The dielectric material is often the bottleneck against fully printed high-performance RF and MW devices. This is often a neglected challenge as the emphasis is mainly on the conductive layer. Indeed, the development of a suitable low-loss digitally-printable dielectric material with high and controlld resolution is a technical challenge. In this 3-min video, Yuri Piro from University of Massachusetts Lowell explains why this is challenging
Yuri said " So photo polymers are the go to materials and the very common conventional polymers are typically acrylate or methacrylate. Those are initiated with radicals and they have huge dipoles. And those dipoles are actually very important for the kinetics of the reaction. They stabilize the radical during propagation and if you were to start to remove those dipoles to make your material less lossy, you would destroy the kinetics of the reaction. You couldn't form this polymers on the spot. Now Cationic polymerization is also very common for things like Epoxies, but again you have the electron withdrawing groups to stabilize the cation propagation and removing those again really hurt your kinetics. So there's a couple of ways around that.
One of the ways is you use pre-formed non-polar materials and you just dissolve them in some sort of solvent. There's two big issues with that approach. The first issue is you have to drive off the solvent during curing, and that introduces a lot of stress into the film. And the other issue is the viscosity. Typically, those are high risk materials. And to get the viscosity down to where you can use high resolution printing, you need to use large fractions of solvent. And a lot of the solvents to get the non-polar materials dissolve have to be things like Taliban or xylene. And those are pretty toxic and hard to use in manufacturing environment. So there's this big need to use to make non-polar polymers on the spot that have this low dielectric constant and minimal processing conditions.
Now, there are some solutions happening right now. You'll see in printed electronics the POLYAMIDE is a common material people use that has a low CTE and has a pretty low dissipation factor. But one of the issues with Polyimide is the material has to be immunizes on the spot. So you have to heat up your material past 180 degrees. And once you start doing that, you have to disqualify some of the flexible materials that are kind of the most exciting parts of integrating print electronics in the first place. Another common material people use is benzos butane. But again, that requires processing conditions that are very long and hot and destroy the ability to introduce flexible materials.
So coming up with a non-polar material that you can form on the spot with low processing conditions and low polarity is difficult and you really can't use these conventional approaches."
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