Challenges of Pd-coated Cu wire bonding on C65 Al bond pad low-k wafer with BOA structure

Abstract

This paper presents the challenges of time zero bond integrity for Palladium-coated Copper (Pd-Cu) wire bonding on C65 Aluminium (Al) bond pad low dielectric constant (low-k) wafer with Bond Over Active (BOA) pad structure. The LQFP-176 7.9mm×8.5mm, Rough micro Pre-Plated Frame (RµPPF) lead-frame design with the bonding temperature of 240°C on thin 1.45µm Al bond pad thickness test vehicle and 25µm Pd-Cu wire diameter was evaluated in this study. The latest Shinkawa model UTC3000 wire bonder retrofitted with newest Cu kit design and software feature was used to ensure good bondability and Free Air ball (FAB) formation. In consideration of sensitive low-k wafer, 3 types of latest capillary technology from 3 different suppliers were assessed. In combination of the latest Multi-step (M-step) software with gradual bond force steps feature, it demonstrated a promising outcome that fulfilled the time zero bond integrity criteria particularly on one of the capillary. The cross-sectional analysis showed good ball bond of >25% Al remnant and uniform bond profile. The essential >75% of Inter-metallic Compound (IMC) coverage is achievable denoting a good ball bond integrity and Al push-out is controllable without shorting to adjacent pad. By de-layering process, no pad damage underneath the bond pad was observed at sample size of 8k pads, suggesting that a lower stress bonded ball was manageable. A spherical shape of FAB was formed as an indication of good ball formation. Other wire bonding process control was also examined and managed to meet the requirements.