Drop-on-demand inkjet printing of functional inks has received a great deal of attention for realizing printed electronics, rapidly prototyped structures, and large-area systems. Although this method of printing promises high processing speeds and minimal substrate contamination, the performance of this process is often limited by the rheological parameters of the ink itself. Effective ink design must address a myriad of issues, including suppression of the coffee-ring effect, proper drop pinning on the substrate, long-term ink reliability, and, most importantly, stable droplet formation, or jettability. In this work, by simultaneously considering optimal jetting conditions and ink rheology, we develop and experimentally validate a jettability window within the capillary number−Weber number space. Furthermore, we demonstrate the exploitation of this window to adjust nanoparticle-based ink rheology predictively to realize a jettable ink. Finally, we investigate the influence of mass loading on jettability to establish additional practical limitations on nanoparticle ink design.
Inkjet‐printed gold nanoparticle pillars are investigated as a high‐performance alternative to conventional flip‐chip interconnects for electronic packages, with significant advantages in terms of mechanical/chemical robustness and conductivity. The process parameters critical to pillar fabrication are described and highly uniform pillar arrays are demonstrated. More generally, this work underscores the impact of sintering on the electrical, mechanical, structural, and compositional properties of three‐dimensional nanoparticle‐based structures. Using heat treatments as low as 200 °C, electrical and mechanical performance that outcompetes conventional lead‐tin eutectic solder materials is achieved. With sintering conditions reaching 300 °C it is possible to achieve pillars with properties comparable to bulk gold. This work demonstrates the immense potential for both inkjet printing and metal nanoparticles to become a viable and cost‐saving alternative to both conventional electronic packaging processes and application‐specific integration schemes.
We present a novel technique for heterogeneous integration using gold filled Through Silicon Vias (TSV) and thermocompression bond bumps formed in a single fabrication step, using gold nanoparticles dispensed by inkjet printing. Gold-filled TSV arrays (12 x 12, via radius 50µm, pitch 250µm) have been demonstrated using this method. Void free, filled TSVs are reported, and thermocompression bonding yielded seamless interfaces. Die shear tests show good bond strength, and sub-Ω via resistances were measured. The presented integration process exhibits a low temperature budget (≤250°C), allows good alignment, and is compatible with substrates of different sizes and materials, enabling the heterogeneous integration of known-good-dies (KGD) which may have been fabricated in different technologies.
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.
Using metal nanoparticle-based inks and droplet-on-demand inkjet printing, we have demonstrated the ability to fully fill, bump, and bond through-silicon vias (TSVs). Standard thermo-compression flip-chip bonding is used to provide both robust mechanical as well as electrical contact while processing below conventional back-end-of-line (BEOL) thermal budgets. This novel process enables both the filling and bumping of TSVs in a single process step, making this process unique among conventional TSV process flows and providing a cost-effective alternative for three-dimensional integration and chip stacking at both the die-level and wafer-scale.
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