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76 FLEX007 MAGAZINE I APRIL 2019 three millimeters, and we're working on larger widths yet. For instance, high-temperature fuel cells run at hundreds of degrees and have mul- tiple layers; further, they are not expansion matched, so they tend to crack at the interface. We print very thin layers and slowly grade it from A to B so that there isn't a sudden change from large to small expansion, but something in the middle. Matties: In your presentation, you talked about the harsh environments that you're able to withstand. That's quite an achievement too. Christenson: Yes, that's because we can tailor the materials and structure. Metamaterials is a buzz word now. For example, I mentioned the idea of a BGA bump between the circuit board and chip that was compliant. If you form the bump out of something rubbery and put silver or some other ductile, moveable conductor on top of it, you don't have to worry about expansion and con - traction. Imagine connecting stiff silicon to a stiff circuit board that expands at different rates with even a solder drop; it will fail, so we're really excited about some of these technologies. Matties: That's an elegant solution. What types of challenges are customers coming to you with? What's the typical desire for this tech- nology? Christenson: I'm in the applications lab, so peo- ple come to us when they're considering buy- ing equipment, saying, "I'd like you to make me one of my parts or a hundred of my parts first." We get requests for a serial number or a specialty marking printed on six-foot, 100- pound turbine blades to the microcircuits we're talking about here. We also get a lot of stuff for medical, such as printing on catheters, and antenna work. IoT is making everything smaller and smaller, so antennas are getting smaller as well. And they're small, but they're still big enough that they could be screen printed—but they're wrapped around the inside or the sur- face of the device. Thus, much of our business is three-dimensional where something else can print the pattern but not on a given shape. Matties: Are there any limits to what you can print on? Christenson: We have to be able to harden whatever it is we print. If it's a dielectric, which a lot of those are UV cured, that's fine. But if it's a metal, we don't have any inks right now that cure below 80°C, and those still have compromises. Companies that make inks are making rapid progress in metal inks, but there are always trade-offs, and there's still more progress to be made. For instance, I tested an ink last week that had fantastic conductivity, 120°C sintering, but it fell off the substrate. And when you add adhesion promoters, that degrades the conductivity. Also, people want to use copper instead of silver because it should be cheaper in the long run, for example, so that's just coming online. For substrates, our biggest problem is stretch - ing. We don't have a very conductive ink that is stretchable. Wires don't stretch well, and that's just the way life goes, and the medi- cal segment loves silicone. If you've ever tried gluing anything to silicone, it's not a good thing. Matties: Are there applications where the mili- tary is using this in the field? Christenson: I can't comment on in the field, but I can tell you what they're looking at. One of the things I showed was a nose cone antenna on a missile nose cone—the spiral—so we put a lot of antennas in weird places for military use. We also talked about the thermal expan- sion coefficient earlier; their stuff gets stored in the desert and in space, so they have huge tem- perature swings. Further, they are the leaders for high frequency, 10–20 gigahertz and above, and wire bonds, and those little wires that arc between the circuit board and silicon have a high inductance. We normally think of wires as being perfect with maybe a little bit of resistance, but they have inductance and capacitance too, which causes them a real problem. We can print con- ductors that are more ribbon-shaped, which is what they need for their ultra-high frequency

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