering method, or a CVD method can be used. From the viewpoint of forming a uniform transparent conductive film over a large area, it is preferable to use a DC magnetron sputtering method.

  In the DC magnetron sputtering method, a film is formed using an alloy target having a desired composition of a thin film layer. For example, when a nickel-chromium alloy thin film containing 15 to 25% by weight of chromium is formed, it is preferable to use an alloy target to which chromium is added by the content. Hereinafter, a film forming method in the case of forming a nickel-chromium alloy containing 15 to 25% by weight of chromium will be described in detail.

For example, when the thin film layer is formed by a DC magnetron sputtering method using a sintered target, the pressure (back pressure) in the vacuum chamber is first set to 1.3 × 10 ⁻⁴ Pa or less and then an inert gas is introduced. To do. Once the pressure in the vacuum chamber is 1.3 × 10 ⁻⁴ Pa or less, it is possible to reduce the influence of molecular species that remain in the vacuum chamber and may affect the characteristics of the transparent conductive layer. Therefore, it is preferable. More preferably, it is 5 × 10 ⁻⁵ Pa or less, and further preferably 2 × 10 ⁻⁵ Pa or less.

  As the inert gas to be introduced next, for example, helium, neon, argon, krypton, or xenon can be used. However, it is said that an inert gas having a larger atomic weight has less damage to the formed film and improves surface flatness. It has been broken. However, it is preferable to use argon in view of its availability and cost.

  In the present invention, since the aromatic polyamide is used for the base film, the film temperature cannot be increased above the decomposition temperature of the aromatic polyamide. Therefore, the temperature at which the thin film layer is formed is preferably about room temperature or lower to the softening temperature or lower or the decomposition temperature or lower. In the present invention, it is preferable to form the thin film layer while keeping the temperature of the substrate at a temperature of 450 ° C. or lower. More preferably, the film is formed at a temperature of 80 ° C. or lower, more preferably 60 ° C. or lower.

  In the present invention, a copper thin film layer is formed on a nickel-chromium thin film layer. As a method for forming the copper thin film layer, it is preferable to use a magnetron sputtering method because it is possible to stably produce a dense and homogeneous film. In spite of the vacuum thermal evaporation method or ion plating method, the sputtering film forming method in this method irradiates the target with charged particles accelerated in a low-pressure gas and recoils atoms and molecules on the target surface to form a sheet. This is a method of depositing a thin film on the substrate surface, and there are various methods such as 2-pole to 3-pole DC sputtering, 2-pole RF sputtering, ion beam sputtering, and magnetron sputtering depending on the generation method of charged particles. In the case of a nickel-chromium alloy, any method can be used. In particular, it is desirable to use a DC magnetron sputtering method capable of high-speed sputter evaporation at a low temperature.

For example, DC magnetron sputtering can be generally performed by adjusting the operating pressure to 5 × 10 ^{−4 to} 5 × 10 ⁻² Torr in a high-purity argon gas atmosphere and applying a voltage from a DC power source. There are a cylindrical type, a sputter gun type, and a planar type as devices that are actually put into practical use. The base material in the present invention is an aromatic polyamide sheet, which can be continuously produced while being supplied in parallel. The planar type is desirable because it is possible and has sufficient output to ensure the desired adhesion.

  The thickness of the copper thin film layer may be determined in consideration of the conductivity necessary for the subsequent step (electrolytic copper plating or the like), but is generally about 50 nm to 500 nm, preferably 150 to 300 nm. If it is thinner than 50 nm, the conductivity required for electrolytic plating cannot be obtained, and the plating thickness becomes non-uniform. Further, when the copper thin film is made thicker than 500 nm, depending on the laminating speed, stress of the copper thin film layer is greatly applied to the base film, and curling and warping are strongly generated, so that handling is impaired. Further, forming a copper thin film with such a film thickness by a sputtering method requires a lot of production time, and the production cost increases, which is a commercially disadvantageous production method.

  Since the nickel-chromium alloy thin film layer and the copper thin film layer described above can be formed on both sides of the sheet-like base material in the same manner, in this case, it is obtained as a double-sided flexible printed circuit board.

  In the flexible printed circuit board of the present invention, a copper plating layer is formed on the copper thin film layer by electrolytic or electroless plating. As a method for forming the copper plating layer, a conventional method can be used.

  The thus obtained circuit board on which the copper plating layer is formed is particularly effectively used as a flexible printed circuit board, but the printed circuit production method therefor is not particularly different from a general copper foil-clad FPC. Absent. That is, an etching resist film is produced by a printing method, a dry film method, etc., and unnecessary copper thin film and nickel-chromium alloy thin film are etched with ferric chloride, cupric chloride, permanganate compound, cerium compound, or the like. Then, removal, masking of the obtained copper circuit with an insulating film, soldering of terminals, drilling, and the like are performed in the same manner. However, unlike rolled copper foil, etching is fast because the copper layer is thin. Therefore, since there is no undesirable phenomenon such as side etching, there is a great merit that the reproducibility of the circuit pattern is extremely excellent and a finer circuit having a pitch of 15 μm or less can be produced. It can be considered that this fine circuit can be produced due to the easy etching property of nickel contained in the adhesion layer.

  Moreover, the board | substrate for flexible printed circuits obtained by this invention is excellent in the adhesive force retention after a heat test compared with the 2-layer sputter-plating type copper clad laminated board known until now. This is because the modulus of elasticity of the polyamide film used in the present invention is much higher than other heat-resistant sheet-like base materials, which suppresses metal diffusion during heat treatment, resulting in retention of adhesion. It is thought that there is.

Examples of the present invention will be described below, but the present invention is not limited to these examples.
[Example 1]
A 9 μm thick polyamide film “Aramika” 090R (manufactured by Teijin Advanced Films Ltd .: polyparaphenylene terephthalamide) was used as the base material, and the surface of the polyamide film was formed by sputtering to form a 5 nm thick nickel-chromium alloy thin film layer. An alloy was laminated, and then a copper thin film was laminated with a thickness of 200 nm. Further, this film is electroplated under an aqueous copper sulfate solution, and 8 μm thick copper is laminated on the copper thin film layer. Specifically, it was produced as follows.

A film substrate having a width of 1100 mm, a length of 1000 m, and a thickness of 9 μm was unwound in a vacuum apparatus and used in the order of rolls. For the formation of the alloy metal layer on the substrate surface, a roll-up DC magnetron sputtering film forming apparatus to which a sputtering target having a length of 1600 mm can be attached was used. Prior to the formation of the metal layer, using a linear ion beam source made by Advanced Energy Japan, a gas mixed with argon and nitrogen was introduced into the apparatus at a pressure of 2 Pa, and the film surface was subjected to plasma treatment with the ion beam. Was roughened and nitrogen atoms were implanted. Vacuum layer was vacuumed to 2 × 10 ⁻⁴ torr or less using a nickel-chromium alloy (chromium content 20% by weight) manufactured by Sumitomo Metal Mining Co., Ltd. as a sputtering target for the formation of the adhesion improving layer thin film. Then, 8 × 10 ⁻¹ torr of argon gas was introduced, a direct current voltage was applied to the nickel-chromium target to cause plasma discharge, and a nickel-chromium thin film was laminated on the film substrate by 5 nm. Further, oxygen-free copper (purity 99.95% or more) is used as a sputtering target for laminating a copper thin film layer, argon gas is introduced at 8 × 10 ⁻¹ torr, and a direct current voltage is applied to the copper target for plasma discharge. Then, a copper thin film was further laminated to 200 nm on the surface on which the nickel-chromium thin film was laminated. For electrolytic plating, a copper sulfate aqueous solution with a copper content of 70 g / liter and a sulfuric acid content of 200 g / liter as a plating solution and a chloride ion content equivalent to 50 mg / liter as an additive, Top Lucina SF manufactured by Okuno Pharmaceutical Co., Ltd. -Using a standard amount of -M added, and using a precision plating device for 4-inch wafers manufactured by Yamamoto Kakin Tester Co., Ltd. as the plating device, stirring at a temperature of 25 ° C, a current density of 2 A / dm Electrolytic plating was performed at ² and copper was laminated until the thickness became 8 μm.

[Measurement of adhesion]
Based on JIS C-5016 (1994), the peel strength was measured by a 90-degree peeling method at a speed of 50 mm / min. Table 1 shows the evaluation results of the samples prepared in the examples.

[Etching characteristics]
The printed circuit board produced in Example 1 was masked in the shape of the test pattern of FIG. 1 with JIS C-5016, and the non-masking portion was stirred while stirring a 40 wt% aqueous iron chloride solution at 40 ° C. Etching was performed. After confirming removal of the copper layer of the non-masking portion by visual inspection, etching is further performed for 30 seconds. After removing the mask and cleaning, a DC voltage of 500 V is applied between the two electrodes, and there is no flashover. did.

  The flexible printed circuit board of the present invention is excellent in heat resistance and thus has high reliability, and is therefore extremely useful as a material for a flexible printed circuit board.

Claims (2)

  A flexible printed circuit board in which a copper thin film layer and a base material made of an aromatic polyamide are laminated via an alloy layer made of nickel and chromium, and the proportion of chromium in the alloy layer is 15 to 25% by weight.   The flexible printed circuit board according to claim 1, wherein the alloy layer has a thickness of 2 to 10 nm.