Extended Isothermal Ageing of Cu-Al Intermetallic Joints: Comparative Wear-Out Study and Characterization Methods for Pure and Alloyed Copper Wires

In wire bonding interconnections, copper has replaced gold due to its cost and performance advantages, particularly when coupled with aluminum bond-pads. Alloyed copper wires can further improve reliability performance. This study investigates how the wire alloying elements can affect the time to wear-out of the Inter-Metallic Compounds (IMCs) between Cu wire and Al bond-pads when submitted to extended isothermal ageing (HTS). In previous experimental works, a delaying effect of the alloying elements on the IMC evolution was observed, but the ageing level reached was not sufficient to detect loss of integrity in the joint, which characterizes the wearout stage. The investigation has therefore been continued through destructive Wire Pull Test (WPT) and inspection of cross-sectioned samples by SEM and TEM to monitor the structural evolution of the bonds in front of an extension of the HTS trials until 10000 equivalent hours at 150° C. IMC stoichiometry has also been studied on TEM lamellas through EDX tools when the typical wear-out symptoms have been detected. The results confirmed the lifetime advantage hypothesized in the previous study for alloyed Cu, with a significant benefit for reliability-demanding applications and aggressive geometry scale-down needing adoption of very thin wires.