Machine Design
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Printed Copper Inks Now Available for Conductive Patterns

PV Nano Cell's Fernando de la Vega talks with Machine Design about how copper-based inks stack up against their silver counterparts.

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Printed circuits have gone through significant advancements over the past decades and are now a growing sector in the electronics industry. Growth in printed electronics is expected to continue growing and be at almost a 22.5% annual growth rate by 2021. To date, most printed circuits have used silver inks for conductive patterns, a relatively expensive proposition. PV Nano Cell, an Israel-based firm, recently announced it had developed a copper-based ink that performs similarly to silver-based versions.

To get a better idea of what these inks are capable of and what the new cooper-based version means, we talked with Fernando de la Vega, CEO of PV Nano Cell. He holds a Ph.D. in applied chemistry and a strong technical and management background with more than 20 years in industry performing different tasks (R&D management, QA, VP operations, and general manager at Chemada, Tosaf, and Cima NanoTech). He is a co-inventor with more than 11 patents (nano materials and alternative energy), and an author and co-author of many papers and publications, including a book chapter in conductive inks for ink jet printing.

Can you tell us a little about printed inks? What are in the inks other than the metal (silver or copper)? Can the printed “wires” ever be repaired or replaced if damaged?

Here at PV Nano Cell, engineers have developed inks to print conductive patterns, not wires. The inks are made of a high percentage of metal nanoparticles, which let companies create inks with low viscosities and lower sintering temperatures. This lets the inks be used in temperature-sensitive substrates like plastics and paper. 

Metal concentrations in the inks are generally 50% and above. However, they can be from 25% to 60% (by weight). In addition to the metal content, the inks include solvents and additives specific to the requirements of individual applications. With this customization we can to do many things, such as enhance adhesion to substrates or make inks compatible with solar cells.

If the printed pattern becomes damaged it is likely it can be repaired, though it depends on the individual situation.

For the silver-based inks: How thin can the wires they create be? How long can they be? How quickly do the lay down these wires? How much distance can be between wires, or how closely can the wires be placed to one another? How much current can they carry? Can they be used in mass-produced circuit boards? In what industries and applications are they used the most?

The dimensions of the printed patterns are determined mostly by the printer and printing process. To date, we have been able to produce patterns with unlimited length, widths of 100 microns, and in some cases even 40 to 50 microns wide. In addition, the thickness can range from 0.5 micron to 10 microns or higher, depending on the throughput. In general, the distance between patterns is similar to the width of those patterns.

Our Sicrys copper-based inks are designed to support high throughputs; therefore, the limit depends mostly on the printers. Printers that have been designed for solar cells can print 1,600 cells per hour, whereas printers for printed-circuit boards have been designed to print 20 one-layer 18 ×21-in. boards per hour.

The amount of current that can run through the printed patterns depends on the thickness of the patterns themselves. A thicker pattern can carry more current than a thinner one.

At PV Nano Cell, we are focusing on mass production in various fields, including printed electronics, printed circuit boards, antennae for mobile phones, solar cells, and 3D printing applications.

Copper based inks can be used to inkjet print conductive patterns on a variety of substrates. The patterns are also obviously flexible.

In what applications can printed inks not replace traditional copper and silver traces or wires?

We predict that in time digital printed inks will be able to replace screen printing and photolithography. At this time there are still limitations caused by the geometry of the patterns which limits the ability to print widths below 40 to 50 microns, as well as additional limitations related mainly to specific requirements for each application.

What are the advantages of printed inks?

The general advantages of printed inks are narrower patterns, cleaner processes, short set-up times, lower costs, and the ability to print small and large batches. There are additional advantages to printed inks specific to each application. PV Nano Cell focuses on additive and digital processes, which have various advantages. Printed inks in additive processes benefit from a more efficient, shorter, and cleaner process with no hazardous waste. Digital processes that use printed inks allow for quick set-up times and greater flexibility in design, which makes the only limitation our imaginations. 

Why was silver the basis for the first widely used printed inks?

The printed electronics industry already uses silver pastes to print conductive patterns through screen printing. Silver was chosen because it offers ease of use, stability, good electrical properties, and competitive pricing. Although copper is less expensive than silver, copper oxidizes quickly, making it difficult to work with. Alternative options such as gold, palladium, and platinum are far more expensive.

Why was there a push for copper-based inks or at least an alternative to silver? Was it for lower cost, or better performance, or safety and reduced environmental risks?

The main demand for silver ink alternatives is cost. Copper inks offer nearly identical electrical properties to their silver counterparts. Although copper inks were hard to produce due to their unstable nanoparticles, we have been able to overcome this challenge to create a copper ink that has a shelf life of one year in ambient conditions.

What new technology or technical capability did it require to make copper-based inks viable and economical?

To produce a stable copper-based ink, we needed to create a process that produced single crystal copper nanoparticles. We used the same production process that creates our silver-based inks which uses metal efficiently, does not produce hazardous wastes, has low energy requirements, and can be scaled up.

How do copper-based inks compare to silver ones in terms of performance (size, spacing, durability/lifetime, cost, safety, and environmental risk)?

In time, we are aiming to provide a full range of copper inks that have the same properties and performance of silver inks. Our current copper inks have demonstrated the same properties and advantages of their silver-based counterparts.

Do copper based inks have any advantages over silver ones? Do they have any disadvantages, or do they not work in as many applications and situations?

Copper-based inks’ largest advantage is their increased cost efficiency. They cost only a third that of silver-based inks. The largest struggle facing the copper ink industry currently is the after-printing processes. Because sintering needs to be performed at higher temperatures, the printing process may be less efficient. However, we are working to find a solution.

What will copper-based printed inks mean for the PCB industry?

Copper-based inks will let circuit-board makers use less expensive substrates that do not have to be coated with metal that will eventually be washed away in the photolithographic process. Inkjet printing also eliminates the wet chemistry and hazardous waste associated with current photolithography. The process is also quick and flexible, with no wasted metal higher yields and less manpower needed. All of this means capital expenses can be lower.

In the future copper-based inks and inkjet-printed conductive patterns will be improved to handle two-sided and multi-layer PCBs. They will also be able to create passive components such as resistors and capacitors, which will reduce costs and assembly steps.

What does the future hold for printed inks? Will they ever carry relatively large voltages and amps? Will they replace regular wiring in houses and industry? Could they ever carry really high voltages and power, such as high-tension wires? Will carbon nanotubes/particles ever be used instead of the copper and silver?

The future for printed inks has yet to be determined, as the many applications which may benefit from the use of them continue to grow each day. The particulars of printed inks carrying large voltages and amps for use in high-tension wires or in home wiring are undetermined. At the moment, I do not envision it coming to fruition—at least not in the near future. Carbon nanotubes have been on the market for some time. Their length would make them difficult to digitally print and their lower electrical properties in comparison to metals would make them inefficient specifically for thick layers. However, they may have advantages when creating transparent conductive products.

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