Issue link: https://iconnect007.uberflip.com/i/1108006
24 FLEX007 MAGAZINE I APRIL 2019 They use these thicker blank core stiffeners in their islands. Then, we only bring out two of the layers in the tethered regions—very thin layers. You can tie the circuit in a knot, and it works just fine. It's a powerful platform upon which to build a wearable, and at the same time, we haven't had any reliability issues in terms of flexing of these stiff and rigidized flex regions. It's super thin, so the form factor is very efficient in terms of volume—a minimum of both surface area and stack height. Matties: At what point in your design process did you reach out to Lenthor—early on, or after you had the basic circuit designed? Flannery: We had the basic circuit designs in our architecture. Lenthor was not necessar- ily a participant in the circuit design; we dis- cussed the PCB platform a lot more as well as some of the assembly requirements. In the accompanying article in this magazine, I detail some of the challenges of the wearables mar- ket. For example, when you talk about circuit design and platform in the wearables market, the form factor is every bit as important as the schematic. In a lot of conventional circuit boards, it's a square, circle, or maybe an L-shape because you have to turn a corner to make a connection. But in the wearables space, the form factor and shape of your layout will determine the shape of your final product on the body. You may be designing a lay- out that goes around an ankle or a radial bone or across the collarbone or some other piece of physiology. Your layout isn't so much determined by your circuit needs as it is by your product needs. If you look at our PCB, it's barbell-shaped with an island in the middle—very non-orthogonal with curvy lines (Figure 2). Matties: You mentioned materials. When you did get Lenthor involved, what other impacts did that have on your thinking regarding the circuit? Flannery: It gave us design rules to work with. This is a new platform; we had to come up with a whole new set of design rules, such as if you're going to make this floppy circuit and move it through production, what's the mini- mal spacing? Can you still get three-mil spac- ing and a three-mil feature on the floppy tether that must go through the manufacturing pro- cess? One of the things that Lenthor did that we're happy about is they figured out how to hold a lot of their design tolerances. Even though we were doing this weird multilayer board, there was some definite innovation on their part in terms of how to get this device through their manufacturing process using practical meth- ods. They gave us limits, and through those, we determined how narrow we could make the tethers. We care about every extra millime- ter of space around the edge. We're trying to push the envelope on circuit density while still using off-the-shelf parts and keeping the cost reasonable. Matties: Lenthor, from your point of view, how was this process of what sounds like an R&D project and new product introduc- tion combined? Rich Clemente: Even though this is a unique product and wear- ables is a new and growing market, taking a design and working with the customer and customizing it for the end user is not totally unique. I can Figure 2: Final shape of the Graftworx SmartPatch PCB.