Investigation of Cu-Cu Stitch Bonding Reliability on Cu Leadframe Substrate Using Ultrathin Cu-Selective Passivation

Despite advances in 2.5D and 3D packaging, wire bonding remains the industry’s most widely used interconnect method due to its cost-effectiveness. Ultrasonic wedge bonding, also known as stitch bonding, is a critical interconnection technique that links IC dies to external circuits for power and signal transmission, where precise control of bonding parameters is essential to ensure reliable connectivity. However, as the industry transitions from traditional gold (Au) wire to cost-effective copper (Cu) and palladium-coated copper (PCC) wire, new challenges have emerged in ensuring second bond (stitch) reliability, particularly on modern leadframe substrates with silver (Ag) coatings. While Ag plating helps suppress Cu oxidation, it adds to material cost and raises environmental concerns due to its bioaccumulation potential. To address this, we developed a low-cost, ultrathin Cu-selective passivation coating that effectively suppresses Cu oxidation and enables direct Cu-Cu stitch bonding without the need for Ag. This passivation layer is fully compatible with standard IC packaging processes and does not interfere with surrounding dielectrics or interconnects. This paper discusses the application of this ultrathin coating on copper-based leadframe (LF) and is subjected to stitch bonding with 25 μm (1 mil) aluminum (Al-1wt%Si) and palladium-coated copper wire (PCC) under different heating conditions (170°C-200°C). The bonding power, time, and force are kept constant throughout the work. Coated Cu leadframes demonstrated a substantial improvement in pull strength and reliability. Notably, bonds on bare-Cu LF failed at the interface due to oxidation, while passivated-Cu LF exhibited failure at the wire neck, indicating stronger bond integrity. Preliminary data with PCC wire at 125°C further support these findings. This work highlights a promising, environmentally sustainable alternative to Ag plating, enabling reliable Cu-Cu stitch bonding for next-generation, low-cost IC packaging solutions.