Nanostructure-dictated bonding strength: thermal transfer & diffusion barrier in palladium coatings on copper wires

Abstract

The redistribution of the palladium coating on the surface of palladium-coated copper wires during the bonding process is a critical factor influencing the bond connections of chips and wires. In this paper, a micro-area coating technique was used to obtain palladium-coated copper wires with varying characteristics by controlling the heating temperature. Ball formation and ball bonding experiments were conducted to investigate the evolution process of the palladium coating and its impact on bonding strength. The results indicate that at 400 °C, the thickness distribution of the palladium coating on the coated copper wire is uniform, with larger nanocrystal sizes and narrower amorphous band widths, forming larger free air ball (FAB) sizes and thicker redistribution coatings. The bonding interface exhibits a continuous and uniformly distributed palladium coating, with higher bond connection strength. For copper wires subjected to secondary heating at 300 °C, there is a significant fluctuation in coating thickness with a wider range of nanocrystal sizes and a broader width of the amorphous band. At this juncture, thinner and discontinuous coatings form at the bonding interface alongside Al₂Cu intermetallic compounds emerging, which leads to reduced bonding connection strength. The distribution of nanocrystals and amorphous regions within the coating influences the melting behavior and redistribution dynamics of coatings by changing thermal transfer behavior. The strength of the bonding interface is modified by the palladium coating at the interface due to blocking the diffusion between copper and aluminum.