A key advance in recent years in the field of additive manufacturing of electronics has been in the quality and consistency of conductive inks for inkjet printing. This advance is a main reason why scalable printed PCBs, FPCBs, and myriad Flexible Hybrid Electronics (FHE) are now within reach.
Materials Compatibility and Roadblocks
A key roadblock to current printed electronic technology is material compatibility with printing systems. For example, to use dielectric materials in production, they need high tensile strength, impact resistance, compatibility with silver ink, a dielectric constant (εr) of 3.5-4.5, deformation resistance to above 200ºC, good printing performance through a piezo inkjet head, electrical insulating properties, and be able to withstand thermal fluctuation from 20ºC to 140ºC.
Advancements in Silver Conductive Inks
Historically, nanoparticle silver inks have been used for additive manufacturing, however, they tend to clog inkjet heads, and resistivity for nanoparticle inks ranges from 10-4 Ω.m to 10-6 Ω.m with sintering temperatures of 150ªC and above for 60 minutes and longer, making them unsuitable for most commercial and manufacturing applications. Advances with reactive, particle-free silver conductive inks have proven to have high performance and reliability in inkjet applications.
Incorporating Carbon Nanotubes (CNTs)
Our research has produced ink that is near the bulk conductivity of silver with volume resistivity in the range of 10-8 Ω.m. The ink fully sinters at a nominal temperature of 90 – 100ºC (as low as 80ºC) and has exceptional printing reliability without clogging. Therefore, this ink can be used in scalable manufacturing for PCBs and FHEs. By incorporating carbon nanotubes (CNT) in the printing of the dielectric layers we have also developed a mechanically superior dielectric material and additional research that shows CNTs can act as a radiation shield with metals.
Mechanism and Benefits of Nanotube Integration
We have discovered that adding very small amounts of nanotubes to a curable matrix drastically changes the properties if the material is cured in thin layers of less than 20µm, just as an inkjet printer would deposit. The mechanism is such that, by laying down a thin film, the nanotubes align themselves in the x-y plane thereby creating a strong layering structure by which a few nanotubes on the surfaces of each layer interface in the z-direction. The tubes must be singular, maximizing surface area to have the desired effect and prevent inkjet nozzles from clogging in high-use manufacturing lines. If the nanotubes agglomerate or bunch up, they will have the opposite effect, and make the material weaker. Instead of composing a singular solution with CNTs, we can print the dielectric at the same time as the CNT solution, causing layer mixing before curing resulting in nanocomposite material that adheres to current manufacturing standards. We have been able to reduce the CTE from over 200 (10–6/K) to below 30 (10–6/K) and significantly improve mechanical properties.
Acknowledgments
This groundbreaking work has been made possible through sponsorship from a NIST SBIR Phase I grant, with Phase II endeavors underway and patents pending. Additionally, we extend our gratitude to the NextFlex community for their invaluable support throughout this journey.
Conclusion
In conclusion, the optimization of conductive inks and the integration of nanocomposites mark significant strides in advancing additive manufacturing capabilities, particularly in the realm of electronics. These developments not only overcome existing roadblocks but also pave the way for scalable production of complex electronic components, ushering in a new era of innovation and accessibility in the field.
Original Publication
