Yuan Gu and Ayan Maity -
Senior Scientists, Electroninks
As the world of electronics continues to change shape – literally, and products become wearable, flexible, foldable, and capable of processing data at the same time, the demand for the tiny circuitry that makes this all possible has hit a tipping point, creating a need for innovative solutions.
Advancements in additive manufacturing are dictating the shape, size and even purpose of new products yet to be created by consumer and industrial companies. Electroninks' particle-free inks - along with the next generation of NanoJet provided by the Integrated Deposition Solution (IDS) system - allows for the fabrication of sensors and circuits. The combination of Electroninks’ particle-free inks and IDS NanoJet provides a higher mass output, better printing quality, and lower cost to fabricate electronics compared with other traditional digital printing technologies.
Particle-free Cu, Ni, Ag, Au and Pt conductive inks from Electroninks, Inc. (based in Austin, TX) are metallo-organic-based inks that provide superior performance and reliability compared to nanoparticle-based inks. Particle-free inks consisting of metal-organic precursors typically have better performance because they reduce metal films more cleanly, often at lower temperature. Final films do not contain organic components and tend to survive stress/adhesion tests which increase their reliability to moisture sensitivity level (MSL) and aerospace standards.
Particle-free inks consisting of metal-organic precursors decompose more cleanly and often at lower temperature. Particle-free inks also have better stability during the printing compared with nano-particle based inks, thus the printing uniformity and longevity is better. The shelf life of metallo-organic based inks is usually longer than the nanoparticle based inks.
Table 1: Comparison of NPs based Ag, Au and Pt inks with Electroninks particle-free Ag, Au and Pt inks
The inks can be deposited using aerosol jet printing which is a non-contact deposition method. The schematic of this printing process involves jetting out an ink stream produced by ultrasonic or pneumatic atomization. Electroninks’ Ag, Au and Pt inks are specifically designed for ultrasonic atomization, so they attain higher printing resolution than the more conventional pneumatic atomization process. This new advanced process of printing circuits, sensors, or interconnects with an aerosol jet printer (OPTOMEC system or IDS NanoJet system) using Electroninks’ particle-free Ag, Au or Pt ink is highly flexible, high conductivity and reliability. This system also gives customers complete control and tremendous latitude in customizing their application and choosing substrates.
IDS has developed the next generation aerosol jetting print head technology. This jetting process, trademarked NanoJet, relies on both hydrodynamic and aerodynamic focusing. It enables a collimated and focused broad aerosol droplet size distribution resulting in printed lines with exceptional printed edge quality. In comparison, traditional alternative approaches have historically resulted in overspray at the printed line edges.
Electroninks’ particle-free Au ink has 4-10 wt.% solid fraction with a mass output that is competitive with NP-based inks. The particle-free ink is concentrated in the atomization as the high vapor pressure solvents are removed during the atomization. This ensures an appropriate mass output. Due to this novel ink formulation design, Electroninks’ particle-free Au ink can be printed at room temperature with a high mass output, which is critical when printing on conformal surfaces. The printed structure is thermal cured at 300 °C for 1 hour to achieve the best electrical conductivity.
Figure 2: Au ink printing on glass (a) before thermal curing; (b) after curing at 300C for 1h
Overspray and satellite spots are inevitable in aerosol jet printing. The overspray and satellite spots are caused by less weight droplets with insufficient inertia to be focused by the central sheath gas and deposited along the edge of designed lines. In contrast, particle-free Au ink has excellent atomization. It exhibits limited overspray and satellite spots in fine features printed by the IDS system, resulting in approximately 60μm linewidth. This improved line edge definition and overall line quality makes Electroninks’ particle-free Au ink suitable for dense interconnects printing in compact electronics.
Electroninks’ particle-free silver ink has 14% silver loading and the same ink formulation mechanism as Electroninks’ gold ink to ensure a high mass output. Most of the existing alternative silver inks require printing at an elevated temperature to prevent spreading. This limits printing on conformal surfaces or printing on 3D substrates. One of the unique attributes of Electroninks’ silver ink is it can be used to print on high aspect ratio structures without heated plates. The printed features have minimized overspray and satellite spot and line width is measured at 15μm with approximate 2μm thickness.
Electroninks Ag and Au inks can be processed at room temperature, making them ideal for printing on complex surfaces. Its Ag ink provides 5-7 μΩ-cm resistivity while Au can provide 6 μΩ-cm resistivity; this low resistivity requires fewer printing passes and saves time compared with alternative inks. Pilot testing has shown the IDS’s next generation aerosol jetting technology - which uses Electroninks particle-free inks - can print feature size ranging from 20μm wide by 2μm tall to 20μm tall on even conformal surfaces with improved edge quality, less satellite spots and minimal overspray.
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