WO2009002083A2

WO2009002083A2 - Manufacturing method of fpcb

Info

Publication number: WO2009002083A2
Application number: PCT/KR2008/003622
Authority: WO
Inventors: Kwang Choon Chung; Myoung Seon Gong; Jin O Lee; Myoung Hee Joo; Hee Yong Ahn
Original assignee: Haeun Chemtec Co Ltd
Current assignee: Haeun Chemtec Co Ltd
Priority date: 2007-06-25
Filing date: 2008-06-25
Publication date: 2008-12-31
Anticipated expiration: 2009-12-25
Also published as: CN101682994B; KR100945039B1; JP5021806B2; TW200922407A; TWI406608B; CN101682994A; JP2010522442A; KR20080113709A; WO2009002083A3

Abstract

The present invention relates to a flexible printed circuit board (FPCB) using a foam sheet that is foamed at a high temperature. The foam sheet is formed by stacking, sequentially, a substrate, a surface treating layer, an adhesive layer and a release film, and the adhesive layer contains thermally expandable microspheres and an interactive copolymer. By employing the interactive copolymer resin of the foam sheet as an adhesive layer, the foam sheet has an excellent adhesive strength and can be simply separated from an adherend by high temperature heating.

Description

[DESCRIPTION]

[invention Title]

MANUFACTURING METHOD OF FPCB

[Technical Field]

The present invention relates to a flexible printed circuit board (FPCB) using a foam sheet that is foamed at a high temperature. More particularly, the present invention relates to a foam sheet for a FPCB that has an excellent adhesive strength by adoption of an interactive copolymer resin as an adhesive layer and can be simply separated from an adherend by high temperature heating, and a method for manufacturing a FPCB using the same.

[Background Art]

In recent, demands for a FPCB have been rapidly increased as an electronic product such as a mobile phone, a digital camera, a MP3 player, a DMB player or a multifunctional product which incorporates two or more functions of these apparatuses is miniaturized and light weighted.

In a conventional process for manufacturing a FPCB, due to flexibility of the FPCB, the process is carried out after securing the FPCB onto a substrate called as a carrier or a backup board using an adhesive tape. In recent, a method for manufacturing a FPCB using a foam sheet has been studied to facilitate the separation of the FPCB from the adhesive tape after completion of the process.

In general, the foaming sheet is mainly used as a support for adhesion or cut when manufacturing a multi- layer chip condenser or an inductor chip. At this time, a condition of losing the adhesive force is at a temperature range of 120 through 130⁰C and a normal pressure, and the adhesive force and foaminess are the important physical factors. However, in the case of manufacturing the FPCB, the foam sheet should have no break out of foam cells at a condition of over 40 minutes at

150 to 160°C and 40kg/cm², which is a condition for hot press lamination and should be foamed at higher temperature, i.e. approximately 180°C after completion of the hot press lamination. Also, it is necessary to avoid contamination of the FPCB due to transcription of adhesive resin during the process of manufacturing the FPCB.

However, since the conventional foam sheet begins to be foamed at about 150°C even how high the foaming temperature of the foam sheet is, the conventional foam sheet has a limitation to be used at higher temperature.

Meanwhile, besides the method for manufacturing a FPCB using the foam sheet foamed at a high temperature as described above, there has been proposed a method of manufacturing FPCBs by attaching the FPCBs to both sides of a double-sided foam sheet for raising the profitability of the FPCB manufacturer. In this method, a double- sided foam sheet is used to secure FPCBs during the process. By using this method, two products are produced at the same time through a single process without changing existing process and thus the productivity per unit time can be doubled in the process for manufacturing the FPCB. In order to give flexibility in the process of manufacturing a multi- layered FPCB or a rigid FPCB, it is inevitable to manufacture a single- sided FPCB. Though there is a manufacturer equipped with a manufacturing facility capable of working simultaneously with respect to two sides at the same time, only single side can be processed and thus there is a limitation in enhancing the productivity. However, in current process for manufacturing the FPCB, since it is possible to simultaneously process two sides by attaching FPCBs onto both sides of a foam sheet using a double-side adhesive foam sheet, it is possible to double the productivity. Also, it is possible to reduce consumption of secondary materials used in the process of manufacturing the FPCB since the foam sheet can replace the role of a carrier film or a cushion film in forming process .

The foam sheet employed to proceed a process by attaching single-sided FPCBs to both sides of the foam sheet is required not to be foamed or thermally deformed at a condition of over

40 minutes at a temperature of 16O⁰C and a pressure of 40kg/cm², to have a chemical resistance to chemicals that come in contact with the foam sheet during the process of manufacturing the FPCB, to be easily foamed at 170 to 190°C after completion of the process of manufacturing the FPCB, and to avoid contamination of the FPCB due to transcription of adhesive resin during the process of manufacturing the FPCB.

Conventionally, as an adhesive for a foam sheet product, an acrylic adhesive mostly containing cross-linking functional group and a cross-linking agent are used. In order to raise the foaming temperature by causing this adhesive to cohesively act on microspheres and restrict expansion of the microspheres, it is necessary for the adhesive to contain a high ratio of the cross-linking function group and is also necessary to use a large amount of the cross-linking agent. Therefore, viscosity is noticeably changed and gelation is proceeded as the cross-linking is proceeded with lapse of time, and physical properties at the beginning portion and the ending portion of a coated product become different when the coating is carried out for more than four hours. Use of the foam sheet for reducing processes of a double access type FPCB, a single- sided FPCB and a thin FPCB has been proposed in Korean Patent Application Laid-Open Nos . 10-2002-0060659, 10-2002-0060656 and 10-2002-0060657, and Korean Patent No. 10-0514611. However, the manufacture of a FPCB is carried out through the steps of deburring - attaching a dry film - exposing - developing the dry film - corroding Cu - drying - winding - roll cutting - tag bonding - forming a coverlay (hot pressing) - deburring - surface treating - electroless plating - punching. Particularly, the coverlay forming process is carried out under a condition of over 40 minutes at 150 to 160°C and 40kg/cm², and the existing foam sheet can be employed only to the step prior to the coverlay forming process since the existing foam sheet is foamed during the coverlay forming process under this condition. Consequently, the conventional foam sheet is not practical.

Also, Japanese Patent Laid-Open No. 2003-338678 by Nitto electric company, Ltd. (Japan) discloses use of a double-sided foam sheet as a carrier film in a process by attaching single- sided FPCBs to both sides of the foam sheet. However, this can be partially applied only in initial stages of the manufacturing process using currently available foam sheet.

[Disclosure] [Technical Problem]

An object of the present invention, to solve the above problem, is to provide a foam sheet for high temperature foaming, which has an excellent adhesive strength by adoption of an interactive copolymer resin as an adhesive layer and can be simply separated from an adherend by heating at more than

18O⁰C, and a method for manufacturing a FPCB using the same. Another object of the present invention is to provide a method for manufacturing a FPCB, which can produce two single- sided FPCB in a single process and thus improve productivity in the FPCB manufacturing process, as compared to conventional process for manufacturing a single-sided FPCB, by way of using a double-sided foam sheet that maintains adhesive force even under a high temperature and high pressure condition of over

40 minutes at 160°C and 40kg/cm² during a hot pressing process and has a chemical resistance not allowing penetration of an etchant liquid during an etching process.

[Technical Solution]

The present invention relates to a foam sheet for a FPCB usable at a high temperature, which has an excellent adhesive strength by adoption of an interactive copolymer resin as an adhesive layer and can be simply separated from an adherend by heating at more than 180⁰C, and a method for manufacturing a FPCB using the same.

Hereinafter, a method for manufacturing a foam sheet for a FPCB usable at a high temperature and a method for manufacturing a FPCB using the same will be described in detail. With respect to the method for manufacturing a FPCB, a method for manufacturing a single-sided FPCB and a method for manufacturing FPCBs by attaching the FPCBs to both sides of a foam sheet will be sequentially described.

The foam sheet for a high temperature foaming of the present invention is an adhesive sheet, in which an adhesive layer is formed using adhesive resin mixed with interactive copolymers containing thermally expandable microspheres and the adhesive layer can be separated at a high temperature as the adhesive layer is foamed or expanded by heating.

Fig. 1 is a cross-sectional view illustrating a single- sided foam sheet for the manufacture of a single-sided foam- sheet, and Fig. 2 is a cross-sectional view illustrating a double-sided foam sheet for the manufacture of FPCBs by attaching the FPCBs to both sides of the foam sheet.

As shown in Fig. 1, in the single-sided foam sheet in accordance with the present invention, a substrate 1, a surface treatment layer 2, an adhesive layer 3 and a release film 4 are sequentially stacked and the adhesive layer 3 contains thermally expandable microspheres and interactive copolymers .

The substrate 1 may include a appropriate commercialized thin film such as PET and has a thickness of preferably less than 250 μm and most preferably 25 to 100 μm, but not limited thereto. The surface treatment layer 2 that causes strong chemical bond between the substrate and the adhesive layer is formed by performing, on the substrate 1, a chemical surface oxidation treatment such as chromic acid, ozone, corona, flame and ionizing radiation treatments or treating, on the substrate 1, a high molecular compound with high polarity such as hydrolyzed ethylene vinyl acetate and poly vinyl butyral . The surface treatment layer 2 is required not to contaminate an adherend at the time of separation after heating and not to be changed by chemicals used during the process of manufacturing the FPCB or under a forming condition. The surface treatment layer has a thickness of less than 5 μm, and preferably less than 1 μm . Larger thickness of the surface treatment layer results in smaller deformation of the adhesive layer and less lowering in adhesive force after heat treatment to the foam sheet. The surface treatment layer acts to maintain good surface shape and provide large adhesion area when adhering the heat separable adhesive sheet to the adherend, and at the same time, to lessen restriction to foaming or expanding in a surface direction of the adhesive sheet and improve the deformation of the adhesive layer in a waveform when foaming or expanding the adhesive sheet by heating for separating the adhesive sheet from the adherend. In a case of forming the foam sheet by directly coating the foam adhesive layer on the substrate, the foam layer and the substrate are separated from each other when foaming and 5 expanding if no adhesive force exists between the adhesive layer and the substrate. Therefore, it is impossible to achieve the desired object.

The adhesive layer 3 contains thermally expandable microspheres in order to simply separate the adhesive sheet

10 adhered to the adherend from the adherend by heating. By heating the adhesive layer, the thermally expandable microspheres are foamed and expanded to perforate out of the adhesive layer. Therefore, an adhesion area adhesive layer in contact with the adherend is reduced and thus the adhesive

If) sheet can be separated.

The thermally expandable microsphere is a material such as isobutane, propane, pentane, which is easily gasified and shows thermal expandability, and the shells thereof are made of an appropriate material such as vinylidene chloride and

20 acrylonitrile copolymers, which is thermally meltable or is breakable by thermal expansion.

The microspheres having an average particle diameter of

10 to 25 μm facilitates dispersion into the adhesive resin by heating and results in large deformation of the adhesive layer and noticeable lowering in adhesive force. Also, the adhesive layer is formed thicker than the average particle diameter of the microspheres, preferably thicker than the maximum particle diameter of the final microspheres. Further, it is preferable to smooth the surface of the adhesive layer and achieve stable adhesive force before heating. In order to lower the adhesive force of the adhesive layer by heating, it is preferable that the thermally expandable microspheres are foamed and expanded with a volume expansion ratio of more than 10 times and have such a strength that the thermally expandable microspheres do not break at the volume expansion ratio.

Furthermore, since a high temperature of 160⁰C is necessarily required in the process of manufacturing a FPCB, it is preferable to use the microspheres having a foaming start temperature as high as possible.

The content of the thermally expandable microsphere is suitably determined in accordance with desired expansion ratio of the adhesive layer or desired degree of lowering in the adhesive force, but is generally less than 50 parts by weight, preferably 5 to 20 parts by weight, based on 100 parts by weight of the adhesive constituting the adhesive layer.

In order that the adhesive layer can employ the thermally expandable microspheres which start to foam at near 140°C so as to be heated at a temperature range of 170 to 190°C durable in the FPCB manufacturing process, it is necessary to raise the foaming temperature by restricting the expansion of the microspheres .

The interactive copolymers constituting the adhesive layer is an adhesive resin, in which a first copolymer copolymerized with vinyl monomer, vinyl comonomer and carboxyl group-containing vinyl monomer and a second copolymer copolymerized with vinyl monomer, vinyl comonomer and oxazoline group-containing vinyl monomer are mixed. The vinyl monomer and vinyl comonomer constituting the first and second copolymers are used for the purpose of giving adhesive force, cohesion, heat resistance, flexibility, retention, elasticity and so on to the adhesive, if necessary, and may be independently one selected from one or more alkyl group-containing vinyl monomer selected from methylacrylate, methylmethacrylate, ethylacrylate, ethylmethacrylate, propylacrylate, propylmethacrylate, butylacrylate, butylmethacrylate , hexylacrylate, hexylmethacrylate, octylacrylate , 2-ethylhexylacrylate,- one or more hydroxyl group-containing vinyl monomer selected from hydroxyethylacrylate , hydroxyethylmethacrylate , hydroxypropylacrylate , hydroxypropylmethacrylate, hydroxybutylacrylate , hydroxybutylmethacrylate , hydroxyhexylacrylate , hydroxyhexylmethacrylate ; one or more N- substituted amide vinyl monomer selected from N, N- dimethylacrylamide and N, N-dimethylmethacrylamide; one or more alkoxyalkylacrylate vinyl monomer selected from methoxyethylacrylate , methoxyethylmethacrylate , ethoxyethylacrylate and ethoxyethylmethcrylate; one or more vinyl monomer selected from vinylacetate, vinylpropionate, N- vinylpyrrolidone, methylvinylpyrrolidone, vinylpyridine, vinylpiperidone , vinylpyrimidine, vinylpiperazine, vinylpyrazine , vinylpyrrole, vinylimidazole, vinyloxazole, vinylmorpholine , N-vinylcarboxamides , styrene, α-methylstyrene, and N-vinylcaprolactam; one or more cyanoacrylate monomer selected from acrylonitrile and methacrylonitrile ; one or more epoxy group-containing acrylic monomer selected from glycidylacrylate and glycidylmethacrylate ; one or more glycol acryl ester monomer selected from polyethyleneglycolacrylate, polyethyleneglycolmethacrylate , polypropyleneglycolacrylate, polypropyleneglycolmethacrylate , methoxyethyleneglycolacrylate , methoxypolyethyleneglycolmethacrylate, methoxypolypropyleneglycolacrylate and methoxypolypropyleneglycolmethacrylate; one or more acrylic acid ester monomer selected from tetrahydrofurfurylacrylate, tetrahydrofurfurylmethacrylate and 2 -methoxyethylacrylate; one or more monomer selected from isoprene, butadiene, isobutylene and vinyl ether; or a mixture of two or more thereof. The carboxyl group-containing vinyl monomer of the first copolymer is employed for the purpose of giving cross-linking property by radical polymerization, and examples thereof include 2, 3 or more kinds of copolymers selected from a functional monomer such as one or more carboxyl group- containing monomer selected from acrylic acid, methacrylic acid, carboxyethyl acrylate, carboxypentyl acrylate, itaconic acid, maleic acid, fumaric acid and crotonic acid; and one or more acid anhydride monomer selected from maleic anhydride or itaconic anhydride.

The first copolymer is preferably manufactured from vinyl monomer, vinyl comonomer and carboxyl group-containing vinyl monomer at a ratio by weight of 1: 0.5 to 1.5: 0.05 to 0.3. The oxazoline group-containing vinyl monomer of the second copolymer is employed for the purpose of giving cross-linking property by radical polymerization, and examples of the oxazoline group-containing vinyl monomer include 2 -vinyl-2- oxazoline, 2-vinyl-4-vinyl-2-oxazoline, 2 -vinyl-5-vinyl-2- oxazoline, 2-isopropenyl-2-oxazoline, 2-isopropenyl-5-ethyl-2~ oxazoline, 2 -isopropenyl-5-methyl-2 -oxazoline, 2- (vinylbenzyloxy-1-methylethyl) -2-oxazoline, 2- (2-hydroxy-l- methylethyl) acrylate , 2- (2-hydroxy-l-methylethyl) methacrylate or a mixture of two or more thereof .

The second copolymer is preferably manufactured from vinyl monomer, vinyl comonomer and oxazoline group-containing vinyl monomer at a ratio by weight of 1: 0.5 to 1.5: 0.05 to 0.3.

The coating liquid for the adhesive layer of the foam sheet for FPCB in accordance with the present invention is prepared by dispersing the micorspheres into the adhesive mixed with the first and second copolymer after preparing respectively the first copolymer containing carboxyl group and the second copolymer containing oxazoline group. The prepared coating liquid for the adhesive layer is stable without secular change despite long-term storage at room temperature, and can raise the foaming temperature up to 10 to 30°C as compared to that of the conventional foam sheet.

The adhesive layer is required not to be foamed or thermally deformed within 40 minutes at a temperature of 160°C and a pressure of 40kg/cm², to have a chemical resistance to chemicals that come in contact with the adhesive layer during the process of manufacturing the FPCB, and not to contaminate the FPCB due to transcription of adhesive resin during the process of manufacturing the FPCB. Also, adhesive layer is required to be easily foamed within 5 to 10 minutes in a convection oven or 10 seconds to 3 minutes in a hot plate at a temperature of 170 to 190°C that is a heating condition for easy separation from the adherend after completion of the process of manufacturing the FPCB.

Meanwhile, as shown in Fig. 2, in a double-sided foam sheet in accordance with the present invention, a release film 4', an adhesive layer 3', a surface treatment layer 2', a substrate 1, a surface treatment layer 2, an adhesive layer 3 and a release film 4 are sequentially stacked and the adhesive layers 3 and 3' includes thermally expandable microspheres and interactive copolymers. Details of respective components are the same as that of the single-sided foam sheet for the FPCB. Hereinafter, a method for manufacturing the FPCB using the foam sheet in accordance with the present invention will be described.

The manufacture of a single-sided FPCB using a single- sided foam sheet is carried out through the steps of laminating a FCCL and a foam sheet - deburring - laminating a dry film - exposing - developing - etching - separating the dry film - tag bonding - hot pressing - surface treating - punching - separating foam sheet.

Meanwhile, a method for manufacturing FPCBs by attaching the FPCBs to a double-sided foam sheet is also carried out, as described above, through the steps of laminating a FCCL and a foam sheet - deburring - laminating a dry film - exposing - developing - etching - separating the dry film - tag bonding - hot pressing - surface treating - punching - separating foam sheet. However, in the step of laminating the FCCL and the foam sheet, a double-sided foam sheet is used to laminate the FCCLs onto both sides of the foam sheet so that the process can be carried out with respect to two sides of the adhered FCCLs simultaneously, and then the rest steps are proceeded. Consequently, through the separating the foam sheet, two single-sided FPCB can be manufactured at the same time with a single process.

[Advantageous Effects]

The present invention can provides a method for manufacturing a FPCB using a foam sheet that maintain adhesive force even under a condition of high temperature and high pressure during hot pressing process, has a chemical resistance not allowing penetration of an etchant during an etching process and can be simply separated from an adherend by heating of high temperature.

[Description of Drawings] The above and other objects, features and advantages of the present invention will become apparent from the following description of preferred embodiments given in conjunction with the accompanying drawings, in which:

Fig. 1 is a cross-sectional view illustrating a single- sided foam sheet in accordance with the present invention.

Fig. 2 is a cross-sectional view illustrating a double- sided foam sheet in accordance with the present invention.

Fig. 3 is a graph showing a working condition for hot pressing employed in the present invention.

Fig. 4 is a graph showing a rate of dimensional change upon working using the foam sheet in accordance with the present invention.

Fig. 5 is a graph showing a rate of dimensional change upon working using a conventional single-sided foam sheet. [Detailed Description of Main Elements] 1 : substrate

2 : surface treatment layer

3 : foam sheet using mixture of interactive copolymer 4: release film

[Best Mode]

Hereinafter, a foam sheet for a FPCB that has an excellent adhesive strength by using an interactive copolymer resin as an adhesive and can be simply separated from an adherend by heating treatment at more than 18O⁰C, and a method for manufacturing a FPCB using the same in accordance with the present invention will be described in detail with reference to accompanying drawings. However, the embodiment is for illustrative purposes only, and the scope of the present invention is not limited thereto.

[Manufacturing Example 1] Manufacture of First Copolymer

In 2L glass reactor equipped with a stirrer, a condenser, a dropping funnel, a thermometer, and a jacket were mixed 45g of ethylacrylate as a monomer, 45g of n-butylacrylate as a comonomer and 1Og of acrylic acid, and O.Olg of α,α'- azobisisobutyronitrile as an initiator was further put together with lOOg of ethylacetate and 2Og of toluene, followed by a radical polymerization reaction at 7O⁰C. After lapse of 30 minutes, 135g of ethylacrylate, 135g of n- butylacrylate as a comonomer and 3Og of acrylic acid were added and mixed. 0.5g of α, α' -azobisisobutyronitrile as an initiator was dissolved in lOOg of ethylacetate and 40g of toluene, followed by dropping of the resulting solution for about 90 minutes using the dropping funnel. Temperature was maintained constant during dropping. To eliminate residual monomers after completion of the reaction, Ig of radical initiator was dissolved in 5Og of ethylacetate and 5Og of ethanol and then dropped for 60 minutes, followed by further reaction for 3 hours, thereby manufacturing the first copolymer . Manufacture of Second Copolymer

In 2L glass reactor equipped with a stirrer, a condenser, a dropping funnel, a thermometer, and a jacket were mixed 45g of ethylacrylate as a monomer, 45g of n-butylacrylate as a comonomer and 1Og of 2-isopropenyl-2-oxazoline . Into the reactor, O.Olg of α, α' -azobisisobutyronitrile as an initiator was dissolved in lOOg of ethylacetate and 2Og of toluene and then added, followed by a reaction at 70⁰C by the same manner as the polymerization of the copolymer containing carboxyl group. A radical polymerization reaction was conducted. After lapse of 30 minutes, 135g of ethylacrylate, 135g of n- butylacrylate as a comonomer and 3Og of acrylic acid were added and mixed. 0.5g of α, α' -azobisisobutyronitrile as an initiator was dissolved in lOOg of ethylacetate and 4Og of toluene, followed by dropping of the resulting solution for about 90 minutes using the dropping funnel. Temperature was maintained constant during dropping. To eliminate residual monomers after completion of the reaction, Ig of radical initiator was dissolved in 5Og of ethylacetate and 5Og of ethanol and then dropped for 60 minutes, followed by further reaction for 3 hours, thereby manufacturing the second copolymer .

Manufacture of Adhesive resin The first and second copolymers manufactured as described above were mixed at a ratio by weight of 1:1. Into lOOg of the mixed resin, 1Og of microsphere (product name: F80VSD, available from Matsumoto, starting to foam at 150 to 160⁰C) was put and dispersed, thereby manufacturing an adhesive resin.

[Manufacturing Example 2]

The adhesive resin was manufactured by the same manner as described in Manufacturing Example 1, except that the ratio by weight of the monomers used in manufacture of the first and second copolymers was ethylacrylate/n-butylacrylate/acrylic acid = 4.5/4.5/1.5 and ethylacrylate/n-butylacrylate/2- isopropenyl-2-oxazoline = 4.5/4.5/1.5 respectively.

[Manufacturing Example 3]

The adhesive resin was manufactured by the same manner as described in Manufacturing Example 1, except that the ratio by weight of the monomers used in manufacture of the first and second copolymers was ethylacrylate/n-butylacrylate/acrylic acid = 4.5/4.5/0.6 and ethylacrylate/n-butylacrylate/2- isopropenyl-2-oxazoline = 4.5/4.5/0.6 respectively.

[Manufacturing Example 4] The adhesive resin was manufactured by the same manner as described in Manufacturing Example 1, except that the monomers used in manufacture of the second copolymer of Manufacturing Example 1 were 2-ethylhexylacrylate/ methylmethacrylate/n- butylacrylate/2-isopropenyl-2-oxazoline = 2.0/2.5/4.5/1.0.

[Manufacturing Example 5]

The adhesive resin was manufactured by the same manner as described in Manufacturing Example 4, except that the ratio by weight of the monomers used in manufacture of the second 10 copolymer of Manufacturing Example 4 was 2-ethylhexylacrylate/ methylmethacrylate/n-butylacrylate/2-isopropenyl-2-oxazoline = 2.0/2.5/4.5/1.5.

[Examples 1 to 5]

If) A foam sheet was manufactured through applying the adhesive resin manufactured by Manufacturing Examples 1 to 5 on PET film (50μm) , forming 37μm foam adhesive layer thereon and attaching 36μm release sheet thereto, and was then aged for 7 days. Overall process for manufacturing single-sided FPCBs

'ZO by attaching the FPCBs to both sides of the foam sheet was carried out through the following steps of: laminating a FCCL and a foam sheet - deburring - laminating a dry film exposing - developing - etching - separating the dry film - tag bonding - hot pressing - surface treating - punching - separating foam sheet. Then, the performance was evaluated respective steps .

A. Selection of FCCL

In general, FCCL for a single-sided FPCB is classified as a copper foil, an adhesive, a thickness of a polyimide film and a kind of the copper foil. In the present embodiment, a copper foil having a specification shown in Table 1 was used. A thickness of a coverlay film used in manufacture of the product was 25μm at an adhesive layer and 25/jm at polyimide layer .

[Table 1]

B. Lamination (FCCL and foam sheet) In this step, the foam sheet and the FCCL are laminated to adhere the FCCL to the foam sheet. The lamination was conducted using a general laminator in such a manner that the copper foil of a single-sided FCCL faces outwardly and a surface of the polyimide is adhered to the foam sheet. Generation of bubbles and rumples of the polyimide in the adhered product was avoided and the adhered state of peripheral portion was confirmed so that the various chemicals do not penetrate into the adhered product due to lowering in adhesive force at peripheries in the follow-up steps.

C. Deburring In the deburring step, rust inhibitor that is treated onto the surface of the copper foil to prevent corrosion of the surface of the copper foil is removed, and unevenness is generated on the surface of the copper foil to increase the surface area of the copper foil and enhance adhesive force to the dry film (photosensitive resin) . This step is carried out by way of corroding weakly the surface of the copper with a chemical method, and a mixed solution of sulfuric acid (H₂SO₄) and hydrogen peroxide (H₂O₂) was used as the chemicals.

D. Laminating dry film

The dry film lamination is a step of adhering a photosensitive resin on the copper foil for forming a circuit on the copper foil. Upon manufacturing the single-sided FPCB, the surface of the copper foil was laminated with the dry film and the surface of the polyimide in opposite side was attached to a carrier film in order to prevent rumple or tearing in the follow-up steps since the FCCL is thin.

E. Exposing In the exposing step, when irradiating ultraviolet rays (UV) , for forming the circuit, onto a mask film placed on the FCCL adhered with the photosensitive resin, the UV cannot transmit dark portion of the mask film and can transmit only transparent portion of the mask film. Therefore, the cross- linking occurred only at UV-received portion of the dry film.

F. Developing

The dry film was exposed in 3% sodium carbonate solution. As the result, a portion at which the cross-linking occurred by receiving UV was not dissolved but a portion at which the cross -linking did not occur by not receiving UV was dissolved to expose the copper.

G. Etching

The dry film covered the copper foil as a resist at the portion to be formed with the circuit by the developing and the etchant (CuCl₂, HCl, H₂O₂) was put into the exposed copper foil to corrode the copper foil at the rest portion other than the circuit

The chemicals and processing condition used in the individual step is shown in Table 2. [Table 2]

H . Tag bonding

After the circuit has been formed in the FCCL through the etching, the coverlay film is adhered, for protecting the circuit, to the circuit portions other than the portions to be connected to connectors and the portions to be soldered for mounting parts. Since the coverlay film is coated with a thermosetting adhesive, the adhesive force is weak. Therefore, before conducting the setting of the adhesive by the hot pressing, the coverlay film and the FCCL formed with the circuit were partially and temporarily fixed using a soldering to prevent the coverlay film from being pushed.

I. Hot pressing The hot pressing is for adhering the coverlay film by thermosetting and was conducted under the condition shown in Table 3. In this step, the adhesive of the coverlay is thermoset using a hot pressing for a predetermine time in a heated and pressurized condition. In order to use the foam sheet, it is necessarily required that the foam sheet is not foamed and the foam sheet and the FPCB have no deformation in external shape or dimensional variation under the working condition for this step. [Table 3]

J. Surface treating

The surface treatment is generally conducted using electroless plating and the electroless plating was a batch type and was conducted under a condition shown in Table 4. Gold plating is a detailed process and preprocesses using the chemicals listed in the Table below was conducted prior to the gold plating. [Table 4]

K. Forming external shape (Punching)

In order to accomplish the final product, the FPCB processed with attached foam sheet was subject to punching for forming external shape. At this time, guide holes were accurately arranged with location of the mold to form the external shape .

L. Separating foam sheet In this step, it is required to separate the FPCBs attached to both sides of the foam sheet. The characteristic of the foam sheet is that it is separated by lowering of the adhesive force due to expansion of the foam cells at a preset temperature. The foaming temperature has been designed so that the foam sheet is foamed at at least 170°C. The product to which the surface treatment was completed was separated using a heating device such as a convection oven.

[Comparative Manufacturing Example 1] In 2L glass reactor equipped with a stirrer, a condenser, a dropping funnel, a thermometer and a jacket, 48.5g of n- butylacrylate, 48.5g of ethylacrylate, 2g of acrylic acid, Ig oi hydroxypropylacrylate and O.Olg of α,α'- azobisisobutyronitrile were dissolved in 16Og of ethylacetate and 27Og of toluene and then were put into the dropping funnel, followed by a radical polymerization for 8 hours while dropping at 80°C with continuous flow after substitution with nitrogen. In the end of the reaction, an excessive amount of radical initiator was put therein to eliminate residual monomers, and then 1Og of microsphere (product name: F80VSD, available from Matsumoto, starting to foam at 150 to 160⁰C) was put in and dispersed, followed by adding isocyanate cross- linking agent (product name: AK-75, Aekyung Chemical) in an amount of two times of that of hydroxyl functional group in the adhesive resin and stirring, thereby manufacturing the adhesive resin.

[Comparative Manufacturing Example 2]

The adhesive resin was manufactured by the same manner as described in Comparative Manufacturing Example 1, except that the used amount of hydroxypropylacrylate having a cross- linking functional group was increased to 2.Og.

[Comparative Manufacturing Example 3]

The adhesive resin was manufactured by the same manner as described in Comparative Manufacturing Example 1, except that the isocyanate cross-linking agent (product name: AK-75, Aekyung Chemical) was added in an amount of four times of that of hydroxyl functional group in the adhesive resin.

[Comparative Manufacturing Example 4] The adhesive resin was manufactured by the same manner as described in Comparative Manufacturing Example 2, except that the isocyanate cross-linking agent (product name: AK-75, Aekyung Chemical) was added in an amount of four times of that of hydroxyl functional group in the adhesive resin.

[Comparative Examples 1 to 4]

A foam sheet was manufactured through applying the adhesive resin manufactured by Comparative Manufacturing Examples 1 to 4 on PET film (50μm) , forming 37μm foam adhesive layer thereon and attaching 36μm release sheet thereto. The manufactured foam sheet was aged for 7 days and then used in the final test. Overall process for manufacturing single-sided FPCBs by attaching the FPCBs to both sides of a foam sheet was carried out through the following steps of: laminating a FCCL and a foam sheet - deburring - laminating a dry film exposing - developing - etching - separating the dry film - tag bonding - hot pressing - surface treating - punching - separating foam sheet. Then, the performance was evaluated respective steps. In order to test the process of manufacturing single- sided FPCBs by attaching the FPCBs to both sides of a foam sheet using the adhesive resins and foam sheets manufactured in accordance with the above Manufacturing Examples, Comparative Manufacturing Examples, Examples and Comparative Examples, reliability as a foam sheet for the high temperature foaming was evaluated under following material and processing condition in every processes of manufacturing the FPCB.

1. Selection of raw sheet

In order to be used in manufacture of a FPCB, the raw sheet for the foam sheet was designed so that the foaming agent is foamed to an optical volume at at least 180°C.

Accordingly, since it is required to maximize heat transfer upon foaming and to have flexibility in consideration of workability or roll to roll method, a PET film with a thickness of 25 to lOOjL/m was used. Further, since heating to high temperature and cooling to room temperature are repeated, it is required to have no thermal dimensional change. To meet this condition, the substrate with a thickness of 5θμm and a release film with a thickness of 36jt/m were used.

2. Condition and method of foaming

In general, a convection oven and hot plate were used to separate the foam sheet. A foam sheet with minimum width of more than 250mm is used in manufacture of FPCB, and the foaming agent as itself should have a physical property of a high foaming temperature since the foam sheet is required to be durable in the step of hot pressing. Therefore, a foam sheet separation equipment was selected in consideration of thermal conduction efficiency in accordance with desired foaming force and dimensional change of the FPCB.

3. Evaluation of reliability

The foam sheet was applied to the process of manufacturing a single-sided FPCB, and then it was confirmed whether it is possible to double process efficiency due to manufacture of FPCBs by attaching the FPCBs to both sides of a foam sheet and to reduce rumple or tearing generated in the manufacturing process resulted from a thin thickness of the FPCB. Particularly, it was additionally experimented whether, upon working using the foam sheet, the foam sheet itself gives nutrition to the quality of the FPCB and negative factor is generated in the process of manufacturing the FPCB to reduce productivity and cause production of poor products.

Lamination of the FCCL onto both sides of the foam sheet was conducted with the temperature being maintained at 70°C. During the laminating step, bubbles and rumples of the polyimide were not generated in the adhered product. Particularly, the adhered state of peripheral portion was confirmed so that the various chemicals do not penetrate into the adhered product due to lowering in adhesive force at peripheries and the result showed a strong adhesive force. Separation of the foam sheet and the copper foil was not found even when the foam sheet adhered with the copper foil is bended by being wound at a roll. The chemicals used in deburring was a mixed solution of sulfuric acid (H₂SO₄) and hydrogen peroxide (H₂O₂) , and it was able to confirm that the solution did not penetrate into the surface adhered to the foam sheet. In this step, the thickness was thickly maintained as the FCCLs were attached to both sides of the foam sheet and it was thus not required a carrier film that is a secondary material. Also, it was able to confirm that the work process was reduced to 50% as the dry film was laminated simultaneously at both sides. Further, it was able to confirm that the exposer irradiates UV from the top and the bottom so as to expose two sides simultaneously in the exposing step and the productivity is improved to two times since it is possible to simultaneous expose with respect to two sides when the exposing step is conducted using the foam sheet. The separate process of FCCLs by attaching the FCCLs to both sides of a foam sheet was confirmed not to have an influence on the product quality and the ease of working was also confirmed to be increased. In the developing step, the separation phenomenon due to the penetration of the developing liquid (Na^CO₃) was not found. In the etching step, like the developing step, possibility of separation of the FCCL and the foam sheet due to penetration of strong acidic solution may be issued, but the separation phenomenon was not found in the actual test. In the tag bonding step, the iron is used and this is the same when the working is conducted using the foam sheet. Therefore, it is required that the foam sheet is not foamed by the iron, and it was able to confirm that partial foaming was not generated after the tag bonding step.

In order to confirm dimensional stability during the hot pressing step, dimensions of products manufactured with and without use of the foam sheet were measured using a three- dimensional measurer. Target value of the dimensional stability was set to less than 0.03%.

The separation of the foam sheet due to penetration of the chemicals is the important matter in the electroless plating, i.e. the surface treating step. In the electroless plating step in the present invention, a chemical treatment of a deposition manner was used and it was able to confirm that the liquid was not penetrated from the result of visual testing .

In this step, the FPCBs attached to both side of the foam sheet should be separated. The foam sheet comes to be separated by lowering in adhesive force due to expansion of the foaming cell at the preset temperature. The foaming cells are designed so as to be foamed at at least 170⁰C. The product to which the surface treatment was completed was separated using a heating device such as a convection oven. Qualifying factor required for the completely separated FPCB is as follows. First, the adhesive of the foam sheet should not to be transcribed to the rear side of the FPCB. Second, rumple on the product should not be generated in the foaming and separation step. Third, the foam sheet should avoid stain due to the penetration of liquid in the overall process using the chemicals. Actually, rumple on the product was generated after foaming from non-unifomity in grain sizes of the foam cells of the foam sheet. However, it was able to produce the final product having the same quality as in existing process using a single- sided foam sheet by making the grain sizes and the size distribution of the foam sheet to be uniform. Table 5 shows results of applying samples obtained from Examples and Comparative Examples to the process for manufacturing the FPCB. [Table 5 ]

To confirm the dimensional stability after the hot pressing, dimensions of a product manufactured with use of the foam sheet and a product manufactured by existing process using a single-sided foam sheet were measured using a three- dimensional measurer. The measured result showed, as shown in Figs. 4 and 5, the rate of dimensional change of less than 0.03% that is the target of the dimensional stability. This means that the dimensional problem does not affect the manufacture of the product upon working with use of the foam sheet . When conducting the process for manufacturing FPCBs by attaching the FPCBs to both sides of a foam sheet, various chemicals are inputted in the manufacturing process. It was confirmed whether the adhesive of the foam sheet is dissolved in and penetrated by used chemicals. If the adhesive is dissolved and penetrated, it is impossible to conduct the process by the separation of the foam sheet and the FCCL due to lowering in the adhesive force of the adhesive and contamination of the FCCL due to the penetration of the liquid may be generated. However, the foam sheet in accordance with the present invention had no quality problem due to the penetration of chemicals such as etchant liquid, etc. Also, the effect of increase in the productivity in accordance with the process of single-sided FPCBs by attaching the FPCBs to both sides of the foam sheet can be confirmed from Table 6. [Table 6]

Process Description Effect

Laminating laminating FCCL and foam sheet Work process is foam sheet for processing of FCCLs by added by 100% attaching the FCCLs to both sides of a foam sheet

Laminating Laminating simultaneously two Work process is dry film sides reduced to 50%

Attaching carrier film to No carrier film is opposite side upon processing required

Examining Process of examining final - The same product is the same upon processing using single-sided foam sheet

[Industrial Applicability]

As described above, in accordance with the present invention, it is possible to provide a method for manufacturing a FPCB using a foam sheet, which maintains adhesive force under a condition of high temperature and high pressure during hot pressing process, has a chemical resistance not allowing penetration of an etchant liquid during etching process, and can be separated from an adherend by high temperature heating.

Also, in the present invention, since a double-sided foam sheet is employed, it is possible to produce two single-sided

FPCB in a single process and thus improve productivity in the

FPCB manufacturing process, as compared to conventional process for manufacturing a single-sided FPCB.

Claims

[CLAIMS]

[Claim l]

A method for manufacturing a flexible printed circuit board (FPCB) , comprising the steps of: attaching a FPCB to either or both sides of a foam sheet containing thermally expandable microspheres and an interactive copolymer; and separating the foam sheet from the DPCB by foaming the foam sheet .

[Claim 2]

The method for manufacturing a FPCB as set forth claim 1, wherein the foam sheet is formed by stacking, sequentially, a substrate, a surface treating layer, an adhesive layer and a release film, and the adhesive layer contains thermally expandable microspheres and an interactive copolymer.

[Claim 3]

The method for manufacturing a FPCB as set forth claim 1, wherein the foam sheet is formed by stacking, sequentially, a release film, a adhesive layer, a surface treating layer, a substrate, a surface treating layer, an adhesive layer and a release film, and the adhesive layer contains thermally expandable microspheres and an interactive copolymer.

[Claim 4]

The method for manufacturing a FPCB as set forth claim 1, wherein the interactive copolymer is a mixture of a first copolymer copolymerized with vinyl monomer, vinyl comonomer and carboxyl group-containing vinyl monomer, and a second copolymer copolymerized with vinyl monomer, vinyl comonomer and oxazoline group-containing vinyl monomer.

[Claim 5]

The method for manufacturing a FPCB as set forth claim 4, wherein the vinyl monomer of the first and second copolymers are independently selected from methylacrylate, methylmethacrylate , ethylacrylate, ethylmethacrylate, propylacrylate, propylmethacrylate, butylacrylate, butylmethacrylate, hexylacrylate, hexylmethacrylate, octylacrylate, 2-ethylhexylacrylate, hydroxyethylacrylate, hydroxyethylmethacrylate , hydroxypropylacrylate , hydroxypropylmethacrylate , hydroxybutylacrylate, hydroxybutylmethacrylate , hydroxyhexylacrylate , hydroxyhexylmethacrylate , N, N-dimethylacrylamide, N, N- dimethylmethacrylamide , methoxyethylacrylate , methoxyethylmethacrylate , ethoxyethylacrylate , ethoxyethylmethcrylate , vinylacetate, vinylpropionate, N- vinylpyrrolidone , methylvinylpyrrolidone, vinylpyridine, vinylpiperidone, vinylpyrimidine, vinylpiperazine, vinylpyrazine, vinylpyrrole, vinylimidazole, vinyloxazole, vinylmorpholine, N-vinylcarboxamides , styrene, α-methylstyrene, N-vmylcaprolactam, acrylonitrile, methacrylonitrxle, glycidylacrylate , glycidylmethacrylate , polyethyleneglycolacrylate , polyethyleneglycolmethacrylate , polypropyleneglycolacrylate , polypropyleneglycoltnethacrylate , methoxyethyleneglycolacrylate , methoxypolyethyleneglycolmethacrylate, methoxypolypropyleneglycolacrylate , methoxypolypropyleneglycolmethacrylate , tetrahydrofurfuryl acrylate, tetrahydrofurfuryl methacrylate, 2-methoxyethyl acrylate, isoprene, butadiene, isobutylene and vinyl ether, or a mixture of two or more thereof .

[Claim 6]

The method for manufacturing a FPCB as set forth claim 4, wherein the carboxyl group-containing vinyl monomer of the first copolymer is selected from one or more carboxyl group- containing monomer selected from acrylic acid, methacrylic acid, carboxyethyl acrylate, carboxypentyl acrylate, itaconic acid, maleic acid, fumaric acid and crotonic acid; and one or more acid anhydride monomer selected from maleic anhydride or itaconic anhydride.

[Claim 7]

The method for manufacturing a FPCB as set forth claim 4, wherein the first copolymer is manufactured from vinyl monomer, vinyl comonomer and carboxyl group-containing vinyl monomer at a ratio by weight of 1: 0.5 to 1.5: 0.05 to 0.3.

[Claim 8]

The method for manufacturing a FPCB as set forth claim 4, wherein the oxazoline group-containing vinyl monomer of the second copolymer is any one selected from 2-vinyl-2-oxazoline,

2-vinyl-4 -vinyl-2-oxazoline, 2-vinyl-5-vinyl-2-oxazoline, 2- isopropenyl-2-oxazoline, 2-isopropenyl-5-ethyl-2-oxazoline, 2- isopropenyl-5-methyl-2-oxazoline, 2- (vinylbenzyloxy-1- methylethyl) -2-oxazoline , 2- (2 -hydroxy-1-methylethyl) acrylate and 2- (2 -hydroxy- 1-methylethyl) methacrylate or a mixture of two or more thereof .

[Claim 9]

The method for manufacturing a FPCB as set forth claim 4, wherein the second copolymer is manufactured from vinyl monomer, vinyl comonomer and oxazoline group-containing vinyl monomer at a ratio by weight of 1: 0.5 to 1.5: 0.05 to 0.3.

[Claim 10]

The method for manufacturing a FPCB as set forth claim 2, wherein adhesive layer is not foamed or thermally deformed within 40 minutes at a temperature of 16O⁰C and a pressure of 40kg/cm², and is foamed within 10 seconds to 3 minutes in a hot plate or 5 to 10 minutes in a convection oven at a temperature of 170 to 190⁰C.

[Claim 11] The method for manufacturing a FPCB as set forth claims 2 or 3, wherein the interactive copolymer of the adhesive layer is a mixture of a first copolymer copolymerized with vinyl monomer, vinyl comonomer and carboxyl group-containing vinyl monomer, and a second copolymer copolymerized with vinyl monomer, vinyl comonomer and oxazoline group-containing vinyl monomer .

[Claim 12]

The method for manufacturing a FPCB as set forth claim 11, wherein the adhesive layer is formed by mixing the first copolymer and the second copolymer and then dispersing the microspheres therein.