KR20050089243A - Flexible printed board and the manufacturing method thereof - Google Patents

Abstract

The present invention relates to a flexible circuit board and a method for manufacturing the same, more specifically copper foil; A first polyimide resin layer positioned on one surface of the copper foil and having a linear expansion coefficient of 10 × 10 ⁻⁶ to 17 × 10 ⁻⁶ / K; And a second polyimide composed of the same component as the first polyimide resin layer, positioned on one surface of the first polyimide resin layer, and having a linear expansion coefficient of 17 × 10 ⁻⁶ / K to 20 × 10 ⁻⁶ / K. The present invention relates to a flexible circuit board including a mid resin layer and a method of manufacturing the same.

The flexible circuit board of the present invention is formed of two kinds of polyimide layers having different linear expansion coefficients by using one kind of polyamic acid varnish, which has excellent reliability in dimensional stability, planarity after etching, etc., and excellent workability in circuit formation. In particular, as one kind of polyamic acid varnish, it is possible to manufacture a flexible circuit board having a polyimide resin layer having a two-layer structure having a different linear expansion coefficient, thereby reducing the effort of varnish manufacturing. .

Description

Flexible circuit board and its manufacturing method {FLEXIBLE PRINTED BOARD AND THE MANUFACTURING METHOD THEREOF}

[Industrial use]

The present invention relates to a flexible circuit board and a method of manufacturing the same, and more particularly, to a two-layer copper-clad flexible circuit board and a method of manufacturing the same, in which two polyimide resin layers having different linear expansion coefficients are formed on a copper foil. .

[Private Technology]

As electronic devices become more complex, much of the wiring has been replaced by circuit boards, which are known to save space, weight, labor and are more reliable than wires. In addition, as the light and short size of electronic devices has progressed, many of them have been replaced by flexible printed circuits (FPCs) from conventional printed circuit boards (PCBs). In particular, the FPC market is expanding with the advent of mobile phones, notebooks, camcorders, and liquid crystal displays (LCDs).

Single-sided flexible circuit boards can be roughly divided into three-layer copper clad laminates (3CCL: 3-copper-clad-laminates) and two-layer copper clad laminates (2CCL: 2-copper-clad-laminates). The three-layer copper-clad laminate refers to a flexible circuit board obtained by thermally compressing a copper foil and a polyimide film using an epoxy or acrylic adhesive, and the two-layer copper-clad flexible circuit board is formed of only polyimide and copper foil without using an adhesive. Non-adhesive type flexible circuit board.

Existing three-layer copper-clad flexible circuit boards have a problem in that curvature occurs during cooling while adding thermal history such as thermocompression to the adhesive, and the flame retardancy or heat resistance is degraded due to the adhesive layer.

In an effort to solve this problem, research has been conducted on a two-layer copper laminated flexible circuit board that does not include an adhesive layer.

There are two ways to make a 2-layer copper clad flexible printed circuit board. One of them is a method of depositing and stacking copper (Cu) on the surface of a polyimide (PI) film once formed (sputtering method), and the other is polyamic acid, which is a precursor of polyimide to copper foil. After varnish is applied and dried, it is thermally imidized or chemically imidized (cured) to form a laminate (casting method).

Japanese Patent Laid-Open No. 8-250860 discloses a flexible circuit board which is intended to reduce the occurrence of curl by forming a laminated polyimide insulating layer having a three-layer structure on a two-layer copper-clad flexible printed circuit board manufactured by a casting method. Is proposed. The flexible circuit board is formed by applying a polyamic acid varnish to one surface of a copper foil as a conductor and imidizing the first polyimide resin layer having a linear expansion coefficient of 20 × 10 ⁻⁶ / K or more, and a linear expansion coefficient formed thereon by the same method. A polyolefin having a three-layer structure in which a second polyimide resin layer of less than 10 × ⁶ / K and a third polyimide resin layer having a linear expansion coefficient of 20 × 10 ⁻⁶ / K or more coated thereon are formed. It includes a mid type resin layer. When the linear expansion coefficients of the first, second, and third polyimide resin layers are K ₁ , K ₂ , and K ₃ , K ₃ ≧ K ₁ > K ₂ is satisfied.

However, in the flexible circuit board having the polyimide resin layer having the three-layer structure, since a plurality of polyimide resins having different linear expansion coefficients are used, three types of polyamic acid varnishes, which are polyimide precursors, must be synthesized and coated. There is a problem in the process.

The present invention is to solve the above problems, an object of the present invention is to form a polyimide layer of two layers having different linear expansion coefficients with one kind of polyamic acid varnish, so that less bending and excellent workability 2 It is to provide a layer copper clad laminated flexible circuit board and a method of manufacturing the same.

The present invention, in order to achieve the above object, copper foil; A first polyimide resin layer positioned on one surface of the copper foil and having a linear expansion coefficient of 10 × 10 ⁻⁶ to 17 × 10 ⁻⁶ / K; And a second polyimide composed of the same component as the first polyimide resin layer, positioned on one surface of the first polyimide resin layer, and having a linear expansion coefficient of 17 × 10 ⁻⁶ / K to 20 × 10 ⁻⁶ / K. Provided is a flexible circuit board including a mid resin layer.

The present invention also comprises the steps of first coating a polyamic acid varnish on one surface of the copper foil; Drying the primary coated polyamic acid varnish to a temperature of two or more steps in a temperature range of 100 to 250 ° C. to form a first polyamic acid layer; Second coating on the dried polyamic acid varnish a varnish of the same component as the primary coated polyamic acid varnish again; Drying the secondary coated polyamic acid varnish at a temperature of 100 to 200 ° C. to form a second polyamic acid layer; Imidating the first polyamic acid layer and the second polyamic acid layer at a temperature of 200 to 400 ° C. to form a first polyimide resin layer and a second polyimide resin layer; And it provides a method of manufacturing a flexible circuit board comprising the step of etching the copper foil.

Hereinafter, the present invention will be described in more detail.

MEANS TO SOLVE THE PROBLEM This inventor noticed that the linear expansion coefficient of a polyimide resin changes with drying of polyamic-acid varnish which is a precursor of polyimide-type resin, and imidation conditions, and coats one kind of polyamic-acid varnish twice, after each coating, By adjusting the drying process conditions, it has been found that the curvature of the flexible circuit board can be prevented and the workability of the flexible circuit board manufacture is improved, and the present invention has been completed.

In the manufacture of flexible circuit boards, if the polyamic acid varnish is coated and then dried at a relatively high temperature to partially cure, the polyimide formed through the imidization process has a relatively low coefficient of linear expansion. On the other hand, if the polyamic acid varnish is coated and then dried at a relatively low temperature to prevent hardening during drying, the polyimide formed through the imidization process has a relatively high coefficient of linear expansion. This means that when drying at a relatively low temperature, the chains can be constantly arranged because there is sufficient time to arrange enough before the polyamic acid is cured, so that the coefficient of linear expansion is larger.

For convenience, the polyimide resin first coated on the copper foil side is called a first polyimide resin, and the polyimide resin coated on the first polyimide resin for the second time is called a second polyimide resin.

The linear expansion coefficient K _c of the copper foil usually has a value of 17 × 10 ⁻⁶ to 19 × 10 ⁻⁶ / K. In order to prevent curls occurring after the manufacture of the flexible circuit board, first, the linear expansion coefficient K ₁ of the first polyimide resin layer should preferably have a value of 10 × 10 ⁻⁶ to 17 × 10 ⁻⁶ / K. At the same time, the linear expansion coefficient K ₂ of the second polyimide resin layer is higher than the linear expansion coefficient K ₁ of the first polyimide resin layer in order to suppress the curl of the first polyimide resin layer itself generated after etching the copper foil. It should be large and preferably have a value of 17 × 10 ⁻⁶ to 20 × 10 ⁻⁶ / K.

In the flexible circuit board of the present invention, it is preferable that the first polyimide resin layer is formed to have a thickness of 3 to 30 µm, and the second polyimide resin layer is formed to have a thickness of 5 to 30 µm. Preferably, the sum of the thicknesses of the first polyimide resin layer and the second polyimide resin layer is preferably formed in a thickness of 10 to 50 μm.

In the flexible circuit board of the present invention, the first polyimide resin layer and the second polyimide resin layer are coated with a polyamic acid varnish obtained through the imidization reaction between dianhydride and diamine on copper foil, and then dried. It can be manufactured by the casting method which reacts and forms.

Examples of dianhydrides used in the preparation of the polyamic acid include pyromellitic dianhydride (PMDA), 3,3 ', 4,4'-biphenyltetracarboxylic dianhydride (BPDA: 3). , 3 ', 4,4'-biphenyltetracarboxylic dianhydride), 3,3'4,4'-benzophenonetetracarboxylic dianhydride (BTDA: 3,3', 4,4'-benzophenonetetracarboxilic dianhydride), 4, 4'-oxydiphthalic anhydride (ODPA: 4,4'-oxydiphthalic anhydride), 4,4 '-(4,4'-isopropylidenediphenoxy) -bis- (phthalic anhydride) ( BPADA: 4,4 '-(4,4'-isopropylidenediphenoxy) -bis (phthalic anhydride), 2,2'-bis- (3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA: 2, 2'-bis- (3,4-dicarboxyphenyl) hexafluoropropane dianhydride), ethylene glycol bis (anhydro-trimellitate), or 3,4,3 ', 4' -Diphenylsulfone tetracarboxylic dianhydra De (DSDA: 3,4,3 ', 4'-diphenylsulfone tetracarboxylic dianhydride) is preferably used by selecting one or more from the dianhydride and the like.

In addition, examples of the diamine used in the preparation of the polyamic acid are p-phenylene diamine (p-PDA: p-phenylene diamine), m-phenylene diamine (m-PDA: m-phenylene diamine), 4,4 ' -Oxydianiline (4,4'-ODA: 4,4'-oxydianiline), 3,4'-oxydianiline (3,4'-ODA: 3,4'-oxydianiline), 2,2-bis ( 4-4 [aminophenoxy] -phenyl) propane (BAPP: 2,2-bis (4- [4-aminophenoxy] -phenyl) propane), 2,2'-dimethyl-4,4'-diamino biphenyl (m-TB-HG: 2,2'-Dimethyl-4,4'-diamino biphenyl), 1,3-bis (4-aminophenoxy) benzene (TPER: 1,3-bis (4-aminophenoxy) benzene ), 2,2-bis (4- [3-aminophenoxy] phenyl) sulfone (m-BAPS: 2,2-bis (4- [3-aminophenoxy] phenyl) sulfone), 4,4'-diamino In diamines such as benzanilide (DABA: 4,4'-diamino benzanilide) or 4,4'-bis (4-aminophenoxy) biphenyl (4,4'-bis (4-aminophenoxy) biphenyl) It is preferable to select and use species or more.

Flexible circuit board of the present invention comprises the steps of first coating a polyamic acid varnish on one surface of the copper foil; Drying the primary coated polyamic acid varnish to a temperature of two or more steps in a temperature range of 100 to 250 ° C. to form a first polyamic acid layer; Second coating the polyamic acid varnish again on the dried first polyamic acid layer; Drying the secondary coated polyamic acid varnish at a temperature of 100 to 200 ° C. to form a second polyamic acid layer; Imidating the first polyamic acid layer and the second polyamic acid layer at a temperature of 200 to 400 ° C. to form a first polyimide resin layer and a second polyimide resin layer; And it is produced through the step of etching the copper foil.

First, the polyamic acid varnish coated on one surface of copper foil is pyromellitic dianhydride (PMDA), 3,3 ', 4,4'-biphenyltetracarboxylic dianhydride (PMDA) in the presence of a solvent. BPDA: 3,3 ', 4,4'-biphenyltetracarboxylic dianhydride), 3,3'4,4'-benzophenonetetracarboxylic dianhydride (BTDA: 3,3', 4,4'-benzophenonetetracarboxilic dianhydride) , 4,4'-oxydiphthalic anhydride (ODPA: 4,4 '-(4,4'-isopropylidenediphenoxy) -bis- (phthalic anhydride) Ride) (BPADA: 4,4 '-(4,4'-isopropylidenediphenoxy) -bis (phthalic anhydride), 2,2'-bis- (3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA : 2,2'-bis- (3,4-dicarboxyphenyl) hexafluoropropane dianhydride), ethylene glycol bis (anhydro-trimellitate), or 3,4,3 ' , 4'-diphenylsulfone tetracarboxylic dian One or more dianhydrides selected from hydrides (DSDA: 3,4,3 ', 4'-diphenylsulfone tetracarboxylic dianhydride), p-phenylene diamine (p-PDA) and m-phenylene diamine (m-PDA: m-phenylene diamine), 4,4'-oxydianiline (4,4'-ODA: 4,4'-oxydianiline), 3,4'-oxydianiline (3,4'-ODA : 3,4'-oxydianiline), 2,2-bis (4-4 [aminophenoxy] -phenyl) propane (BAPP: 2,2-bis (4- [4-aminophenoxy] -phenyl) propane), 2 , 2'-dimethyl-4,4'-diamino biphenyl (m-TB-HG: 2,2'-Dimethyl-4,4'-diamino biphenyl), 1,3-bis (4-aminophenoxy) Benzene (TPER: 1,3-bis (4-aminophenoxy) benzene), 2,2-bis (4- [3-aminophenoxy] phenyl) sulfone (m-BAPS: 2,2-bis (4- [3 -aminophenoxy] phenyl) sulfone), 4,4'-diamino benzanilide (DABA: 4,4'-diamino benzanilide), or 4,4'-bis (4-aminophenoxy) biphenyl (4,4 It can be prepared by imidization of one or more diamines selected from '-bis (4-aminophenoxy) biphenyl) and the like.

Solvents that may be used to prepare the polyamic acid varnish include N-methylpyrrolidinone (NMP: N-methylpyrrolidinone), N, N-dimethylacetamide (DMAc: N, N-dimethylacetamide), and tetrahydrofuran (THF: tetrahydrofuran), N, N-dimethylformamide (DMF: N, N-dimethylformamide), dimethyl sulfoxide (DMSO: dimethylsulfoxide), cyclohexane, acetonitrile or a mixture thereof is preferred. However, the kind of the solvent is not limited thereto. Moreover, it is also possible to add a small amount of other dianhydride, another diamine, or another compound other than the said compound as needed.

The polyamic acid varnish is preferably prepared to have a viscosity of 2,000 to 50,000 cps.

When apply | coating the polyamic-acid varnish manufactured as mentioned above to copper foil, copper foil with a thickness of 5-40 micrometers can be used. In addition, the thickness of the polyamic acid varnish to be applied may vary depending on the concentration, but finally, the first polyimide resin layer is adjusted to have a thickness of 3 to 30 μm after the imidization reaction is finished.

The flexible circuit board of the present invention is formed with two layers of polyimide resin layers having different linear expansion coefficients in order to prevent curl. The linear expansion coefficient of the polyimide resin layer may be adjusted by drying conditions, and the linear expansion coefficient of the second polyimide resin layer is preferably larger than the linear expansion coefficient of the first polyimide resin layer. At the time of formation, the drying temperature of the polyamic acid varnish must be higher than the polyamic acid varnish drying temperature of the second polyimide resin layer.

More specifically, the primary coated polyamic acid varnish is dried at a temperature of two or more stages in the temperature range of 100 to 250 ℃, in this process the semi-curing of the polyamic acid occurs. At this time, the first drying temperature is 120 to 180 ℃, the second drying temperature is preferably 180 to 250 ℃. If the primary drying temperature is less than 120 ℃, there may be a problem that the coating surface and the copper foil surface is adhered during winding due to insufficient drying does not occur, and if it exceeds 180 ℃, the coating surface becomes uneven or wrinkles on the copper foil side Problems may arise. In addition, when the secondary drying temperature is less than 180 ° C., the semi-curing does not occur sufficiently, so that the coefficient of linear expansion is difficult to control, and when it exceeds 250 ° C., the curing occurs excessively, so that the polyamic acid coating and imidization reaction of the second polyimide resin layer occurs. It is not preferable that peeling with the first polyimide resin layer occurs.

As described above, the first polyimide resin layer is dried at a temperature range of at least two or more stages, and then the same kind of polyamic acid varnish is secondarily applied thereon. In this case as well, the thickness of the second polyamic acid varnish layer to be applied may vary depending on the concentration. However, the thickness of the second polyimide resin layer after the imidization reaction is finished is 5 to 30 μm. The second applied polyamic acid varnish is dried at a temperature of 100 to 200 ℃.

After the polyamic acid varnish of the first polyimide resin layer and the polyamic acid varnish of the second polyimide resin layer are dried, the imidation reaction is performed at a temperature of 200 to 400 ° C. to form the first polyimide resin layer. And a second polyimide resin layer are formed.

In the flexible circuit board manufactured as described above, the linear expansion coefficient of the formed first polyimide resin layer is 10 × 10 ⁻⁶ to 17 × 10 ⁻⁶ / K, and the linear expansion coefficient of the second polyimide resin layer is 17 ×. is from 10 ^-6 to 20 × 10 ^-6 / K is preferable.

Hereinafter, preferred embodiments of the present invention are described. However, the following examples are only preferred embodiments of the present invention, and the present invention is not limited to the following examples.

Example 1

16.722 g of p-phenylene diamine (p-PDA) (0.154 mol) and nitrogen were flown into a 1000 mL four-necked round bottom flask equipped with thermometer, stirrer and nitrogen inlet and powder dispensing funnel. 500 mL of N-methylpyrrolidinone (NMP) was added to 13.272 g of 4,4'-oxydianiline (4,4'-ODA) (0.066 mol) and stirred to dissolve completely. While cooling the solution below 15 ° C, 65.006 g of 3,3 ', 4,4'-biphenyltetracarboxylic dianhydride (BPDA) (0.22 mol) was slowly added and polymerized while stirring to give a viscosity of 15,000 cps. The polyamic acid varnish of was obtained.

The prepared polyamic acid varnish was coated on a rolled copper foil (Japan energy) having a thickness of 18 μm using a doctor blade. At this time, the coating thickness was adjusted so that the final polyimide resin layer after the curing process was 10 μm thick. After the coating of the polyamic acid varnish was finished, it was dried for 3 minutes at 140 ℃, and then dried for 5 minutes at 200 ℃ to form a first polyamic acid layer.

The final polyimide resin layer after the curing process was coated on the first polyamic acid layer in the same manner so as to have a thickness of 15 μm, and then dried at 140 ° C. for 5 minutes to form a second polyamic acid layer.

After the second polyamic acid layer was formed, the temperature was raised to 350 ° C. to proceed with the imidization reaction, and copper foil was etched to prepare a two-layer copper clad laminate (2CCL) flexible circuit board.

In this case, the thickness of the total polyimide resin layer including the first polyimide resin layer and the second polyimide resin layer was 25 μm, and the manufactured flexible circuit board was almost flat, and almost no curl after etching. There was no.

The linear expansion coefficient of the polyimide resin layer after the etching was measured at 18 × 10 ⁻⁶ / K, and the linear expansion coefficient of the first polyimide resin layer near the copper foil was 16 × 10 ⁻⁶ / K and the second polyimide. The linear expansion coefficient of the resin layer was 19 × 10 ⁻⁶ / K.

Example 2

14.802 g of p-PDA (0.137 mol) and 18.275 g of 4,4 'were flowed into a 1000 mL four-necked round bottom flask equipped with thermometer, stirrer, nitrogen inlet and powder dispensing funnel. 500 mL of NMP was added to -ODA (0.091 mol) and stirred to dissolve completely.

46.992 g of BPDA (0.160 mol) and 14.930 g of PMDA (0.069 mol) were slowly added to the solution while cooling to 15 ° C. or lower, followed by polymerization while stirring to obtain a polyamic acid varnish having a viscosity of 17,000 cps.

2 in the same manner as in Example 1, except that the polyamic acid varnish was coated on a rolled copper foil having a thickness of 18 μm so that the final first polyimide resin layer was 12 μm and the thickness of the second polyimide resin layer was 13 μm. A layer copper clad laminated flexible circuit board was manufactured.

In the manufactured flexible circuit board, the total thickness of the first polyimide resin layer and the second polyimide resin layer was 25 µm, and the obtained flexible circuit board was almost flat before and after etching. The linear expansion coefficient of the polyimide resin layer after the etching was measured at 17.5 × 10 ⁻⁶ / K, and the first polyimide resin layer near the copper foil was 15 × 10 ⁻⁶ / K and the second polyimide thereon. The resin layer was 19.5 × 10 ^-6 / K.

Comparative Example 1

A flexible circuit board was manufactured in the same manner as in Example 1, except that the polyamic acid varnish was coated at a time so that the final polyimide resin layer had a thickness of 25 μm on the rolled copper foil having a thickness of 18 μm. The coefficient of linear expansion of the polyimide resin layer measured after the etching was 16 × 10 ⁻⁶ / K, the curvature radius of the curl before copper foil etching was 400 mm toward the copper foil, and the curvature radius of the curvature after copper foil etching was 30 toward the copper foil. mm.

Comparative Example 2

The polyamic acid varnish was coated on a rolled copper foil having a thickness of 18 μm so that the thickness of the final first polyimide resin layer was 10 μm, and firstly dried at 140 ° C. for 3 minutes, and then secondarily dried at 260 ° C. for 5 minutes to obtain a first thickness. After forming the polyamic acid layer, the same method as in Example 1 was applied except that the polyamic acid varnish was coated and dried at 140 ° C. for 5 minutes so that the thickness of the final second polyimide resin layer was 15 μm. A circuit board was prepared.

In the manufactured flexible circuit board, the total polyimide resin layer had a thickness of 25 μm, but peeling occurred between the two layers, so that a two-layer copper clad flexible circuit board could not be obtained.

Table 1 below describes the manufacturing conditions, the thickness of the polyimide resin layer, the coefficient of linear expansion, and the degree of curl of the two-layer copper-clad flexible circuit boards prepared by Examples 1 and 2 and Comparative Examples 1 and 2. It was.

TABLE 1

Example 1 Example 2 Comparative Example 1 Comparative Example 2 Primary drying temperature (° C.) of the first polyamic acid layer 140 140 140 140 Secondary drying temperature (° C.) of the first polyamic acid layer 200 200 200 260 Drying temperature of the second polyamic acid layer (℃) 140 140 - 140 Thickness of the First Polyimide Layer (µm) 10 12 25 10 Thickness of the Second Polyimide Layer (μm) 15 13 15 Coefficient of Linear Expansion of First Polyimide Layer 16 × 10 ^-6 / K 15 × 10 ^-6 / K 16 × 10 ^-6 / K Unable to measure due to peeling Coefficient of Linear Expansion of Second Polyimide Layer 19 × 10 ^-6 / K 19.5 × 10 ^-6 / K Radius of curvature of the flexible circuit board Before etching plane plane 400 mm towards copper foil After etching plane plane 30 mm towards copper foil

As shown in Table 1, Comparative Example 1 in which only one polyimide layer was formed was severely warped before and after etching, and the second drying temperature of the first polyamic acid layer was 260 ° C. In this case, the delamination occurred, but it can be seen that the flexible circuit boards manufactured according to Examples 1 and 2 of the present invention are in a planar state in which warpage does not occur before and after etching.

The flexible circuit board of the present invention is formed of two kinds of polyimide layers having different linear expansion coefficients by using one kind of polyamic acid varnish, which has excellent reliability in dimensional stability, planarity after etching, etc., and excellent workability in circuit formation. In particular, as one kind of polyamic acid varnish, it is possible to manufacture a flexible circuit board having a polyimide resin layer having a two-layer structure having a different linear expansion coefficient, thereby reducing the effort of varnish manufacturing. .

Claims (9)

Copper foil; A first polyimide resin layer positioned on one surface of the copper foil and having a linear expansion coefficient of 10 × 10 ⁻⁶ to 17 × 10 ⁻⁶ / K; And a second polyimide composed of the same component as the first polyimide resin layer, positioned on one surface of the first polyimide resin layer, and having a linear expansion coefficient of 17 × 10 ⁻⁶ / K to 20 × 10 ⁻⁶ / K. Mid resin layer Flexible circuit board comprising a. The thickness of the first polyimide resin layer is 3 to 30 ㎛, the thickness of the second polyimide resin layer is 5 to 30 ㎛, the first polyimide resin layer and the second polyimide resin A flexible circuit board having a sum of the thicknesses of the layers of 10 to 50 µm. The flexible circuit board of claim 1, wherein the copper foil has a thickness of 5 to 40 μm. The method of claim 1, wherein the first polyimide resin layer and the second polyimide resin layer Pyromellitic dianhydride (PMDA), 3,3 ', 4,4'-biphenyltetracarboxylic dianhydride (BPDA: 3,3', 4,4'-biphenyltetracarboxylic dianhydride), 3 , 3'4,4'-benzophenonetetracarboxylic dianhydride (BTDA: 3,3 ', 4,4'-benzophenonetetracarboxilic dianhydride), 4,4'-oxydiphthalic anhydride (ODPA: 4, 4'-oxydiphthalic anhydride), 4,4 '-(4,4'-isopropylidenediphenoxy) -bis- (phthalic anhydride) (BPADA: 4,4'-(4,4'-isopropylidenediphenoxy) ) -bis (phthalic anhydride), 2,2'-bis- (3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA: 2,2'-bis- (3,4-dicarboxyphenyl) hexafluoropropane dianhydride ), Ethylene glycol bis (anhydro-trimellitate) (TMEG: ethylene glycol bis (anhydro-trimellitate)), and 3,4,3 ', 4'-diphenylsulfone tetracarboxylic dianhydride (DSDA: 3,4,3 ', 4'-diphenylsulfone tetracarboxylic dianhydride) One or more dianhydrides selected p-phenylene diamine (p-PDA: p-phenylene diamine), m-phenylene diamine (m-PDA: m-phenylene diamine), 4,4'-oxydianiline (4,4'-ODA: 4, 4'-oxydianiline), 3,4'-oxydianiline (3,4'-ODA: 3,4'-oxydianiline), 2,2-bis (4-4 [aminophenoxy] -phenyl) propane (BAPP : 2,2-bis (4- [4-aminophenoxy] -phenyl) propane), 2,2'-dimethyl-4,4'-diamino biphenyl (m-TB-HG: 2,2'-Dimethyl- 4,4'-diamino biphenyl), 1,3-bis (4-aminophenoxy) benzene (TPER: 1,3-bis (4-aminophenoxy) benzene), 2,2-bis (4- [3-amino Phenoxy] phenyl) sulfone (m-BAPS: 2,2-bis (4- [3-aminophenoxy] phenyl) sulfone), 4,4'-diamino benzanilide (DABA: 4,4'-diamino benzanilide) And at least one diamine selected from the group consisting of 4,4'-bis (4-aminophenoxy) biphenyl (4,4'-bis (4-aminophenoxy) biphenyl) A flexible circuit board which is an imidation reaction product of a polyamic acid varnish obtained from the product. Firstly coating a polyamic acid varnish on one surface of the copper foil; Drying the primary coated polyamic acid varnish to a temperature of two or more steps in a temperature range of 100 to 250 ° C. to form a first polyamic acid layer; Second coating on the dried polyamic acid varnish a varnish of the same component as the primary coated polyamic acid varnish again; Drying the secondary coated polyamic acid varnish at a temperature of 100 to 200 ° C. to form a second polyamic acid layer; Imidating the first polyamic acid layer and the second polyamic acid layer at a temperature of 200 to 400 ° C. to form a first polyimide resin layer and a second polyimide resin layer; And Etching the copper foil Method of manufacturing a flexible circuit board comprising a. The method of claim 5, wherein the thickness of the first polyimide resin layer is 3 to 30 ㎛, the thickness of the second polyimide resin layer is 5 to 30 ㎛, the first polyimide resin layer and the second polyimide resin A method of manufacturing a flexible circuit board in which the sum of the thicknesses of the layers is 10 to 50 µm. The method of claim 5, wherein the copper foil has a thickness of 5 to 40 μm. The linear expansion coefficient of the first polyimide resin layer is 10 × 10 ^-6 to 17 × 10 ^-6 / K, and the linear expansion coefficient of the second polyimide resin layer is 17 × 10 ^-6 to 20 Method for producing a flexible circuit board of 10 × ⁶ / K. The method of claim 5, wherein the polyamic acid varnish (varnish) Pyromellitic dianhydride (PMDA), 3,3 ', 4,4'-biphenyltetracarboxylic dianhydride (BPDA: 3,3', 4,4'-biphenyltetracarboxylic dianhydride), 3 , 3'4,4'-benzophenonetetracarboxylic dianhydride (BTDA: 3,3 ', 4,4'-benzophenonetetracarboxilic dianhydride), 4,4'-oxydiphthalic anhydride (ODPA: 4, 4'-oxydiphthalic anhydride), 4,4 '-(4,4'-isopropylidenediphenoxy) -bis- (phthalic anhydride) (BPADA: 4,4'-(4,4'-isopropylidenediphenoxy) ) -bis (phthalic anhydride), 2,2'-bis- (3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA: 2,2'-bis- (3,4-dicarboxyphenyl) hexafluoropropane dianhydride ), Ethylene glycol bis (anhydro-trimellitate) (TMEG: ethylene glycol bis (anhydro-trimellitate)), and 3,4,3 ', 4'-diphenylsulfone tetracarboxylic dianhydride (DSDA: 3,4,3 ', 4'-diphenylsulfone tetracarboxylic dianhydride) One or more dianhydrides selected p-phenylene diamine (p-PDA: p-phenylene diamine), m-phenylene diamine (m-PDA: m-phenylene diamine), 4,4'-oxydianiline (4,4'-ODA: 4, 4'-oxydianiline), 3,4'-oxydianiline (3,4'-ODA: 3,4'-oxydianiline), 2,2-bis (4-4 [aminophenoxy] -phenyl) propane (BAPP : 2,2-bis (4- [4-aminophenoxy] -phenyl) propane), 2,2'-dimethyl-4,4'-diamino biphenyl (m-TB-HG: 2,2'-Dimethyl- 4,4'-diamino biphenyl), 1,3-bis (4-aminophenoxy) benzene (TPER: 1,3-bis (4-aminophenoxy) benzene), 2,2-bis (4- [3-amino Phenoxy] phenyl) sulfone (m-BAPS: 2,2-bis (4- [3-aminophenoxy] phenyl) sulfone), 4,4'-diamino benzanilide (DABA: 4,4'-diamino benzanilide) And at least one diamine selected from the group consisting of 4,4'-bis (4-aminophenoxy) biphenyl (4,4'-bis (4-aminophenoxy) biphenyl) Method for producing a flexible circuit board obtained from.