A lack of conventional PCB rigidity may mean custom tooling and stencils.

Wearable electronics really aren’t that new. For many years now there have been commercial products that would be classified as wearable electronics – everything from GPS watches often used by runners for tracking distance and time to mobile heart monitors. What is new and the cause for the recent buzz is the convergence of everything – fitness tracking, medical monitoring, phone and internet connectivity, e-mail capability – into a single device. While the first touted wearable – Google Glass – didn’t quite have the predicted market appeal, the much-anticipated launch of wearable watches are a different story. It’s not only the merging of all the electronics capability, but also the intersection of function and fashion that makes the new generation of wearables so exciting. Unlike a smartphone, wrist-worn wearables aren’t just black boxes. Consumers now have a choice of colors, metal finishes, sizes and shapes, in addition to 24/7 connectivity and convenience.

For the naysayers who argue wearables are a solution without a problem, I submit the rise of the tablet as evidence of broad market viability. I’ll admit that when the first tablet was introduced, I didn’t see the need for it. “Why would you need this to do what you can do with other devices?” I questioned. I stood corrected. And I think the same is true for wearables. I was speaking with several colleagues about why you would need a phone on your wrist. Is it that inconvenient to take your phone out of your pocket? The overriding opinion among the group was that a phone integrated into a watch was something definitely worth having.

If one looks at the market projections for wearables, it would appear that many consumers feel the same as my colleagues. The predicted growth for wrist-worn wearables, according to CCS Insights, is significant – going from just under 10 million devices in 2013 to over 135 million in 2017. The wearables market has indeed arrived and so have the challenges for manufacturers.

For years I have used this space to discuss miniaturization and the printing complexities that come with it. With wearables, all the preparation we’ve been doing in terms of miniaturization is here; it’s reality. Integrating exceptionally small form factors in very high density on flexible substrates is what wearables manufacturers must achieve, in high volume at high yield.

Wearable device dimensions and flexibility add additional complexity to the printing process. While the dimensions are challenging, tools exist to manage the current product sets. Wearables manufacturers must, however, have a very close relationship with their printing supplier. Every device is different, and off-the-shelf solutions probably won’t work.

Substrates are a case in point. The majority of wearable devices are being built with multi-level flexible substrates. This dynamic dictates out-of-the-box approaches to substrate support, stencil architecture and material, and substrate transfer, among other considerations. A complete understanding of the board technology and board tolerances is absolutely critical to ensuring the proper tooling solution, as the lack of conventional rigidity (as with a standard FR-4 printed circuit board) may present challenges in terms of positional stability. Customized tooling solutions that incorporate pallets and virtual panel tooling (see my June 2013 column on this topic) are probably the most viable approaches for robust wearables substrate support during printing. Remember, not only are we dealing with very thin and very flexible substrates, but exceptionally fine pitches as well. Spot-on support and extreme precision are absolute necessities.

In addition to tooling modifications, alterations to conventional stencil designs may be required. For wearables products, not only might superior stencil materials such as fine grain (details can be found in my April 2015 column), stainless steel and nanocoatings be necessary, but the stencil design could incorporate multi-level printing in the form of a 3D stencil. The substrate may not be completely flat, with pockets where material deposition is required. To achieve solder paste printing in a single stroke for optimal throughput, a 3D stencil design is the best solution.

And, of course, there is the sheer challenge of printing ultra-fine print deposits through ever-decreasing area ratios and still achieving robust transfer efficiency.
Newer stencil materials and coatings, Type 4.5 and Type 5 solder pastes, transfer efficiency enhancing tools such as activated squeegees and, without question, print platforms with exceptional accuracy and process capability are all part of the wearables printing proposition.

With this market sector, miniaturization is here. We don’t have the luxury of the five- to 10-year learning curve as we did with smartphone iterations. Applying everything that’s been learned from the mobile market progression is what has made wearables a reality. Miniaturization has been the “what’s coming” for years. It’s now the “what’s here!” With wearables, miniaturization’s time has come and there is no room for error in the print process.

Clive Ashmore is global applied process engineering manager at ASM Assembly Systems, Printing Solutions Division, DEK (asmpt.com); clive.ashmore@asmpt.com. His column appears bimonthly.

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