In this paper, the electrical and reliability properties of various solder ACF joints for Flex-On-Board (FOB) assembly using ultrasonic bonding were assessed in terms of short circuits, contact resistance, current handling capability, wetted solder areas, and reliability evaluation. It was found that the wetted solder areas increased and reliability was enhanced as the size and content of the solder ball increased. Regardless of solder ball size and content, the contact resistances of solder ACF joints were almost the same, because the current went through the electrode/electrode contact, which is like a short circuit to the solder joint. And the exact value of current handling capability of all the ACF joints could not be evaluated, because metal lines of PCB were burnt before ACF joint failure. After 120 hours of PCT (Pressure Cooker Test), the open circuit failures were mainly observed at Ni ACFs, and 5-15 μm solder ball size from 10 wt% to 30 wt% solder content and 25-32 μm size and 10 wt% solder content of solder ACFs , presumably due to lower wetted solder areas on metal electrodes. In addition, the crack was noted after 120 hours of PCT in the solder ACF joints who has lower wetted solder areas. As a result of this study, the solder ball size and content of solder ACFs joints were optimized at 25-32 μm sizes and 30 wt% content for 300-μm-pitch FOB assembly.
본 연구에서는, 초음파 본딩을 이용한 Flex-On-Board (FOB) 어셈블리에서 솔더 ACF 조인트의 전기적 특성을 short circuit, 컨택저항, current handling capability, 솔더 wetting 면적, 신뢰성 관점에서 평가하였다.
솔더 볼의 크기와 함량이 증가함에 따라서 솔더 wetting 면적이 증가하였으며, 솔더 ACF 조인트의 신뢰성이 향상되었다. 한 편, 본딩 후 전류가 상부전극과 하부전극이 직접적으로 맞닿은 곳으로 흐르게 됨에 따라, 솔더볼의 크기와 함량에 관계없이 솔더 ACF 조인트의 컨택저항은 거의 일정했다. 그리고 솔더 ACF 조인트의 current handling capability는 평가할 수가 없었는데, 그 이유는 PCB의 도선이 ACF 조인트보다 먼저 고열에 의해 손상되었기 때문이다. Pressure cooker test 120시간 이후, conventional Ni ACF, 5~15 μm 솔더볼을 10~30 wt% 함유하는 솔더 ACF와 25~32 μm 솔더볼을 10 wt% 함유하는 솔더 ACF에서만 open failure가 발생했는데, 이는 낮은 솔더 wetting 면적과 그것으로 인한 crack 때문이었다. 본 연구의 결과를 종합하여, 300 μm 피치 FOB 어셈블리용 솔더 ACF 내의 솔더볼 크기와 함량은 25~32 μm, 30 wt%로 최적화 할 수 있었다.
