Innovating in the Technology of Stretchable PCBs

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Hill: All sorts, using low-temperature solders with melting points about 140°C. Here’s an example of a demonstrator we made as part of the last project, which has a matrix of pressure sensors that pick up direct points of pressure without any crosstalk. Our partners have incorporated this type of circuit into appliances for measuring and monitoring how a person is sitting, for purposes of correcting posture.

Example_Demonstrator.jpgSince coming to the end of that project, we are in the early stages of the funding process for a second Eurostars project with an international consortium based in the Netherlands, and also some UK companies. That project is to develop tactical vests for the police force, for monitoring of vital signals and for tracking.

Starkey: I see some examples of what would appear to be more conventional flexible circuit designs, but they don’t look or feel like conventional flex substrates. They’re quite rubbery and extremely flexible—completely different from flex built from Kapton. Can you tell me something about them?  

Extremely_Flexible_Circuit.jpgMakwana: Yes, they’re based on a PEEK (polyether-ether-ketone) fabric which we coat with a resin and foil to produce a thin reinforced flexible copper-clad laminate, but which contains no glass. And again, the coverlay is polyurethane. These are complementary to stretchable circuits in applications like military tactical vests, where they have been used as connectors. They have been extensively tested and have out-performed alternative materials. There has been a lot of interest and we have made samples for evaluation by companies as far away as New Zealand. The applications have moved beyond the military, into wearables for police and emergency services. From the feedback we have had, they are proving to be very durable and survive many cycles of washing, for example. And we are anticipating a rapid ramp-up in demand—into the thousands. That’s going to give us a headache, but a good headache!

Starkey: What sort of partnering opportunities are there, to cope with that increasing demand?

Makwana: We have explored a couple of partnering options. One of them was a laminate supplier. The trickiest part is putting the resin on the fabric, and we thought they would be able to automate that operation, but it didn’t work well their end—they were not able to do it! In the meantime, we are producing the material in-house, but not in roll form – only as panels.

Hill: Outside of the stretchable projects we have discussed, we have been working with universities—three of them in the United States. And one of the companies we worked with in the original STELLA project still sends us a range of designs for different applications. It seems from the feedback we get that there is significant interest in medical applications, and from the sportswear industry.

Sport_Shoe.jpgStarkey: Looking back to productronica 2011, the first of your examples I saw was an insole for a sports shoe.

Makwana: Yes, now that has developed further we manufacture insoles for a company in California, which our sister company assembles. We currently have units in production in the factory here. Another example is this test circuit we have made on 50-micron polyurethane for one of the American universities that Mike mentioned. It has tracks as fine as 50 microns—you can borrow my glass to take a look! The copper foil thickness is 12 microns and the finish is standard electroless nickel/immersion gold.

Test_Circuit.jpgStarkey: That’s a very impressive example, although I wouldn’t like the job of handling material that flimsy through process machinery!

Changing the subject, do any standards exist yet for stretchable circuits, and is anyone working on standards?

Hill: We’re not aware of any industry standards. Everything we’ve done so far has been to specifications agreed within the project, or with individual customers. We inspect to basic IPC acceptability standards, and release against whatever performance requirement has been agreed to with the customer before we commit to accepting the purchase order.

Starkey: How do you manage design—where a lot of design features are unique and exclusive? Do you produce your own designs, or do you licence your design features to third-party designers?

Hill: We have basic capability guidelines, and there is close cooperation between the customer’s engineers and our own engineering team to agree design-for-manufacture rules for a specific project. There is one contract designer we know who has developed his own software, and we sometimes direct enquiries to him. But as far as the major EDA vendors are concerned, this area is so specialised that there is not yet enough commercial potential to be of real interest to them.

Starkey: But if we look at some of the projections for wearable technology in general, the growth forecasts are spectacular: 10% annually to 2018, then accelerating to 23% by 2023. The analysts reckon that the overall market for electronic products designed to be worn on the person will have grown from $30 billion in 2016 to more than $150 billion by 2026. And I am sure that stretchables will at least follow this trend.

Vin, Mike, thank you for sharing your knowledge and experience—a most enlightening half-hour. I realise that stretchables represents only part of your PCB manufacturing capability, but you seem to have established an exclusive niche technology. I wish you every success for the future.



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