High power and fine pitch assembly using solder Anisotropic Conductive Films (ACFs) combined with ultrasonic bonding technique

In this study, in order to improve the electrical properties and the reliability of ACF joints, we propose the simultaneous fluxless solder joining and adhesive bonding technology. This technology utilizes fluxless soldering within an adhesive matrix on metal electrodes combined with room temperature ultrasonic (U/S) ACF bonding technique advantages. According to the experimental results, the temperature of the solder ACF joints showed rapid heating rates up to 400 °C/s and peak values above 250°C by applying ultrasonic vibration. The ACF temperature could be precisely controlled ranging from 75°C to 260°C by adjusting U/S vibration amplitudes from 4 um to 13 um. At above the melting temperatures of solder particles, U/S bonded solder ACF joints showed higher than 80% soldering ratios and no void formation with optimized U/S parameters. The soldering ratio at the solder ACF joints increased as the ACF temperature increased and it was presumably due to the viscosity decrease of the ACF adhesive matrix. On the other hand, thermocompression (T/C) bonded solder ACF joints showed poor soldering ratios lower than 30% and severe void formation at above 200°C. At the same time, U/S bonded solder ACF joints showed 30% reduced electrical contact resistances and twice better reliability in an unbiased autoclave test (121°C, 2 atm, 100%RH) compared with conventional ACF joints. Significance of this result is that fluxless solder joining and adhesive bonding can be simultaneously achieved within 5 seconds by using solder ACFs combined with the room temperature U/S bonding technique.