Wed, 02/21/2018 - 9:00am 3 Comments by Laura Panjwani - Editor
Thanks to recent breakthroughs, researchers and manufacturers are now able to successfully demonstrate 3D printed electronics using silver nanoparticle inks. While this technology has proven to be quite promising for lower power electronic systems, it is not feasible for systems that require high-current density, known as “power electronics.”
“The same properties that make silver appealing for printed electronics—that ability to sinter at low temperatures because it’s very unstable and the atoms migrate with low temperatures and low energy—this can lead to premature failures when you have higher current densities running through it,” said Greg Fritz, a material scientist in the Charles Stark Draper Laboratory. “So much of the technology of printed electronics has been on printed, lower power circuits that work fine with silver nanoparticle inks, but Draper has products that are high power and quite small so we are not able to use the printed silver for these products.”
Because of this, Fritz and his team at Draper are currently working on alternative techniques for printing high power electronics.
Fritz recently discussed this work at the 2018 FLEX Conference, held Feb. 12-15 in Monterey, CA.
Prior to his presentation—titled “Enabling Printed Power Electronics through Collaboration”—Fritz spoke with R&D Magazine about what he and his colleagues are doing to make printed power electronics a reality.
R&D Magazine: What falls under the “power” electronics umbrella?
Fritz: The term ‘power’ is kind of a misnomer in that it is really referring to high power density. When we say power, we mean powering something without exacerbating the size of the product beyond a meaningless point and still maintaining the performance. The more technical way to describe what we are doing is that we are trying to create a printable material that can handle high-current densities without undergoing high temperature processing.
R&D Magazine: What are the applications for printed power electronics? Fritz: We are finding that drone boards are actually pretty high power so we are trying to print the ‘guts’ of the drone. Between the battery and the rest of the drone— the lights and the coms and the fan and all the other things—there is one board that gets all the power. It can require a single amp or more than ten amps of current, and that’s a lot of power for silver to endure when you scale down the size of the conductor. We are also looking at high power antennas. We have 3D antennas that we are making now with wire or copper or other bulky materials, but it would be really nice if we could just print these materials, and today we can’t do that with the available printed inks. R&D Magazine: Why are the currently used silver nanoparticle inks not effective for high-powered printed electronics? Fritz: Silver works for printed electronics because it does a really good job of going from an ink to continuous lines. This is because the surface atoms of a silver particle will move around until they find a low energy spot. That generally means they can sinter into these electrically continuous traces pretty readily. Unfortunately, once they form that continuous trace, this property is not changed so now when I put energy into it, in the form of temperature or even more commonly high current, you will get migration leading to failure. It’s the same properties, so the atoms will be diffused amongst the surface and it will open up voids. That is why silver is not used in your cell phone or your laptop as a power conductor—silver is this great conductor, but it has this problem of being unstable, especially with high currents. R&D Magazine: What research avenues are you exploring to print power electronics? Fritz: What we want to do is be able to print something—like silver where it sinters well— but once it sinters it stops being unstable. That is quite tricky because on top of that we are adding on a requirement that we don’t want high pressure processing. People around the world are working on printing with copper and gold and other materials, and to really get a good quality copper or gold, you also need to have high pressure or high temperature during the sintering process because unlike silver it’s always stable relatively. What we’ve come up with is a way to have this tradeoff. We start off with a material that is nano-layered—so it is unstable as an ink, it has a nano-spaced reactants. Once we print the ink and we sinter it, we are able to form a new material, which is an alloy of the layered structure we started off with, but now this new alloy has completely different properties. It is inherently high-temperature stable. We’ve demonstrated this, and we’ve made conductors this way. The downfall of this is that we need to keep improving the other properties—the mechanical stability for flexible electronics, the electronic connectivity to make it more directly competitive with the available silver. However, we’ve been able to make these new inks and we are currently printing them in all kinds of commercial printers. We are off to a promising start. R&D Magazine: Are there barriers to making this technology successful? Fritz: There is some optimization to be done in this area and there are some new chemistries that we need to be aware of. We are trying to improve the resistivity. We understand there is a trade-off. We are not going to beat silver in terms of resistivity. But even in your cell phone they used copper wires, and you trade off the better resistivity of silver, for the reliability of copper. That is all we are asking the market to do as well. I don’t think it is that crazy to say we are going to have a worse resistivity than silver, with the added benefit of it being printable and handling high power, but we need to make sure that we are not experiencing too much of a penalty in resistivity. We’ve proven our concept and I think now we need to understand how much more improvement in resistivity we need to make this adoptable. R&D Magazine: What are the biggest advantages of a printable approach for these high-power electronics? Fritz: The real challenge with some of our products is that we don’t have a lot of space. We need to put something in there that is going to be conformal and fit in an odd spot. It is easier in some ways to print. For our size of company, a lot of times what we are doing is starting with commercially available products that are made in the billions of units a year and then taking some of these chips or other components we want and adding something to them. We also want to connect multiple chips on a single module in a way that makes a whole module have added value. We are printing these packaging interconnects, these interconnects between chips or commercial products, because we want to be able to do that quickly, in-house, and we want do it a way that we can trust the quality. Printing has the benefits of both conformability and the ability to work on smaller, custom parts that you couldn’t afford to run in a fabrication plant. The fact that you control it means anyone can do this. If we can deliver this to market anybody can print their own circuits. This interview was edited for length and clarity