High Power and High Reliability Flex-On-Board Assembly Using Solder Anisotropic Conductive Films Combined With Ultrasonic Bonding Technique

To realize an ultrathin and fine-pitch flex-on-board module assembly with high power handling capability and excellent reliability, ultrasonic (US) bonding technology with solder anisotropic conductive films (ACFs) was successfully demonstrated. This technology provides fluxless soldering within a polymer adhesive matrix on metal electrodes using solder ACFs combined with the room temperature US bonding technique. According to experimental results, the temperature of the solder ACF joints showed rapid heating rates up to 400°C/s and reached at above 250°C under US vibration. The ACF temperature could be precisely controlled from 75°C to 260°C by adjusting US vibration amplitudes from 4 to 13 μm. At above the melting temperatures of solder particles, US-bonded solder ACF joints showed stable solder metallurgical joints alloyed with metal electrodes and more than 90% curing of ACFs within 5 s bonding time. At the same time, US-bonded solder ACF joints showed about 30% reduced electrical joint resistances, more than 90% improved current handling capability and significantly enhanced reliability in an unbiased autoclave test (121°C, 2 atm, 100%RH) compared with conventional Ni ACF joints. The significance of this paper is that the fluxless solder metallurgical joining and adhesive bonding can occur at the same time, and realizes not only enhanced ACF joint properties but also reduced ACF bonding time within 5 s at room temperature.