Innovating in the Technology of Stretchable PCBs
Express Circuits is a long-established PCB fabricator located in Coleshill, near Birmingham in the Midlands of England, specialising in quick-turn complex prototype and small- to medium-volume microvia multilayer and rigid-flex. With the benefit of associate companies focused on design and assembly, Express is recognised for its exemplary customer service and technical support, as well as for its innovative manufacturing solutions.
Back in the days when I considered myself something of an innovator in PCB technology, I had the pleasure of working with Vin Makwana, who for several years now has been technical manager at Express and with whom I have maintained occasional contact. Meeting Vin at a recent Institute of Circuit Technology Seminar, and knowing of his interest in the development of stretchable circuits, I asked for an update and he invited me to visit the Express factory to see for myself. So I had the opportunity to sit down with Vin and Operations Manager Mike Hill to talk about stretchable developments.
Pete Starkey: Vin, as work colleagues we go back the best part of 30 years, and I recognise you as one of the pioneers of microvia technology. But it was not until I met you at productronica 2011 that we had any serious conversation about stretchable circuits—although it was clear that you had already built an impressive history in that technology, and were exhibiting actual examples. Here we are nearly six years later. Give me an idea how far you have moved on in that time.
Vin Makwana: We’ve come a long way since that meeting at productronica. We have attended dedicated exhibitions for wearable technology in San Francisco and Santa Clara, California, and Miami, Florida, and formed working relationships with several potential customers. One of these is based in Europe, and we have just completed a two-year Eurostars project with them, which Mike will tell you about.
Mike Hill: The project was co-funded by the EU Horizon 2020 Framework programme and InnovateUK. We have been cooperating with a Spanish company based in Barcelona, Sensing Tex, who develop and commercialise smart textiles. They primarily work with fibre optic materials and wanted to develop pressure-sensor mapping through stretchable technology for truly-wearable-tech. The project basically involved us bringing in stretchable product and working with them to integrate it into clothing for mapping posture and movement.
Starkey: Was this for health-monitoring applications?
Hill: Yes, the project was based around a garment for measuring and monitoring the posture of patients with muscle imbalance and postural dysfunction problems. After proving the concept would work, the aim of the project was to produce a development kit which could be offered to a variety of markets for companies to incorporate into their own product. That could be sportswear, military, in fact any kind of truly-wearable-tech that needed some form of pressure sensor. Our primary involvement in the project was to develop the actual circuitry, but also to increase the capability of our stretchable product. By trialing different designs provided by Sensing Tex with different material types, we achieved a balance between material and meander design which enabled up to 50% stretch on their connectors. That project has now reached its conclusion and they are putting together final demonstration kits for marketing in Europe, and for us explore commercial opportunities.
Starkey: Can we talk about substrate materials? I understood from previous conversations that the key to your initial developments was your ability to make your own copper-clad substrate. Do you still have exclusive rights to it?
Makwana: Yes, although this particular customer found that the elastomer we use didn’t give the strength they were looking for. But they had been working with similar materials which met their specification, so that’s what we used in the project. This is ours, (he held up a sheet of rubbery transparent film, shown here) and they have a slightly different version of it.
Starkey: Can you disclose what it is?
Makwana: It’s actually an off-the-shelf thermoplastic polyurethane that we buy locally, although it’s not used elsewhere in the PCB industry. We can specify whatever thickness we want, but we typically use 50-micron and 100-micron.
Starkey: So the proprietary knowledge is in how you convert it into a copper-clad laminate?
Makwana: Yes, putting the copper on is the tricky part, but once we have made the material we can use standard PCB processes—drilling, metallising, imaging, plating, etching, etc—to fabricate stretchable circuits. We have a special press, and it’s our lay-up technique and press cycles that are confidential.
Starkey: Then your base layer is foil rather than an electroless deposit?
Makwana: Yes, it’s standard ED copper foil. We would prefer to use RA copper for our application, if we could get it, but it’s not readily available other than to the major flexible laminate manufacturers.
Starkey: So your process is basically part-subtractive rather than full-additive?
Makwana: Yes, we are able to drill and metallise our substrates with electroless copper, so we can make double-sided PTH, or even a 4-layer. We can bond it three times, although obviously, that reduces its stretchability, but we have certainly made stretchable 4-layers.
Starkey: So the polyurethane becomes its own adhesive if you’ve got the right press cycle; you’re effectively fusion-bonding.
Makwana: Yes, and our solder mask is also polyurethane—it’s effectively a coverlay—so we can maintain a homogeneous elastomeric construction.
Hill: And we have moved into the assembly of these units as well—in small quantities, a mixture of hand assembly and on-line assembly.
Starkey: Are you assembling active components, or just connectors and passives?
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.
Since 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?
Makwana: 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.
Starkey: 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.
Starkey: 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.