A report from the part of IS&T's Digital Fabrication conference, concerned with "Printed Electronics and Devices," held on 18 Sept, Alaska.
The day started with a plenary session given by John Rogers of the University of Illinois. His topic was the use of a technique known as electrohydrodynamic printing to build flexible electronics. This is an interesting alternative to the "normal" piezo and thermal drop on demand inkjet systems and has the interesting attribute that it is capable of a sort of "burst" mode where it acts more like continuous inkjet. The resolution looks good too but we are not sure how you would make a practical multi-nozzle system.
We then got into the Printed Electronics session proper. This took up a full day with 16 technical papers on a wide variety of topics in this application area. There were a good variety of authors too, from academics, students, large and small corporations. A strong showing by the Eastern nations perhaps indicates where a lot of the action is but it was good to see us Europeans well represented too!
The concept of digital printing taking some market share from conventional presses was echoed in this session with digital techniques providing the route to personalised electronics. There was some emphasis placed on the fact that inkjet is an additive process and thus is perceived to have environmental and materials (cost) benefits.
So what was new this year? There were some simple demonstration circuits such as ring oscillators being fabricated as proof of principle. Optical feedback techniques are being used to help interlayer and front to back registration. And there was a lot of confidence that PE was really happening by non-impact printing.
Various ways of sintering metallic inks were discussed. One was the use of laser sintering of the conductive inks which delivers heat where required and thus reduces the total heat load on the other components and the substrate. Another was the use of microwave heating. These methods certainly hold promise for the future but the issues do not seem to be resolved yet.
Another reoccurring theme was the coffee ring effect. This is the problem you get when a spherical drop of fluid is placed on a non-absorbing substrate and dries predominantly by evaporation. Instead of a continuous dot with reasonable uniformity you tend to get the solid load dried in a peripheral ring and often with little or no material near the centre. It was pointed out that this is a particular issue if you are trying to produce an insulator as such an area is likely to develop a pin hole in the centre.
As you would perhaps expect from a conference co-located with a Non Impact Printing (NIP) meeting there was a lot of inkjet in this session. One feature of inkjet is that it prints thin layers. However, when thicker layers are needed, such as those needed for significant conductivity in PV this means multiple passes to secure lower resistance. In addition to the issues this raises of vertical layer registration one paper noted differentiation of the layers, presumably due to drying in between passes. Hybrid printing techniques were another feature of this session and it does look to be the way forward. Electrostatics and screen printing have a lot to offer too.
It is easy to dismiss thin layers as simply a result of the relatively small drops that inkjet currently produces. However, there is rather more to this to do with the dynamics of ink/media interaction so there are still tricks we can play. However, if you want really big drops a paper on inkjet by ultrasonic drop production may have the answer!
While we are on inkjet don't fall into the trap that piezo head technology is the only option for PE. Thermal inkjet is alive and well here, particularly with the power of HP behind it. Indeed there were applications in this session that only seemed to work with thermal. My opinion is that thermal has a lot to offer, particularly as we run into biological sensor applications (which will still incorporate PE) and we need it in our toolbox.
There was not only printed electronics in this session; Photovoltaics also made it in. On displays, both electrophoretic with an inkjet active matrix backplane and LCD filters by inkjet also featured.
There was an interesting variant on printed electronics presented - the wiring up of the conventionally fabricated components of an integrated circuit package by inkjetting conductive inks and insulators.
So where are we going next? Printed sensors looks to be a big one for the future and these will incorporate PE. There was a paper on textile sensors at this meeting, continuing on from work I saw presented at a conference in the UK. This and another paper featured electrodeless plating, a technique to increase the conductivity of metal tracks from a plating solution.
Issues? Yes, it isn't going to be plain sailing. We are going to have to do some more work on some elements of the system. As we move into more exotic fluids, such as those containing nanowires the issue of fluid management will need to be solved as some of the components will just fall out of suspension if you let them. And we need to do more on substrates for these applications. I did a paper on this at the 2006 Digital Fabrication conference but I guess I just have to keep shouting!
Key points? Inkjet is not going to be the only technology. In inkjet, it isn't all piezo technology. There are some interesting opportunities arising for substrates.
The day finished with an interactive discussion on Intellectual Property. As I do some work as an Expert Witness this was of particular interest to me. We had a couple of very interesting presentations of fostering innovation and the interpretation of the term "originality". More than enough to fill an already stuffed brain!
By Alan Hodgson, an independent consultant firstname.lastname@example.org.
For more attend Printed Electronics USA.
Source of top image: www.ent.ohiou.edu