High-frequency wirebonding: Process and reliability implications

Most of the current wirebonding machines operate at a nominal frequency of 60 kHz. The choice of 60 kHz was made several decades ago based on appropriate transducer dimensions for the product sizes of the era and stability during the bonding load. A wide range of frequencies (25 to over 300 kHz) has been used to ultrasonically attach wires. Today's interest in higher-frequency wirebonding stems from reports that using higher ultrasonic frequencies produces better welding at lower temperatures in shorter bonding times (dwell times). It has also been indicated that higher-frequency wirebonding improves bonding to pads placed over soft polymers such as Teflon(R) and unreinforced polyimide. Despite these reports, few, if any, systematic side-by-side studies using controlled conditions have been performed. The current work continues our systematic efforts to evaluate the effects of using higher bonding frequency on bond quality and reliability. Using two identical bonding machines (except for ultrasonic frequency), we have investigated the bonding process on a variety of metallizations and substrates. In this study, statistical methods were used to determine whether the differences in the means and variances between comparable samples sets (one bonded at 60 kHz and the other bonded at 100 kHz) were significant. Results of our studies indicate that significant differences exist between bonding at nominally 60 kHz and bonding at 100 kHz. In particular, we describe effects associated with: (1) the ball shear strength before and after thermal aging (150degreesC for 120 hours) for both 60-kHz and 100-kHz bonds, (2) the influence of substrate-metallizations combinations on the geometry and strength of the bonds at the different frequencies, and (3) the sensitivity and control of the overall bonding processes.