The world of flexible and hybrid electronics (FHE) was on display during FLEX 2019, organized by SEMI-FlexTech. One of the keys to FHE is the conductive inks and other materials, from silver inks and nanowires and carbon nanotubes to even more unique materals.
 
Inks were the topic of FLEX Session 17, and attendees learned about the current state-of-the-art of the industry, as well as what may be coming next. Robert Praino Jr., COO and co-founder, Chasm Advanced Materials, opened the session with “Transparent Conductive Films: Hybrid Material Structures vs. Single Material Composition.”
 
Praino said that after working on developing single material, Chasm saw there were challenges to be overcome. Crating a hybrid system solved these issues.
 
“We looked at a new hybrid structure,” Praino noted. “We discovered a ‘vehicle’ as a viscosity modifier, which opened the door to a surfactant-free ink. Silver nanowires (AgNW) is currently the leading ITO alternative but patterning cost and stability are challenges. We combined carbon nanotubes (CNT) and ink to create our proprietary carbon nanotubes for screenprinting.
 
“AgeNT is coated silver nanowire film with screenprinting CNT ink,” Praino added. “The AgNW is then etched away. We have created an aftermarket Clear Film for cell phones that redirects RF energy, a wearable device for queue management, LED surfaces in automotive moonroofs. We also are creating a transparent heater for defogging headlight lenses. Being able to deliver is critical – we have production, and we are enhancing our labs.”
 
Praino also discussed key lessons the company learned along the way.
 
“We learned to be objective about materials and material sets – we realized that CNTs alone would not work,” he said. “Be willing to pivot, and execute applications development to deliver solutions, not just materials. Look out for customers that aren’t on your radar – we were looking at touch screens, but customers came to us with heaters and other applications.”
 
Christopher Tabor, research scientist, Air Force Research Laboratory, talked about th development of gallium-based  liquid metals to enable highly stretchable electronics in his talk, “Liquid Metal Networks for Stretchable Electronics.”
 
“These are alternatives to mercury that use gallium,” Tabor reported. “It is non-toxic. The key is controlling the oxide so you can control the metal, which we developed to control switches and also for self-healing electronics that creates a shell over particles.
 
“Our newest area is ultra-stretchable electronics to create airman/machine interfaces and conductive hinges,” added Tabor. “We can print these polymerized liquid metal networks (LMN). We found that the conductivity increased during elongation with no fatigue or hysteresis over 1,000 cycles. We have successfully reached elongation of more than 700%. It is easy to inkjet or aerosol jet because it is a colloidal solution.”
 
Don Veri, sales and business development, Meyer Burger, covered “Inkjet Printing for Hybrid Printed Electronics and Semiconductor Applications.”
 
Veri observed that inkjet printing offers numerous advantages.
 
“Inkjet is flexible and very precise,” Veri said. “It is low cost, with less process steps and low material consumption as it is an additive process, and provides high yields. We see most interest in dielectric inks, masking inks (hotmelt and UV curable), and conductive inks (silver of copper nanoparticles in solvent, cured by drying and sintering).
 
Veri talked about some of the various projects Meyer Burger is working on with its customers.
 
“We have printed  PE flexible solder masks using acrylate-based inks, which eliminates several process steps,” Veri added. “We are working on flexible electronics using silver nanoparticle and polymer inks, moisture barriers using UV-curable inks, stress buffers using solvent-based dielectric and polyimide ink, and wafer-edge protection with polyimide or other dielectric.”
 
Andrew Moon, director of marketing, C3Nano, discussed “Engineering the Highest Performance Flexible Transparent Conductor.” Moon noted thatnew form factors require new material; for example, Royole recently launched its first foldable smartphone, with Samsung also announcing its first foldable phone.
 
“Our material is ideal for touch sensors. Most of our customers are using our slot-die ink for mass production,” Moon said “The main advantages are the coating and drying temperature, and it doesn’t require vacuum process. We add silver nanowire synthesis and purification to create our Activegrid ink. Our Nanoglue bonds nanostructures together into a nanoscale network or mesh that we call Activegrid. Fusing leads to a dramatic lowering of resistance and formation of the nanoscale grid.”
 
Bruce Kahn, chief scientist and director of business development, RIT, concluded the Inks session with “Preparation, Properties, and Applications of Aligned Ni Nanowires.” Kahn discussed RIT’s AMPrint Center, which features additive manufacturing and multifunctional printing. One difference is that RIT is using nickel-based conductive materials.
 
“The printing work was done on the NanoJet printhead, on an aerosol printing process,” Kahn noted. “The inks we developed can be used in various other ways. We used nickel, as it is oxidation stable and has the highest temperature coefficient of resistance of any metal.”