Issue link: https://iconnect007.uberflip.com/i/1073397
12 FLEX007 MAGAZINE I JANUARY 2019 Stackup Planning Most experienced PCB designers consider the layer stack as a cross-section of the board build-up of materials, and that makes perfect sense. We think of the lamination cycles, and how the PCB will be constructed at the fabricator. But there's more to it than layers of dielectric and foil—especially when it comes to flex and rigid-flex. Will components be mounted on the flex stack portion? Will you have vias in both the rigid and flex sections of the board? Will you use plated through-hole parts that require plated holes all the way through? Modelling your stack for a rigid-flex board is the first part of the design. Before you come to parts placement and routing, you at least need to develop and visualize the layer stack as a starting point. Yes, it may change during the design—if you realize you can reduce the layers of copper, you'll definitely want to try— but you need to at least begin with something not too different from what is actually going to be constructed to place and route the board efficiently. Being able to visualize the full rigid-flex stackup in 3D with thickness scal- ing is an indispensable check for validating what you had mentally pic- tured your rigid-flex con- struction to be (Figure 1). When you model the layer stack cross-section in 3D along with your defined via structures, some very interesting things are revealed: • Thicknesses of materi- als may look very dif- ferent in reality than what you imagined; for instance, you may not have accurately envi- sioned just how thick adhesive layers in a flex stack can be, which has implications for thermal properties, assembly, and signal integrity you would perhaps not otherwise explore before prototyping • You may be counting on flex layers for ground and power capacitance, but without a field solver, even just the visual check can alert your intuition that it's not going to be enough (unless you make the layer stack asymmetric to draw copper pours closer) • Placement and adhesion of stiffeners may not be what you expected • The 3D view can show you how physically difficult it might be to manufacture it; seeing clear cross-sectioned views can help you understand the additional manufacturing steps Rigid-flex Board From Assembly Like any PCB design, begin with the end-prod- uct in mind. The most common practice that I anecdotally encounter—even to this day—is that the industrial designer feeds mockups to the mechanical engineering team/designer who defines the 3D mechanical enclosure from which Figure 1: 3D layer stack in a proportional visualization.