Ultrasonic-Assisted Thermocompression Bonding Method of Solder Anisotropic Conductive Film Joints for Reliable Camera Module Packaging

In this paper, to improve the reliability of anisotropic conductive film (ACF) joints in the camera module packaging without any mechanical damages, ultrasonic-assisted thermocompression (TC) bonding was investigated using solder ACFs. This technology uses small lateral-direction ultrasonic vibration during the TC ACF bonding to break solder oxide layers for sufficient solder wetting on the electrodes at ACF joints. Bonded solder ACF joints were investigated in terms of solder ACF joint morphology, resistance, adhesion strengths, and reliabilities. In addition, the stability of camera module components after bonding process was also observed. Ultrasonic-assisted TC bonded solder ACF joints showed excellent solder wetting compared with conventional TC bonded solder ACF joints due to broken solder oxide layers by applied ultrasonic vibration. The Au contents diffused from electroless nickel immersion gold-finished Cu pads to solders was about 12 atomic% in the ultrasonic-assisted TC bonding. The Au contents were three times higher than that of the solders in the conventional TC bonding. In the ultrasonic-assisted TC bonding, ultrasonic amplitude was optimized. At the optimized amplitude, solders were well wetted and showed no mechanical damages of camera module components. Adhesion strength of ultrasonic-assisted TC bonded solder ACF joints was almost the same as that of TC bonded ACF joints due to fully cured resins in these samples. During the unbiased autoclave test (121 ^°, 2 atm, 100%RH), ultrasonic-assisted TC bonded ACF joints showed no open-circuit failure compared with conventional TC bonded Ni ACF joints due to metallurgical alloy joints between solder balls and electrodes by sufficient solder wetting. These results prove that ultrasonic-assisted TC bonding using solder ACFs can be applied for highly reliable ACF assembly in the camera module packaging without any mechanical damages.