Demonstration of Inkjet Printed Nanoparticle-based Inks for Solder Bump Replacement

Conventional eutectic solders suffer from scaling limitations related to their relatively poor conductivity, electrochemical and thermochemical stability, and toxicity/regulatory restrictions. Novel materials and systems that aim to replace conventional eutectic solders for future packaging applications must be able to meet both the throughput and the thermal demands of standard bumping processes while simultaneously providing the electrical and mechanical properties characteristic of high performance and extended lifetimes. Inkjet printing is a fast-growing deposition technique aptly suited for wafer bumping processes because the feature sizes and pitches capable are appropriate for solder bumps and the droplet-on-demand functionality is an attractive alternative to blanket films processing. Metallic nanoparticle-based inks represent the functional material for eutectic solder replacement, and these inks are attractive due to their relative ease in synthesis and dispensation, their low thermal budget, and electrical conductivities that routinely exceed those of even the best lead-based solders. Furthermore, such material systems have been shown to deliver very good reliability due to their bulk-like properties and avoidance of intermetallic complications as seen in eutectic systems. In this work, we present the capability of printing three dimensional features with nanoparticle inks and demonstrate the critical processing parameters that dictate size control. We fabricate micropillars using a commercially available gold nanoparticle ink, demonstrating feature sizes and techniques appropriate for solder bump replacement. Further, we investigate, for the first time, the process of sintering in three-dimensional nanoparticle features. Using extracted conductivity and mechanical strength measurements, we suggest a model for the sintering in three-dimensional features using our printed micropillars as test structures.