The anisotropic conductive films (ACFs) which consist of thermosetting polymer matrix resin and conductive balls provide a green interconnection method commonly used in flat panel display applications, which allow electrical and mechanical connection between two electrical parts such as chips, glasses, flexible printed circuits (FPCs) and printed circuit boards (PCBs). In conventional ACFs, current flows through an ACF joint formed by a physical contact between electrodes and conductive balls such as Au coated Ni balls or Au and Ni coated polymer balls. However, there have been constant needs for alternative ACFs which have lower contact resistance, higher power handling capacity, improved reliability and fine pitch assembly requirements. In order to satisfy these needs of ACF joints, we have proposed a solder ACF demonstrated. In the previous study, soldering on metal pads in an adhesive matrix using the ultrasonic ACF bonding technique was reported. In this method, solder oxide layers around the solder ball could be broken by applying ultrasonic vibrations above the melting temperature of solder balls. As a result, solder wetting on the metal pads while forming stable metallurgical joints were successfully demonstrated using solder ACFs. However, there may be potential damage issues for mechanically sensitive applications when using ultrasonic bonding. Accordingly, there have been needs for alternative bonding methods to remove solder oxides resulting in an excellent solder wetting while applying conventional thermo-compression. In order to make stable solder metallurgical joints in solder ACF joints, a chemical reaction changing Sn oxides into Sn ion is suggested using thermal acid generators (TAGs).
In this study, the reaction between Sn oxide and acid at high temperature (200°C) was investigated. To evaluate whether the reaction can be used in ACF bonding or not, theoretical estimations were conducted in terms of chemical equilibrium and reaction speed. To investigate chemical equilibrium of Sn oxide reduction, a pourbaix diagram and Nernst equation which are well known in the field of electro-chemistry were used. From the result of theoretical calculations, the amount of a TAG for the chemical equilibrium was about 2 wt%. And the obtained time for Sn oxide removal was 5 s according to the results about reaction speed. On the other hand, adding TAG in solder ACFs did not cause property degradation in terms of curing speed, modulus, shelf life, and moisture absorption. However, solder wettability improved significantly, as the amount of the TAG increased in the solder ACFs due to solder oxide removal. As a result, the TAG caused significant improvement of electrical properties of solder ACF joints such as contact resistance, power handling capacity, and reliability.
Using the results, to confirm a performance of the solder ACFs with the TAG, a comparative investigation of reliability was conducted among conventional ACF joints, ultrasonic bonded solder ACFs, and the TAG applied solder ACFs. Before fabricate the solder ACFs, the amount of solder balls in the ACF was optimized in terms of open and short circuit ratio. And then, contact resistances, power handling capacity, several types of reliability, and microwave characteristics of ACF joints were evaluated using the optimized solder ACFs. According to the results, solder ACFs, regardless of bonding method, had 3.3 times higher power handling capacity than conventional polymer ball ACFs. In PCT reliability results, there was no perfectly failed ACF joint in all types of ACF samples. However, it was obvious that contact resistances of conventional polymer ball ACF showed the highest increment. In terms of microwave characteristics, at frequency range from 200 MHz to 5 GHz, a signal loss at ACF joints can be ignored regardless of ACF types. However, in higher frequency range, solder ACFs had about 0.3 dB lower insertion loss than polymer ball ACFs. It meant power loss of solder ACFs were 6.6% lower than that of polymer ball ACFs. Therefore, regardless of bonding method, solder ACFs showed better properties compared with conventional polymer ball ACFs in terms of power handling capacity, reliability, and microwave characteristics.
열경화성 폴리머 수지와 도전볼으로 이루어져 있는 이방성 전도성 필름 (ACF)은 칩, 유리, Flex-On-Board (FOB) 등의 기판간 전기적, 기계적 접합을 만드는 환경 친화적 재료이다. ACF에 쓰이는 도전볼은 보통 금 도금된 니켈볼이나, 금과 니켈이 도금된 폴리머볼인데, 이 종류의 도전볼은 단지 전극과 물리적으로 접촉되어 있을 뿐이기 때문에, 저항, 출력전달, 신뢰성 등에 취약하다. 따라서 이를 해결하기 위해서 솔더볼을 이용한 ACF가 제안되었다. 지난 연구에서는 솔더의 산화막을 제거하기 위해서 초음파 본딩 방법을 이용했는데, 그 결과 기판의 패드에 안정적인 금속간 결합을 만들 수 있었다. 그러나 초음파 본딩 방법을 이용했을 때에는 진동으로 인한 기판 내부의 잠재적인 데미지가 문제가 되었고, 따라서 이를 대체할 수 있는 다른 방법이 필요하게 되었다.
본 연구에서는 산(acid)을 이용한 솔더 산화막 제거에 대한 실험이 진행되었다. 일반적인 산을 이용하면 에폭시 계열 폴리머의 보관성에 문제를 야기하기 때문에, 본 연구에서는 고온에서만 산을 발생시키는 열산발생제(TAG)를 이용하였다. ACF에 직접적으로 TAG를 첨가하여 실험하기 전에, 산과 산화막의 반응에 대한 기초적인 실험이 선행되었는데, pourbaix diagram을 이용하여 산화막 환원 반응의 화학평형 조건은 TAG가 1.5 wt% 필요한 것으로 나타났으며, 산화막 제거 속도 측정 결과 10 nm 솔더 산화막의 제거에는 5초가 필요한 것으로 나타났다. 이 결과를 토대로, TAG를 솔더 ACF에 적용한 결과, 모듈러스, Tg, 상온 보관성, 흡습성 측면에서 ACF 물성 저하가 없었으며, 솔더의 웨팅성 증가로 인한 ACF 조인트의 출력 허용량 및 신뢰성이 개선되었다.
이 결과를 이용해서 실제 TAG가 첨가된 솔더 ACF의 성능을 기존의 ACF의 결과와 비교하는 실험이 진행되기 전에, 최적화된 솔더 ACF를 제작하기 위해서, ACF 내부 솔더볼 함량이 Open, short circuit 가능성에 미치는 영향이 수학적, 실험적으로 확인되었으며, 최적 솔더볼 함량은 15%로 결정되었다. ACF 조인트의 출력허용량, 신뢰성, 고주파 특성 관점에서 폴리머볼 ACF, 초음파 본딩된 솔더 ACF, TAG가 첨가된 솔더 ACF를 비교하였는데, 초음파 본딩된 솔더 ACF와 TAG가 첨가된 솔더 ACF는 같은 특성을 나타냈으며, 폴리머볼 ACF에 비해서 모든 특성이 더 좋은 것으로 나타났다.
