High-speed flex-on-board assembly method using anisotropic conductive films (ACFs) combined with room temperature ultrasonic (US) bonding for high-density module interconnection in mobile phones

In this study, a high-speed FOB assembly method using ACFs combined with room temperature US bonding was investigated for high-density module interconnection in mobile phones. US bonding parameters were optimized for high-speed FOB assembly using ACFs and the US bonded FOB assemblies were evaluated in terms of the long-term reliability as well as electrical and mechanical properties. At the same time, real module interconnection in a mobile phone was demonstrated using US bonding with optimized bonding parameters. The minimum US bonding time was 1 sec at room temperature at 3MPa bonding pressure and 60% vibration amplitude. It was demonstrated that the ACF bonding process can be significantly enhanced by utilizing US vibration compared with conventional TC bonding at 190°C. Using the optimized US bonding parameters, the ACF joints showed similar bonding characteristics as TC bonding in terms of the adhesion strength, the daisy-chain resistance, and stable electrical resistances during 125°C high temperature storage test, 85°C/85 % RH test, and -40°C~125°C thermal cycling test. In addition, US bonding was successfully demonstrated for FOB interconnection in a real mobile phone by replacing conventional TC ACF bonding. The significance of these results is that ACF bonding temperature can be reduced from typical 190°C to room temperature, and bonding time can be also reduced from typical 7~15 seconds up to 1 second by using the US bonding method.