임베디드 기판 적용을 위한 폴리페닐렌에테르 기반의 저유전손실 복합소재 및 감광성 저항 페이스트의 제조와 특성평가

Abstract

IT 산업의 발전에 따라 휴대 기기에는 더 높은 사용 주파수 대역이 필요하게 되고, 기기 사이즈의 소형화와 기기에 사용되는 전자부품 패키지의 고집적화도 계속 요구되고 있다. 이러한 요구 사항들은 기존의 인쇄회로기판(PCB, printed circuit board) 기술로는 해결하기 어려운 여러 난제들을 제시하고 있다. 더 높은 주파수의 사용에 의한 전송신호의 손실 방지를 위하여, 향상된 기판소재에 대한 요구, 수동 부품에 있어서의 기생 인덕턴스나 기생 캐패시턴스의 저감, 사이즈의 소형화에 수반하는 배선 밀도의 증대, 그리고 표면 실장의 한계 극복 등 새로운 소재 및 공정 기술을 필요로 하고 있다. 기판소재가 기가 헤르츠 이상의 고주파 대역에 활용되기 위해서는 통상의 PCB 소재와 달리 유전체 기판 자체의 손실값이 가능한한 작아야한다. 또한 기생 효과의 저감과 배선, 소자의 고집적화를 위해서는 배선의 총 길이를 짧게 하고 기판의 표면 활용성을 높이는 기술이 필요하다. 이러한 기술적 요구로부터 임베디드(embedded) PCB 기술이 출현하게 되었으며, 이 기술은 저항, 코일, 캐패시터와 같은 수동부품들을 PCB의 내층에 내장하는 기술이다. 고주파 대역에서 임베디드 PCB 기술이 적용되기 위해서는 성능을 뒷받침할 수 있는 핵심 소재의 개발이 필수 요소이다. 본 연구에서는 핵심 소재 중에서도 유전손실이 작은 기판소재와 임베디드 저항소자용 페이스트 소재에 대하여 연구하였다. 특히, 기존의 기판소재와 저항소재가 갖고 있던 공정 및 특성의 한계를 극복하기 위한 새로운 소재 조성물들을 설계하고, 제작 및 평가하였다. 저유전손실 기판소재의 개발을 위하여 폴리페닐렌에테르(polyphenylene ether, PPE)를 기반으로 한 수지 경화물 조성에 대한 연구를 실시하였다. 폴리페닐렌에테르는 엔지니어링플라스틱으로 내열성, 강인성이 우수하면서도 유전 특성이 뛰어난 장점을 가지고 있으나, 가공성이 떨어지고 내약품성과 동박접합성이 약한 단점이 있다. 이를 보완하기 위하여 열경화성분을 부여하여 하는 방법을 사용하였는데, 경화성분으로는 상용의 소재로 dicyclopentadiene 구조를 주 골격으로 가진 시아네이트 에스터 올리고머와 이중결합을 가진 2관능성 모노머 중 하나인 N,N'-m- phenylenedimaleimide (PDMI)를 사용하였다. 한편으로는 경화물에 유연성을 부가하기 위하여 hexamethylene 구조를 가진 3관능성 아크릴레이트 모노머를 직접 합성하여 PPE의 경화성분으로 활용하였다. Dicyclopentadienyl cyanate 올리고머를 PPE의 경화성분으로 사용하여서는 동박의 박리강도가 1.7 kN/m 이상으로 우수하고 1 GHz에서의 유전손실이 0.004로 작은 PPE 기판소재를 얻었으며, PDMI를 경화성분으로 사용하여서는 1 GHz에서 유전손실이 0.002 보다 작은 저유전손실 기판소재를 얻었다. 경화성분에 따른 물성의 차이, 특히 gel fraction에 큰 차이가 나타나는 원인을 알아보기 위하여 두 복합물의 점탄성 거동, 추출 후 미세구조의 분석 등을 통하여 경화 후 형성되는 구조에 대한 고찰을 실시하여, cyanate 올리고머 수지를 사용한 경우는 semi- interpenetrating polymer network (semi-IPN) 구조를 형성하고, PDMI의 경우는 분자사슬의 가교를 통하여 3차원 망상구조를 형성하는 것으로 결론지었다. 마지막으로 고유의 3관능기 유연 아크릴레이트 모노머 2종을 합성하고 FT-IR, NMR 등을 이용하여 합성 후 구조를 확인하였으며, PPE의 경화 성분으로 이용하였을 때, 1 GHz에서의 유전손실이 0.003 보다 작은 유연한 PPE 기판소재를 얻을 수 있었다. 임베디드 후막 저항소재의 개발을 위해서는 광경화형의 수지를 매트릭스 소재로 도입하는 연구를 실시하였다. 일반적인 폴리머 후막 저항은, 카본블랙과 같은 전도성 필러와 에폭시 또는 페놀 수지를 사용하여 만들어진 페이스트를 인쇄 및 열경화하여 제작되는데, 기본적으로 가지고 있는 해상도의 한계와 패턴정밀도의 저하, 또는 저항치의 허용편차가 큰 문제들을 안고 있다. 본 연구에서는 이러한 단점들을 해결하기 위하여 고정밀 패터닝이 가능한 광경화형 수지를 매트릭스로 활용하여, 광경화와 열경화가 동시에 가능한 후막형 저항 페이스트의 개발을 실시하였다. 목적에 부합하는 수지소재로 일액형 또는 이액형의 상용 포토솔더레지스트(PSR, photo solder resist)를 이용하고, 여기에 전도성 필러로 카본블랙을 도입하여 감광성 저항페이스트를 제작하고, 노광 및 현상 공정을 이용하여 저항체를 정밀 패터닝하였다. 포토공정을 이용하여 제작된 저항체는 스크린 인쇄를 이용하여 만들어진 저항체보다 형상이 정밀하고 미세회로 구현이 가능하여 좁은 영역에서도 큰 저항치를 구현할 수 있었다. 또한 페이스트의 롤러 코팅법을 도입하여 테스트 기판 상에서 두께의 균일도를 향상시킴으로써, 저항치의 허용편차(3σ)를 스크린 인쇄시의 40~50%에서 최저 4.9%까지 대폭 줄일 수 있었다. 테스트 기판상 위치별 두께 분포와 저항치 분포를 도시하여 그 상관관계를 확인할 수 있었다. 한편 에폭시 아크릴레이트 올리고머를 기반으로 노광 및 현상이 가능한 광경화형 조성물을 조합하여, 각 구성성분이 감광성 저항소재의 현상성 및 저항치 변화에 주는 영향을 고찰하였다. 실험결과 크레졸노볼락에폭시 변성 아크릴레이트 사용시 현상성이 가장 우수하였고, 감광성 모노머로 분자량이 작은 TMPTA, PETA 등을 사용한 페이스트의 시트저항이 DPHA를 모노머로 사용한 페이스트보다 더 낮게 나타나는 것을 확인하였다.| The higher operation frequencies have been needed in mobile terminals. The size miniaturization, the high integration of electronic components, and packages in the terminals are also required. These requirements produce many difficult problems which cannot be solved easily by conventional PCB (printed circuit board) technology. So, the novel materials and the process technologies such as an improved substrate, the reduction of parasitic inductance or capacitance in passive components, the increase of circuit density accompanying the size miniaturization, the technologies overcoming the limit of surface mounting, are now requested to prevent the transmission signal loss by using higher frequency. For using the high frequency in range of giga hertz and over, the dielectric loss value of substrate, unlike conventional PCB substrates, has to be minimized. Moreover, the technology to be shorten the total circuit length and to promote the applicability of substrate surface is needed for the reduction of parasitic effect and the high integration of circuits and devices. The embedded PCB technology due to above technical requests is the one equipped the passive components like resistor, coil, and capacitor into the inner layer of PCB. The development of the core materials to support the performance is the essential factor in order to apply the embedded PCB in high frequency range. Among the indispensable core materials, in present study, substrate material with low dielectric loss and paste for embedded resistor were studied. In particular, new material compositions were designed, fabricated, and characterized to overcome the limitations of process and property of the conventional substrate and resistor materials. Polyphenylene ether(PPE)-based cured resin compositions were studied for the development of low dielectric substrate material. PPE as an engineering plastic has excellent dielectric properties as well as high heat resistance and high strength, while it has the demerits such as weak machinability, low chemical resistance and low Cu peel strength. To compensate these weak points, the method to impart the thermally curable ingredients to PPE was used here. Commercially available cyanate ester oligomer with dicyclopentadienyl structure in main chain or N,N'-m-phenylenedimaleimide (PDMI), one of the difunctional monomers having two double bonds, were used as the curable ingredients. On the other hand, trifunctional acrylate monomers with hexamethylene chain structure were synthesized and used as curing ingredients to impart the flexibility to cured PPE substrates. When the dicyclopentadienyl cyanate oligomer was used as a curing ingredient, Cu peel strength was excellent as over 1.7 kN/m and the PPE substrate with low dielectric loss of 0.004 at 1 GHz was obtained. Using PDMI, the substrate with lower dielectric loss below 0.002 at 1 GHz was obtained. To investigate the reason of the different properties depends on the curing ingredients, especially the big difference in the gel fraction, the structures created after curing in two composites were compared through analyzing the viscoelastic behaviors and the microstructures after Soxhlet extraction. So it was concluded that a semi-interpenetrating polymer network (semi- IPN) structure was formed when the cyanate ester oligomer was used, but a three dimensional network structure by crosslinking of molecular chains was formed when the PDMI was used. Finally, two kinds of unique trifunctional flexible acrylate monomers were synthesized and their structures were confirmed by FT-IR and NMR. And then, when the synthesized monomers were used as curing ingredients of PPE, the flexible substrate material of which the dielectric loss was lower than 0.003 at 1 GHz was obtained. To develop the embedded-type thick-film resistor, a photocurable resin as a matrix material was studied. A conventional polymer thick-film resistor is fabricated by printing and curing a paste which is made from the combination of conductive filler like carbon black and epoxy or phenol resin. But it has some big problems such as intrinsic limit of printing resolution, lack of pattern accuracy, or large tolerance of resistance value. To solve these shortcomings, a photocurable resin which is enabled high precise patterning was used as a matrix. So the development of thick-film resistor paste enabled both photocuring and thermal curing was conducted. One or two liquid-type commercial photo solder resists were used as the resin material which satisfied the above purpose and the photosensitive resistor pastes were made by incorporating carbon blacks as conductive fillers into the photocurable resin. The resistors were precisely patterned by paste printing, UV exposure and aqueous development. The photo-patterned resistors showed more accurate shape and enabled the formation of fine line circuit, so that large resistance could be obtained in spite of small area. Moreover, by introducing the roller coating method of the resistor paste, the thickness uniformity on resistance test board was improved, and the tolerance of resistance (3σ) was greatly reduced from 40~50% in screen printing to 4.9% in results. The thickness distribution and the resistance distribution according to the position on test board were showed in graphic forms so that the relationship between them could be confirmed. Meanwhile, the unique photocurable compositions based on several epoxy acrylate oligomers were formulated, and then the effects of each ingredient of the photocurable resistor paste on the change of its developability and resistance value was investigated. The developability was the best when the cresol novolac epoxy-modified acrylate was used as a main oligomer. In addition, the resistor paste using TMPTA or PETA as a monomer showed lower sheet resistance than that of the resistor paste using DPHA.; The higher operation frequencies have been needed in mobile terminals. The size miniaturization, the high integration of electronic components, and packages in the terminals are also required. These requirements produce many difficult problems which cannot be solved easily by conventional PCB (printed circuit board) technology. So, the novel materials and the process technologies such as an improved substrate, the reduction of parasitic inductance or capacitance in passive components, the increase of circuit density accompanying the size miniaturization, the technologies overcoming the limit of surface mounting, are now requested to prevent the transmission signal loss by using higher frequency. For using the high frequency in range of giga hertz and over, the dielectric loss value of substrate, unlike conventional PCB substrates, has to be minimized. Moreover, the technology to be shorten the total circuit length and to promote the applicability of substrate surface is needed for the reduction of parasitic effect and the high integration of circuits and devices. The embedded PCB technology due to above technical requests is the one equipped the passive components like resistor, coil, and capacitor into the inner layer of PCB. The development of the core materials to support the performance is the essential factor in order to apply the embedded PCB in high frequency range. Among the indispensable core materials, in present study, substrate material with low dielectric loss and paste for embedded resistor were studied. In particular, new material compositions were designed, fabricated, and characterized to overcome the limitations of process and property of the conventional substrate and resistor materials. Polyphenylene ether(PPE)-based cured resin compositions were studied for the development of low dielectric substrate material. PPE as an engineering plastic has excellent dielectric properties as well as high heat resistance and high strength, while it has the demerits such as weak machinability, low chemical resistance and low Cu peel strength. To compensate these weak points, the method to impart the thermally curable ingredients to PPE was used here. Commercially available cyanate ester oligomer with dicyclopentadienyl structure in main chain or N,N'-m-phenylenedimaleimide (PDMI), one of the difunctional monomers having two double bonds, were used as the curable ingredients. On the other hand, trifunctional acrylate monomers with hexamethylene chain structure were synthesized and used as curing ingredients to impart the flexibility to cured PPE substrates. When the dicyclopentadienyl cyanate oligomer was used as a curing ingredient, Cu peel strength was excellent as over 1.7 kN/m and the PPE substrate with low dielectric loss of 0.004 at 1 GHz was obtained. Using PDMI, the substrate with lower dielectric loss below 0.002 at 1 GHz was obtained. To investigate the reason of the different properties depends on the curing ingredients, especially the big difference in the gel fraction, the structures created after curing in two composites were compared through analyzing the viscoelastic behaviors and the microstructures after Soxhlet extraction. So it was concluded that a semi-interpenetrating polymer network (semi- IPN) structure was formed when the cyanate ester oligomer was used, but a three dimensional network structure by crosslinking of molecular chains was formed when the PDMI was used. Finally, two kinds of unique trifunctional flexible acrylate monomers were synthesized and their structures were confirmed by FT-IR and NMR. And then, when the synthesized monomers were used as curing ingredients of PPE, the flexible substrate material of which the dielectric loss was lower than 0.003 at 1 GHz was obtained. To develop the embedded-type thick-film resistor, a photocurable resin as a matrix material was studied. A conventional polymer thick-film resistor is fabricated by printing and curing a paste which is made from the combination of conductive filler like carbon black and epoxy or phenol resin. But it has some big problems such as intrinsic limit of printing resolution, lack of pattern accuracy, or large tolerance of resistance value. To solve these shortcomings, a photocurable resin which is enabled high precise patterning was used as a matrix. So the development of thick-film resistor paste enabled both photocuring and thermal curing was conducted. One or two liquid-type commercial photo solder resists were used as the resin material which satisfied the above purpose and the photosensitive resistor pastes were made by incorporating carbon blacks as conductive fillers into the photocurable resin. The resistors were precisely patterned by paste printing, UV exposure and aqueous development. The photo-patterned resistors showed more accurate shape and enabled the formation of fine line circuit, so that large resistance could be obtained in spite of small area. Moreover, by introducing the roller coating method of the resistor paste, the thickness uniformity on resistance test board was improved, and the tolerance of resistance (3σ) was greatly reduced from 40~50% in screen printing to 4.9% in results. The thickness distribution and the resistance distribution according to the position on test board were showed in graphic forms so that the relationship between them could be confirmed. Meanwhile, the unique photocurable compositions based on several epoxy acrylate oligomers were formulated, and then the effects of each ingredient of the photocurable resistor paste on the change of its developability and resistance value was investigated. The developability was the best when the cresol novolac epoxy-modified acrylate was used as a main oligomer. In addition, the resistor paste using TMPTA or PETA as a monomer showed lower sheet resistance than that of the resistor paste using DPHA.