Amid the current trend for wearable electronics assembly, flex-on-board (FOB) assembly is attracting great attention because of its important role in replacing conventional physical contacts-based socket type interconnections. In the case of conventional socket type interconnections, there are three main drawbacks, which include: physical contact, large package size, and low packaging density. For reducing the package thickness from 4mm to 0.1mm, and fine pitch capability to less than 100um, FOB application is an obvious choice in the replacement of conventional socket type interconnectors.
Anisotropic conductive films (ACFs) are the interconnection material used in FOB assembly. ACFs generally consist of thermos-setting polymer resins and micron-sized conductive particles, with the cruel requirement in fine pitch applications of providing electrical paths only in the z-axis, and insulation property by polymer resins in the x-y plane. Physical contacts between conductive particles and metal electrodes are the main electrical paths for micron-sized Au/Ni coated polymer ball joints and metal ball joints, but they will easily fail with the thermal expansion of polymer resin in high-power applications. Therefore, micron-sized solder particles are added to polymer resin matrixes to replace micron-sized Au/Ni coated polymer ball joints and metal ball joints to form metallurgical joints for lower joint resistance, higher power-handling capability, and better reliability.
Recently, two bonding methods have normally been used to remove the solder oxide layer and get better solder wettability. One method uses ultrasonic bonding, and the other method adds flux materials in adhesives using thermo-compression bonding. However, polymer resins show a low modulus at high temperature when the pressure is automatically released at the end of the thermo-compression bonding, and adhesive rebound may crack solder joints after FOB assembly. For highly reliable solder ACFs FOB assembly, and consequent improved electrical performance, it is necessary to investigate and eliminate solder ACF joint cracks.
The effects of thermo-compression bonding temperatures and pressures, as well as ACF resin properties on the solder ACF joint cracks, are discussed in Chapter 2. It was found that SnBi58 solder wettability was so poor below 200$\circ C$, and there were no solder ACF joint cracks at the optimized 200$\circ C$. However, solder ACF joints cracked above 225$\circ C$, and could not be eliminated by using higher bonding pressures. On the other hand, even at the optimized 200$\circ C$ bonding temperature, solder joint cracks were obtained when using lower modulus ACF resin. Finally, adhesive rebounds were tested by a thermal mechanical analyzer (TMA) as a function of temperatures and pressures. Then, solder ACF joint cracks were due to adhesive rebound when bonding pressure was released at the end of thermo-compression bonding. In addition, this rebound can be reduced using a higher resin modulus.
The effects of delaying bonding tool lift time and adding silica fillers, which were carried out to increase the ACF resin modulus and remove solder cracks, are detailed in Chapter 3. For maintaining the bonding pressure on the FOB in the cooling process until room temperature, a conventional thermo-compression bonding could not be used, and a novel ultrasonic bonding was used instead. Even though cracks could disappear when maintaining bonding pressures until cooling to polymer Tg, over 30 seconds was too long for assembly. Therefore, 7nm silica fillers were added in the adhesives to reduce adhesive rebound using the conventional thermos-compression bonding. It was found that solder cracks can be eliminated by adding 10wt% silica fillers.
AC and DC electrical performances of optimized solder ACF joints are investigated in Chapter 4. In AC conditions, a network analyzer in a 100MHz？20GH high-frequency test was compared between an Au/Ni coated polymer ball and solder ball joints. The polymer ball was 0.1 dB lower than solder joints above 17 GHz. In the DC condition, the power-handling capability was compared between an Au/Ni coated polymer ball, Ni ball, SnBi58, and Sn-3Ag-0.5Cu (SAC305) solder ball ACF joints. The Sn-3Ag-0.5Cu ball showed two times better power-handling capability than the polymer ball due to a higher mechanical property under its melting points.
The failure mechanism of solder ACF joints on an Au/Ni surface finish in a pressure cooker test (121$\circ C$ 100% humidity 2atm) is demonstrated in Chapter 5, and the best ACF resin candidate and solder materials were optimized in terms of hydroscopic swelling and solder property for higher FOB reliability. Cationic epoxy was selected due to having the lowest hydro-swelling, and SAC305 solder was chosen due to better mechanical property in the pressure cooker test. As a result, there is no resistance change after five days of the pressure cooker test using cationic epoxy-based SAC305 solder ACF joints after the optimization of the FOB interconnection.
ACFs 는 상호접속 재료로 FOB조립에 쓰인다. ACFs는 일반적으로 thermos-setting 폴리머와 미크론 크기의 전도성 입자로 조성된다. 전도성 입자와 전극사이의 물리적 접촉은 미크론 크기의 폴리머 볼 연결부위와 솔더 볼 연결부위의 주요한 전기적 통로이다. 전동성 입자와 전극사이의 물리적 접촉은 열 팽창에서 쉽게 무효화된다. 미코론 크기의 파티클을 폴리머 수지 메트릭스에 넣어 미크론 크기의 미크론 크기의 폴리머 볼 연결부위와 솔더 볼 연결부위를 대체한다.
솔더 산화층을 제거하는데 일반적으로 두가지 연결 방법이 쓰인다. 하나는 초음파 연결이고 다른 하나는 접착제에 유동 재료를 넣어주는 방법이다. 성능이 높은 솔더 ACFs FOB 조립부품을 만들기 위하여 솔더 ACF 연결부위가 균열하는것을 방지하여야 한다.
Thermo-compression의 연결온도, 압력 그리고 ACF수지 성질등은 제2장에서 설명하였다. 접착제의 탄성은 TMA로 테스트하였다. 접착제의 탄성 더 높은 수지 계수로 감소시킬수 있다.
FOB 냉각중 연결 압력을 유지하는데는 일반적인 thermo-compression는 사용할수 없기 때문에 새로운 초음파 연결방법이 사용된다. 균열이 사라지기는 하나 30초가 넘는 시간은 조합에 긴 시간이다. 7nm 실리카 충전제를 접착제에 넣어 접착제의 탄성를 감소시킬수 있다.
AC조건에서 네트워크 분석기 테스트를 폴리머 볼과 solder ball 연결부분에서 비교하였다. 17GHz이상에서 폴리머 볼은 솔더 연결부분보다 0.1 dB 낮다. DC조건에서 전력처리능력을 폴리머 볼과 솔더 볼 ACF 연결부분에서 비교하였다. 솔더 볼은 폴리머 볼과 비교하였을때 두배 높은 전기처리능이 있다.
